By
Sdr. Tan Chee Fai
On 21st June 2003, G&S Section had organized a
talk One-Day Course in “Programmable Logic Controller (PLC)
Application and Training” at the IEM. The speaker for the talk
were Mr Danny Lim and Mr. Song Boon Hock. Mr Danny Lim has more
than 15 years of experience in electrical control systems. He is
the founder of Intellogic Technology Sdn. Bhd. and Intellogic
System Sdn. Bhd. A leading group of companies actively involve in
advance control technology. In addition, Mr. Song Boon Hock has
many years of experience as a project manager in Intellogic System
Sdn. Bhd. He has successfully completed many control system
projects in many big establishments in Malaysia as well as in
overseas. He is one of the experts for NAIS (Formally known as
National/Panasonic) PLC in Malaysia.
This one-day
course aims to provide a fundamental understanding of PLC
application in Building and Manufacturing services and hands-on
PLC training. It covers the popular applications like lift control
system, parking system, automatic storage & retrieval system, data
acquisition and robotics. In addition, the course will also
include half-day workshop on PLC ladder programming with computer
and basic electrical wiring training.
Introduction of PLC
Imagine an automated manufacturing line in
1960's and 1970's. There was always a huge wiring panel to control
system. The wiring panel could cover an entire wall. Inside the
panel were messes of electromechanical relays. These relays were
all hardwire together to make the system work. Hardwiring means
that an electrician had to install wires between the connection of
the relays. An engineer would have to design the logic of the
system and the electricians would be given a blueprint of the
logic and would have to wire the components together. There were
hundreds of electro-mechanical relays in a system, before
Programmable Logic Controller (PLC) was developed.
A PLC, which is usually called a PLC or, more
commonly, simply a programmable controller, is a solid-state,
digital, industrial computer. Upon first glance, a programmable
controller may seem to be no more than a black box with wires
bringing signals in and other wires sending signals out. It might
also appear there is some magic being done inside that somehow
decides when field devices should be turned on. In actuality,
there is no magic. The PLC is a computer, and someone had told it
what to do. The PLC knows what to d through a program that was
developed and then entered into its memory. The PLC is a computer,
however, without a set of instructions telling it what to do, it
is nothing more than a box full of electronic components. The user
program is the list of instructions that tells the PLC what to do.
Computers such as PLCs can be wonderful tools; however, although
it might appear otherwise, they only do exactly what human
programmer told them to do. Refer to Table 1, it shows the
advantages of PLC over Relay System.
Table 1: Advantages of PLC over Relay System
| 1. |
Space |
Save pace required for mounting |
|
2. |
Cost |
In a system requiring about 6 to 8
control relays, a PLC can become more cost effective. |
|
3. |
Maintenance
|
Because a PLC is an intelligent device,
it has the ability to provide diagnostics to aid in
troubleshooting in the event a failure occurs. |
|
4. |
Flexibility |
PLC can be fitted into a wide range of
application. If the application changes. So can the PLC with
some program changes as well as the possible addition of
other I/O modules |
|
5. |
Compatibility |
Most PLCs can be easily interfaced with
the entire control products line including drivers,
pushbuttons, limit switches, proximity switches,
photo-sensor and so on. |
|
6. |
Usability |
PLCs are programmed using symbols, which
are familiar to people in industry. It is not a high level
language such as Pascal, Fortran, etc. Relay ladder logic is
the industry standard. |
|
7. |
Reliability |
Because PLCs are solid-state devices, it
has no moving parts. This differs greatly form the standard
relay system which are subject to mechanical failure due to
wearing out of mechanism along with and dirt build-up.
Generally, the longer a solid-state device operates, the
longer it will continue to operate. |
|
8. |
Visual observation |
A PLCs circuit's operation can be seen
during the operation directly on CRT screen. The operation
or miss-operation of a circuit can be observed as it
happens. |
|
9. |
Pilot Running |
A PLCs programmed circuit can be pro-run
and evaluated in the office or lab. The program can be typed
in, tested, observed and modified of needed, saving valuable
factory time. |
|
10. |
Documentation |
An immediate printout of the true PLCs
circuit is available in minutes, if required. There is no
need to look for the blueprint of the circuit in the remote
files. |
What is Control
Control is the process in a system in which one
or several input variables influence other variables as a result
of the laws pertaining to the system. (DIN 19226)
Figure 1: Input and output configuration
Basic of Programmable Logic Controllers
An apt definition of programmable logic
controller (PLC) is that it is a digital electronic device that
uses a programmable memory to store instructions and to implement
specific function such as logic, sequence, timing, counting and
arithmetic to control machines and process.
Figure 2: Basic of PLC
The main differences between PLC and , say, a
microcomputer are that:
(i)
Programming is predominantly connected
with logic and switching operations.
(ii)
The interfacing for input and output
devices is inside the controller.
(iii)
They are rugged being packaged to
withstand vibration, temperature, humidity and noise.
Types of Inputs and Outputs
Figure
3: Type of input and output signal
There are three types of
input and output signals.
(i) Bit
signal
Figure
4: Signal from switch or sensors
(i) Digital
signal
Code signal, which
consists of series of On and OFF signals
For example: signal from counter to PLC
(ii) Analog
signal
Example: signal from
temperature sensor or tacho generator (Speed measurement)
The component of PLC
Solutions to
control problems realized technically with PLC applications may
vary in their complexity. However, the following basic components
are always necessary:
(i)
Hardwire
By hardwire, we understand the electronic
modules, through which all functions of the installation or
machine to be controlled are addressed and actuated in the logical
sequence.
(ii)
Software
By software, we mean the programs in which
logic operations and the related of components within the
installation or machine are precisely specified. Software is filed
in a special hardwire memory and can be modified if required. The
control sequence changes with the new program. It is not necessary
to change the hardwire.
(iii)
Sensorics (Sensors)
By attaching these components directly to the
machine or installation to be controlled, information regarding
the current status of the devices can be transmitted to the PLC.
Sensors are, e.g., limit or proximity switches.
(iv)
Actorics (Actuators)
These are the components, fitted directly to
the machine or installation to be controlled, which can be used to
change statuses via the PLC. In other words, sequences can be
influenced or changes in the status announced. Examples of the
actuators are buzzers and solenoid valves.
(v)
Programmar
The programmer is used to created the software
and to transfer it to the PLC memory. In most cases, it can also
be used for testing software, sensorics and actorics.
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