Analysis of a transmission-based robotic servoactuator concept

This research was a pioneering effort toward the investigation of incorporating multi-speed transmissions into electric actuation devices for robotic and industrial applications. This new idea has been conceptually developed from first principles using an application-based approach to parametric study. First, the full gamut of literature, both from the academic and research domains, was exhaustively surveyed. The design space criteria of an existing robot manipulator design was revisited for the purpose of studying feasibility for the transmission-based actuator (TBA) concept. This initial study verified that with conservative assumptions concerning gross transmission parameters, size, and weight, distributed actuation is possible for a much wider class of manipulator mechanisms than is currently available. Multi-speed transmissions inherently allow smaller, high power motors to also deliver high torque at low speeds, effectively allowing the overall actuator power density to be increased to a level more on par with electrohydraulics.

This work also emphasized a methodology for developing logical, optimized, shifting algorithms for both continuously variable transmission (CVT) and discrete speed transmission (DST) concepts. The resulting algorithms were fully defined and presented in closed form. When optimized with respect to the acceleration delivered to a load, all motor-transmission combinations (both CVT and DST) required shifting profiles that were definitively related to the maximum power operating point of the driver motor. It was found that, for maximum acceleration, the transmission should manipulate mechanical advantage in terms of torque and speed capacity such that the motor can achieve and maintain an operating point within a close vicinity of maximum power. During shifting, the CVT-based actuators proved to be capable of placing the motor at exactly the maximum power point in minimum time; the DST-based actuators, having only a finite number of gear ratios available, operate within a "speed bandwidth" around the maximum power point defined by the discrete shifting points.

The optimal shifting profiles (for either CVT's or DST's) proved to form a logical basis for evaluating motor potential for a given application in that the associated algorithms clearly define an upper bound on the performance potential for a given motor with any transmission system. The optimization results are shown to provide concise and logical methods for selecting motor-transmission combinations and designing transmission parameters to meet a required actuator payload/response requirement.

Fundamental mechanical design concepts for multi-speed transmissions are discussed and expanded upon in detail. The next steps must address the feasibility of mechanical implementation and effective servo-control of TBA's. These investigations should include a prototype testbed, further examination into low-level actuator control, quantitative parametric studies, and further development and verification of the transmission shifting algorithms.