Fundamental control concepts for implementation of transmission-based actuators in robotics and automation

This paper is a continued investigation of a new concept for extending the payload-power range of electrical servoactuators. The fundamental idea is to combine small, high power motors with multispeed or continuously variable transmissions such that the effective torque-speed characteristics of the motor are “spread” across a wider output speed range. This has the effect of allowing smaller, high speed motors to also deliver high torque at low speeds. Use of a multispeed transmission allows motor size to be reduced while increasing overall actuator power density in the process. This power density increase can be used to allow a much greater range of electrically-actuated robots to be implemented with distributed actuators and can be used to drastically reduce overall manipulator complexity. This paper discusses the transmission-based actuator concept from a low-level control standpoint, and demonstrates that the “shifting action” of the transmission occurs in a way that is minimally obtrusive to the overall joint servo-control. It is shown that the gear-ratio only changes in situations where high acceleration is desired; the shifting of the transmission happens early in the move, occurs very rapidly, and settles to a final, fixed gear ratio very quickly. For the majority of the move, control objectives are essentially identical to those for available actuators. These results encourage further development of the concept for both robotics and automation applications