This paper describes the development of an angle-sensorless exoskeleton with a tap water-driven artificial muscle actuator. The artificial muscle actuator consisted of an elastic rubber tube reinforced by braided fiber. Such actuators are highly flexible, lightweight, and water-resistant, and thus are inherently safe even for operations in direct contact with humans. An estimation system for the displacement of the artificial muscle actuator based on the water flow rates detected by flowmeters was constructed for the water-hydraulic exoskeleton. In addition, estimators of the velocity and acceleration of the actuator based on the estimated displacement and the measured flow rates were derived and incorporated into the estimation system. With this system, our previous wearable upper-limb assistive exoskeleton prototype was converted into an angle-sensorless version with higher safety in wet conditions. Its assistive performance was evaluated through experiments with research participants. Experimental results demonstrated that muscle activity could be reduced, although an assistive control strategy was executed with the variables estimated, excluding force.

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