Any weighing device, whether it is a bathroom scale or a WIM system, must be calibrated at some time to relate the output from the device doing the measuring to some unit of weight. This is achieved by loading the platform with objects of known mass and recording the various outputs from equipment so that a mathematical relationship between the two can be derived.
A mass sensor produces a signal whose value depends on the instantaneous dynamic wheel weight of a moving vehicle. When the output for the sensor is properly calibrated, a mass measurement is produced. Calibration is the process of adjusting the combined output of a mass sensor and site characteristics to match the measurements of the static mass.
WIM calibration as such has two components:
- calibration of the mass sensor
- calibration of the site.
The following process, based on ASTM 1994, is recommended for the calibration of any WIM system:
- WIM system settings should be adjusted to the vendor's recommendations or to a best estimate of proper setting based on previous experience.
- Vehicles used for calibration should be weighted on a static scale near the site to obtain static weight data for referencing against.
- Tire loads and axle spacing should be recorded at the static scales.
- Speed data should be recorded as vehicles move through the WIM sensors.
- The difference between the WIM system estimate and the reference values for speed, wheel loads, axle loads, axle group loads, gross vehicle weights and axle spacing measurements should be calculated and expressed in percentage and a mean value, for each set of measurements.
- The calibration factors should be entered into the WIM system.
- Whether the calibrated system can be expected to perform at the necessary tolerances should be determined. If there is a large number of differences between the reference and the data measured by the WIM system, it is more likely that the system would not perform to a beneficial level.
It is recommended to conduct on site calibrations annually and anytime when the data appears to be shifting.
In between field calibrations, automatic desk-top calibrations can be conducted by looking at the rate of change of the ratio of the vehicle steer axle mass to GVM.
The software is designed to detect common types of WIM system failures and possible drifts in calibration, by comparing the data being reported for each site with expected ranges of data for that site.
The LTPP QA software is WIM system independent and performs the following:
- preliminary format and range check
- GVM distribution of five axle articulated trucks (A122) check (common heavy vehicle in the US).
A concern with the LTPP QA software, and any automatic calibration system, is that the possible changing nature of vehicular loading at any particular site could be incorrectly interpreted or mistaken as a drift in calibration. This would have the effect of re-calibrating a WIM site to achieve an expected or predetermined result. The use of a steer axle, that is, a standard non-changing load, as in the case of the Australian A123 vehicle would alleviate this problem.
The Australian developed WIMLINK software system has a QA component which is modelled on the US based LTPP study, but uses 'continual' calibration techniques (in addition to the field based methods) that are fine-tuned for individual WIM site characteristics (Austroads 2000).
As with other operational characteristics, the calibration requirement varies among specific WIM systems. In general, it is desirable for WIM systems to be routinely (automatically or at regular intervals) checked for possible calibration drift or equipment failure.
Austroads 2000, Weigh-in-motion technology, AP-R168-00, Austroads, Sydney, NSW.