The calibration should not be performed within 72 hours of the installation of the sensors. It is recommended that calibration take place after two weeks of traffic having passed over the WIM site.
The calibration procedure is dependent upon the end-users' requirements. It should clearly state the type of vehicles, the number of runs, the loads, the equipment to be used and other relevant issues.
Piezoelectric WIM calibration is relatively simple. The following covers some general issues of Hi-Trac piezoelectric WIM calibration.
Calibration procedure and calibration vehicles
Basic calibration consists of passing a number of vehicles (normally three different types) over the sensors in each lane and recording each axle and gross vehicle weight on each pass.
Each vehicle is loaded to known axle and gross vehicle weights and checked at a calibrated static weighbridge prior to calibrating the HSWIM system.
Wherever possible it is recommended that three vehicles are used:
- 2 axle rigid vehicle loaded to 16 000 kg (approx.)
- 3 axle rigid vehicle loaded to 25 000 kg (approx.)
- 5-6 axle articulated vehicle loaded to 35 000 kg (approx.).
Where these vehicles are not available it is recommended that at least one vehicle with three or more axles is used. The recorded axle and gross vehicle weights are compared to the weights recorded on the static weighbridge and corrections are made to ensure results are to an acceptable level of accuracy.
Factors or parameters
Note that weight results are subject to variation due to a number of parameters that are independent of the electronic system; to this end site selection is paramount to system accuracy.
These parameters include:
• road condition and smoothness of the surface
• geometric parameters (road is on an incline or curve)
• vehicle change of speed (accelerating or braking).
To provide the most accurate results a Hi-Trac system (TDC Systems-2004) should be installed, commissioned and left collecting data for approximately one or two weeks after which corrections utilising the temperature non-linearity functions are made prior to the actual calibration.
System re-calibration should be carried out on a 6-8 month basis.
Refer to TDC Systems-2005 for further information.
TDC Systems 2004, ‘Hi-Trac 100 high speed weigh-in-motion data collection and pre-selection system utilizing Hi-Comm 100 software, system overview’, TDC Systems, North Somerset, United Kingdom
TDC Systems 2005, ‘BL piezo electric sensor installation’, TDC Systems, North Somerset, United Kingdom