PLC Application in Industry

I am taking an introduction course to PLC's and am stuck on a question in one of my homework assignments. Unfortunately being in the military and the format I have to take this course in, I do not have a teacher or advisor I can consult with questions. The question I am asked is below:

Choose two examples of the PLC's purpose in industry. For each describe the advantages and disadvantages.

From the book I believe they are relating to either single ended, mutlitask, or control management PLCs. The book does not describe any of these very well or give any real world examples. I am not asking anyone to do my homework for me, but any information concerning the above applications of PLC and examples or your personal interpretation of what the question wants would be greatly appreciated.

Thank you.

single ended, mutlitask, or control management PLCs

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Is this the successor to unitary, modular or rack mounted PLCs that we've had so much fun disparaging over the years?
Seriously, I've been involved in manufacturing automation for more than 30 years and I've never encountered those terms. If you're pretty sure the homework question relates to those terms, you'll need to provide your textbook's definition of them.

This is a very broad wide-open question. Almost any use in industry would be a right answer. Basically the PLCs purpose is to automate equipment and machinery. It allows better control, faster operation, and lower costs than all-manual controls. PLCs are used now in just about all types of plants. It would be hard to list a type of manufacturing or industrial plant that did not use a PLC in some part of the facility.

From the book I believe they are relating to either single ended, mutlitask, or control management PLCs.

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Those would be more like control types or control situations, not "examples of the purpose of PLCs in industry".

The unitary/modular stuff was out of UK. I hope we don't follow suit. This type of test/question are a waste of time for everyone involved and needs to be taken out the back and buried before it gets a start to infect others.

I could be wrong, but here are my thoughts.

single ended
you are usually referring to an analog input, but since you are talking plc's, it may mean a plc application with a specific purpose / task.

multi tasking
a plc that is operating multiple workstations or a plc designed to run different programs based on the program selected.

control management plc
Flexible management system / DCS that is capable of running multiple lines in a plant.

As I said, I could be wrong. Just my thoughts.
Maybe someone from across the pond can add their thoughts.

regards,
james

Could we be talking Relay Replacement and Automation as purposes for PLCs?

Nick

"The PLC was originally designed for the Auto Industry, so that they could change fore each new model and year without extensive rewiring and downtime associated with Old Relay Logic panels"

My former mentor and partner (RIP Dave) had a roll in the beginnings of the PLC. An interesting read Inside the competition for the first PLC

jstolaruk said:

My former mentor and partner (RIP Dave) had a roll in the beginnings of the PLC. An interesting read

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Yes, it is! Thanks for posting the link.

The book's categories are apparently too poorly defined to pursue as answers, so just answer the question as it's posed, "Choose two examples of the PLC's purpose in industry. For each describe the advantages and disadvantages."

One thing that the classroom book I used 10 years ago did not cover was the HMI. Any PLC needs an HMI. There has to be some form of HMI, an On/Off LED or start/stop switch on one end of the spectrum to full blown multiple touch screen control rooms on the other end. An HMI is neither an advantage nor a disadvantage, it just is.

The other thing is the misplaced expectation of having a PLC store process data; they generally don't at all or don't do it very well. A PLC works in real time. Storing process data is the job of an HMI panel, HMI/SCADA software or a recorder. That's by design.

The advantages of a PLC are likely spelled out in a book on PLCs.

'Disadvantages' of PLCs that seem to come up on this forum, but are not likely in your text book are

- not having the right category of PLC for the job (needing complex calculations or communications in a PLC that doesn't support such)

- costs and maintenance of the associated programming development software tools (and cables) which are generally Windows PC based tools. Maintenance on a PLC that originally used a Windows 98 development program (and where is that special programming serial cable?) might be a serious challenge in 2015 or 2020.

- "The PLC program is the problem". Lots of threads here citing fuses, E-stops, a damaged limit switch or similar situations that caused a fault or shutdown that have nothing to do with the PLC or its program.

- PLCs are not PCs. This frustrates cube programmers who want to use their favorite language for control or for data access for HMI purposes.

- service is limited to someone with the right software and cables. The HVAC guy in town probably doesn't have the tools or experience to service a PLC.

- governments can remotely hack into a PLC and its HMI and change its operation; Stuxnet.

- a different skill set is needed to program a PLC vs discrete components

- the pounding a square peg into a round hole phenomenon; substituting a PLC for a commonly available dedicated control device, like using a PLC for a home thermostat.

So pick a couple apps - automate or traffic lights at an intersection or a car wash are common classroom projects and answer the question. (FYI, traffic lights use dedicated controllers, not PLCs, but the concept is sound).

single-ended.................stand-alone single PLC controlling one process

multitask......................PLC application involves one PLC controlling several processes

control management.......PLC application that involves one PLC controlling several others

JRB said:

single-ended.................stand-alone single PLC controlling one process

multitask......................PLC application involves one PLC controlling several processes

control management.......PLC application that involves one PLC controlling several others

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First, thank you everyone for responding with help!

For the above, that is all the book really has on each type of PLC. One of the biggest areas I am stuck on is finding a real world example for each type that I could explain advantages and disadvantages for. The advantages all seem easy, but the disadvantages are not. Anybody have any good examples of each type.

Cost can be a disadvantage for a small system.
The requirement for programming tools/programmer to change or fault find may be viewed as a disadvantage.

Nick

Hodak04 said:

First, thank you everyone for responding with help!

For the above, that is all the book really has on each type of PLC. One of the biggest areas I am stuck on is finding a real world example for each type that I could explain advantages and disadvantages for. The advantages all seem easy, but the disadvantages are not. Anybody have any good examples of each type.

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I don't see any relation of the terms (single ended, multitask, or control management) to the question asked.
I would think you only had to provide 2 examples of a PLC in use in industry, then describe advantages and
disadvantages of each PLC, related to each use.


Revisit danw's post, he painstakingly provided lots of disadvantages, and here is one of the biggest;

You don't have the software, cables, replacement parts or backup program, and all your processor lights are blinking red.

http://www.plctalk.net/qanda/showthread.php?t=94182

Besides being used in amusement rides, other PLC application examples are in transportation industry. When businesses
have medium to large scale transportation needs, each vehicle is equipped with special PLC system so whenever they set
off or return to the base, the system will automatically take note of their check in and check out process. The
system is also equipped with mapping system that allows the manager in charge of everything to locate each vehicle.

PLC's are used on conveyors, packaging machines, material transport, building automation, water authority systems,
entire lines of production equipment, home automation, extruders, molding machines, or any automated process that
requires more than a few relays, or anywhere that decisions based on numerous or variable input and output criteria
is present. PLC's save a lot of wiring space and replacement costs by eliminating external relays, timers, and counters.
They provide flexibility of control changes through software, eliminating downtime ordinarily incurred through wiring
changes.

Pop tarts, potato chips, breads, buns, vehicles, and nearly everything else is produced by machines controlled by PLC's.

Howerver if you have never been in industry, or seen or experienced any PLC actually in use, it would be difficult to
provide examples of their actual use, or their advantages or disadvantages.

Even though the question doesn't ask for examples of each PLC type, this is how I see these types used;

"Single ended" PLC is any individual PLC controlling one machine, such as a small conveyor recieving product
from a packaging machine.

"Multitask" PLC is a PLC controlling more than one process, such as mulitple production lines being controlled by
one PLC feeding the packaging machines.

"Control management" is a PLC controlling other PLC's, such as multiple PLC's controlling multiple production lines,
and all are communicating together, with one PLC having master control over others.


http://www.iamechatronics.com/notes/general-engineering/257-plc-examples

Good luck.

I don't see any relation of the terms (single ended, multitask, or control management) to the question asked.

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Correct, and that is Hodak's big stumbling block - he thinks the answer must be in the text book somewhere.... Look past this barrier into the real world, and you will find plenty of examples, many already mentioned. But first you must answer the question being asked, not the question you found an answer for! The real question has no mention of 3 types of PLC - in the real world, those types are murky as many PLC systems have attributes of all 3. This is mainly a teaching aid, a way to classify devices that really do not usually fit easily into those categories.

Choose two examples of the PLC's purpose in industry. For each [example], describe the advantages and disadvantages.

Example 1 of an industrial purpose for a PLC: Control the scrap metal collection conveyor in automative metal stamping plant.
Advantages of Example 1: Makes the scrap system easily customizable for the customer's needs.
Disadvantages of Example 1: Customer had difficulty training operators and maintenance personnel on automated scrap system.

Example 2 of an industrial purpose for a PLC: Control all the motors, valves, solenoids, and other equipment in a coated fertilizer plant.
Advantages of Example 2: Made the entire plant fully automated, so that only 4 operators and 6 maintenance men were needed (not including the sales staff that handled the weighing of fertilizer trucks going out). Also customer was able to easily duplicate the system to double the size of the plant.
Disadvantages of Example 2: As with any automated system, when something breaks - even a small unimportant sensor or switch, the plant may have to shut down until repairs are made. Also, the plant employed many fewer unskilled workers than other plants operated by the customer. This meant that the local job market for unskilled labor was less, but the need for skilled and trained maintainence technicians was greater.