Shou-Kun Wang*, Jun-Zheng Wang and Da-Wei Shi
The Journal of Mechanical Science and Technology, vol. 25, no. 6, pp.1595-1602, 2011

Abstract : The movement precision of the hydraulically driven 6-six degrees of freedom (6-DOF) parallel manipulator is determined mainly by the precision of the valve-controlled asymmetrical cylinder (VCAC). Unfortunately, the asymmetrical movement of the VCAC caused by its asymmetrical structure can significantly compromise control precision. Owing to this asymmetry and, more fundamentally, the inherent nonlinearity of hydraulic systems as well as complicated load variations, it is very difficult to achieve ideal control precision with traditional (PID) control. In the present study, the working principle and characteristics of VCAC were analyzed, with particular focus on the asymmetry problem. In order to improve the precision of both VCAC control and 6-DOF parallel manipulator movement, this paper presents a new, cerebellar model articulation control (CMAC)-based control method. Experiments on both the single VCAC system and the parallel manipulator were developed to verify the validity and effectiveness of the new compound control method. The theoretical analysis and testing results, compared with those for the PID control, proved that the proposed CMAC-based control method can acquire high movement precision on the 6-DOF motion simulator while eliminating the need to build a mathematical model or obtain accurate loading conditions.

Keyword : 6-DOF parallel manipulator; Asymmetry; Cerebellar model articulation control; Compound control; Valve-controlled asymmetrical cylinder