Compensating Control of a Flexible Robot Arm

Since the characteristics of flexible robot arm motion is far more complex than that of rigid arm motion due to its link elastic deflections, the flexible arm end-effector positioning problem also becomes more complex. The problem is finally resolved into the following three subproblems: (1) how to suppress the link elastic vibration, (2) how to achieve accurate joint positioning, and (3) how to compensate the end-effector positioning errors due to the link deflections. The problem (1) is being solved by many pieces of work. The problem (2) arises also in the case of rigid arms but, since the joint positioning and link vibration suppressing are coupled, it becomes more complex for the case of flexible arms. The problem (3) is important in order for the arms to perform tasks but no effective method has been presented so far to solve it. This paper presents a hierarchical control system which incorporates organically three control functions: joint positioning, link vibration suppression, and end-effector positioning error compensation. The convergence condition for the compensating control is derived theoretically for the condition of static gravitational loads. The effectiveness of the proposed control system is proved by experiments using a two-link flexible arm. The link deflections are measured by a newly devised and developed sensor consisting of a semiconductor laser and a position sensitive detector (PSD).