Page 152 - Understanding Automotive Electronics

2735 | CH 4 Page 139 Tuesday, March 10, 1998 11:06 AM

MICROCOMPUTER INSTRUMENTATION AND CONTROL 4

Closed-Loop Control System

Recall the basic closed-loop control system block diagram of Chapter 2.
The error amplifier compares the command input with the plant output and
sends the error signal to the control logic. The control logic uses the error
signal to generate a plant control signal that causes the plant to react with a
new output so that the error signal will be reduced toward zero. The control
logic is designed so that the plant’s output follows or tracks the command
input. A microcomputer can replace the error amplifier and the control logic.
The computer can compare command input and plant output and perform
the computation required to generate a control signal.
Limit-Cycle Controller
The limit-cycle controller, discussed in Chapter 2, can be readily
implemented with a microcomputer. Recall that the limit-cycle controller
controls the plant output so that it falls somewhere between an upper and lower
limit, preferably so that its average value is equal to the command input. The
controller must read in the command input and the plant output and decide
what control signal to send to the plant based on those signals alone.
Using a microcomputer, the upper and lower limit can be determined
from the command input by using a lookup table similar to that discussed later
in this chapter. The plant output is compared against these two limits. If the
plant output is above the higher limit or below the lower limit, the
microcomputer outputs the appropriate on/off signal to the plant to bring the
output back between the two limits.
With proper software, a Recall that in Chapter 2 the concept of a feedback control system was
microcomputer can introduced. There it was shown that a control system compares the value of
replace the error ampli- some controlled variable with a desired value (or set point) for that variable. In
fier and control logic such a control system, the difference between the desired and actual value is
used in the closed-loop first obtained, then an electrical signal is generated. The resulting error signal is
control system. processed electronically, thereby generating a control signal that operates an
actuator. The actuator changes the controlled system in such a way as to reduce
the error. In Chapter 2, it was presumed that analog electronics were used for
the control.
A feedback control system can also be implemented using digital
electronics. Figure 4.24 is a block diagram of a control system employing a
computer. In this diagram, there is a physical system, or plant, that is to be
controlled. The specific variable being controlled is denoted X. For example, in
an automobile, the plant might be the engine and the controlled variable might
be brake torque.
The desired value for X is the set point S. An error signal e is obtained:

e = S – X

UNDERSTANDING AUTOMOTIVE ELECTRONICS 139