Assessment of the Adaptive Sliding Mode Control of an Active Ankle Foot Orthosis with an Impedance Reference

Abstract

Patients suffering from impairment in the ankle lose the ability to walk properly. The requirements of the ankle during a healthy gait cycle, accurately follow an impedance profile. The properties of the environment the orthosis is in contact with however are unknown and may change. Disturbances from both the user and the environment are also common. The ankle requirements have been found to imitate an impedance profile and an adaptive sliding mode controller with an impedance reference is proposed in order to assess these issues. The effectiveness of the controller has been assessed by carrying out simulations on a model of the ankle orthosis developed previously. Various bodies (soft, hard and medium stiffness) have been considered with and without the sliding mode control. The results show that the adaptive control responds very well and manages to model the body the actuator is in contact with, despite the presence of an external force. The results also show the controller manages to follow the desired impedance reference after the adaption is complete. The sliding mode control however, is found to be inessential in these working conditions due to the actuator motor reaching its limits.