Back again with project 2

bryan372002

Member
Join Date
Apr 2005
Posts
72
ok Back to see how much poo i can stir with this One.lol

right off the bat, here is exactly what the project sheet reads, Word for Word.

The Plant Engineer has given Us instructions to fix the operation of the index Machine so it cannot be started without both hands and feet are in the clear. the problem has been in the past that people have been getting there arms and legs in the machine and the machine breaks them off. this is a great expense to the company having to train new people and pay insurance settlements. our job is to put an end to this expense by putting an anti-tie-down circuit in. the only way the machine can start is 2 foot switches must be made and 2 palm switches must be made at the same time before the machine will Index.

Material need for this project is

2- start buttons
2- timers
2- foot switches
1- relay

Now before we go any further this is additional info i have been able to coax from the instructor.

if the 2 foot switches, OR the 2 palm switches are made for more than 3 seconds timer 1 will time out and not allow the machine to index. this way neither the foot switches or palm switches can be tied down and allow the machine to index.

Guys i dont want solutions to the ladder logic, Just help with trying to figure Out, a good layout of the logic statements
 
bryan372002 said:
if the 2 foot switches, OR the 2 palm switches are made for more than 3 seconds timer 1 will time out and not allow the machine to index.

Look at your sentence there. See if you can construct logic from it. You have two implied AND's and on explicit OR. Start from there.
 
HI
A light curtain control system can be used to prevent any harm to operator. Just for the fun of it
 
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yes I did reply in that post but that operation is not what i Have concieved from this assignment, i think Darren was right on the monet with his analysis of that problem in that what is basically wanted is that if the the operator presses the 2 palm switches he has 2 second to press the 2 foot switches and release them all 4 or the machine will not index. once the machine indexes once the cyle must be restarted all over again.
 
bryan372002 said:
yes I did reply in that post but that operation is not what i Have concieved from this assignment, i think Darren was right on the monet with his analysis of that problem in that what is basically wanted is that if the the operator presses the 2 palm switches he has 2 second to press the 2 foot switches and release them all 4 or the machine will not index. once the machine indexes once the cyle must be restarted all over again.

Bryan..One of us has to re read my post!!! I dont think i said the switches have to be released..Just that after the index starts they could be released..

I think the key to these questions is the words..has anyone looked up what is ment my "Index"..I know what i THINK it means..(One cycle) but i could be wrong..

D
 
Rick Densing said:
Look at your sentence there. See if you can construct logic from it. You have two implied AND's and on explicit OR. Start from there.

This is good advice for any project you are going to have. Make it a practice to write down what you want to do in exactly this fashion.

For example IF this limit switch AND NOT this float switch OR this pushbutton THEN the desired result.

EDIT: Not talking down to RD, pointing out to the OP that what RD said is a good idea that works wonders.
 
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Read a book

I think you need to read your books, being an electrical maintenance guy, this stuff should not be so hard for you. After this course in PLC, do you think that your company will let you program away? Doubt it! That’s right, it’s for troubleshooting. A good troubleshooter learns his machines and processes inside and out, when a problem comes to life he or she should be able to grab the I/O schematic, look at the idiot lights on the PLC, and say hey "found the problem." There won’t be any laptop in your hands. Read your damn books and think outside your bubble!
 
Absolutely, the need is for the ability to discriminate the hardware and software problems. The guy with the laptop is very protective as it's his *** if the program kills somebody or sets the place on fire. He's gonna make sure nobody screws with the program unless he makes the changes requested. Besides, a properly placed jumper can fake a lot of faulty digital inputs.
 
Okay, you asked for no solutions, just the framework.

Use 3 rungs, one for each timer, and the last for the relay. Call the relay "MACHINE INDEX". Do not seal in the relay. It triggers the machine and thats all.

On the first rung arrange the inputs so that Timer 1 can be started by either PALM switch. Set the Timer for 3 seconds per your instructions. That is too long for real-world machines, but necessary to allow you time to flip the switches on the LogixPro simulator, because you can only flip one at a time.

On the second rung arrange the inputs so that Timer 2 can be started by either FOOT switch.

On the third rung, add 2 timer contacts and 4 switch inputs so that only if all are closed at the same time, the MACHINE INDEX relay is energized.

I could have typed the rungs quicker! I did type the rungs into LogixPro to test it and the above will prevent the switches from being tied down, and also prevent the machine from indexing unless all switches are pressed, only if all logic is entered correctly.

As many have said, this safety circuit should not actually be built using a PLC. This is serious stuff. People get killed because of improper safety designs. My wife's cousin's brother-in-law stuck his head under a stamping press and it indexed and crushed his skull into a bloody pulp.
 
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Hi Bryan...I stayed out of Round 1, but did follow it ever day...stay with it, you are doing great!!!!

As Lancie1 said, you have to be ( in real live applications) very careful as to what you allow a PLC control...this is one of the times you should NOT use a PLC, unless it is a specially listed and designed unit, meant to be used as a SAFETY control.

I just had to make that clear so someone doesnt just read part of this thread and use what will, most likely, be post here in an unsafe manner.

Keep up the good work...WE all learn things from following and joining in on this site.:site:

David
 
Since the spec you have been given is a bit vague, before you write a single line of ladder logic, you should write a more detailed spec and tell the instructor that it defines what your program will do.

For a first draft, I suggest:

1. Upon receipt of an "index" command, the machine will begin operating and will continue to operate through one and only one cycle.
2. The "index" command must be repeated in order for the machine to go through another cycle.
3. The machine must be stopped before an "index" command can be issued.
4. All operator devices (palm buttins and foot pedals) must be in their non-actuated positions before an "index" command can be issued.
5. Once any one of the operator devices is actuated, all of the other operator devices must be actuated within 3 seconds.
6. If condition 5 is not satisfied, the "index" command can not be issued and all operator devices must be returned to their non-actuated state in order to create a new 3-second "window of opportunity".
7. Once the "index" command has been issued and the machine starts, all operator devices must remain actuated for xxx seconds or the machine will stop.
 
Steve, good call about the use of the second timer. That would be better than my idea. It does give the second timer something useful to do. Bryan, try using a TOF (Off Delay) timer for the second one, and fix it to operate as Steve outlined. That would be closer to the way presses and punches operate. Up until a critical time, the indexing can be stopped if the operator removes his hand or foot. After the "point of no return" the machine cannot be stopped (due to inertia) until it finishes the stamping, punching, or whatever.
 
Hi David, Just want to say a special thanks for The Vote of confidence, tha means a Lot.

A thanks to all of you for your help and Replies to this thread. Looks like I have been given some great guidance, however im a bit to tired to work on this project so far, as I am on call this weekend at work and spent all night there last night, got about 4 hrs sleep and had to go back in again this afternoon, so I'll prolly give it a go tommorow on the project.

I understand that A plc shouldnt be used as a safety circuit and that light curtains are the way to go now. safety gates and light curtains are numerous where I work and not a anti-tie-down anywhere....lol Thanks Everyone and I'll be Back.
 
Adam James said:
I think you need to read your books, being an electrical maintenance guy, this stuff should not be so hard for you. After this course in PLC, do you think that your company will let you program away? Doubt it! That’s right, it’s for troubleshooting. A good troubleshooter learns his machines and processes inside and out, when a problem comes to life he or she should be able to grab the I/O schematic, look at the idiot lights on the PLC, and say hey "found the problem." There won’t be any laptop in your hands. Read your damn books and think outside your bubble!

Feel Better Now????
 

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