Plate-in-Road: Capacitance WIM

The main components of a capacitance WIM system (also known as capacitive WIM system) are:

  • a traffic data logger
  • inductive loops (two per lane)
  • capacitance weigh sensors (one or two per lane)
  • optional piezo axle detectors to enhance detection of very light axles and provide detection of axles in lanes where no WIM sensor is installed.

Configuration options chosen for a specific logger determine the sensor configuration. Each logger is configured for a specific application. Therefore, not all the interfaces are automatically included.

Typical loop dimensions are 0.100 m x 0.300 m, spaced 2 m apart (other dimensions can also be used). Refer to sensor configuration diagrams on page 4 of the Mikros Systems Capacitance HSWIM Product Specification.

Figure: Example of a capacitance pad or mat

Fig 5.5

Source: Mikros 2008a

Capacitance pads, or mats, consist of two metal sheets separated by a dielectric material. An outer surface layer surrounds the sensor, protects the steel plates and allows the sensor to be placed on the pavement or in a mounting frame. A voltage is applied across the two metal plates. When a vehicle crosses over the plate system it causes the distance between the two plate systems to decrease which increases the capacitance of the system. Measurement of the resonance frequency of this circuit allows the estimation of the weight of each tyre as it is applied to the sensor system.

Permanently mounted capacitance mats differ from portable mats in that the former mats are placed in steel frames that are installed in the pavement surface. This allows the surface of the mats to be flush with the roadway and improves the accuracy of the system. It also reduces the impact load on the sensor itself, both increasing sensor life and decreasing the potential for the sensor to be dislodged from the roadway.

Most capacitance mat systems rely on weighing only one wheel path. This limitation makes them slightly less accurate than other WIM system alternatives. However, vendors do sell permanent capacitance mat systems that use mats in both wheel paths.

The Mikros Systems capacitance high-speed-weigh-in-motion (HSWIM) system is the major capacitance WIM system in Australia.

Figure: Mikros capacitance weigh sensor

Fig 5.6

Source: Mikros 2008a

Further reading

Mikros 2008a, ‘Capacitive HSWIM product specification MS001-51000-40’, issue 7, Mikros Systems, Pretoria, South Africa.

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