PAVAS Wheel Sensor
PAVAS uses a magnetic pickup aimed at the wheel bolts on the hub. The pickup can sense any number of bolt heads passing by and the usual number is two or four. The distance the wheel rotates between bolt sensing points is called the Wheel Pulse Distance (WPD). The gap between the pickup probe and the bolt head is typically 1 mm. Magnetic probes are less affected by environmental conditions such as dust and water. They can be affected by excessive heat but are usually mounted in a relatively cool location.
The wheel chosen is usually an undriven wheel since they are less likely to slip on a surface. Driven wheels can be used but they are best applied on a all wheel drive vehicles.
There are errors in this technique due to calibration, tyre wear, tyre pressure,bag radius and slippage but compared to GPS measurements, the technique is more accurate and less dependent on the geometry of the vehicle track or path. The electrical pulse produced by the bolt passing may not be well defined and PAVAS uses techniques to ensure the pulse is captured and spurious signals are ignored. PAVAS also allows for a range of pulse voltages and uses circuit protection to ensure that damage cannot occur on the input circuitry. PAVAS has a lower speed limit of approximately 4 Kph but still measures distances to zero speeds.
PAVAS uses a number of calibration techniques to measure the WDP and so it can be easily installed in a range of vehicle types.