BirdBot achieves energy-efficient gait with minimal control using avian-inspired leg clutching

Abstract

Designers of legged robots are challenged with creating mechanisms that allow energy-efficient locomotion with robust and minimalistic control. Sources of high energy costs in legged robots include the rapid loading and high forces required to support the robot’s mass during stance and the rapid cycling of the leg’s state between stance and swing phases. Here, we demonstrate an avian-inspired robot leg design, BirdBot, that challenges the reliance on rapid feedback control for joint coordination and replaces active control with intrinsic, mechanical coupling, reminiscent of a self-engaging and disengaging clutch. A spring tendon network rapidly switches the leg’s slack segments into a loadable state at touchdown, distributes load among joints, enables rapid disengagement at toe-off through elastically stored energy, and coordinates swing leg flexion. A bistable joint mediates the spring tendon network’s disengagement at the end of stance, powered by stance phase leg angle progression. We show reduced knee-flexing torque to a 10th of what is required for a nonclutching, parallel-elastic leg design with the same kinematics, whereas spring-based compliance extends the leg in stance phase. These mechanisms enable bipedal locomotion with four robot actuators under feedforward control, with high energy efficiency. The robot offers a physical model demonstration of an avian-inspired, multiarticular elastic coupling mechanism that can achieve self-stable, robust, and economic legged locomotion with simple control and no sensory feedback. The proposed design is scalable, allowing the design of large legged robots. BirdBot demonstrates a mechanism for self-engaging and disengaging parallel elastic legs that are contact-triggered by the foot’s own lever-arm action.

Gastrocnemius and Power Amplifier Soleus Spring-Tendons Achieve Fast Human-like Walking in a Bipedal Robot

“…The CoT values of the GAS + SOL, SOL and GAS configurations were 55 %, 50 % and 119 % of the approximated CoT of a natural walker with the same weight as these configurations (calculation method in [15]). Many factors could influence the CoT, a freely walking robot on natural terrain and with trunk control would probably consume more power.…”

“…A simulation study showed that combining mono-and bi-articular foot prosthesis actuation reduces peak power requirements [14]. BirdBot's springtendon network improved cost of transport, and enabled robust engagement and disengagement of the leg's parallel elasticity [15]. So far, the exact role and function of plantarflexor spring-tendons as part of the swing leg catapult during walking has not been studied in robots.…”

Learning-Based Design and Control for Quadrupedal Robots With Parallel-Elastic Actuators

“…In a lab experiment with simulated moon gravity, PEAs reduced the energy required for a jump by a factor of two on this robot [10]. Another, more recent example of using PEA is BirdBot [11], which has a parallel elastic spring clutching mechanism, spanning multiple joints. The avian-inspired leg design shows self-stable and robust bipedal locomotion while requiring 10 % of the knee-flexing torque compared to a nonclutching parallel spring setup.…”

“…Building upon intuitive design, a common approach starts with mimicking nature's counterparts [13]. Atrias [4] and BirdBot [11] for instance are inspired by ostriches and the emu. The problem with bio-inspired design is the high amount of variables that need to be taken into account to fully model the targeted animal accurately.…”

Slack-based tunable damping leads to a trade-off between robustness and efficiency in legged locomotion

“…Instead of regulating the damper's force by adjusting the orifice size, we propose damping control by adjustment of the damper tendon slack. Tendon slack has been observed in biology, with tendon stretch up to 2% of the nominal tendon length before starting to produce considerable force [39][40][41] . This is known as the 'toe region' in the tendon's stress-strain diagram.…”