Robot assisted knee joint RoM exercise: A PID parallel compensator architecture through impedance estimation

Knee joint rehabilitation exercise refers to a therapeutic procedure of a patient having dysfunctions in certain abilities to move knee joint due to some medical conditions like trauma or paralysis. The exercise is basically a series of repeated assistive physical movements within the range of motion (RoM) of the joint. Reflex action of limbs during RoM exercise causes inappropriate balance of load which may cause secondary injuries, such as damages of muscle or tendon tissues. Establishing correlation between impedance data and limb motions is important to solve this problem. This paper aims to design and modeling of a robotic arm with an original approach in control strategy which is developed based on the correlation in between the joint-impedances and joint-motion characteristics during exercise. The knee joint impedances are estimated based on the internal feedback of the system dynamics, that lead to design the torque compensator to improve the overall control signals in real time. This paper also demonstrates the characteristics of various responses of the system during exercise with human subject. Results have reflected good performances with low position and velocity tracking errors, $\pm 0.{02}^{\circ }$ and $0.04rad.se{c}^{-1}$ during hold phase; and $\pm 0.{14}^{\circ }$ and $0.17rad.se{c}^{-1}$ during motion phse. Though, the limitation of the prototype is its current RoM (limited to $0^{\circ }$–${25}^{\circ }$), the system has potential in the application of RoM exercise for paraplegic or monoplegic patients.