PD Plus Dynamic Pressure Feedback Control for a Direct Drive Stewart Manipulator

Abstract

In order to ensure good dynamic characteristics, servo valve is usually adopted as the drive part of Stewart manipulator which causes huge power consumption, while direct drive electro-hydraulic servo system has the advantages of energy saving, simple structure, convenient installation, and low failure rate. But its dynamic characteristics are so poor that it can only be applied to occasions where quick response is not needed. On the consideration above, following works are done in this paper. Since current coupling exists in the control system based on the speed of the servo motor as the control input, the control system of the direct drive Stewart manipulator is established based on the current of the servo motor as the control input in which the current coupling can be solved. In order to improve the dynamic characteristics of the direct drive Stewart manipulator, a Proportion Differentiation (PD) plus dynamic pressure feedback control strategy is also put forward in this paper, which is verified by using a simulated hydraulically driven Stewart manipulator. Simulation results show that both dynamic coupling and current coupling are solved and the control strategy proposed in this paper can significantly increase the bandwidths of all degrees of freedom.

Experimental Study of the Direct Drive Hydraulic System with the Torque Mode

“…Since both Equation (3) and Equation (4) apply to the speed mode and the torque mode, the parameters of the second-order element with the speed mode and the dynamic secondorder element with the torque mode are quite similar. According to [21], both proportion differentiation (PD) control strategy and dynamic pressure feedback (DPF) control strategy which are designed to increase the bandwidth of the direct drive 6-DOF parallel mechanism should also be adopted to improve the dynamic performances of the direct drive hydraulic system in Figure 2 with the torque mode, experimental verification is also carried out.…”

“…The structure and the direct drive hydraulic system of the direct drive 6-DOF parallel mechanism in [21] are shown in Figure 1. Both dynamic coupling and current coupling among motors exist in the direct drive 6-DOF parallel mechanism.…”

“…Combined with modal space coordinate transformation, the speed mode can only solve the dynamic coupling, while the torque mode can solve both dynamic coupling and current coupling with modal space coordinate transformation. So, the torque mode is more suitable for the direct drive 6-DOF parallel mechanism [21]. However, the key parameters and the dynamic characteristics of the direct drive hydraulic system for the direct drive 6-DOF parallel mechanism with the torque mode have never been studied, which are important and are studied by the combination of the transfer function and the experiment of the direct drive hydraulic system with the torque mode, compared with those with the speed mode in this paper.…”

“…The structure and the direct drive hydraulic system of the direct drive 6-DOF parallel mechanism in [21] are shown in Figure 1. The motion of the upper platform which is usually made of steel is generated by six symmetrical direct drive legs.…”

“…Usually, smooth joints which can eliminate the effects of the joint friction on its dynamics characteristics are often adopted in the parallel mechanism, so the load of the parallel mechanism is mainly composed of the inertia load. Since the hydraulic system of each leg of the direct drive 6-DOF parallel mechanism is the same, and the complete coupled transfer function with the torque mode can be transformed into six independent single DOF control systems with modal space coordinate transformation, which are quite similar to that of a single DOF direct drive hydraulic system with inertia load [21], the hydraulic system of the direct drive 6-DOF parallel mechanism can be simplified to a single DOF hydraulic system which costs much less and the study of this paper can also be carried out with the simplified system, which should be exactly the same with the hydraulic system of each leg of the direct drive 6-DOF parallel mechanism. It is worth noting that the hydraulic and electric components of the single DOF hydraulic system should be selected according to the requirements of the direct drive 6-DOF parallel mechanism.…”