Invented and designed by the Bundesanstalt fuër Strassenwesen (BAST), Cologne, Germany in the late 60s, bending plate technology has been widely used in numerous long-term traffic and pavement monitoring programs.
NZTA is using the PAT bending plate technology at four TMS sites and two further sites at the Auckland Harbor Bridge for special studies with a total of 20 lanes. All sites are continuously collecting individual truck records and statistics downloaded weekly, or daily if VBV data of all vehicles are recorded.
The bending plate has also been used in Australian road networks such as Eastlink in Melbourne.
Figure: Example of bending plate site (Eastlink, Melbourne)
Source: VicRoads 2008
The sensor of a bending plate generally consists of a high strength steel plate either 1 250 or 1 750 mm wide and 500 mm long.
Strain gauges as used for precision weighing in commercial scales are bonded to the bottom of the plate in milled grooves and wired to a wheatstone bridge. The gauges are sealed with a proprietary coat and the whole plate is covered with a vulcanised rubber sheet. A cable connects the bending plate with the electronics. The bending plate is a load cell where the load receiving plate is combined with the load measuring device. This makes it different from all conventional constructions where load cells and platform are two separate components. Designs using separate components have, due to the interface problems, long been taken off the market.
The bending plate allows more accurate measurement of wheel load as its whole footprint, typically up to 250 mm in the direction of travel, is measured at the same time. Strip sensors are less accurate as they only see a small part of the footprint.
The natural frequency of the bending plate's vibration is about 10 times higher than the frequency introduced by multiple axles with short spacing, thus ensuring the bending plate does not resonate after the axle load is applied. The slight bending under load (approx 1 mm under 4 tonne wheel load) is negligible. The shocks introduced by wheels are mainly absorbed by the rubber bearing strips at each edge of the plate. The plate rests in a foundation frame and is secured with steel rails.
Besides the inductive loops for speed and length measurement, the bending plate WIM does not need additional sensors for axle detection and separation.
PAT DAW 100 is one of the most popular bending plate WIM systems applied in Australia and New Zealand. The figure below provides the physical configuration of sensors for a typical DAW 100 system.
Figure: Typical layout of plate-in-road sensors and electronics
Source: IRD 2007
The DAW 100 WIM system is based on 16-bit technology with multiprocessor functions imbedded in each of the vehicle detector and piezo detector processing boards. Each of the two separate WIM systems is housed within a separate roadside control cabinet, and powered by a 12 V battery and charger. A separate serial WIM data output is used for additional data logging in ASCII VBV (vehicle by vehicle) format.
One pair of fibre optic modems is installed on each system for data downloading and maintenance purposes and vehicle data logging, separate individual fibre node connection being made to the control PC at a remote location.
There is remote monitoring of sensor and power failures via the VBV data packet sent from Com 2 on each WIM system.
The logging program monitors any sensor failure triggers on a daily basis and provides a separate logged count as well as a visual indication. The logging program also synchronises the WIM system clock to the server clock on a daily basis. All logging, time syncing and close/open new file functions are carried out at 00:00:50 AM nightly.
The PAT/IRD system has recently updated its electronics, known as iSINC.