Page 213 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

cylinders reaches TDC on its power stroke. It should be noted that if the disk is
mounted on the crankshaft, then the number of tabs for this crankshaft
position sensor always will be half the number of cylinders because it takes two
crankshaft rotations for a complete engine cycle.

Engine Speed Sensor
Engine speed can be cal- An engine speed sensor is needed to provide an input for the electronic
culated in a number of controller for several functions. The position sensor discussed previously can
ways. Digital circuits use be used to measure engine speed. The reluctance sensor is used in this case as
counters and crankshaft an example; however, any of the other position sensor techniques could be
sensors to calculate used as well. Refer to Figure 6.6 and notice that the four tabs will pass
actual engine speed. through the sensing coil once for each crankshaft revolution. Therefore, if
we count the pulses of voltage from the sensing coil in one minute and
divide by four, we will know the engine speed in revolutions per minute
(RPM).
This is easy to do with digital circuits. Precise timing circuits such as those
used in digital watches can start a counter circuit that will count pulses until the
timing circuit stops it. The counter can have the divide-by-four function
included in it, or a separate divider circuit may be used. In many cases, the
actual RPM sensor disk is mounted near the flywheel and has many more than
four tabs; in such cases, the counter does not actually count for a full minute
before the speed is calculated, but the results are the same.

Timing Sensor for Ignition and Fuel Delivery
In electronic engine control it is often desirable to measure the angular
position of the engine relative to a specific point in the cycle. For such
measurement it is normally necessary to measure the position of the camshaft.
The measurement of engine position via crankshaft and camshaft position
sensors (as well as its use in timing fuel delivery and ignition) is described in
Chapter 7. Normally it is sufficient to measure camshaft position at a fixed
point. Such a sample of camshaft position is readily achieved by a magnetic
sensor similar to that described above for the crankshaft position
measurement.
This sensor detects a reference point on the angular position of the
camshaft that defines a beginning to a complete engine cycle (e.g., power stroke
for all cylinders). Once this reference point has been detected, crankshaft
position measurements (as described above) provide sufficient information for
timing fuel injection pulses and ignition.
In one scheme a variable-reluctance sensor is located near a
ferromagnetic disk on the camshaft. This disk has a notch cut (or it can have
a protruding tab), as shown in Figure 6.9. The disk provides a low-reluctance
path (yielding high magnetic flux) except when the notch aligns with the
sensor axis. Whenever the notch aligns with the sensor axis, the reluctance of

200 UNDERSTANDING AUTOMOTIVE ELECTRONICS