Tasks in munitions response such as remote surveys, retrieval, and disabling of potentially hazardous unexploded ordnance (UXO) require traversal of amphibious terrain. Surf zones, areas of the ocean with shallow water, are a particularly critical area. Accessing UXO in these areas may be prioritized to prevent them from reaching land. Mobile robots have the potential to make these tasks safer and more efficient. However amphibious locomotion is challenging in surf zones. The design space is limited by sinking in sand at high body weights, and disruptions from waves at low body weights.

The main objective of this project was to determine the degree to which crab-like legs increase the force required to dislodge the robot, effectively increasing weight. This required developing amphibious robot platforms with crab-like legs, and a lab wave tank test set-up. In addition to characterizing static gripping behavior, it was important to validate that the gripping legs could also locomote on natural terrain in both controlled lab substrates and in local beaches. The project team also shows preliminary sensor integration. They can build on the leg designs in future work to determine the simplest effective legs and create a more developed platform to compare trafficability with wheeled robots in surf zone terrains.

Inspired by biological crabs, the project team built and modified legged robots (mass 1kg to 4kg). In controlled lab tests, they compared pointed, crab-like dactyls with traditional rounded robot feet and compared effects of gripping the ground by moving legs inward from stance position. They measured maximum vertical grip forces with a digital force gauge and a constant speed winch. With visual tracking, they measured displacement due to waves generated with a hydraulic piston. They validated the robot outdoors in freshwater lake waves. 

The project team showed that by using a curved sharp dactyl and pulling the feet inward, the effective weight can be increased by a third in wet submerged sand. The sharp dactyls can eliminate travel from waves that would otherwise displace the robot several centimeters and reduce travel due to larger waves. The dactyls can walk consistently on rocks and sand, but the speed is less than with original rounded feet (likely due to slip on hard surfaces and sinkage on sand). In outdoor tests, they demonstrated walking into waves that crash over the body, and walking submerged at depths of approximately 1m. They also showed that sensors can be integrated into the feet to collect ground contact data which consistently responds to load distributions.

This work expands the design space for future amphibious legged robots with reduced weight. This is expected to help amphibious robots better navigate waves, handle increased payload and deploy from aerial vehicles. Selectively grasping ground could also enable ultra-efficient gaits in which robots exploit wave forces. Applications of such robots could include gathering samples along waterline, accessing sites with hazardous materials, and potentially using additional legs to move or disable individual pieces of UXO.

Crab robot concepts for surf zones (left) and crab robot platform with pointed dactyls during amphibious testing (right).