Author name cluster

Alin Albu-Schäffer

Possible papers associated with this exact author name in Arrow. This page groups case-insensitive exact name matches and is not a full identity disambiguation profile.

122 papers

1 author row

IROS Conference 2025 Conference Paper

Extraction of Robotic Surface Processing Strategies from Human Demonstrations

Thomas Eiband
Lars Leimbach
Korbinian Nottensteiner
Alin Albu-Schäffer

Learning from Demonstration (LfD) is a widely used approach for teaching robot motion, but more sophisticated strategies are required to address complex tasks such as surface processing. Sanding is an example where comprehensive strategies are necessary to ensure complete and efficient coverage of the surface of a workpiece. In this paper, we present a system that captures human motions and contact forces during surface processing using a powered sanding tool. We provide a publicly available dataset that consists of demonstrations for various geometric shapes with the goal to extract robot execution strategies through LfD from a variety of users. This is in contrast to conventional LfD, which generates a policy directly from one or multiple trajectories provided by a single user. Further, we provide a data analysis that reveals key insights into how humans adapt their strategies to different surface geometries and extract robot execution strategies from it. Finally, we conduct two basic robotic experiments justifying the approach of strategy extraction. Our findings contribute to the understanding of human surface-processing behavior and lay the foundation for developing more effective robotic surface processing strategies.

ICRA Conference 2025 Conference Paper

RACCOON: Grounding Embodied Question-Answering with State Summaries from Existing Robot Modules

Samuel Bustamante-Gomez
Markus Knauer
Jeremias Thun
Stefan Schneyer
Alin Albu-Schäffer
Bernhard M. Weber
Freek Stulp

Explainability is vital for establishing user trust, also in robotics. Recently, foundation models (e. g. vision-language models, VLMs) fostered a wave of embodied agents that answer arbitrary queries about their environment and their interactions with it. However, naively prompting VLMs to answer queries based on camera images does not take into account existing robot architectures which represent the robot's tasks, skills, and beliefs about the state of the world. To overcome this limitation, we propose RACCOON, a framework that combines foundation models' responses with a robot's internal knowledge. Inspired by Retrieval-Augmented Generation (RAG), RACCOON selects relevant context, retrieves information from the robot's state, and utilizes it to refine prompts for an LLM to answer questions accurately. This bridges the gap between the model's adaptability and the robot's domain expertise.

ICRA Conference 2025 Conference Paper

VSB - Variable Stiffness Based on Bowden Cables: A Simple Mechanism for Soft Robotic Hands

Steffen Puhlmann
Alin Albu-Schäffer
Hannes Höppner

Soft robotic hands compensate for uncertainty in perception and actuation by leveraging passive deformation in their intrinsically compliant hardware, facilitating robust and dexterous interactions with their environment. The ability to adjust the level of compliance during operation has the potential to further improve the performance of these hands by enabling novel interaction strategies. However, achieving variable stiffness mechanically typically requires significant engineering complexity, making these systems difficult to manufacture, prone to error, and expensive. We present a novel, very simple mechanism for achieving variable stiffness. This mechanism employs tendon-driven antagonistic actuation, with Bowden cables connecting elastic elements to servomotors. It supports compact actuator designs, while the Bowden cables facilitate flexible component placement within a robotic system. Following our approach, variable stiffness actuators can be easily manufactured at low-cost from readily available materials. Despite its simplicity, we demonstrate that our mechanism provides consistent and precise control over stiffness levels and contact torques, showcasing its potential for a broad range of applications in soft robotic systems.

ICRA Conference 2024 Conference Paper

Singularity-Robust Prioritized Whole-Body Tracking and Interaction Control With Smooth Task Transitions

Xuwei Wu
Alin Albu-Schäffer
Alexander Dietrich

In this work, we propose a singularity-robust whole-body control framework that ensures smooth task transitions while maintaining strict priorities. The weighted generalized inverse is adopted to derive a hierarchical control law compatible with singular and redundant tasks. Moreover, a smooth activation matrix is proposed to continuously shape both null-space projectors and task-level control actions. Validation has been conducted in MATLAB/Simulink and MuJoCo simulations with Rollin’ Justin.

ICRA Conference 2022 Conference Paper

Kinematic Transfer Learning of Sampling Distributions for Manipulator Motion Planning

Peter Lehner
Máximo A. Roa
Alin Albu-Schäffer

Recent research has shown that guiding sampling-based planners with sampling distributions, learned from previous experiences via density estimation, can significantly decrease computation times for motion planning. We propose an algorithm that can estimate the density from the experiences of a robot with different kinematic structure, on the same task. The method allows to generalize collected data from one source manipulator to similarly designed target manipulators, significantly reducing the computation time for new queries for the target manipulator. We evaluate the algorithm in two experiments, including a constrained manipulation task with five different collaborative robots, and show that transferring information can significantly decrease planning time.

IROS Conference 2022 Conference Paper

Multi-Phase Multi-Modal Haptic Teleoperation

Maximilian Mühlbauer 0001
Franz Steinmetz
Freek Stulp
Thomas Hulin
Alin Albu-Schäffer

Virtual Fixtures facilitate teleoperation, for in-stance by guiding the human operator. Developing these Virtual Fixtures in tasks with tight tolerances remains challenging. Fixtures with a high stiffness allow for more precise guidance, whereas a lower stiffness is required to allow for corrections. We observed that many assembly operations can be split into different phases - approaching, positioning, in-contact manipulation - each with different accuracy requirements. Therefore, we propose to use multi-modal fixtures, satisfying the different requirements of these phases: i. e. a position-based Trajectory Fixture for approaching and a more accurate Visual Servoing Fixture for the positioning phase. A state estimation and arbitration component ensures smooth transitions between the fixtures to provide optimal support for the operator and to achieve global availability paired with local precision at the same time. It also allows a high stiffness to be used throughout, thus achieving good guidance for all phases. The approach is validated in an application from a space scenario, consisting of the assembly of a CubeSat subsystem. The empirical results from a pilot study on this task show that our approach is faster and requires less interaction force from the operator than the baseline method.

ICRA Conference 2022 Conference Paper

Planning Natural Locomotion for Articulated Soft Quadrupeds

Mathew Jose Pollayil
Cosimo Della Santina
George Mesesan
Johannes Englsberger
Daniel Seidel
Manolo Garabini
Christian Ott 0001
Antonio Bicchi

Embedding elastic elements into legged robots through mechanical design enables highly efficient oscillating patterns that resemble natural gaits. However, current trajectory planning techniques miss the opportunity of taking advantage of these natural motions. This work proposes a locomotion planning method that aims to unify traditional trajectory generation with modal oscillations. Our method utilizes task-space linearized modes for generating center of mass trajectories on the sagittal plane. We then use nonlinear optimization to find the gait timings that match these trajectories within the Divergent Component of Motion planning framework. This way, we can robustly translate the modes-aware centroidal motions into joint coordinates. We validate our approach with promising results and insights through experiments on a compliant quadrupedal robot.

ICRA Conference 2021 Conference Paper

Collision Detection, Identification, and Localization on the DLR SARA Robot with Sensing Redundancy

Maged Iskandar
Oliver Eiberger
Alin Albu-Schäffer
Alessandro De Luca 0001
Alexander Dietrich

Physical human-robot interaction is known to be a crucial aspect in modern lightweight robotics. Herein, the estimation of external interactions is essential for the effective and safe collaboration. In this work, an extended momentum-based disturbance observer is presented which includes the sensing redundancy related to additional force-torque measurements. The observer eliminates the need for acceleration measurements/estimates and it is able to accurately reconstruct multiple simultaneous contact locations. Moreover, it provides uncoupled, configuration-independent, and singularity-free estimates of the external forces. The performance of the approach is experimentally validated on the SARA robot, the new generation of DLR lightweight robots, involving high resolution force-torque sensors in a redundant arrangement.

IROS Conference 2021 Conference Paper

Embedding a Nonlinear Strict Oscillatory Mode into a Segmented Leg

Anna Sesselmann
Florian Loeffl
Cosimo Della Santina
Máximo A. Roa
Alin Albu-Schäffer

Robotic legs often lag behind the performance of their biological counterparts. The inherent passive dynamics of natural legs largely influences the locomotion and can be abstracted through the spring-loaded inverted pendulum (SLIP) model. This model is often approximated in physical robotic legs using a leg with minimal mass. Our work aims to embed the SLIP dynamics by using a nonlinear strict oscillatory mode into a segmented robotic leg with significant mass, to minimize the control required for achieving periodic motions. For the first time, we provide a realization of a nonlinear oscillatory mode in a robotic leg prototype. This is achieved by decoupling the polar task dynamics and fulfilling the resulting conditions with the physical leg design. Extensive experiments validate that the robotic leg effectively embodies the strict mode. The decoupled leg-length dynamic is exhibited in leg configurations corresponding to the stance and flight phases of the locomotion task, both for the passive system and when actuating the motors. We additionally show that the leg retains this behavior while performing jumping in place experiments.

ICRA Conference 2020 Conference Paper

Adaptive Authority Allocation in Shared Control of Robots Using Bayesian Filters

Ribin Balachandran
Hrishik Mishra
Matteo Cappelli
Bernhard M. Weber
Cristian Secchi
Christian Ott 0001
Alin Albu-Schäffer

In the present paper, we propose a novel system-driven adaptive shared control framework in which the autonomous system allocates the authority among the human operator and itself. Authority allocation is based on a metric derived from a Bayesian filter, which is being adapted online according to real measurements. In this way, time-varying measurement noise characteristics are incorporated. We present the stability proof for the proposed shared control architecture with adaptive authority allocation, which includes time delay in the communication channel between the operator and the robot. Furthermore, the proposed method is validated through experiments and a user-study evaluation. The obtained results indicate significant improvements in task execution compared with pure teleoperation.

ICRA Conference 2020 Conference Paper

Closing the Force Loop to Enhance Transparency in Time-delayed Teleoperation

Ribin Balachandran
Jee-Hwan Ryu
Mikael Jorda
Christian Ott 0001
Alin Albu-Schäffer

In the present paper, we first adopt explicit force control from general robotics and embed it into teleoperation systems to enhance the transparency by reducing the effect of the perceived inertia to the human operator and simultaneously improve contact perception. To ensure stability of the proposed teleoperation system considering time-delays, we propose a sequential design procedure based on time domain passivity approach. Experimental results of master-slave teleoperation system, based on KUKA light-weight-robots, for different values of delays are presented. Comparative analysis is conducted considering two existing approaches, namely 2-channel and 4-channel architecture based bilateral controllers, and its results clearly indicate significant improvement in force transparency owing to the proposed method. The proposed system is finally validated considering a real industrial assembly scenario.

IROS Conference 2020 Conference Paper

EDAN: An EMG-controlled Daily Assistant to Help People With Physical Disabilities

Jörn Vogel
Annette Hagengruber
Maged Iskandar
Gabriel Quere
Ulrike Leipscher
Samuel Bustamante-Gomez
Alexander Dietrich
Hannes Höppner

Injuries, accidents, strokes, and other diseases can significantly degrade the capabilities to perform even the most simple activities in daily life. A large share of these cases involves neuromuscular diseases, which lead to severely reduced muscle function. However, even though affected people are no longer able to move their limbs, residual muscle function can still be existent. Previous work has shown that this residual muscular activity can suffice to apply an EMG-based user interface. In this paper, we introduce DLR's robotic wheelchair EDAN (EMG-controlled Daily Assistant), which is equipped with a torque-controlled, eight degree-of-freedom light-weight arm and a dexterous, five-fingered robotic hand. Using electromyography, muscular activity of the user is measured, processed and utilized to control both the wheelchair and the robotic manipulator. This EMG-based interface is enhanced with shared control functionality to allow for efficient and safe physical interaction with the environment.

IROS Conference 2020 Conference Paper

Joint-Level Control of the DLR Lightweight Robot SARA

Maged Iskandar
Christian Ott 0001
Oliver Eiberger
Manuel Keppler
Alin Albu-Schäffer
Alexander Dietrich

Lightweight robots are known to be intrinsically elastic in their joints. The established classical approaches to control such systems are mostly based on motor-side coordinates since the joints are comparatively stiff. However, that inevitably introduces errors in the coordinates that actually matter: the ones on the link side. Here we present a new joint-torque controller that uses feedback of the link-side positions. Passivity during interaction with the environment is formally shown as well as asymptotic stability of the desired equilibrium in the regulation case. The performance of the control approach is experimentally validated on DLR's new generation of lightweight robots, namely the SARA robot, which enables this step from motor-side-based to link-sided-based control due to sensors with higher resolution and improved sampling rate.

ICRA Conference 2020 Conference Paper

Robust, Locally Guided Peg-in-Hole using Impedance-Controlled Robots

Korbinian Nottensteiner
Freek Stulp
Alin Albu-Schäffer

We present an approach for the autonomous, robust execution of peg-in-hole assembly tasks. We build on a sampling-based state estimation framework, in which samples are weighted according to their consistency with the position and joint torque measurements. The key idea is to reuse these samples in a motion generation step, where they are assigned a second task-specific weight. The algorithm thereby guides the peg towards the goal along the configuration space. An advantage of the approach is that the user only needs to provide: the geometry of the objects as mesh data, as well as a rough estimate of the object poses in the workspace, and a desired goal state. Another advantage is that the local, online nature of our algorithm leads to robust behavior under uncertainty. The approach is validated in the case of our robotic setup and under varying uncertainties for the classical peg-in-hole problem subject to two different geometries.

ICRA Conference 2019 Conference Paper

A Coordinate-based Approach for Static Balancing and Walking Control of Compliantly Actuated Legged Robots

Dominic Lakatos
Yuri Federigi
Thomas Gumpert
Bernd Henze
Milan Hermann
Florian Loeffl
Florian Schmidt 0001
Daniel Seidel

The paper addresses the static balancing and walking of elastically actuated legged robots. The control is realized by commanding the motor positions only and exploiting the bijective relation between motor and link positions at equilibrium under static external forces. The approach is formulated in a quite general framework first. The main implementation contribution is the definition of a body coordinate system and of an appropriate set of constraints, which leads to a fully determined system of equations. In addition to the desired COM and the vertical foot positions, which are defined by the walking task and the terrain, the imposed constraints are related to distances between individual legs. The controller is experimentally validated on a compliantly actuated quadruped.

ICRA Conference 2019 Conference Paper

Exact Modal Characterization of the Non Conservative Non Linear Radial Mass Spring System

Cosimo Della Santina
Dominic Lakatos
Antonio Bicchi
Alin Albu-Schäffer

Since the spread of robotic systems embedding in their mechanics purposefully designed elastic elements, the interest in characterizing and exploiting non-linear oscillatory behaviors has progressively grown. However, few works so far looked at the problem from the point of view of modal analysis. This is particularly surprising if considered the central role that modal theory had in the development of classic results in analysis and control of linear mechanical systems. With the aim of making a step toward translating and extending this powerful tool to the robotic field, we present the complete modal characterization of a simple yet representative non-linear elastic robot: the 2D planar mass-spring-damper system. Generic non-linear elastic forces and dissipative effects are considered. We provide here exact descriptions of the two non-linear normal modes of the system. We then extend the analysis to generic combinations of the modes in conservative case and for small damping. Simulations are provided to illustrate the theoretical results. This is one of the very firsts applications of normal mode theory to dynamically coupled non-linear systems, and the first exact result in the field.

ICRA Conference 2019 Conference Paper

Experiments with Human-inspired Behaviors in a Humanoid Robot: Quasi-static Balancing using Toe-off Motion and Stretched Knees

Bernd Henze
Máximo A. Roa
Alexander Werner
Alexander Dietrich
Christian Ott 0001
Alin Albu-Schäffer

Humanoid robots typically display locomotion patterns that include walking with flat foot-ground contact, and knees slightly bent. However, analysis of human gait indicate that several physiological mechanisms like stretched knees, heel-strike and toe push-off increase the step length and energetic efficiency of locomotion. This paper presents an implementation of two of those mechanisms, namely stretched knees and push-off, on a quasi-static whole-body balancing controller. The influence of such mechanisms on the kinematic capabilities of the DLR humanoid robot TORO is analyzed in different experiments, and their benefits are thoroughly discussed. As a result, the energetic savings of balancing with stretched knees are shown to be of reduced magnitude with respect to the overall power consumption of the robot, and the ability of TORO for negotiating stairs is greatly enhanced.

ICRA Conference 2019 Conference Paper

Visual Repetition Sampling for Robot Manipulation Planning

En Yen Puang
Peter Lehner
Zoltán-Csaba Márton
Maximilian Durner
Rudolph Triebel
Alin Albu-Schäffer

One of the main challenges in sampling-based motion planners is to find an efficient sampling strategy. While methods such as Rapidly-exploring Random Tree (RRT) have shown to be more reliable in complex environments than optimization-based methods, they often require longer planning times, which reduces their usability for real-time applications. Recently, biased sampling methods have shown to remedy this issue. For example Gaussian Mixture Models (GMMs) have been used to sample more efficiently in feasible regions of the configuration space. Once the GMM is learned, however, this approach does not adapt its biases to individual planning scene during inference. Hence, we propose in this work a more efficient sampling strategy to further bias the GMM based on visual input upon query. We employ an autoencoder trained entirely in simulation to extract features from depth images and use the latent representation to adjust the weights of each mixture components in the GMM. We show empirically that this improves the sampling efficiency of an RRT motion planner in both real and simulated scenes.

IROS Conference 2018 Conference Paper

Data-Driven Discrete Planning for Targeted Hopping of Compliantly Actuated Robotic Legs

Daniel Seidel
Dominic Lakatos
Alin Albu-Schäffer

Motion planning for fast locomotion of compliantly actuated robotic legs is generally considered to be a challenging issue, posing considerable real-time problems. This is at least the case if time-continuous trajectories need to be generated online. In this paper we take advantage of a simple controller structure, which reduces the motion planning to a discrete-time planning problem, in which only a small set of input parameters need to be determined for each step. We show that for a planar leg with serial elastic actuation, hopping on a ground with stairs of irregular length and height can be planned online, based on a parameter mapping which has been learned in a data-driven manner by performing hopping trials with an adaptive exploration algorithm to evenly sample the parameter space. Experiments on a planar hopping leg prototype validate the approach.

IROS Conference 2018 Conference Paper

Elastic Structure Preserving Impedance (ESπ)Control for Compliantly Actuated Robots

Manuel Keppler
Dominic Lakatos
Christian Ott 0001
Alin Albu-Schäffer

We present a new approach for Cartesian impedance control of compliantly actuated robots with possibly nonlinear spring characteristics. It reveals a remarkable stiffness and damping range in the experimental evaluation. The most interesting contribution, is the way the desired closed-loop dynamics is designed. Our control concept allows to add a desired stiffness and damping directly on the end-effector, while leaving the system structure intact. The intrinsic inertial and elastic properties of the system are preserved. This is achieved by introducing new motor coordinates that reflect the desired spring and damper terms. Theoretically, by means of additional motor inertia shaping it is possible to make the end-effector interaction behavior with respect to external loads approach, arbitrarily close, the interaction behavior that is achievable by classical Cartesian impedance control on rigid robots. The physically motivated design approach allows for an intuitive understanding of the resulting closed-loop dynamics. We perform a passivity and stability analysis on the basis of al physically motivated storage and Lyapunov function.

IROS Conference 2018 Conference Paper

Inferring Semantic State Transitions During Telerobotic Manipulation

Adrian Simon Bauer
Peter Schmaus
Alin Albu-Schäffer
Daniel Leidner

Human teleoperation of robots and autonomous operations go hand in hand in today's service robots. While robot teleoperation is typically performed on low to medium levels of abstraction, automated planning has to take place on a higher abstraction level, i. e. by means of semantic reasoning. Accordingly, an abstract state of the world has to be maintained in order to enable an operator to switch seamlessly between both operational modes. We propose a novel approach that combines simulation based geometric tracking and semantic state inference by means of so called State Inference Entities to overcome this issue. We also demonstrate how Evolutionary Strategies can be employed to refine simulation parameters. All experiments are demonstrated in real-world experiments conducted with the humanoid robot Rollin' Justin.

ICRA Conference 2018 Conference Paper

Workspace Fixation for Free-Floating Space Robot Operations

Alessandro M. Giordano
Davide Calzolari
Alin Albu-Schäffer

When a space robot accidentally or voluntarily comes in contact with a target object, a workspace shift happens due to exchange of momentum between the objects. The problem of workspace adjustment is addressed herein. A novel controller is derived to simultaneously adjust the workspace and control the end-effector pose. The controller is based on a center-of-mass (CoM) regulation which fixes the workspace in the inertial space while leaving the base free to move, resulting in fuel efficiency. The control is validated on hardware using a robotic simulator composed of a seven degree-of-freedom (DOF) arm mounted on a 6DOF moving base.

ICRA Conference 2017 Conference Paper

Haptic intention augmentation for cooperative teleoperation

Michael Panzirsch
Ribin Balachandran
Jordi Artigas
Cornelia Riecke
Manuel Ferre
Alin Albu-Schäffer

Multiple robotic agents, autonomous or teleoperated, can be employed to synergise and cooperate to achieve a common objective more effectively. Tasks using robotic manipulators can be eased and improved in terms of reliability, adaptability and ergonomics via robot cooperation. In spite of visual and haptic feedback, cooperative telemanipulation of multiple robots by distant operators can still be challenging due to practical limitations in synchronisation and supervision. This paper presents a new control approach for haptic intention augmentation between two human operators handling objects via teleoperation in a cooperative manner. The force feedback to each operator is enhanced by information on the motion intention of the other operator observed by a force sensor at the input devices. Besides on-ground experiments, an experiment is presented that involves the cooperative teleoperation of an on-ground robot by a cosmonaut on the International Space Station and another distant operator on ground.

IROS Conference 2017 Conference Paper

Repetition sampling for efficiently planning similar constrained manipulation tasks

Peter Lehner
Alin Albu-Schäffer

We present repetition sampling, a new adaptive strategy for sampling based planning, which extracts information from previous solutions to focus the search for a similar task on relevant configuration space. We show how to generate distributions for repetition sampling by learning Gaussian Mixture Models from prior solutions. We present how to bias a sampling based planner with the learned distribution to generate new paths for similar tasks. We illustrate our method in a simple maze which explains the generation of the distribution and how repetition sampling can generalize over different environments. We show how to apply repetition sampling to similar constrained manipulation tasks and present our results including significant speedup in execution time when compared to uniform sampling.

ICRA Conference 2016 Conference Paper

A passivity-based approach for trajectory tracking and link-side damping of compliantly actuated robots

Manuel Keppler
Dominic Lakatos
Christian Ott 0001
Alin Albu-Schäffer

This paper presents a control method to implement trajectory tracking and disturbance rejection characteristics for the link-side dynamics of compliantly actuated robots with nonlinear spring characteristics. This is achieved by introducing new motor coordinates reflecting the damping and feedforward terms and shaping the dynamics of the motor such that it structurally equals the dynamics in the original coordinates. Thus, the approach achieves the control goal while changing the original plant dynamics only to a minimum extent. Passivity, stability, and convergence properties of the closedloop dynamics are proven. The performance of the control approach has been experimentally evaluated on the variable stiffness robot arm DLR Hand Arm System, where the stiffness in each of the joints is highly nonlinear. To our best knowledge, this is the first experimentally validated tracking controller for compliantly actuated robots with nonlinear elastic elements.

ICRA Conference 2016 Conference Paper

KONTUR-2: Force-feedback teleoperation from the international space station

Jordi Artigas
Ribin Balachandran
Cornelia Riecke
Martin Stelzer
Bernhard M. Weber
Jee-Hwan Ryu
Alin Albu-Schäffer

This paper presents a new robot controller for space telerobotics missions specially designed to meet the requirements of KONTUR-2, a German & Russian telerobotics mission that addressed scientific and technological questions for future planetary explorations. In KONTUR-2, Earth and ISS have been used as a test-bed to evaluate and demonstrate a new technology for real-time telemanipulation from space. During the August 2015' experiments campaign, a cosmonaut teleoperated a robot manipulator located in Germany, using a force-feedback joystick from the Russian segment of the International Space Station (ISS). The focus of the paper is on the design and performance of the bilateral controller between ISS joystick and Earth robot. The controller is based on a 4-Channels architecture in which stability is guaranteed through passivity and the Time Delay Power Network (TDPN) concept. We show how the proposed approach successfully fulfills mission requirements, specially those related to system operation through space links and internet channels, involving time delays and data losses of different nature.

ICRA Conference 2016 Conference Paper

Optical-inertial tracking of an input device for real-time robot control

Florian Steidle
Andreas Tobergte
Alin Albu-Schäffer

Minimally invasive robotic surgery systems are usually controlled by input devices, that are mechanically linked to the environment. These input devices often have a limited workspace, which makes intuitive operation difficult. This paper presents a tracking algorithm of a handheld input device, which combines inertial and optical measurements to obtain accurate and robust state estimates with high update rates and low latency. It is based on the fusion of inertial and optical data in an error state extended Kalman filter. To achieve a high degree of robustness with respect to partial device occlusions, active optical markers are tracked and their 2D positions in the camera planes are directly forwarded to the fusion process. The algorithm can handle partial occlusions of the device in one or all of the cameras. A quality measure is defined, which indicates if tracking performance is sufficient to control a robot. An exemplary task in a medical robotics context verifies the assumption that the tracking system can be used for real-time robot control despite frequent marker occlusions.

IROS Conference 2016 Conference Paper

Robotic simulation of on orbit servicing including hard impacts

Friedrich Lange
Gerhard Grunwald
Alin Albu-Schäffer

Industrial robots are often used for the simulation of satellites during on orbit servicing. In order to cover also the docking phase, both robots are equipped with force-torque sensors, and the measured forces and torques are taken to compute the desired motion of the position controlled robots. Since the system dynamics of robots and of free floating bodies obviously differ, for each robot we distinguish between the really executed and the assumed satellite motion. The difference between the two motions is used to adapt the measured forces in such a way that they correspond to the satellite's trajectory. In this way the docking procedure can be visualized by two robots which closely follow the satellites' trajectories. Stability of the robot control is not compromised even if the dynamics of the satellites and the robots are totally different. Simulation results verify the approach.

IROS Conference 2015 Conference Paper

Classifying compliant manipulation tasks for automated planning in robotics

Daniel Leidner
Christoph Borst 0001
Alexander Dietrich
Michael Beetz
Alin Albu-Schäffer

Many household chores and industrial manufacturing tasks require a certain compliant behavior to make deliberate physical contact with the environment. This compliant behavior can be implemented by modern robotic manipulators. However, in order to plan the task execution, a robot requires generic process models of these tasks which can be adapted to different domains and varying environmental conditions. In this work we propose a classification of compliant manipulation tasks meeting these requirements, to derive related actions for automated planning. We also present a classification for the sub-category of wiping tasks, which are most common and of great importance in service robotics. We categorize actions from an object-centric perspective to make them independent of any specific robot kinematics. The aim of the proposed taxonomy is to guide robotic programmers to develop generic actions for any kind of robotic systems in arbitrary domains.

ICRA Conference 2015 Conference Paper

Passivity of virtual free-floating dynamics rendered on robotic facilities

Marco De Stefano
Jordi Artigas
Wolfgang Rackl
Alin Albu-Schäffer

This paper describes a control strategy to achieve high fidelity dynamics simulation rendered on admittance controlled robotic facilities. It explores the reasons for an increasing energy found in the virtual dynamics of a free-floating satellite rendered on a six degree of freedom robot, which can lead the system to become unstable and proposes a method to cope with it. The proposed method identifies the sources of intrinsic instability provoked by time delays that are found in the computational loop of the rendered dynamics and counteracts their destabilizing effects using the passivity criteria. The performance of the system and the benefits of the method are shown in simulations and are verified experimentally.

IROS Conference 2015 Conference Paper

Robotic agents capable of natural and safe physical interaction with human co-workers

Michael Beetz
Georg Bartels
Alin Albu-Schäffer
Ferenc Balint-Benczedi
Rico Belder
Daniel Beßler
Sami Haddadin
Alexis Maldonado

Many future application scenarios of robotics envision robotic agents to be in close physical interaction with humans: On the factory floor, robotic agents shall support their human co-workers with the dull and health threatening parts of their jobs. In their homes, robotic agents shall enable people to stay independent, even if they have disabilities that require physical help in their daily life - a pressing need for our aging societies. A key requirement for such robotic agents is that they are safety-aware, that is, that they know when actions may hurt or threaten humans and actively refrain from performing them. Safe robot control systems are a current research focus in control theory. The control system designs, however, are a bit paranoid: programmers build “software fences” around people, effectively preventing physical interactions. To physically interact in a competent manner robotic agents have to reason about the task context, the human, and her intentions. In this paper, we propose to extend cognition-enabled robot control by introducing humans, physical interaction events, and safe movements as first class objects into the plan language. We show the power of the safety-aware control approach in a real-world scenario with a leading-edge autonomous manipulation platform. Finally, we share our experimental recordings through an online knowledge processing system, and invite the reader to explore the data with queries based on the concepts discussed in this paper.

IROS Conference 2015 Conference Paper

Targeted jumping of compliantly actuated hoppers based on discrete planning and switching control

Dominic Lakatos
Daniel Seidel
Werner Friedl
Alin Albu-Schäffer

We address the operation of robotic legs with intrinsic elasticity in hopping cycles determined by the mechanical resonant properties of the system. This ensures energy efficiency and high jumping velocity and distance. Recently, we have shown in simulation that a simple, biologically inspired bang-bang controller operating in the local coordinate of the first resonant mode leads to limit cycles which are robust with respect to leg model uncertainties and ground properties. In this paper we address the velocity control of the hopping and the planning of the bang-bang control parameters for the case that the systems should not move at steady state velocity, but should have different step lengths and heights. We exploit the discrete structure and the small number of parameters of the controller to develop a fast optimization procedure for generating an arbitrary sequence of steps. This approach can provide high motion performance, robustness and substantial computational time saving compared to continuous trajectory and controller gain planning. The stationary and the aperiodic hopping is validated by experiments on a new planar elastic leg.

ICRA Conference 2015 Conference Paper

The OOS-SIM: An on-ground simulation facility for on-orbit servicing robotic operations

Jordi Artigas
Marco De Stefano
Wolfgang Rackl
Roberto Lampariello
Bernhard Brunner
Wieland Bertleff
Robert Burger
Oliver Porges

On-orbit servicing involves a new class of space missions in which a servicer spacecraft is launched into the orbit of a target spacecraft, the client. The servicer navigates to the client with the intention of manipulating it, using a robotic arm. Within this framework, this work presents a new robotic experimental facility which was recently built at the DLR to support the development and experimental validation of such orbital servicing robots. The facility allows reproducing a close-proximity scenario under realistic three-dimensional orbital dynamics conditions. Its salient features are described here, to include a fully actuated macro-micro system with multiple sensing capabilities, and analyses on its performance including the amount of space environment volume that can be simulated.

ICRA Conference 2014 Conference Paper

A model-free approach to vibration suppression for intrinsically elastic robots

Florian Petit
Christian Ott 0001
Alin Albu-Schäffer

Robots with joint elasticity find increasing interest in many research areas. A common design goal is to achieve as little mechanical joint damping as possible. To still achieve system damping often control systems are used. Here, we present a model-free approach to achieve damping via exploiting the kinetic to potential energy transformation process of the robot mass and the joint elasticity. The controller acts in an energetically passive way and is applicable to multi-joint systems. The theoretical findings and simulations are substantiated by experiments on the DLR Hand Arm System.

ICRA Conference 2014 Conference Paper

Jumping control for compliantly actuated multilegged robots

Dominic Lakatos
Gianluca Garofalo
Alexander Dietrich
Alin Albu-Schäffer

A feedback control to generate jumping motions for compliantly actuated multilegged robots is proposed. The method allows to specify the direction of the jumping motion. This is achieved by a constraint that defines a one-dimensional submanifold and a bang-bang control which generates a limit cycle on this submanifold. The approach is based on classical impedance control with the difference that the stiffness on the submanifold and the force to preserve a predefined nominal body configuration result from the intrinsic mechanical springs in the joints. Furthermore, we propose two controller implementations: the first implementation does not require to detect the contact state, while the second implementation requires contact state detection, but accounts in addition for Coulomb friction constraints. The controller is validated in simulation with a compliantly actuated quadruped.

ICRA Conference 2014 Conference Paper

Object-centered hybrid reasoning for whole-body mobile manipulation

Daniel Leidner
Alexander Dietrich
Florian Schmidt 0001
Christoph Borst 0001
Alin Albu-Schäffer

Many houseworks such as cleaning the floor or wiping the windows require to manipulate tools over wide areas. It is necessary to move along a path while manipulating a tool with the whole body and applying exactly the right amount of force to successfully accomplish the task. So mastering such a challenge demands detailed knowledge about the involved objects and the underlying process models. Reasoning about an appropriate parameterization of the task is thereby essential. In this paper we propose a combination of object-centered hybrid reasoning and compliant force control to solve complex whole-body mobile manipulation issues. Depending on the objects involved in the task, an appropriate controller is selected and automatically parameterized. The methods are validated in an elaborate experiment on the humanoid robot Rollin' Justin.

ICRA Conference 2014 Conference Paper

The Hardware Abstraction Layer - Supporting control design by tackling the complexity of humanoid robot hardware

Stefan Jörg
Jan Tully
Alin Albu-Schäffer

Humanoid robots are highly integrated robotic systems with many sensors and actuators. The DLR Hand Arm system has 52 motors and 430 sensors. Design goal is to operate the robot with high-performance feedback control at cycles beyond 1kHz but to retain the flexibility of prototyping control algorithms. This is achieved with the concept of a Hardware Abstraction Layer (HAL) that provides a convenient high-level interface to the entire robotic hardware. The requirements for the design of a HAL in terms of functionality and implementation are presented that improve the integration of complex robot hardware with prototype control applications. Experiments with the HAL of the DLR Hand Arm System demonstrate the high performance of a Simulink Interface implementation. A 3 kHz control loop yields a latency below 333/is and a jitter below 50/xs.

IROS Conference 2014 Conference Paper

Trajectory generation for continuous leg forces during double support and heel-to-toe shift based on divergent component of motion

Johannes Englsberger
Twan Koolen
Sylvain Bertrand
Jerry E. Pratt
Christian Ott 0001
Alin Albu-Schäffer

This paper works with the concept of Divergent Component of Motion (DCM), also called ‘(instantaneous) Capture Point’. We present two real-time DCM trajectory generators for uneven (three-dimensional) ground surfaces, which lead to continuous leg (and corresponding ground reaction) force profiles and facilitate the use of toe-off motion during double support. Thus, the resulting DCM trajectories are well suited for real-world robots and allow for increased step length and step height. The performance of the proposed methods was tested in numerous simulations and experiments on IHMC's Atlas robot and DLR's humanoid robot TORO.

ICRA Conference 2014 Conference Paper

Workspace analysis for a kinematically coupled torso of a torque controlled humanoid robot

Alexander Dietrich
Melanie Kimmel
Thomas Wimböck
Sandra Hirche
Alin Albu-Schäffer

The workspace and performance of a humanoid robot is decisively influenced by the design of its torso. The joints or spinal discs are usually the weak points due to the high stress they are exposed to, e. g. when lifting heavy objects. One way to circumvent the necessity of large motors is to use parallel mechanisms to optimize the distribution of loads. Here, we analyze the workspace of the humanoid robot Rollin' Justin of the German Aerospace Center (DLR) w. r. t. the constraints imposed by kinematic coupling of torso joints via tendons. The results of the analysis can be used for planning and reactive control to efficiently exploit the torso performance capabilities of the robotic system. As an application, we design a potential field based controller to avoid violating these constraints and implement it on the real robot.

IROS Conference 2013 Conference Paper

A modally adaptive control for multi-contact cyclic motions in compliantly actuated robotic systems

Dominic Lakatos
Martin Görner
Florian Petit
Alexander Dietrich
Alin Albu-Schäffer

Compliant actuators in robotic systems improve robustness against rigid impacts and increase the performance and efficiency of periodic motions such as hitting, jumping and running. However, in the case of rigid impacts, as they can occur during hitting or running, the system behavior is changed compared to free motions which turns the control into a challenging task. We introduce a controller that excites periodic motions along the direction of an intrinsic mechanical oscillation mode. The controller requires no model knowledge and adapts to a modal excitation by means of measurement of the states. We experimentally show that the controller is able to stabilize a hitting motion on the variable stiffness robot DLR Hand Arm System. Further, we demonstrate by simulation that the approach applies for legged robotic systems with compliantly actuated joints. The controlled system can approach different modes of motion such as jumping, hopping and running, and thereby, it is able to handle the repeated occurrence of robot-ground contacts.

IROS Conference 2013 Conference Paper

A robust sagittal plane hexapedal running model with serial elastic actuation and simple periodic feedforward control

Martin Görner
Alin Albu-Schäffer

In this article we present a sagittal plane, sprawled posture hexapedal running model with distributed body inertia, massless legs and serial elastic actuation at the hips as well as along the telescoping legs. We show by simulation that simple, periodic, feedforward controlled actuation is sufficient to obtain steady period 1 running gaits at twice the actuation frequency. We observe a nearly linear relation of average running speed and actuation frequency. The ground reaction profiles of the legs show leg specialization as observed in running insects. Interleg phasing has a strong influence on the foot fall sequence and thus the overall body dynamics. While the single leg ground reaction force profiles show little dependency on interleg actuation phase the total reaction force does. Thus, depending on the interleg actuation phase body motions without flight phase are observed as well as body motions and total ground reaction forces that show similarities to those obtained for the spring loaded inverted pendulum model. Further, we show that including leg damping and a ground friction model the periodic orbits have a large region of attraction with respect to the initial conditions. Additionally, the model quickly rejects step up and step down disturbances as well as force impulses. Finally, we briefly discuss the energetics of the hexapedal running model.

IROS Conference 2013 Conference Paper

Dynamic optimality in real-time: A learning framework for near-optimal robot motions

Roman Weitschat
Sami Haddadin
Felix Huber
Alin Albu-Schäffer

Elastic robots have a distinct feature that makes them especially interesting to optimal control: their ability to mechanically store and release potential energy. However, solving any kind of optimal control problem for such highly nonlinear dynamics is feasible only numerically, i. e. offline. In turn, optimal solutions would only contribute a clear benefit for dynamic environments/tasks (apart from rather general insights), if they would be accessible/generalizable in real-time. In this paper, we propose a framework for executing near-optimal motions for elastic arms in real-time. We approach the problem as follows. First, we define a set of prototypical optimal control problems. These represent a reasonable set of motions that an intrinsically elastic robot arm is sought to execute. Exemplary, we solve the optimal control problem for some of these prototypes in a roughly covered task space. Then, we encode the resulting optimal trajectories in a dynamical system via Dynamic Movement Primitives (DMPs). Finally, a distance and cost function based metric forms the basis to generalize from the learned parameterizations to a new unsolved optimal control problem in real-time. In short, we intend to overcome the well known problems of optimal control and learning with associated generalization: being offline and being suboptimal, respectively.

ICRA Conference 2013 Conference Paper

Evaluation of human safety in the DLR Robotic Motion Simulator using a crash test dummy

Karan Sharma
Sami Haddadin
Sebastian Minning
Johann Heindl
Tobias Bellmann
Sven Parusel
Tim Rokahr
Alin Albu-Schäffer

The DLR Robot Motion Simulator is a serial kinematics based platform that employs an industrial robot (as opposed to the conventional ‘Hexapod’) to impart motion cues to the attached simulator cell. This simulation platform is the culmination of ongoing research on motion simulation at the Robotics and Mechatronics Center, German Aerospace Center (DLR). Safety tests were undertaken to ascertain the effects of critical motions and subsequent emergency stop procedures on the prospective human passengers of the simulator cell. To this end, an Anthropomorphic Test Device (ATD) aka ‘crash test dummy’ was used as a human surrogate for these tests. Several severity indices were evaluated for the head-neck region, which was found to be more susceptible to injuries compared to the rest of the body. The results of this study are discussed in this paper.

IROS Conference 2013 Conference Paper

First analysis and experiments in aerial manipulation using fully actuated redundant robot arm

Felix Huber
Konstantin Kondak
Kai Krieger
Dominik Sommer
Marc Schwarzbach
Maximilian Laiacker
Ingo Kossyk
Sven Parusel

In this paper we describe a system for aerial manipulation composed of a helicopter platform and a fully actuated seven Degree of Freedom (DoF) redundant industrial robotic arm. We present the first analysis of such kind of systems and show that the dynamic coupling between helicopter and arm can generate diverging oscillations with very slow frequency which we called phase circles. Based on the presented analysis, we propose a control approach for the whole system. The partial decoupling between helicopter and arm — which eliminates the phase circles — is achieved by means of special movement of robotic arm utilizing its redundant DoF. For the underlying arm control a specially designed impedance controller was proposed. In different flight experiments we showcase that the proposed kind of system type might be used in the future for practically relevant tasks. In an integrated experiment we demonstrate a basic manipulation task — impedance based grasping of an object from the environment underlaying a visual object tracking control loop.

ICRA Conference 2013 Conference Paper

Modal limit cycle control for variable stiffness actuated robots

Dominic Lakatos
Gianluca Garofalo
Florian Petit
Christian Ott 0001
Alin Albu-Schäffer

This paper presents a control approach to stabilize limit cycle motions along a mechanical mode of variable stiffness actuated (VSA) robots. Thereby, first a PD controller with gravity and Coriolis/centrifugal compensation shapes a desired dynamics, which is decoupled in terms of modal coordinates. Then an asymptotically stable limit cycle is generated on the link side dynamics for a selected mode. Finally, the modal control approach first introduced for rigid robots is extended to the VSA case. This is done by a joint torque controller, which decouples the torque dynamics from the link side dynamics. Stability and convergence are proven for the dynamics resulting from each feedback control. Furthermore, the energy efficiency of the proposed approach is verified by simulation and experiments on the VSA robotic arm DLR Hand Arm System.

IROS Conference 2013 Conference Paper

Multi-objective compliance control of redundant manipulators: Hierarchy, control, and stability

Alexander Dietrich
Christian Ott 0001
Alin Albu-Schäffer

Robots with a large number of actuated degrees of freedom are usually redundant w. r. t. a given task. That kinematic redundancy can be utilized to execute additional tasks simultaneously, e. g. via null space projection techniques. We introduce a new representation of hierarchical robot dynamics which are based on a set of particular null space velocities. Dynamic consistency is preserved, and strict compliance with the order of priority is ensured at all times due to a power-conserving cancellation of coupling terms by active control. No external force measurements have to be performed. We show asymptotic stability of the generic closed-loop system with an arbitrary number of hierarchy levels. Several simulations confirm our results.

ICRA Conference 2013 Conference Paper

Nonlinear oscillations for cyclic movements in variable impedance actuated robotic arms

Dominic Lakatos
Florian Petit
Alin Albu-Schäffer

Biologically inspired Variable Impedance Actuators (VIA) offer the capability to execute cyclic and/or explosive multi degree of freedom (DoF) motions efficiently by storing elastic energy. This paper studies the preconditions which allow to induce robust cyclic motions for strongly nonlinear, underactuated multi DoF robotic arms. By experimental observations of human motor control, a simple control law is deduced. This controller achieves intrinsic oscillatory motions by switching the motor position triggered by a joint torque threshold. Using the derived controller, the periodic behavior of the robotic arm is analyzed in simulations. It is found that a modal analysis of the linearized system at the equilibrium point allows to qualitatively predict the periodic behavior of this type of strongly nonlinear systems. The central statement of this paper is that cyclic motions can be induced easily in VIA systems, if the eigenfrequencies and modal damping values of the linearized system are well separated. Validation is given by simulation and experiments, where a human controls a simulated robotic arm, and the developed regulator controls a robotic arm in simulation and experiments.

IROS Conference 2013 Conference Paper

On the closed form computation of the dynamic matrices and their differentiations

Gianluca Garofalo
Christian Ott 0001
Alin Albu-Schäffer

In this paper we review and extend some classic results on rigid body dynamics, in order to give a symbolic expression of the different derivatives of the matrices of the dynamic model of a general tree-structured robot. In what follows the matrices are differentiated with respect to time, state and dynamic parameters. Obviously from the derivatives of the single matrices it is possible to recover the derivatives of the direct and inverse dynamic functions and classic results like the regressor matrix. Moreover an iterative algorithm is sketched which allows to compute all these derivatives as well as the kinematics and dynamics of the robot.

IROS Conference 2013 Conference Paper

Optimal control for haptic rendering: Fast energy dissipation and minimum overshoot

Thomas Hulin
Ricardo Gonzalez Camarero
Alin Albu-Schäffer

Controlling haptic devices in an optimal way is crucial to achieve both, best performance and most realistic haptic feedback. The present article investigates control design of a single degree of freedom haptic device that is interacting with a human operator and rendering a virtual wall affected by time delay. To this end, it suggests different optimization criteria based on the step response of the haptic system. These criteria cover fundamental requirements for efficiently using haptic devices, particularly fast settling and minimum overshoot. For each criterion an optimal path and point inside the stable region of the virtual wall parameters is derived. These optima depend mainly on the system mass, sampling time and time delay. This approach is supported by experiments on two devices, a Falcon haptic device and a DLR/KUKA Light-Weight Robot arm.

IROS Conference 2013 Conference Paper

Task-specific evaluation of kinematic designs for instruments in minimally invasive robotic surgery

Bastian Deutschmann
Rainer Konietschke
Alin Albu-Schäffer

In minimally invasive robotic surgery, slender instruments are used that provide additional degrees of freedom inside the human body. Kinematic limitations due to the instrument could endanger the secure execution of a surgical task. Numerous design alternatives are proposed in literature whereas few work is done that evaluates the performance of these instruments objectively. This paper presents a new method to evaluate alternative designs of instrument kinematics with respect to their ability to perform surgical tasks. Two specific criteria are set up accounting for the limited space during a minimally invasive intervention as well as for the ability to execute the desired task. The evaluation is based on task specific reference trajectories which are recorded in one orientation. During robotic surgery, arbitrary orientations of this area can occur. The method is able to handle this by rotating the acquired reference trajectories within software. The presented method is independent from the setup, i. e. the relative position and orientation of the area of interest with respect to the trocar location. Four different examples demonstrate the application of the method to show its usefulness.

IROS Conference 2013 Conference Paper

Three-dimensional bipedal walking control using Divergent Component of Motion

Johannes Englsberger
Christian Ott 0001
Alin Albu-Schäffer

In this paper, we extend the Divergent Component of Motion (DCM, also called ‘Capture Point’) to 3D. We introduce the “Enhanced Centroidal Moment Pivot point” (eCMP) and the “Virtual Repellent Point” (VRP), which allow for the encoding of both direction and magnitude of the external (e. g. leg) forces and the total force (i. e. external forces plus gravity) acting on the robot. Based on eCMP, VRP and DCM, we present a method for real-time planning and control of DCM trajectories in 3D. We address the problem of underactuation and propose methods to guarantee feasibility of the finally commanded forces. The capabilities of the proposed control framework are verified in simulations.

IROS Conference 2013 Conference Paper

Towards a robust variable stiffness actuator

Sebastian Wolf 0001
Alin Albu-Schäffer

Robots with Variable Stiffness Actuators (VSA) are intrinsically flexible in the joints. The built-in mechanical spring not only has the advantage of a higher peak performance, but also leads to a more robust robot. This paper presents and analyzes the threats to a VSA equipped robot that arise from external or internal origin. Influences of mechanical, moisture, electrical, thermal, radiation, and chemical nature are identified. Protection methods from these threats are discussed and the results presented. The results are separated into hardware, observation, control limiters, and reaction strategies. A hierarchical implementation of the control limiters and reaction strategies is presented. The reaction strategies use a motor position deviation and a change in the stiffness setup to reduce the load at high passive deflections in the VSA. Control limiter and reaction strategies have been implemented in the DLR Hand Arm System and evaluated experimentally with impacts on the system.

IROS Conference 2012 Conference Paper

A truly safely moving robot has to know what injury it may cause

Sami Haddadin
Simon Haddadin
Augusto Khoury
Tim Rokahr
Sven Parusel
Rainer Burgkart
Antonio Bicchi
Alin Albu-Schäffer

Enabling robots to safely interact with humans is an essential goal of robotics research. The developments achieved over the last years in mechanical design and control made it possible to have active cooperation between humans and robots in rather complex situations. In these terms, safe behavior of the robot even under worst-case situations is crucial and forms also a basis for higher level decisional aspects. In order to quantify what safe behavior really means, the definition of injury, as well as understanding its general dynamics are essential. This insight can then be applied to design and control robots such that injury due to robot-human impacts is explicitly taken into account. In this paper we approach the problem from a medical injury analysis point of view in order to formulate the relation between robot mass, velocity, impact geometry, and resulting injury qualified in medical terms. We transform these insights into processable representations and propose a motion supervisor that utilizes injury knowledge for generating safe robot motions. The algorithm takes into account the reflected inertia, velocity, and geometry at possible impact locations. The proposed framework forms a basis for generating truly safe velocity bounds that explicitely consider the dynamic properties of the manipulator and human injury.

ICRA Conference 2012 Conference Paper

A versatile biomimetic controller for contact tooling and haptic exploration

Gowrishankar Ganesh
Nathanaël Jarrassé
Sami Haddadin
Alin Albu-Schäffer
Etienne Burdet

This article presents a versatile controller that enables various contact tooling tasks with minimal prior knowledge of the tooled surface. The controller is derived from results of neuroscience studies that investigated the neural mechanisms utilized by humans to control and learn complex interactions with the environment. We demonstrate here the versatility of this controller in simulations of cutting, drilling and surface exploration tasks, which would normally require different control paradigms. We also present results on the exploration of an unknown surface with a 7-DOF manipulator, where the robot builds a 3D surface map of the surface profile and texture while applying constant force during motion. Our controller provides a unified control framework encompassing behaviors expected from the different specialized control paradigms like position control, force control and impedance control.

IROS Conference 2012 Conference Paper

Adaptive friction compensation in trajectory tracking control of DLR medical robots with elastic joints

Luc Le Tien
Alin Albu-Schäffer

In this paper we introduce an adaptive control scheme for robots with elastic joints (in particular for the DLR medical robot) in order to increase the positioning accuracy and the performance of control with respect to uncertainties of the parameters of the robot dynamics. In order to design control and analyze system stability a static friction model is applied which describes Coulomb, viscose and load dependent friction. A stability analysis is done for this adaptive control scheme, allowing a Lyapunov based convergence analysis in the context of the nonlinear robot dynamics. Experimental results validate the practical efficiency of the approach.

ICRA Conference 2012 Conference Paper

Direct force reflecting teleoperation with a flexible joint robot

Andreas Tobergte
Alin Albu-Schäffer

This paper presents a high fidelity force feedback teleoperation control for surgical applications. Advanced control methods, such as flexible joint tracking control and passivity observation, are introduced in the direct force reflecting control architecture. A full state feedback controller of the flexible joint slave robot controls the motor position, velocity, the joint torque, and the torque derivative. The pose of the haptic device and the first three derivatives are observed to generate reference states for the robot control using the robot's inverse dynamics model. Interaction forces of the slave and the environment are measured with a force/torque sensor and directly sent back to the master device. Stability is guaranteed with a passivity observer that monitors the energy in the teleoperation system online and disconnects master and slave if the system operates beyond its stable region. The proposed control architecture is implemented with the sigma. 7 haptic device and the MIRO robot. It is experimentally shown, that appropriately considering elasticities with full state reference and control of the slave, increases the dynamic range of the system enabling transparent and stable interaction with hard and soft environments.

IROS Conference 2012 Conference Paper

Intrinsically elastic robots: The key to human like performance

Sami Haddadin
Felix Huber
Kai Krieger
Roman Weitschat
Alin Albu-Schäffer
Sebastian Wolf 0001
Werner Friedl
Markus Grebenstein

Intrinsically elastic robots, which technically implement some key characteristics of the human muskoskeletal system, have become a major research topic in nowadays robotics. These novel devices open up entirely new control approaches. They base on temporary storage of potential energy and its timed transformation into kinetic energy. In legged locomotion, such considerations have been a common tool for unveiling the respective fundamental physical processes. However, in arm control, elasticities were typically considered parasitic. In this video we outline our efforts in exploiting the inherent capabilities of intrinsically elastic robots in order to bring them closer to human performance. Instead of applying purely kinematic learing-by-demonstration approaches, which are certainly suboptimal, we argue for using model based techniques in order to optimally exploit the system dynamics such that highly dynamic motion and manipulation capabilities can be achieved. In particular, the explicit use of elasticities as temporary energy tanks can be fully exploited, if they are modeled adequately as an integral part of the mechanism. We also believe that such approaches can substantially contribute to the understanding of human motion biomechanics.

ICRA Conference 2012 Conference Paper

On continuous null space projections for torque-based, hierarchical, multi-objective manipulation

Alexander Dietrich
Alin Albu-Schäffer
Gerhard Hirzinger

The technological progress in the field of robotics results in more and more complex manipulators. However, having an increasing number of degrees of freedom raises the question of how to use them effectively. In turn, establishing manipulators in human environments, e. g. , as service robots, calls for the fulfillment of various constraints and tasks at the same time. In the context of torque controlled robotic systems, we provide an approach to simultaneously deal with a multitude of tasks and constraints which are arranged in a hierarchy, utilizing the large number of actuated joints of the manipulator. To this end, we propose a continuous null space projection technique to consider unilateral constraints, singular Jacobian matrices and dynamic variations of the priority order within the hierarchical structure. We show that activating and deactivating tasks as well as crossing singularities does not lead to a discontinuous control law. Simulations and experiments on the humanoid Justin of the German Aerospace Center (DLR) validate our approach. The presented concept is supposed to contribute to whole-body control frameworks.

IROS Conference 2012 Conference Paper

On impact decoupling properties of elastic robots and time optimal velocity maximization on joint level

Sami Haddadin
Kai Krieger
Nico Mansfeld
Alin Albu-Schäffer

Designing intrinsically elastic robot systems, making systematic use of their properties in terms of impact decoupling, and exploiting temporary energy storage and release during excitative motions is becoming an important topic in nowadays robot design and control. In this paper we treat two distinct questions that are of primary interest in this context. First, we elaborate an accurate estimation of the maximum contact force during simplified human/obstacle-robot collisions and how the relation between reflected joint stiffness, link inertia, human/obstacle stiffness, and human/obstacle inertia affect it. Overall, our analysis provides a safety oriented methodology for designing intrinsically elastic joints and clearly defines how its basic mechanical properties influence the overall collision behavior. This can be used for designing safer and more robust robots. Secondly, we provide a closed form solution of reaching maximum link side velocity in minimum time with an intrinsically elastic joint, while keeping the maximum deflection constraint. This gives an analytical tool for determining suitable stiffness and maximum deflection values in order to be able to execute desired optimal excitation trajectories for explosive motions.

ICRA Conference 2012 Conference Paper

Optimal control for exploiting the natural dynamics of Variable Stiffness robots

Sami Haddadin
Felix Huber
Alin Albu-Schäffer

In contrast to common rigid or actively compliant systems, Variable Stiffness Arms are capable of storing potential energy in their joint and convert it into kinetic energy, respectively speed. This capability is well known from humans and is a good example for the outstanding performance of biological systems. However, only since some years intrinsic compliance is considered as a key feature and not a drawback in robot design. Therefore, only very little work has been carried out on exploiting the natural dynamics of elastic arms for such explosive motion sequences. In this paper, we treat the problem of how to optimally achieve maximum link velocity at a given final time for Variable Stiffness Arms. We show that solutions to this problem lead to excitation motions, which enable the robot to move on the link side at much higher speed than on the motor side. In particular, the robot uses the dynamic transfer of elastic joint energy into link side kinetic energy for further acceleration. In our work we consider the practically relevant input and state constraints, and give experimental verification of the developed methods on the new DLR Hand-Arm system.

IROS Conference 2012 Conference Paper

Optimal torque and stiffness control in compliantly actuated robots

David J. Braun
Florian Petit
Felix Huber
Sami Haddadin
Patrick van der Smagt
Alin Albu-Schäffer
Sethu Vijayakumar

Anthropomorphic robots that aim to approach human performance agility and efficiency are typically highly redundant not only in their kinematics but also in actuation. Variable-impedance actuators, used to drive many of these devices, are capable of modulating torque and passive impedance (stiffness and/or damping) simultaneously and independently. Here, we propose a framework for simultaneous optimisation of torque and impedance (stiffness) profiles in order to optimise task performance, tuned to the complex hardware and incorporating real-world constraints. Simulation and hardware experiments validate the viability of this approach to complex, state dependent constraints and demonstrate task performance benefits of optimal temporal impedance modulation.

IROS Conference 2012 Conference Paper

Rigid vs. elastic actuation: Requirements & performance

Sami Haddadin
Nico Mansfeld
Alin Albu-Schäffer

Intrinsically elastic joints have become increasingly popular over the last years. Commonly, they are considered to outperform rigid actuation in terms of peak dynamics, robustness, and energy efficiency. In particular, the possible increase of link speed by adequate motor excitation trajectories, such that the elastic transmission temporarily stores elastic energy and then timely converts it into kinetic link energy, is a new control problem in robotics. However, despite being a popular argument in favor of elastic actuation, it was not shown yet that this potential speed gain is truly inherent to the physical properties of the mechanism. In order to argue that “elasticity is superior to input torque”, i. e. size and weight, it still needs to be derived that this new feature does not come at the cost of increasing weight for a given actuation technology. Therefore, we analyze, under which circumstances “extracting” a certain amount of mass from a rigid joint and “investing” this into an elastic mechanism in the drive train leads to such a performance increase. For this, we derive the general scaling behavior of rigid joints and compare their capabilities in terms of maximum velocity to the performance behavior of an elastic joint, while taking into consideration the most important real-world constraints.

IROS Conference 2012 Conference Paper

Variable impedance actuators: Moving the robots of tomorrow

Bram Vanderborght
Alin Albu-Schäffer
Antonio Bicchi
Etienne Burdet
Darwin G. Caldwell
Raffaella Carloni
Manuel G. Catalano
Gowrishankar Ganesh

Most of today's robots have rigid structures and actuators requiring complex software control algorithms and sophisticated sensor systems in order to behave in a compliant and safe way adapted to contact with unknown environments and humans. By studying and constructing variable impedance actuators and their control, we contribute to the development of actuation units which can match the intrinsic safety, motion performance and energy efficiency of biological systems and in particular the human. As such, this may lead to a new generation of robots that can co-exist and co-operate with people and get closer to the human manipulation and locomotion performance than is possible with current robots.

ICRA Conference 2012 Conference Paper

Walking control of fully actuated robots based on the Bipedal SLIP model

Gianluca Garofalo
Christian Ott 0001
Alin Albu-Schäffer

The goal of this paper is to generate and stabilize a periodic walking motion for a five degrees of freedom planar robot. First of all we will consider a biped version of the spring loaded inverted pendulum (SLIP), which shows openloop stable behavior. Then we will control the robot behavior as close as possible to the simple model. In this way we take advantage of the open-loop stability of the walking pattern related to the SLIP, and additional control actions are used to increase the robustness of the system and reject external disturbances. To this end an upper level controller will deal with the stabilization of the SLIP model, while a lower level controller will map the simple virtual model onto the real robot dynamics. Two different approaches are implemented for the lower level: in the first one, we aim at exactly reproducing the same acceleration that a SLIP would have when put in the same condition, while in the second one, we aim at a simpler control law without exactly reproducing the aforementioned acceleration. The latter case is equivalent to considering a SLIP with additional external disturbances, which have to be handled by the upper level controller. Both approaches can successfully reproduce a periodic walking pattern for the robot.

IROS Conference 2011 Conference Paper

A human-centered approach to robot gesture based communication within collaborative working processes

Tobias Ende
Sami Haddadin
Sven Parusel
Tilo Wüsthoff
Marc Hassenzahl
Alin Albu-Schäffer

The increasing ability of industrial robots to perform complex tasks in collaboration with humans requires more capable ways of communication and interaction. Traditional systems use separate interfaces such as touchscreens or control panels in order to operate the robot, or to communicate its state and prospective actions to the user. Transferring human communication, such as gestures to technical non-humanoid robots, creates various opportunities for more intuitive human-robot-interaction. Interaction shall no longer require a separate interface such as a control panel. Instead, it should take place directly between human and robot. To explore intuitive interaction, we identified gestures that are relevant for co-working tasks from human observations. Based on a decomposition approach we transferred them to robotic systems of increasing abstraction and experimentally evaluated how well these gestures are recognized by humans. We created a human-robot interaction use-case in order to perform the task of handling dangerous liquid. Results indicate that several gestures are well perceived when displayed with context information regarding the task.

IROS Conference 2011 Conference Paper

Bipedal walking control based on Capture Point dynamics

Johannes Englsberger
Christian Ott 0001
Máximo A. Roa
Alin Albu-Schäffer
Gerhard Hirzinger

This paper builds up on the Capture Point concept and exploits the simple form of the dynamical equations of the Linear Inverted Pendulum model when formulated in terms of the center of mass and the Capture Point. The presented methods include (i) the derivation of a Capture Point (CP) control principle based on the natural dynamics of the linear inverted pendulum (LIP), which stabilizes the walking robot and motivates (ii) the design of a CP tracking and a CP end-of-step controller. The exponential stability of the CP control law is proven. Tilting is avoided by proper projection of the commanded zero moment point. The robustness of the derived control algorithms is analyzed analytically and verified in simulation and experiments.

IROS Conference 2011 Conference Paper

Cartesian impedance control for a variable stiffness robot arm

Florian Petit
Alin Albu-Schäffer

The variable stiffness actuation (VSA) technology has been recently developed and applied in robotic arms. Mechanism robustness, high peak torque and velocity, and stiffness adjustment flexibility are key benefits of VSA joints. However, the achievable Cartesian stiffness by uncoupled VSA joints is limited. Therefore we suggest and analyze the use of an active impedance controller in combination with the passive joints to further increase the stiffness range. An algorithm to optimize the passive and active Cartesian stiffness is proposed to achieve a desired Cartesian stiffness as precise as possible. The algorithm was implemented and tested on the VSA robot DLR Hand Arm System. Experimental results and measurements of the active/passive impedance algorithm are shown.

ICRA Conference 2011 Conference Paper

Designing optimally safe robot surface properties for minimizing the stress characteristics of human-robot collisions

Jung-Jun Park
Sami Haddadin
Jae-Bok Song
Alin Albu-Schäffer

Modeling of low severity soft-tissue injury due to unwanted collisions of a robot in collaborative settings is an important aspect to be treated in safe physical Human-Robot Interaction (pHRI). Up to now, safety evaluations for pHRI were mainly conducted by using safety criteria related with impact forces and head accelerations. These indicate severe injury in the robotics context and leave out low severity injury such as contusions and lacerations. However, for the design of an intrinsically safer robot arm, a reliable evaluation of the collision between a human and a robot that is based on skin injury criteria is essential. In this paper, we propose a novel human-robot collision model with and without covering, which is based on the impact stress distribution. The reliability of the proposed collision model is verified by a comparison with various cadaver experiments taken from existing biomechanical literature. Since the stress characteristics acting on the human head can be analyzed with this new collision model, the occurrence of certain soft-tissue injury can be estimated. Furthermore, the method serves for selecting the appropriate covering parameters, as e. g. elastic modulus and thickness, by evaluating the chosen skin injury indices.

IROS Conference 2011 Conference Paper

Dynamic whole-body mobile manipulation with a torque controlled humanoid robot via impedance control laws

Alexander Dietrich
Thomas Wimböck
Alin Albu-Schäffer

Service robotics is expected to be established in human households and environments within the next decades. Therefore, dexterous and flexible behavior of these systems as well as guaranteeing safe interaction are crucial for that progress. We address these issues in terms of control strategies for the whole body of DLR's humanoid Justin. Via impedance control laws, we enable the robot to realize main tasks compliantly while, at the same time, taking care of aspects like physical limitations and collision avoidance with its own structure and the environment autonomously. The controller provides a natural redundancy resolution between the arms, the torso and the wheeled platform. A low-dimensional task space interface is proposed that can be used by planning tools. Thereby, planning time can be saved significantly. Experimental results on DLR's Justin are presented to validate our approach.

IROS Conference 2011 Conference Paper

Exploiting potential energy storage for cyclic manipulation: An analysis for elastic dribbling with an anthropomorphic robot

Sami Haddadin
Kai Krieger
Mirko Kunze
Alin Albu-Schäffer

For achieving dynamic manipulation capabilities that are comparable to human performance in terms of speed, energetic properties, and robustness, intrinsic elasticity is widely proposed as a necessary robot design element. In this paper we show how passive compliance can be exploited for a 6-degree-of-freedom (DoF) cyclic ball dribbling task with a 7-DoF articulated Cartesian impedance controlled DLR Lightweight Robot III. For this, the robot is equipped with an elastic hand, which extends the contact time and therefore, also enlarges both, observability and controllability of the ball. We show via simulation and experiment that it is possible to achieve a stable dynamic cycle based on a 1 DoF analysis from [1] for the main axis together with control strategies for the secondary translations and rotations of the task. The scheme allows also the continuous tracking of a desired dribbling height and horizontal position. As a human is able to dribble blindly, we decided to solve the task by force sensing only, i. e. no vision is used for our approach, however, it could be easily incorporated.

ICRA Conference 2011 Conference Paper

Extensions to reactive self-collision avoidance for torque and position controlled humanoids

Alexander Dietrich
Thomas Wimböck
Holger Täubig
Alin Albu-Schäffer
Gerhard Hirzinger

One of the fundamental demands on robotic systems is a safe interaction with their environment. For fulfilling that condition, both collisions with obstacles and the own structure have to be avoided. We address the problem of self-collisions and propose an algorithm for its avoidance which is based on artificial repulsion potential fields and applicable to both torque and position controlled manipulators. To this end, we design a damping that incorporates the configuration dependance of the robot. For a maximum level of safety, an additional emergency brake strategy based on kinetic energy considerations is introduced for situations in which self-collisions are not avoidable by the controller. Experiments are performed on DLR's humanoid Justin.

IROS Conference 2011 Conference Paper

Impedance control of a non-linearly coupled tendon driven thumb

Maxime Chalon
Werner Friedl
Jens Reinecke
Thomas Wimböck
Alin Albu-Schäffer

A large workspace and proper force capabilities of a robotic thumb can be obtained using a tensegrity structure for the actuation, similar to the human thumb base muscles. Using nonlinear stiffness elements and an antagonistic architecture, the joint stiffness can be adjusted by variation of the tendon pre-tension. However, the highly nonlinear actuation creates new control challenges and in particular the nonlinear tendon kinematics must be accounted for. Despite the challenges, the nonlinear structure is required to achieve the desired torques. In this paper, the dynamic equations of a tendon driven thumb are established. An efficient formulation is proposed to generate the pretension forces in order to preserve the torques and approximate the stiffness matrix. A cascaded structure is used for the controller. The equations for the inner tendon force control loop and the outer impedance control loop are presented. Because of the absence of link side position sensors, an iterative estimation algorithm is proposed and implemented in real-time. It is shown that, using the mechanical joint flexibility, the controller impedance gain can be adjusted to improve the steady-state effective impedance. The search algorithm robustness is evaluated through a set of simulations. Finally, experimental results and equivalent simulations demonstrate the effectiveness of our controller.

ICRA Conference 2011 Conference Paper

Modular state-based behavior control for safe human-robot interaction: A lightweight control architecture for a lightweight robot

Sven Parusel
Sami Haddadin
Alin Albu-Schäffer

In this paper we present a novel control architecture for realizing human-friendly behaviors and intuitive state based programming. The design implements strategies that take advantage of sophisticated soft-robotics features for providing reactive, robust, and safe robot actions in dynamic environments. Quick access to the various functionality of the robot enables the user to develop flexible hybrid state automata for programming complex robot behaviors. The real-time robot control takes care of all safety critical aspects and provides reactive reflexes that directly respond to external stimuli.

ICRA Conference 2011 Conference Paper

Singularity avoidance for nonholonomic, omnidirectional wheeled mobile platforms with variable footprint

Alexander Dietrich
Thomas Wimböck
Alin Albu-Schäffer
Gerhard Hirzinger

One characteristic attribute of mobile platforms equipped with a set of independent steering wheels is their omnidirectionality and the ability to realize complex translational and rotational trajectories. An accurate coordination of steering angle and spinning rate of each wheel is necessary for a consistent motion. Since the orientations of the wheels must align to the Instantaneous Center of Rotation (ICR), the current location and velocity of this specific point is essential for describing the state of the platform. However, singular configurations of the controlled system exist depending on the ICR, leading to unfeasible control inputs, i. e. , infinite steering rates. Within this work we address and analyze this problem in general. Furthermore, we propose a solution for mobile platforms with variable footprint. An existing controller based on dynamic feedback linearization is augmented by a new potential field-based algorithm for singularity avoidance which uses the tunable leg lengths as an additional control input to minimize deviations from the nominal motion trajectory. Simulations and experimental results on the mobile platform of DLR's humanoid manipulator Justin support our approach.

ICRA Conference 2011 Conference Paper

State feedback damping control for a multi DOF variable stiffness robot arm

Florian Petit
Alin Albu-Schäffer

The concept of variable stiffness actuation (VSA) for robotic joints promises advantages regarding robustness, energy efficiency, and task adaptability. The VS joints developed at DLR show very low intrinsic damping for efficient energy storage and retrieval whereas the desired damping behavior for task execution needs to be implemented in control. Robotic arms with multiple VS joints, as for example the DLR Hand Arm System, ask for advanced control algorithms which can cope with the elastic joints and the multi-input multi-output (MIMO) system properties of the mechanical setup. We propose a MIMO controller for flexible joint robots based upon an eigenmode decoupling approach. For robustness reasons, the controller is designed to modify the intrinsic plant properties as little as possible while attaining the desired damping. A gain design algorithm is proposed. The controller is validated in simulations and experiments.

IROS Conference 2011 Conference Paper

Static calibration of the DLR medical robot MIRO, a flexible lightweight robot with integrated torque sensors

Julian Klodmann
Rainer Konietschke
Alin Albu-Schäffer
Gerhard Hirzinger

This paper presents a method to calibrate the model of serial and flexible lightweight robots with joint sided torque sensors in the assembled state. The calibration is done in an iterative three-step process, based on static robot poses. In the first step the kinematics and stiffnesses of the flexible components are calibrated. Second the models of the integrated torque sensors are identified in a linear least square solution. In the third step the masses, the centers of gravity and the torque sensor offsets are estimated using linear regression. The calibration steps are repeated stepwise to account for their dependencies. The calibration procedure is simulated and experimentally performed with the medical lightweight robot MIRO of the German Aerospace Center. Through the iterative procedure the pose accuracy improves from about 5mm translational error and 2. 5 ° rotational error to 1mm and 0. 3 ° regarding the entire workspace.

ICRA Conference 2011 Conference Paper

The DLR hand arm system

Markus Grebenstein
Alin Albu-Schäffer
Thomas Bahls
Maxime Chalon
Oliver Eiberger
Werner Friedl
Robin Gruber
Sami Haddadin

An anthropomorphic hand arm system using variable stiffness actuation has been developed at DLR. It is aimed to reach its human archetype regarding size, weight and performance. The main focus of our development is put on robustness, dynamic performance and dexterity. Therefore, a paradigm change from impedance controlled, but mechanically stiff joints to robots using intrinsic variable compliance joints is carried out.

IROS Conference 2011 Conference Paper

The sigma. 7 haptic interface for MiroSurge: A new bi-manual surgical console

Andreas Tobergte
Patrick Helmer
Ulrich Hagn
Patrice Rouiller
Sophie Thielmann
Sébastien Grange
Alin Albu-Schäffer
François Conti

This paper presents the design and control of the sigma. 7 haptic device and the new surgical console of the MiroSurge robotic system. The console and the haptic devices are designed with respect to requirements in minimally invasive robotic surgery. Dedicated left and right handed devices are integrated in an operator console in an ergonomic configuration. The height of the whole console is adjustable, allowing the surgeon seated and standed operation. Each of the devices is fully actuated in seven degrees of freedom (DoF). A parallel mechanism with 3 DoF actuates the translational motion and an attached wrist with 3 intersecting axis drives the rotations of the grasping unit. This advantageous design leads to inherently decoupled kinematics and dynamics. Cartesian forces are 20 N within the translational workspace, which is a sphere of about 120 mm diameter for each device. The rotational wrist of the device covers the whole workspace of the human hand and provides maximum torques of about 0. 4 Nm. The grasping unit can display forces up to 8 N. An integrated force/torque sensor is used to increase the transparency of the devices by reducing inertia and friction. It is theoretically shown that the non-linear closed loop system behaves like a passive system and experimental results validate the approach. The sigma. 7 haptic devices are designed by Force Dimension in cooperation with the German Aerospace Center (DLR). DLR designed the surgical console and integrated the haptic devices in the MiroSurge system.

ICRA Conference 2010 Conference Paper

Bidirectional antagonistic variable stiffness actuation: Analysis, design & Implementation

Florian Petit
Maxime Chalon
Werner Friedl
Markus Grebenstein
Alin Albu-Schäffer
Gerhard Hirzinger

The variable stiffness actuation concept is considered to provide a human-friendly robot technology. This paper examines a joint concept called the bidirectional antagonistic joint which is a extension of antagonistic joints. A new operating mode called the helping mode is introduced, which increases the joint load range. Although the joint can not be pretensioned in the helping mode, it is shown that a stiffness variation is possible, assuming a suitable torque-stiffness characteristic of the elastic elements. A methodology to design such characteristics is presented along with several example cases interpreted in a torque-stiffness plot. Furthermore, a stiffness adaptation control scheme which ensures mechanism safety is described. Finally, the design methodology and the control are evaluated on an implementation of a bidirectional antagonistic joint.

ICRA Conference 2010 Conference Paper

Biomimetic motor behavior for simultaneous adaptation of force, impedance and trajectory in interaction tasks

Gowrishankar Ganesh
Alin Albu-Schäffer
Haruno Mashiko
Mitsuo Kawato
Etienne Burdet

Interaction of a robot with dynamic environments would require continuous adaptation of force and impedance, which is generally not available in current robot systems. In contrast, humans learn novel task dynamics with appropriate force and impedance through the concurrent minimization of error and energy, and exhibit the ability to modify movement trajectory to comply with obstacles and minimize forces. This article develops a similar automatic motor behavior for a robot and reports experiments with a one degree-of-freedom system. In a postural control task, the robot automatically adapts torque to counter a slow disturbance and shifts to increasing its stiffness when the disturbance increases in frequency. In the presence of rigid obstacles, it refrains from increasing force excessively, and relaxes gradually to follow the obstacle, but comes back to the desired state when the obstacle is removed. A trajectory tracking task demonstrates that the robot is able to adapt to different loads during motion. On introduction of a new load, it increases its stiffness to adapt to the load quickly, and then relaxes once the adaptation is complete. Furthermore, in the presence of an obstacle, the robot adjusts its trajectory to go around it.

ICRA Conference 2010 Conference Paper

Dynamic modelling and control of variable stiffness actuators

Alin Albu-Schäffer
Sebastian Wolf 0001
Oliver Eiberger
Sami Haddadin
Florian Petit
Maxime Chalon

After briefly summarizing the mechanical design of the two joint prototypes for the new DLR variable compliance arm, the paper exemplifies the dynamic modelling of one of the prototypes and proposes a generic variable stiffness joint model for nonlinear control design. Based on this model, the design of a simple, gain scheduled state feedback controller for active vibration damping of the mechanically very weakly damped joint is presented. Moreover, the computation of the motor reference values out of the desired stiffness and position is addressed. Finally, simulation and experimental results validate the proposed methods.

IROS Conference 2010 Conference Paper

Holistic design and analysis for the human-friendly robotic co-worker

Sami Haddadin
Sven Parusel
Rico Belder
Jörn Vogel
Tim Rokahr
Alin Albu-Schäffer
Gerhard Hirzinger

In this overview paper we present current work on safety analysis for physical Human-Robot Interaction (pHRI) and motion control methods for robotic co-workers. In particular, we introduce the analysis tools for investigating the potential injury a human would suffer during robot-human impacts. Furthermore, we outline our concept for establishing a procedure towards standardized crash testing in robotics with automobile crash-test dummies. Since it is only possible to investigate blunt impacts with these devices, we developed a drop testing setup for analyzing soft-tissue injury in robotics from a biomechanics perspective. In the second part of the paper, some of our methods for task preserving and task relaxing motion schemes are described, which enable collision avoidance in real-time. The algorithms are well suited to work in an integrated fashion with the soft robotics control developed for the DLR Lightweight Robot III (LWR-III). In addition, it is shown how the torque sensing capabilities of the robot can be used to support reactive motion schemes. Finally, an overview of our human-friendly control architecture for the LWR-III is given, which unifies the rich bundle of developed methods for this manipulator

IROS Conference 2010 Conference Paper

New insights concerning intrinsic joint elasticity for safety

Sami Haddadin
Alin Albu-Schäffer
Oliver Eiberger
Gerhard Hirzinger

In this paper we present various new insights on the effect intrinsic joint elasticity has on safety in pHRI. We address the fact that the intrinsic safety of elastic mechanisms has been discussed rather one sided in favor of this new designs and intend to give a more differentiated view on the problem. An important result is that intrinsic joint elasticity does not reduce the Head Injury Criterion or impact forces compared to conventional actuation with some considerable elastic behavior in the joint, if considering full scale robots. We also elaborate conditions under which intrinsically compliant actuation is potentially more dangerous than rigid one. Furthermore, we present collision detection and reaction schemes for such mechanisms and verify their effectiveness experimentally.

ICRA Conference 2010 Conference Paper

On joint design with intrinsic variable compliance: derivation of the DLR QA-Joint

Oliver Eiberger
Sami Haddadin
Michael Weis
Alin Albu-Schäffer
Gerhard Hirzinger

In this paper we introduce a classification of intrinsically compliant joint mechanisms. Furthermore, we outline design considerations for realizing such devices in order to match the requirements for robust and performant actuation. Based on this elaboration, a new design concept is presented, the DLR QA-Joint. Its performance is investigated by various experiments, covering velocity increase using the elastic energy, joint protection capabilities, and control performance.

IROS Conference 2010 Conference Paper

Real-time reactive motion generation based on variable attractor dynamics and shaped velocities

Sami Haddadin
Holger Urbanek
Sven Parusel
Darius Burschka
Jürgen Roßmann
Alin Albu-Schäffer
Gerhard Hirzinger

This paper describes a novel method for motion generation and reactive collision avoidance. The algorithm performs arbitrary desired velocity profiles in absence of external disturbances and reacts if virtual or physical contact is made in a unified fashion with a clear physically interpretable behavior. The method uses physical analogies for defining attractor dynamics in order to generate smooth paths even in presence of virtual and physical objects. The proposed algorithm can, due to its low complexity, run in the inner most control loop of the robot, which is absolutely crucial for safe Human Robot Interaction. The method is thought as the locally reactive real-time motion generator connecting control, collision detection and reaction, and global path planning.

ICRA Conference 2010 Conference Paper

Soft-tissue injury in robotics

Sami Haddadin
Alin Albu-Schäffer
Gerhard Hirzinger

Up to now, mostly blunt human-robot impacts were investigated in the robotics literature. In this context, the influence of robot mass and velocity during rigid impacts with and without the possibility of the human being clamped was quantified. In this paper an analysis of soft-tissue injuries caused by sharp tools, which are mounted on/grasped by a robot is carried out as the next step down the road to a full safety analysis of robots for HRI. We conducted an analysis of soft-tissue injuries based on available biomechanical and forensic data and to our knowledge for the first time in robotics present various experimental results with biological tissue for validation. Furthermore, possible countermeasures are evaluated quantitatively based biomechanically relevant quantities.

IROS Conference 2009 Conference Paper

Experimental study on dynamic reactionless motions with DLR's humanoid robot Justin

Thomas Wimböck
Dragomir N. Nenchev
Alin Albu-Schäffer
Gerhard Hirzinger

The capabilities of DLR's multi-DOF humanoid robot Justin are extended with the help of a dynamic torque control component for base reaction minimization. Since the mobile base of the robot comprises springs, reactions induced by arm/torso motions lead to vibrations and deteriorate the performance. The control component is derived from the equation of motion of the robot, represented as an underactuated system, and partitioned into a “driven” subsystem (one of the arms), and a “compensating” subsystem (the other arm, with or w/o torso contribution). The control component is then embedded into the existing sophisticated controller structure of Justin, as a feedforward component, with additional control signals from an augmented PD feedback controller. It was possible to obtain satisfactory performance with a very “soft” compensatory subsystem. The experimental results confirmed the potential of this model-based approach for use in a complex multi-DOF system. As far as we know, this is the first time that a dynamic-coupling compensating controller is applied to a real system of such complexity, utilizing thereby a torque control interface.

ICRA Conference 2009 Conference Paper

On the kinematic modeling and control of a mobile platform equipped with steering wheels and movable legs

Paolo Robuffo Giordano
Matthias Fuchs
Alin Albu-Schäffer
Gerhard Hirzinger

Mobile platforms equipped with several steering wheels are known to be omnidirectional, i. e. , able to independently translate and rotate on the plane. As an improvement to this design, the Justin mobile platform also possesses the ability to vary its footprint over time by extending/retracting the wheel legs during motion. In this paper, we discuss the kinematic modeling and control issues for such a platform. The goal is to obtain a tracking controller which is able to realize an arbitrary linear/angular platform motion while, at the same time, independently expanding/retracting each leg. Experimental results support the proposed approach.

ICRA Conference 2009 Conference Paper

Rollin' Justin - Mobile platform with variable base

Christoph Borst 0001
Thomas Wimböck
Florian Schmidt 0001
Matthias Fuchs
Bernhard Brunner
Franziska Zacharias
Paolo Robuffo Giordano
Rainer Konietschke

Research on humanoid robots for use in servicing tasks, e. g. fetching and delivery, attracts steadily more interest. With “Rollin' Justin” a mobile robotic system and research platform is presented that allows sophisticated control algorithms and dexterous manipulation. This video gives an overview of the mobile humanoid robotic system “Rollin' Justin” with special emphasis on mechanical design features, control issues and high-level system capabilities such as human robot interaction.

ICRA Conference 2009 Conference Paper

The "DLR Crash Report": Towards a standard crash-testing protocol for robot safety - Part I: Results

Sami Haddadin
Alin Albu-Schäffer
Mirko Frommberger
Jürgen Roßmann
Gerhard Hirzinger

After analyzing fundamental impact characteristics of robot-human collisions in our previous work, the intention in the present paper is to augment existing knowledge in this field, verify previously given statements with standardized equipment of the German Automobile Club (ADAC), and provide a crash-test report for robots in general. Various new insights are achieved and a systematic and extensive set of data is provided. The presented work is divided into two papers. The main purpose of Part I is to give, similarly to reports known from the automobile world1, a fact based and result oriented view on our newest robot crash-test experiments. In Part II detailed discussions of the results listed in the present paper and recommendations towards a standard crash-test protocol for robot safety are carried out.

ICRA Conference 2009 Conference Paper

The "DLR crash report": Towards a standard crash-testing protocol for robot safety - Part II: Discussions

Sami Haddadin
Alin Albu-Schäffer
Mirko Frommberger
Jürgen Roßmann
Gerhard Hirzinger

After giving a rich data basis of our impact tests with standardized crash-test dummies in Part I of this work we address in Part II various aspects related to these tests in a case based discussion. The presented facts, the knowledge gained from our previous work, and the data from Part I lead us to recommendations for standardized crash-testing procedures in robotics. The proposed impact procedures will help to compare blunt robot-human impacts on a common basis. We will discuss additional requirements which will enhance the completeness of testing procedures.

ICRA Conference 2009 Conference Paper

The DLR MiroSurge - A robotic system for surgery

Rainer Konietschke
Ulrich Hagn
Mathias Nickl
Stefan Jörg
Andreas Tobergte
Georg Passig
Ulrich Seibold
Luc Le Tien

This video presents the in-house developed DLR MiroSurge robotic system for surgery. As shown, the system is suitable for both minimally invasive and open surgery. Essential part of the system is the MIRO robot: The soft robotics feature enables intuitive interaction with the robot.

IROS Conference 2008 Conference Paper

Collision detection and reaction: A contribution to safe physical Human-Robot Interaction

Sami Haddadin
Alin Albu-Schäffer
Alessandro De Luca 0001
Gerhard Hirzinger

In the framework of physical Human-Robot Interaction (pHRI), methodologies and experimental tests are presented for the problem of detecting and reacting to collisions between a robot manipulator and a human being. Using a lightweight robot that was especially designed for interactive and cooperative tasks, we show how reactive control strategies can significantly contribute to ensuring safety to the human during physical interaction. Several collision tests were carried out, illustrating the feasibility and effectiveness of the proposed approach. While a subjective “safety” feeling is experienced by users when being able to naturally stop the robot in autonomous motion, a quantitative analysis of different reaction strategies was lacking. In order to compare these strategies on an objective basis, a mechanical verification platform has been built. The proposed collision detection and reactions methods prove to work very reliably and are effective in reducing contact forces far below any level which is dangerous to humans. Evaluations of impacts between robot and human arm or chest up to a maximum robot velocity of 2. 7 m/s are presented.

IROS Conference 2008 Conference Paper

Friction observer and compensation for control of robots with joint torque measurement

Luc Le Tien
Alin Albu-Schäffer
Alessandro De Luca 0001
Gerhard Hirzinger

In this paper we introduce a friction observer for robots with joint torque sensing (in particular for the DLR medical robot) in order to increase the positioning accuracy and the performance of torque control. The observer output corresponds to the low-pass filtered friction torque. It is used for friction compensation in conjunction with a MIMO controller designed for flexible joint arms. A passivity analysis is done for this friction compensation, allowing a Lyapunov based convergence analysis in the context of the nonlinear robot dynamics. For the complete controlled system, global asymptotic stability can be shown. Experimental results validate the practical efficiency of the approach.

IROS Conference 2008 Conference Paper

Impedance control for variable stiffness mechanisms with nonlinear joint coupling

Thomas Wimböck
Christian Ott 0001
Alin Albu-Schäffer
Andreas Kugi
Gerhard Hirzinger

The current discussion on physical human robot interaction and the related safety aspects, but also the interest of neuro-scientists to validate their hypotheses on human motor skills with bio-mimetic robots, led to a recent revival of tendon-driven robots. In this paper, the modeling of tendon-driven elastic systems with nonlinear couplings is recapitulated. A control law is developed that takes the desired joint position and stiffness as input. Therefore, desired motor positions are determined that are commanded to an impedance controller. We give a physical interpretation of the controller. More importantly, a static decoupling of the joint motion and the stiffness variation is given. The combination of active (controller) and passive (mechanical) stiffness is investigated. The controller stiffness is designed according to the desired overall stiffness. A damping design of the impedance controller is included in these considerations. The controller performance is evaluated in simulation.

ICRA Conference 2008 Conference Paper

Injury evaluation of human-robot impacts

Sami Haddadin
Alin Albu-Schäffer
Michael Strohmayr
Mirko Frommberger
Gerhard Hirzinger

Currently, large efforts are unertaken to bring robotic applications to domestic environments. Especially physical human-robot cooperation is a major concern and various design and control methodologies were developed on the way to achieve this task. In particular, this necessitates the evaluation of injury risks a human is exposed to in case he is hit by a robot. In this video several blunt impact tests are shown, leading to an assessment of which factors dominate injury severity. We will illustrate the effect robot speed, robot mass, and constraints in the environment have on safety in human-robot impacts. It will be shown that the intuition of high impact loads being transmitted by heavy robots is wrong. Furthermore, the conclusion is induced that free impacts are by far less dangerous than being crushed.

IROS Conference 2008 Conference Paper

Robotic assembly of complex planar parts: An experimental evaluation

Paolo Robuffo Giordano
Andreas Stemmer
Klaus Arbter
Alin Albu-Schäffer

In this paper we present an experimental evaluation of automatic robotic assembly of complex planar parts. The torque-controlled DLR light-weight robot, equipped with an on-board camera (eye-in-hand configuration), is committed with the task of looking for given parts on a table, picking them, and inserting them inside the corresponding holes on a movable plate. Visual servoing techniques are used for fine positioning over the selected part/hole, while insertion is based on active compliance control of the robot and robust assembly planning in order to align the parts automatically with the hole. Execution of the complete task is validated through extensive experiments, and performance of humans and robot are compared in terms of overall execution time.

ICRA Conference 2008 Conference Paper

The role of the robot mass and velocity in physical human-robot interaction - Part I: Non-constrained blunt impacts

Sami Haddadin
Alin Albu-Schäffer
Gerhard Hirzinger

The desired coexistence of robotic systems and humans in the same physical domain, by sharing their workspace and actually cooperating in a physical manner, poses the very fundamental problem of ensuring safety to the user. In this paper we will show the influence of robot mass and velocity during blunt unconstrained impacts with humans. Several robots with weights ranging from 15–2500 kg are impacted at different velocities with a mechanical human head mockup. This is used to measure the so-called Head Injury Criterion, mainly a measure for brain injury. Apart from injuries indicated by this criterion and a detailed analysis of chest impacts we point out that e. g. fractures of facial bones can occur during collisions at typical robot velocities. Therefore, this injury mechanism which is more probable in robotics is evaluated in detail.

ICRA Conference 2008 Conference Paper

The role of the robot mass and velocity in physical human-robot interaction - Part II: Constrained blunt impacts

Sami Haddadin
Alin Albu-Schäffer
Mirko Frommberger
Gerhard Hirzinger

Accidents occurring with classical industrial robots often lead to fatal injuries. Presumably, this is to a great extent caused by the possibility of clamping the human in the confined workspace of the robot. Before generally allowing physical cooperation of humans and robots in future applications it is therefore absolutely crucial to analyze this extremely dangerous situation. In this paper we will investigate many aspects relevant to this sort of injury mechanisms and discuss the importance to domestic environments or production assistants. Since clamped impacts are intrinsically more dangerous than free ones it is fundamental to discuss and evaluate metrics to ensure safe interaction if clamping is possible. We compare various robots with respect to their injury potential leading to a main safety requirement of robot design: Reduce the intrinsic injury potential of a robot by reducing its weight.

ICRA Conference 2007 Conference Paper

A humanoid upper body system for two-handed manipulation

Christoph Borst 0001
Christian Ott 0001
Thomas Wimböck
Bernhard Brunner
Franziska Zacharias
Berthold Bäuml
Ulrich Hillenbrand
Sami Haddadin

This video presents a humanoid two-arm system developed as a research platform for studying dexterous two-handed manipulation. The system is based on the modular DLR-Lightweight-Robot-III and the DLR-Hand-II. Two arms and hands are combined with a three degrees-of-freedom movable torso and a visual system to form a complete humanoid upper body. The diversity of the system is demonstrated by showing the mechanical design, several control concepts, the application of rapid prototyping and hardware-in-the-loop (HIL) development as well as two-handed manipulation experiments and the integration of path planning capabilities.

ICRA Conference 2007 Conference Paper

An Analytical Method for the Planning of Robust Assembly Tasks of Complex Shaped Planar Parts

Andreas Stemmer
Alin Albu-Schäffer
Gerhard Hirzinger

The paper addresses the automatic assembly of planar parts with complex geometry. Its main focus is on the automatic generation and parameterization of the assembly sequence, which should provide maximal robustness with respect to positioning errors of the robot and residual position uncertainties of vision based object localization. The assembly utilizes active or passive compliance of the robot in order to align the parts automatically. Success of the automatic alignment, i. e. the convergence of the assembly process can be guaranteed using the means of regions of attraction (ROA). The planning optimizes the assembly trajectories and parameters in such a way that the ROA is maximized for a given part geometry. For the convergence analysis, passivity properties of the robot and the environment are used. The method is validated through extensive experiments and can be successfully applied also for the automated assembly planning with passive compliance devices, as widely used today in industrial automation.

IROS Conference 2007 Conference Paper

Influence of sensor quantization on the control performance of robotics actuators

Renat Iskakov
Alin Albu-Schäffer
Manfred Schedl
Gerhard Hirzinger
Vitaly Lopota

In this paper the effect of sensor quantization on the control performance of robotics actuators in the steadystate condition is considered. First, the existence of a limit cycle mode due to the limited sensor resolution in the systems with P-controller is shown in analogy to [1]. Because of the poor transient response of the P-controlled system the extension to the PD-controller is thereafter taken into consideration. A simple solution for limit cycles avoidance in terms of modification of controller structure is provided. The experimental data confirm the theoretical analysis for the robotics actuators.

ICRA Conference 2007 Conference Paper

MIMO State Feedback Controller for a Flexible Joint Robot with Strong Joint Coupling

Luc Le Tien
Alin Albu-Schäffer
Gerhard Hirzinger

The paper describes the modeling and control of a robot with flexible joints (the DLR medical robot), which has strong mechanical couplings between pairs of joints realized with a differential gear-box. Because of this coupling, controllers developed before for the DLR light-weight robots cannot be directly applied. The previous control approach is extended in order to allow a multi-input-multi-output (MIMO) design for the strongly coupled joints. Asymptotic stability is shown for the MIMO controller. Finally, experimental results with the DLR medical robot are presented.

IROS Conference 2006 Conference Paper

A Cartesian Compliance Controller for a Manipulator Mounted on a Flexible Structure

Christian Ott 0001
Alin Albu-Schäffer
Gerhard Hirzinger

In this paper the Cartesian compliance control of a manipulator mounted on a flexible base is considered. The proposed control law aims at achieving a desired stiffness and damping in Cartesian coordinates while taking account of the base flexibility. The controller does not use any measurement of the base motion, however a model of the base stiffness is required. For the closed loop system, asymptotic stability in case of free motion is proven. Furthermore, considering interaction tasks, it is shown that the controlled manipulator system has a useful passivity property

ICRA Conference 2006 Conference Paper

A Hands-on-robot for Accurate Placement of Pedicle Screws

Tobias Ortmaier
Holger Weiss
Ulrich Hagn
Markus Grebenstein
Matthias Nickel
Alin Albu-Schäffer
Christian Ott 0001
Stefan Jörg

This paper presents a novel system for accurate placement of pedicle screws. The system consists of a new light-weight (<10 kg), kinematically redundant, and fully torque controlled robot. Additionally, the pose of the robot tool-center point is tracked by an optical navigation system, serving as an external reference source. Therefore, it is possible to measure and to compensate deviations between the intraoperative and the preoperatively planned pose. The robotic arm itself is impedance controlled. This allows for a new intuitive man-machine-interface as the joint units are equipped with torque sensors: the robot can be moved just by pulling/pushing its structure. The surgeon has full control of the robot at every step of the intervention. The hand-eye-coordination problems known from manual pedicle screw placement can be omitted

IROS Conference 2006 Conference Paper

Collision Detection and Safe Reaction with the DLR-III Lightweight Manipulator Arm

Alessandro De Luca 0001
Alin Albu-Schäffer
Sami Haddadin
Gerhard Hirzinger

A robot manipulator sharing its workspace with humans should be able to quickly detect collisions and safely react for limiting injuries due to physical contacts. In the absence of external sensing, relative motions between robot and human are not predictable and unexpected collisions may occur at any location along the robot arm. Based on physical quantities such as total energy and generalized momentum of the robot manipulator, we present an efficient collision detection method that uses only proprioceptive robot sensors and provides also directional information for a safe robot reaction after collision. The approach is first developed for rigid robot arms and then extended to the case of robots with elastic joints, proposing different reaction strategies. Experimental results on collisions with the DLR-III lightweight manipulator are reported

IROS Conference 2006 Conference Paper

Robotic On-Orbit Servicing - DLR's Experience and Perspective

Klaus Landzettel
Carsten Preusche
Alin Albu-Schäffer
Detlef Reintsema
Bernhard Rebele
Gerhard Hirzinger

The increasing number of launched satellites per year, calls for solutions to keep free operational space for telecommunication systems in geo-synchronized orbit, as well as to avoid the endangering of space systems in LEO (low-Earth orbit) and of the public living in the habited parts on Earth. Examples for such dangerous stranded space systems in the past are Skylab and MIR. In the future, the uncontrolled and accidental de-orbiting of other huge satellites is expected, where parts of these will hit the surface of the Earth. A feasible way to handle such problems might be to enforce the operational requirement to use some dedicated residual fuel for a controlled de-orbiting, or in case of GEO (geostationary orbit), to lift the satellites at their end of life into the graveyard orbit. Despite these measures, malfunctions of solar generators, control systems or thrusters cannot be avoided. Therefore, on-orbit servicing (OOS) will be a mandatory and challenging topic for space robotics in the near future. The outcome of national German projects like ROTEX, ESS and GETEX/ETS-VII represent a know-how which can be directly applied for the development of OOS-robotic systems. Control structures and several possible operational modes are discussed within this paper. The recently started national project ROKVISS already provides the necessary space-qualified hardware as well as the very powerful telepresence operational mode. The paper will concentrate on a description of the ROKVISS mission

ICRA Conference 2006 Conference Paper

ROKVISS - Robotics Component Verification on ISS Current Experimental Results on Parameter Identification

Alin Albu-Schäffer
Wieland Bertleff
Bernhard Rebele
Bernd-Helge Schäfer
Klaus Landzettel
Gerhard Hirzinger

ROKVISS, the German new space robotics technology experiment, was successfully installed outside at the Russian Service Module of the International Space Station (ISS) during an extravehicular space walk at the end of January 2005. Since February 2005 a two joint manipulator can be operated from ground via a direct radio link. The aim of ROKVISS is the in flight verification of highly integrated modular robotic joints as well as the demonstration of different control modes, reaching from high system autonomy to force feedback teleoperation. A main goal of the experiment is the evaluation of the dynamical parameters (especially friction, motor constant and stiffness), as well as the monitoring of their evolution over the duration of the mission, in order to validate the long term performance of the system. The paper gives first a short overview of the experiment and in particular a description of the applied control structures. The main focus of the paper is on the joint parameter identification results obtained so far, during one year of operation

ICRA Conference 2005 Conference Paper

Constructive Energy Shaping Based Impedance Control for a Class of Underactuated Euler-Lagrange Systems

Alin Albu-Schäffer
Christian Ott 0001
Gerhard Hirzinger

The paper presents an impedance controller for a class of underactuated Euler-Lagrange systems based on energy shaping. For shaping the potential energy, feedback variables are introduced, which are functions of the collocated state variables only, but which are statically equivalent to the noncollocated state variables. In this way the passivity of the system can be ensured, while exactly satisfying all steady state requirements formulated in terms of the noncollocated states (such as desired stiffness and desired equilibrium configuration). The method is constructive, allowing the direct formulation of the controller and of the corresponding energy function. The controller additionally uses the noncollocated feedback to shape the kinetic energy. Under some conditions on the potential energy of the plant, the closed loop system can then be seen as a feedback interconnection of passive systems and is proven to be asymptotically stable. Experimental results for a flexible joint robot validate the proposed controller.

ICRA Conference 2004 Conference Paper

A Passivity based Cartesian Impedance Controller for Flexible Joint Robots - Part I: Torque Feedback and Gravity Compensation

Christian Ott 0001
Alin Albu-Schäffer
Andreas Kugi
Stefano Stramigioli
Gerhard Hirzinger

In this paper a novel approach to the Cartesian impedance control problem for robots with flexible joints is presented. The proposed controller structure is based on simple physical considerations, which are motivating the extension of classical position feedback by an additional feedback of the joint torques. The torque feedback action can be interpreted as a scaling of the apparent motor inertia. Furthermore the problem of gravity compensation is addressed. Finally, it is shown that the closed loop system can be seen as a feedback interconnection of passive systems. Based on this passivity property a proof of asymptotic stability is presented.

ICRA Conference 2004 Conference Paper

A Passivity based Cartesian Impedance Controller for Flexible Joint Robots - Part II: Full State Feedback, Impedance Design and Experiments

Alin Albu-Schäffer
Christian Ott 0001
Gerhard Hirzinger

The paper presents a Cartesian impedance controller for flexible joint robots based on the feedback of the complete state of the system, namely the motor position, the joint torque and their derivatives. The approach is applied to a quite general robot model, in which also a damping element is considered in parallel to the joint stiffness. Since passivity and asymptotic stability of the controller hold also for varying damping matrices, some possibilities of designing those gain matrices (depending on the actual inertia matrix) are addressed. The passivity of the controller relies on the usage of only motor side measurements for the position feedback. A method is introduced, which provides the exact desired link side stiffness based on this motor position information. Experimental results are validating the proposed controller.

IROS Conference 2004 Conference Paper

Learning from demonstration: repetitive movements for autonomous service robotics

Holger Urbanek
Alin Albu-Schäffer
Patrick van der Smagt

This paper presents a method for learning and generating rhythmic movement patterns based on a simple central oscillator. It can be used to generate cyclic movements for a robot system which has to solve complex tasks. The system is laid out in such a way that multiple motion dimensions, or degrees of freedom of the robot, are represented independent of each other; therefore, an extension to higher-dimensional problems is easily possible. Guiding the robot by holding its end-effector, the user teaches simple movement primitives forming the basis for a more complex task. Each movement primitive is represented in the system using an oscillator combined with a learned nonlinear mapping. These primitives are then optimally combined to a complete solution to the posed problem. Said optimality is obtained using simulated annealing with the A* global search algorithm. Our approach is demonstrated on the problem of wiping a table, but can be used for many typical problems in service and household robotics.

IROS Conference 2004 Conference Paper

Soft robotics: what Cartesian stiffness can obtain with passively compliant, uncoupled joints?

Alin Albu-Schäffer
Max Fischer
Günter Schreiber
Florian Schoeppe
Gerhard Hirzinger

In the field of service robotics, whole arm contact with an unstructured environment or human beings becomes a major issue. Therefore soft robots, which mean robots with passively (or mechanically) compliant joints, become more and more important. In this work we analyze what Cartesian stiffness at the tool center point one can achieve with a passively compliant, redundant robot with variable joint stiffness. We restrict this work to the special case of uncoupled joint stiffness only, as coupling of joint stiffness seems to be mechanically difficult to realize. Finally we discuss a Cartesian controller, which incorporates the compliance of the joints and ensures the correct stiffness behavior also for high displacements from the desired position.

ICRA Conference 2003 Conference Paper

Cartesian impedance control of redundant robots: recent results with the DLR-light-weight-arms

Alin Albu-Schäffer
Christian Ott 0001
Udo Frese
Gerhard Hirzinger

This paper addresses the problem of impedance control for flexible joint robots based on a singular perturbation approach. Some aspects of the impedance controller, which turned out to be of high practical relevance during applications are then addressed, such as the implementation of nullspace stiffness for redundant manipulators, the avoiding of mass matrix decoupling and the related design of the desired damping matrix. Finally, the proposed methods are validated through measurements on the DLR robot.

ICRA Conference 2003 Conference Paper

Decoupling based Cartesian impedance control of flexible joint robots

Christian Ott 0001
Alin Albu-Schäffer
Andreas Kugi
Gerhard Hirzinger

This paper addresses the impedance control problem for flexible joint manipulators. An impedance controller structure is proposed, which is based on an exact decoupling of the torque dynamics from the link dynamics. A formal stability analysis of the proposed controller is presented for the general tracking case. Preliminary experimental results are given for a single flexible joint.

ICRA Conference 2002 Conference Paper

Cartesian Impedance Control Techniques for Torque Controlled Light-Weight Robots

Alin Albu-Schäffer
Gerhard Hirzinger

The paper compares various approaches to implementing a compliant Cartesian behavior for robotic manipulators: impedance, admittance and stiffness control. A new controller structure is proposed, which consists of an impedance controller enhanced by local stiffness control. This structure consistently takes into account the two time scale property of the joint and Cartesian control loops. The DLR light-weight robot, with its position, torque and impedance interfaces on joint level, is an adequate platform for the implementation of the presented methods. The experimental results are discussed and a critical comparison of the performance with different controllers is made. As an application for the new control structure the fast and intuitive teaching of an insertion task (piston into a motor block) is described.

IROS Conference 2002 Conference Paper

Comparison of adaptive and nonadaptive tracking control laws for a flexible joint manipulator

Christian Ott 0001
Alin Albu-Schäffer
Gerhard Hirzinger

This paper addresses the adaptive tracking control of flexible joint manipulators based on the singular perturbation approach. Comparisons between adaptive and nonadaptive tracking controllers are given. Experimental results with the DLR's light weight robot are given for the 2-DOF case.

ICRA Conference 2002 Conference Paper

DLR's Torque-Controlled Light Weight Robot III - Are We Reaching the Technological Limits Now?

Gerhard Hirzinger
Norbert Sporer
Alin Albu-Schäffer
Matthias Hähnle
R. Krenn
A. Pascucci
Markus Schedl

A third generation of torque-controlled light weight robots has been developed in DLR's robotics and mechatronics lab which is based on all the experiences that have been had with the first two generations. It aims at reaching the limits of what seems achievable with present day technologies not only with respect to light-weight, but also with respect to minimal power consumption and losses. One of the main gaps we tried to close in version III was the development of a new, robot-dedicated high energy motor designed with the best available techniques of concurrent engineering, and the renewed efforts to save weight in the links by using ultralight carbon fibres.

ICRA Conference 2001 Conference Paper

On a New Generation of Torque Controlled Light-Weight Robots

Gerhard Hirzinger
Alin Albu-Schäffer
Matthias Hähnle
Ingo Schäfer
Norbert Sporer

The paper describes the recent design and development efforts in DLR Robotics Lab towards the second generation of light-weight robots. The design of the light weight mechanics, integrated sensors and electronics is outlined. The fully sensory joint, with motor and link position sensors as well as joint torque sensors enables the implementation of effective vibration damping and advanced control strategies for compliant manipulation. The mechatronic approach incorporates a tight collaboration between mechanics, electronics and controller design. The authors hope that important steps towards a new generation of service and personal robots have been achieved.

ICRA Conference 2001 Conference Paper

Parameter Identification and Passivity Based Joint Control for a 7DOF Torque controlled Light Weight Robot

Alin Albu-Schäffer
Gerhard Hirzinger

We propose a method for identifying the parameters of a flexible joint robot based on the motor position, current and the additional joint torque sensor information. We make some theoretical remarks on the passivity property of the new controller used in the experiments. Simulation results for the movement of the complete robot are compared to measurements in order to validate the quality of the model and the performance of the controller.

IROS Conference 2000 Conference Paper

State feedback controller for flexible joint robots: a globally stable approach implemented on DLR's light-weight robots

Alin Albu-Schäffer
Gerhard Hirzinger

Addresses the problem of controlling the joints of a flexible joint robot with a state feedback controller and proposes a gradual way of extending such a controller towards feedback linearization. The global asymptotic stability for the state feedback controller with gravity compensation is proven. Experimental results on the DLR light-weight robots validate the method.

ICRA Conference 1998 Conference Paper

Key Issues in the Dynamic Control of Lightweight Robots for Space and Terrestrial Applications

Jing-Xin Shi
Alin Albu-Schäffer
Gerhard Hirzinger

Compared with industrial robots, the DLR lightweight robot characterizes a new milestone in the field of robot design. The module structure, the highly integrated electrical and mechanical components, the very low own-weight/load ratio, the high motion speed as well as the joint-torque feedback capability are the main features of this new robot generation aiming towards the functionality of a human arm. The paper attempts to integrate the newest results in the field of robotics, nonlinear control theory and electric drive systems to formulate the possible solutions of the dynamic control issues, Some experimental or simulation results are given to confirm the effectiveness of the proposed control approaches.