In this paper, we develop a small-scale knee exoskeleton by using springs of shape memory alloy (SMA). In a simplified design, two groups of four SMA springs are arranged on the front and back of the leg to mimic the human muscular architecture actuating the human knee. The front SMAs provides the extension motion of the knee, when heated. The rear SMAs group provides the flexion motion of the knee, when it is heated. Our prototype is fabricated by using SMA springs with soft pads and rigid mounting parts that protect the other components from the overheated SMA spring. We use two inertial measurement units (IMU) to feedback the angles of the leg parts (thigh and shin), and hence to calculate the knee joint angle for the control action. We applied system identification and PID tuning procedure. Then proportional integrator derivative (PID) controller are tuned experimentally to track a desired trajectory of the knee angle and the temperature. The experiments show that the knee joint of the 1/6 mannequin prototype follows the desired trajectories as designed. |
