Page 59 - Understanding Automotive Electronics

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2 THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION

system response. The step input is an abrupt change occurring at time t .
o
However, the output change is gradual relative to the input. The output
asymptotically approaches a steady value HX.
The rate at which the output approaches the final value varies inversely
with bandwidth, as illustrated in Figure 2.12 (where bandwidth is denoted f ).
o
Although the actual response of the system depends on the characteristics of the
system, the response time of any system generally varies inversely with its
bandwidth.

INSTRUMENTS
Instrumentation sys- An instrument (or instrumentation system) is a device for measuring
tems, whether electrical, some specific quantity. Automotive instruments have traditionally been
mechanical, or a combi- mechanical, pneumatic, hydraulic, electrical, or combinations of these.
nation of both, measure However, modern automotive instrumentation is largely electronic. These
a physical quantity and electronic instruments or instrumentation systems are used to measure a
provide a report of that variety of physical quantities, including
measurement.
1. Vehicle speed
2. Total distance traveled
3. Engine angular speed (rpm)
4. Fuel quantity and/or flow rate
5. Oil pressure
6. Engine coolant temperature
7. Alternator charging current
For an understanding of measurement instrumentation, it is helpful to review a
definition of measurement.

Measurement
A measurement is defined as a numerical comparison of an unknown
magnitude of a given physical quantity to a standard magnitude of the same
physical quantity. In this sense, the result of a measurement is normally a
numerical value expressing the indicated value of the measurement as a
multiple of the appropriate standard. However, other display devices are
possible in which simple messages are given. Automotive instrumentation
does not always display measured quantities. Rather, a warning lamp (or
buzzer) is activated whenever a given quantity falls outside of allowed
limits. For example, it is common practice not to provide a display of
measured values for engine oil pressure or coolant temperature. Warning
lamps are activated by the electronic instrumentation system whenever oil
pressure is too low or coolant temperature is too high.

46 UNDERSTANDING AUTOMOTIVE ELECTRONICS