WeighingThe SPx fiber optic axle sensors used with CLASSAX deliver analogous output signals which are load-dependent and show a clear difference between light vehicles and heavy ones. This fact is enticing people to assume that it should be possible to determine the weight of passing vehicles this way.
Weigh-In-Motion (WIM), however, is a cumbersome workscope. Even if a vehicle is run over a scale which accepts it completely the quality of the measurements is far from being comparable with that of static ones. Not for nothing scales often carry a sign requiring to stand still during weighing. Cable-shaped axle sensors can not even accept the whole footprint of a single axle. Thus it is not even guaranteed that the sensor is touched at all: If the tire profile happens to have a groove at the wrong place it may perform a downright step over the sensor. This example may appear a little overstated but, nevertheless, since the tire is always also supported by the pavement beside the sensor there is nothing which limits errors caused by peculiarities in tire profile, tire diameter, tire width, rubber composition etc to some certain value.
With this in mind remaining difficulties associated with temperature effects, pavement deformations, reproducibility of manufacturing and installation and so on appear marginal. Still, these difficulties alone would be far sufficient to reliably confound any stable calibration.
There is, however, a WIM application left which appears quite feasible: For law enforcement there are static scales installed at motorways allowing to check the weight of passing vehicles which must be directed out of the traffic flow for this purpose. These weigh stations have limited capacity; often it is impossible to inspect all passing vehicles, not even all passing trucks. This creates a need for some means to pre-select vehicles which appear to be overloaded for being called out.
The attempt to tell for example how many times heavier a semi-truck than a passenger car appears quite hopeless with cable shaped axle sensors because the different footprints lead to different sensor characteristics. What appears possible, however, is telling which one of two passenger cars or which one of two semi-trucks is heavier than the other, and at this point it must be remembered that CLASSAX is able to classify vehicles – even according to nothing but their footprint.
So everything which is needed to feed a weigh station in some optimized manner is a CLASSAX system with a suitable classification scheme and a particular weight threshold value for every vehicle class. If a vehicle of a certain class appears heavier than the threshold value for its class it is called out, otherwise ignored. If the number of vehicles waiting at the weigh station is growing the threshold values are increased a little, in the opposite case decreased. This way it should easily be possible to operate a weigh station constantly at its maximum capacity with only the most suspect vehicles of the passing traffic.
It has not yet been investigated what is to be understood as a "suitable" classification scheme. Whether, for example, any truck may be compared with any semi-truck or whether even two-axle trucks must be handled different from three-axle trucks are questions to be answered. Designing a good control algorithm for the threshold values is also a challenging but feasible job for the future.