Page 207 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

A pressure sensor having the configuration of Figure 6.3 is also used for
measuring absolute atmospheric pressure. It will be shown in Chapter 7 that
this absolute pressure can be used in engine control applications, as can the
manifold pressure.
The resistors in the An electrical signal that is proportional to the manifold pressure is
strain gauge MAP sensor obtained by connecting the resistors in a circuit called a Wheatstone bridge, as
are connected in a shown in the schematic of Figure 6.4a. Note the similarity in the Wheatstone
Wheatstone bridge cir- bridge of Figure 6.4a with that employed in the MAF sensor of Figure 6.2. The
cuit. Output voltage of voltage regulator holds a constant dc voltage across the bridge. The resistors
the circuit varies as the diffused into the diaphragm are denoted R , R , R , and R in Figure 6.4a.
1
2
4
3
resistance varies in When there is no strain on the diaphragm, all four resistances are equal, the
response to manifold bridge is balanced, and the voltage between points A and B is zero. When
pressure variations. manifold pressure changes, it causes these resistances to change in such a way
that R and R increase by an amount that is proportional to pressure; at the
1
3
same time, R and R decrease by an identical amount. This unbalances the
4
2
bridge and a net difference voltage is present between points A and B. The
differential amplifier generates an output voltage proportional to the difference
between the two input voltages (which is, in turn, proportional to the pressure),
as shown in Figure 6.4b.
ENGINE CRANKSHAFT ANGULAR POSITION SENSOR
Crankshaft angular posi- Besides pressure, the position of shafts, valves, and levers must be sensed
tion is an important for automotive control systems. Measurements of the angular position or
variable in automotive velocity of shafts are common in automotive electronics. It is highly desirable
control systems, particu- that these measurements be made without any mechanical contact with the
larly for controlling igni- rotating shaft. Such noncontacting measurements can be made in a variety of
tion timing and fuel ways, but the commonest of these in automotive electronics use magnetic or
injection timing. optical phenomena as the physical basis. Magnetic means of such
measurements are generally preferred in engine applications since they are
unaffected by oil, dirt, or other contaminants.
The principles involved in measuring rotating shafts can be illustrated by
one of the most significant applications for engine control: the measurement of
crankshaft angular position or angular velocity (i.e., RPM). Imagine the engine
as viewed from the rear, as shown in Figure 6.5. On the rear of the crankshaft is
a large, heavy, circular steel disk called the flywheel that is connected to and
rotates with the crankshaft. Let’s mark a point on the flywheel, as shown in
Figure 6.5, and draw a line through this point and the axis of rotation. Let’s
draw another line through the axis of rotation parallel to the horizontal center
line of the engine as a reference line. The crankshaft angular position is the
angle between the reference line and the mark on the flywheel.
Imagine that the flywheel is rotated so that the mark is directly on the
reference line. This is an angular position of zero degrees. For our purposes,
assume that this angular position corresponds to the No. 1 cylinder at TDC
(top dead center). As the crankshaft rotates, this angle increases from zero to

194 UNDERSTANDING AUTOMOTIVE ELECTRONICS