A McKibben artificial muscle is a typical soft actuator, and it has features of flexibility, lightweight, and low cost. It consists of a rubber tube and a sleeve which is woven with spiral fibers, and contracts axially by applying pneumatic pressure to the rubber tube. It is expected to be used in robot mechanisms that can guarantee mechanical safety because of its flexibility. For controlling the artificial muscle, a sensor system for detecting the contractile displacement is required. However, the general conventional sensors do not match the advantages of the artificial muscle mentioned above.
Therefore, we have developed the combination structure of the McKibben artificial muscle and the optical fiber which works as a contractile displacement sensor. The optical fiber can be braided into the sleeve which is the necessary component of the artificial muscle, which means that the optical fiber works as both the sensor and actuator element. In addition, the optical fiber can be combined during the fabrication process of the artificial muscle by a braider machine. As the artificial muscle contracts, the spiral curvature of the sleeve fibers, including the optical fiber, changes. Then, the light intensity propagating into the optical fiber changes due to the bending loss of the optical fiber. Therefore, the displacement of the artificial muscle can be estimated by measuring the propagating light intensity. In the previous sensor system, the light-receiving part (Photo Diode) and the light-emitting part (LED) were located at the base and tip sides of the artificial muscle, respectively. Therefore, the tip of the artificial muscle had the rigid part (LED), and electrical lines had to be wired from the base to the tip. These have a limitation in the applications and electrical line troubles. In this report, the LED and the Photo Diode are arranged at the base end of the artificial muscle. For this configuration, we established the novel process during braiding. Through the process, the optical fiber from the base can be returned to the base again via the tip, and the LED and Photo Diode can be located at the base side of the artificial muscle. Experimentally the relation between the sensor output and contractile displacement of the artificial muscle was confirmed.
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