Beginer level

Can any one please tell me about learning the PLCs. I am a Mechanical Engineer and have a thorough knowledge for the mechanical systems. I want to learn the PLCs and its programming and am having very less knowledge about the electrical stuffs etc. Is it worthy for me to use the PLCs.nets DVD pack to get a sufficient knowledge about the PLC and its programming.
Thank you in advance for your answers. I would really appreciate your suggestions.

Shalin,

At one time I would have said that you first need to understand some basic electrical principles, before trying to learn PLC programming. PLC program methods were originally based on electrical control circuits.

Now, I see that many programmers posting here do not have even the smallest understanding of relay operation, switch contact operation, or any other part of electrical circuits. So I guess if you study programming first, you will be as good as most other modern programmers, who understand how the program instructions work, but have very little knowledge of the underlying electrical circuits.

It appears that the newer PLC programs are dis-associating with the electrical part of the process, so that programmers can only have knowledge of the Boolean logic, but still be able to write a program. Then some other specialist will have to translate that into a working electrical circuit. It will require two sets of specialists where before one was enough. Such is the modern process that stands in for "progress".

I disagree with Lancie1 that PLC ladder programming was ever exactly like actual relay wiring. The rungs of the ladder are not actually in parallel. Instead, the logic is processed from top to bottom. If you command an output off on the last rung, it will be off regardless of what the higher rungs tried. I don't think any outputs are actually set until the final rung is processed. In that sense, it acts like any text-based programming language (C, Basic, ST).

However, I didn't grow up with PLC's and my only experience with ladder logic is Allen-Bradley ControlLogix, so maybe it was different long ago. I do know relay logic and we still have much of that in operation at my company (octal-base type relays). I program PLC's in Structured Text, which is much easier for me to read, and easily allows For loops and such. I do know wiring and even circuit board design, though my degrees are ME/AE. I use mostly Beckhoff Automation EtherCAT, which is easy hardware to learn and the highest performing, but they are a bit weak on software examples. They do offer a learning package w/ hardware, though I haven't tried it.

I disagree with Lancie1 that PLC ladder programming was ever exactly like actual relay wiring.

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No, it was never exactly the same. For example, in an electrical relay logic circuit, all relays are being scanned all the time (no delay). But the original PLC software simulated this as closely as possible with the available processors.

For example, in a relay circuit, you could have many relays connected to the same input controls (switches, other relay contacts, timer contacts), but they had to be wired in parallel, never in series. Now ControlLogix allows serial output instructions, which resembles nothing in the real physical world, but does look like instructions in the C programming language.

Now, the programming software is quickly migrating away from any physical wiring connections, in order to work and look like higher-level programming languages. Eventually, it will be impossible for us electrician-types to relate the PLC program to any physical device (unlike the older software). I suppose this is deliberate so that electricians will not be able to easily pick up PLC programming. I can only conclude that none of the programmers putting out this c r a p ever worked with or wired up any real devices. They are desk jockey nerds only, not getting out very much.

I would say get a digital logic book and read that then you would have a good understanding of PLC for ladder logic if you want to know STL then I would suggest program a micro controller using assembly language. Other than that program to learn programming.

I would say get a digital logic book and read that then you would have a good understanding of PLC for ladder logic

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Yes, great advice. The same way as when you are building a skyscrapper building, you should start with the penthouse first, then when that is completed, go back and build the basement. That is what it is like to first learn PLC programming, then go back and learn electrical circuit design (when you find that most of your programs do not work).

Most ME degrees provide very little electrical circuit design, usually a course called "Circuits for non-Electrical Degrees" or something like that, a sop for the electrically challenged.

No easy way...Period

I like most started in the trenches working as an electrical technician and spending countless hours of troubleshooting and learning a craft. Mostly from others experience and good old trial by fire.

The basic premise of the Ladder Logic was to mimic the industrial wiring diagrams as closely as possible to make it familiarly similar. The made it both easy and comfortable to make the transition to the controllers from relay logic.

The capabilities and functions of the PLC allowed systems to expand and grow. The one thing that needs to be noted. Although all PLC's may seem to be the same, they are not! Even the same manufacturer will have vast differences in the models of PLC's and how they function. Controllogix does not process I/O the same as a compactlogix even though they use the same programming software you have to be cautious as to how you program the I/O.

Again, no easy way to learn this. It all boils down to taking the time to learn a craft.

I've said this many times. There aren't very many jobs where all you do is program PLCs. A PLC is just one of many components in an automated system.

You also need to to be familiar with the sensors that connect ot PLC inputs. These include limit switches, inductive and capacitive proximity sensors, photoelectric sensors, vision systems, analog transmitters, thermocouples, RTDs, just to name a few.

You also need to be familiar with the devices connected to the PLC's outputs. These include solenoid valves (both pneumatic and hydraulic), motor starters, contactors, VFDs, stepper and servo drives, just to name a few.

You need to understand enough about circuits, especially Ohm's law, to be able to evaluate when you can connect a PLC output directly to a load and when you must use an interposing relay. You ought to know enough physics to be able to select the correct size motor for an application, or to affirm or question someone else's choice.

Don't think for a minute that knowing how to program a PLC (in any language) will be enough to land you a job in automation.

Don't think for a minute that knowing how to program a PLC (in any language) will be enough to land you a job in automation.

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It shouldn't, and 10 years ago it couldn't. Now I am not so sure. The world has lost sanity. Up and down have swapped places. Judging by the new programmer posts now, as opposed to 8 years ago, it is daylight versus dark.

Dear All,

Thank you very much for your valuable suggestions on my doubt to learn the PLC programming.
I would also like to tell you that the reason of my being so interested in learning the PLC logics is nothing more than self knowledge. To tell you frankly I am a business man and I own a CNC machining shop which includes couple of turning and millings machines. I also know CNC programming for the component to be made on both Turning or Milling. But now due to my interest to learn more things I had got some idea of making some special purpose machines for my own use and that was the reason which pulled me in to this PLC thing.I think that may be it will be little bit difficult for me to understand certain electrical terms in the beginning, but I have made up my mind and want to learn the PLCs any how, so I will not fall back on this.
I was just surfing for some good learning sources for the PLC and ended at this place, which I found that can really help me understand the PLCs. Also the 5 DVD pack of this site I think can help me for the beginner level of the learning, if the site what it says in the sections details of their learning levels is up to the point which I guess it should be. Also you people are experts in the PLCs I would say, so you have commented very well and explained about the path that I should use to learn the PLCs.

Thank you, once again.

I think that may be it will be little bit difficult for me to understand certain electrical terms in the beginning, but I have made up my mind and want to learn the PLCs any how, so I will not fall back on this.

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I think that you have an accurate picture. The desire to learn is the main requirement for success. I think your CNC experience will help.

On another related subject, what do you think about the "3-D printing" machines that can produce workable objects? Are these going to be competition for CNC operations?

I have heard and seen some online videos of the 3D printing machines which can produce workable objects. But it seems that it will not have any competition with the CNC machines, as the 3D printing machines which are available in the market are basically used for prototyping and not for production due to the long production time for printing any kind of object, secondly these machines as seen in videos are working on plastic and that kind of very soft materials, where as regular CNCs are able to machine any kind of component with high speed machining ability which is mainly used for high production output. I might be wrong in this matter as I have not practically seen the 3D printing machine in person. Let me me know if I am wrong. But as of my idea there might not be any competetion in these two.

Thanks for the info. That is about what I thought. However, improvements are being made, and there are some 3-D machines that now work with metals. It could be that eventually they will be able to create objects in any material. It might be an area to keep your eye on for possible additions to your business.

Lancie1 said:

It shouldn't, and 10 years ago it couldn't. Now I am not so sure. The world has lost sanity. Up and down have swapped places. Judging by the new programmer posts now, as opposed to 8 years ago, it is daylight versus dark.

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wow thats a long time ago how old is this site?

Tommycai said:

wow thats a long time ago how old is this site?

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I think Phil started this site in the mid '90s, although it has been revamped a few times since. I originally joined in the late '90s. The wayback machine has captures dating back to 1998... :site:

http://wayback.archive.org/web/19980101000000*/http://www.plcs.net



-Eric