Plc?

[curtis rosche]

any one do any work with PLC's? (programmable logic controls) im takin a class on them this semester, but im taking the introduction class at the same time, so im trying to learn the advanced and basics at the same time,, it can get a little confusing. any suggestions for good sites that explain them in simple terms?

[Dan Hintz]

Don't know of any sites, offhand, but if you have a specific question that might be easier to deal with...

Ladder logic was always a pain... the logic is simple enough, but as the ladder gets bigger things get tricky to keep straight.

[curtis rosche]

the other guy i work with in class is in the same situation. we both know "C" programming and some others, but we have no idea how to input what we want into the computer cause its so different in the input. its really frustrating cause we know what we want to do, but not how to put it into ladder

[Stephen Cherry]

One thing to remember about the PLC is that it essentially implements a state flow diagram rather than a flowchart. Generally, it's every rung, on every iteration. I had used thees in chem/pharma applications, and they work great. Dan's absolutely right though, as the program gets bigger, it's harder to manage. To handle this, I would try to handle each device in about the same way, and then build automation on top of that.


The hardest thing for me was to get all of the little doodads communicating. Maybe this is easier now.

If I were starting out with this, I would try to input the code to start and stop a motor. Plus, you need to consider the operator interface, or multiple operator interfaces. What sort of devices are you learning about-- plc, with what sort of field devices and opertor interface?

[Matt Meiser]

What brand and flavor of PLC? That does make some difference. For example, Modicon scans the logic differently than Rockwell.

The hardest thing for me was to get all of the little doodads communicating. Maybe this is easier now.

When I listen to the controls engineers I work with, sound like -- NO!

[curtis rosche]

allen bradley micrologix 1000.

then for out fianl project we are linking multiples together and adding a few other model controllers and making our 4 samsung aj2 robotic arms do the dance for styx- mr roboto

[Anthony Whitesell]

Now you're speaking my langauge. I work for a local AB system integrator. When I'm not playing with wood, I'm playing with PLCs especially AB. PLC-5, micrologix 1000, 1100, 1200, 1500, controllogix, compactlogix. What do you have for comminication module(s)? What are the inputs for the robotic arm?

Getting AB PLC to talk to AB PLC is easy. Getting any two other manufacturer's PLCs to talk is like trying to mix oil and water. It can be done, but it may not be easy or pretty.

What class and degree are you taking. If you have any specific questions, feel free to PM me.

[curtis rosche]

Anthony, i have only just started the course. im not sure the other details yet, but i will deffinatly be asking questions along the way. ive looked at some of the strictly PLC forums out there, and they never seem to give students a straight awnser, they just like to give them a really hard round about awnser

[Anthony Whitesell]

I'll help as much as I can, but just remember that I can't see what you have or are doing and I'm not going to do your homework for you (ha ha ha!). The first thing to get used to is that PLCs run in a loop (or scan) and all "variables" are global and all values are persistent (starting the next scan as the value they finished the last one, unless intentionally reset at the top). So when you're ready fire away.

P.S. Oh yeah, another thing to keep in mind when dealing with Allen-Bradley, is they are the center of the universe. Everything is relative to their PLCs. Download goes to them, upload comes from (reverse of most other transfer programs). Inputs and outputs are the same, sinking and sourcing is relative to the I/O module not what's attached.

[curtis rosche]

Anthony, any good places to look to learn how to write the code? i understand the use of loops and variables from doing C programing, but that is a text based language. i know how to make it do what i want it to, but i have no idea how to transform what i want from text into the ladder symbols. i understand the logic, i just dont know how to read the ladder 100% of the time

[Stephen Cherry]

Curtis- the ladder diagrams translate directly into text based statement, for example, this rung

----| |----| |----------( )--
a b c

is a boolean assignment statement c:= a and b.


----| |----| |----------(L )--
a b c

is a boolean condition statement, if (a and b) then c:=1

Also, I would start with just some basic functions, for example starting and stopping a motor. This code will correspond very closely to a wiring diagram of a motor starter circuit. Plus you can add alarms, interlocks, etc.

For the variables, you generally just name a memory location. This is just like any other language. MOst basic machine functions do not require loops. They can come in later as automation is added into the system. I would always code a "push button" machine. (I used typically wonderware as an interface, there were not many real buttons). Then, if necessary, automation is coded on top of that.

and so on and so forth.

[Anthony Whitesell]

Unfortunately, I learned PLCs before the information was on the web. So I don't know of any good websites/tutorials on the web. I found one of two of my books. Of course the one I found, I wouldn't recommend. I will look again for the other book. in a PLC loops can be difficult. In the micrologix and PLC 5 they aren't so bad, but are completely different in the ControlLogix/CompactLogix world (which you don't have to worry about). Best to avoid them if you can.

[Dan Hintz]

ive looked at some of the strictly PLC forums out there, and they never seem to give students a straight awnser, they just like to give them a really hard round about awnser

This doesn't surprise me... most forums (such as certain engineering-related ones I'm on) are meant for practicing professionals, not students. It always works the same... if we started answering student questions, then group would quickly be flooded by students asking basic questions that should be answered by their professors or in their class notes/book.

On that note... you have a professor, so if your notes or book do not answer your question, ask. You're paying him a good salary (hopefully), so use him like any other resource.

[jeremy levine]

Give this a try
http://www.plcs.net/contents.shtml

[Bruce Pratt]

or this? http://www.allaboutcircuits.com/vol_4/chpt_6/1.html