Evaluation of Force Feedforward Control for Actuators with Limited Dynamics and Time Delay

Abstract

This paper investigates the effects of time delay and actuator dynamics on the stability and effectiveness of a force feedforward controller for an impedance controlled system, and the applicability of this control strategy to support astronauts during manipulation tasks against the resistance of the gloves of a space suit by means of a hand exoskeleton. To this end, two simulation studies were conducted. To obtain general results, stability was investigated first on the basis of a simple linear system. This first study revealed a highly detrimental effect of time delay on the critical gain factor of the force feedforward controller. Slower actuator dynamics, however, led for the most part to higher stable gain factors. The second simulation study analyzed the applicability of the control approach on an impedance controlled hand exoskeleton. It was shown that the feedforward controller clearly decreases the effort required by the human to move along predefined minimum-jerk trajectories.