Earthworm-inspired robotic locomotion in sand: an experimental study with X-ray tomography

Self-motile probes are an emerging technology that will enable multidirectional in situ testing of soils with methodologies currently unavailable. Several of these technologies use bio-inspiration from annelids to develop the locomotion systems for these devices. In this study, the soil response to the anchor–push mechanism employed by earthworms is investigated utilizing robotic analogues and X-ray microtomography. The robotic analogue consists of two soft actuators capable of anchoring and penetration. The device is buried in Hostun sand and scanned using a tomograph before and after actuation. The displacement field is obtained using three-dimensional digital image correlation. The preliminary experimental programme reported herein evaluates the effects of relative density, overburden stress, tip shape and different actuator designs on the observed soil response. The device was able to reproduce the anchor–push mechanics and advance into the specimen. In dense samples, anchoring by inflating a chamber in the soil causes a kinematic block to form that significantly reduces anchor capacity. In looser samples, anchoring effects are more localised and provide sufficient resistance to backsliding. Additionally, it is observed that a robust design of the pusher chamber is essential to sustaining actuation pressures and achieving high advancement rates into the soil.