Page 320 - Understanding Automotive Electronics

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AUTOMOTIVE INSTRUMENTATION 9

Several fuel quantity sensor configurations are available. Figure 9.10
illustrates the type of sensor to be described, which is a potentiometer
connected via mechanical linkage to a float. Normally, the sensor is mounted so
that the float remains laterally near the center of the tank for all fuel levels. A
constant current passes through the sensor potentiometer, since it is connected
directly across the regulated voltage source. The potentiometer is used as a
voltage divider so that the voltage at the wiper arm is related to the float
position, which is determined by fuel level.
The sensor output voltage is not directly proportional to fuel quantity in
gallons because of the complex shape of the fuel tank. The computer memory
contains the functional relationship between sensor voltage (in binary number
equivalent) and fuel quantity for the particular fuel tank used on the vehicle.
The computer reads the binary number from the A/D converter that
corresponds to sensor voltage and uses it to address a particular memory
location. Another binary number corresponding to the actual fuel quantity in
gallons for that sensor voltage is stored in that memory location. The computer
then uses the number from memory to generate the appropriate display
signal—either analog or digital, depending on display type—and sends that
signal via DEMUX to the display.
The computer compen- Computer-based signal processing can also compensate for fuel slosh. As
sates for fuel slosh by the car moves over the road, the fuel sloshes about and the float moves up and
averaging float sensor down around the average position that corresponds to the correct level for a
readings over a period of stationary vehicle. The computer compensates for slosh by computing a
time. running average. It does this by storing several samples over a few seconds and
computing the arithmetic average of the sensor output. The oldest samples are
continually discarded as new samples are obtained. The averaged output

Figure 9.10
Fuel Quantity Sensor

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 307