Hybrid drive parallel arm and its motion control

A hybrid cylinder/cable drive parallel arm and its motion control are discussed. The basic idea of the hybrid parallel arm is to make the best use of the mixture of the high power cable drive and the cylinder actuation to obtain a heavy-duty manipulator in construction and ship building applications. A prototype arm has been designed and built based on the analysis and the comparative study of several types of hybrid drive mechanisms, the Stewart platform, and a wire drive mechanism. The tension of wire has to be maintained properly in the hybrid mechanism during its motion. The proposed hybrid arm is a kind of actuation-redundant mechanism and its internal force can be freely controlled. The proposed control algorithm is to give wires proper tension during its motion by exploiting the internal force, The required tension and cylinder force can be obtained by compensating each link position command based on its compliance. The repeatability tests show the effectiveness of the proposed algorithm in maintaining the overall stiffness of the arm.