PROBLEMS OF FIBRE OPTIC SENSOR APPLICATION IN WEIGHT-IN-MOTION (WIM) SYSTEMS

In the last decade the weight fibre optic sensors, based on the change in the optical signal parameters due to optic fibre strain under the weight of a passing transport vehicle [2], have gained popularity. These sensors are more durable, they are relatively cheap in manufacture and operation. However, they are mainly used as detectors for vehicles because of the low accuracy of weight measurement (especially weight-in-motion) and high dependence on weather conditions. In the 1990s the weighing and control of transport in motion systems using fibre optic force sensors appeared. Their usage was justified by the low cost and ease of installation on roads with heavy traffic. Fibre optic cables are placed in narrow grooves across the road and are filled with resilient rubber, transmitting the tyre pressure on the cord. The traffic flow should not be interrupted for a long time, so the ease and speed of the sensors’ installation outweighed its drawback – low measurement accuracy. The tasks of weight-in-motion systems are the following [2]: • Sorting of vehicles by type with the accumulation of statistics. This allows us to determine the traffic load and load on the road surface during the specified periods of time; • Selection from the traffic flow of vehicles, axes of which, according to system indicated value, exceed the axle weight limit. After weighing the vehicle on more accurate scales, it is released, but it is assessed upon violation of rules. • Transfer of the statistical data into a centralized system of transport accounting and traffic management in the region. Analysis of current trends on WIM issues indicates that optical sensors, which are more reliable and durable in comparison with tensor and piezoelectric, are based on two main principles: • Bragg grating (the change of diffraction in a channel under deformations) • Change under deformations of the fibre optical properties (transparency, frequency, phase, polarization). It is the change of transparency (the intensity of the light signal) which is used in the SENSOR LINE SPT experimental sensors [5] on which these studies were carried out.