A shear and vertical stress sensor for physiological measurements using fibre Bragg gratings

In-shoe shear stress sensors are a required tool for the investigation of plantar ulcer development after the onset of diabetes. Recently, several transducers have been developed for measuring in-shoe shear stress. Common drawbacks are the relatively large size of the sensors and the difficulty in interrogating many sensors simultaneously in order to achieve distributed sensing. Fibre Bragg gratings (FBGs) are an attractive method for this application because of their small size, and their multiplexing capability, which enables quasi-distributed sensing of shear stress on the plantar surface by interrogating a large number of identical sensors. The authors have designed a novel sensor using 3 BBGs embedded in a 10×1093 mm block of elastic material. This sensor is capable of measuring vertical stress as well as the magnitude and the direction of shear stress on its top surface. The sensor design is based on the theory of elasticity of solid polymers. Application of stress produces axial strain on the embedded fibres, which can be calculated by measuring the Bragg wavelength shift of the reflected light from each FBG using a CCD spectrometer. Preliminary experimental results have shown linear response of the sensor, good repeatability, and high resolution (<5 kPa) in both vertical and shear stress measurements