Keywords: optical fibre sensors, fibre Bragg grating sensor, condition monitoring.

Unprecedented demands on modern rail systems and the historical context of limited maintenance and servicing has driven a requirement to develop novel techniques for monitoring and managing the networks with minimal disruption to service. Traditional maintenance techniques are incompatible with both the current loading of these networks and the business models employed to run them. This paper reports on the deployment of optical fibre sensors to monitor sub components of a tramway network; fishplates, switchblades and stretcher bars.

While optical fibres are widely used as the transmission line in high bandwidth communication systems, the properties of the light travelling within the optical fibre can be perturbed by local environmental parameters, such as strain and temperature, allowing for the development of sensing techniques in which the optical fibre can acts as both the transmission line and sensing element. The optical fibre sensor technology exploited here is the fibre Bragg grating (FBG), which offers a means for labelling a short section of an optical fibre to allow it to be uniquely identified and interrogated to measure the local strain and temperature. It is possible to form an array of strain sensors within a single optical fibre. Optical fibres, which have a diameter of 125 µm, can transmit light over many tens of kilometres with low attenuation, which offers the potential for monitoring large structures or networks from a central sensor interrogation unit. The typical gauge length of the sensor is of the order of mms, and the sensors can be fabricated with arbitrary physical separation, allowing a high density of sensors to be deployed where large strain gradients are expected, or a sparse array to monitor widely spaced points on the network. FBGs do not suffer from issues of zero drift, are immune to electrical interference and the data collection and processing units can be configured to offer strain resolutions of order 1 µm/m, with data rates ranging from Hz to 100s kHz.

In the work described in this paper, arrays of FBG sensors have been attached to fishplates, switchblades and stretcher bars on the Sheffield Stagecoach Supertram Network to monitor the response of the components to loading by passing trams, with the aim of the identification of signatures that may be used as indicators of damage or wear. The principles of the sensing technique, properties of the sensor interrogation and demultiplexing unit and methods for attachment of the sensors will be discussed. Results obtained from field trials on the tram network will be presented that indicate that the measurement of strain on a fishplate can provide information of the torque of the bolts. The dynamic responses of stretcher bars and switchblades measured using FBG sensor arrays will also be discussed, showing that the data can reveal issues related to the configuration and status of the switch.

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