INDUSTRY
AUTOMATIC CONTROL FUNDAMENTALS
The
concepts and terminology described here are intended to provide a general explanation
of automatic control fundamentals and their applications in the production processes
of the industry , including a brief view of related instruments
characteristics . The following document is intended for training purposes only
for industry workers or students . Specific process conditions may require
considerable variation from procedures and specifications described herein.
Industrial controllers , basic theory
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| The most fundamental element of any automatic control system is the basic
feedback control loop. The concept of feedback control is not new; the first
such industrial loop was applied in 1774 by James Watt for controlling the speed
of an early steam engine. Although understanding of feedback control loops
developed slowly at first , pneumatic transmission systems did not become common
until the 1940's -the past few years have seen extensive study and development
in the theory and application of feedback control loops and relative instruments
to measure , record , indicate , transmit , analyze , and control process
variables ( among them : flow , temperature , pressure , level , moisture ,
acidity , composition and more). Today the application of feedback control loops is an essential element in successfully and economically manufacturing virtually every industrial product from steel to softball bats to breakfast foods. Yet this feedback control loop which is so important to
industry is based on a few very simple and easily understood principles. This
article discusses this control loop, its basic elements, and the basic
principles of its application. |
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Function of automatic control .
The basic idea of a feedback control loop is most easily understood by
imagining what an operator would have to do if automatic control did not exist.
Figure I shows a common application of automatic control found in many
industrial plants, a heat exchanger which uses steam to heat cold water. In
manual operation, the amount of steam entering the heat exchanger depends on the
air pressure to the valve which is set on the manual regulator. To control the
temperature manually, the operator would watch the indicated temperature, and by
comparing it with the desired temperature, be would open or close the valve to
admit more or less steam . When the temperature had reached the desired value,
the operator would simply hold that output to the valve to keep the temperature
constant. Under automatic control, the temperature controller performs the same
function. The measurement signal to the controller from the temperature
transmitter is continuously compared to the set point signal entered into the
controller. Based on a comparison of the signals, the automatic controller can
tell whether the measurement signal is above or below the setpoint and move the
valve accordingly until the measurement (temperature) comes to its final value.
The feedback loop .
This simple feedback control loop serves to illustrate the four major
elements of any feedback control loop
(figure 2).
The
measurement
Measurement must be made to indicate the current value
of the variable controlled by the loop. Common measurements used in industry
include flow rate, pressure, level, temperature, analytical measurements
such as pH, ORP and conductivity and many others particular to specific
industries.
The
final actuator
For every process there must be some final actuator,
which regulates the supply of energy or material to the process and changes
the measurement signal. Most often this is some kind of valve, but it
might also be a belt or motor speed, louver position, etc.
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The process
The kinds of processes found in industrial plants are as varied as the
materials they produce. They range from the simple and commonplace, such as
loops to control flow rate, to the large and complex such as distillation
columns in the petro-chemical industry.
The automatic controller
The last element of the loop is the automatic controller. Its job is to
control the measurement. To "control" means to keep the measurement
within acceptable limits. In this article, the mechanisms inside the automatic
controller will not be considered. Therefore, the principles to be discussed can
be applied equally well to both pneumatic and electronic controllers and to the
controllers from any manufacturer. All automatic controllers use the same
general responses, although the internal mechanisms and the definitions given
for these responses may be slightly different from one manufacturer to another.
One basic concept is that for the automatic feedback control to exist, the
automatic control loop must be closed. This means that information must be
continuously passed around the loop. The controller must be able to move the
valve, the valve must be able to affect the measurement, and the measurement
signal must be reported to the controller. If this path is broken at any point,
the loop is said to be open. As soon as the loop is opened, as for example, when
the automatic controller is placed on manual, the automatic unit in the
controller is no longer able to move the valve. Thus signals from the controller
in response to changing-measurement conditions do not affect the valve and
automatic control does not exist.
CONTROLLING THE PROCESS
In performing the control function, the automatic controller uses the
difference between the set point and measurement signals to develop the output
signal to the valve. The accuracy and responsiveness of these signals is a basic
limitation on the ability of the controller to correctly control the
measurement. If the transmitter does not send an accurate signal, or if there is
a lag in the measurement signal, the ability of the controller to manipulate the
process will be degraded. At the same time, the controller must receive an
accurate set point signal. In controllers using pneumatic or electronic set
point signals generated within the controller, miscalibration of the set point
transmitter will necessarily result in the automatic control unit in the
controller bringing the measurement to the wrong value. The ability of the
controller to accurately position the valve is yet another limitation. If there
is friction in the valve, the controller may not be able to move the valve to a
specific stem position to produce a specific flow and this will show up as a
difference between measurement and set point. Repeated attempts to exactly
position the valve may lead to hunting in the valve and in the measurement. or,
if the controller is only able to move the valve very slowly, the ability of the
controller to control the process will he degraded. One way to improve the
response of control valves is to use a valve positioner, which acts as a
feedback controller to position the valve at the exact position corresponding
the controller output signal. Positioners, however, should be avoided in favor
of volume boosters on fast responding loops such as flow arid liquid pressure.
To control the process, the change in output from the controller must be in
such a direction as to oppose any change in the measurement value.
Figure 3 shows a direct connected valve to control level in a tank at
midscale. As the level in the tank rises, the float acts to reduce the flow rate
coming in thus, the higher the liquid level the more the flow will be shut off.
In the same way, as the level falls, the float will open up the valve to add
more liquid to the tank. The response of this system is shown graphically.
As the level goes from 0% to 100%, the valve goes from fully open to fully
closed. The function of an automatic controller is to produce this kind of
opposing response over varying ranges; in addition, other responses are
available to more efficiently control the process.
