PAVAS Calibrate Wheel with Survey Distance

This menu is used to calibrate the wheel by driving over a known distance. In the PAVAS system, the wheel bolt passing a magnetic sensor generates a pulse which is fed to PAVAS. By simply counting the pulses over a know distance it is easy to determine the distance the wheel has travelled between one set of pulses. This measurement technique takes into account the “bag radius” of the tyre. Put in another way, it allows for the deflection in the tyre as it absorbs the vehicle weight.

When the menu is entered, the distance measured is set to zero and a messages to press Select to start is displayed. If the vehicle is moving, the uncalibrated distance and calibrated speed will be displayed but will not form part of the calibration.

To start the procedure from the menu, press Select. PAVAS displays the uncalibrated distance in meters and the uncalibrated speed. Drive the surveyed distance. The vehicle speed may be quite fast since this will affect the “bag radius”.

During the calibration run, pressing the Left or Right buttons can lead to an Abort procedure. In this case the Abort message is displayed along with a count down. To execute the Abort, the Select button is pressed. If any other button is pressed, the Abort procedure to cancelled. It a button is not pressed in six seconds, the Abort procedure is cancelled.

To reduce the survey distance, some competitors prefer to drive back over the same survey distance and add it to total distance traveled. PAVAS allows the calibration distance measured having a Pause function. This is engaged by pressing the Up button. To resume the calibration distance measurement, the Down button is pressed.

The Select button is pressed once the survey distance is complete the uncalibrated distance and and an accuracy value is displayed based on the number of pulses counted. The survey distance travelled is the entered with the cursor editing techniques and then the Select button is pressed again. PAVAS now display the WPD in millimeters.

There are a number of considerations with regard to the accuracy of Survey distance technique. The first point is what should be the length of the surveyed distance. Wheels Pulses Distance (WDP) will vary with each vehicle but a usual value is approximately one meter. To achieve and accuracy of 0.1% the distance to be driven would one thousand times this or 1 Km. Greater distances will achieve greater accuracy but the whole procedure depends on the accuracy of the survey. The Pause/Resume technique described above also introduces errors when the vehicle is turned around because the wheel may not be in the same position as when the calibrated distance measurement was paused. Hence the error maybe increased by +/- one pulse. Some roads have “speedo check” zones which show distances up to 5 Kms. These can be used as well but the accuracy is a function of the survey. The Survey Distance technique can also allow for dynamic “bag radius” changes at increased speeds.

Depending the procedures used, Survey Distance calibration results tend to be more accurate than GPS based comparison measurements.