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.
Highlights
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
Londhe et al [7] proposed a robust nonlinear PID-like fuzzy control scheme for a 3-DOF parallel manipulator, by combining a feed forward term and PID-like fuzzy logic control
To reduce the dynamic tracking error in a highly real time application, Yang et al [17] proposed a computed force and velocity controller for hydraulic 6-DOF parallel manipulators, which is composed of a conventional PID feedback controller and a desired driven force and actuator velocity as feed forward
Summary
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. Londhe et al [7] proposed a robust nonlinear PID-like fuzzy control scheme for a 3-DOF parallel manipulator, by combining a feed forward term and PID-like fuzzy logic control These methods cannot achieve a high tracking accuracy. To reduce the dynamic tracking error in a highly real time application, Yang et al [17] proposed a computed force and velocity controller for hydraulic 6-DOF parallel manipulators, which is composed of a conventional PID feedback controller and a desired driven force and actuator velocity as feed forward. Huang et al [36] applied incremental nonlinear dynamic inversion (INDI) to design a robust force controller based on the pressure/ force sensor and obtained better force tracking performance than that of the feedback linearization method.
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