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Yunjiang Lou

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31 papers

1 author row

IROS Conference 2025 Conference Paper

4D-ROLLS: 4D Radar Occupancy Learning via LiDAR Supervision

Ruihan Liu
Xiaoyi Wu
Xijun Chen
Liang Hu
Yunjiang Lou

A comprehensive understanding of 3D scenes is essential for autonomous vehicles (AVs), and among various perception tasks, occupancy estimation plays a central role by providing a general representation of drivable and occupied space. However, most existing occupancy estimation methods rely on LiDAR or cameras, which perform poorly in degraded environments such as smoke, rain, snow, and fog. In this paper, we propose 4D-ROLLS, the first weakly supervised occupancy estimation method for 4D radar using the LiDAR point cloud as the supervisory signal. Specifically, we introduce a method for generating pseudo-LiDAR labels, including occupancy queries and LiDAR height maps, as multi-stage supervision to train the 4D radar occupancy estimation model. Then the model is aligned with the occupancy map produced by LiDAR, fine-tuning its accuracy in occupancy estimation. Extensive comparative experiments validate the exceptional performance of 4D-ROLLS. Its robustness in degraded environments and effectiveness in cross-dataset training are qualitatively demonstrated. The model is also seamlessly transferred to downstream tasks BEV segmentation and point cloud occupancy prediction, highlighting its potential for broader applications. The lightweight network enables 4D-ROLLS model to achieve fast inference speeds at about 30 Hz on a 4060 GPU. The code of 4D-ROLLS will be made available at https://github.com/CLASS-Lab/4D-ROLLS.

IROS Conference 2025 Conference Paper

Learning-Based Passive Fault-Tolerant Control of a Quadrotor with Rotor Failure

Jiehao Chen
Kaidong Zhao
Zihan Liu
YanJie Li
Yunjiang Lou

This paper proposes a learning-based passive fault-tolerant control (PFTC) method for quadrotor capable of handling arbitrary single-rotor failures, including conditions ranging from fault-free to complete rotor failure, without requiring any rotor fault information or controller switching. Unlike existing methods that treat rotor faults as disturbances and rely on a single controller for multiple fault scenarios, our approach introduces a novel Selector-Controller network structure. This architecture integrates fault detection module and the controller into a unified policy network, effectively combining the adaptability to multiple fault scenarios of PFTC with the superior control performance of active fault-tolerant control (AFTC). To optimize performance, the policy network is trained using a hybrid framework that synergizes reinforcement learning (RL), behavior cloning (BC), and supervised learning with fault information. Extensive simulations and real-world experiments validate the proposed method, demonstrating significant improvements in fault response speed and position tracking performance compared to state-of-the-art PFTC and AFTC approaches. Video and code will be available at https://github.com/HITSZcjh/uav_ftc.

IROS Conference 2025 Conference Paper

Safe and Efficient Navigation for Differential-Drive Robots in Dynamic Pedestrian Environments

Wenhao Liu
Letian Fu
Chen Li
Wanlei Li
Yunjiang Lou

Differential-drive robots are widely used in dynamic pedestrian environments, such as hospitals, for time-sensitive tasks like medication delivery, which require high navigation efficiency to ensure timely arrivals. However, existing methods tend to overemphasize safety, resulting in overly conservative behaviors and prolonged navigation times, which in turn lead to reduced efficiency. To address this issue, this paper proposes a novel navigation framework that integrates a pedestrian risk map, modeled using asymmetric Gaussian distributions, into B-spline trajectory optimization. Rather than strictly avoiding high-risk regions, the method balances collision risk and trajectory length minimization, leading to more effective navigation. Additionally, multiple planning modes enhance adaptability in complex environments, ensuring both safety and efficiency. Furthermore, kinematic constraints specific to differential-drive robots are incorporated to ensure the feasibility of the generated trajectories. Simulations and real-world experiments validate the proposed method’s effectiveness in achieving safe and efficient navigation in dynamic pedestrian environments. The video is available at https://youtu.be/S9qJmXyPEzw.

IROS Conference 2025 Conference Paper

Whole-Body Admittance Control of Anti-Saturation for Quadruped Manipulators with Impact Force Observer

Fenghao Lin
Tianlin Zhang
Xiaogang Xiong
Yunjiang Lou

Quadruped manipulators require precise detection of external impact forces to ensure safe and compliant responses during environmental interactions. However, these systems often lack tactile sensors on their body surfaces or force/torque sensors at critical joints. This study introduces a whole-body admittance control framework for quadruped manipulators, utilizing a novel external impact force observer that estimates impact forces acting on the manipulator or the quadruped’s base without relying on dedicated force sensors. The observer leverages the robustness of a super-twisting algorithm (STA) based on the momentum model of quadruped manipulators. Model uncertainties are mitigated using a low-pass filter (LPF) and compensated by ground reaction forces, significantly reducing estimation oscillations during dynamic gaits. By integrating these estimated impact forces, the whole-body admittance control framework enables compliant interactions with the environment and mitigates unsafe behaviors caused by torque saturation through a set-valued feedback loop that constrains command torques within actuation limits, including joint torque boundaries and friction cone constraints of the ground reaction force. Experimental validation across diverse scenarios confirms the effectiveness of this approach in facilitating safe and adaptive interactions between quadruped manipulators and external forces.

IROS Conference 2024 Conference Paper

RTTF: Rapid Tactile Transfer Framework for Contact-Rich Manipulation Tasks

Qiwei Wu 0001
Xuanbin Peng
Jiayu Zhou
Zhuoran Sun
Xiaogang Xiong
Yunjiang Lou

An increasing number of robotic manipulation tasks now use optical tactile sensors to provide tactile feedback, making tactile servo control a crucial aspect of robotic operations. This paper presents a rapid tactile transfer framework (RTTF) that achieves optical-tactile image sim2real transfer and robust tactile servo control using limited paired data. The sim2real aspect of RTTF employs a semi-supervised approach, beginning with pretraining the latent space representations of tactile images and subsequently mapping different tactile image domains to a shared latent space within a simulated tactile image domain. This latent space, combined with the proprioceptive information of the robotic arm, is then integrated into a privileged learning framework for policy training, which results in a deployable tactile control policy. Our results demonstrate the robustness of the proposed framework in achieving task objectives across different tactile sensors with varying physical parameters. Furthermore, manipulators equipped with tactile sensors, allow for rapid training and deployment for diverse contact-rich tasks, including object pushing and surface following.

IROS Conference 2024 Conference Paper

Whole-body Compliance Control for Quadruped Manipulator with Actuation Saturation of Joint Torque and Ground Friction

Tianlin Zhang
Xuanbin Peng
Fenghao Lin
Xiaogang Xiong
Yunjiang Lou

In normal operations, when quadruped manipulators with impedance control experience external disturbances, they may become unstable and lose balance due to actuation saturation, affecting their stability, safety, and compliance with the environment. To address this issue, we propose a whole-body compliance controller to prevent unstable behaviors like slip, oscillation, and overshoot, which arise from actuation saturation. The controller includes an admittance scheme with a set-valued operator as the internal feedback, to constrain joint torques within actuators’ limits and ground reaction forces within friction cones to ensure stability against disturbances. Then, it formulates a hierarchical optimization problem using the Hierarchical Quadratic Programming (HQP) to impose the output of the admittance scheme while ensuring physical consistency to maintain compliance behaviors. Unlike traditional compliance control with one-dimensional torque limitations, our approach considers both joints torque limits of manipulator joints and friction cones of quadruped ground reaction as actuation saturation. This ensures overall compliance and stability for the quadruped manipulators, even under significant external forces, regardless of where they are exerted on the robot. We demonstrate through experiments involving variable stiffness environments and external forces during normal operations how effective our approach is in enhancing the safety of quadruped manipulators.

IROS Conference 2023 Conference Paper

A Safety Filter for Realizing Safe Robot Navigation in Crowds

Kaijun Feng
Zetao Lu
Jun Xu
Haoyao Chen
Yunjiang Lou

It is challenging to realize the safe navigation of mobile robots in crowds. Most of the previous studies may lead to unsafe robot navigation in crowds, as safety guarantee is lacked. To solve this problem, we devise a safety filter (SF) that enables realization of safe robot navigation in crowds, and provides safety guarantees by verifying whether the optimal action recommended by an unsafe method is safe and, if not, corrects the action. The three main processes performed by the SF applied to given robot are (1) construction of the safe state constraints of the robot using a safe set; (2) construction of the safe action constraints of the robot based on discrete-time generalized velocity obstacles (DGVOs); and (3) determination of a feasible solution of the SF design problem, or, if none can be found, replacement of the above hard constraints with heuristic soft constraints. We used the SF with a reaction-based method and three learning-based methods in simulation experiments of random and non-random crowds, and the results showed that the SF decreases the collision rates and danger rates and thereby increases the success rates of these methods. We also deployed the SF with three learning-based methods on an mr1000 robot in real-world experiments, and the results showed that the SF enabled the robot using learning-based methods to navigate to its goal without colliding with humans.

IROS Conference 2023 Conference Paper

Dynamic Object Tracking for Quadruped Manipulator with Spherical Image-Based Approach

Tianlin Zhang
Sikai Guo
Xiaogang Xiong
Wanlei Li
Zezheng Qi
Yunjiang Lou

Exactly estimating and tracking the motion of surrounding dynamic objects is one of important tasks for the autonomy of a quadruped manipulator. However, with only an onboard RGB camera, it is still a challenging work for a quadruped manipulator to track the motion of a dynamic object moving with unknown and changing velocities. To address this problem, this manuscript proposes a novel image-based visual servoing (IBVS) approach consisting of three elements: a spherical projection model, a robust super-twisting observer, and a model predictive controller (MPC). The spherical projection model decouples the visual error of the dynamic target into linear and angular ones. Then, with the presence of the visual error, the robustness of the observer is exploited to estimate the unknown and changing velocities of the dynamic target without depth estimation. Finally, the estimated velocity is fed into the model predictive controller (MPC) to generate joint torques for the quadruped manipulator to track the motion of the dynamical target. The proposed approach is validated through hardware experiments and the experimental results illustrate the approach's effectiveness in improving the autonomy of the quadruped manipulator.

IROS Conference 2022 Conference Paper

Flexible and Precision Snap-Fit Peg-in-Hole Assembly Based on Multiple Sensations and Damping Identification

Ruikai Liu
Xiansheng Yang
Ajian Li
Yunjiang Lou

Snap-fit peg-in-hole assembly widely exists in both industry and daily life, especially for consumer electronics. The buckle mechanism leads to a damping zone inside the port where insertion force needs to be increased. It is much difficult to automate this process by robots, for size and clearance of the components are always small, and the damping buckle should be perceived and distinguished from solid inner walls of the port. End-effector position control might be invalid, since grasping error will make it difficult to locate the plug accurately. In this article, we undertake this assembly challenge by taking advantage of fingertip tactile perception combined with visual images and force feedback. Raw sensor data is collected, processed, and fused together to be state input of a reinforcement learning network, generating continuous action vectors. We also propose a novel damping zone predictor through feature extraction and multimodal fusion, which is able to identify whether the plug has touched the buckle mechanism, so as to adjust the insertion force. The whole framework is implemented through a common USB Type-C insertion experiment on Franka Panda robot platform, reaching a success rate of 88%. Furthermore, system robustness is verified, and comparisons of different modalities are also conducted.

IROS Conference 2022 Conference Paper

IMU Dead-Reckoning Localization with RNN-IEKF Algorithm

Hang Zhou
Yibo Zhao
Xiaogang Xiong
Yunjiang Lou
Shyam Kamal

In complex urban environments, the Inertial Navigation System (INS) is important for navigating unmanned ground vehicles (UAVs) for its environment-independency and reliability of real-time localization. It is usually employed as the baseline in the case of other sensors failures, such as the GPS, Lidar, or Cameras. However, one problem for the INS is that its estimation error of localization accumulates over time, and thus the estimated trajectories of the UAVs continue to drift away from their ground truths. To solve this problem, this paper proposes an improved algorithm based on the Invariant Extended Kalman Filter (IEKF) for dead-reckoning of autonomous vehicles, which dynamically adjusts the process noise and the observation noise covariance matrixes through Attention mechanism and Recurrent Neural Network (RNN). The algorithm achieves more robust and accurate dead-reckoning localization in the experiments conducted on the KITTI dataset, reducing the translational error by about 45%compared to the baseline.

IROS Conference 2021 Conference Paper

A Compliant Five-Bar Legged Mechanism for Heavy-Load Legged Robots by Using Magneto-Rheological Actuators

Guangzeng Chen
Jiangtao Ran
Chenguang Bai
Pengyu Jie
Yunjiang Lou

In this paper, a compliant five-bar leg mechanism is proposed, designed and manufactured for heavy-load legged robots, by using two magneto-rheological actuators (MRAs) that are capable of offering a maximal torque of 78Nm. To address the rate-dependent hysteresis of the MRA, a hybrid rate-dependent hysteresis model is derived based on the idea of mappings between different hysteresis loops. With integrating the classical Preisach model and the NARX neural network, the hybrid model is able to model hysteresis nonlinearity of the magneto-rheological clutch (MRC). It is then used to estimate and control the output torque of the MRA at the absent of external force/torque sensors. High fidelity force control and variable compliance of the leg mechanism are realized and validated in various experiments with using the MRAs.

ICRA Conference 2018 Conference Paper

Visual Grasping for a Lightweight Aerial Manipulator Based on NSGA-II and Kinematic Compensation

Linxu Fang
Haoyao Chen
Yunjiang Lou
Yanjie Li
Yun-Hui Liu 0001

The grasping control of an aerial manipulator in practical environments is challenging due to its complex kinematics/dynamics and motion constraints. This paper introduces a lightweight aerial manipulator, which is combined with an X8 coaxial octocopter and a 4-DoF manipulator. To address the grasping control problem, we develop an efficient scheme containing trajectory generation, visual trajectory tracking, and kinematic compensation. The NSGA-II method is utilized to implement the multiobjective optimization for trajectory planning. Motion constraints and collision avoidance are also considered in the optimization. A kinematic compensation-based visual trajectory tracking is introduced to address the coupled nature between manipulator and VAV body. No dynamic parameter calibration is needed. Finally, several experiments are performed to verify the stability and feasibility of the proposed approach.

IROS Conference 2017 Conference Paper

Contouring error vector and cross-coupled control of multi-axis servo system

Ran Shi
Xiang Zhang 0006
Yunjiang Lou

The contouring error and cross-coupled gains calculation have always been the critical issues in the application of cross-coupled control. Traditionally, the linear approximation and circular approximation are widely used to determine the contouring error and cross-coupled gains. However, for linear approximation and circular approximation, the contouring error and cross-coupled gains are calculated sophisticatedly, especially in three-dimensional applications. In this paper, a contouring error vector is established under task coordinate frame, then the contouring error and cross-coupled gains can be easily obtained based on the magnitude and orientation of the contouring error vector. The experimental results on a three-axis CNC machine indicate the proposed approach simplifies the calculation of contouring error and cross-coupled gains.

IROS Conference 2016 Conference Paper

A novel contouring error estimation for position-loop cross-coupled control of biaxial servo systems

Ran Shi
Yunjiang Lou
Yongqi Shao 0002
Jiangang Li
Haoyao Chen

How to achieve the required contouring tracking accuracy especially during high-speed and large-curvature contouring tasks, has always been an important problem in manufacturing applications. In this paper, a contouring error estimation method based on natural local approximation is used, and then the position-loop cross-coupled controller is proposed to reduce the estimated contouring error. The effectiveness and superiority of the natural local approximation method using on the position-loop cross-coupled control scheme are demonstrated through experiments on a biaxial linear motor drive servo system.

IROS Conference 2015 Conference Paper

Design and analysis of parallel robots for a flexible fixturing system with performance atlases

Bing Li 0015
Peng Xu 0012
Hongjian Yu
Yunjiang Lou
Xiaojun Yang

According to the automobile industry's flexible manufacturing requirements, a novel flexible fixturing system for sheet metal assembly is proposed with parallel robots. A methodology of the structure synthesis is presented by taking account simultaneously several performance indices. Taking the 3UPU/UPS parallel robot in the system as an example, models of inverse kinematics, Jacobian matrix and design space are developed. Thus, the structure synthesis of the parallel robot is simplified to a two-dimensional problem. Once the performance indices (workspace, singularity and stiffness) are established, the corresponding indices atlases are expressed in the design space. The structure parameters of the parallel robot are obtained by analyzing these atlases. The prototype of the fixturing system is developed, and the relevant clamping and stiffness experiments are conducted. The experimental results generally agree well with the simulation results and satisfy the flexible fixturing systems' requirements.

IROS Conference 2013 Conference Paper

Natural local approximation based contouring control for free-form contours

Hao Meng
Yunjiang Lou
Jiangpeng Zhou

In this paper, a novel contouring control method based on natural local approximation of desired contour in task frame is proposed for multi-axis control systems. Based on local geometry properties, natural local approximation can achieve more accurate contouring error estimation compared with other local approximation methods for both planar and spatial contouring tasks. The contouring controller, integrated with a PD controller cooperated with the feedback linearization technique and a feedforward compensation, is designed to realize the decoupling control of estimated errors in the task frame. Contouring performance can then be improved directly by increasing corresponding controller parameters. Simulations of 3-axis system and experiments on biaxial XY-stage verified that our proposed method can reduce the contouring errors dramatically in high speed and large curvature cases compared with first-order based method.

ICRA Conference 2010 Conference Paper

Quotient kinematics machines: Concept, analysis and synthesis

Yuanqing Wu 0001
Hong Wang
Zexiang Li 0001
Yunjiang Lou
Jinbo Shi

In this paper, we identify a class of structurally distinguished machines, called quotient kinematics machines (QKM). A QKM realizes a motion task, typically characterized by a subgroup G of rigid transformation group SE(3), through coordinated motion of two mechanisms called modules. One is referred to as a subgroup module H and the other a complementary or quotient module G/H of H in G. Since QKM can retain both large workspace/rotation range of SKMs and speed/accuracy of PKMs by appropriate choice of modules, it is often implemented in high end machine design for semiconductor die/wire-bonding and 5-axis machining, etc. To promote QKM technology beyond occasional studies and applications, we use differential geometric techniques to develop a rigorous and precise treatment of QKMs, including: (i) modeling and analysis of QKMs; (ii) classification and synthesis of QKMs; (iii) PKM realization of quotient modules.

IROS Conference 2009 Conference Paper

Improved and modified geometric formulation of POE based kinematic calibration of serial robots

Yunjiang Lou
Tieniu Chen
Yuanqing Wu 0001
Zhibin Li
Shilong Jiang

The authors proposed in this paper an improved geometric formulation of POE (product of exponential) based kinematic calibration of serial robots. We use both joint offset-free formulation and adjoint transformation errors of joint screws, and apply it to the calibration of an elbow manipulator. Our formulation explains why the original POE calibration always fails with the existence of joint offset errors; the adjoint formulation of joint screw errors eliminates joint screw constraints that was imposed in the original iterated least square calibration algorithm. The second contribution of this paper is the proposal of a modified POE formulation which adopts point measurement data instead of frame measurement data of the end-effector, which can be more realistic and convenient for practical implementation. Simulation results show that the proposed method is plausible and effective. An experiment is under preparation to verify the effectiveness of the proposed calibration method on an elbow manipulator built by Googol Technology.

ICRA Conference 2009 Conference Paper

Natural frequency based optimal design of a two-link flexible manipulator

Yunjiang Lou
Wei Gong
Zexiang Li 0001
Jianjun Zhang 0003
Guilin Yang

Modern industries, e. g. , semiconductor packaging, imposes increasing stringent requirement on equipment with very high acceleration and high precision. Traditionally, arm linkage and drive mechanism are first designed followed by control design. The integrated design method is proposed as a preferable technique of the traditional one. In this paper, a general framework of the integrated design method for a point-to-point control is presented. The dynamic model for a flexible planar two-link manipulator is derived by the finite element method. The PD control strategy is applied in the closed-loop system. The structural parameters and control parameters are optimized simultaneously by solving the integrated design problem. The differential evolution (DE) technique, a global optimization technique, is used to solve the optimal design problem. A simulation shows the integrated design method gives improved system performance.

IROS Conference 2008 Conference Paper

Quotient kinematics machines: Concept, analysis and synthesis

Yuanqing Wu 0001
Zexiang Li 0001
Han Ding 0001
Yunjiang Lou

In mechanism and machine design, the notion of serial kinematics machine (SKM), parallel kinematics machine (PKM) and hybrid kinematics machine (HKM) is well understood. In this paper, we introduce a fourth type of kinematics machine, known as quotient kinematics machine(QKM). A QKM generating a subgroup motion G consists of two mechanisms (or motion modules) acting in unison, one synthesizing a subgroup H of G, and another that of a complement of G/H. Apparently, the two motion modules of a QKM have simpler kinematic structures than that of a SKM, PKM or HKM with the same motion type G, and thus is expected to have performance advantages in terms of stiffness (speed and accuracy), modularity and etc, over its SKM/PKM/HKM counterparts. The formulation of the QKM concept and its analysis and synthesis are considered in this paper.

ICRA Conference 2007 Conference Paper

Development of a Novel 3-DoF Purely Translational Parallel Mechanism

Yunjiang Lou
Jiangang Li
Jinbo Shi
Zexiang Li 0001

In view of the successful application of planar parallelogram in the Delta robot and its variants, we are interested to investigate mechanisms consisting of spatial parallelograms. The spatial parallelogram, denoted by Pscr a *, is a 2-SS (S stands for a spherical joint) parallel mechanism having identical length for opposite links. We show that a 3-PPscr a * mechanism is generically undergoes 3-dimensional purely translational motion. Based on the 3-PPscr a * topology, an integrated optimal design on both architecture and geometry design is carried out. Using the formulation of maximizing effective cubic workspace, the Orthopod, which has three orthogonally arranged linear joint axes, is found to be the best in our settings. A prototype machine of the Orthopod is thus designed and manufactured.

IROS Conference 2006 Conference Paper

A Novel 3-DoF Purely Translational Parallel Mechanism

Yunjiang Lou
Zexiang Li 0001

A novel 3-DoF purely translational parallel mechanism, the Orthotripod, is proposed. It is a variant of the tripod based parallel machine and has a similar architecture to the Orthoglide. In order to reduce the number of passive joints and remove the effect of ease of abrasion of revolute joints, spherical joints are applied in the parallelogram. A mathematic mobility analysis shows the mechanism is indeed 3-DoF purely translational. We optimally design the Orthotripod and the tripod based parallel machine by maximizing the well-conditioned workspace. The optimized Orthotripod possesses a nearly ball-shaped workspace and has much better kinematic performance than the optimized tripod based parallel mechanism. The proposed mechanism is adaptable for machine tool applications

IROS Conference 2006 Conference Paper

Geometric Contouring Control on the Smooth Surface

Dongjun Zhang
Yunjiang Lou
Zexiang Li 0001

In this paper we concentrate on contouring control for surface machining. The object of the motion control system is tracking the spatial curve lying on the surface. Observing that the contour error can be approximated by the tracking error (projected to the normal subspace of the surface), we propose a new design procedure based on the geometrical properties of the curves and surfaces. Essentially the controllers look ahead using the information provided by the curvature of the curves and surfaces. The simulation results show the efficiency of the design method

IROS Conference 2006 Conference Paper

Grasping Force Optimization for Whole Hand Grasp

Jijie Xu
Yunjiang Lou
Zexiang Li 0001

In tasks of grasping and manipulation, the hand sometimes uses not only fingertips but also fingers' inner links and the palm to achieve more robust grasp. This kind of grasp is called whole hand grasp, or power grasp. One property of whole hand grasp is that the hand may not be able to generate grasping forces in any directions, so previous fingertip grasping analysis is no longer suitable for whole hand grasp. In this paper, concepts of active force and passive force are introduced. With these concepts, the contact force space is decomposed into four orthogonal subspaces. Considering the roles of both active force and passive force, a new cost index is proposed for the whole hand grasping force optimization, which is then reformulated into a convex optimization problem involving LMIs. Finally, numerical example and simulation results verify the validity and performance of our formulation of the problem with that new proposed cost index

IROS Conference 2006 Conference Paper

Hybrid Automaton: A Better Model of Finger Gaits

Jijie Xu
Yunjiang Lou
Zexiang Li 0001

Large-scale motion of the grasped object is one of the tasks, which is involved in practical dextrous manipulation of multifingered robotic hand. When the large-scale motion can not be accomplished only by rolling and sliding of the finger, finger gaiting, or regrasping, is used. In this paper, two primitives of finger gaits are introduced. Based on the characteristic of finger gaits, we model finger gaits as a hybrid automaton. Finally, we do simulations on a three fingered hand to verify the validity of our model

IROS Conference 2006 Conference Paper

Task Space Based Contouring Control of Parallel Machining Systems

Yunjiang Lou
Ni Chen
Zexiang Li 0001

Since the tracking error does not truly reflect product quality, the contouring error is introduced in the dynamic control of parallel machining systems. For real-time computation reason, the contouring error is approximated by the distance from the actual position to the tangent plane of the desired contour at the corresponding desired position, i. e. , the error in normal direction. By attaching a moving task frame to each point on a desired trajectory, the tracking error is decomposed into tangential error and normal error. By the transformation introduced by the task frame, we obtain error dynamics in the task frame. The error dynamics is decoupled into error dynamics in tangential and normal directions by applying the computed torque control and choosing appropriate system matrices. Simulation shows that a larger bandwidth of the normal dynamics leads to smaller contouring error given fixed natural frequency for the tangential dynamics. By a comparison with the PD control in the world frame, the task space based contouring control exhibits much better performance in contouring accuracy

ICRA Conference 2005 Conference Paper

Optimal Design of a Parallel Machine Based on Multiple Criteria

Yunjiang Lou
Dongjun Zhang
Zexiang Li 0001

This paper proposes to optimally design a parallel machine based on multiple criteria. Many criteria, workspace, condition number, accuracy, stiffness, maximum velocity, and maximum force, are considered. The optimal design problem is proposed as to find a set of design parameters such that (a) the Cartesian workspace generated by the resulting manipulator contains a prescribed workspace; (b) the resulting manipulator possesses a good condition number at each points in the prescribed workspace; (c) the resulting manipulator possesses good performance on accuracy, stiffness, velocity/force transmission factor. By some manipulations, the requirements on the latter four criteria are reduced to constraints on singular values of the kinematic Jacobian. A trade-off must be made since there're opposite requirements among those four criteria. The singular values of kinematic Jacobian are limited in a given interval to guarantee good properties. All the requirement are finally reduced to polynomial inequalities with respect to design parameters. The optimal design problem is transformed into a Max-Det optimization problem that can be ef ficiently solved. The Orthoglide is used as an example to demonstrate the procedure.

IROS Conference 2005 Conference Paper

Optimal design of parallel manipulators for maximum effective regular workspace

Yunjiang Lou
Guanfeng Liu 0002
Ni Chen
Zexiang Li 0001

Kinematic design of parallel manipulators is addressed in this paper. By observation that regular (e. g. , hyper-rectangular) workspaces are desirable for most machines, we propose the concept of effective regular workspace, which reflects both requirements on the workspace shape and quality. Dexterity index is utilized to characterize the effectiveness of the workspace. The optimal design problem is then formulated to find a manipulator geometry that maximizes the effective regular workspace. Since the optimal design problem is a constrained nonlinear optimization problem without explicit analytical expressions, the controlled random search (CRS) technique, which was reported robust and reliable, is applied to numerically solve the problem. The commonly-used Stewart-Gough platform is employed as an example to demonstrate the design procedure.

ICRA Conference 2004 Conference Paper

A General Approach for Optimal Kinematic Design of Parallel Manipulators

Yunjiang Lou
Guanfeng Liu 0002
Jijie Xu
Zexiang Li 0001

This paper deals with the problem of optimal geometry design of parallel manipulators. In order to reduce the main drawbacks of parallel manipulators, relatively small workspace and more singularities, two requirements, workspace and condition number, are considered. The design problem is thus formulated to find a parallel mechanism such that its Cartesian workspace contains a prescribed workspaces with good condition numbers in it. By observing that those requirements can be locally cast into Linear Matrix Inequalities (LMIs), we formulate the design problem locally as a convex optimization problem subject to LMIs with a max-det function as its objective function. Hence, at each node of discretized space of design parameters, there is an LMI-based convex optimization problem. A two-level algorithm can be applied to solve for a set of optimal design parameters: (1) Discretize the space of design parameters into a set of discrete nodes; (2) At each node the Newton algorithm is applied to solve the max-det optimization problem. By comparing all the locally optimal costs, we can obtain a corresponding set of globally optimal design parameters correspondingly. Simulation results verify the effectiveness of the proposed approach.

IROS Conference 2003 Conference Paper

An LMI based optimal design of parallel manipulators

Yunjiang Lou
Guanfeng Liu 0002
Zexiang Li 0001

This paper deals with the problem of optimal design of parallel manipulators which have no singularity, have high stiffness and manipulability and are the most economic. By observing that those requirements can be cast into linear matrix inequalities (LMIs), we formulate the design problem as a convex optimization problem subject to LMIs with either a linear function or a max-det function as its objective function. The variables x associated with LMIs are nonlinear functions of some key kinematic parameters /spl alpha/. If the dimension of x is equal to the number of independent kinematic parameters, a two-level algorithm can be applied to solve for a set of optimal kinematic parameters: (1) applying the interior-point algorithm for solving of x; (2) applying the Newton method to a set of nonlinear algebraic equations for solving of /spl alpha/. If the dimension of x is greater than the number of independent kinematic parameters (i. e. , x are not linearly independent), we consider the constrained semi-definite programming problems and the constrained max-det problems by taking account of an additional set of nonlinear constraints. We propose a simplified constrained gradient algorithm for solving of x in such cases, /spl alpha/ derives from x using Newton method. Simulation results verify the effectiveness of the proposed algorithms.

ICRA Conference 2003 Conference Paper

Optimal design of parallel manipulators via LMI approach

Yunjiang Lou
Guanfeng Liu 0002
Zexiang Li 0001

This paper deals with the problem of optimal design of parallel manipulators which are singularityless, of high stiffness and manipulability and the most economic. By observing that those requirements can be cast into Linear Matrix Inequalities (LMIs), we formulate the design problem as a convex optimization problem subject to LMIs with either a linear function or a max-det function as its objective function. The variables x associated with LMIs are nonlinear functions of some key kinematic parameters /spl alpha/. If the dimension of x, t, is equal to the number of kinematic parameters, l/sub 0/, a two-level algorithm can be applied to solve for a set of optimal kinematic parameters: (1) Applying the interior point algorithm for solving of x; (2) Applying Newton method to a set of nonlinear algebraic equations for solving of /spl alpha/. If the dimension of x is greater than the number of kinematic parameters (i. e. , x are not linearly independent), we consider the constrained semi-definite programming problems and the constrained max-det problems by taking account of an additional set of nonlinear constraints. We propose a simplified constrained gradient algorithm for solving of x in such cases, /spl alpha/ derives from x using Newton method. Simulation results verify the effectiveness of the proposed algorithms.