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 in a switch 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 and takes of the order of 50 milliseconds (msec).

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. PAVAS uses a slicing circuit to transform the waveform into a pulse and is triggered at approximately 1.8 volts. PAVAS only triggers on the rising edge of the waveform so the falling edge is ignored. If a small spike was to occur after the rising edge of the waveform, then all the detection conditions may be met and an extra pulse may be detected.

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 ranges that must be accommodated.

The best approach is to recognize the very valid first pulse and to ignore all others for a period of time. This time where pulses are ignored, is called the Debounce Time. This delay allows the waveform sufficient time 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. The minimum pulse width is about 2 microseconds (usec) If valid wheel pulses are occurring at high rate due to the vehicle speed, they would be ignored if they fall in the Debounce duration leading to speed reading errors. So determining the optimum Debounce duration is complicated.

The Debounce duration 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 Time programmable via the Max Speed menu. Once the Debounce Duration exceeds the time between valid wheel pulses at a given speed, PAVAS will ignore the valid pulse. So, instead of setting the Debounce Time values directly, PAVAS sets the maximum speed values and then calculates the Debounce Duration at that speed for a given Wheel Distance calibration.The Maximum speed is displayed in kph with a resolution of 0.1 kph and a maximum speed of 500 kph. Above that speed, the PAVAS warranty is void. The optimum Debounce Duration is the lowest maximum 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.

Allowing the Maximum Speed and, consequently 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.

Note: Since the Debounce Duration is dependent on the Wheel Distance, the Wheel Distance should be set first.