Highly sensitive measurement of finger joint angle based on a double-U tapered POF embedded in PDMS film

Finger joint angle measurements are important for the fields such as treatment of joint diseases, health monitoring, wearable devices, etc. As an effective detecting device, the joint angle monitoring sensor should have wide measurement angle range, and high sensitivity. In addition, the sensor should have good skin affinity, so as to enhance the degree of comfort for the user. In this paper, high-sensitivity bending angle sensing based on tapered plastic optical fiber (POF) configuration is proposed. The sensing performance is enhanced by tapering the POF and bending the POF taper to form a double U-shaped configuration then encapsulated with Polydimethylsiloxane (PDMS) film, therefore, the POF taper will be bent at three positions simultaneously, so that the bend induced loss will increase. The experimental preparation process of tapered POF and PDMS film package are presented in detail. The influence of taper waist diameter, and bending diameter on the bending loss has been investigated and optimized to enhance the angle sensitivity of the double U-shaped configuration. The results show that the sensitivity can reach 0.070 dB/° with a linearity of 0.995 in a wide bending angle range of 0-105°. The POF sensor is then applied for the joint bending detection. The experimental results show that the sensor can quickly respond to small pressures, and the variation of bending angles can be distinguished in the detection of joint bending. The proposed POF bending sensor shows the advantages of wide measurement angle range, high sensitivity, low cost and good portability, and shows important potential applications in the fields such as human joint movement detection.