Gait studies for a quadrupedal microrobot reveal contrasting running templates in two frequency regimes

Benjamin Goldberg, Neel Doshi, Kaushik Jayaram, Robert J Wood
2017 Bioinspiration & Biomimetics  
Performance metrics such as speed, cost of transport, and stability are the driving factors behind gait selection in legged locomotion. To help understand the effect of gait on the performance and dynamics of small-scale ambulation, we explore four quadrupedal gaits over a wide range of stride frequencies on a 1.43 g, biologicallyinspired microrobot, the Harvard Ambulatory MicroRobot (HAMR). Despite its small size, HAMR can precisely control leg frequency, phasing, and trajectory, making it an
more » ... xceptional platform for gait studies at scales relevant to insect locomotion. The natural frequencies of the body dynamics are used to identify frequency regimes where the choice of gait has varying influence on speed and cost of transport (CoT). To further quantify these effects, two new metrics, ineffective stance and stride correlation, are leveraged to capture effects of foot slippage and observed footfall patterns on locomotion performance. At stride frequencies near body resonant modes, gait is found to drastically alter speed and CoT. When running well above these stride frequencies we find a gait-agnostic shift towards energy characteristics that support "kinematic running", which is defined as a gait with a Froude number greater than one with energy profiles more similar to walking than running. This kinematic running is rapid (8.5 body lengths per second), efficient (CoT=9.4), different from widely observed SLIP templates of running, and has the potential to simplify design and control for insectscale runners. 1 as with wood ants described by Reinhardt et al. Furthermore, some of the fastest legged locomotion relative to body size occurs at scales smaller than those previously studied. For example erythracarid mites (body length of 1 mm) can run at speeds up to 192 body lengths per second (BL s −1 ) (Rubin et al., 2016) . So far, few explanations for these speeds exist, in no small part due to the challenges in measuring the underlying mechanics at this scale. Given the nascent phase of designing small scale, bio-inspired robots to approach these remarkable speeds observed in biology, research is typically focused on the manufacturing aspects, for example the flexure-based approaches overviewed in , and less on performance characterization . Exemplary flexure-based mobile microrobots that implement new design and manufacturing processes include robots that can walk (Hoover et al.,
doi:10.1088/1748-3190/aa71dd pmid:28485300 fatcat:zo7hiuwl6zaslmpn2on73b4yx4