Design of a Powered Leg Prosthesis with a Custom Embedded Load Cell

The lower limbs support the body’s weight and are responsible for human balance and mobility. The number of lower limb amputations is significant and the amputee population increases every year. Assistive technologies, such as prostheses, are fundamental to restoring bipedal locomotion and independence. To avoid extra weight, the current design of leg prostheses employs a single-axis load cell to measure ground force reaction. This paper presents the mechanical design and control of a powered lower limb prosthesis with the knee and ankle joints using a coaxial motor-reducer composed of a 200 W EC motor directly coupled to a harmonic drive reducer with a 51:1 ratio. Additionally, we present the design and calibration of a four-bar structure with integrated sensors capable of functioning as a load cell for ground reaction force and also serving as a linking mechanism for both robotic joints. The controller architecture employs a finite state machine and impedance controller to manage the prosthesis dynamics in each phase. Data from the linking structure is used to identify gait phase transitions in the state machine. Tests with transfemoral amputees demonstrated that this design is suitable to assist the user during walking with the powered leg.