Learning Image-Conditioned Dynamics Models for Control of Under-actuated Legged Millirobots

Millirobots are a promising robotic platform for many applications due to their small size and low manufacturing costs. Legged millirobots, in particular, can provide increased mobility in complex environments and improved scaling of obstacles. However, controlling these small, highly dynamic, and underactuated legged systems is difficult. Hand-engineered controllers can sometimes control these legged millirobots, but they have difficulties with dynamic maneuvers and complex terrains. We present an approach for controlling a real-world legged millirobot that is based on learned neural network models. Using less than 17 minutes of data, our method can learn a predictive model of the robot's dynamics that can enable effective gaits to be synthesized on the fly for following user-specified waypoints on a given terrain. Furthermore, by leveraging expressive, high-capacity neural network models, our approach allows for these predictions to be directly conditioned on camera images, endowing the robot with the ability to predict how different terrains might affect its dynamics. This enables sample-efficient and effective learning for locomotion of a dynamic legged millirobot on various terrains, including gravel, turf, carpet, and styrofoam. Experiment videos can be found at https://sites.google.com/view/imageconddy

HIGH-FORCE ELECTROSTATIC INCHWORM MOTORS FOR MILLIROBOTICS APPLICATIONS

Due to scaling laws and ease of fabrication, electrostatic actuation offers a promising opportunity for actuation in small-scale robotics. This dissertation presents several novel actuator and motor designs as well as new techniques by which to characterize electrostatic gap closing actuators. A new motor architecture that uses in-plane electrostatic gap-closing actuators along with a flexible driving arm mechanism to improve motor force density is introduced, optimized, manufactured, and tested. This motor operates similarly to other inchworm-based microactuators by accumulating small displacements from the actuators into much larger displacements in the motor. Using an analytical model of the inchworm motor based on the static force equilibrium condition, optimizations of a full motor design were performed to maximize motor force density. In addition, force losses from supporting flexures were included to calculate the theoretical motor efficiency for different motor designs. This force density optimization analysis of the gap-closing actuators and supporting motor structures provided the basis for designing and manufacturing inchworm motors with flexible driving arms and gap-closing actuators. The motor required only a single-mask fabrication and demonstrated robust performance, a maximum speed of 4.8mm/s , and a maximum force on the shuttle of 1.88mN at 110V which corresponds to area force density of 1.38mN/mm2. In addition, instead of estimating motor force based on drawn or measured dimensions which often overestimates force, the demonstrated maximum motor force was measured using calibrated springs. The efficiency of the manufactured motor was measured at 8.75% using capacitance measurements and useful work output. To further increase force output from these motors, several new designs were proposed, analyzed, and tested. Thick film actuators that take advantage of a through-wafer etch offered a promising opportunity to increase force given the linear increase in force with actuator thickness. However, fabrication challenges made this particular approach inoperable with current manufacturing capabilities. New actuator designs with compliant and zipping electrodes did demonstrate significant increases in force, but not the order of magnitude increase promised by modeling and analysis. In order to study and understand this discrepancy, several new techniques were developed to electrically and electromechanically characterize the force output of these new actuator designs. The first technique identifies parameters in an equivalent circuit model of the actuator, including actuator capacitance. By monitoring change in capacitance along the travel range of the motor, electrostatic force in equilibrium can be estimated. Charge transferred to and from the actuator can also provide an estimate of actuator efficiency. The second technique uses a constant rate spike to more thoroughly explore the rapid dynamics of actuator pull-in and zipping. New characterization methods allowed for collecting large amounts of data describing performance of motors with zipping and compliant electrodes. The data was used to back up the main hypothesis of force output discrepancy between theory and practice. Also, it was used to highlight extreme sensitivity of proposed motors toward manufacturing process and its tolerances