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Rotary sense

I'm sure you'll agree when I say the environment alongside a race engine isn't the most electronics-friendly of places. Heat and high levels of vibration are traditionally the enemies of anything electronic, and consequently the technology of producing robust sensors for engine management systems must be a particular challenge. Because of its critical function therefore, a prime example here is the engine speed/timing sensor.

Designed to provide the engine control ECU with information relating to how fast the engine is rotating as well as the position of the crank (or cam) throughout its cycle, the crankshaft sensor is essential in determining the amount of fuel to be injected as well as the timing of its ignition. Typically using some form of disc or wheel with slots cut into it at known angles relative to the ignition firing of individual cylinders, the two most common sensor technologies used are either impedance-based, known generally as variable reluctance (VR), or those based on the Hall effect. But if both rely on sensing the instantaneous changes in a magnetic field, that is surely the limit of their similarity.


The simplest form of VR sensor consists of a coil of wire wrapped around a magnetic former. As the slots in the toothed disc or wheel pass the magnet, the flux surrounding it changes and causes an EMF (electromotive force) to be generated in the coil; the frequency with which this is generated is proportional to the speed of rotation of the disc or wheel. The higher the rotational speed the greater the amplitude of the signal, which is fine for high-speed engines but at low speed the circuitry to produce the clean signal necessary for the engine ECU is difficult to design. So despite the low cost of the sensor, the rugged nature of its construction and its extreme high-temperature capability (up to 300 C in some cases), VR sensors are increasingly being replaced by those that use the Hall effect.

Using a principle discovered by 19th century physicist Edwin Hall, the Hall effect sensor relies on a semiconductor element between the magnet and the toothed wheel. Sensing the change of magnetic flux, rather than the rate of change as in the VR method, the Hall effect sensor not only measures the crankshaft speed in terms of revolutions per minute but actually counts individual revolutions.

That makes it particularly useful if engines are running slowly - as in the case of initial cranking, say, or in setting the ignition timing of the engine when the digital on-off nature of the signal makes this much more precise when the engine is rotated by hand. With semiconductors at the heart of the sensor, signal conditioning is generally also incorporated, so the output at the connector will be compatible with the digital logic used in the engine's ECU. The presence of these electronics, however, makes the unit less robust than its VR counterpart, so Hall effect sensors are usually rated only up to 150 C.

Being less sensitive to electromagnetic interference as well, the Hall effect makes modern engine management so much more functional, despite having been around for more than 140 years.

Fig. 1 - An older style VR pick-up

Written by John Coxon

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