Design of Redundant Drive Joints with Double Actuation using springs in the second actuator to avoid excessive active torques

This paper discusses the design of a Redundant Drive Joint with Double Actuation (RDJ-DA) to produce controlled compliant motions over a higher bandwidth. First, our strategies on mechanical and controller designs to produce compliant motions are described, and the basic structure and impedance control of the RDJ-DA with internal serial structure explained. The standard form of the basic structure is introduced using a set of parameters which can be used to express the inertia property of RDJ-DA. Second, a problem statement for the design of RDJ-DAs is described after pointing out that required torques of the second actuator of the RDJ-DA could be large. Then, a basic idea of introducing springs into the second actuator in parallel is proposed as a part of the structural design of RDJ-DA to reduce the large torques. Simulations are conducted to find out a set of the design parameters of RDJ-DA which obtains a higher natural frequency of the output joint admittance satisfying both the desired output joint admittance and the limitations of two identical motors on their performance. Finally, a prototype design of RDJ-DA using the obtained set of the design parameters is presented.