Page 151 - Understanding Automotive Electronics

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4 MICROCOMPUTER INSTRUMENTATION AND CONTROL

The level at that point will change as fuel is consumed, but it also will change as
the car slows, accelerates, turns corners, and hits bumps. The sensor’s output
voltage varies widely because of fuel slosh even though the amount of fuel in
the tank changes slowly. If that voltage is sent directly to the fuel gauge, the
resulting variable indication will fluctuate too rapidly to be read.
The measurement can be made more readable and more meaningful by
using a low-pass filter to smooth out the signal fluctuations to reduce the
effects of sloshing. The low-pass filter can be implemented in a
microcomputer by programming the computer to average the sensor signal
over several seconds before sending it to the display. For instance, if the fuel
level sensor signal is sampled once every second and it is desirable to average
the signal over a period of 60 seconds, the computer saves only the latest 60
samples, averages them, and displays the average. When a new sample is
taken, the oldest sample is discarded so that only the 60 latest samples are
kept. A new average can be computed and displayed each time a new sample
is taken.
Digital filters are imple- Digital filtering (averaging) can be performed by a computer under the
mented completely control of the software. Sometimes the section of code that performs the
through the use of soft- filtering task is simply called “the filter.’’ Digital signal processing is very
ware, thus their charac- attractive because the same computer can be used to process several different
teristics can be easily signals. Also, since digital filters require no extra hardware, the filters can be
changed. Because digital made much more complex with relatively little increase in cost. In addition, the
filters require no extra characteristics of the digital filter can be changed by changing the software.
hardware, they are low Changing the characteristics of an analog filter usually requires changing the
in cost. hardware components. Another feature of digital filters (true for digital signal
processing in general) is that they don’t change with age or temperature, unlike
analog filters.
There are limitations to the use of digital filters, however. The frequency
range of digital filters is determined by the speed of the processor. The
microcomputer must be able to sample each signal at or above the rate required
by the Nyquist sampling theorem. It must also be fast enough to perform all of
the averaging and linearization for each signal before the next sample is taken.
This is an important limitation, and the system designer must be certain that
the computer is not overloaded by trying to make it do too many things too
quickly.

MICROCOMPUTERS IN CONTROL SYSTEMS
Computers can also be Microcomputers are able to handle inputs and outputs that are either
used in control applica- digital or converted analog signals. With the proper software, they are capable
tions. of making decisions about those signals and can react to them quickly and
precisely. These features make microcomputers ideal for controlling other
digital or analog systems, as discussed in the following sections.

138 UNDERSTANDING AUTOMOTIVE ELECTRONICS