Design, development and testing of a lightweight and compact locking mechanism for a passive knee prosthesis

In knee prosthetics and orthotics, there is the need to have a change in stiffness within the gait cycle. Doing this using a locking mechanism requires locking high forces using a small amount of energy. This paper presents a novel compact and light-weight locking mechanism which combines a ratchet-and-pawl and a singular position locking mechanism. It is used as a lock in a passive knee prosthesis and allows a change in compliance of the joint. The mechanism is transparent during the swing phase, allowing the joint to flex, and during the weight acceptance phase it provides the same stiffness as a natural knee joint. It is explained that this high stiffness characteristic can be approximated by a linear spring put in parallel with the knee joint. The mechanism uses a small servo motor to unlock the ratchet, other than this the operation is fully passive. The prosthesis has been tested during walking tests with amputee test subjects. The passive knee prosthesis is part of the CYBERLEGs-Project.