A Compliant Leg Structure for Terrestrial and Aquatic Walking Robots

The legs and transmissions of walking robots need to fulfill dynamic and load-bearing movements while also enabling more applications by being resistant to challenging outdoor- and aquatic environments. Furthermore, the physical behaviour of the robot legs needs to be predictable throughout their service life to enable torque-dependent control. Therefore, weathering and wear of leg components must not alter the movement resistance in order to avoid frequent maintenance. Hence, we present a new leg design that obtains low inertia and fluid resistance by using four-bar linkages with compliant mechanisms of either stainless spring steel or super-elastic nickel-titanium. The leg mechanisms were tested to formulise their torque behaviour, axial deflections, and fatigue life in saltwater (≈ 12% salinity). Conventional sealed stainless steel ball bearings were also tested to provide data references. Our experiments show that nickel-titanium outperform stainless spring steel with more predictable behaviour, less resistance, and longer fatigue life.