Development of Millimeter Scale Motors for Miniature Direct Drive Robots

Abstract

The twentieth century marked a period of rapid expansion of technology associated with miniaturization of engineering systems. A recent theme in this trend is the development of miniature, distributed robots that mimic insect behavior and locomotion. This research addresses the need for millimeter-scale, direct drive, high force/torque motors to support these platforms. Among the technologies currently available, scalable motors based on piezoelectric transducers are the focus. The specific contributions of this work are as follows. (1.) The design, analysis, and characterization of a macro-scale linear piezomotor constructed with a parallel arrangement of stressed unimorph piezoelectric transducers are presented. The prototype demonstrates a novel application of passive mechanical latches to produce inchworm motion while eliminating the need for multiple control signals. (2.) A dimensional analysis is conducted to reveal scale factors that govern the relationship between stressed unimorph performance parameters and size. The results support a millimeter-scale version of the linear piezomotor that incorporates transducers with alternative annular geometry for improved stiffness. (3.) The development of a miniature mode conversion rotary ultrasonic motor based on a piezoelectric stack transducer is reported. Results of a dynamic analysis lead to scale factors for static torque and rotor velocity. Lastly, the linear and rotary piezomotor systems are compared in the context of scalability to determine the most effective system for miniature direct drive robotics.

Blocked force performance of the miniature linear piezomotor was limited to 0.25 N by back slip in the passive latches, and transducer displacement losses leading to greater compliance in the assembly. Since displacement of the annular stressed unimorph transducer decreases with the square of the outside radius, precision engineering is required to avoid these losses. The rotary ultrasonic motor proved to be a more effective choice for driving the robotic locomotion system. Dimensional analysis results indicate that static torque scales with the square of the rotor contact radius. Using alternative designs, a static torque density of 0.37 Nm/kg was measured in the prototype.