Unidirectional Compliance to Improve Natural Dynamics Modification for Energy Efficiency

Parallel compliances can be very efficient in modifying robots' natural dynamics provided that, compliance force profiles match the required joint forces. To contribute to this area, firstly by presenting the optimality condition of a parallel compliance for energy efficiency, we show that a conventional compliance does not fulfill this condition in general because its force profile is invariant with respect to its movement direction. To overcome this, we secondly introduce the concept of "unidirectional compliance" which produces torque in only one movement direction of a reciprocating joint. Furthermore, we propose a method to implement a desired unidirectional compliance profile. Thirdly, we theoretically show that there exists a combination of unidirectional and conventional compliances to produce two arbitrary torque profiles for moving back and forth in a reciprocating task. Fourthly, we propose a framework to find and design a suitable combination of unidirectional and conventional compliances in mono- and bi-articular configurations for increasing the energy efficiency of an n-DoF serial manipulator. Also, theoretical supports for the presented method are developed. Finally, we demonstrate the applicability of the proposed approach as well as its efficacy in energy consumption reduction in a set of simulations and experiments.