Page 211 - Understanding Automotive Electronics

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6 SENSORS AND ACTUATORS

circuit to the magnetic field intensity is described by the second quantity,
which is called magnetic flux. A line of constant magnetic flux is a closed path
through the magnetic material. The magnetic flux is similar to the current
that flows when a resistor is connected across a battery to form a closed
electrical circuit.
The voltage generated by As we shall see, the voltage generated by the reluctance sensor is
the magnetic reluctance determined by the strength of this magnetic flux. The strength of the
position sensor is deter- magnetic flux is, in turn, determined by the reluctance of the magnetic
mined by the strength of circuit. Reluctance is to a magnetic circuit what resistance is to an electrical
the magnetic flux. When circuit.
a tab on the steel disk The path for the magnetic flux of the reluctance sensor is illustrated in
passes through the gap, Figure 6.7. The reluctance of a magnetic circuit is inversely proportional to the
the flow of the magnetic magnetic permeability of the material along the path. The magnetic
flux changes signifi- permeability of steel is a few thousand times larger than air; therefore, the
cantly. reluctance of steel is much lower than air. Note that when one of the tabs of the
steel disk is located between the pole pieces of the magnet, a large part of the
gap between the pole pieces is filled by the steel. Since the steel has a lower
reluctance than air, the “flow” of magnetic flux increases to a relatively large
value.
On the other hand, when a tab is not between the magnet pole pieces, the
gap is filled by air only. This creates a high-reluctance circuit for which the
magnetic flux is relatively small. Thus, the magnitude of the magnetic flux that
“flows” through the magnetic circuit depends on the position of the tab, which,
in turn, depends on the crankshaft angular position.
The magnetic flux is least when none of the tabs is near the magnet pole
pieces. As a tab begins to pass through the gap, the magnetic flux increases. It

Figure 6.7
Magnetic Circuit of
the Reluctance
Sensor

FPO

198 UNDERSTANDING AUTOMOTIVE ELECTRONICS