A Compact Series Elastic Actuator for Bipedal Robots with Human-Like Dynamic Performance

Series-elastic actuation offers several important benefits to dynamic robots, including high-bandwidth force control and improved safety. While this approach has become common among legged robots, the lack of commercial series-elastic actuators and the unique design requirements of these robots leaves custom-built actuators as the only option. These custom actuators are often designed for nominal behavior rather than an extended performance envelope, and thus lack the capacity for high dynamic behavior and large-scale disturbance rejection outside of controlled operating conditions. This paper details the design and construction of a compact series elastic actuator for use in the knee of a robotic biped built to emulate the dynamic performance of a half-scale human. The actuator specifications are defined by the combined performance envelope of scaled human knee extensor muscles. The design uses two DC motors geared in parallel for improved weight, compactness, and power density. The constructed actuator is designed for a no-load speed of 210 rpm and a nominal torque of 13 Nm. In addition, a unique approach to series-elastic actuator torque control is proposed which uses servo-controlled motor velocity to modulate spring deflection. Several key advantages of our proposed method over traditional torque-embedded control are demonstrated, including increased torque bandwidth, rejection of nonlinear effects from gear dynamics, and zero steady-state error.