The present paper investigates the effect of compliance on the locomotion of a biologically-inspired soft-body pedundulatory robotic system, employing lateral undulations of its elongated body, which are augmented by the oscillation of sets of lateral appendages (parapodia), to propel itself on unstructured granular substrates. We explore control strategy alternatives for the robot to generate two different locomotor gaits by employing direct or retrograde lateral body waves, combined with appropriately coordinated parapodial motion (pedundulatory modes). Computational models of this class of robots have been devised, which demonstrate the effects of joint compliance on gait generation and on the characteristics of robot propulsion. A new three-segment soft-body robotic prototype has been developed, whose body was fabricated by molding polyurethane elastomers, and was tested extensively on an experimental sandbox, on various formations of the granular substrate, to compare the performance of stiff and compliant joints. Body and joint compliance were found to enhance the adaptability of the robot to environmental irregularities, however they may deteriorate the proper formation of the un-dulatory body wave, degrading somewhat system performance in terms of the attained velocities.
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