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In this paper, we design a new controller of power-assist wheelchair. Conventional power-assist controllers only focus on the amplification of the human input force using torque sensor. Our proposed controller does not use a torque sensor but an electromyogram(EMG) sensor. Our designed controller is composed of EMG filter block, disturbance observer, direction discrimination function and fuzzy controller. We estimate whether the driver wants to propel forward or backward by the EMG signal placed at the triceps brachii muscle and the biceps brachii muscle. Then we provide power assist by combining the filtered myoelectric signal of the adductor pollicis muscle and the estimated human torque signal calculated by the disturbance observer. If the driver wants to stop or decelerate the wheelchair, our designed controller detects the changes of EMG signal and step down assist torque and realize vibration-free speed reduction. Experimental results show that the proposed power-assist method realizes quick and smooth power assist according to the driver's will.