Two leading qualities of skeletal muscle that produce good performance in uncertain environments are damage tolerance and the ability to modulate impedance. For this reason, robotics researchers are greatly interested in discovering the key characteristics of muscles that give them these properties and replicating them in actuators for robotic devices. This paper describes a method to harness the redundancy present in muscle-like actuation systems composed of multiple motor units and shows that they have these same two qualities. By carefully choosing which motor units are recruited, the impedance viewed from the environment can be modulated while maintaining the same overall activation level. The degree to which the impedance can be controlled varies with total activation level and actuator length.
Discretizing the actuation effort into multiple parts that work together, inspired by the way muscle fibers work in the human body, produces damage-tolerant behavior. This paper shows that this not only produces reasonably good resolutions without inordinate numbers of units, but gives the control system the ability to set the impedance along with the drive effort to the load.