Advantages of Using Programmable Logical Controller [PLC] in the Construction Industry

chillibreeze writer — Siddharth Jha

Chasing deadlines and optimizing cost of operations are perhaps the most discussed issues in the construction industry today. High efficiency and higher productivity has become the centre of this discussion and all project managers are looking for finding means to curb rising costs.

What is a PLC?

A programmable logic controller, also called a PLC or programmable controller, is a computer- type device that is used to control equipment in an industrial facility. The kinds of equipment that PLCs can control are as varied as the industrial facilities themselves. E.g.:- Conveyor systems, food- processing machinery, auto assembly lines.

How does PLC help in removing complexities?

In aPLC system, the PLC replaces the wiring between the devices. Thus, instead of being wired directly to each other, all equipment s are wired to the PLC. Then, the control program inside the PLC provides the “wiring” connection between the devices. The use of a PLC to provide the wiring connections between system devices is called ‘soft wiring’.

In a control system, the push button would be wired directly to the motor. In a PLC system, both the push button and the motor would be wired to the PLC. It is one of the most important features of PLCs. Soft wiring makes changes in the control system easy and cheap. In a control system, if we have to make some changes in the circuit, we have to rewire the entire circuit. In a PLC however, instead of rewiring the whole circuit, we can make small changes in the control program.

PLC BASICS:-

In the 1960s and 1970s, the construction industry began to see the need for automation. There was a need to improve quality and increase productivity. Flexibility had also become a major concern. Companies needed to be able to change processes quickly to meet the needs of the consumer. The industry either had to find a means to make amendments to the system without actually shutting down the system, or reduce the time consumed in making the changes to a bare minimum. They thought about troubleshooting the system without actually running the system. This would enable them to avoid damage to hardware in case there were some connection errors.

The old method:-

There was always a huge wiring panel to control the system.
Inside the panel, there were masses of electromechanical relays, which were hardwired to make the system work.
Hardwiring meant that an electrician had to install wires between the connections of the relays.
The only way to check whether everything was correct was to run the system.
Troubleshooting was done by running the actual system— a very time-consuming process.
No product could be manufactured while the wiring was being changed and the system had to be disabled for wiring changes. This meant that all the production personnel associated with that production line were without work until the system was repaired.
The control system was based on mechanical relays. Mechanical devices were usually the weakest links in the systems since they had moving parts that were subject to wear and tear. If one relay failed, the electrician would have to troubleshoot the whole system, which would be down again until the problem was found and corrected.

Need for PLC:-

Due to the disadvantages of the hardwired control panel, the industry saw a need to replace them and introduce PLCs. There was increased competition among manufacturers to improve both quality and productivity. Flexibility, rapid changeover and reduced downtime became important factor.

If changes in the system logic or sequence of operations were needed, the program of the computer could be changed instead of rewiring. This meant that all the downtime associated with wiring changes could be eliminated. How the system operated could be changed completely just by changing the software in the computer.

WHAT IS A 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 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 when to do it.

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

A contact is a computer code that monitors the status of an input coil.

A coil is a set of 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.

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. Coil makes the field device to switch on or off depending upon the PLC’s instructions.

Example: -

Let’s say that turning on the switch is supposed to turn on the light. In this situation, the PLC’s control program would contain a contact that examines the input device of the wall switch for an ‘on’ condition and a coil that references the light. When the switch turns on, the contact will energize, meaning that it will tell the PLC that the condition it has been looking for has happened. The PLC will relay this information to the coil instruction by energizing it. This will let the coil know that it needs to tell its referenced output, the light, to turn on.

Programmable Logical Controller

PLC BASIC COMPONENTS:-

Programmable Logical Controller

PLC programming devices

There are many devices used to program the PLCs .These devices do not need to be attached to the PLC once the ladder is written. The devices are just used to write the user program for the PLC.
They may also be used to troubleshoot the PLC

Dumb terminal:-

Device with keyboard and a monitor.
No intelligence.
Uses processor & operating system of PLC.
Inexpensive.
Can be used with variety of devices
Cannot upload, download or store programs

Dedicated industrial terminal:-

Dedicated to one brand of PLC.
ON line programming, Offline programming with some PLCs also
Trouble shooting while PLC is running
Forcing ON/OFF of I/Os.
Expensive

Power Supply:-

The power supply is used to supply the power for the central processing unit.
Most of the PLCs operate on 115VAC. This means that input voltage to power supply is 115VAC.
On some PLCs power supply is a separate module and some have integrated on it.

Input section:-

This section performs two vital tasks.
It takes input from the outside world and also protects CPU from the ouworld.
Input devices are often called Field Devices.

Output section:-

The output section of PLC provides connection to the real world output devices.
The output devices might be motor starters, lights, coils, valves etc. These are also called the Field Devices.

They can be used to output analog or digital signals.

ADVANTAGES OF PLC:-

High reliability
Small space requirements
Computing capabilities
Reduced costs
Ability to withstand harsh environments
Expandability

CONCLUSION

In present time, the industries need higher quantity with the best quality at a reliable speed and also with minimum maintenance cost. Thus, the PLC based automation system will not only guarantee reduced production time but also a higher productivity both in terms of quantity and quality. The offering comes optional for all customers and is poised to gain importance with ever increasing pressures in project execution and the best part is that this can be offered to the existing plants also.

Editor's note: Most articles submitted to Chillibreeze go through a selection process. Only 30 percent of submitted articles are accepted for publication on the Chillibreeze.com featured article list. All accepted articles are edited and proofread for glaring errors of punctuation and grammar. Sentence structure is changed in certain cases and sometimes, entire sections are rewritten. If you notice any errors that have slipped through the cracks, do let us know! (Email us at info at chillibreeze dot com).

Chillibreeze's disclaimer: This is a contributed article and was published on Chillibreeze in August, 2010. The views and opinions expressed in this article are those of the author(s) and do not reflect the views of Chillibreeze as a company. Chillibreeze has a strict anti-plagiarism policy. Please contact us to report any copyright issues related to this article. The relevance of the facts and figures cited (if any) could change after a period of time.

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—About our writer:

A final year Electrical Engineering student, Siddharth Jha has experience in technical writing, report writing and editing. He has worked with professors in their research and has never failed to meet the deadlines. He has a knack for helping people determine how they can get more with less by focusing and refining their work.

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