PAVAS -Debounce Time

Debounce is a common term used in Electronic design. It describes the behavior of a switch contact when it is opened or closed. The simple minded approach is that contact or connection is made just once. In reality, due to a spring forcing the contacts closed, the contacts “bounce”, that is they are open and closed repetitively for a short period of time. So, instead of just one closure event, several are observed. From an electronics view point, this results in several pulses before a steady state contact condition occurs.

PAVAS uses a magnetic pickup that detects a bolt head passing the sensing head. There is no mechanical contact but due to the different voltage levels induced, which are speed dependent, the voltage waveform is not necessarily “clean”. So, instead of one pulse being detected, many may occur. PAVSAS uses a slicing circuit to transform the waveform into a pulse and is triggered at approximately 3.2 volts. PAVAS only triggers on the rising edge of the waveform so the falling edge is ignored. If a small spike was to occur on the rising edge of the waveform, then all the detection conditions may be met and an extra pulse may be generated.

The other scenario is that the overall voltage pulse maybe small and so that the inherent noise voltages may be sufficient to trigger PAVAS. As can be appreciated, solutions to these spurious pulses can be very difficult. Filtering, used a fixed network of resistors and capacitors, is often proposed as a solution but the design is always a compromise due to the vehicle speed variations that occur.

The best approach is to recognize the very first pulse and ignore all others for a period of time. This time where pulses are ignored, is called the Debounce time. This delay allows the pulse to fall below a level where it cannot trigger the PAVAS wheel pulse circuit. If multiple pulses occur, they just ignored. This approach does not limit the response time of the pulse detection circuit in anyway. If valid wheel pulses are occurring at high rate due to the vehicle velocity, they could be ignored if they fall in the Debounce period leading to speed errors. So determining the optimum Debounce period is complicated.

The Debounce period is a function of how fast the vehicle will travel (Max speed) and the distance the wheel travels between pulses. Wheel distance is complicated by tyre wear and other factors. PAVAS solves these problems by making the Debounce Period programmable in the Debounce Time Wheel Pulse menu. The Debounce time is displayed in milliseconds and, using the calibrated Wheel Distance, the maximum speed is calculated and displayed concurrently. The maximum vehicle speed is displayed up to 999 kph. Above that speed, the PAVAS warranty is void. The optimum Debounce time is such that the lowest max speed can never be achieved in the vehicle being monitored.

For example, a vehicle with a wheel distance of 1000 mm and a Debounce time of 13.38 milliseconds with have a maximum speed of 269 kph. Above the maximum speed, there is a probability that wheel pulses will be ignored and the indicated speed will be less than the actual speed. If the Debounce Time was reduced to 10.0 milliseconds then the Maximum Speed would increase to 359 kph. If the Debounce time is set to zero, the debounce period is ignored.

Allowing the Debounce time to be programmable provides an efficient and flexible solution to the problem of spurious pulses and can be applied to a wide variety of vehicles.