Rapid and Agile Locomotion with Power-dense Millirobots

Author(s): Haldane, Duncan | Advisor(s): Fearing, Ronald S; Agogino, Alice M | Abstract: The development of legged robots can serve two purposes. The first is to enable more mobility for robotic platforms and allow them greater flexibility for moving through complex real-world environments. The second is that the legged robot is a scientific tool. It can be used to design new experiments that drive insights both for the development of new robotic platforms and the characteristic of animal locomotors from which they are inspired. This work presents a design methodology that targets the creation of extreme robotic locomotors. These are robots that outperform all others at a particular task. They are used to study locomotion at the edge of the current performance envelope for robotic systems. The design methodology focuses on maximizing the power-density of the platform. We apply it to create first a rapid running robot, the X2-VelociRoACH, and two versions of a jumping robot, Salto and Salto-1P. In all of these robots, we centralize the actuation such that one actuator provides all the power for the energetic locomotory tasks. A kinematic coupling is designed for each platform, such that the correct behavior (running or jumping) happens by default when the energetic actuator is driven open-loop. The design methodology successfully created two robots at the edge of their respective performance envelopes.The X2-VelociRoACH is a 54 gram experimental legged robot developed with this methodology that was developed to test hypotheses about running with unnaturally high stride frequencies. It is capable of running at stride frequencies up to 45 Hz, and velocities up to 4.9 m/s, making it the fastest legged robot relative to size. The top speed of the robot was limited by structural failure. High-frequency running experiments with the robot shows that the power required to cycle its running appendages increase cubically with the stride rate. Our findings show that although it is possible to further increase the maximum velocity of a legged robot with the simple strategy of increasing stride frequency, considerations must be made for the energetic demands of high stride rates. For the development of the jumping robot Salto, we first devise the vertical jumping agility metric to identify a model animal system for inspiration. We found the most agile animals outperform the most agile robots by a factor of two. The animal with the highest vertical jumping agility, the galago (Galago senegalensis), is known to use a power-modulating strategy to obtain higher peak power than that of muscle alone. Few previous robots have used series-elastic power modulation (achieved by combining series-elastic actuation with variable mechanical advantage), and because of motor power limits, the best current robot has a vertical jumping agility of only 55% of a galago. Through use of a specialized leg mechanism designed to enhance power modulation, we constructed a jumping robot that achieved 78% of the vertical jumping agility of a galago. The leg mechanism also has constraints which assure rotation-free jumping motion by default. Agile robots can explore venues of locomotion that were not previously attainable. We demonstrate this with a wall jump, where the robot leaps from the floor to a wall and then springs off the wall to reach a net height that is greater than that accessible by a single jump. Our results show that series-elastic power modulation is an actuation strategy that enables a clade of vertically agile robots.We extend the work with Salto to see how the locomotory capacity of an extreme robotic locomotor can be extended without compromising the power density of the platform. Salto-1P uses aerodynamic thrusters and an inertial tail to control its attitude in the air. A linearized Raibert step controller was sufficient to enable unconstrained in-place hopping and forwards-backwards locomotion with external position feedback. We present studies of extreme jumping locomotion in which the robot spends just 7.7% of its time on the ground, experiencing accelerations of 14 times earth gravity in its stance phase. An experimentally collected dataset of 772 observed jumps was used to establish the range of achievable horizontal and vertical impulses for Salto-1P.