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Programmable Logic Controllers In Practical Application

Posted February 10, 2019 12:01 AM by ahorner_22
Pathfinder Tags: controllers logic controllers PLC

A Programmable Logic Controller (PLC) is an electronic device used to monitor or control production processes. It is a device that a user can program to perform a series or sequence of events. These events are triggered by inputs received at the programmable logic controller through delayed actions such as time delays.

Once an event triggers, it actuates in the outside world by switching on or off electronic control gear or the physical actuation of devices. A Programmable Logic Controller will continually loop through its user defined program, waiting for inputs and giving outputs at the specific programmed times. They are designed to function in industrial environments and can withstand extreme temperatures, humidity, vibration and electrical noise.

Generally, PLCs are tasked with monitoring or controlling a group of sensors and actuators. PLCs are basically like industrial computers because they have microprocessors but are more robust than the average home computer. PLCs use a different language than home PCs as well.

PLCs eliminate the need to rewire or add additional hardware to make things work. They increase the functionality of controls and don’t take up a large amount of physical space. They are also flexible and easy to modify as application needs change. Utilizing programmable logic controllers is a cost-effective method of controlling complex systems. PLCs are best used in applications where machinery can work on its own without human labor having to constantly monitor.

What kind of industries utilize PLCs? Some of the many examples of practical PLC applications are listed below:

Leisure: Amusement Rides

Transportation Industry: Trains

Travel Industry: Escalators, Elevators

Textile industry: Textile Shrinkage Systems

Plastics: Injection moulding control

Material Handling: Conveyors, Automated Storage and Retrieval Systems

In addition to the items listed above there are endless manufacturing operations that utilize PLCs to control their systems.

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#1

Re: Programmable Logic Controllers In Practical Application

02/11/2019 3:30 PM

The next step up is to move to a DCS (Distributed Control System) where the PLCs become bigger with more analog or digital inputs, outputs and connect them to a SCADA (Supervisory Control And Data Acquisition) OIU (Operator Interface Unit) and you have an integrated system for control of a complete process.

We used the above DCS to control and report on the operations of a BE1370 dragline where the control from the operator sticks and pedals fed the mode control and there were separate systems for generator control, sync motor control and all alarm handling.

Alarm handling was prioritised for removal of alarm flooding and all parameters could be viewed remotely by a multi hop radio link.

The whole coal handling plant was run by 16 separate MFCs (Multi Function Controllers) to clean the coal, handle raw coal stacking and product storage.

4 small PLCs ran the coal washing jigs and had high speed loops running in the coal jigs where control changes in the high speed air pulsation section ran in the mS range while the slower segment controlled the gate opening control from the float positions.

Then there was the power station which used the coal where everything was run by a DCS system.

Regards Stef

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#8
In reply to #1

Re: Programmable Logic Controllers In Practical Application

02/14/2019 5:34 AM

Stef, you and I are of the same generation in that we started on relays and had to design and wire systems to avoid "relay races" that could lock up the controls if two inputs changed together. What a pleasure when in the early 1970s I moved on to using PLCs that were deterministic and you could write a program to dictate the desired response even if two inputs changed status within the same scan time. More recently new PC based products have appeared onto the market that suppliers claim has outgrown PLCs and those same suppliers argue that scan speeds are now so fast that determinism is no longer required. But the use of PLCs has not decreased, so it is clear that those of us who understand how retrograde a step dispensing with determinism is do not agree. Ladder logic may no longer be in fashion but it should still be learned as it instills precision into programming. I see so much badly written software often made up of cut and pasted badly written sub routines. The use of patches to plaster over the cracks is no substitute for breaking down the brief into timing charts and Boolean Equations and understanding precisely what is required to write software without the need for patches.

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#2

Re: Programmable Logic Controllers In Practical Application

02/11/2019 6:43 PM

PLC use with a PC friendly front End:

"PLCs use a different language than home PCs as well." Few people want to have to learn "ladder logic" and have to cram into it the broad functionality that they already have in the PC world. Now there is a full operating system which can run on a super cheap family of boxes and which supports legacy "ladder logic" which the user may want to incorporate into a new creation which can use the full range of Linux applications to provide Human Interface, communications, associated niche applications, etc. and still support PLC type function in an integrated fashion.

The OpenPLC plays well with raspberry pi and arduino boxes. A developer fluent in some popular language (Perl, Python, C, C++, ...) can install OpenPLC runtime on their pi and marry the best parts of both worlds on their favorite full operating system low-cost platform. If they know ladder logic for process control, then they can write the parts they wish in that vernacular. If they are weak there they can obtain fragments written by others and wrap them in their custom user interface. If they want checkboxes, buttons, and other HI widgets and gizzies they can layer them on. If they want communications via low energy bluetooth they can write that in Python and hook it up to their PLC functions. If they want to dedicate an arduino to some process control function and talk to it with a pi, or a laptop, or a cellphone, the capability is there.

Specific purpose languages should not be prodigally extended to encompass ever broader function requiring life time learning. General purpose languages should be used to knit together packages with narrower goals. Linux is a Swiss army knife style operating system with selectable packages. PLC functionality is now one of the packages one can choose to include in some of your boxes.

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#4
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Re: Programmable Logic Controllers In Practical Application

02/12/2019 9:52 PM

Ah but the world has moved on since ladder logic but some PLCs have Ladder Diagram for those living in the past and fluent in ladder logic for now there is Function Block diagram, Structured Text and other forms.

It is even possible to write a program in Structured Text, and have it appear as a Function Block with configured inputs and outputs. This block then does the processing internally and presents results based on the input values.

While I have nothing against Pi raspberry or other flavour, the problem comes about when someone other than the original programme writer has to decipher the program when they have no knowledge on the program.

A propriety PLC with its software comes as a stand alone unit. The manufacturer looks after the improvements and the legacy of existing systems. If others have to repair the system and the programmer is not available then the KISS principle will always win for Murphy was an optimistic programmer. A case in point one PLC was written in C++ and used inputs of pressure to frequency on a critical process. What electrician knows C++ and who has an meter that will read frequency, not very many. The system was replaced with a PLC in function Block and the Pressure to HZ were changed to P to I. Now every meter is capable of measuring 4 to 20 mA or 1 to 5 volt when the current is terminated in a 250 ohm resistor, problem solved.

So learning a new method of doing the logic is no different than someone having to learn Pi or Picaxe or C in its various forms. The function block method is really no different old time IC gate logic but they are in the form of Blocks of logic. Like AND OR exclusive OR Delay on/off/pulse, S/R etc.

Yes Linux works, that is why Siemens used it in the power station for the T3000 interface which used a PLC written with Function Block logic.

Regards Stef

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#3

Re: Programmable Logic Controllers In Practical Application

02/12/2019 9:24 AM

Sounds like not all applications of PLC is practical.

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#5
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Re: Programmable Logic Controllers In Practical Application

02/12/2019 10:09 PM

On the contrary If a application exists using other means to control an output then the PLC equivalent can be written and interfaced to the real world.

A PLC will run until told to stop doing this in the background reading inputs setting outputs and will do so at various speeds which can be programmed to fit the required application.

If a number of PLC can control a dragline, en electric truck, washplant or power station, sequencing traffic lights or garden watering system then the mind is the only limit to the creativity and application. I once created a Function Block written in structured text containing 300 lines of code to provide the average and instantaneous efficiency of Drag vs Hoist to gain optimum hoisting with a full bucket of dirt. Or on the other end of the scale accepting a radio signal to start a pump and provide the motor control for flow and tripping.

As I say the mind is the limit.

Regards Stef

I say this for I have worked with Siemens, Bailey, GE, Omron, Komatsu and several other's PLC's or DCS systems, all are different but all can do the same

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#6
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Re: Programmable Logic Controllers In Practical Application

02/13/2019 12:15 AM

Hello Stef, could you point me in the direction of a good beginners book on PLC's. I have a couple of ideas that would probably run using these devices. I need to start at a pre-beginners level being senior in age. My brain refuses to stop wondering what if.! I once designed a system for a hospital evacuation system, it worked perfectly but was not an approved device. To have it passed for use would have cost me more money than I have, it was therefore shelved. It used discrete I.C's back then, but I wish to redesign the system before I shuffle off the mortal coil.

Regards, Rod

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Re: Programmable Logic Controllers In Practical Application

02/14/2019 12:54 AM

Hi Rod,

Luckily for me I started with control systems back in 1964 working on relay the real mechanical devices when the internet was not even a dream. Remember the computers were microbee (microbes?) and things were written basic and who ever thought you would fill a 10Mb hard disk, ridiculous!

Now to the question. I did find this site on the internet which appears to be of help but there are many others out there. Queensland TAFE has a PLC course but that is probably of no help to you where ever you live.

https://www.myplctraining.com/beginners-plc-overview-part-1-of-4-introduction-to-plcs/

When you are comfortable with this then might I suggest choosing a PLC brand and downloading the instruction manual and read away, we are here to help you while we are here!

I taught my boys with a product from RevolutionEd from the UK which offers the Picaxe programmer chips. You can build the boards, plug the chips in and program with the free software. You can make all sorts of gizmos for your amusement. I built a radio repeater controller with cross linking UHF/VHF for our local club, now defunct, too many deperching mortal coil shufflers. Remember this was back 18 years ago I started them on the learning curve.

PI and other newer products abound but I have old PLCs bought for scrap if the urge to make something really interesting comes to pass but now I repair/restore old tractors and earth moving machinery. Nothing like getting your hands on a 5ft breaker bar to undo a nut compared to a 10BA spanner and if it is dropped it is easier to find.

Regards Stef

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#9
In reply to #7

Re: Programmable Logic Controllers In Practical Application

02/14/2019 5:37 AM

Great reply Stef,

I had a good smile while reading it. I found what I hope will be a good book on PLC's, finally, It comes from Amazon, It looked cheap but I keep forgetting the P & P from America is horrendous. However, I ordered it and it is due in March 4th. Regarding Queensland, it is at the top of the country, I am at (or near) the bottom of the country. I noted that you have been restoring tractors. I don't know if you have heard of the old grey Ferguson tractor, model TEA. Dad purchased this new when they first came to Australia, I still use it to saw wood for the stove, it has a couple of small problems but it goes good enough for me.

Anyway, I am starting to waffle on so I'll get off to bed and hopefully greet a new tomorrow.

Thanks again for your help Stef. very much appreciated.

Regards, Rod.

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#11
In reply to #5

Re: Programmable Logic Controllers In Practical Application

02/22/2019 11:01 AM

That's great stef. What I mean is if you use that device with uneconomical purpose. It would not be practical isn't it? Just a sarcasm, you know

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#10

Re: Programmable Logic Controllers In Practical Application

02/16/2019 10:29 AM

I started with a Siemens MC16 to control a 10 axis automated welding system, using jumps to machine routines where additional speed was required in arc sampling routines, and ladder logic controlling open loop motor routines...this was replaced with off the shelf servo controls and high speed analog and digital boards in STD format and a compiled FORTH supervisor program running out of a 8k rom on an 8086 processor at 8mhz. Sometimes PLC's are too limited. It is sad that the STD hardware is no longer used. It is amazing the IO and control available today for $20 in formats like Arduino.

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#12

Re: Programmable Logic Controllers In Practical Application

03/09/2019 5:32 AM

OK guys, I guess I am the Ancient One.

I started in controls when they were all pneumatic,communications were vacuum tube,and relays (Either pneumatic or electrical) were the final control element.

It is still amazing to me what was done using purely analog,physical methods.

They had pneumatic devices that could add,subtract, divide,multiply,square root extraction,ratio,bias, all without a single wire,only air.

Some devices incorporated all functions into one device that was programmed by moving input/output ports.

It was difficult for me to make the transition to solid state,then binary,Boolean Algebra,Logic gates,PLC's (long before personal computers).

But I persevered and learned to program in Pascal,Basic,C,and variants,Linux,FORTRAN,COBOL,and lots of proprietary languages along the way.

A very good book for learning logic from the beginning was written by Forrest M Mims. Not certain of the title,but something like Understanding Digital Computers.

It starts at the very basics of logic gates and how very simple gates can be connected to form complicated tasks,and takes you all the way to small computers.

I was able to trace all logic states from a keyboard input to the output result on the display after studying his material.

I recommend it for anyone desiring to learn logic and computers from scratch.

Computing has come a very long ways in the last 50 years,doing things that were considered impossible in the past.

The main thing I see was making them user friendly with point and click interface(Mouse) and putting computer power in the hands of the general public.

They do not know how many lines of code are required just to generate a single icon on screen and to make it function as it does and they do not need to.

However,the rush to market by developers has led to very sloppy programming.

When memory capacity was limited,and precious,and programs were compact and efficient one had to write good,bug free code.

Now memory is virtually unlimited,and programmers are pressured to "Get it to market now,we will work out the bugs later."

Any one can turn around an 18 wheeler if you give him a 100 acre field to do it in.

It takes real skill to parallel park one in a crowded city.

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