Low-Cost Coil-Shaped Optical Fiber Displacement Sensor for a Twisted and Coiled Polymer Fiber Actuator Unit

This study proposes a low-cost coil-shaped optical fiber (COF) sensor that can be used in the twisted and coiled polymer fiber (TCPF) actuator unit. The TCPF is a thermal-driven artificial muscle with large stroke and large power ratio. To improve its response, several actuator units, which combine the TCPF with a cooling system, were developed. However, the displacement sensors for these units were not established, unlike the encoder for the motor. Although several methods are available to estimate the TCPF displacement, they are based on the resistance of the heating wire and are affected by TCPF actuation and load change. Therefore, to accurately measure displacement, in this study, we focus on the bending loss of optical fiber, which is often used in soft robotics. Because the bending loss of COFs is caused by the change in length of the coil, the displacement can be obtained by measuring the light intensity through the optical fiber. This study experimentally demonstrates that the COF is available, even when driving the TCPF actuator and changing the load.