Assist-as-Needed Rehabilitation With Bumpless Transfer Torque Command Shaping for a 2-DOF Planar Robot

Active participation in rehabilitation exercise is encouraged to improve the rate of recovery in patients suffering from muscular weakness resulting from injury or stroke. In robotic rehabilitation, active patient participation is achieved through an assist-as-needed (AAN) approach, where robotic assistance is provided only when the patient is unable to complete a rehabilitation task due to insufficient muscular torque or fatigue. AAN robotic rehabilitation usually involves switching from human-in-charge (HIC) to robot-in-charge (RIC) mode when the patient requires assistance to complete a rehabilitation task. A major concern when switching from HIC to RIC mode is bump transfer (high torque), which causes motor jerking and discomfort to the patient at the instant of switching. The present study addresses this problem through bumpless torque command shaping (BTCS) to improve patient safety and comfort in AAN robotic rehabilitation. The proposed BTCS method achieves bumpless transfer around the point of switching to robotic assistance by suppressing the high control torque using an appropriately designed critically damped feedforward torque shaper. The effectiveness of the proposed control approach is assessed via simulations and real-time experiments using a 2-degree-of-freedom planar robot with compliant trajectory tracking realized via an admittance model and a position controller. Comparisons with PID and active disturbance rejection control (ADRC) demonstrate that the proposed PID + BTCS approach significantly suppresses the bumps and prevents chattering of the torque signal at switching points, while achieving comparable tracking of the admittance reference trajectory.