Page 318 - Understanding Automotive Electronics

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AUTOMOTIVE INSTRUMENTATION 9

out of the total sample period, T, as illustrated in Figure 9.8. This method is
satisfactory as long as the sample period is small compared with the time in
which any quantity changes appreciably. Certain quantities, such as coolant
temperature and fuel quantity, change very slowly with time. For such variables,
a sample period of a few seconds or longer is often adequate.
Some variables, such as On the other hand, variables such as vehicle speed, battery charge, and
speed and battery fuel consumption rate change relatively quickly and require a much shorter
charge, change much sample period, perhaps every second or every few tenths of a second. To
faster than others. To accommodate the various rates of change of the automotive variables being
effectively monitor these measured, the sample period varies from one quantity to another. The most
differences the com- rapidly changing quantities are sampled with a very short sample period,
puter uses different sam- whereas those that change slowly are sampled with a long sample period.
pling times. In addition to sample period, the time slot allotted for each quantity must
be long enough to complete the measurement and any A/D or D/A conversion
required. The computer program is designed with all of these factors in mind so
that adequate time slots and sample periods are allowed for each variable. The
computer then simply follows the program schedule.

Advantages of Computer-Based Instrumentation
One of the big advantages of computer-based instrumentation is its great
flexibility. To change from the instrumentation for one vehicle or one model to
another requires only a change of computer program. This change can often be
implemented by replacing one ROM (read-only memory) with another.
Remember that the program is permanently stored in a ROM that is typically
packaged in a single integrated circuit package (see Chapter 4).

Figure 9.8
Sequential
Sampling

FPO

UNDERSTANDING AUTOMOTIVE ELECTRONICS 305