Motion control of a tendon-based parallel manipulator using optimal tension distribution

This paper presents the motion control of a six degree-of-freedom tendon-based parallel manipulator, which moves a platform with high speed using seven cables. To control the motion of the platform along desired trajectories in space, nonlinear feedforward control laws in the cable length coordinates are used. Taking account of the effect of redundancy on actuation, the optimal tension distribution should be considered to the advantage of the control laws. Using a method based on the analysis of the workspace condition, tension constraints and limiting torque constraints of actuators, an analytical solution for optimum tension distribution was found and used to compute the force in each cable for compensation of dynamic errors. It is experimentally demonstrated that the proposed control laws reduce the energy consumption of the actuators and satisfy the path tracking accuracy.

I. Introduction

Tendon-Based parallel manipulators using cables (tendons) instead of rigid links are able to generate higher accelerations and higher speeds than parallel manipulators with rigid links, for the mass of the moving parts is extremely low in comparison to a manipulator with rigid links. Further merits of this manipulator are that it is relatively light and has a good flexibility and maneuverability. It can be used for applications such as shipbuilding, positioning telescopes, and camera systems for stadiums where robot arms with heavy rigid links are not appropriate and accuracy is not so important. Another field of application is manipulators for high-speed assembly of lightweight objects such as semiconductors [8].