Page 154 - Understanding Automotive Electronics

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MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

After computing y for each input sample, a digital version of y is
n
n
transmitted through the I/O to the DAC. There it is converted to analog
format, providing a control signal to the actuator (A), which is presumed here
to be analog. The actuator controls the plant in such a way as to cause the error
to be reduced toward zero. Many examples of the application of computer-
based electronic control systems in automobiles are presented in a later chapters
of this book.

Multivariable Systems

With the appropriate A very important feature of microcomputer control logic is the ability to
control program, micro- control multiple systems independently and to control systems with multiple
computers have the abil- inputs and outputs. The automotive applications for microcomputer control
ity to sample and control involve both of these types of multivariable systems. For instance, the
multiple inputs and out- automobile engine has several inputs (such as air/fuel ratio, throttle angle, and
puts independently. This spark timing). All of the outputs must be controlled simultaneously because
type of control is much some inputs affect more than one output. These types of controllers can be very
more difficult to design complicated and are difficult to implement in analog fashion. The increased
when using analog cir- complexity (and cost) of a multivariable microcomputer system is not much
cuitry. higher than for a single-variable microcomputer system, presuming the
microcomputer has the capacity to do the task. It only affects the task of
programming the appropriate control scheme into the microcomputer. This
type of control is discussed in a later chapter.
Table Lookup

One of the important functions of a microcomputer in automotive
applications is table lookup. These applications include:
1. Linearization of sensor data
2. Multiplication
3. Calibration conversion
These applications are explained in detail in later chapters where
appropriate. In this chapter, we explain the basic principle using a specific
example.
The concept of table lookup is illustrated in Figure 4.25, in which a pair
of variables, V and X, are related by the graph depicted therein. Also shown in
o
Figure 4.25 is a table listing certain specific values for the relationship. The
functional relationship between V and X might, for example, be the output
o
voltage of a nonlinear sensor V for measuring a quantity X. If the value for V is
o
o
known, then the corresponding value for X can theoretically be found using the
graph or the tabulated values. In the latter case, the nearest two tabulated values
for V are located, and the corresponding values for X are read from the table.
o
Denoting V and V as the nearest values and X , X as the corresponding
2
1
1
2
UNDERSTANDING AUTOMOTIVE ELECTRONICS 141