Control of a Five-axle, Three-steering Coupled-vehicle System

— Introduction of Virtual Mechanical Elements for Motion Control —

Abstract

This paper presents a novel methodology of introducing virtual mechanical elements for motion control of transferred objects in modeling of a five-axle, three-steering coupled-vehicle system. The coupled-vehicle system consists of two car-like mobile robots, two carriers and one steering unit, and it is capable of forming straight-bed and V-bed carriers depending on shapes of objects. In the kinematical model of the coupled-vehicle system, two virtual car-like mobile robots, a virtual carrier and two virtual revolute joints are newly introduced, although these virtual mechanical elements are redundant on converting the kinematical equations of the coupled-vehicle system into time differential equations in a chained form. The objective of introducing these virtual mechanical elements is to set the position of one of the virtual revolute joints and the orientation of the virtual carrier to be state variables in the chained form. Through the state variables, it is possible to specify the motion of an object placed on the V-bed carrier whose manipulation point is put on the virtual joint as the movement of the manipulation point and the rotation of the virtual carrier linked with the V-bed carrier around the point. Especially, the virtual revolute joint can be located inside a supporting triangle whose vertices are the three real revolute joints between the first car-like mobile robot, the first carrier, the second carrier and the second car-like mobile robot so that stable transportation is achieved when the manipulation point of the object is its center of mass. The validity of the control method based on the model in which the virtual mechanical elements are introduced is experimentally verified in transportation of an object placed on the V-bed carrier by following an 8th-order Bezier curve path while avoiding collision with an obstacle.