Learrning

leitmotif

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L
Join Date
Nov 2004
Location
Seattle Wa. USA
Posts
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As I have said I am trying to learn PLCs.

My first effort was taking a Siemens unit I had out of storage and use it to learn with. The manuals that came with it were not much help and I was chicken and afraid of blowing it up.

One HUGE lesson I learned was that with no input hooked up I got an output LED. This showed me I had a lot to learn to avoid this dangerous condition. If I recall correctly this was with an input configured as a normally closed contact.

I contacted you guys asking for more study material. YOu referred me I used it and it helped. Once again thanks.

I found a class at North Seattle Community College on PLCs, enrolled and start next week.

In meantime I am reading the textbook Introduction to Programmable LOgic Controllers by Gary Dunning.ISBN 0 8273 7866. This is the first version and the class is using the second version. I also intend to buy the second version - I am evaluating the book courtesy of the County Library.

AT LAST the question
on pp 305 309 he discusses the start stop switches (momentary on and momentary off) for motor control. He makes the point the stop (normally closed or momentary off) should be a fail safe such that any failure will shutdown the equipment (broken wire etc etc). Completely agree.

Here is where I get confused and seems to touch on the experience I had with the Siemens (no connected input and an output LED). He is using the Rockwell SLC 500 or micrologix 1000 instructions. He recommends configuring the STOP switch (momentary Norm Closed) as if it were a normally open using ROCWELL XIC (examine if closed) instruction.

I can sort of see the reasoning behind it but keep wondering why I should call a normally closed switch (hardwired) a normally open in the software. If his recommendatin on p 309 work -- why have a normally closed software coniguration in the software if it cannot be used directly to a normally closed switch (hardwired).

I intend to bring my dilemna up in class and get a clear understanding on it.

In the meantime I want to hear from the "guys in the real world" who have been thru the dilemna and how they coame to understand it and not get confused.

Dan
 
leitmotif said:


Here is where I get confused and seems to touch on the experience I had with the Siemens (no connected input and an output LED). He is using the Rockwell SLC 500 or micrologix 1000 instructions. He recommends configuring the STOP switch (momentary Norm Closed) as if it were a normally open using ROCWELL XIC (examine if closed) instruction.

Click to expand...

With the Stop switch wired normally closed at the input, your XIC will always be true unless stop is pushed or wired breaks going to the input. You would put the XIC in series with the motor start, stop pushed or if the wire breaks the motor will stop.
 
ooooops, got beat out by Martin...

I don't like the idea of the NC in the internal ladder logic. If you are familiar with how a MCR works, why not write some logic so that when the plc powers up, the start button latches in the internal MCR circuit, but if the power is lost on the external NC contact, the internal MCR circuit opens up.

So, I would have an OR circuit, with the top rung being the NO input, with the bottom rung being the internal relay coil, these are the then ANDed to the external NC contact, then to the internal coil. Easier drawn then typed, so if I can do ladder logic, it will appear below.

>--[ I1 ]---v---[ I2 ]---( Q1 )--<
>--[ Q1 ]--^

I1 is NO Start
I2 is NO Stop
Q1 is Coil, and also contact that latches in until I2 momentarily opens

I don't have that book readily available. I got the older copy from interlibrary loan last year.

My guess is that there would still be an e-stop external to the plc to kill all power to the polc outputs,motors, valves, etc.

I have seen some machines where the start-stop circuit was the external MCR (master control relay),and that it killed power to the plc outputs, and also to a plc input. in addition, the e-stop was in this circuit, as maintained stop buttons instead of momentary.

regards.....casey
 
Last edited:
Code: 
   Start          Stop                 Motor
   I:1/0         I:1/1                 O:2/0
----] [-----+-----] [-------------------( )--
   Motor    |
   O:2/0    |
----] [-----|

When momentary Start is pushed and stop NC input is made motor starts and runs as long as I:1/1 is still true, If wires is broken motor never starts or will stopif running.
 
It's not the syntax, it's the sentence fragments....

Newbies to PLCs have always had problems distinguishing between the NO/NC contacts on PBs and relays, and the -| |- and -|/|- symbols used in ladder logic (AB calls these XIC and XIO, but since that only AB, I'll just stick with -| |- and -|/|- when talking about ladder.)

The problem starts with the traditonal two-button motor control circuit. When hardwired it looks like this:


     START          STOP

      PB             PB             MOTOR

------| |------+-----|/|-------------( )

               |

     MOTOR     |

------| |------+



But, since you wire the NO of the Start PB and the NC of the Stop PB to different PLC inputs, the ladder logic looks like this:


     START          STOP

      PB             PB             MOTOR

------| |------+-----| |-------------( )

               |

     MOTOR     |

------| |------+



Ultimately, the confusion is caused by poor annotation. If the above bits were annotated using complete sentences (including verbs), then it might be a little clearer:


    START PB       STOP PB            RUN THE   

    IS PRESSED     IS NOT PRESSED     MOTOR

------| |------+-----| |---------------( )

               |

     RUN THE   |

     MOTOR     |

------| |------+



This ladder translates (almost literally) to:
When the START PB IS PRESSED, then RUN THE MOTOR. Once you RUN THE MOTOR, you keep doing so as long as the STOP PB IS NOT PRESSED.

Which is what it's supposed to be. If you were to use the NO contact of the Stop PB instead, then the annotation of it would read STOP PB IS PRESSED. You'd then use -|/|- in the above logic, and the ladder would read:
When the START PB IS PRESSED, then RUN THE MOTOR. Once you RUN THE MOTOR, you keep doing so as long as the STOP PB IS PRESSED doesn't happen.


Like almost any topic, this one's been covered before.
CLICK HERE
HERE,
OR HERE .
 
THANK YOU GENTLEMEN FOR ASSISTANCE.
Martin T was a huge help in that he confirmed the book was correct.

I think Mr Nelson came closest to describing my confusion. He said
QUOTE
This ladder translates (almost literally) to:
When the START PB IS PRESSED, then RUN THE MOTOR. Once you RUN THE MOTOR, you keep doing so as long as the STOP PB IS NOT PRESSED.

Which is what it's supposed to be. If you were to use the NO contact of the Stop PB instead, then the annotation of it would read STOP PB IS PRESSED. You'd then use -|/|- in the above logic, and the ladder would read:
When the START PB IS PRESSED, then RUN THE MOTOR. Once you RUN THE MOTOR, you keep doing so as long as the STOP PB IS PRESSED doesn't happen.
QUOTE

I really hate this learning stuff. It is hard to describe your confusion when you don't know what you are talking about.

I think part of the problem is I am looking at it with an electrician perspective and think the programming operates just like hard wired.
I am REALLY getting my eyes opened on this and need to learn a new way of thinking.
I am having a hard time primarily because I am only working out of books (Icluding Melores) and need hands on. Tonite is first class and I will get hands on. I think after some hands on and class work this will all sort it self out.

IN MEANTIME
1. Thanks all for help.
2. DO NOT take any more of your time responding. Let me come back later with a little better understanding so I can ask better questions.

Dan Bentler
 
leitmotif said:
It is hard to describe your confusion when you don't know what you are talking about.
Click to expand...

I find it hard to describe my confusion even when I DO know what I'm talking about!... :p

Best of luck with the class! I'm sure you'll do fine... :nodi:

beerchug

-Eric
 
Class

Well went to class. Looks like it will be fun.
They have several demo modules can crusher elevator and a heat exchange system incorporating thermocouples and a pump. I like the heat exchanger because of ability to learn/do analog (thermocouples) and motor control all at once. Also because am interested in energy conservation by reusing heat normally wasted.

Dan Bentler
 
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