The Practical Implementation of Time-optimal Control for Robotic Manipulators

This paper presents experimental results for time-optimal control of robotic manipulators along specified paths. The implementation of time-optimal control represents several unique problems: (1) the control is generally discontinuous (bang-bang), (2) actuator dynamics are usually ignored in order to reduce the system order, and (3) the optimal control leaves no control authority to compensate for tracking errors caused by unmodeled dynamic and the delays introduced by the on-line feedback controller. To overcome these difficulties, we compensate for motor dynamics using a simplified friction model, and account for the dynamics of the feedback controller using trajectory preshaping. Implemented for the UCLA Direct Drive Arm, this is shown to drastically reduce the tracking errors compared to the errors obtained with no preshaping and no compensation for motor dynamics. The experimental results demonstrate the merit of time optimal control for reducing motion time as well as for increasing tracking accuracy.