Page 209 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

Figure 6.5
Engine Crankshaft
Angular Position
Measurement

FPO

Crankshaft angular posi- In automobiles having electronic engine control systems, angular
tion can be sensed position can be sensed on the crankshaft directly or on the camshaft. Recall
directly at the camshaft, that the piston drives the crankshaft directly, while the valves and the
since the camshaft distributor for the spark ignition are driven from the camshaft. The camshaft
rotates at exactly one- is driven from the crankshaft through a 1:2 reduction drivetrain, which can be
half the speed of the gears, belt, or chain. Therefore, the camshaft rotational speed is one-half that
crankshaft. of the crankshaft, so the camshaft angular position goes from zero to 360˚ for
one complete engine cycle. Either of these sensing locations can be used in
electronic control systems. Although the crankshaft location is potentially
superior for accuracy because of torsional and gear backlash errors in the
camshaft drivetrain, many production systems locate this sensor such that it
measures camshaft position. At the present time, there appears to be a trend
toward measuring crankshaft position directly rather than indirectly via
camshaft position. In fact, it is sufficient for engine control purposes to
measure crankshaft/camshaft position at a small number of fixed points. The
number of such measurements (or samples) is determined by the number of
cylinders.
It is desirable to measure engine angular position with a noncontacting
sensor to avoid mechanical wear and corresponding changes in accuracy of the
measurement. The two most common methods for noncontact coupling to a
rotating shaft employ magnetic fields or optics. Let’s consider the concepts used
for magnetically coupled sensors first.

196 UNDERSTANDING AUTOMOTIVE ELECTRONICS