Adaptive Task-Space Control of Flexible-Joint Manipulators

This paper considers the motion control and compliance control problemsfor uncertain rigid-link, flexible-joint manipulators, and presents newadaptive task-space controllers as solutions to these problems. The motioncontrol strategy is simple and computationally efficient, requires littleinformation concerning either the manipulator or actuator/transmissionmodels, and ensures uniform boundedness of all signals and arbitrarilyaccurate task-space trajectory tracking. The proposed compliant motioncontrollers include an adaptive impedance control scheme, which isappropriate for tasks in which the dynamic character of theend-effector/environment interaction must be controlled, and an adaptiveposition/force controller, which is useful for those applications thatrequire independent control of end-effector position and contact force. Thecompliance control strategies retain the simplicity and model independenceof the trajectory tracking scheme upon which they are based, and are shownto ensure uniform boundedness of all signals and arbitrarily accuraterealization of the given compliance control objectives. The capabilities ofthe proposed control strategies are illustrated through computer simulationswith a robot manipulator possessing very flexible joints.     

Adaptive hybrid force/position control of robot manipulators using an adaptive force estimator in the presence of parametric uncertainty

This study is devoted to sensorless adaptive force/position control of robot manipulators using a position-based adaptive force estimator (AFE) and a force-based adaptive environment compliance estimator. Unlike the other sensorless method in force control that uses disturbance observer and needs an accurate model of the manipulator, in this method, the unknown parameters of the robot can be estimated along with the force control. Even more, the environment compliance can be estimated simultaneously to achieve tracking force control. In fact, this study deals with three challenging problems: No force sensor is used, environment stiffness is unknown, and some parametric uncertainties exist in the robot model. A theorem offers control laws and updating laws for two control loops. In the inner loop, AFE estimates the exerted force, and then, the force control law in the outer loop modifies the desired trajectory of the manipulator for the adaptive tracking loop. Besides, an updating law updates the estimated compliance to provide an accurate tracking force control. Some experimental results of a PHANToM Premium robot are provided to validate the proposed scheme. In addition, some simulations are presented that verify the performance of the controller for different situations in interaction.     

Noncontact impedance control for redundant manipulators

Proposes an impedance control method that can regulate a virtual impedance between a robot manipulator and external objects using visual information. The conventional impedance control method is not useful in some cases where no interaction force between the arm and its environment exists, although it is one of the most effective control methods for manipulators in contact with the environment. Using the proposed method, we can control the manipulator motion based on the virtual impedance before contact with the objects. The validity of the proposed method is verified through computer simulations and experiments using a direct-drive robot     

Nonlinear feedback control of robot manipulator and compliant wrist

Many robotic applications require the direct contact of the end-effector with the environment. Passive compliance attached to the robot wrist, hand, or finger is desirable to produce smooth transitions between the free motion and contact, as well as to allow self-correction in order to accommodate geometric uncertainties in assembly and manufacturing. However, the use of passive compliance degenerates the positioning capability of the manipulator when the robot moves in free space. When the robot makes contact on workpiece, active adjustment of stiffness for various tasks in different directions is needed. We proposed to use passive compliance that is instrumented so that the system provides the necessary flexibility and also sensing to actively control the contact forces or to compensate the positioning error during motion and contact. In this article, the dynamic control of the manipulator with a compliant wrist is addressed. The measured deformation information of the instrumented compliant wrist is utilized in the feedback loop to increase the stiffness of the overall system in position control and to decrease the stiffness in force control. The dynamics model for both unconstrained and constrained cases is established. Applying nonlinear feedback control techniques, the dynamics of the manipulator-wrist system is linearized and decoupled, which allows the controller design to be carried out by using the linear system theory. Editor: J.M. Skowronski     

Adaptive compliant motion control of manipulators without velocity measurements

This article presents two adaptive schemes for compliant motion control of uncertain manipulators. The first strategy is developed using an adaptive impedance control approach and is appropriate for tasks in which the dynamic character of the end-effector/environment interaction must be controlled, while the second scheme is an adaptive position/force controller and is useful for those applications which require independent control of end-effector position and contact force. The proposed controllers are very general and computationally efficient, require virtually no information regarding the manipulator dynamic model or the environment, and are implementable without velocity measurements. It is shown that the schemes ensure semiglobal boundedness of all signals in the presence of bounded disturbances, and that the ultimate size of the system errors can be made arbitrarily small. The capabilities of the proposed control strategies are illustrated through both computer simulations and laboratory experiments with a 6 degree-of-freedom (DOF) IMI Zebra Zero manipulator. ©1997 John Wiley & Sons, Inc.     

Reasoning about nature of contact for planning manipulators tasks

Extracting information about contact between two convex bodies from the measured force vector is a prerequisite for any fine compliant motion control strategy. Contact information contains the direction and orientation of the contact surface normal and its relative location and orientation with respect to the compliant reference frame system.A method for interpreting the contact force feedback during compliant robot motion control, using kinematic screws, is presented. Domain specific rules combined with partial a priori knowledge of mating parts geometry and interpreted force signals are used to reason and make inferences about the initial contact configuration. The likely contact surfaces are predicted and point(s) or line(s) of contact are fully defined. These surfaces are idealized and represented by quadratic equations or polyhedral surfaces. The geometric properties of surfaces at the contact location are used to select the contact configuration when multiple solutions exist.An algorithm for predicting the Expected Contact Configuration (ECC) has been developed and is illustrated here with examples. Experimental validation of the developed expert system prototype, using a 6R manipulator, a six-axis force sensor, and a host computer is described.     

A biologically inspired active compliant joint using local positive velocity feedback (LPVF).

Starting from studies which revealed that positive feedback is found in the control system for walking in arthropods, we have constructed a new positive feedback driven joint that can be used for solving compliant motion tasks. We propose two different joint constructions each of which shows passive compliance. Based on these joints we introduce three different local positive velocity feedback (LPVF) controllers and discuss their properties in the context of motion generation in closed kinematic chains. The third circuit named undelayed dLPVF is used for the control of a compliant planar manipulator which turns a crank. Our concept is of highly decentralized nature and follows the idea of embodiment. In our case this means that a process which is controlled by LPVF controllers reveals its nature when the controllers interact with this process.     

A compliant end-effector coupling for vertical assembly: design and evaluation

Many manipulation tasks require compliance, i.e. the robot's ability to comply with the environment and accomplish force as well as position control. Examples are constrained motion tasks and tasks associated with touch or feel in fine assembly. Few compliance-related tasks have been automated, and usually by active means of active compliance control: the need for passive compliance offered by the manipulator itself has been recognized and has led to the development of compliant end-effectors and/or wrists. In this paper we present a novel passively compliant coupling, the compliant end-effector coupling (CEEC), which aids automated precision assembly. It serves as a mechanical interface between the end of the robot arm and the end-effector. The coupling has 6 degrees of freedom. The design of the coupling is based on a “lock and free” assembly idea. The coupling is locked and behaves like a stiff member during robot motion, and is free (compliant) during constrained motion. It features an air bearing, a variable stiffness air spring and a center-locking mechanism. The end-effector assembly, being centrally unlocked, will float within the designed compliance limits assisted by the air bearing. These frictionless and constraint-free conditions facilitate a fast correction of any initial lateral and angular misalignments. In a peg insertion assembly, such accommodation is possible provided that the tip of the peg is contained within the chamfer of the hole. A variable stiffness air spring was incorporated in the design to allow variable and passive vertical compliance. This vertical compliance allows the accommodation of angular and vertical errors. The center-locking mechanism will return the end-effector assembly to its initial position upon an error correction. In a robot application program, the CEEC can be locked during rapid motion to securely transport a part or be set free during assembly or disassembly processes when the motions are constrained.     

Impedance control of a direct-drive manipulator without using force sensors

This research is concerned with impedance control of a manipulator which carries out stable contact tasks. The method controls the dynamic interaction between a robot and its environment by changing the apparent mechanical impedance of the manipulator. Conventional impedance control methods required force or torque sensors, which made the manipulator system very complex. In this paper a new method is proposed for controlling the impedance of a manipulator without using force or torque sensors. The angular velocity and angular acceleration of the manipulator joints are estimated, and by using a computer model of the manipulator, the necessary torque for each joint is calculated and applied to the joint to attain the desired impedance. The feasibility of the method is verified by surface-following experiments and collision experiments using a two-degree-of-freedom direct-drive manipulator.     

Non-linear position-force control of objects grasped by multifingered mechanical hands

This paper presents a new non-linear position-force control scheme for control of interaction of objects, grasped by multifingered mechanical hands, with the environment. Using the theory of input-output linearization, the hand-object dynamics in joint space is first projected along the position and force control directions, input-output linearized and decoupled. Along the position control direction(s), a state feedback controller (SFC) is used for trajectory tracking. A new dynamic SFC, which programs a variable compliance, is used along the force control direction(s). The variable compliance is appropriately modulated on the basis of the force feedback information to maintain a desired interaction force level. An optimal grasp force decomposition scheme is also developed for the control of grasping forces. The proposed scheme ensures grasp stability under dynamic conditions. The efficacy of the overall control scheme is demonstrated through numerical simulation.     

An inverse kinematics algorithm for interaction control of a flexible arm with a compliant surface

This paper deals with the problem of controlling the interaction of a multilink flexible arm in contact with a compliant surface. For a given tip position and surface stiffness, the joint and deflection variables are computed using a closed-loop inverse kinematics algorithm. This is based on a suitable Jacobian matrix which includes terms accounting for the static deflections due to gravity and contact force. The computed variables are used as the set-points for a simple joint PD control, thus achieving regulation of the tip position and contact force via a joint-space controller. The scheme is tested in a simulation case study for a planar two-link manipulator.     

基于运动预测和阻抗控制的未知受限机构操作

针对服务机械臂操作各种未知受限运动机构的需要,提出了一种基于运动预测和阻抗控制的控制方法,本方法采用试探一预测一修正的滚动操作策略,通过阻抗控制保证机械臂与未知受限机构交互的柔性.首先试探受限机构的运动能力,根据已有操作臂运动轨迹估计受限机构的运动模型,然后预测受限机构的运动方向,进而修正机械臂的运动轨迹,并根据交互作...     

Development of the Cartesian arm exoskeleton system (CAES) using a 3-axis force/torque sensor

This paper presents the development of the exoskeleton system for amplifying human strength, which we call the Cartesian arm exoskeleton system (CAES). The CAES is a highly geared manipulator equipped with electric motors and encoders. In addition, we developed a 3-axis force/torque (FT) sensor to measure the operator’s force. The Cartesian computed torque method is applied to the CAES for compliance control. The explicit force control for amplifying the operator’s force was explored. This method provides back-drivable compliant control with force amplification. Rather than theoretical improvements, we focused on the experimental realization and evaluation of the Cartesian computed torque method with explicit force control for improving joint compliance and amplifying the operator’s force. The experimental results demonstrated the effectiveness of the proposed control method.     

空间机器人捕获航天器操作的避撞柔顺复合自抗扰控制

讨论空间机器人在轨捕获非合作航天器过程避免关节受冲击力矩破坏的避撞柔顺控制问题.在机械臂与关节电机之间配置一种弹簧类柔顺装置—–旋转型串联弹性执行器(RSEA),其作用在于:1)在捕获碰撞阶段,可通过其内置弹簧的变形吸收碰撞产生的能量;2)在镇定运动阶段,结合避撞柔顺策略适时开、关电机,以保证关节所受冲击力矩受限在安全...     

基于变刚度自适应导纳机制的机械臂恒力控制

工业机械臂在诸如打磨抛光等接触式作业任务中对环境刚度信息存在一定的依赖性, 未知环境刚度信息将严重影响机器人的力位控制精度, 使得作业效果难以得到保证. 为解决环境信息不足或未知情况下的力/位置精确控制问题, 本文首先提出了一种新的自适应环境刚度在线估计方法, 针对时变的环境刚度进行实时估计, 由此预测生成后继的机械臂参考轨迹点, 随后提出了一种根据力跟踪误差实时调整末端工具手刚度系数的变刚度导纳恒力控制方法, 并结合李雅普诺夫稳定性理论给出了整体控制律的收敛性证明. 针对刚柔两种末端工具手和多种不同的曲面工件开展了实验研究, 并与传统PID控制方法和传统导纳控制方法进行了对比, 其结果表明本文所提出的复合控制方法可在不同工况条件下实现机器人运动过程中接触力的快速柔顺调节, 并获得4.55%以内的最优力控误差效果, 证明了本文所提出方法的有效性与可行性.     

Control strategies for rodotic contact tasks: An experimental study

In this article, we study the contact instability problem encountered in robotic manipulators while trying to make contact with an environment, such as grasping or pushing against objects, and propose a unified control strategy capable of achieving a stable contact against both stiff and compliant environments. The problem has three distinct stages of the contact task. In the first stage, free-space motion, the robot is approaching the environment; in the second stage, post-contact force regulation; in the third, impact stage, the transition from the first stage to the second. We make an experimental comparison of the control schemes that may be used for the three stages. For example, during impact, the manipulator should not lose contact with the environment, nor exert high impulsive forces on the environment, and in the post-impact phase, the robot should have a fast force trajectory tracking. The best strategies for the above stages are experimentally determined and then combined into a single unified controller that can achieve stable contact as well as a fast force trajectory tracking response for surfaces of variable stiffnesses. This control scheme does not require a priori knowledge of the stiffness of the environment, and is able to estimate the environmental stiffness and tune gains accordingly so as to achieve the best response. Also experimentally compared is the use of such a scheme with impedance control, another method proposed in the literature for robotic contact task control. © 1995 John Wiley & Sons, Inc.     

Posture measure and control for robot compliance tasks

Various measures have been proposed for evaluating the compatibility of manipulator postures with respect to task requirements from kinematic and dynamic standpoints. In most previous studies, the measures were used to determine optimal postures for manipulators in advance, and their effects on system performance were generally examined statically. When posture measures are applied to controlled dynamic systems, however, their effects are usually not evident, because deficiencies (merits) caused by bad (good) postures can be compensated for by the controller. On the other hand, postures determined according to proper measures can still alleviate the controller's load and be helpful in control strategy realization. In this paper, we propose that planned compliant motion trajectories should be accompanied by proper postures for compliance tasks. Thus, we analyze manipulator dynamic behavior by using postures specified according to various measures. And, because different postures are used in different phases of the compliance task, a posture selection and control scheme is also proposed to govern the sequence of postures selected according to task requirements and environments. Redundant robot manipulators were used for investigation because of their better manipulability. Simulations that demonstrate the effectiveness of the proposed scheme are described. © 1998 John Wiley & Sons, Inc.     

A passive mechanism that improves robotic positioning through compliance and constraint

This paper presents the design of a passive robotic wrist that is capable of establishing and maintaining an accurate position relative to a workpart edge through compliance and constraint (force guidance).In previous work, we have shown that, through proper selection of a manipulator's impedance, a manipulator's end-effector can be guided to its desired relative position despite errors in its commanded position. The selected proper impedance is attained here through the design of a passive micromanipulator that is mounted on the end-effector of a conventional manipulator. The micromanipulator consists of three linkages connected by revolute joints and torsional springs. The outermost linkage contacts the workpart at multiple locations providing multidirectional unilateral kinematic constraint. This kinematic constraint in conjunction with the compliance provided by the torsional springs causes the linkage to be re-positioned so that any existing misalignment (that inevitably occurs) is eliminated and a unique planar position/orientation with respect to the workpart edge is attained.Here, we present the procedure used in the parametric design of this mechanism. The desired compliant properties identified in task space (using Cartesian variables (x, y, and θ) for force and motion) are extended here to joint space (using joint variables (θ₁, θ₂), and θ₃) for torque and motion). The appropriate micromanipulator link lengths, initial linkage angles, and the appropriate torsional spring constants are selected using an optimization procedure. Computer simulation of the constrained manipulator/workpart interaction demonstrates that the desired force guidance behavior is attained.     

Robust implementation of generalized impedance control for robot manipulators

Compliant manipulation requires the robot to follow a motion trajectory and to exert a force profile while making compliant contact with a dynamic environment. For this purpose, a generalized impedance in the task space consisting of a second-order function relating motion errors and interaction force errors is introduced such that force tracking can be achieved. Using variable structure model reaching control, the generalized impedance is realized in the presence of parametric uncertainties. The proposed control method is applied to a multi-d.o.f. robot for an assembly task of inserting a printed circuit board into an edge connector socket. It is suggested that an assembly strategy which involves a sequence of planned target generalized impedances can enable the task to be executed in a desirable manner. The effectiveness of this approach is illustrated through experiments by comparing the results with those obtained using a model-based control implementation.     

基于Matlab机械臂力控系统仿真研究

以位置控制为主的机械臂控制方法已不能满足某些复杂环境（装配、抛光、去毛刺）的应用要求,控制机械臂与环境间的接触力已成为机器人学研究的一个热点。提出一种在Matlab/SimMechanics环境下平面二自由度机械臂力控制的仿真研究方法。在平面中模拟机械臂与环境的接触面,设计振荡抑制控制器,实现机械臂与环境间接触力的控制,以及机械臂与刚性环境碰撞接触过程中冲击振荡阶段的振荡抑制,生成机械臂期望的运动轨迹。仿真结果表明,该方法可实现特定作业下机械臂与环境间接触力的控制。