Operation of WIM systems
Before an end-user can use the mass data for the intended application first the data from the WIM systems needs to be collected, stored and verified. Due to the nature of WIM systems and the ‘hostile’ environment in which they have to operate, damage, wear and tear on the system’s hardware and the surrounding pavement are part of normal operation. This means that the performance of all WIM systems will degrade over time, normally slowly because of wear and tear but sometimes more quickly in the case of damage to key parts of the system. In the operation of a WIM system three periodical processes can be identified:
- Visual inspection, this includes a quick visual inspection of the sensors and the pavement directly surrounding the sensors, without closing the lane for traffic. The goal is to look for any visible signs of damage to either the sensors or the pavement. Normally, visual inspection is not sufficient to detect internal defects, but it may well be sufficient to observe some serious installation problems. The sooner such a problem is detected, the sooner the issue can be resolved.
- Sensor maintenance, the frequency and nature of the sensor maintenance strongly depends on the type of WIM sensor technology, the type of pavement, and the traffic and environmental conditions. WIM sensors with moving parts may become filled with dust or mud that will have to be removed after a certain period. The top layer of certain strip and bar sensors installed in asphalt pavements will have to be ground flush to the pavement surface, to compensate for increased rutting.
- System calibration, the objective of a system calibration is to establish the average measurement error of the system. The measurement error is the difference between the value measured/calculated by the WIM system and a static reference value. Once the average error has been noted this value can be used to make adjustments to the system. Strictly speaking, the term calibration means just the act of comparison, and does not include any subsequent adjustment. In daily WIM practice ‘calibration’ means both, calibration and justification.
- Data Quality Control, the quality (accuracy, reliability and stability) of the WIM data used in any application directly determines the quality of the results. Data quality checks will look at the stability of certain elements or characteristics of the measured data.