Air cylinders are actuators that slide a piston inside cylinders by applying air pressure. We propose an air-cylinder-type artificial muscle that can be flexibly bent by using a flexible tube for the cylindrical part. The actuator output was a string connected to a piston. When the air-cylinder-type artificial muscle bends, the inner wall of the tube and the string come into contact, causing output fluctuations owing to friction. In this study, we investigated the output when an artificial muscle was bent. After describing the structure of the air-cylinder-type artificial muscle, the measurement results of the resistance force at each part of the actuator are presented. A theoretical output inspired by the capstan equation was derived, and its validity was verified by comparison with experimental results.

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