Design, Development, and Validation of a Lightweight Nonbackdrivable Robotic Ankle Prosthesis

Robotic ankle prostheses can imitate the biomechanical function of intact legs at the cost of a larger weight and size compared to conventional passive prostheses. Unfortunately, increased weight and size negatively affect comfort and socket stability, ultimately limiting their clinical viability. Alternatively, a nonbackdrivable transmission system can be used to actively regulate the ankle position during nonweight bearing activities only. This semiactive design can be made smaller and lighter as a result of the lower actuation power requirements. However, the transmission system must withstand high loads during stance and standing. Thus, available semiactive prostheses are still significantly heavier and have a larger build height than passive ankle prostheses. In this paper, we present the design, development, and validation of a semiactive ankle prosthesis with a nonbackdrivable cam-follower mechanism designed to lower the load on moving components and align with the foot longitudinally as necessary to reduce the prosthesis weight and size. The proposed ankle mechanism is ~50% shorter, has ~40% wider range of motion (ROM), and is estimated to be ~27% lighter than available semiactive prostheses. Experiments with a transtibial subject show that the semiactive prosthesis can increase foot clearance up to 142% and reduce the load on the residual limb as low as 32% compared to passive prostheses.