CHAPTER 1
Process control system
Introduction
1.1 Conventional control panel
1.2 Control panel with a PLC controller
1.3 Systematic approach to designing a process control system
Introduction
Generally speaking, process control system is made up of a group of
electronic devices and equipment that provide stability, accuracy and
eliminate harmful transition statuses in production processes. Operating
system can have different form and implementation, from energy supply units
to machines. As a result of fast progress in technology, many complex
operational tasks have been solved by connecting programmable logic
controllers and possibly a central computer. Beside connections with
instruments like operating panels, motors, sensors, switches, valves and
such, possibilities for communication among instruments are so great that
they allow high level of exploitation and process coordination, as well as
greater flexibility in realizing an process control system. Each component
of an process control system plays an important role, regardless of its
size. For example, without a sensor, PLC wouldn’t know what exactly goes on
in the process. In automated system, PLC controller is usually the central
part of an process control system. With execution of a program stored in
program memory, PLC continuously monitors status of the system through
signals from input devices. Based on the logic implemented in the
program, PLC determines which actions need to be executed with output
instruments. To run more complex processes it is possible to connect more
PLC controllers to a central computer. A real system could look like the one
pictured below:
1.1 Conventional control panel
At the outset of industrial revolution, especially during sixties and
seventies, relays were used to operate automated machines, and these were
interconnected using wires inside the control panel. In some cases a
control panel covered an entire wall. To discover an error in the system
much time was needed especially with more complex process control systems.
On top of everything, a lifetime of relay contacts was limited, so some
relays had to be replaced. If replacement was required, machine had to be
stopped and production too. Also, it could happen that there was not enough
room for necessary changes. control panel was used only for one particular
process, and it wasn’t easy to adapt to the requirements of a new system. As
far as maintenance, electricians had to be very skillful in finding errors.
In short, conventional control panels proved to be very inflexible. Typical
example of conventional control panel is given in the following picture.
In this photo you can notice a large number
of electrical wires, time relays, timers and other elements of automation
typical for that period. Pictured control panel is not one of the more
“complicated” ones, so you can imagine what complex ones looked like.
Most frequently mentioned disadvantages of a classic control panel are:
- Too much work required in connecting wires
- Difficulty with changes or replacements
- Difficulty in finding errors; requiring skillful work force
- When a problem occurs, hold-up time is indefinite, usually long.
1.2 Control panel with a PLC controller
With invention of programmable controllers, much has changed in how an
process control system is designed. Many advantages appeared. Typical
example of control panel with a PLC controller is given in the following
picture.
Advantages of control panel that is based on a PLC controller can be
presented in few basic points:
1. Compared to a conventional process control system, number of wires
needed for connections is reduced by 80%
2. Consumption is greatly reduced because a PLC consumes less than a
bunch of relays
3. Diagnostic functions of a PLC controller allow for fast and easy
error detection.
4. Change in operating sequence or application of a PLC controller to
a different operating process can easily be accomplished by replacing a
program through a console or using a PC software (not requiring changes in
wiring, unless addition of some input or output device is required).
5. Needs fewer spare parts
6. It is much cheaper compared to a conventional system, especially
in cases where a large number of I/O instruments are needed and when
operational functions are complex.
7. Reliability of a PLC is greater than that of an electro-mechanical
relay or a timer.
1.3 Systematic approach in designing an
process control system
First, you need to select an instrument or a system that you wish to
control. Automated system can be a machine or a process and can also be
called an process control system. Function of an process control system is
constantly watched by input devices (sensors) that give signals to a PLC
controller. In response to this, PLC controller sends a signal to external
output devices (operative instruments) that actually control how system
functions in an assigned manner (for simplification it is recommended that
you draw a block diagram of operations’ flow).
Secondly, you need to specify all input and output instruments that will be
connected to a PLC controller. Input devices are various switches, sensors
and such. Output devices can be solenoids, electromagnetic valves, motors,
relays, magnetic starters as well as instruments for sound and light
signalization.
Following an identification of all input and output instruments,
corresponding designations are assigned to input and output lines of a PLC
controller. Allotment of these designations is in fact an allocation of
inputs and outputs on a PLC controller which correspond to inputs and
outputs of a system being designed.
Third, make a ladder diagram for a program by following the sequence of
operations that was determined in the first step.
Finally, program is entered into the PLC controller memory. When finished
with programming, checkup is done for any existing errors in a program code
(using functions for diagnostics) and, if possible, an entire operation is
simulated. Before this system is started, you need to check once again
whether all input and output instruments are connected to correct inputs or
outputs. By bringing supply in, system starts working.
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