WIM systems generally comprise the following basic components:
- vehicle classification and/or identification sensor
- vehicle classification and/or identification sensor
- processor and data storage unit
- user communication unit
Figure: Components and layout of a typical WIM system
The mass sensor (and the output signal it produces) is the most fundamental and important component of a WIM system. The other components are peripheral to it, in that their abilities are ultimately dependent and indirectly judged by its performance.
Mass sensors are positioned on or within the road structure. Based on this positioning, they can be temporary, semi-permanent, or permanent.
Temporary mass sensors are usually surface mounted and fixed so that they can be fully removed, transported and re-installed at another location. Semi-permanent mass sensors are similar to temporary ones, but only one component of the sensor can be removed and re-installed. Usually the housing or cradle is permanently fixed and the gauge, transducer or signal processor is removable. Permanent mass sensors are designed to be installed once (surface or in-depth) and not removed, at least without causing damage.
Mass sensor technology is constantly evolving with new sensor types being tested and evaluated and some past failures being revisited given advances in technology.
The most widely used mass sensor mechanisms in Australia and New Zealand are as follows:
- Bending plate technology incorporates steel/rubber plates with strain gauges permanently attached to their underside such that the strain gauges behave as one with the plates. The strain gauges develop a strain signal proportional to the deflection of the plate under a vehicle axle. The strain signal is amplified and processed to produce the vehicle axle mass. Some examples of bending plate systems used in Australia and New Zealand are the PAT bending plates; DAW 100, 190 and 200.
- A capacitance pad mass sensor is a rubber and steel mat device comprising three sheets of steel separated by a soft rubber dielectric material. Compression of the pad under a vehicle axle produces an increase in capacitance, which is interpreted as a mass. One of the best known capacitance pad WIM systems is Mikros.
- Strain gauges mounted to an existing road structure, such a culvert, use a flexible insulating material, typically a copper wire, adhered to a rigid material. As a vehicle axle passes over the structure or housing, the induced strain is measured and related to mass. One of the most popular stain gauge systems is Culway. Culway weighs and classifies traffic by using mechanical strain gauges deployed in a box culvert. The Culway system employs two piezoelectric sensors, placed 10 m apart in each instrumented lane to detect vehicle passage and synchronise the strain measurement at each axle.
- Piezoelectric cable is usually mounted in an aluminium U shaped channel. The channel is placed into the road surface. The passage of a vehicle axle over the cable creates a wave form of current that is proportional to the axle mass. Kistler Lineas TDC Hi Track and MSI Sensors Roadtrax BL are the most popular peiezoelectric WIM in use in Australian and New Zealand.
In addition to axle mass determination, most WIM systems offer the ability to fully or partially classify vehicles in a traffic stream.
Classification is accomplished via the use of a variety of different vehicle detection sensors, placed adjacent to the mass sensor. Typical vehicle detection sensors include loops, piezoelectric cables, treadle switches, and tubes.
Vehicle identification takes the form of a picture or video image. The camera or video is placed off the road in a specialised housing, and is designed to identify or capture the vehicle's registration number or other unique feature. More advanced technologies allow identification of a weighed vehicle through so-called 'smart cards' carried on board the vehicle via a base station adjacent to the WIM system.
The processor and data storage unit is usually installed at the roadside.
Depending on the WIM system it is either secured to a fixed roadside item or in a protected cabinet.
The processor is connected to the mass and vehicle classification and/or identification sensors, receiving and analysing the incoming signals to produce individual vehicle by vehicle and/or user defined summary reports. This unit also powers the WIM system.
This power can be mains, rechargeable battery or solar based.
The processor and data storage unit can also be the user communication unit in that it can directly display the collected data.
Usually the user interfaces with the processor and data storage unit via a communication link as follows:
- specially designed retrieving unit linking directly into the processor and data storage unit
- personal computer linking directly into the processor and data storage unit
- modem and telemetry linking the processor and data storage unit into an offsite user unit.
Although the pavement is not a component of a WIM system, it is integral to the accuracy of a WIM system.
Pavement roughness, rutting, rigidity and porosity, in addition to a road's geometry, gradient and drainage, all have a bearing on the horizontal and vertical forces vehicles apply as they pass over sensors.
Pavement specifications, such as maximum allowable roughness and rutting, are outlined for each sensor technology in Site Selection.