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. This technique is the recommended calibration technique.

When the menu is entered, the Survey Distance is displayed. If the distance is not correct then it can be edited using the Up/Down and Left/Right key combinations. To end the editing mode, Select is pressed. This saves the Survey Distance to memory. PAVAS then displays a distance and speed values using the uncalibrated WPD ( Wheel Pulse Distance)

The next part of the procedure is to drive to the commencement of the Surveyed Distance. Many rally organizers provide marks on the road to show the start and end of the surveyed distance and this is used as reference to ensure all competitors navigation instruments are calibrated to the same distance standard. This makes the calibration procedure simple.

At the start of the Survey Distance, Select is pressed. This resets the distance measurement and pulse count to zero and then the vehicle can drive over the surveyed distance while PAVAS displays the uncalibrated distance in meters and the uncalibrated speed. The vehicle speed used may be quite fast since this will affect the “bag radius” and promotes better accuracy.

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 additional calibration distance measured by having a Pause function. This is engaged by pressing the Up button. To resume the calibration distance measurement, the Down button is pressed. PAVAs allows the Pause/Resume cycle to be executed multiple times.

At 200 meters prior to the end of the Survey Distance, a beep tone is sounded to alert the driver to slow down for the end position. At the end position, the vehicle should be stopped for the most accurate reading.

The Select button is pressed once the survey distance is complete. PAVAS now calculates the WPD based on the known distance travelled and the number of wheel pulses counted in the trip. PAVAS now displays the WPD in millimeters and the error percentages based on the number of pulses counted. Experience has shown that WPD should be measured to 1/100^th of a millimeter.

The calibration method allows for easy calibration checks. This would involve entering the menu item, pressing Select to accept the previously stored Survey Distance, driving to the start of the surveyed distance, pressing Select to reset the distance value and then driving over the surveyed distance. At the end, the displayed distance can be compared to the Survey Distance as an error check. If the error is satisfactory, then the Left/Right button can be pressed to abort the procedure and not affected the stored WDP. If the error is too great, then pressing the Select button will calculate a new value for the WPD.

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. WDP will vary with each vehicle but a usual value is approximately one meter. To achieve an 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 allows 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 and simple to perform.