Page 60 - Understanding Automotive Electronics

2735 | CH 2 Page 47 Tuesday, March 10, 1998 10:55 AM

THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION 2

Issues

In any measurement made with any instrument there are several
important issues, including
1. Standards
2. Precision
3. Calibration
4. Accuracy
5. Errors
6. Reliability
Each of these issues has an important impact on the performance of the
instrumentation.
The standard magnitudes of the physical variables measured by any
instrument are maintained by the National Institute of Standards &
Technology in the United States. These standard magnitudes and the
fundamental relationships between physical variables determine the units for
each physical quantity.
The precision of any instrument is related to the number of significant figures
that is readable from the display device. The greater the number of significant
figures displayed, the greater the precision of the instrument.
Calibration is the act of setting the parameters of an instrument such that
the indicated value conforms to the true value of the quantity being measured.
The accuracy of any measurement is the conformity of the indicated value
to the true value of the quantity being measured. Error is defined as the
difference between true and indicated values. Hence, accuracy and error vary
inversely. The required accuracy for automotive electronic systems varies with
application, as will be shown in later chapters. In general, those instruments
used solely for driver information (e.g., fuel quantity) have lower accuracy
requirements than those used for such applications as engine control or
diagnosis.
Systematic errors in the The errors in any measurement are generally classified as either systematic
accuracy of instruments or random. Systematic errors result from known variations in instrument
are due to known imper- performance, for which corrections can be made if desired. There are many
fections in an instru- sources of systematic error, including temperature variations in calibration,
ment; random errors are loading, and dynamic response. Since virtually any component in an
caused by outside distur- instrument is potentially susceptible to temperature variations, great care must
bances. be exercised in instrument design to minimize temperature variations in
calibration. As will be seen later in this book, most automotive instruments
have relatively low precision requirements, so that temperature variations in
calibration are negligible. Random errors are essentially random fluctuations in
indicated value for the measurement. Most random measurement errors result
from noise.

UNDERSTANDING AUTOMOTIVE ELECTRONICS 47