Publications

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.

Abstract 4D printing can address time-evolving structural functions that are unattainable by conventional 3D printing. Despite the advance in materials and printing techniques, however, 4D printing of continuity of shape representation that generally characterizes 3D matters is still challenging, because the existing methodologies mostly rely on a few discrete levels of strain and their spatial distributions. Here, a 4D printing strategy of shape memory polymers (SMPs) that can program continuous levels of shape-recovery strain is proposed. It is found that the irrecoverable state of the SMP and the corresponding recovery strain can be controlled in a continuous and precise manner by a single printing parameter. Importantly, the continuity of strain programming provides an opportunity for the translation into mathematical function representation (F-rep), which allows the systematic derivation and implementation of 4D-printed bilayer strain functions that are matched to the continuously varying curvatures of the target geometry. Combined with the custom-built software, the F-rep 4D printing strategy can produce 4D-printed architectures that involve continuously varying strain profiles of almost any function type. The effectiveness of the framework is highlighted by a set of 3D face masks with facial feature transformation driven by a function operator.