Author name cluster

David J. Braun

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.

18 papers

1 author row

IROS Conference 2024 Conference Paper

Energy Minimization using Custom-Designed Magnetic-Spring Actuators

Yue Yang Fu
Ali U. Kilic
David J. Braun

This study introduces an innovative actuator that resembles a motor with a non-uniform permanent magnetic field. We have developed a prototype of the actuator by combining a standard motor, characterized by a uniform magnetic field, with a custom rotary magnetic spring exhibiting a non-uniform magnetic field. We have also presented a systematic computational approach to customize the magnetic field to minimize the energy consumption of the actuator when used for a user-defined oscillatory task. Experiments demonstrate that this optimized actuator significantly lowers energy consumption in a typical oscillatory task, such as pick-and-place or oscillatory limb motion during locomotion, compared to conventional motors. Our findings imply that incorporating task-optimized non-uniform permanent magnetic fields into conventional motors and direct-drive actuators could enhance the energy efficiency of robotic systems.

IROS Conference 2023 Conference Paper

A Novel Approximation for the Spring Loaded Inverted Pendulum Model of Locomotion

Ali U. Kilic
David J. Braun

The Spring-Loaded Inverted Pendulum (SLIP) is one of the simplest models of robot locomotion. SLIP is commonly used to predict the center of mass motion and derive simple control laws for stable locomotion. However, the SLIP model is not integrable, which means that no closed-form relation can be derived to understand how the design and control parameters of the SLIP model affect stable locomotion. There exist a number of different analytical approximations to the SLIP model when considering small step lengths and symmetric steps. In this paper, we present a novel approximation to the SLIP model without relying on the small step length and the symmetric step assumption. The model was found to accurately predict the stability of the SLIP model for large and asymmetric steps and was used to design a controller to stabilize the SLIP model in a couple of steps.

ICRA Conference 2023 Conference Paper

Controllable Mechanical-domain Energy Accumulators

Sung Y. Kim
David J. Braun

Springs are efficient in storing and returning elastic potential energy but are unable to hold the energy they store in the absence of an external load. Lockable springs use clutches to hold elastic potential energy in the absence of an external load, but have not yet been widely adopted in applications, partly because clutches introduce design complexity, reduce energy efficiency, and typically do not afford high fidelity control over the energy stored by the spring. Here, we present the design of a novel lockable compression spring that uses a small capstan clutch to passively lock a mechanical spring. The capstan clutch can lock over 1000 N force at any arbitrary deflection, unlock the spring in less than 10 ms with a control force less than 1% of the maximal spring force, and provide an 80% energy storage and return efficiency (comparable to a highly efficient electric motor operated at constant nominal speed). By retaining the form factor of a regular spring while providing high-fidelity locking capability even under large spring forces, the proposed design could facilitate the development of energy-efficient spring-based actuators and robots.

ICRA Conference 2023 Conference Paper

Design of a Variable Stiffness Spring with Human-Selectable Stiffness

Chase W. Mathews
David J. Braun

Springs are commonly used in wearable robotic devices to provide assistive joint torque without the need for motors and batteries. However, different tasks (such as walking or running) and different users (such as athletes with strong legs or the elderly with weak legs) necessitate different assistive joint torques, and therefore, springs with different stiffness. Variable stiffness springs are a special class of springs which can exert more or less torque upon the same deflection, provided that the user is able to change the stiffness of the spring. In this paper, we present a novel variable stiffness spring design in which the user can select a preferred spring stiffness similar to switching gears on a bicycle. Using a leg-swing experiment, we demonstrate that the user can increment and decrement spring stiffness in a large range to effectively assist the hip joint during leg oscillations. Variable stiffness springs with human-selectable stiffness could be key components of wearable devices which augment locomotion tasks, such as walking, running, and swimming.

ICRA Conference 2023 Conference Paper

Novel Spring Mechanism Enables Iterative Energy Accumulation under Force and Deformation Constraints

Cole A. Dempsey
David J. Braun

Springs can provide force at zero net energy cost by recycling negative mechanical work to benefit motor-driven robots or spring-augmented humans. However, humans have limited force and range of motion, and motors have a limited ability to produce force. These limits constrain how much energy a conventional spring can store and, consequently, how much assistance a spring can provide. In this paper, we introduce an approach to accumulating negative work in assistive springs over several motion cycles. We show that, by utilizing a novel floating spring mechanism, the weight of a human or robot can be used to iteratively increase spring compression, irrespective of the potential energy stored by the spring. Decoupling the force required to compress a spring from the energy stored by a spring advances prior works, and could enable spring-driven robots and humans to perform physically demanding tasks without the use of large actuators.

ICRA Conference 2021 Conference Paper

Human Driven Compliant Transmission Mechanism

Tiange Zhang
David J. Braun

Energetically-passive robot exoskeletons, mimicking the function of the bicycle, could enable humans to reach previously unprecedented mobility. However, energetically-passive robot exoskeletons require a sophisticated mechanism to enable the human to supply energy, similar to what is enabled by the variable gear transmission mechanism of the bicycle. In this work, we present a new type of human-driven compliant transmission mechanism that could enable humans to supply energy when the leg is in the air, store the supplied energy, and release the stored energy when the leg is on the ground, in order to amplify the leg force and power. The compliant transmission mechanism presented in this paper is the first prototype and key component of a future human-driven artificial limb that aims to augment human mobility without using external energy.

IROS Conference 2021 Conference Paper

Novel Variable Stiffness Spring Mechanism: Modulating Stiffness Independent of the Energy Stored by the Spring

Sung Y. Kim
David J. Braun

Theory suggests a linear relation between stiffness and the energy stored by a linear helical spring at constant deformation. This relation implies that increasing the stiffness of a helical spring upon deformation requires more energy at larger deformations. State-of-the-art variable stiffness spring actuators, used to drive robots and human assistive and augmentation devices, are characterized by a similar relation: increasing stiffness as the spring is deformed costs more energy as more energy is stored by the spring. This feature imposes an apparently fundamental limitation on variable stiffness spring actuation in demanding tasks, such as lifting more, jumping higher, or running faster, because, in all these tasks, the variable stiffness spring should store a considerable amount of energy and provide different stiffness to accommodate different weights in lifting, heights in jumping, and speeds in running. Here, we present an innovative variable stiffness spring design, where the energy cost of changing stiffness is independent of the energy stored by the spring. The key element of the new design is a novel floating spring which changes stiffness without changing the energy stored by the spring. Springs possessing the aforementioned feature could pave the way towards variable stiffness robot actuation and human augmentation using smaller motors and smaller battery packs.

IROS Conference 2021 Conference Paper

Parallel Variable Stiffness Actuators

Chase W. Mathews
David J. Braun

In this paper, we introduce a new type of compliant actuator named the Parallel Variable Stiffness Actuator (PVSA) which consists of a variable stiffness spring placed in parallel with a direct-drive motor. Parallel variable stiffness actuators provide (i) high-fidelity force control and (ii) controllable energy storage, as they inherit the benefits of direct-drive motors and variable stiffness springs. We present a compact design of the PVSA using a flat motor connected to an adjustable mechanical advantage torsional spring. We show that this PVSA is (1) not subject to the fundamental force control bandwidth limitation of series elastic and variable stiffness actuators, and most notably, (2) enables resonant energy accumulation despite the limited deformation of the spring and the constrained motion of the load attached to the actuator. The latter differentiates parallel variable stiffness actuators from fixed-stiffness parallel elastic actuators. PVSAs may be used with smaller direct-drive motors to match the peak power of larger motors without compromising force control fidelity. PVSAs may be used to implement resonant forcing under joint angle limitations in walking, jumping, running, swimming robots, or robotic exoskeletons used to augmented human motion in the aforementioned tasks.

ICRA Conference 2020 Conference Paper

Variable Stiffness Springs for Energy Storage Applications

Sung Y. Kim
Tiange Zhang
David J. Braun

Theory suggests an inverse relation between the stiffness and the energy storage capacity for linear helical springs: reducing the active length of the spring by 50% increases its stiffness by 100%, but reduces its energy storage capacity by 50%. State-of-the-art variable stiffness actuators used to drive robots are characterized by a similar inverse relation, implying reduced energy storage capacity for increased spring stiffness. This relation limits the potential of the variable stiffness actuation technology when it comes to human performance augmentation in natural tasks, e. g. , jumping, weight-bearing and running, which may necessitate a spring exoskeleton with large stiffness range and high energy storage capacity. In this paper, we theoretically show that the trade-off between stiffness range and energy storage capacity is not fundamental; it is possible to develop variable stiffness springs with simultaneously increasing stiffness and energy storage capacity. Consistent with the theory, we experimentally show that a controllable volume air spring, has a direct relation between its stiffness range and energy storage capacity. The mathematical conditions presented in this paper may be used to develop actuators that could bypass the limited energy storage capacity of current variable stiffness spring technology.

ICRA Conference 2019 Conference Paper

Algorithmic Resolution of Multiple Impacts in Nonsmooth Mechanical Systems with Switching Constraints

Yangzhi Li
Haoyong Yu
David J. Braun

We present a differential-algebraic formulation with switching constraints to model the nonsmooth dynamics of robotic systems subject to changing constraints and multiple impacts. The formulation combines a single structurally simple governing equation, a set of switching kinematic constraints, and the plastic impact law, to represent the dynamics of robots that interact with their environment. The main contribution of this formulation is a novel algorithmic impact resolution method which provides an explicit solution to the classical plastic impact law in the case of multiple simultaneous impacts. This method serves as an alternative to prior linear-complementarity-based formulations which offer an implicit impact resolution through iterative calculation. We demonstrate the utility of the proposed method by simulating the locomotion of a planar anthropometric biped.

ICRA Conference 2019 Conference Paper

Constrained Feedback Control by Prioritized Multi-objective Optimization

Linfeng Li 0001
David J. Braun

Prioritized multi-objective optimization has been widely used within the operational space inverse dynamics control framework. In this paper, we present a constrained prioritized multi-objective optimization-base control formulation that extends to impedance control, including the `simple' impedance controller, which does not require the dynamic model. The main contribution of this paper is the dynamic-model-free prioritized feedback control formulation which encompasses arbitrary number of priority levels and takes the saturation constraints on the control inputs rigorously into account. The utility of the proposed formulation is demonstrated by a combined inverse dynamics impedance controller used to simulate stable locomotion of a planar anthropometric biped robot.

ICRA Conference 2018 Conference Paper

Stiffness Modulator: A Novel Actuator for Human Augmentation

Hong Fai Lau
Amanda Sutrisno
Tze Hao Chong
David J. Braun

Stiffness modulators are devices that promote a novel means of actuation; they provide stiffness modulation without deliberately doing mechanical work. These type of compliant actuators may be used for human augmentation to complement co-contracted antagonistic muscles and as such reduce muscle activity and metabolic energy cost. Despite the theoretical appeal of this concept, its implementation remains elusive in practical applications. This is particularly true for human augmentation which requires a portable stiffness modulator. In this paper, we present a compact, lightweight, and self-contained stiffness modulator. Using this device, we demonstrate stiffness augmentation of the human knee joint in a sit to stand task. The experimental results indicate that the proposed device is able to assist a human by reducing muscle activity while drawing minimal battery power.

ICRA Conference 2017 Conference Paper

Analytical conditions for the design of variable stiffness mechanisms

Tze Hao Chong
Vincent Chalvet
David J. Braun

This paper introduces an analytical approach for the design of variable stiffness mechanisms. The basis of this approach is a general model - representing the potential energy function and the physical constraints - covering the design space of variable stiffness mechanisms. Using this model, we present a systematic procedure to analytically define classes of variable stiffness mechanisms from first principles. Consequently, we identify mechanisms capable of infinite range stiffness modulation using bounded motor forces, and define the simplest mathematical model representing mechanisms in this class. A prototype mechanism consistent with this canonical model is designed, fabricated and experimentally tested. The experimental data are consistent with our theoretical predictions showing constant motor force independent of the output deflection and output stiffness when the mechanism is subject to external load.

ICRA Conference 2017 Conference Paper

Efficiently tunable positive-negative stiffness actuator

Abhinav Dahiya
David J. Braun

Compliant actuators have found their place in areas of prosthetics, rehabilitation and robot locomotion because they enable safe human-robot and stable robot-environment interaction, both non-trivial to achieve using conventional rigid actuation. These actuators are capable of varying their equilibrium position and apparent output stiffness in a way humans change the resting position and compliance of their limbs. Just like antagonistically actuated human joints, these actuators require two motor units to provide control over the equilibrium position and the positive joint stiffness. Here we present a novel compliant actuation concept which affords control over the equilibrium position and joint stiffness using a single motor unit. In order to achieve this unconventional functionality, the actuator combines a passive positive feedback (negative stiffness) mechanism with an efficiently tunable negative feedback (positive stiffness) mechanism. This provides a novel design with two distinct operation modes, one leading to unprecedented stiffness tunability, while the other enabling equilibrium point controllability. We present the first practical implementation of this actuator using a prototype prosthetic limb design along with experimental data testifying the range of tunability, covering compliant to rigid behaviour, without paying much on the power input.

ICRA Conference 2016 Conference Paper

Compliant actuation for energy efficient impedance modulation

David J. Braun
Salil S. Apte
Olzhas Adiyatov
Abhinav Dahiya
Neville Hogan

Energy efficient compliant actuation is the missing ingredient and key enabler of next-generation autonomous systems, domestic robots, prosthetic devices, orthotic devices, and wearable exoskeletons, to name a few. For all these devices, one would wish to develop actuators enabling wide range impedance modulation with low energy cost. Using conventional and biologically-inspired compliant actuation, previous research led to functional devices but with high energy cost. Here we introduce a minimalistic compliant actuator to realize impedance modulation with low energy cost. Using this actuator we demonstrate stiffness augmentation in human-machine collaboration. We argue that the non-biologically-inspired actuation concept presented here may effectively complement a biological system, by restoring or extending its functionality, with negligible energy cost.

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.

ICRA Conference 2011 Conference Paper

Constraint-based equilibrium and stiffness control of variable stiffness actuators

Matthew J. Howard 0001
David J. Braun
Sethu Vijayakumar

Considerable research effort has gone into the design of variable passive stiffness actuators (VSAs). A number of different mechanical designs have been proposed, aimed at either a biomorphic (i. e. , antagonistic) design, compactness, or simplified modelling and control. In this paper, we propose a (model-based) unified control methodology that is able to exploit the benefits of variable stiffness independent of the specifics of the mechanical design. Our approach is based on forming constraints on commands sent to the VSA to ensure that the equilibrium position and stiffness of the VSA are tracked to the desired values. We outline how our approach can be used for tracking stiffness and equilibrium position both in joint and task space, and how it may be used in the context of constrained local optimal control. In our experiments we illustrate the utility of our approach in the context of online teleoperation, to transfer compliant human behaviour to a variable stiffness device.

IROS Conference 2009 Conference Paper

A controller for dynamic walking in bipedal robots

David J. Braun
Michael Goldfarb

This paper presents an approach for the closed-loop control of actuated biped that allows natural looking and energy efficient walking. Rather than prescribe kinematic trajectories or kinematic constraints, the approach is based on the prescription of state dependent torques that “encourage” patterned movement. Some of the prescribed torques are referenced to the inertial reference frame, which largely decouples the angular dynamics of the robot, and as such greatly simplifies the selection of control parameters. Implementation of torques from the inertial coordinate frames is enabled by a joint torque computation which is motivated by Gauss's principle of least constraint. The proposed approach is implemented in simulation on an anthropomorphic biped, and is shown to quickly converge to a natural looking gait limit cycle. Simulations are conducted with various control parameters and different initial conditions. The authors also show that walking speed can be altered in a simple manner by varying two intuitive controller parameters. The mechanical cost of transport computed on a representative dynamic walk is used to validate energy efficiency of the proposed control approach.