Controlling pneumatic artificial muscles in exoskeletons with surface electromyography

Powered exoskeletons are gaining more interest in the last few years, as useful devices to provide assistance to elderly and disabled people. Many different types of powered exoskeletons have been studied in the past. In this research paper, a soft lower limb exoskeleton driven by pneumatic artificial muscles (PAMs) is presented. It features a lightweight, flexible solution to attach actuators to human limbs without imposing restrictions on limb movements. Ankle plantarflexion and knee extension by both legs are assisted. Surface electromyography (sEMG) sensors are used to measure muscle activity, which are amplified and used as input to control the pressure in the corresponding PAMs proportionally to sEMG activity level. No force or torque measurements/estimations are performed, this is not strictly necessary because of the similarity between human muscles and PAMs. Employing the principle of assistance-as-needed, allows for a range of activities to be supported, such as walking, standing up and climbing stairs. Metabolic measurements in different scenarios did not register a significant change in energy expenditure when the (passive) exoskeleton is worn, even not when it is activated although the assistance can be experienced physically. More research is needed to optimize the whole system in different aspects, in order to obtain a significant reduction in energy expenditure and make the system practical.