Development of a Four DOF High Speed Parallel Mechanism Based on Virtual Force Redundancy Concept

Abstract

This paper discusses development of a four DOF (Degree of Freedom) high speed parallel mechanism for electronic part mounters. First, the concept of the Virtual Force Redundancy (VFR) is proposed based on an analysis of a typical four DOF pick-and-place motion. VFR is defined as; when the DOF of a robot's endpoint motion is decreased in a certain region, the robot is regarded to have VFR in the region. Second, a noble structure of a four DOF high speed parallel mechanism is proposed using the newly designed Constrained Differential Drive Mechanism (CDDM) based on the VFR concept. The dynamics of CDDM is analyzed to investigate the theoretical maximum acceleration. Then, a trajectory planning and a position controller with a low level velocity controller are embodied for the proposed mechanism. Usually, adopting velocity controller for all the joints of parallel mechanism with force redundancy could cause large internal force because of the over-constraint property. However, the proposed parallel mechanism with VFR does not cause such problems because it does not have the over-constraint property. Finally, the maximum acceleration of CDDM and the cycle time of pick-and-place motion are measured using a prototype in order to investigate the validity of the proposed high speed parallel mechanism and the VFR concept.