Design and control of powered lower limb prostheses

Abstract

In the development of powered lower-limb prostheses, providing sufficient power and torque to support amputees’ locomotion is a major challenge, considering prostheses’ weight and size limits. Furthermore, regulating the power delivery during locomotion is equally important that gives amputees safe and natural movements. This dissertation aims to address these challenges by investigating new approaches in the actuation and control of powered lower-limb prostheses, with the overarching objective to obtain compact, powerful lower-limb prostheses that interact with amputees and the environment in a coordinated manner. The initial efforts were focused on the design and control of transfemoral (TF, also known as above-knee) prostheses powered by pneumatic muscles, an extraordinary actuator with superb power-to-weight ratio. The first prototype incorporates powered knee and ankle joints in a volumetric profile similar to that of human leg. The unique feature is a single-acting-spring-return mechanism, in which a single pneumatic muscle drives the motion in the torque-demanding direction, while a set of mechanical springs drives the motion in the opposite direction. A finite-state impedance controller has been developed for this prosthesis, which was demonstrated to provide a natural gait. Based on previous success, a novel type of pneumatic muscle, namely double-acting sleeve muscle (DASM), was examined to replace traditional pneumatic muscle. Incorporating a second chamber, the DASM is able to provide additional extensional force without using return springs. Therefore, the prosthesis can be significantly simplified into a more compact and lightweight device. Compared with pneumatic muscles, traditional cylinder-type actuators are more technologically mature. Therefore, the subsequent efforts were to develop a pneumatic cylinder-actuated TF prosthesis, which has great potential for real-world applications. All peripheral components were integrated, including a carbon fiber air tank as the energy source, and the prosthesis’ capability of independent, untethered operation was demonstrated in human walking test. In addition to the improvement of prosthetic design, control methods were also investigated. The results include an integrated walking – stair climbing controller and a sit-to-stand controller. Both were developed based on biomechanical analysis of the knee dynamics in human locomotion. In the walking – stair climbing control system, an improved finite state impedance controller was constructed, which incorporates a unique time function to enable gradual energy injection during weight acceptance phase. An intuitive thigh position-based switching condition was introduced to merge the walking and stair climbing controllers into one system. In the sit-to-stand controller, a similar controller was established, which eliminates the need for a state machine and significantly simplifies the controller tuning and implementation. The human testing was conducted with results demonstrating the effectiveness of both control systems.