Analysis and Compensation of Hydroxypropyl Cellulose Sensors Degradation in Force Assessment Applications

This article presents the development and degradation analysis of a liquid crystal (LC) sensor using a hydroxypropyl cellulose (HPC)-water mixture to achieve the Bragg effect in visible wavelength region. In this case, the LC can be used as a sensor for force monitoring. The tests are performed by means of applying controlled forces at the center of the sensor, where the red, green, and blue (RGB) components are acquired from computer vision algorithms. The results indicated the correlation between the RGB components and the applied force, where it is possible to infer the applied force from the RGB components using a multifeature linear regression. Furthermore, the degradation of the sensor is evaluated from the tests along 19 days using the same protocols in all tests, which indicated a variation on the sensor response as a function of the degradation. Another analysis indicated that the LC sensor degradation can be estimated from the RGB components in the border of the sensor region, which resulted in the possibility of compensating this effect using the degradation state as one of the inputs of the force estimation model. Results show that the proposed compensation technique can lead to force errors as small as 0.3 N.