I3SA: The Increased Step Size Stability Assessment Benchmark and its Application to the Humanoid Robot REEM-C

The implementation of stable locomotion on humanoid robots is a difficult task. This is complicated by the fact that there is no uniform method for analyzing a robot and its control architecture and for calculating indicators to quantify performance of flat ground walking. Moreover, there is no widely accepted indicator do distinct between a stable and unstable state of the robot. We propose the Increased Step Size Stability Assessment (I3SA) as a testing protocol and standardized procedure for data collection and evaluation of stability for locomotion on flat terrain. We apply this test to the humanoid robot REEM-C. The biped must cover a set distance of four meters with predefined step sizes. The initial step size is defined as 20% of the total leg length of the robot. After three successful trials, the step size is continuously increased until REEM-C's last successful trial at 40% of its total leg length, leading to REEM-C’s I3SA rating of 40. The recorded data are evaluated using metrics known from the literature, such as the capture point, foot placement estimator, and the angular momentum acting at the center of mass. We illustrate the experimental setup, data collection and processing, the calculation of performance indicators for several step sizes and the trial which resulted in a fall of the robot. The trend towards decreasing stability with increased step size and the available key assumptions are reported.