Non–invasive electromyography–based sensing for proportional prosthesis control

Abstract

The best design of a multi-function tool is the human hand. Normal limb functionality is taken for granted until the day it is lost. Maslow’s theory of human motivation suggests self-actualisation and control of one’s own situation being most needed. The psychological implications of any disability are described in Maslow’s theory of human motivation, based on human hierarchy of needs. “Self-actualisation” is placed on top of all needs. By having the ability to function normal and independent and the feeling of being in control of one’s own life or actions, usually associated with being successful in life. An amputation has a major impact on a person’s self-esteem and affects their life style. People tend to have the urge to replace what they had, at least with a counterpart with equal performance. Should patients have a limb amputated, the question is, what functionality remains in the surrounding muscles and nerves? Biomechatronics is introduced at the North West University (NWU), with the aim to research a complete proportional powered prosthetic hand. The versatility of the human hand suggests that it is a complex part of the body, and the future goal of the development of proportional prosthesis control is divided into several studies. This particular study focuses on the human-machine interface (HMI) or the sensing component for prosthesis control. The HMI has to be able to provide Matlab®/Simulink® with the sensed data as Matlab®/Simulink® will be used for future research. The HMI makes use of surface electromyography (sEMG). sEMG could be the most elegant design approach, as no medical surgery procedures are required to have a device implanted. By considering the rate at which technology improves, it would also be unwise to insert an implant that becomes outdated in a short time. The sEMG electrodes consist of a set of five electrodes in a wristband fitted to a patient’s forearm. This interfaces the patient with the sEMG platform. The electrodes sense antagonist muscle activity through the patient’s skin, and is regarded as a non-invasive HMI. The sEMG sensing platform is an interface board that acts as a serial emulator (COM port) that connects the sEMG sensors to the Matlab®/Simulink® environment via USB. The platform’s circuitry converts the dual-channel analogue input sEMG signals into digital format. A calibration algorithm calibrates the sensors with the push of a button, using automatic gain control (AGC). A pulse duration modulation (PDM) servo is used to test the effect of visual feedback on the accuracy of performing a gesture according to an animation. The proportional control algorithm is implemented in Simulink® and has the capability of decoding dual-channel antagonist muscles’ sEMG signal into position and force information. The algorithm and platform is evaluated by making use of a gesture animation that asks the user to mimic the gesture. The power of visual feedback on the accuracy of human gestures should not be underestimated, and is demonstrated in this study. The results obtained from this study verify the functionality of the sEMG platform and demonstrates the possibility of proportional control through sEMG.