Lightweight Torque Sensor Based on a Gradient Grating Period Guided-Mode Resonance Filter

This paper describes the design, fabrication, and testing of a lightweight and compact torque sensor system based on a gradient grating period guided-mode resonance (GGP-GMR) filter and a flexure-elastic-force-sensing element. The GMR filter exhibits a characteristic resonant reflection, when illuminated with a broadband light source at normal incidence. Instead of a fixed grating period, the GGP-GMR filter consists of grating periods varying from 250 to 550 nm with an increment of 2 nm. Given the flexibility of the plastic-based GGP-GMR filter, it can be bent conform to the cylindrical surface of the flexure. The applied torque induced deformation of the flexure and angular displacement of the attached GGP-GMR. For a stationary light source, the angular displacement of the GGP-GMR filter results in illumination at different locations (grating periods), leading to a shift of the resonant reflection wavelength. The magnitude of shift in the reflection wavelength can be correlated to the magnitude of deformation and the applied torque. In addition, commercial software based on the finite element method was used to simulate the proposed design, which indicates that the flexure made of medium-carbon steel can withstand the torque of 35 Nm without yielding. Furthermore, the simulation results (torque-induced deformation) were consistent with those obtained using the proposed torque sensor system. Torque measurements from 0 to 25 Nm showed good linearity. The limit of detection achieved was 0.77 Nm.