Actuation and Control of Lower Limb Prostheses

Abstract

Millions of people are suffering from lower limb loss all around the world. Passive ankle prostheses in the market cannot fully restore ankle function and will cause asymmetrical walking gaits. Several powered ankle prostheses, which provide net power in the stance phase to assist walking, have been developed by the researchers, but their walking range is significantly limited by the power requirement. In this thesis, an electrohydrostatic actuator (EHA) powered ankle prosthesis is proposed. This is intended to actively assist walking at certain points in the gait cycle, namely the plantarflexion (PF) before toe-off and dorsiflexion (DF) in the early swing phase for toe-lifting. In the rest of the gait, the ankle prosthesis actuation system can operate passively with controllable damping. This approach can increase the working time range compared to a continually powered ankle and ensure safe passive prosthetic function after the battery discharged. A prototype of the EHA powered ankle prosthesis has been developed. A 100 W brushless DC motor is used driving a 0.45 cc/rev bi-directional gear pump. The damping ratios of the ankle PF and DF are controlled by bypass restriction valves. The EHA system and the foot springs at the ankle joint weigh 2.2 kg. The controller and a 2 Ah battery are held in a backpack. Walking characteristics with a passive ankle were studied in an amputee trial to gather ankle sensor signals for the controller design. A timing control method is proposed which uses the foot spring strain gauge signals to detect heel strike. A middle stance time delay is added between the end of the heel strike and the start of the powered PF phase. This delay time length can be adjusted to fit different walking speeds. Heel strike detection using hydraulic pressure signals is also studied. The EHA powered ankle prosthesis and its controller has been tested by a 70 kg transtibial amputee. According to the amputee trial results, the EHA can provide sufficient power to assist walking in the terminal stance and the energy consumption in the passive phases are proximately zero. The on-board battery is able to power over 5500 level walking steps. In the amputee trial, the ankle prosthesis controller correctly recognises the heel strike and triggers the powered PF phase. According to feedback from the amputee, the EHA powered ankle prosthesis provided beneficial level walking assistance and a very natural walking gait. The characteristics of the powered ankle prosthesis are analysed by comparing with the healthy ankle and by testing at different walking speeds. A simulation model was developed to help analyse the performance characteristics of the EHA. This includes a brushless DC motor model and a symmetric hydraulic actuation model. The laboratory-based experiment results and amputee trial results are used to analyse and validate the simulation model. The model can be used for future development and refinement of EHA powered ankle prostheses.

Date of Award	2 Nov 2017
Original language	English
Awarding Institution	University of Bath
Supervisor	Andrew Plummer (Supervisor) & Pejman Iravani (Supervisor)