Speed sensing system with automatic sensitivity adjustment

Abstract

An electronic circuit measures a pulse frequency and/or a pulse period of pulses in a sensor signal originating in a sensor coil, the pulses being zero crossing pulses. The circuit includes a filter stage having an input connected to the sensor coil, the filter stage being for removing electrical noise from the sensor signal to provide a filtered sensor signal. The circuit also has a signal strength sensing circuit connected to receive at least one of the sensor signal and the filtered sensor signal, the signal strength sensing circuit producing a variable threshold signal, which is a substantially monotonically non-decreasing function of a strength of the sensor signal. A variable threshold zero crossing detector is connected to receive the filtered sensor signal and the variable threshold signal. The variable threshold zero crossing detector has an upper threshold to test for zero crossing in a positive direction and a lower threshold to test for zero crossing in a negative direction. One or both of the thresholds is dependent on the variable threshold signal. The variable threshold zero crossing detector generates a clean square-wave pulse train indicative of positive and negative zero crossings. The circuit also includes an output circuit generating a signal indicative of the pulse frequency and/or the pulse period.