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Load Measuring Mat and Weigh-in-Motion

The development of weigh-in-motion sensors and the TRG's Load Measuring mat. Multipl-sensor Weigh-in-motion technology.

In 1986 it was recognised that the standard methods then in use for measuring vehicle dynamics (i.e. instrumenting individual vehicles) significantly limited the scope of research into road damaging potential. Each vehicle had to be equipped with extensive and costly instrumentation, and complex data logging and analysis procedures were necessary. This limited the tests that could be performed to a few specially prepared vehicles, tested on special purpose test tracks.

It was proposed to instrument the road rather than the vehicle, and the idea of a 'wheel load measuring mat' was borne. Unfortunately, no vehicle weighing technology, available at that time, was found to be sufficiently accurate for the requirements of the research. So over the following 4 or 5 years, the researchers patented and developed a new type of capacitance strip sensor in collaboration with Golden River Traffic Limited. This sensor has many advantages over other types of WIM sensors - in particular its low cost, high accuracy, linearity, and insensitivity to temperature, vehicle speed variations, and tyre configuration.

Two large scale installations of these sensors were undertaken for research into the road-damaging characteristics of heavy vehicles: one funded by the US Strategic Highway Research Program (SHRP) and one in the UK funded by the EPSRC. In the UK tests, 155 WIMstrip sensors, mounted in 12mm thick polyurethane tiles, were installed on the A34 trunk road near Oxford. Over a period of three days of testing, the dynamic tyre forces generated by some 2000 heavy goods vehicles were recorded. The wheel load data was analysed to investigate the patterns of wheel loading generated by mixed traffic, and the effects of various suspension types on dynamic tyre forces.

Load Measuring Mat

Wheel load measuring mat with 150 capacitance strip sensors, installed on the A34 near Oxford

One interesting discovery was that some of the highest dynamic tyre forces were generated by vehicles with (so called) 'road-friendly' air suspensions. The reason for this apparently anomalous result is that air suspensions require well maintained shock absorbers to provide their damping - otherwise their bouncing motion can be lightly damped and they can generate very high dynamic loads. Existing EC regulations for road-friendly suspensions encourage the use of air suspensions by providing a 1 tonne payload incentive to operators. However the regulations do not control their in-service dynamic loading performance, or suspension and damper maintenance. It seems, therefore, that current regulations have the opposite effect to that intended by the European Commission... In a significant proportion of cases, they increase rather than decrease dynamic loads and the resulting road damage.

The load measuring mat has also been used to devise strategies for improving the accuracy of WIM systems. Using arrays of 2, 3 or more sensors, it is possible to compensate for the weighing errors caused by dynamic loads generated by the vehicles as they 'bounce' over the WIM site. WIM accuracy can be improved markedly. The researchers have shown that it is possible to measure individual static axle loads with only 5% RMS error. This compares with (typically) 12-15% RMS error for single sensor WIM systems.


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