Knuckles that buckle: compliant underactuated limbs with joint hysteresis enable minimalist terrestrial robots

Underactuated designs of robot limbs can enable these systems to passively adapt their joint configuration in response to external forces. Passive adaptation and reconfiguration can be extremely beneficial in situations where manipulation or locomotion with complex substrates is required. A common design for underactuated systems often involves a single tendon that actuates multiple rotational joints, each with a torsional elastic spring resisting bending. However, a challenge of using those joints for legged locomotion is that limbs typically need to follow a cyclical trajectory so that feet can alternately be engaged in stance and swing phases. Such trajectories present challenges for linearly elastic underactuated limbs. In this paper, we present a new method of underactuated limb design which incorporates hysteretic joints that change their torque response during loading and unloading. A double-jointed underactuated limb with both linear and hysteretic joints can thus be tuned to create a variety of looped trajectories. We fabricate these joints inside a flexible legged robot using a modified laminate based 3D printing method, and the result shows that with passive compliance and a mechanically determined joint sequence, a 2-legged minimalist robot can successfully walk through a confined channel over uneven substrates.