Research: Series Elastic Actuators

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Actuator characteristics are currently a major limitation in the design of biomimetic robots. Electromagnetic actuators commonly used in legged robots and manipulators are dissimilar to their biological counterpart, muscle. Muscle is able to deliver high torque at low speeds whereas conventional electromagnetic actuators are only able to achieve high torque by spinning at high speeds and using gear-reduction to trade speed for torque, which severely degrades the quality of control. Muscle also yields a high torque per mass, referred to as torque-density. While brushless electromagnetic actuators are capable of delivering high torque at low speeds, their low torque-densities make them too massive for use in the legs or arms of robots. Piezoelectric motors are capable supplying high torque at low speeds without gearing and have a torque density nearly as high as muscle. Heretofore, piezoelectric motors have not been employed as actuators in series elastic actuators, the robotic equivalent of muscle-tendon systems. This research compares the quality of torque control and the torque-speed characteristics of series elastic actuators using piezoelectric versus conventional electromagnetic actuation. Both types of motors were tested in a rotary series elastic actuator which consisted of a torsional spring in series with a torque transducer between the motor and load. The torque transducer provided feedback to a closed-loop control system that used proportional control in order to achieve a prescribed time-varying torque. The two systems were challenged with sinusoidal torque patterns of variable amplitude and frequency and the dynamic results were compared in absolute as well as dimensionless terms established for series elastic actuators.