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Plant brochure designed by Charles L.
Woodruff 1999 and revised in June 2004.
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SCADA System PLC & RTU
Controllers |
So
What is a PLC?
PLC - A programmable logic controller or "PLC" is a computer used for
automation of real-world processes, such as control of machinery controlling
process operations. Where older automated systems would use hundreds or
thousands of relays, a single PLC can be programmed as a replacement.
The PLC is a microprocessor based device with either modular or integral
input/output circuitry that monitors the status of the field connected
"sensor" inputs and controls the attached output "actuators" (motor
starters, solenoids, pilot lights/displays, speed drives, valves, etc.)
according to a user-created, logic program stored in the microprocessor's
battery-backed RAM memory. The functionality of the PLC has evolved over the
years to include capabilities beyond typical relay control; sophisticated
motion control, process control, DCS and complex networking have now been
added to the PLC's list of functions. |
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Princeton Oxidation
Ditch PLC
Control Panel |
A PLC basically consists
of two elements:
• the central processing unit
• the input/output system
The Central Processing Unit
The central processing unit (CPU) is the part of a programmable controller
that retrieves, decodes, stores, and processes information.
It also executes the control program stored in the PLC’s memory. In essence,
the CPU is the “brains” of a programmable controller. It functions much the
same way the CPU of a regular computer does, except that it uses special
instructions and coding to perform its functions.
 The
CPU has three parts:
• the processor
• the memory system
• the power supply
The processor is the section of the CPU that codes, decodes, and computes
data. The memory system is the section of the CPU that stores both the
control program and data from the equipment connected to the PLC. The power
supply is the section that provides the PLC with the voltage and current it
needs to operate.
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Princeton ATAD PLC
Control Panel Power Supply |
The
Input/Output System
The input/output (I/O) system is the section of a PLC to which all of the
field devices are connected. If the CPU can be thought of as the brains of a
PLC, then the I/O system can be thought of as the arms and legs. The I/O
system is what actually physically carries out the control commands from the
program stored in the PLC’s memory.
The I/O system consists of two main parts:
• the rack
• I/O modules |
The rack is an enclosure
with slots in it that is connected to the CPU.
I/O modules are devices with connection terminals to which the field devices
are wired. Together, the rack and the I/O modules form the interface between
the field devices and the PLC. When set up properly, each I/O module is both
securely wired to its corresponding field devices and securely installed in
a slot in the rack. This creates the physical connection between the field
equipment and the PLC. In some small PLCs, the rack and the I/O modules come
pre-packaged as one unit.
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Princeton Oxidation
Ditch PLC
Rack |
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A LITTLE MORE ABOUT
INPUTS AND OUTPUTS
All of the field devices connected to a PLC can be classified in one of
two categories:
• inputs
• outputs
Inputs are devices that supply a signal/data to a PLC. Typical examples of
inputs are push buttons, switches, and measurement devices.
Basically, an input device tells the PLC, “Hey, something’s happening out
here…you need to check this out to see how it affects the control program.”
Outputs are devices that await a signal/data from the PLC to perform their
control functions. Lights, horns, motors, and valves are all good examples
of output devices. These devices stay put, minding their own business, until
the PLC says, “You need to turn on now” or “You’d better open up your valve
a little more,” etc.There
are two basic types of input and output devices:
• discrete
• analogue
Discrete devices are inputs and outputs that have only two states: on and
off. As a result, they send/receive simple signals to/from a PLC. These
signals consist of only 1s and 0s. A 1 means that the device is on and a 0
means that the device is off. Analogue devices are inputs and outputs that
can have an infinite number of states. These devices can not only be on and
off, but they can also be barely on, almost totally on, not quite off, etc.
These devices send/receive complex signals to/from a PLC. Their
communications consist of a variety of signals, not just 1s and 0s. |
In a traditional industrial control system, all control devices are wired
directly to each other according to how the system is supposed to operate.
In a PLC system, however, the PLC replaces the wiring between the devices.
Thus, instead of being wired directly to each
other, all equipment is wired to the PLC. Then, the control program inside
the PLC provides the “wiring” connection between the devices.
The control program is the computer program stored in the PLC’s memory that
tells the PLC what’s supposed to be going on in the system. The use of a PLC
to provide the wiring connections between system devices is called soft
wiring.
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EXAMPLE:
Let’s say that a push button is
supposed to control the operation of a motor. In a traditional control
system, the push button would be wired directly to the motor. In a PLC
system, however, both the push button and the motor would be wired to the
PLC instead. Then, the PLC’s control program would complete the electrical
circuit between the two, allowing the button to control the motor. |
| A PLC works by
continually scanning a program. We can think of this scan cycle as
consisting of 3 important steps. There are typically more than 3 but we can
focus on the important parts and not worry about the others. Typically the
others are checking the system and updating the current internal counter and
timer values. |
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Step 1-CHECK INPUT STATUS -
First the PLC takes a look at each input to determine if it is on or off. In
other words, is the sensor connected to the first input on? How about the
second input? How about the third... It records this data into its memory to
be used during the next step.
Step 2-EXECUTE PROGRAM -
Next the PLC executes your program one instruction at a time. Maybe your
program said that if the first input was on then it should turn on the first
output. Since it already knows which inputs are on/off from the previous
step it will be able to decide whether the first output should be turned on
based on the state of the first input. It will store the execution results
for use later during the next step.
Step 3 - UPDATE OUTPUT STATUS -
Finally the PLC updates the status of the outputs. It updates the outputs
based on which inputs were on during the first step and the results of
executing your program during the second step. Based on the example in step
2 it would now turn on the first output because the first input was on and
your program said to turn on the first output when this condition is true.
After the third step the PLC goes back to step one and repeats the steps
continuously. One scan time is defined as the time it takes to execute the 3
steps listed above. |
A LITTLE MORE ABOUT THE
CONTROL PROGRAM
We talked a little bit earlier about the control program. The control
program is a software program in the PLC’s memory. It’s what puts the
control in a programmable controller.
The user or the system designer is usually the one who develops the control
program. The control program is made up of things called instructions.
Instructions are, in essence, little computer codes that make the inputs and
outputs do what you want in order to get the result you need.
There are all different kinds of instructions and they can make a PLC do
just about anything (add and subtract data, time and count events, compare
information, etc.). All you have to do is program the instructions in the
proper order and make sure that they are telling the right devices what to
do and voila!…you have a PLC-controlled system.
And remember, changing the system is a snap. If you want the system to act
differently, just change the instructions in the control program.
Different PLCs offer different kinds of instructions. That’s part of what
makes each type of PLC unique. However, all PLCs use two basic types of
instructions:
• contacts
• coils
Contacts are instructions that refer to the input conditions to the control
program—that is, to the information supplied by the input field devices.
Each contact in the control program monitors a certain field device. The
contact waits for the input to do something in particular (e.g., turn on,
turn off, etc.—this all depends on what type of contact it is). Then, the
contact tells the PLC’s control program, “The input device just did what
it’s supposed to do. You’d better check to see if this is supposed to affect
any of the output devices.”
Coils are instructions that refer to the outputs of the control program —
that is, to what each particular output device is supposed to do in the
system. Like a contact, each coil also monitors a certain field device.
However, unlike a contact, which monitors the field device and then tells
the PLC what to do, a coil monitors the PLC control program and then tells
the field device what to do. It tells the output device, “Hey, the PLC just
told me that the switch turned on. That means that you’re supposed to turn
on now. So let’s go!”
Below is an example of the development of a control program . |
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Example of Control
Program Development |
In the example above, a
motor can be started in one of two different ways.
It can be started by selecting it and pushing the start button. That will
activate CR1 which will latch in the start button and start the motor
provided a limit switch, LS102, is closed.
If LS102 opens or the select switch is opened, the motor will stop.
The motor can also start automatically
if a pressure switch, PS103 closes provided LS102 is also closed. The motor
will stop if either PS103 or LS102 opens.
In the illustration above, this
application is shown as, (a) - a logic diagram, (b) - a relay diagram, and
(c) - a PLC ladder programming diagram. |
THE RTU
RTU's - Remote Telemetry (or Terminal) Units, rugged industrial computers
which provides intelligence in the field, and allows the central SCADA
master to communicate with the field instruments. It is a stand alone data
acquisition and control unit. Its function is to control process equipment
at the remote site, acquire data from the equipment, and transfer the data
back to the central SCADA system.
The RTU is similar to the PLC
except as its name suggests, it is used where the equipment being controlled
is more remote from the central unit. |
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Illustration of RTU
System Components & Connections |
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