Comparison of the Discrete Gait Control and Phase-Difference Control for Earthworm-like Locomotion Robots

From the perspectives of the locomotion characteristics, the steady-state average locomotion speed, and the wave properties, a comparative study between the discrete phase control and the phase-difference control of the earthworm-like locomotion robots is carried out. Firstly, an earthworm-like robot segment which can deform antagonistically is designed and manufactured based on a composite structure of servomotor and spring-steel belts by learning the morphology characteristics of the earthworm. By connecting 8 segments in series, an 8-segment earthworm-like locomotion robot is developed. Taking the developed robot as a platform, the locomotion characteristics and the average locomotion speed corresponding to the discrete gait control and the phase-difference control are examined both theoretically and experimentally. The results reveal that the experimentally obtained average locomotion speeds qualitatively agree with the theoretical predictions for both control strategies. However, some robot segments may slip backward obviously during locomotion such that, quantitatively, the experimental average speeds are lower than the theoretical ones. Then the two control strategies are compared in terms of wave propagation. Both control strategies can propagate the deformations of the robot segments along the robot body in the form of a wave, with the direction being opposite to the locomotion, which agrees with the real earthworm's retrograde peristalsis wave. For the discrete gait control, the waveform, the wave speed, the wavelength, and the period closely relate to the gait parameters. For the phase-difference control, the waveform and the period are determined by the actuation law and cannot be tailored via the phase difference, while the wavelength and the wave speed can be tuned by the phase difference. In terms of the control effect, the optimum identical phase-difference (IPD) control shows some advantages, because the robot can output a high average locomotion speed and is consistent with the earthworm's continuity feature. © 2020, Science Press. All right reserved.