Fiber-optic sensors for weigh-in-motion application

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Virginia Polytechnic Institute and State University


Automated techniques to acquire weight and traffic data are indispensable to effective management and maintenance of the vast network of highways. Weigh-In-Motion (WIM) systems have the potential to greatly reduce the cost and improve the accuracy associated with weight data collection.

The existing WIM systems utilizing piezoelectric cables have been shown to result in rather large random errors of up to 12% and need to be installed at permanent sites. In addition, the exponential decay of the output signal of the piezoelectric cable with time can cause complications in signal processing and possibly further errors.

In this thesis, a fiber-optic sensor is proposed which measures the pressure generated by the weight of a vehicle. The system consists of a pneumatic tube filled with an incompressible fluid, a rubber pad embedding the tube, a diaphragm to convert pressure into displacement, and an optical displacement sensor. A prototype of the proposed sensor is designed, manufactured and tested in the laboratory.

Both piezoelectric cable and the optical sensor are tested under varying load-frequencies~ It is shown that the piezoelectric cable sensor shows considerable dependence on the load frequency, whereas the response of the proposed system is much less frequency dependent and, unlike the piezoelectric cable has a waveform similar to that of the applied load. This latter property can significantly reduce the difficulties associated with signal processing. Besides, the linearity of response over the range of loads applied is better than that of the piezoelectric cable. This implies that the proposed fiber-optic sensor with its high rate of accuracy can be implemented under conditions where piezoelectric sensor does not deliver accurate results. For example, when equally loaded axles at different vehicle speeds and axle configurations are used.