Silicone Rubber-based Flexible Optical Sensor for Precise Twist Angle Measurement

A novel torsion or twist angle measurement sensor, utilizing a flexible silicone rubber material, has been designed, fabricated, and experimentally evaluated. The operation of this sensor relies on the principle of optical polarization, involving two linear optical polarizers placed sequentially with their polarization axes perpendicular to each other. These polarizers are separated by a transparent cylindrical duct structure made from rigid epoxy resin, which efficiently guides the light rays from the source-side polarizer to the detector-side polarizer. Under normal, untwisted conditions, the orthogonal alignment of the polarizers prevents any light transmission. However, when the sensor experiences torsion or twisting around the axis aligned with the direction of light propagation, the relative angle between the polarizers' axes decreases from 90°, allowing a portion of the light to pass through. The intensity of this transmitted optical power, detected by a phototransistor, is directly proportional to the square of the sine of the twist angle, enabling accurate twist angle measurement by monitoring the photocurrent output. A comprehensive analytical model has been developed to precisely correlate the photocurrent with the twist angle, applicable for both narrow and wide spectrum sensing configurations. Validation of this model through experimental measurements across a twist angle range from –90° to +90° demonstrates strong agreement. Furthermore, repeated cyclic twist tests indicate minimal degradation in measurement accuracy, highlighting the sensor's robustness. This sensor structure achieves an effective balance among accuracy, reliability, scalability, and long-term stability, making it particularly suitable for applications involving frequent cyclic twisting.