A temperature-compensated fiber Bragg grating sensor system based on digital filtering for monitoring the pantograph–catenary contact force

Abstract

Optical sensors based on fiber Bragg gratings have become an important type of sensing element for strain measurements. In this study, fiber Bragg grating strain sensors and corresponding reference temperature sensors were integrated into a pantograph strip. The results of an online test indicated that the temperature of the pantograph strip impacting the strain measurement reaches about 80  ℃ while the vehicle was running. In addition, Fourier analysis showed that the energies of the temperature measurement signals dominated the relatively low-frequency band (less than 0.02 Hz), whereas the strain signals prevailed in the relatively high-frequency band. Thus, a novel method based on a digital filtering technique was proposed for temperature compensation in fiber Bragg grating sensor systems for monitoring the conditions in electrified pantograph–catenary systems. A Butterworth high-pass filter was designed to reject the temperature-related signal component with a stopband and to capture the real strain in the passband in frequency domain. To achieve this filtering, the cutoff frequencies and the filter order were calculated adaptively according to the frequency-domain characteristics of the measured temperature signal. With the designed filter, the temperature effect to the strain signals can be eliminated so that the strain can be estimated accurately. In comparison with the traditional temperature compensation technique, the proposed method is more effective in terms of estimating the real strain of the pantograph strip in practical applications. Compensated by the proposed method, the time-domain and frequency-domain analyses of the contact force denote the frequencies corresponding to the support span, and the dropper can be distinguished. Further, the first-order (4 Hz) and second-order (8.3 Hz) natural frequencies of the pantograph are visible in the measurements from catenary sections, which demonstrates the sufficiency and rationality of the proposed temperature compensation method for fiber Bragg grating sensor systems.