Sensor Cell Network for Pressure, Temperature and Position Detection on Wheelchair Users

Abstract

This work proposes an optical sensing network to monitor pressure and temperature in specific areas of a wheelchair to prevent pressure ulcers and to monitor the position of the wheelchair user by analyzing its pressure distribution. The sensing network is composed of six optical fiber Bragg grating (FBG)-based sensor cells. Each sensor cell is built from a polylactic acid (PLA) base and has two FBGs, one embedded in epoxy resin to monitor pressure variations (FBG_P) and another without resin to monitor temperature (FBG_T). Once produced, all sensor cells were experimentally characterized for pressure and temperature variations, resulting in an average pressure sensitivity of 81 ± 5 pm/kPa (FBG_P) and -5.0 ± 0.4 pm/kPa (FBG_T), and an average temperature sensitivity of 25 ± 1 pm/°C (FBG_P) and 47.7 ± 0.7 pm/°C (FBG_T). The sensor cells were then placed in six specific areas of a wheelchair (four in the seat area and two in the shoulder blade area) to carry out experimental tests, wherein the response of the sensors to a specific sequence of relief positions was tested. During the execution of the test, the optical signal of all sensors was monitored, in real time, with the pressure and temperature values detected in each zone of the wheelchair. In addition, random position changes were performed in order to evaluate the precision of the proposed sensing network in the identification of such positions.

Keywords: fiber Bragg grating; position detection; pressure and temperature sensors; pressure ulcers; wheelchair user.

Figure 1

Schematic representation of the most common areas for the development of pressure ulcers, highlighting the areas studied in this work.

Figure 2

3D design of sensor cell with: (a) all components; (b) components dimensions; and (c) final assembled sensor cell with its dimensions.

Figure 3

Determination of the response time of the IR sensor cell to pressure variations: (a) entire test; (b) zoom of first peak to show the methodology used to find the rise and fall times.

Figure 4

Determination of the response time of the SL sensor cell to 15 °C of temperature variation.

Figure 5

FR sensor cell calibration test to pressure variations. The symbols correspond to experimental data, and the lines to the linear fits.

Figure 6

Temperature IL sensor cell calibration test.

Figure 7

Photography of the wheelchair with the identification of six FBG-based sensor cells for pressure and temperature monitoring.

Figure 8

Positions performed during the test: (A) reference position, person sitting normally; (B) leaning slightly forward with hands on knees; (C) leaning strongly forward with hands on feet; (D) slight tilt to the right; (E) too tilted to the right; (F) slight tilt to the left; (G) too tilted to the left.

Figure 9

Scheme showing the sequence of pressure relief positions and their time at the first test.

Figure 10

Bragg wavelength shift for the FBG_P (a) and FBG_T (b) of the SR sensor cell. In the first graph (a), each color refers to the position that was exercised at that moment: position A in white; B in red; C in blue; D in orange; E in green; F in dark blue; and G in yellow. The white area with pattern corresponds to a random movement of the volunteer (identified as X).

Figure 11

Pressure with temperature compensation (a) and temperature (b) of the SR sensor cell during the test. In the first graph, each color refers to the position that was exercised at that moment: position A in white; B in red; C in blue; D in orange; E in green; F in dark blue; and G in yellow. The white area with pattern corresponds to a random movement of the volunteer (identified as X).

Figure 12

Pressure variation for all sensor cells during the test. In the graphs, each color refers to the position that was exercised at that moment: position A in white; B in red; C in blue; D in orange; E in green; F in dark blue; and G in yellow. The white area with pattern corresponds to a movement of the volunteer (identified as X).

Figure 12

Pressure variation for all sensor cells during the test. In the graphs, each color refers to the position that was exercised at that moment: position A in white; B in red; C in blue; D in orange; E in green; F in dark blue; and G in yellow. The white area with pattern corresponds to a movement of the volunteer (identified as X).

Figure 13

Pressure variation for SR, IL, and FR sensor cells during the random test of relief positions: position A in white; B in red; C in blue; D in orange; E in green; F in dark blue; and G in yellow. The white area with pattern corresponds to a movement of the volunteer (identified as X).