Page 66 - Understanding Automotive Electronics

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THE SYSTEMS APPROACH TO CONTROL AND INSTRUMENTATION 2

Signal Processing

Any changes performed Signal processing, as defined earlier, is any operation that is performed on
on the signals between signals traveling between the sensor and the display. Signal processing converts
the sensor and the dis- the sensor signal to an electrical signal that is suitable to drive the display. In
play is considered to be addition, it can increase the accuracy, reliability, or readability of the
signal processing. measurement. Signal processing can make a nonlinear sensor appear linear, or it
can smooth a sensor’s frequency response. Signal processing can be used to
perform unit conversions such as converting from miles per hour to kilometers
per hour. It can perform display formatting (such as scaling and shifting a
temperature sensor’s output so that it can be displayed on the engine temperature
gauge either in centigrade or in Fahrenheit), or process signals in a way that
reduces the effects of random system errors.
Signal processing can use Signal processing can be accomplished with either a digital or an analog
either analog circuitry or subsystem. The trend in automotive electronic systems toward fully digital
digital circuitry, depend- instrumentation means that the majority of automotive electronic signal
ing on the application. processing is accomplished with a digital computer.

DIGITAL SIGNAL PROCESSING
The block diagram of a digital instrumentation system is shown in Figure
2.5. In this figure the sensor is assumed to be analog and is measuring a physical
variable (which we call x in this figure). This continuously varying quantity is
sampled (as described earlier) and quantized, yielding a sequence of binary-
valued numbers (which we call x when n = 1, 2, 3, . . . ). In more formal
n
mathematical terms, this sequence is given by
x = x(nT) n = 1, 2, 3, . . .
n
where T is the sample period. That is, each x is the value of the input at discrete
n
time nT. This sequence is the input to a digital computer that performs the
digital signal processing (DSP). The output from the computer (which we call
y ) is a sequence of digital data that is input to the display. The display is
n
assumed to be digital since such display devices are commonplace in automotive
electronic instrumentation systems. It should be noted that in the event the
sensor is digital, the sample and quantizer (ADC) are not required because the
digital sensor output is in a form that can be read directly by the computer.
The actual signal processing computation is specific to a given
application. Perhaps the most general statement that can be made concerning
the DSP operation is that each output from the computer is made by a series of
computations performed by the computer on one or more input samples. The
mathematical formula or rule for these computations is called an algorithm. The
number of inputs used to compute each output is specific to a given algorithm,
which, in turn, is specific to a given application.
The DSP operates on the samples x under program control to perform
n
arithmetic and logical operations (as explained in Chapter 4) and generate an
output y for each input x . The set of steps performed on x to yield y is
n n n n

UNDERSTANDING AUTOMOTIVE ELECTRONICS 53