A reliable low-cost wireless and wearable gait monitoring system based on a plastic optical fibre sensor

Abstract

A wearable and wireless system designed to evaluate quantitatively the human gait is presented. It allows knee sagittal motion monitoring over long distances and periods with a portable and low-cost package. It is based on the measurement of transmittance changes when a side-polished plastic optical fibre is bent. Four voluntary healthy subjects, on five different days, were tested in order to assess inter-day and inter-subject reliability. Results have shown that this technique is reliable, allows a one-time calibration and is suitable in the diagnosis and rehabilitation of knee injuries or for monitoring the performance of competitive athletes. Environmental testing was accomplished in order to study the influence of different temperatures and humidity conditions.

Compensation technique for environmental and light source power variations applied in a polymer optical fiber curvature sensor for wearable devices

“…A common compensation technique is based on a reference fiber that remains on a static position during the measurement. For this purpose, Bilro et al (2011) and Moraleda et al (2013) proposed the use of a reference fiber in the same direction of the POF sensor. Such solution may not be suitable for wearable applications, since this reference fiber may move or bend with the POF sensor, presenting signal attenuations that could result in measurement errors.…”

“…The reference works that report the temperature influence on the sensor response employ temperature variation of about 50°C for the curvature analysis Bilro et al (2011), Peters (2011. Since the POF power variation on the temperature range tested is not too large, temperatures within the comfort zone range for wearable applications (Moreno et al, 2008) will not lead to high errors on the sensor response.…”

“…Moreover, the polymer higher strain limits and fracture toughness enable the fiber bent in angles with the same order of magnitude of the human joints range of movement (Peters, 2011). Different operation principles for POF sensors have been proposed throughout the years such as light coupling based sensors (Antunes et al, 2013;Zawawi et al, 2013), specklegrams (Rodriguez-Cobo et al, 2013), Fiber Bragg Gratings (Marques et al, 2015) and intensity variation detection (Bilro et al, 2011). Among them, intensity variation is one of the most commonly used principles for joint angle measurement due to its ease of implementation, good price-quality ratio, and simplicity in signal processing (Bilro et al, 2012).…”

“…The intensity modulation principle for angle assessment is the variation of the optical power with the fiber curvature due to radiation losses (Bilro et al, 2011). The power attenuation can be measured with photodiodes and is related with the fiber curvature angle.…”

“…Nevertheless, the reference fiber signal has low intensity due to reflection losses. Bilro et al (2011) and Moraleda et al (2013) employed a self-referencing system, which lowers the source power deviations to acceptable values. This system comprises of a sensor fiber and a reference fiber with the same properties as the sensor fiber in a static position.…”