Power Assist Unit with Variable Assist Gain using Add-On End-Effector

Abstract

This paper proposes a power assist unit using an add-on effector so as to easily construct a power assist system. In the proposed system, the assisting force of the actuator is transmitted to the environment via a pair of assist springs. Thus, the desired assisting force can be generated by controlling the spring displacement in accordance with the operator-applying force. The controller is designed taking into consideration explicitly both the human and environmental mechanical impedances in order to analyze the stability for the parameter variations and to make the system more robust against those perturbations. The system also allows the operator to adjust the amplitude of assisting ratio, i.e., the assist gain, online. This function could improve usability and widen its application field such as rehabilitation and welfare equipment. Performance of a prototype of the proposed system was evaluated by experiments in both time and frequency domain for different back drivability conditions. Experimental results showed that the prototype has robustly worked against perturbations of the environment, keeping the power assist control bandwidth approximately 1 to 3 Hz, and the validity of the variable assist gain control has also confirmed.