Spring-Brake! Handed Shearing Auxetics Improve Efficiency of Hopping and Standing

TL;DR

Legged robots suffer from high energy costs due to motor Joule heating and static power consumption. The authors introduce Handed Shearing Auxetics (HSA) as a parallel elastic actuator that provides both compliant energy storage and a jam-based braking function, enabling low-power operation in hopping and standing. The study demonstrates that a 3D-printed HSA in parallel with a low-reduction motor reduces hopping energy costs by roughly 24–32% and provides substantial static braking power savings, with cost-of-transport comparable to state-of-the-art compliant hoppers like SPEAR. This multi-functional, compact metamaterial-based approach offers practical benefits for energy-efficient legged robots, while highlighting trade-offs in energy dissipation and suggesting paths for improvement through material and modelling enhancements.

Abstract

Energy efficiency is critical to the success of legged robotics. Efficiency is lost through wasted energy during locomotion and standing. Including elastic elements has been shown to reduce movement costs, while including breaks can reduce standing costs. However, adding separate elements for each increases the mass and complexity of a leg, reducing overall system performance. Here we present a novel compliant mechanism using a Handed Shearing Auxetic (HSA) that acts as a spring and break in a monopod hopping robot. The HSA acts as a parallel elastic actuator, reducing electrical power for dynamic hopping and matching the efficiency of state-of-the-art compliant hoppers. The HSA\u2019s auxetic behavior enables dual functionality. During static tasks, it locks under large forces with minimal input power by blocking deformation, creating high friction similar to a capstan mechanism. This allows the leg to support heavy loads without motor torque, addressing thermal inefficiency. The multi-functional design enhances both dynamic and static performance, offering a versatile solution for robotic applications.

Figures (5)

• Figure 1: System Overview. We use Handed Shearing Auxetics (HSA) as a combined Spring and Break mechanism in a hopping leg to improve system efficiency in hopping and standing.
• Figure 2: HSA average stiffness over five-centimeters stroke is shown for different twist angles. Data was collected on an instron UTM. For large twist angles (red region), the HSA jams against the inner cylinder, dramatically increasing its stiffness.
• Figure 3: Exploded view of the HSA assembly, illustrating the twisting mechanism at its base. The HSA is fixed at its right end to a gear carrier, which is driven by a compact servomotor ("Twist Motor"). The left end is coupled to the foot via a 30 cm steel rod passing through a linear bearing embedded in the base. A shaft collar and thrust bearings secure the gear assembly to the cart (not shown). Inside the HSA, a rigid cylindrical insert (occluded in orange) enables the braking mechanism by limiting auxetic deformation.
• Figure 4: Comparison of the electrical power drawn vs. the force blocked by the leg motor compared to the HSA twist motor. The power consumed by the primary motor (squares) is fit by a quadratic function of the blocked force (purple dotted line). The power consumed by the HSA twist motor (diamonds) follows a linear trend (gold dashed line). The solid gold line predicts the power that would be consumed by the twist servo at 1:8 gear reduction.
• Figure 5: Part (a): Hopping cost-of-transport with and without an HSA compared to results from the SPEAR hopping robot. Part (b): Changes to the mechanical and thermal components of COT (with an HSA minus without an HSA), shown in absolute value, i.e. the blue curve is the decrease in the thermal cost, the red curve is the increase in mechanical cost. The reduced hopping COT in (a) is due to reduced thermal cost, while the mechanical cost increases, suggesting energy is dissipated by the HSA.