A new approach to robust position/force control of flexible-joint robot manipulators

In this paper a new approach employing smooth robust compensators is proposed for the control of uncertain elastic-joint robot manipulators during contact tasks. It is assumed that the flexible-joint manipulators consist of two subsystems: the rigid subsystem and the flexible subsystem. The output of the flexible subsystem is assumed to be the input of the rigid subsystem. The control design is carried out in two steps. First, a desired input is designed for the rigid subsystem, which can robustly stabilize it. Second, a robust controller is designed to stabilize the flexible subsystem so that it generates the necessary torque designed for the rigid subsystem. By using this approach, the robot manipulator can exert a preset amount of force on the environment while tracking a desired trajectory with global asymptotic stability. Lyapunov's direct method is used here to prove the global asymptotic stability of the closed-loop system. The assumption of weak joint elasticity is relaxed and exact knowledge of joint stiffness is not required for the control design. Also, exact knowledge of robot kinematic and dynamic parameters and actuator parameters are not required. Unlike other approaches, this approach takes the environmental stick-slip friction as well as its dependency on normal contact force into consideration. It compensates for the adverse effects of the stick-slip friction. The proposed controller produces a smooth control action, and ensures smooth motion on the contact surface. The efficacy of the proposed controller is illustrated with the help of a numerical example of a two-link flexible-joint robot. © 1996 John Wiley & Sons, Inc.     

Exponential trajectory tracking control in the workspace of a class of flexible robots

In this paper, we present a control strategy that ensures the exponential stability of the tracking error in the virtual joint space of a class of mechanical systems made up of rigid links that form a chain that ends with a flexible beam. Virtual joints are defined so as to be related kinematically to the workspace. Thus, when the inverse kinematics is nonsingular, trajectory tracking in the virtual joint space is equivalent to trajectory tracking in the workspace. The method proposed in this paper calls for the transformation of the trajectory from the virtual joint space to the joint and deformation space. The robot is a non-minimum-phase system in the virtual joint space. However, this transformation, which involves the dynamics of the flexible part, can be solved using a causal–anticausal iterative approach. The controller is then designed using an input–output feedback linearization scheme, with regard to the joints, and two linear control laws with regard to the joint and to the deformation variable tracking errors. Analysis based on the passivity theorem, hierarchical systems stability, and linear matrix inequalities then allows us to determine the controller gains that ensure that the tracking errors in the virtual joint space are well damped and exponentially stable. Finally, the strategy is validated by simulating a controller that incorporates the proposed laws and that drives a two-link manipulator that has one rigid and one flexible link. The simulation results demonstrate the good performance of the proposed control system. © 1998 John Wiley & Sons, Inc.     

基于直齿轮传动和双环解耦的柔性手腕原理与运动学分析

由于ｐｉｔｃｈ、ｙａｗ运动耦合干涉问题一直没有被提出和解决，使得现有的柔性手腕机构中必须采用复杂、加工困难的球齿轮传动以满足ｐｉｔｃｈ、ｙａｗ机构运动的特殊要求．基于作者提出的双环解耦原理解决了不需球齿轮传动实现ｐｉｔｃｈ－ｙａｗ－ｒｏｌ柔性腕机构的关键问题，因而采用通常的直齿圆柱齿轮传动方式不需特殊加工即避开了球齿轮传动复杂和加工困难的问题．此外，提出和设计的柔性手腕在机器人柔性臂设计方面也有参考价值．     

Dynamic hybrid position/force control of a two degree-of-freedom flexible manipulator

In this article, a method is proposed whereby both contact force exerted by a flexible manipulator and position of end-effector while in contact with a surface are controlled. We approximate elastic deformations by means of B-spline functions and derive dynamic equations of joint angles, vibration of the flexible link, and constraint force. A controller for the hybrid position/force control of the flexible manipulator is designed on the basis of the singular perturbation method. Simulation results confirm that the controller performs remarkably well. © 3994 John Wiley & Sons, Inc.     

Dynamics modeling and analysis of a biped walking robot actuated by a closed-chain mechanism

We developed a new type of human-sized biped walking robot (BWR) driven by the closed-chain type of joint actuator. Each leg of the robot is composed of three pitch joints and one roll joint. In all, a 15 degree-of-freedom robot including four arm joints and three joints for the head was developed. The BWR was developed to walk autonomously such that all leg joints are actuated by small 90 W dc motors/drivers and dc batteries and controllers which are boarded. The joint actuator for the BWR is composed of the four-bar-link mechanism driven by the ball screw which has high strength and high gear ratio. A dynamics modeling of the developed BWR for forward walking is presented in which the revolute joint dynamics are transformed into the prismatic joint dynamics of the ball screw. Also, an analysis on the four-bar-link mechanism applied to the joint actuator and on the structure of the BWR is shown. The design specification of the actuating motor for the BWR is analyzed through the torque analysis of the four-bar-link actuator. Through walking experiments of the BWR, the walking performance and trajectory tracking ability is shown. © 2004 Wiley Periodicals, Inc.     

基于行星齿轮机构的牵引式欠驱动机械手设计

为实现夹持力调节和目标物体抓取功能,设计了一种基于行星齿轮机构的牵引式欠驱动机械手。该机械手采用不固定输出轴和内齿圈的行星齿轮机构来分配2种互斥的运动,一种用于手指的转动,实现机械手的夹持功能;另一种用于皮带轮的无限转动,通过皮带实现目标物体的拉入功能。相对于腱绳式和连杆式欠驱动机械手,这种单输入双输出形式不仅能够保持机械手的自适应性,同时还能降低机械手的耦合程度。根据行星齿轮机构的这种传动特点,设计了阻力矩调节机构,实现了机械手的夹持力调节功能。夹持力测试实验表明,在阻力矩调节机构的作用下,机械手能有效调节夹持力。抓取实验结果表明,机械手能够实现对刚性和柔性目标物体的抓取操作,验证了机械手设计的有效性。     

Sensing mechanism for estimation of the physical properties of an object avoiding failure of the sensors

Active sensing, in which a robot pushes an object and senses the reaction force or joint angle by means of the force sensor at the point of the contact or on the joint, is one of the effective approaches to estimate the physical properties of an object, such as its compliance. A compliant joint driven by elastic actuators has an advantage over a rigid joint driven by a motor with a high gear ratio in that it absorbs the reaction force, and thus avoids any joint damage during active sensing. However, this approach is not suitable for either rigid joint or a compliant joint because the sensors attached to the contact point and the joint tend to break, owing to iterative contact or an excessive force. Here, this paper adopts a one-degree-of-freedom joint mechanism driven by elastic pneumatic actuators, and focuses on the passivity of the elastic pneumatic actuator, in which the pressure is changed when force is applied, after which it is deformed. By utilizing the passivity of the actuators under a number of conditions, this paper derives multiple regression models of the force and the angle, using the pressures before and after force is applied to the joint mechanism. Experimental results present that the contact information can be estimated from the pressure values and that the joint mechanism can detect the elasticity of an object using the regression models. We also observe the range of the elasticity of the object by tuning the joint compliance. This approach provides a robot hand that can estimate the contact information, including the force and joint displacement, avoiding the failure of the sensors.     

Stability analysis and robust composite controller synthesis for flexible joint robots

In this paper the control of flexible joint manipulators is studied in detail. The model of N-axis flexible joint manipulators is derived and reformulated in the form of singular perturbation theory and an integral manifold is used to separate fast dynamics from slow dynamics. A composite control algorithm is proposed for the flexible joint robots, which consists of two main parts. Fast control, u _f, guarantees that the fast dynamics remains asymptotically stable and the corresponding integral manifold remains invariant. Slow control, u _s, consists of a robust PID designed based on the rigid model and a corrective term designed based on the reduced flexible model. The stability of the fast dynamics and robust stability of the PID scheme are analyzed separately, and finally, the closed-loop system is proved to be uniformly ultimately bounded (UUB) stable by Lyapunov stability analysis. Finally, the effectiveness of the proposed control law is verified through simulations. The simulation results of single- and two-link flexible joint manipulators are compared with the literature. It is shown that the proposed control law ensures robust stability and performance despite the modeling uncertainties.     

基于扰动和摩擦补偿的柔性机械臂系统非奇异快速终端滑模控制

本文针对系统中存在的关节摩擦、动力学参数不确定性和外部负载干扰等因素引起的柔性机械臂系统控制性能下降的问题,提出了一种基于扰动和摩擦补偿的非奇异快速终端滑模控制方法(NFTSMC-DE-FC).首先,设计扰动估计器(DE)对系统未知动态参数和负载干扰进行估计.然后,针对扰动估计器不能精确估计的关节摩擦力矩进行辨识.最后,利用滑模控制技术设计非奇异快速终端滑模控制器,并将扰动估计值和摩擦力辨识值以前馈的方式进行补偿,实现对柔性机械臂系统给定参考轨迹跟踪的准确性以及对外界扰动的鲁棒性.值得注意的是,与传统只使用扰动估计器的方法相比,本文考虑到了摩擦力等非线性因素的影响,并利用辨识技术对摩擦力进行辨识,提高了控制精度.利用Lyapunov稳定性定理从理论上证明了所设计的控制器可以保证闭环系统的稳定性.实验结果表明,相较于非奇异快速终端滑模控制方法(NFTSMC)和基于扰动估计器的非奇异快速终端滑模控制方法(NFTSMC-DE),所提方法提高了柔性机械臂系统的轨迹跟踪性能.     

On the force control problem for flexible joint manipulators

It is shown, using a singular perturbation model of the elastic joint manipulator dynamics and the concept of corrective control. how force control techniques developed for rigid manipulators can be extended to the flexible joint case. It is shown that the overall control law can be implemented in an inner loop/outer loop structure, where the inner loop is a nonlinear control that linearizes the system restricted to a suitable integral manifold in state space and the outer loop is a linear control that can be designed independently of the nonlinear inner loop, using any number of force control schemes designed for rigid manipulators to extend all of the standard techniques for force control of rigid manipulators to the flexible joint case, including hybrid position/force control, impedance control, or any other suitable design     

机器人柔性关节准椭球面齿轮传动—节曲面的优化设计

本文针对“东方”喷漆机器人柔性关节上装用的球面齿轮传动装置存在的问题,研制出了一种新型的可实现精密全方位变传动比的准椭球面齿轮传动.本篇依据空间双自由度啮合条件,对准椭球面齿轮的传动比函数及节曲面有关参数进行了优化设计.     

基于新型补偿控制策略的柔性关节控制器设计

柔性关节机器人控制系统中存在不确定性扰动、摩擦力、参数变化以及建模误差等问题,而常规PID控制无法兼顾稳定性和控制精度的要求,为此设计了一种基于新型补偿控制策略的柔性关节鲁棒控制器.控制器设计中,将摩擦力分为线性和非线性两部分,扰动分为确定性和不确定性两部分:机器人自身结构相关量、确定性扰动以及摩擦力的线性部分可以通过...     

基于力阻抗模型的上肢康复机器人交互控制系统设计

传统上肢康复机器人交互控制系统受到奇异位形影响,导致系统控制精准度较低,为此提出基于力阻抗模型的上肢康复机器人交互控制系统;设计上肢康复机器人交互控制系统结构,选取双串口12CSA60S2系列单片机作为下位机控制核心模块,利用椎齿轮改变驱动力方向,设计机械臂肘部结构,通过同步带传动,将器件隐藏于空手柄中;设计机械臂腕部结构,满足临床康复时上肢患者站姿与坐姿训练需求;选择箔式应变片BF350力传感器,设计电阻应变片桥接电路,处理传输信号;构建机器人目标阻抗模型,设计基于力阻抗控制策略,调节位置、速度和关节;为改善奇异位形情况,在奇异位形附近关节角速度指令直接由各个关节力矩阻尼控制得到,实现角速度精准输出,完成系统控制;由实验结果可知,该系统直线运动位置、旋转关节位置和伸缩关节位置跟踪结果与标准值基本一致,满足系统设计需求。     

基于混沌遗传算法的柔性机械手滑模控制器优化设计

针对柔性机械手动力学方程的非最小相位特点,本文提出一种柔性机械手的终端滑模控制方法,将关节电机转角和柔性模态变量的线性组合定义为柔性机械手系统的输出.通过输入输出线性化,将系统分解为输入输出子系统和零动态子系统.设计终端滑模控制策略,使输入输出子系统在有限时间收敛到零;利用混沌遗传算法优化控制器的设计参数,使零动态了系统在甲衡点附近渐近稳定,从而保证整个系统的渐近稳定.本文提出的方法设计过程简单,易于实现.仿真结果证明了设计的有效性.     

具有不完全微分的最优模糊PID控制器及其在智能人工腿中应用的仿真研究

提出了一种新的最优模糊PID控制器, 它由两部分组成, 即在线模糊推理机构和带有不完全微分的常规PID控制器. 在模糊推理机构中, 引入了三个可调节因子x_p,x_i 和x_d,其作用是进一步修改和优化模糊推理的结果, 以使控制器对一个给定对象具有最优的控制效果. 可调节因子的最优值采用ITAE准则及Nelder和Mead提出的柔性多面体最优搜索算法加以确定. 这种PID控制器被用来控制由作者设计的智能人工腿中的一个直流电机. 仿真结果表明该控制器的设计是非常有效的, 它可被用于控制各种不同的对象和过程.     

Investigation of joint clearances in a large-scale flexible solar array system

This paper is concerned with the effect of joint clearances in a large-scale flexible solar array system. The adopted solar array system is introduced and dynamics equation of the solar array system is presented by the Jourdain velocity variation principle firstly. Then detection of clearance contact and calculation of contact force are discussed. And finally the effect of joint clearances on the solar array system is studied in detail during the deployment process. Simulation results indicate that joint clearances will affect dynamic behaviors of the deployable mast, the container and the sub-panels in the solar array system. Furthermore, the presented method and simulation result have innovation and reference value for future researches.     

Use of coherence analysis for the evaluation and adjustment of robot gear performance

Previous work by our group has demonstrated that it is possible to use the robot joint excitation technique and coherence analysis function to adjust robot joint gears for maximum robot precision. A robot joint gear box equipped with gear adjustment mechanism was instrumented so that the gap between the teeth of a pair of gears (backlash) could be measured. The translational and rotational components of robot link motion were decoupled using multiple sensors so that the relationship between the coherence analysis function of each type of motion and gear backlash could be investigated. The sensitivity of the adjustment technique to various input signals and sensor positions was also examined. Results show that coherence analysis can be used as a measurement technique to determine the precision with which the robot joint link responds to control signal input.     

A coupled sliding-surface approach for the trajectory control of a flexible-link robot based on a distributed dynamic model

This paper proposes a coupled sliding-surface method for the design of trajectory control of a flexible-link robot. First, a sliding surface, coupling the joint velocity with the link bending moment at the joint, is defined based on the energy dynamics of the flexible link. Then a new trajectory–tracking control scheme is designed based on the coupled sliding surface, and extended to an adaptive scheme to cope with parametric uncertainties, where the Lyapunov stability theorem is used as a mathematical design tool. The proposed control is a collocated control designed based on a distributed-parameter dynamic model and hence is free from the so-called spillover instability. Using only the joint actuator, the proposed control guarantees stability throughout the entire trajectory control and asymptotic stability at desired goal positions. The proposed control is a PID control for the rigid dynamics and a proportional control for the flexible dynamics, with feed-forward and dynamics compensation. As a result, the proposed control guarantees zero steady-state joint-tracking errors even in the presence of low-frequency disturbances due to unavoidable mechanical inaccuracies in application. The theoretical results have also been proven by experimental studies.     

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.