Development of an optical fibre pressure sensor for F1 applications.

Abstract

This study investigated the development of optical fibre sensing technologies for the measurement of pressure in aerodynamic applications, in particular on F1 racing cars. The specific application placed constraints on the dimensions and geometry of the sensor, and provided the specification for a resolution of order 20 Pa over a range 0 to 35 kPa, with a requirement for means for compensation for the temperature response of the sensor. The light weight and minimally intrusive nature of optical fibres has aroused great interest for embeddment in the thin composite structures that comprise the aerodynamic elements of F1 cars. The sensor design is based on a diaphragm that deflects under pressure, with an in-plane optical fibre sensor bonded to the diaphragm to measure the induced strain. The initial design was informed by analytical calculations, with Finite Element Analysis (FEA) subsequently providing information on the strain distribution over the diaphragm’s surface and revealing a nonlinear response, which were key to the final design decisions. Two optical fibre sensing technologies were investigated: Fibre Bragg Grating (FBG) and Fibre Segment Interferometry (FSI). Experimental analysis of sensors of differing dimensions made from candidate materials and formed using different manufacturing and sensor integration processes allowed refinement of the material choice and sensor fabrication approach. A sensor of diameter 25 mm and thickness 0.27 mm with an FBG sensor mounted on its surface exhibited a sensitivity of 56.7 ± 7.6 pm/kPa, equivalent to a resolution of 17.6 Pa, over a pressure range of 0 to 35 kPa. The sensor diaphragm was made from Glass Fibre Reinforced Polymer (GFRP), fabricated using a press mould with the integration of the FBG during the cure. The performance of FBG and FSI based strain sensing with the optical fibres bonded to the same GFRP diaphragm of diameter 21.5 mm and thickness 0.23 mm was analysed. This is the first reported use of FSI for pressure measurement. Over the pressure range of 0 to 35 kPa, the FBG sensor exhibited a sensitivity of 95 ± 4.4 pm/kPa, equivalent to a resolution of 10.5 Pa. The FSI sensor exhibited a sensitivity of 6.3 ± 0.3 rad/kPa, equivalent to a resolution of 0.61 Pa. The strain distribution on the diaphragm surface revealed by the FEA highlighted an annulus of compressive strain around the edge of the diaphragm, which was exploited using an optical fibre with FSI segments that covered the regions of compressive and tensile strain with a view to demonstrating temperature compensation. In addition to the measurement of pressure, there is an interest in the measurement of strain on composite suspension components using embedded optical fibre sensors. The process of embedding the fibre sensors and its influence on the signal quality were investigated, culminating in the calibration of the instrumented components and their testing on track. Several processes developed during this work are now in routine use at Mercedes AMG F1.