Wearable Wrist Movement Monitoring Using Dual Surface-Treated Plastic Optical Fibers

Abstract

Regarding high-sensitivity human wrist joint motion monitoring in exercise rehabilitation; we develop a pair of novel wearable and sensitivity-enhanced plastic optical fiber (POF) strain sensors consisting of an etched grating fiber and a side-polished fiber stitched into a polyamide wrist brace. The two flexible and surface-treated fibers are; respectively; featured with an etched periodic gratings with a pitch of 6 mm and a depth of 0.5 mm and a D-shaped side-polished zone of ~300 µm depth and ~30 mm length; which, correspondingly, show the sensitivities of around 0.0176/° and 0.0167/° in a normalized bending angle by far larger than a conventional commercial POF, because it achieves a more sensitive strain-induced evanescent field interaction with the side-machined fibers. Moreover, in terms of the sensor response to bending deformation in the range of -40°~+40°, the former exhibits a better sensitivity in lower angle change, while the latter is superior as the bending angle increases; thereby arranging the two modified POFs separately at the side and back of the human wrist, in order to decouple the wrist joint behaviors induced by typical flexion-extension or abduction-adduction movements. Then, the circular and pentagonal wrist motion trajectory patterns are investigated, to demonstrate the maximum average single-axis motion error of 2.94° via the transformation of spatial angle to plane coordinate for the fabricated couple of POF sensors, which is lower than a recognized standard of 5°, thus suggesting the great potential in wearable exercise rehabilitation of human joints in the field of medical treatment and healing.

Keywords: etched grating POF; motion trajectory recovery; optical fiber wearable senor; side-polished POF; wrist movement detection.

Figure 1

(a) Etching periodic grating and (b) side-polishing the cladding surface on a plastic optical fiber (POF). Microscope view of (c) the etched grating and (d) the side-polished fibers. (e) Side view and (f) front view of a wearable wrist brace with the two surface-treated POFs.

Figure 2

Schematic illustration of angle calibration experiment for the developed POF sensor.

Figure 3

(a) The response to bending angle for the POFs with different sensitive structures. The measured normalized sensor output voltage at various angle ranges: (b) −40°~+40°, (c) −32°~+32° and (d) −20°~+20°.

Figure 4

(a) Schematic illustration of wrist motion angle detection. (b) The transformation of the spatial angle of wrist motion to the plane coordinate (X, Y).

Figure 5

The measured responses to wrist flexion-extension and abduction-adduction movements for (a) the side-polished POF and (b) the etched grating POF.

Figure 6

The extracted wrist (a) circular and (b) pentagonal motion trajectory patterns for the developed wearable sensor, where yellow dashed line and blue solid line represent the predetermined and recovered patterns, respectively.