Dynamic control of free-floating coordinated space robots

The position and force control of coordinated robots mounted on spacecraft, manipulating objects with closed kinematic chain constraints, represents an important class of control problem. In this article, the kinematics and dynamics of free-floating coordinated space robotic system with closed kinematic constraints are developed. An approach to position and force control of free-floating coordinated space robots with closed kinematic constraints is proposed for the first time. Unlike previous coordinated space robot control methods which are for open kinematic chains, the method presented here addresses the main difficult problem of control of closed kinematic chains. The controller consists of two parts, position controller and internal force controller, which regulate, respectively, the object position and internal forces between the object and end-effectors. The stability of the closed-loop coordinated robotic system is analyzed using the error models of the object position and internal forces. It is proved that the errors in the object position and internal forces asymptotically converge to zero under the assumption of exact kinematic and dynamic models. © 1998 John Wiley & Sons, Inc.     

On cooperative manipulation of dynamic objects

This paper proposes a decentralized position/internal force hybrid control approach for multiple robot manipulators to cooperatively manipulate an unknown dynamic object. In this approach, each autonomous robot has its own controller and uses its own sensor information in performing the fast cooperation. This approach eliminates a lot of information communications between each robot and reduces numerous computations. The influences of the position and the internal force estimation errors to the overall control system is analyzed. A cooperative identification method for each autonomous robot to identify the object's complex dynamics, cooperatively, is presented. In addition, the trade-off between the unilateral force constraint and the robots' position response is studied. Experiments show the effectiveness of this control approach.     

Multiple-arm space free-flying robots for manipulating objects with force tracking restrictions

Multiple Impedance Control (MIC) is an algorithm that enforces designated impedance at various levels, i.e. on the manipulated object, all cooperating manipulators, and the moving platform of a robotic system. In this paper, a force tracking strategy inspired by a human control system is added to the MIC algorithm and the general formulation is revised to fulfill a desired force tracking strategy for object manipulation tasks. The stability analysis of the MIC algorithm based on the Liapunov Direct Method, besides error analysis, shows that a good tracking of cooperative manipulators and the manipulated object is guaranteed. Next, using MAPLE and MATLAB tools, a system of three manipulators mounted on a space free-flying robot is simulated. The task is moving an object based on given trajectories which come across an obstacle, to examine the performance of the developed control law. The results show that, even in the presence of both external disturbances and an impact due to collision with the obstacle, the response of the MIC algorithm is smooth. Moreover, based on the embedded force tracking strategy, the contact force is confined to follow a desired trajectory. Also, it is shown that decreasing the values of the controller mass matrix elements results in reducing both the object position and force tracking error.     

Adaptive neural controller for cooperative multiple robot manipulator system manipulating a single rigid object

In this article, an adaptive neural controller is developed for cooperative multiple robot manipulator system carrying and manipulating a common rigid object. In coordinated manipulation of a single object using multiple robot manipulators simultaneous control of the object motion and the internal force exerted by manipulators on the object is required. Firstly, an integrated dynamic model of the manipulators and the object is derived in terms of object position and orientation as the states of the derived model. Based on this model, a controller is proposed that achieves required trajectory tracking of the object as well as tracking of the desired internal forces arising in the system. A feedforward neural network is employed to learn the unknown dynamics of robot manipulators and the object. It is shown that the neural network can cope with the unknown nonlinearities through the adaptive learning process and requires no preliminary offline learning. The adaptive learning algorithm is derived from Lyapunov stability analysis so that both error convergence and tracking stability are guaranteed in the closed loop system. Finally, simulation studies and analysis are carried out for two three-link planar manipulators moving a circular disc on specified trajectory.     

Adaptive hybrid control of manipulators on uncertain flexible objects

This paper presents an adaptive hybrid control approach for a robot manipulator to interact with its flexible object. Because of its flexibility, the object dynamics influence the robot's control system, and since it is usually a distributed parameter system, the object dynamics as seen from the robot change when the robot moves. The problem becomes further complicated such that it is difficult to decompose the robot's position and contact force control loops. In this paper, we approximate the object's distributed parameter model into a lumped 'position state-varying' model. Then, by using the well-known nonlinear feedback compensation, we decompose the robot's control space into a position control subspace and object torque control subspace. We design the optimal state feedback for the position control loop and control the robot's contact force through controlling the resultant torque of the object. We use the model-reference simple adaptive control strategy to control the torque control loop. We also study the problem on how to select a reasonable reference model for this control loop. Experiments of a PUMA robot interacting with an aluminum beam show the effectiveness of our approach.     

Robot docking based on omnidirectional vision and reinforcement learning

We present a system for visual robotic docking using an omnidirectional camera coupled with the actor critic reinforcement learning algorithm. The system enables a PeopleBot robot to locate and approach a table so that it can pick an object from it using the pan-tilt camera mounted on the robot. We use a staged approach to solve this problem as there are distinct subtasks and different sensors used. Starting with random wandering of the robot until the table is located via a landmark, then a network trained via reinforcement allows the robot to turn to and approach the table. Once at the table the robot is to pick the object from it. We argue that our approach has a lot of potential allowing the learning of robot control for navigation and remove the need for internal maps of the environment. This is achieved by allowing the robot to learn couplings between motor actions and the position of a landmark.     

High‐gain nonlinear observer‐based impedance control for deformable object cooperative teleoperation with nonlinear contact model

Unmeasurable object deformation and local communication time delays between the slave robots influence the manipulation effect for multirobot multioperator teleoperation. In this article, a distributed control method based on high‐gain nonlinear observer, interactive identification, and impedance control is proposed for this problem. First, we use Hunt‐Crossley contact model and deduce the desired synchronizing object state in cooperative teleoperation. Second, an impedance item expressed by the internal position errors is presented to decrease object position tracking errors. For the unmeasurable object deformation, an interactive identification method is proposed for estimating unknown variables. Third, we consider both varying communication time delays and local time delays in the slave side. Two mirror high‐gain nonlinear observers are designed for estimating other slave robots' real‐time state. Finally, we build the system controllers and prove the stability of the closed‐loop system and the boundless of estimating errors using Lyapunov functions. Comparable simulation results executed by the physical system present that the position and internal force tracking errors of the object decrease in the designated cooperative tasks.     

State and Impedance Reflection Based Control Interface for Bilateral Telerobotic System with Asymmetric Delay

In this paper, we investigate state and imped-ance reflection based robust control strategy for bilateral shared telerobotic system under unsymmetrical time varying delay. Shared input for both master and slave robot is designed by combining delayed position and position-velocity signals with impedance reflection properties of the interaction between slave and environment and between human and master robot manipulator. Adaptive control algorithm is proposed to estimate the interaction properties between human and master manipulator and between slave and remote environment. Then, the delayed estimated interaction properties are reflected back to the master and slave robot manipulator to match with the estimated impedance properties of the interaction between human and remote environment. We combine robust term with adaptive control term to deal with the uncertainty associated with gravity loading vector, unmodeled dynamic and external disturbance. The stability conditions with time varying delays are derived by using Lyapunov-Krasovskii functional. Experimental results are given to demonstrate the validity of the proposed design for real-time applications.     

Object handling control among two-wheel robots and a human operator: An empirical approach

This article presents object handling control between two-wheel robot manipulators, and a two-wheel robot and a human operator. The two-wheel robot has been built for serving humans in the indoor environment. It has two wheels to maintain balance and is able to make contact with a human operator via an object. A position-based impedance force control method is applied to maintain stable object-handling tasks. As the human operator pushes and pulls the object, the robot also reacts to maintain contact with the object by pulling and pushing against the object to regulate a specified force. Master and slave configuration of two-wheel robots is formed for handling an object, where the master robot or a human leads the slave robot equipped with a force sensor. Switching control from position to force or vice versa is presented. Experimental studies are performed to evaluate the feasibility of the object-handling task between two-wheel mobile robots, and the robot and a human operator.     

Cooperative object manipulation with contact impact using multiple impedance control

Impedance Control imposes a desired behavior on a single manipulator interacting with its environment. The Multiple Impedance Control (MIC) enforces a designated impedance on both a manipulated object, and all cooperating manipulators. Similar to the standard impedance control, one of the benefits of this algorithm is the ability to perform both free motions and contact tasks without switching control modes. At the same time, the potentially large object inertia and other forces are taken into account. In this paper, the general formulation for the MIC algorithm is developed for distinct cooperating manipulators, and important issues are detailed. Using a benchmark system, the response of the MIC algorithm is compared to that of the Object Impedance Control (OIC). It is shown that in the presence of flexibility, the MIC algorithm results in an improved performance. Next, a system of two cooperating two-link manipulators is simulated, in which a Remote Centre Compliance is attached to the second end-effector. As simulation results show, the response of the MIC algorithm is smooth, even in the presence of an impact due to collision with an obstacle. It is revealed by both error analysis and simulation that under the MIC law, all participating manipulators, and the manipulated object exhibit the same designated impedance behavior. This guarantees good tracking of manipulators and the object based on the chosen impedance laws which describe desired error dynamics, in performing a manipulation task.     

4WD-4WS型床椅一体化机器人室内定位与点镇定控制研究

以床椅一体化机器人为研究对象,通过设计相应的运动控制器,并结合所搭建的组合导航定位测控系统,实现了室内环境下的点镇定控制;首先,设计了一种四轮转向-四轮驱动模式的全向床椅机器人样机,并对其运动学进行分析;其次,通过不连续坐标变换,用极坐标形式表示当前位姿与目标位姿间的全局控制误差,并选取合适的位姿误差变量对系统模型进行描述,设计出一种基于位置闭环的全局反馈控制器;继而,根据控制器的需要,设计搭建基于卡尔曼滤波的IMU/UWB组合导航定位系统,实现床椅一体化机器人的全局实时精确定位;最后,采用Lyapunov函数法,对所设计控制器中的控制律进行稳定性分析;MATLAB仿真实验与现场实验均表明所设计的点镇定方法控制效果良好。     

Cooperative Control of a 3D Dual-Flexible-Arm Robot

This paper discusses cooperative control of a dual-flexible-arm robot to handle a rigid object in three-dimensional space. The proposed control scheme integrates hybrid position/force control and vibration suppression control. To derive the control scheme, kinematics and dynamics of the robot when it forms a closed kinematic chain is discussed. Kinematics is described using workspace force, velocity and position vectors, and hybrid position/force control is extended from that on dual-rigid-arm robots. Dynamics is derived from constraint conditions and the lumped-mass-spring model of the flexible robots and an object. The vibration suppression control is calculated from the deflections of the flexible links and the dynamics. Experiments on cooperative control are performed. The absolute positions/orientations and internal forces/moments are controlled using the robot, each arm of which has two flexible links, seven joints and a force/torque sensor. The results illustrate that the robot handled the rigid object damping links' vibration successfully in three-dimensional space.     

基于扰动观测器的机器宇航员协调操作阻抗控制

针对未知干扰环境下机器宇航员执行协调操作任务的高精度控制要求,提出一种基于扰动观测器的协调操作阻抗控制算法．首先分析操作物与机器宇航员的几何约束和力约束关系,建立机器宇航员与操作物的统一动力学模型;其次利用机器人广义动量,设计适用于机器宇航员协调操作系统的动量扰动观测器;然后结合统一动力学模型,设计基于扰动观测器的机器宇航员协调操作阻抗控制算法;最后通过仿真对控制方法开展验证．结果表明,当臂杆受未知干涉力影响时,操作物的位置误差可被控制在10^-5 m的量级内．所提出的算法有效减小了未知干涉力对操作物位姿控制精度的影响,保证了协调操作任务的高精度控制．     

Virtual target tracking of mobile robot and its application to formation control

A virtual target tracking approach is proposed for kinematic control of mobile robot. In the controller, linear and angular velocity inputs are generated by using the local data of robot position and orientation along with the estimated velocity of target object. Applying the proposed approach to a cooperative robot group with arbitrary number of multiple mobile robots, it is possible to create various robot formations for cooperative navigation and tracking of moving object. The developed controller is shown to be stable and convergent through theoretical proof and a series of experiments.     

Robotic execution of everyday tasks by means of external vision/force control

In this article, we present an integrated manipulation framework for a service robot, that allows to interact with articulated objects at home environments through the coupling of vision and force modalities. We consider a robot which is observing simultaneously his hand and the object to manipulate, by using an external camera (i.e. robot head). Task-oriented grasping algorithms (Proc of IEEE Int Conf on robotics and automation, pp 1794–1799, 2007) are used in order to plan a suitable grasp on the object according to the task to perform. A new vision/force coupling approach (Int Conf on advanced robotics, 2007), based on external control, is used in order to, first, guide the robot hand towards the grasp position and, second, perform the task taking into account external forces. The coupling between these two complementary sensor modalities provides the robot with robustness against uncertainties in models and positioning. A position-based visual servoing control law has been designed in order to continuously align the robot hand with respect to the object that is being manipulated, independently of camera position. This allows to freely move the camera while the task is being executed and makes this approach amenable to be integrated in current humanoid robots without the need of hand-eye calibration. Experimental results on a real robot interacting with different kind of doors are presented.     

Direct adaptive impedance control of robot manipulators

This article presents an adaptive scheme for controlling the end-effector impedance of robot manipulators. The proposed control system consists of three subsystems: a simple “filter” that characterizes the desired dynamic relationship between the end-effector position error and the end-effector/environment contact force, an adaptive controller that produces the Cartesian-space control input required to provide this desired dynamic relationship, and an algorithm for mapping the Cartesian-space control input to a physically realizable joint-space control torque. The controller does not require knowledge of either the structure or the parameter values of the robot dynamics and is implemented without calculation of the robot inverse kinematic transformation. As a result, the scheme represents a general and computationally efficient approach to controlling the impedance of both nonredundant and redundant manipulators. Furthermore, the method can be applied directly to trajectory tracking in free-space motion by removing the impedance filter. Computer simulation results are given for a planar four degree-of-freedom redundant robot under adaptive impedance control. These results demonstrate that accurate end-effector impedance control and effective redundancy utilization can be achieved simultaneously by using the proposed controller.     

Optimal target impedance selection of the robot interacting with human

Human–robot interaction is an important issue in robotic researches which is the key in many rehabilitation and robot-assisted therapy applications. Impedance control can properly handle soft interaction of robots with the environment. Optimal target impedance selection can increase the performance of the overall system and guarantee the stability. The target impedance cannot be selected without proper knowledge about the stiffness and inertia parameters of the human. In this paper, a systematic analysis is done to introduce a method to estimate the human stiffness and consequently adjust the robot target stiffness. Then, particle swarm optimization is used to find the damping and inertia parameters of the robot to minimize the peak of the interaction force. Also, no assumption is made for the passivity of the human dynamic. The passivity analysis of the human–robot system is investigated. The novelty of this paper is in introducing a practical approach to select the robot target impedance. Finally, experimental results on a lower limb exoskeleton are provided to validate the proposed approach.     

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。     

Optimal variable stiffness control: formulation and application to explosive movement tasks

It is widely recognised that compliant actuation is advantageous to robot control once dynamic tasks are considered. However, the benefit of intrinsic compliance comes with high control complexity. Specifically, coordinating the motion of a system through a compliant actuator and finding a task-specific impedance profile that leads to better performance is known to be non-trivial. Here, we propose an optimal control formulation to compute the motor position commands, and the associated time-varying torque and stiffness profiles. To demonstrate the utility of the approach, we consider an “explosive” ball-throwing task where exploitation of the intrinsic dynamics of the compliantly actuated system leads to improved task performance (i.e., distance thrown). In this example we show that: (i) the proposed control methodology is able to tailor impedance strategies to specific task objectives and system dynamics, (ii) the ability to vary stiffness can be exploited to achieve better performance, (iii) in systems with variable physical compliance, the present formulation enables exploitation of the energy storage capabilities of the actuators to improve task performance. We illustrate these in numerical simulations, and in hardware experiments on a two-link variable stiffness robot.