First results from in line strain measurements with FBG sensors on the pantograph collector of underground trains

FBG sensors are particularly suitable for strain measurements on mechanical parts subjected to strong electromagnetic disturbances. This is the case of strain measurements on the pantograph collector strip of underground trains, where, due to arcing phenomena caused by contact loss between the pantograph collector and the contact wire of overhead line during the current collection, high electromagnetic disturbances are present. Moreover, an intrinsic advantage of fiber optic sensors is the ability to electrically insulate the sensor from the conditioning and data-gathering unit. In this work the application of FBG sensors on the pantograph collector of an underground train, instrumented for experimental in-line tests, is presented. First, a FEM analysis of the collector to identify the suitable position, in terms of maximum sensitivity and fiber safety, for the strain sensor was performed. The position of a thermal compensating FBG sensor was also selected as a compromise between a place not too far from the main sensor (so that both are subject to the same temperature), and with a small (or negative) strain. The compensated signal is simply computed subtracting the compensator signal from the main one, and this can be considered roughly proportional to the total contact force. A static calibration was conducted in the laboratory with standard masses, and dynamic tests were carried out to dynamically characterize the pantograph. The FBG static measurements were compared with force measurements obtained from traditional sensors positioned on the pantograph collector interface, on each side of the collector, in order to obtain information on the position of vertical contact force. The total vertical contact force from the traditional load cells is compared with the FBG measurement obtained in the center of the collector. The comparison of the total force signals obtained during experimental results for both laboratory and in-line tests showed a good accordance, and these tests can be considered as a validation of the method, so that it can be further developed and used in the future in similar situations.