A Switchable Parallel Elastic Actuator and its Application to Leg Design for Running Robots

Motivated by the role of compliant elements in animal motion, springs are introduced in the driving train of legged robots to improve their locomotion performance. This paper presents the working principle, hardware realization, and experimental evaluation of a switchable parallel elastic actuator design actuating the monopedal switchable parallel elastic actuator robot (SPEAR). In the proposed design, a mechanical switch engages the parallel spring <italic>only</italic> during the stance phase, when it is needed to support body weight and promote energy recovery. During flight, the spring is disengaged to allow for unobstructed joint movement. Furthermore, the proposed design enables online leg stiffness adjustments simply by changing the landing configuration of the knee joint. Experimental results demonstrate the effectiveness of the design in improving energy efficiency without compromising mobility. SPEAR can run with an electrical cost of transport of 0.86 at <inline-formula><tex-math notation="LaTeX">${\text{0.5 }}{\text{m/s}}$</tex-math></inline-formula>, as well as reach a toe clearance of more than <inline-formula><tex-math notation="LaTeX">${\text{45}\%}$</tex-math></inline-formula> of its leg length. The overall design is compact and reliable, and can be easily scaled for legged robots with different sizes.