Experimental Evaluation of a Lightweight and Backdrivable Powered Knee Prosthesis for Transfemoral Amputees

Rehabilitation of amputees is still mostly limited to passive technology that cannot provide energy to the gait and contribute to a better experience and comfort of walking. To address this shortcoming, active prostheses have been introduced with actuators capable of generating and dissipating energy in a controlled way. However, proposed designs of powered leg prosthesis still present heavy and bulky structure or low backdrivable actuators, which reduce the volitional control and comfort of the user. This work presents the development and control of a lightweight and highly backdrivable powered knee prosthesis. The designed actuator is composed by a coaxial motor-reducer with a 200 W brushless DC motor in series with a harmonic drive with a 51:1 ratio. After assembled the leg prosthesis weights just 3.64 kg. Bench tests were performed to validate the model and tune a position-based controller. Then, a custom force sensor was implemented to measure ground reaction force and identify gait phases so an impedance controller can regulate joint torque. The powered prosthesis was experimented in three able-bodied subjects walking on a treadmill using both the position and impedance controller. The prosthesis presented high backdrivability in bench tests and the treadmill results shown that the proposed impedance controller was more effective to follow the healthy knee trajectory compared to the position controller. In future works we plan to evaluate the prosthesis in amputee subjects.