[From Bruce Abbott (950605.1545 EST)]
Well, I finally remembered to bring my text on control-system design in.
The following is from Hostetter, G. H, Savant, C. J., Jr., & Stefani, R. T.
(1989). _Design of feedback control systems (2nd ed.), Section 1.2:
Control System Terminology
. . . A control system is, in the broadest sense, any interconnection of
components to provide a desired function.
The portion of a system which is to be controlled is called the _plant_ or
the _process_. It is affected by applied signals, called _inputs_, and
produces signals of particular interest, called _outputs_. The plant is
fixed insofar as the control system designer is concerned. Whether the
plant is an automobile engine, electrical generator, or nuclear reactor, it
is the designer's job to ensure that the plant operates as required . . .
A _controller_ may be used to produce a desired behavior of the plant. The
controller generates plant input signals designed to produce desired
outputs. Some of the plant inputs are accessible to the designer and some
are generally not available. The inaccessible input signals are often
disturbances to the plant. . . [figure showing open-loop system with
disturbance]. This system is termed _open-loop_ because the control inputs
are not influenced by the plant outputs; that is, there is no _feedback_
around the plant.
Such an open-loop control system has the advantage of simplicity, but its
performance is highly dependent upon the properties of the plant, which may
vary with time. The disturbances to the plant may also create unwanted
responses which it would be desirable to reduce.
As an example, suppose a gasoline engine is used to drive a large pump. The
carburetor and engine comprise a common type of control system wherein
large-power output is controlled with a small-power input. The carburetor
is the controller in this case, and the engine is the plant. The fuel rate
is the control input, and the pump load is a disturbance signal. The
desired plant output, a certain engine shaft speed, may be obtained by
adjusting the throttle angle [using a calibration curve].
. . . This "calibration curve" gives the engine speed for a given throttle
setting, at constant load on the engine. . . . If the engine should become
untuned (a change in plant) or if the load should change (a disturbance),
the calibration curve would change, and an 80 degree throttle angle would no
longer produce a 2300-rpm engine speed.
The Feedback Concept
If the requirements of the system cannot be satisfied with an open-loop
control system, a _closed-loop_ or _feedback_ system is desirable. A path
(or loop) is provided from the output back to the controller. Some of all
of the system outputs are measured and used by the controller. The
controller may then compare a desired plant output with the actual output
and act to reduce the difference between the two. [Extended example of
engine control with feedback given.]
Some of the advantages which feedback control offers to the designer are
1. Increased accuracy. The closed-loop system may be designated to
drive the error between desired and measured response to zero.
2. Reduced sensitivity to changes in components. The system may be
designed to seek zero error despite changes in the plant.
3. Reduced effects of disturbances. The effects of disturbances to the
system may be greatly attenuated.
[Bill P. will be happy to see this next one included:]
4. Increased speed of response and bandwidth. Feedback may be used to
increase the range of frequencies over which a system will respond
and to make it respond more desirably. A satellite booster rocket,
for example, has aerodynamics resembling those of a giant broomstick.
It may, with feedback, behave with beauty and grace.
Regards,
Bruce