Nonlinear computed torque control for a high-speed planar parallel manipulator

A new computed torque (CT)-type controller termed nonlinear CT (NCT) controller is developed and applied to a high-speed planar parallel manipulator. The NCT controller is designed by replacing the linear PD in the conventional CT controller with the nonlinear PD (NPD) algorithm. The stability of the parallel manipulator system with the NCT controller is proven using the Lyapunov theorem, and the proposed controller is further proven to guarantee asymptotic convergence to zero of both tracking error and error rate. The superiority of the proposed NCT controller is verified through the trajectory tracking experiments of an actual high-speed planar parallel manipulator, and the experiment results are compared with the CT controller.     

A new spatial three-DoF parallel manipulator with high rotational capability

This paper concerns the proposal and analysis of a novel spatial parallel manipulator. The parallel manipulator consists of a base plate, a movable platform, and three connecting legs. The moving platform has three degrees of freedom (DoFs), which are two degrees of translational freedom and one degree of rotational freedom, with respect to the base plate. The inverse and forward kinematics problems are described in closed forms and the velocity equation of the new parallel manipulator is given. Three kinds of singularities are presented. The workspace for the manipulator is analyzed systematically. Especially, the indices to evaluate the rotational capability of the moving platform of the manipulator will be defined and discussed in detail. Due to the high rotational capability performance, the proposed manipulator has wide application in the fields of industrial robots, simulators, micromotion manipulators, and parallel kinematic machines.     

Neural network impedance force control of robot manipulator

The performance of an impedance controller for robot force tracking is affected by the uncertainties in both the robot dynamic model and environment stiffness. The purpose of this paper is to improve the controller robustness by applying the neural network (NN) technique to compensate for the uncertainties in the robot model. NN control techniques are applied to two impedance control methods: torque-based and position-based impedance control, which are distinguished by the way of the impedance functions being implemented. A novel error signal is proposed for the NN training. In addition, a trajectory modification algorithm is developed to determine the reference trajectory when the environment stiffness is unknown. The robustness analysis of this algorithm to force sensor noise and inaccurate environment position measurement is also presented. The performances of the two NN impedance control schemes are compared by computer simulations. Simulation results based on a three-degrees-of-freedom robot show that highly robust position/force tracking can be achieved in the presence of large uncertainties and force sensor noise     

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     

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.     

Position domain contour control for multi-DOF robotic system

Contour tracking control is one of the fundamental operations for robotic systems. In this paper, a position domain PD control is developed to control a multi-DOF nonlinear robotic system for improving contour tracking performance. In this new position domain control system, a robotic system is viewed as a master–slave system where the master motion is used as an independent reference through equidistantly sampling, while slave motions are described as functions of the master motion according to contour tracking requirements. A position domain dynamic model of the robotic system based on the master motion is developed through one-to-one mapping of the original dynamic model from time domain to position domain. Stability analysis is conducted for the proposed position domain PD control, the global boundedness of the tracking errors is guaranteed through the Lyapunov method, and the effectiveness is successfully verified through simulation study for linear and nonlinear contour tracking problems. Compared results demonstrate that the position domain PD control is better than its time domain counterpart for contour tracking of multi-DOF robotic systems.     

基于改进型DOB的电液伺服力控制技术研究

考虑到电液伺服力控制系统容易受到外界载荷扰动影响等问题,提出了一种基于改进型DOB算法的电液伺服力控制器。通过对控制系统中的标称控制器P,低通的滤波器K以及控制参数V进行设计,以消除电液伺服力控制系统受到外界扰动信号d和电液伺服力控制系统测量的噪声信号n对系统的影响,提高DOB系统的鲁棒性能。通过仿真试验验证提出的改进型DOB算法对提高电液伺服力控制系统稳定性和抗干扰能力的效果,并与基于模糊算法和基于常规DOB算法的控制系统进行比较。结果表明,提出的改进型DOB算法使得电液伺服力控制系统具有更好的鲁棒性以及稳定性。     

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.     

Experimental study of event-based neural network control on parallel manipulator

In the paper, the event-based switching controller (ESC) is utilized to achieve better tracking performance compared with the neural network controller in circumstance of parameter uncertainty and unknown disturbance for the parallel manipulator. The ESC optimizes the choice of the neural weights by combining the prior knowledge of the system dynamics and the estimation of the system parameters. To implement the controller, a general method of computing the system regression matrix for the PM is proposed and the stability proof is given in circumstance of unbounded disturbance. The ESC is tested by the simulated Delta manipulator and the experimental 5R testbed. The results show the effectiveness of the proposed controller.     

Robust force control with a feed-forward inverse model controller for electro-hydraulic control loading systems of flight simulators

A hybrid control strategy for an electro-hydraulic control loading system (EHCLS) of a flight simulator in the presence of a control mechanism kinetic parameter perturbation is proposed to improve the force tracking accuracy and guarantee robust stability of the EHCLS system. A double-loop model of the EHCLS, including the control mechanism and the hydraulic mechanism, is established and analyzed from the force-displacement impedance perspective. A force closed-loop parameter model of the EHCLS is identified by a recursive-least-squares (RLS) algorithm and its inverse model is designed using a zero phase error compensation technology to expand the frequency bandwidth of the force closed-loop system of the EHCLS. A μ theory of robust control is employed to design a stable controller for enhancing robust stability of the EHCLS in the presence of uncertainties of the inner loop, the control mechanism and the high frequency disturbance force. Simulation and experimental results show that the proposed hybrid control approach can greatly improve the control performance of the EHCLS by expanding the frequency bandwidth of the force closed-loop system and enhancing stability of the EHCLS, which can decrease displacement output response error of the EHCLS from 10.34% to 3.1%.     

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.     

Development and hybrid force/position control of a compliant rescue manipulator

Performing search and rescue tasks in the ruins after disasters demand rescue robots with slender and compliant structure to accommodate the complicated configurations under debris. This paper presents the structural design and system composition of a novel tendon-sheath actuated compliant rescue manipulator with slender and flexible body. The proposed robot can drill into the narrow space where rescuers and traditional rigid robots cannot get in because of size limitation or toxic environment. The self-sensing calibration, dynamic modeling, and hybrid force/position control trajectory of the compliant gripper with integrated position and force monitoring capabilities are analyzed and discussed. With the aim of regulating the gripper displacement and clamping force during operation, a hybrid force/position control strategy is proposed based on a cascaded proportional-integral-derivative (PID) controller and a fuzzy sliding mode controller (FSMC). Experimental setups mainly consisting of servo motor, tendon sheath transmission components, compliant gripper, and real-time control system are established to calibrate the strain gauge sensors and identify the dynamic model parameters. Further experimental investigations involving force tracking experiments, position tracking experiments, and object grasping experiments are carried out. The experimental results demonstrate the effectiveness of the developed self-sensing approach and control strategies during rescue operation.     

Output feedback nonlinear control for electro-hydraulic systems

In this paper we present an output feedback nonlinear control for position tracking of electro-hydraulic systems (EHSs). Although previous nonlinear control methods improved the position tracking performance of EHS, all of the methods require full state feedback. However, due to cost and space limitations, it is not always possible to measure the full state of the EHS. The proposed method consists of a high gain observer and a passivity-based controller. The high gain observer is designed to estimate the full state, and the passivity-based control is implemented for position tracking. In order to design the passivity-based controller with the high gain observer, a defined Lyapunov condition guarantee that the origin of the tacking error dynamics is exponentially stable by selecting the controller gain. The stability of the closed-loop is studied using the singular perturbation theorem. The performance of the proposed method is validated through simulations and experiments.     

Error analysis and control scheme for the error correction in trajectory-tracking of a planar 2PRP-PPR parallel manipulator

This paper presents the end-effector pose error modeling and motion accuracy analysis of a planar 2PRP-PPR parallel manipulator with an unsymmetrical (U-shape) fixed base. The error model is established based on the screw theory with considerations of both configuration (geometrical) errors and joint clearances. It also proposes a robust cascaded control scheme for the end-effector pose (task-space) error correction in trajectory-tracking of the manipulator due to mechanical inaccuracies. The proposed control scheme uses redundant sensor feedback, i.e., individual active joint displacements and velocities and, end-effector positions and orientation are obtained as feedback signals using appropriate sensors. To demonstrate the efficacy and show complete performance of the proposed controller, real-time experiments are accomplished on an in-house fabricated planar 2PRP-PPR parallel manipulator.     

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.     

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.     

Spatial hybrid/parallel force control of a hexapod machine tool using structure-integrated force sensors and a commercial numerical control

Structure-integrated force measurement in hexapod structures or kinematics offers great potential for spatial process force control in six degrees of freedom. Although force control has been studied for many years, research questions remain unanswered, especially when regarding parallel kinematic machine tools with numerical control and integrated sensors. This contribution summarizes the technology of structure-integrated force sensing in hexapod machine tools and develops two approaches to use the measured signals for direct hybrid/parallel force control. For different use-cases, such as teach-in or synchronous force/position control with variable task frame, different approaches of set-point specification and injection of manipulated values are studied from a practical point of view. As a result of the work, the feasibility of the force control with structure-integrated sensors on a commercial CNC can be confirmed through appropriate experiments. Furthermore, the realized G-Code integration represents a practical solution for programming force-controlled machine tools in an easy and concise way from the NC programme.     

Real-time electro-hydraulic hybrid system for structural testing subjected to vibration and force loading

Real-time electro-hydraulic hybrid system (REHS) with shaking table and force loading simulator is an essential experimental facility for evaluating structural performance subjected to simultaneously vibration excitation and force loading. The key feature of this paper is combination of a feedforward force controller including modified force inverse model compensator (MFIMC) and velocity feedforward compensator (VFFC) with an internal model control (IMC) to compensate the surplus force disturbance caused by active motion of shaking table and to obtain high fidelity force loading tracking performance. An acceleration tracking controller is also designed with modified acceleration inverse model compensator (MAIMC) to extend the acceleration tracking frequency bandwidth and to improve the acceleration tracking performance. The acceleration/force closed-loop transfer function model and their inverse model are identified and designed by multi-step recursive extended least squares (RELS) algorithm and zero magnitude error tracking controller (ZMETC) technology respectively because the identified transfer function model of the acceleration and force loading closed-loop systems may be a nonminimum-phase (NMP) system and their inverse model are instable. An acceleration and force modeling error compensator (MEC) are utilized in MFIMC and MAIMC to minimize the effect of the inaccuracy of identified model and designed inverse model. Experimental results obtained on a real uniaxial REHS with xPC rapid prototyping technology clearly demonstrate the benefit of the proposed compensation method.     

Workspace and singularity analysis of a double parallel manipulator

A double parallel manipulator (DPM) is composed of two parallel mechanisms with a central aids. The device is expected to have large workspace by reducing interference between links and to avoid singularity by constraining its motion. To prove this, workspace and singularity are analyzed. The workspace of the device is decoupled into a positional and an orientational workspace, each of which is computed and compared with that of a Stewart platform. The singularities occurring outside the workspace are analytically found at the configurations where a Jacobian matrix becomes rank deficient. To ascertain the analytical results, they are geometrically examined at the configurations where the duty for resisting forces and moments is not properly shared by a central axis and linear actuators due to losing static equilibrium. The geometrical results are coincident to the analytical results, which proves the DPM is free from the singularity problem     

用于肌肉电信号控制的仿生机械手设计与仿真

针对肌肉电信号(EMG)控制假手的需求,介绍了一种仿生机械手的设计和控制仿真.此机械手优势在于关节和手指尺寸完全还原真实人手的大小.首先,在多自由度的基础上,建立了五指的运动学模型并求出其运动学正解和逆解.然后,通过仿真来验证其运动学解的正确性,从而为仿生机械手的运动轨迹规划和进一步的控制提供了理论依据.最后,简要说明...