Development and evaluation of add-on end-effector for linear power assist unit with variable assist gain

This paper proposes a linear power assist unit to be simply designed by using an add-on end-effector and gives a controller design for the unit explicitly taking into consideration mechanical impedances of both a human operator and environment. The proposed end-effector transmits assisting force to the environment via a pair of assisting springs, and the displacement of the assisting springs is controlled to generate a desirable power assisting according to the force applied by the operator. In the proposed system, the assist gain, i.e., the ratio of the sum of the human-applied force and the assisting force to the human-applied force, can be online changed so as to easily adjust a stress loading, and it could be helpful in rehabilitations to tune the load depending on the degree of recovery. A prototype of the proposed power assist unit has been developed and was evaluated from viewpoints of both the time and frequency domains. Experimental results showed that the prototype has robustly worked well against perturbations of the environment, keeping the power assist control bandwidth about 1 to 3 Hz, and the validity of the variable assist gain control has also confirmed.