Page 220 - Understanding Automotive Electronics

2735 | CH 6 Page 207 Tuesday, March 10, 1998 1:10 PM

SENSORS AND ACTUATORS 6

A section of resistance material is placed in an arc around the pivot axis
for the movable contact. One end of the resistor is connected to ground, the
other to a fixed voltage V (e.g., 5 volts). The voltage at the contact point of the
movable contact is proportional to the angle (a) from the ground contact to the
movable contact. Thus,

v(a) = ka
where v(a) is the voltage at the contact point, k is a constant, and a is the angle
of the contact point from the ground connection.
This potentiometer can be used to measure any angular rotation. In
particular, it is well suited for measuring throttle angle. The only disadvantage
to the potentiometer for automotive applications is its analog output. For
digital engine control, the voltage v(a) must be converted to digital format
using an analog-to-digital converter.

TEMPERATURE SENSORS
Temperature is an important parameter throughout the automotive
system. In operation of an electronic fuel control system it is vital to know the
temperature of the coolant, the temperature of the inlet air, and the
temperature of the exhaust gas oxygen sensor (a sensor to be discussed in the
next section). Several sensor configurations are available for measuring these
temperatures, but we can illustrate the basic operation of most of the
temperature sensors by explaining the operation of a typical coolant sensor.
Typical Coolant Sensor
One kind of coolant sen- A typical coolant sensor, shown in Figure 6.16, consists of a thermistor
sor uses a temperature- mounted in a housing that is designed to be inserted in the coolant stream. This
sensitive semiconductor housing is typically threaded with pipe threads that seal the assembly against
called a thermistor. The coolant leakage.
sensor is typically con- A thermistor is made of semiconductor material whose resistance varies
nected as a varying resis- inversely with temperature. For example, at –40˚C a typical coolant sensor has
tance across a fixed a resistance of 100,000 ohms. The resistance decreases to about 70,000 ohms at
reference voltage. As the 130˚C.
temperature increases, The sensor is typically connected in an electrical circuit like that shown in
the output voltage Figure 6.17, in which the coolant temperature sensor resistance is denoted R .
T
decreases. This resistance is connected to a reference voltage through a fixed resistance R.
The sensor output voltage, V , is given by the following equation:
T

R T
V T = V---------------
R + R T
The sensor output voltage varies inversely with temperature; that is, the output
voltage decreases as the temperature increases.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 207