Computed force and velocity control for spatial multi-DOF electro-hydraulic parallel manipulator

A novel dynamic trajectory tracking controller for spatial 6-DOF electro-hydraulic parallel manipulator considering system nonlinearity-computed force and velocity controller is proposed, with a view of improving the control performance with high computational efficiency of control algorithm. The dynamic model of electro-hydraulic parallel manipulator, both mechanical and hydraulic system, is described by using Kane and hydromechanics method. The requisite system states are estimated via forward kinematics based upon global Newton–Raphson with monotonic descent algorithms under the measured actuator position. The desired leg position and velocity required for the proposed controller are calculated by an analytical method corresponding to the desired generalized pose, and the desired driven force is computed with an effectively simplified inverse dynamics. Under feed-forward of the desired driven force and velocity, the computed force and velocity controller is developed with actual leg position as its feedback only, and the desired leg position, velocity and driven force as its input. The control performance of the proposed controller for multi-DOF parallel manipulator is evaluated in theory and experiment, especially for dynamic tracking performance. Experimental results show that the presented controller can greatly improve the dynamic trajectory tracking performance for high real time electro-hydraulic parallel manipulator.     

An Experimental Study of a Redundantly Actuated Parallel Manipulator for a 5-DOF Hybrid Machine Tool

This paper deals with the experimental study of a redundantly actuated parallel manipulator for a 5-DOF hybrid machine tool. Based on the kinematics of the redundantly actuated parallel manipulator, the inverse dynamics is derived by using the virtual work principle. A position and force switching control strategy is used for the two extendible chains of the parallel manipulator. Further, the difference predictive control is presented for the extendible chain with force mode. The parallel manipulator is incorporated into a 5-DOF hybrid machine tool. Linear and circular contouring experiments of both the hybrid machine tool as well as its corresponding nonredundant counterpart with the redundant limb removed are conducted to evaluate the machine performance. Finally, the machine experiment is used to demonstrate the machine's applicability.      

Identification of dynamic and friction parameters of a parallel manipulator with actuation redundancy

The dynamic model is formulated in the joint space for a parallel manipulator with actuation redundancy. The established model is expressed in its linear matrix form with respect to the dynamic and friction parameters, and then the Weighted Least Square (WLS) method is applied to estimate the parameters. In order to get satisfying estimated results, the filters of joint angle and actuated torque are designed for the parallel manipulator. A simple and effective method is presented to design the excitation trajectory by analyzing the mechanism structure characteristics of the actual parallel manipulator. Another type of trajectory which is different from the excitation trajectory is adopted to verify the estimated parameters. The dynamic control experiments based on the identified model with the estimated parameters are implemented on an actual 2-DOF parallel manipulator with actuation redundancy, and the tracking accuracy of the identified model is compared with the result of the so-called nominal model.     

Dynamic modeling and control of a 3-DOF Cartesian parallel manipulator

Dynamic modeling is the basic element for controller design of mechanisms. In this paper, an effective dynamic equation of a 3-DOF translational parallel manipulator for control purpose has been derived by the Lagrange-D’Alembert formulation. The structural properties of the derived dynamic equation were proved so that the vast control strategies developed for the serial counterparts can be easily extended for controlling the CPM (Cartesian parallel manipulator). The derived model also leads to decoupling dynamic characteristics, by which the complexity of the controller design can be significantly reduced. Based on this approach, a model-based computed torque method for positioning control of the CPM is illustrated. Both simulated and experimental results show that the model-based controller can achieve high positioning performance. Furthermore, it is shown that the coupling forces from other limbs play significant roles in the force components of the total dynamics of the CPM.     

Robust learning control of a high precision planar parallel manipulator

End-point positioning accuracy and fast settling time are essential in the motion system aimed at semiconductor packaging applications. In this paper, a novel robust learning control method for a direct-drive planar parallel manipulator is presented. A frequency-domain system identification approach is used to identify the high frequency dynamic of the manipulator. A robust control design method is employed to design a stable, fast tracking response feedback controller with less sensitivity to high frequency disturbance and the control parameters are determined using genetic algorithm. A Fourier-series-based iterative learning controller is designed and used on the feedforward path of the controller to further improve the settling time by reducing the dynamic tracking error of the manipulator. Experimental results demonstrate that the planar parallel manipulator has significant improvements on motion performance in terms of positioning accuracy, settling time and stability when compared with traditional XY-stages. This shows that the proposed manipulator provides a superior alternative to XY-motion stages for high precision positioning.     

Observer-based cascade control of a 6-DOF parallel hydraulic manipulator in joint space coordinate

This paper addresses the joint space control problem of a 6-DOF (degree of freedom) parallel hydraulic manipulator. High precision motion control of a six-degree parallel manipulator is hardly achieved due to the existence of uncertain payload and other disturbance such as coupling force. A disturbance observer for this parallel manipulator is first constructed to estimate and compensate the unknown disturbance. A cascade control algorithm is then applied to separate the hydraulic dynamics from the mechanical part, which can mask the hydraulic dynamics with an inner loop. With such a control structure, known control design methods within the area of manipulator control can be directly used in the outer loop. In this approach, the complex dynamics and direct kinematics of the parallel manipulator are not required and acceleration feedback is also avoided. Experimental results are presented to show the effectiveness of the proposed scheme.     

Control-faced dynamics with deformation compatibility for a 5-DOF active over-constrained spatial parallel manipulator 6PUS–UPU

Taking into account the effect of structural compliance, inverse dynamics of the active over-constrained parallel manipulator 6PUS–UPU with five degrees of freedom is solved in this article. Firstly, the relationship between driving forces and actuated force screws of each limb is derived. Then the coordination of elastic deformation between limbs which consider the effect of gravity and inertia is acquired. Finally the unique solution of driving forces for the active over-constrained parallel manipulator is derived by incorporating the force equilibrium equation of the moving platform. To validate the theoretical derivation, dynamics simulation model of manipulator based on rigid–flexible mixed structure is shown and numerical examples are given. Comparison with the traditional method of dynamics based on pseudo-inverse is also made. Finally, a feasible experimental method, as an effective test to the theoretical calculation, is proposed and applied on the prototype.     

Development of micromanipulator and haptic interface for networkedmicromanipulation

Telemicromanipulation systems with haptic feedback, which are connected through a network, are proposed. It is based on scaled bilateral teleoperation systems between different structures. These systems are composed of an original 6 degree of freedom (DOF) parallel link manipulator to carry out micromanipulation and a 6-DOF haptic interface with force feedback. A parallel mechanism is adopted as a slave micromanipulator because of its good features of accuracy and stiffness. The system modeling and control of the parallel manipulator system are conducted. Parallel manipulator feasibility as a micromanipulator, positioning accuracy and device control characteristics are investigated. A haptic master interface is developed for micromanipulation systems. System modeling and a model reference adaptive controller are applied to compensate friction force, which spoils free motion performance and force response isotropy of the haptic interface. These systems aim to make the micromanipulation more productive constructing a better human interface through the microenvironment force and scale expansion     

Design and Development of a Medical Parallel Robot for Cardiopulmonary Resuscitation

The concept of a medical parallel robot applicable to chest compression in the process of cardiopulmonary resuscitation (CPR) is proposed in this paper. According to the requirement of CPR action, a three-prismatic-universal-universal (3-PUU) translational parallel manipulator (TPM) is designed and developed for such applications, and a detailed analysis has been performed for the 3-PUU TPM involving the issues of kinematics, dynamics, and control. In view of the physical constraints imposed by mechanical joints, both the robot-reachable workspace and the maximum inscribed cylinder-usable workspace are determined. Moreover, the singularity analysis is carried out via the screw theory, and the robot architecture is optimized to obtain a large well-conditioning usable workspace. Based on the principle of virtual work with a simplifying hypothesis adopted, the dynamic model is established, and dynamic control utilizing computed torque method is implemented. At last, the experimental results made for the prototype illustrate the performance of the control algorithm well. This research will lay a good foundation for the development of a medical robot to assist in CPR operation.     

The investigation of the maneuverability deterioration based on acceleration radius theory

Maneuverability is the measure of the dynamic performance of a manipulator in a specific posture or configuration, and acceleration radius is one of the most utilized indices of it. Acceleration radius can be utilized as the reference to judge whether further dynamic analysis should be performed when evaluating the controllability and feasibility of the manipulator following the prescribed path with assigned kinematic and kinetic requirements in the planning phase. When utilizing acceleration radius as the dynamic reference in the planning phase, it can prevent wasting the calculation cost due to these non-necessary dynamic analyses, and it can also be utilized as the benchmark in the on-line control.However, the existence of the configuration errors is inevitable in reality, and it deteriorates the dynamic performance of a manipulator with the ideal configuration parameters and leads to the potential risk of failing to achieve an assigned dynamic task. To investigate the adverse behavior caused by the configuration errors and to provide some clues to avoid or reduce their influence, this article proposes a novel and systematic method which can be used to evaluate the maneuverability deterioration of a non-redundant serial manipulator system due to the influence of configuration errors, and it also provides an index, deterioration rate, to quantitate this kind of deterioration. Deterioration rate can be utilized to quantitate the maneuverability deterioration due to the influence of configuration errors in a prescribed workspace or region and can also be treated as the safety or derating margin when proceeding with the control or path planning.     

基于激光测距的微型机械臂关节控制模型

为了更加深入探究并提升机械臂关节控制效果,提出一种基于激光测距的微型机械臂关节控制模型,通过激光测距技术对微型机械臂周边环境进行感知,同时进行数据点扫描,将全部数据点集进行直线分割以及拟合等相关操作,获取二维环境地图。在上述基础上,通过坐标变换矩阵获取微型机械臂的正运动学模型,利用正运动学模型得到机械臂逆运动学模型。将获取的动力学模型转换为仿射非线性系统的形式,利用输入输出反馈线性化方法选取合适的状态变换和反馈变换,通过滑膜控制方法构建微型机械臂关节控制模型,采用模型进行关节控制。仿真实验结果表明,所提模型可以获取理想的微型机械臂关节控制结果。     

Motion control of a tendon-based parallel manipulator using optimal tension distribution

This paper presents the motion control of a six degree-of-freedom tendon-based parallel manipulator, which moves a platform with high speed using seven cables. To control the motion of the platform along desired trajectories in space, nonlinear feedforward control laws in the cable length coordinates are used. Taking account of the effect of redundancy on actuation, the optimal tension distribution should be considered to the advantage of the control laws. Using a method based on the analysis of the workspace condition, tension constraints and limiting torque constraints of actuators, an analytical solution for optimum tension distribution was found and used to compute the force in each cable for compensation of dynamic errors. It is experimentally demonstrated that the proposed control laws reduce the energy consumption of the actuators and satisfy the path tracking accuracy.     

A motion control of mobile manipulator with external force

Describes a stable motion control of a mobile manipulator with the consideration of external force. As is well known, the mobile manipulator has infinite motion area, which brings several sophisticated advantages to the manipulator control. However, the unexpected external force to the mobile manipulator makes its motion unstable since there are no fixed points in the stationary coordinate. To obtain the stable motion independently of the external force, the cooperative control of the manipulator and vehicle parts is strongly required. To address the above issue, the paper presents a stable control strategy for mobile manipulator in the case that an eternal force exists. The validity of the proposed method is confirmed by several experimental results     

Trajectory generation and tracking control of a multi-level hybrid support manipulator in FAST

The Feed Support System (FSS) of Five-hundred-meter Aperture Spherical radio Telescope (FAST) is a multi-level redundant support manipulator, which consists of a cable driven parallel manipulator, an A–B rotation mechanism and a Gough–Stewart platform. In this article, we report our work on FAST in the following aspects: first, kinematic model and trajectory generation strategy of the FSS are established. Second, considering preventing the pseudo-drag problem of flexible cable and realizing the accurate pose control for the cable driven parallel manipulator, the hybrid position/force control is implemented and validated. Then, the prediction control is adopted in the Gough–Stewart platform to improve the terminal accuracy. Finally, with the 1:15 similarity model of FSS, experiments are carried out to prove the control accuracy of the cable driven parallel manipulator, showing that the terminal error is within 10 mm and the cable tension is kept in the given range. Further experiments on tracking control of the entire FSS illustrate terminal tracking accuracy of the astronomical observation is less than 2 mm, meeting the design requirement. Trajectory generation and tracking control given in this paper lay the foundation for the FAST prototype.     

Torque sensorless control in multidegree-of-freedom manipulator

A torque sensorless control for a multi-degree-of-freedom manipulator is described. In the method, two disturbance observers are applied to each joint. One is used to realize a robust motion controller. The other is used to obtain a sensorless torque controller. A robust acceleration controller based on the disturbance observer is shown. To obtain the sensorless torque control, it is necessary to calculate the reaction torque when the mechanical system performs a force task. The calculation method for the reaction torque is explained. Then the method is expanded to workspace force control in the multi-degree-of-freedom manipulator. Several experimental results are shown to confirm the validity of the proposed sensorless force controller     

Application of a 3-DOF parallel manipulator for earthquake simulations

In this paper a formulation and experimental results are presented for a novel application of a 3-degree of freedom (DOF) parallel manipulator to simulate point seismograms and three-dimensional (3-D) earthquake motion. The rigid body acceleration is analyzed to simulate real 3-D earthquakes. Furthermore, first experimental results are reported to analyze earthquake effects on scaled civil structures.     

Visual servoing for constrained planar robots subject to complex friction

The theoretical framework and the experimental validation of a new image-based position-force control for planar robots are presented in this paper. This scheme produces simultaneous convergence of the constrained visual position and the contact force between the end effector and the constraint surface. Camera, robot, and the visual jacobian parameters are considered unknown. This approach is based on a new formulation of the orthogonalization principle used in the robot force control, termed the visual orthogonalization principle. This allows, under the framework of passivity, to yield a synergetic closed-loop system that fuses accordingly camera, encoder, and the force sensor signals. Furthermore, due to the technological limitations, it can be noticed that the visual servoing contact tasks are characterized by slow motion, typically with frequent velocity reversals along the constraint surface, thus, important friction problems arise at the joints and the contact points. Therefore, visual compensation of the complex dynamic joint friction and the viscous contact friction are also studied. A Linux real-time operating-system-based experimental system is implemented to visually drive a constrained direct-drive planar robot manipulator, equipped with six-axes JR3 force sensor and a digital fixed camera, thus proving the effectiveness of the proposed scheme.     

Interaction control of an industrial manipulator using LPV techniques

This paper presents the design and implementation of a hybrid force/motion control scheme on a six-degrees-of-freedom robotic manipulator employing a gain-scheduled linear parameter-varying (LPV) controller. A nonlinear dynamic model of the manipulator is obtained and the unknown parameters are estimated. The manipulator is decomposed into an inner and a wrist submodel, and a practical way is proposed to investigate the coupling between them. The motion control part of the hybrid controller which is the main focus of this paper is formed by a combination of an LPV controller and a model-based inverse dynamics controller for the inner submodel and the wrist joints, respectively. A quasi-LPV model with a reduced number of scheduling parameters is derived for the inner submodel, and a polytopic LPV gain-scheduled controller is synthesized in a two-degrees-of-freedom structure including feedback and feedforward parts, which is augmented by a friction compensation term. A PD controller with a feedforward path is designed to control the interaction force. The proposed hybrid force/motion scheme is implemented on the 6-DOF CRS A465 robotic manipulator to perform a writing task. Comparison of the results with those of a hybrid force/motion controller with a plain model-based inverse dynamics motion control and the same force control shows that the proposed controller improves the position tracking performance significantly.     

Position control system of a two degree of freedom manipulator witha passive joint

A method is proposed for controlling the position of a manipulator with passive joints that have holding brakes instead of actuators. In this method, the coupling characteristics of manipulator dynamics are used, and no additional mechanisms are required. The effectiveness of the method was verified by experiments using a prototype manipulator. The prototype is a two-degree-of-freedom, horizontally articulated manipulator. The first axis is an active joint, and the second axis is a passive joint. While the brake of the passive joint is released, the passive joint is indirectly controlled by the motion of the active joint through the use of dynamic coupling. While the brake is engaged, the active joint is controlled. By combining these two control modes, the total position of the manipulator is controlled. The experiments show that use of this method makes the precise positioning of the passive joints possible     

Force control of a very lightweight single-link flexible arm based on coupling torque feedback

In this paper, a feedback control law is proposed for regulating the contact force exerted by a very lightweight single-link flexible manipulator when it comes into contact with a motionless object. This control law is based on a lumped-parameter model. The tracking of the desired contact force is obtained by using a feedback loop control of the coupling torque between the motor and the flexible arm. To achieve a good performance on the force control it is only necessary to measure the coupling torque at the root of the arm. Neither the contact force sensor nor the angular position sensor of the motor are used in the control method. A modified PID controller is proposed for this control law. In this work the force control problem is studied for both free and constrained motions of the flexible manipulator, and a collision detection algorithm is also described.