Gear-tooth sensors offer an easy and relatively inexpensive means of measuring the speed of rotating machinery. By detecting moving ferrous teeth, they provide an output pulse train with frequency proportional to rotational speed. While gear-tooth sensors can be implemented with optical or eddy-current sensors, Hall-effect sensors offer several advantages including contamination and EMI immunity, response times less than 100 μsec, and operation from –40 to 85°C.

Hall-effect gear-tooth sensors detect changes in a magnetic field. When a gear tooth or other ferrous object approaches a magnet, it distorts the field surrounding that magnet. The presence or absence of ferrous targets are detected by measuring this distortion. The most common method of measurement places a sensor on one magnet poleface, and observes an increase in the magnetic field as a target approaches the poleface.

Hall-effect gear-tooth sensors typically rely on either single point or differential sensing means. In a single-point sensor, the magnitude of the magnetic field at a poleface is measured, and signal-processing circuits compare this measurement to a threshold. If the value exceeds the threshold, the geartooth sensor output switches to a target-present state, but when the value falls short of the threshold, the output assumes a targetabsent state. Many single-point gear-tooth sensors rely on thresholds which vary dynamically to handle changing operating conditions. Such devices are commonly referred to as ac coupled, meaning that there is a minimum speed at which the target must be moving in order to be sensed — typically a few Hertz.

Differential gear-tooth sensors rely on magnetic-field gradients to detect targets and are sensitive to the leading and trailing edges of gear teeth. The leading edge of a target triggers the sensor into the target-present state, while the trailing edge of a target resets the sensor. Differential gear-tooth sensors are especially useful in applications where it is important to know exactly when the edge of a target tooth has passed by, such as in an automobile ignition timing module. Many differential sensors provide zerospeed sensing capability, which means that the sensor has no lower limit to the speed at which it can detect passing gear teeth.

While gear-tooth sensors were originally designed for detecting gearlike targets used in factory machines, automobile ignition systems, and antilock-braking systems, they are also capable of detecting other ferrous objects. Bolt heads, sheet-metal stampings, roller chains, and splines and keyways on shafts are all potentially useful targets that may exist in a mechanical system.

This article was submitted by Ed Ramsden, Senior Engineer, Cherry Electrical Products, 40 Terrill Pk. Dr., Concord, NH 03301.

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