ServoSEA concept: Cheap, miniature series-elastic actuators for orthotic, prosthetic and robotic hands

For interactive humanoids, rehabilitation robots, and orthotic and prosthetic devices, the human-robot interaction is an essential but challenging element. Compliant Series-Elastic Actuators (SEAs) are ideal to power such devices due to their low impedance and smoothness of generated forces. In this paper we present the ServoSEA, which is a miniature Series-Elastic Actuator (SEA) based on cheap RC servos, and which is useful for actuation of orthotic, prosthetic or robotic hands. RC servos are complete packages that come with rotary motor and sensor and have an integrated control board to control the output angle. In the ServoSEA, a small rotational spring is attached to the output shaft and the internal rotary sensor is relocated to measure the spring deflection. These small modifications immediately make the integrated control board behave as a series-elastic torque controller. Here we present several design alternatives and report on the performance of our implementation that will be used in the active SCRIPT wrist and hand orthosis. The performance measurements showed that feedforward control of the example implementation of the ServoSEA results in acceptable, though not perfect, force tracking behavior. It is clear that although the ServoSEA concept is universal, final performance strongly depends on the quality of the original RC servo.