The capacitance pad weigh sensors and loops are to be installed only after the final pavement has been provided or is available. This ensures that the mounting frames will be perfectly flush with the road surface. This is extremely difficult to achieve if the frames are pre-installed.
For example, the Mikros Systems capacitance sensor is secured to a special flat mounting frame that requires only a 45 mm recess to be excavated into the pavement (shown in the figure below). The sensors can only be installed in suitable pavements. Under normal weather conditions one lane can be fully installed in less than six hours. The sensor is bolted into an installation frame that is secured to the pavement with epoxy grout and a number of road anchors. The sensor is installed perfectly flush with the road surface.
The figure shows the elements of a 2 m capacitance WIM sensor installation. It is possible to replace the sensors with dummy spacers that will allow the sensors to be relocated at other installations.
Figures: Illustration of excavation and 2 m capacitance WIM sensor installation elements
The same procedure applies, whether installing the Mikros Systems capacitive WIM sensor in a 'thin' or 'thick' pavement. The only difference is that, in the case of thin pavements, a support slab is provided directly under the installation frame.
Figure: Drilling the anchor holes into the frame
Figure: Aligning and setting the frame
Figure: Close-up of a road anchor (length = 180 mm, width = 15 mm and hook = 40 mm)
Figure: Close-up of the anchored frame
Note: The frame is initially set lower than the road surface. The sensor is then shimmed to the desired height. The advantage of setting the frame lower than the road surface is that the sensor could be lowered in the case where upward local pavement settlement forces the frame up. In the case of downward settlement, more shims can be added to lift the sensor.
The main installation steps are:
- excavating the recess
- providing the support slab in the case of thin pavements
- grouting the installation frame into the recess
- placing and shimming the sensor to be flush with the road surface
- final sealing of the sensor and frame.
A number of housings are available to suit the local conditions. The RAKTEL 8000 of Mikros WIM is a standard 19” rack and it can be housed in any standard 19” rack system.
Figure: Standard 19” rack system
A more secure housing can be used at sites where security is a concern. Internal heating is provided in cold climates where condensation is a problem.
Figure: A secure housing
Figure: A housing with internal heating
Mikros 2005, ‘Installation of fixed capacitive HSWIM sensors MS001-96100-52’, issue 5, Mikros Systems, Pretoria, South Africa
Mikros 2008, ‘Capacitive HSWIM product specification MS001-51000-40’, issue 7, Mikros Systems, Pretoria, South Africa