Fabrication of Composite Thin-Film Strain Gauge on Flexspline by Photoresist Stencil Lithography

The harmonic reducer is a core component of industrial robots, and the design of thin-film strain gauge (TFSG) for in situ monitoring of the strain state of the harmonic reducer is crucial for the operational accuracy and reliability of industrial robots. However, the current methods for fabricating TFSG primarily rely on planar lithography and thin-film deposition techniques. These techniques have limitations when it comes to creating miniature TFSG on nonplanar rough surfaces. In addition, metallic materials commonly used for TFSG have a large temperature coefficient of resistance (TCR), which affects the applicability of in situ preparation of sensors on harmonic reducers. Here, a fabrication of copper-nickel alloy-nickel–chromium alloy (CuNi-NiCr) composite TFSG by photoresist stencil lithography is reported. The sensing performance of the TFSG was evaluated at room temperature. The gauge factors (GFs) of the TFSG were calibrated to 2.23 at room temperature and a TCR of 5.2 ppm/°C at
$10~^{\circ }$
C–
$60~^{\circ }$
C. In addition, its excellent durability was confirmed by performing cyclic strain tests. Finally, CuNi-NiCr composite TFSG was fabricated on a harmonic reducer flexspline and experimentally tested to successfully acquire the strain conditions that the harmonic reducer is subjected to during normal operation. This approach provides feasibility in the preparation of nondestructive, small-disturbance, and in situ surface strain-monitoring components in nonplanar surfaces.