Comparison of Two Series Elastic Actuator Designs Incorporated into a Shoulder Exoskeleton

Low impedance and torque control are critical for movement rehabilitation using robotic exoskeletons. A grounded 3 degree of freedom shoulder exoskeleton was designed for movement assistance in shoulder abduction/adduction, flexion/extension, and shoulder internal/external rotation. Two series elastic actuators designs were developed using a linear spring arrangement with a global nonlinear stiffness behavior. RMS errors during application of constant torque were less than.06 Nm in shoulder add/abd and less than.04 Nm in arm rotation as the limb was moved in sinusoidal trajectories up to 3.5 Hz. For abd/adduction, the step response rise time was.05 s, and free mode impedance peaked at.007 Nm/deg during 3.5 Hz oscillations. For arm rotation, the step response rise time was.03 s, and impedance peaked at.023 Nm/deg during 3.5 Hz oscillations. Both SEA designs had performance measurements that were similar to other SEA designs in terms of torque tracking, but with much lower impedance than previously reported.