Establishment Of Control System And Research On Impedance Control For Dual-arm Robots

With the rapid development of dual-arm robots, especially the humanoidrobots with redundant manipulators, the increasingly requirement on therobustness of robots in dynamic interaction with uncertain environment and evenwith human makes impedance control become a much important research area. Inthis paper, on the basis of the laboratory pre-research project named Robonaut, adual-arm robot control system based on modern hardware and softwaretechnology is developed, and then researches both on the null-space impedancecontrol of7-DOF redundant manipulators and the object-oriented multi-spaceadaptive impedance control of dual-arm system are carried out. Furthermore,some experiments on the Robonaut experimental platform with the impedancecontrol of maipulators designed in this paper have been conducted.In terms of the achievement of hardware and software in dual-arm robotcontrol system, taking robot astronaut ground experimental platform as aresearch object, this paper analyzed the characteristics of dual-arm robot system.On this basis, a robot control system with hierarchical architecture is established.The central controller based on EPIC, the MMSC bus and the interface boardbased on mLVDS, modular joint controller, the vision computer based on1394b,and the programmable power supply are developed, so that the computing powerand communication speed of modern hardware can be flexibly mapped to certainrobot control system, whose applicability would be significantly broadened.Combined with hardware of control system in middle layer, the software basedon Simulink is designed with rapid prototyping, block abstraction andstandardized communication. Finally, the robot control system which makes useof Simulink to develop is achieved, that allows users to quickly implementsimulation and experiment of robot control algorithm.From the perspective of the impedance control of redundant manipulators,in order to solve the problem of having no way to control the null-space motionbrought by the impedance control method with position inner loop, theconfiguration of7-DOF redundant manipulators is analyzed and on this basiscompliant plane and arm plane are introduced, then the angle between these twoplanes is defined as the arm angle to describe the null-space motion of the redundant manipulator. Furthermore, null-space impedance equation containingstiffness term is derived according to the energy theorem, making its expressionhave a clear physically interpretation, then considering the combination of thisequation and the dynamic equation, the impedance control method of redundantmanipulator with null-space impedance is finally achieved. This control strategyenables7-DOF redundant manipulator to achieve null-space impedance whileretaining the Cartesian space impedance behavior. Simulations using the modelof the7-DOF redundant manipulator system, established by SimMechanics,verify the proposed impedance control strategy, which is also compared withsome other general impedance control methods. The comparison results showedthat when force is applied to the manipulator, this method achieves a compliantbehavior in null space of the manipulator, and at the same time ensures that theposition error in Cartesian space asymptotically converges to zero.In the view of the obstacle avoidance of the impedance control forredundant manipulators, general impedance control cannot achieve null-spaceimpedance behavior. To solve this problem, self-motion of the manipulatorgenerated by a virtual force is used for obstacle avoidance. Virtual force here is arepulsive force applied on manipulator, pointing from obstacle to the manipulator.When the manipulator moves close to the obstacle, with this virtual repulsiveforce, null-space impedance of the redundant manipulator is used for obstacleavoidance, while complete the trajectory tracking of the end-effector.Simulations verify the effectiveness of this method, and the results also showthat the position tracking accuracy of the end-effector is not affected when themanipulator avoids obstacle.In terms of the dual-arm impedance control, considering the problem thatthe force exerted on the object easily saturates when dual-arm system clamps theobject, the human motion control and learning characteristics are analyzed in theview of mechanics. On this basis, the object oriented impedance is introduced asthe outer loop of the impedance control strategy, combined with the impedanceof the manipulator as the inner loop. This strategy ensures that both the objectand the manipulators have impedance behavior. Then the adaptive impedance lawbased on energy cost function is proposed. That the adaptive law allows energycost function converges to a minimum is proved by the Lyapunov method. Simulation results verify the effectiveness of the proposed method and show thatthe object impedance and the manipulator impedance can bound the force exertedon object by the manipulators and the force acting on the environment by object.Also the proposed adaptive law can adaptively adjust parameters of the objectimpedance according to the environmental interference applied on the object,thereby improving the robustness of the system to uncertain interference.From the perspective of applications of dual arm robot impedance control,several experiments with uncertain environment based on the existingmanipulator and experimental platform are designed, including the uncertainspatial location constraints and uncertain human disturbance, and withmanipulator impedance, several control tasks are completed, such as that thehuman interacting with a manipulator to shake hand, the docking between amanipulator and a fixed target, the null-space obstacles avoidance of theredundant manipulator, dual arms holding a object, the human static and dynamicinteracting with the dual-arm robot and its holding object and so on. Theexperiment results show the robustness of the robot impedance control systemwhen interacts with uncertain environment.