Human-centered robotics draws growing interest in utilizing pneumatic artificial muscles (PAMs) for robots to cooperate with humans. In order to address the limited control performance which prevents PAMs from being more widely used, a hybrid actuation scheme has been proposed to combine PAMs and a low inertia DC motor, and presented significantly improved control performance without loss of robot safety. While the DC motor provides high precision and reliability, the small motor has, however, difficulties in dealing with the large stored energies of the PAMs, especially in the events of PAMs failure and large initial load changes. In order to further ensure robot safety, we developed a new hybrid actuation scheme with PAMs (macro) and a particle brake (mini), which provides high torque-to-weight ratio and inherent stability. We then conducted comparative studies between hybrid actuations with (1) a DC motor and (2) a brake in terms of robot safety and performance. Experimental comparisons show that the hybrid actuation with PAMs and a brake provides higher energy efficiency for control bandwidths under 2 Hz, and is capable of effectively reducing large impacts due to the brake's high torque capacity and passive energy dissipation. These comparative studies provide insight that the hybrid actuation with PAMs and a brake can be a competitive solution for the applications that require high efficiency, but accept a relatively low control performance, for example, a waist joint.
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