Help, please explain a PLC program to me

[Adam303]

There’s got to be more to this than just the logic you posted. What is that box that sits beneath the robot. Is this the control box for the robot arm? Can you post a picture of what’s inside the control panel? I got to say this is a very nice set up.

 

[bernie_carlton]

It appears that most of the intelligence for this setup is in the camera and the robot. This program appears to just be handing off the camera decision to the robot and sequencing the part placement on the turntable.

 

[davidgeorge212]

I believe the robot arm is already programmed somehow to do its own thing. I think all that needs to be done is to program the PLC when you want the robot arm to move and do its task. Im sure the program works because it is printed up and put in a binder next to the the computer that runs the this trainer. They printed it out along with all of the other information like inputs and outputs and how the PLC is wired so that students can learn it. I am just interested in how the ladder program itself works. Do you think its necessary to know how the robot arm works or can you just assume it is already programmed to work the way it should. In the ladder logic program, there are outputs that look like they control the robot arm.

 

[davidgeorge212]

The zip file I uploaded seems to not have as specific descriptions above the instructions. Here are a few more descriptive names for certain instructions by the rung numbers.
Rung 0: Equal > counts what part it is on
Rung 9: Go > Connected to Robot input 6
Rung 11: Connected to robot output 6
Rung 12: Connected to robot output 8.
Stop sensor > Device net sensor on conveyor near docking station
Rung 15: Device net sensor in front of camera
Rung 17: device net sensor on conveyor near docking station
Piston > connected to piston on conveyor
Rung 18: connected to camera "Good"
Rung 19: Piston > connected to piston on conveyor
Rung 20: Connected to camera "bad"
Rung 21: connected to piston on conveyor
Rung 22: connected to robot output 10 "part done"
CTU > counts what part it is on
Rung 23: EQU > both count what part it is on
Rung 24: counts what part it is on
MOV > counts what part it is on
Rung 25: Counts what part it is on
MOV > counts what part it is on

 

[OkiePC]

I believe the robot arm is already programmed somehow to do its own thing. I think all that needs to be done is to program the PLC when you want the robot arm to move and do its task. Im sure the program works because it is printed up and put in a binder next to the the computer that runs the this trainer. They printed it out along with all of the other information like inputs and outputs and how the PLC is wired so that students can learn it. I am just interested in how the ladder program itself works. Do you think its necessary to know how the robot arm works or can you just assume it is already programmed to work the way it should. In the ladder logic program, there are outputs that look like they control the robot arm.

It looks like a program intended to teach a bit more than basic instructions using JMPs and LBLs and masked moves to real I/O. I would venture to guess that the program was made intentionally strange like this as a training aid, not intended to be simple to follow or easy to trouble shoot, rather it is intended to make a somewhat simple machine complicated enough to prepare students for what they might run into in the real world with more complex controls and equipment.

It is going to be easier (much much easier) to understand how the program works when you can monitor it online at runtime to view it dynamically while flipping switches and seeing action.

In order to go through it all in its entirety "on paper" if you will, we might need to know how the I/O are wired and how any of the signals connected to the robot are configured. It is probably not necessary to have the entire robot controller program if you know roughly what it does based on descriptions.

 

[Lancie1]

It is most probably a program written by a past, more advanced student. Surely no instructor would use such poorly-documented rungs as examples for PLC students....

 

[davidgeorge212]

I think you are right that it is written by a student. But one of my teachers told me it is the program they use to run this trainer.

 

[Lancie1]

Yes, I bet you are correct. I have seen that before. Some student gets a program working (apparently) and it is used for years before some curious soul discovers (1) either it doesn't work like it is supposed to, or (2) it was not documented so that no one except the original author can figure out how to use it, or (3) it doesn't do anything at all!

 

[OkiePC]

Yes, I bet you are correct. I have seen that before. Some student gets a program working (apparently) and it is used for years before some curious soul discovers (1) either it doesn't work like it is supposed to, or (2) it was not documented so that no one except the original author can figure out how to use it, or (3) it doesn't do anything at all!

And how does that differ from the real world of controls?

 

[Lancie1]

Paul,
You are corret, the situation is about the same. I suppose the main difference is that many students accept instructor-presented programs at face value, never thinking that there could be errors and omissions. If it is in the machine and sponsored by the instructor, they just automatically ASSUME that it is a great, working program.

I once spent 6 months trying to help this PLC student on PLCS.net write a program for a light display built by a former student. After the first week, I was convinced (from the pictures and wiring schematics that she sent me) that the display never worked the first time, and would not work as she wanted, without major rewiring. The problem was that she trusted the instructors opinions more than mine. As a result, her project never worked, and she graduated from that course, not knowing why. Misplaced trust is a difficult obstacle to overcome. There seems to be no good fast way to prove that one opinion is better than another.

 

[davidgeorge212]

So even though its a poorly written program, is anyone able to make sense of any part of it? I would just like to get a general overview of whats going on in the ladder diagram.

 

[harryting]

What's the purpose of doing this excerise?

 

[Lancie1]

So even though its a poorly written program, is anyone able to make sense of any part of it? I would just like to get a general overview of whats going on in the ladder diagram.

It is difficult to say what is going on in the ladder, because none of the rungs have any comments. You know, those little sentences above the rung that says stuff like "This rung is supposed to xxxx, when xxxx = xx". This is a great illustration of why most PLC training instructors have you to fill out that rung comment BEFORE you ever put any instructions on it.

The program could work well, or not at all. The thing here is that the program is only a small part of the system. It appears that the PLC program mainly starts the conveyor, and moves some data values around, and monitors the camera output for Good and Bad Parts and counts those.

Then you have a robot that is a separately controlled machine that is running its own program from somewhere.

Is there a wiring schematic, or any other papers with this machine?

 

[davidgeorge212]

What's the purpose of doing this excerise?

Well, part of it is supposed to be an "independant study" project for me to do. The instructor just showed me the machine and showed me that program and told me to explain how it operates. I barely just got done taking the intro to PLC in a structured class. I think he is expecting me to do this all on my own and he pretty much gave me no instructions or help on how to do it.

 

[Lancie1]

....showed me that program and told me to explain how it operates.

Fair enough. But tell him that you just have one teeny little request before you write up the explanation. You want to see it operate!

If you can see it running with the program open and monitor the Inputs and Outputs, then figuring out how it works (if it does) will be much easier.

He may come back with something like "well, it hasn't actually RUN in years, but what I want is a THEORETICAL explanation of how it would work IF it did work". In which case, say "Okay, since I can't see it actually run, I will write you up a THEORETICAL explanation of how it MIGHT work".

(In THEORY, anything is possible...)