Bending plate WIM calibrations are to be carried out at least once annually as well as after each major repair or adjustment at the bending plates e.g. reinstallation of one or several frames, height adjustment and pavement changes/repairs in the vicinity of the sensors). Replacement of a bolt or fastening of rails does not require a recalibration.
There are main four factors influencing the accuracy of WIM systems:
- pavement quality (evenness), if poor, can lead to 100% error
- vehicle condition (tyres, suspension), up to 30% error
- driver behaviour (braking, accelerating), up to 50% error
- equipment (response to load introduced), typically 2% or less error.
Pavement quality and equipment factors are controllable by the user and are defined in ASTM E1318-02 (ASTM 2002) as follows:
- unevenness 60 m before and 30 m (20 m in New Zealand) after the scale should not exceed 3 mm under 6 m straight edge, measured with a 150 mm diameter, 3 mm thickness disc
- equipment response to load introduced should not deviate more than 2% for type I WIM systems.
The driver and vehicle factors are controlled when the equipment is being calibrated. When all four influence factors are controlled, the accuracy of type I WIM equipment is obtained. This accuracy is defined in ASTM as ±10% of gross weight (GW) at a 95% confidence level (2 Sigma). This may not be obtained with typical traffic when there are stop and go situations, lane changes or sensor bypassing.
The following are the steps for each calibration vehicle (such as multi-axle tractor trailer combination vehicles), loaded with solid material to their legal limit and empty:
- obtain static weight for each single axle, axle group and gross weight from certified weighbridge, measure axle spacing and total length, enter in protocol
- instruct driver to do two runs each at three different speeds; the driver should hold the speed without causing danger to other traffic and complete safe turning manoeuvres
- connect laptop to WIM and go into single vehicle display with logging
- conduct test runs, enter in protocol: measurement number, speed, GW, axle weights, axle spacing, total length and time of passage
- after the runs are completed, correct speeds by using measured spacing
- correct length by using speed and measured length
- enter all gross weights into speed versus weight diagram
- determine deviations for both vehicles to the three speed compensation points for each lane and calculate new correction factor by weighting: loaded 6 axle 50%, loaded 2 axle 20%, empty 6 axle 20% and empty 2 axle 10%
- enter weight compensation factors for each lane
- enter corrected speed (loop distance) factor for each lane
- enter corrected length (loop length) factor for each lane.
As a quality control, the mean of the front axle load of type 851 trucks is calculated weekly. The expected value should be between 4.8 and 5.1 tonnes to give an indication of sensitivity changes. With variations greater than 5% measured consistently over four weeks, the overall sensitivity parameter in the WIM electronics is adjusted remotely.
Common Data Errors
Traditionally data errors are mainly due to faulty sensors:
- Erratic speed, spacing, length and high number of unclassified vehicles can be due to loop problems. Either one or both loops have failed (90% of cases) or the detector is faulty. Usually detected by the higher number of unclassified vehicles or no vehicles at all or less than the usual volume.
- Erratic weights, no weights, high scatter and vehicle misclassification are usually due to pavement or weigh pad problems.
- Pavement problems like potholes will increase the scatter of weights and can lead to misclassification by not detecting bouncing axles.
Once the sensor outputs are digitalised errors in data resulting from processing/transmitting through modem and evaluation are very rare. Communication problems are usually resolved by the telecommunication provider within hours or days after reporting. Due to sufficient onsite data memory the loss of data is very rare.
Currently the general data errors can be caused by:
- 50% pavement
- 40% weight sensor (bending plate/frame/suspension/cable/ converter)
- 5% loop (usually a break is caused by pavement break outs)
- 5% other (electronics/accidents/vandalism/lightning).
Therefore it is required that:
- the pavement is periodically checked for level and rectified
- calibration is carried out with a vehicle of known axle weights and speed
- data is monitored for errors and deviation.
International Road Dynamics 2007, ‘Operational manual Australia PAT DAW 100’, International Road Dynamics, Saskatoon, SK, Canada.