Polymer optical fiber-embedded force sensor system for assistive devices with dynamic compensation

This paper presents the development and the performance analysis of a force sensor using a novel high stretchable polymer optical fiber (POF) fabricated using the light polymerization spinning (LPS) fiber. The system consists of an LPS-POF fiber encapsulated in a flexible material polydimethylsiloxane (PDMS), in addition to a light source and a photodetector. The sensor was characterized by using a commercial 3-axis force sensor K3D60a ±500N/VA (ME Systeme, Germany). Since the LPS-POF and PDMS are viscoelastic materials, tests with loading and unloading cycles were performed to evaluate the sensor response. A viscoelasticity compensation model was proposed to decrease the errors and the sensor phase delay provoked by the viscoelastic behavior. In addition, the LPS-POF force sensor was applied on two different applications, as a gait perturbation system for balance assessment and as a walking cane for gait assistance. Results showed that proposed sensor presents a linear response, with determinant coefficient (R2) of 0:9974, and high sensitivity (S = 40N=V). However, the unload time presented high phase delay and errors, corroborating the viscoelastic behavior. Compensated response presented lower hysteresis, leading to a decrease of the root mean square error (RMSE) of approximately 65%. Moreover, numerical integration was used as performance metric for the results of both applications and presented a decrease of 48% in the first application and up to 46% in the second application when used viscoelasticity compensation. The proposed sensor is small and versatile for different applications, and presents simple fabrication, data acquisition and processing.