Page 62 - Understanding Automotive Electronics

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THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION 2

Figure 2.14
Instrument Dynamic FPO
Error

begins responding, but cannot instantaneously change and produce the new
value. After a time, the indicated value approaches the correct reading
(presuming correct instrument calibration). The greater the bandwidth of an
instrument or instrumentation system, the more quickly it can follow rapid
changes in the quantity being measured.
In many automotive instrumentation applications the bandwidth is
purposely reduced to avoid rapid fluctuations in readings. For example, the type
of sensor used for fuel quantity measurements actually measures the height of fuel
in the tank with a small float. As the car moves, the fuel sloshes in the tank,
causing the sensor reading to fluctuate randomly about its mean value. The signal
processing associated with this sensor has an extremely low bandwidth so that
only the average reading of the fuel quantity is displayed, thereby eliminating the
undesirable fluctuations in fuel quantity measurements that would occur if the
bandwidth were not restricted.
The reliability of an instrumentation system refers to its ability to perform
its designed function accurately and continuously whenever required, under
unfavorable conditions, and for a reasonable amount of time. Reliability must
be designed into the system by using adequate design margins and quality
components that operate both over the desired temperature range and under
the applicable environmental conditions.
BASIC MEASUREMENT SYSTEM
The basic block diagram for an electronic instrumentation system has
been given in Figure 2.1b. That is, each system has three basic components:
sensor, signal processing, and display. Essentially, all electronic measurement
systems incorporated in automobiles have this basic structure regardless of the
physical variable being measured, the type of display being used, or whether the
signal processing is digital or analog.
Understanding automotive electronic instrumentation systems is facilitated
by consideration of some fundamental characteristics of the three functional

UNDERSTANDING AUTOMOTIVE ELECTRONICS 49