Capacitance WIM Calibration

Mikros-2005b recommends the following method for calibration and verification of HSWIM capacitance pad installations.

Ideally, a two-axle truck with high quality suspension (air suspension preferred) should be used for calibration and verification. Alternatively more than one calibration truck can be used. The same procedure must be followed as with one truck.

The detailed data capture and analysis procedures are part of the TELSYS suite of programs; the operation of the calibration data capture procedures are described in Mikros-2008b.

It is of the utmost importance that if trucks of more than two axles are used, all of the axles be weighed on the reference scale in one action. Enough scales must be available to do all the axles (wheels) at once. This means that for a five-axle (tractor semi-trailer) truck, ten wheel scales, or five axle scales, should be used. Should a segmented weighbridge be used it must be kept in mind that axle weights are not available.


Equipment Needed

Calibration truck

Two-axle truck (or multi-axle trucks) with good suspension (air suspension preferred) capable of maintaining the average running speeds of trucks over the HSWIM site are required. The truck(s) should be evenly loaded with a stable load to at least 75% of the legal axle limits. This axle limit must be the legal road load limit.

Reference scales

The truck must be weighed at a legal static weighbridge. In addition, static portable scales conforming to at least OIML No. 76 Class 4 (OIML1995) should be used.

Traffic control

Adequate traffic control must be provided for the turnarounds. Under no circumstances shall turnarounds be permitted without the required authority and prescribed traffic deviations. Should it be more beneficial to close off a whole lane to allow the truck to reverse back to the start of each run adequate length of lane closure must be provided.

Trained driver

A professional driver must be employed to ensure that the truck will be driven smoothly over the site. No sudden braking or gear changes are allowed over the HSWIM site.

Data collection team

A team of two observers at the data logger must be provided; observer one to check for the truck arriving at the WIM site and observer two to tag the vehicle on the laptop connected to the logger. Two-way radios might be needed if the site geometry does not allow for direct communication between the two observers.

Laptop computer

A laptop computer is needed to tag and collect the calibration information.


 Methodology

  • The calibration truck must first be loaded with a stable load of at least 70% of the legal road axle limits and then be weighed at a legal static weighbridge (in total and axle by axle). This step can be substituted by only using the weights from the static wheel scales (minimum of five readings).
  • Close the affected lane and weigh the calibration truck with two static wheel scales. Take five readings per axle (pull off and drive on). This must be done at the position of the sensors to allow for local crossfall and camber conditions.
  • Calculate the average of these values and check with the weighbridge values. It may be that due to the local geometry the axle values can be slightly different. Do not accept a difference larger than 1.5%.
  • Place the observers at the running logger. Connect the laptop and select the calibration mode of TELSYS.
  • Observer A (in view of the roadside) must warn observer B (at the logger) of the approaching calibration truck. Observer A must also closely note any abnormal driving behaviour due to traffic or other conditions (sudden braking, swerving or missing the sensors at the site). Any suspect data due to these abnormal conditions must not be taken into account for calibration and verification.
  • The calibration truck must pass over the sensor area at the normal prevailing speed for trucks at the site. The driver should maintain normal lane position and drive smoothly without gear changes or sudden braking.
  • Turnaround should be done under approved conditions and traffic control.
  • Collect a minimum of 40 sample runs to allow for the discarding of possible erroneous runs. For proper statistics a minimum of 34 valid runs are needed per lane.
  • The data captured in TELSYS is also analysed in TELSYS. Refer to the calibration part of Mikros-2008b.

 


 Further Reading

Mikros 2005, ‘HSWIM calibration instruction TEL equipment MS001-00006-56’, issue 3, Mikros Systems, Pretoria, South Africa

Mikros 2008, ‘Introduction to the mass calibration function of TelWinPro’, quick guide, Mikros Systems, Pretoria, South Africa


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