SMALLBug: A 30-mg Crawling Robot Driven by a High-Frequency Flexible SMA Microactuator

Abstract

We present the design, fabrication and experimental testing of SMALLBug, a 30-mg crawling microrobot that is 13 mm in length and can locomote at actuation frequencies of up to 20 Hz. The robot is driven by an electrically-powered 6-mg bending actuator that is composed of a thin shape-memory alloy (SMA) wire and a carbon-fiber piece that acts as a loading leaf-spring. This configuration enables the generation of high-speed thermally-induced phase transformations of the SMA material to produce high-frequency periodic actuation. During development, several actuator prototypes with different mechanical stiffnesses were tested and characterized by measuring their bending motions when excited with pulse-width modulation (PWM) voltages with a variety of frequencies and duty cycles (DCs). In a similar manner, the displacement-force characteristic of the actuator chosen to drive SMALLBug was identified by measuring its bending displacements under a number of different loads ranging from 4.22 to 83.8 mN. The locomotion capabilities of SMALLBug were experimentally tested at three different input actuation frequencies, which were observed to produce three distinct gaits. At the low frequency of 2 Hz, the robot locomotes with a crawling gait similar to that of inchworms; at the moderate frequency of 10 Hz, the robot advances smoothly at an approximately constant speed using a shuffling gait; and at the high frequency of 20 Hz, the robot generates small and fast jumps in a galloping gait, reaching average speeds of up to 17 $\boldsymbol{\text {mm}} \cdot \boldsymbol{\text {s}}^{\boldsymbol{\text {--}}\boldsymbol{\text {1}}}$, equivalent to 1.3 body-lengths per second (BLPS).