Compound Impedance Control of a Hydraulic Driven Parallel 3UPS/S Manipulator
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Chinese Journal of Mechanical Engineering Open Access
ORIGINAL ARTICLE
Compound Impedance Control of a Hydraulic Driven Parallel 3UPS/S Manipulator Lihang Wang1, Shaofei Cui1, Chong Ma1 and Lijie Zhang1,2*
Abstract The hydraulic parallel manipulator combines the high-power density of the hydraulic system and high rigidity of the parallel mechanism with excellent load-carrying capacity. However, the high-precision trajectory tracking control of the hydraulic parallel manipulator is challenged by the coupling dynamics of the parallel mechanism and the high nonlinearities of the hydraulic system. In this study, the trajectory control of a 3-DOF symmetric spherical parallel 3UPS/S manipulator is evaluated. Focusing on the highly coupling and nonlinear system dynamics, a compound impedance control method for a hydraulic driven parallel manipulator is proposed, which combines impedance control with the spatial motion characteristics of a parallel manipulator. The control strategy is divided into the inner and outer loops. The inner loop controls the impedance of the actuator in the joint space, and the outer loop controls the impedance of the entire platform in the task space to compensate the coupling of the actuators and improve the tracking accuracy of the moving platform. Compound impedance control does not require force or pressure sensors and is less dependent on modeling precision. The experimental results show that the compound impedance control effectively improves the tracking accuracy of the moving platform. This research proposes a compound impedance control strategy for a 3-DOF hydraulic parallel manipulator, which has high tracking precision with a simple and cheap system configuration. Keywords: Parallel manipulator, Impedance control, Hydraulic servo system, Trajectory tracking 1 Introduction Parallel manipulators are widely applied in flight simulators [1, 2], machine tools [3], and six-dimensional force sensors [4] because they have higher stiffness, increased precision, lower inertia of moving parts at a high speed, and higher accuracy than those of their serial counterparts. However, their closed chain structure creates a complicated forward kinematic analysis and highly coupled time-varying nonlinear system dynamics. In addition, the uncertainty in the manipulator parameters and disturbances make it difficult to achieve high trajectory tracking control for parallel manipulators. *Correspondence: [email protected] 1 Hebei Key Laboratory of Heavy Machinery Fluid Power Transmission and Control, Yanshan University, Qinhuangdao 066004, China Full list of author information is available at the end of the article
A number of works have investigated improving the tracking accuracy of parallel manipulators. PID control is the conventional and simplest method used to control the system and is widely used in parallel manipulators. However, because of the highly nonlinear and coupled dynamic behaver of parallel manipulators, the tracking error of pure PID control rapidly increases with speed [5]. To improve
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