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Old December 12th, 2013, 12:47 PM   #16
Karoly
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I dont think you will succeed Lancie, as I had a long chat with the moderator there, as you can read in the talk page
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Old December 12th, 2013, 01:09 PM   #17
dmargineau
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...it has been edited as of 12/12/13 12:00pm CST...
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Old December 12th, 2013, 01:30 PM   #18
Lancie1
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In case there is disagreement and argument, I simulated the exact Tank Fill program on the LogixPro simulator software. Here are the results (attached pictures in order of operation), using two NC Level Switches, but using XIC PLC instructions (those appear to be Normally Open).

With NC level switches (both closed when the switches are lying on the shelf at the manufacturer), this program shows that when the water drops below the low level switch point, both will have closed contacts, and using XIC (NO) instructions, the fill valve will go on.

Reading the comments on Wikipedia, I think the author "North8000" believes that the logic for a PLC program for his Tank Fill is exactly the same as hard-wired switches and relays. For hard-wired level switches, he is correct about how it works. Most of use have been using PLCs long enough to know that PLC logic is not always the same as the hard-wired logic.

Last edited by Lancie1; December 12th, 2013 at 02:00 PM.
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Old December 12th, 2013, 02:42 PM   #19
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Remember the contacts used are picked for failsafe reasons, that is why sometimes they will use nc contacts. Stop button is one example.
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Old December 12th, 2013, 02:51 PM   #20
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Oh-boy. Again, the XIC and XIO instructions in the PLC are NOT the contacts in the Level Switches, or Stop switches, or any other real physical "Look-Mamma-I-Can Touch-the Switch".
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Old December 12th, 2013, 08:53 PM   #21
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Lancie,

I feel you may have misread just the cowboy's statement?

I don't think he's mixing anything up between the physical contacts and the PLC instructions. I think he just didn't state which contacts he's referring to.

I think he's merely pointing out the reason, in this application, that the chosen physical contacts on the float switches are normally closed for fail-safe reasons. Similar to STOP push buttons.

Another point to make for novice readers...

I've often found that some learners struggle with the concept of a signal being 'LOW' to indicate a physical 'HIGH LEVEL' state. Or a signal being 'HIGH' to indicate a physical 'LOW LEVEL' state. A signal being 'HIGH' or 'LOW' has no direct relationship to a physical 'HIGH LEVEL' or 'LOW LEVEL' state. They are similar terms, but not to be confused.

Well done on the edit Lancie. You were more than correct to edit the instructions to N.O. as the very point they stressed, in bold text, in the article clearly states how important it is to use the correct instruction type to suit the physical contacts...

Quote:
...Therefore, the contacts in the PLC program that "read" or look at the physical switch contacts in this case must be "opposite" or open in order to return a TRUE for the closed physical switches...
A simple error in their diagram, but an important one to correct.

G.
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Old December 12th, 2013, 09:56 PM   #22
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Quote:
I feel you may have misread just the cowboy's statement?
If so, I am sorry for my misunderstanding.
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Old December 12th, 2013, 11:33 PM   #23
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Quote:
Originally Posted by Lancie1 View Post
A comment with an error is not much better than no comments at all.
I'd go with worse.
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Old December 13th, 2013, 03:28 AM   #24
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(I refrased all this)

Regarding the "fail-safe".
That a sensor uses N.C. contacts doesnt mean it is fail-safe. One has to take into account what happens if the signal is lost, and chose the version (NO. or N.C., and "above" or "below") that has the least grave consequence and/or the one that can most easily be detected to be in error.

The most fail-safe combination of sensors in the wikipedia example must be:
Low level sensor: Indicates "above low level" with an N.C. contact.
High level sensor: Indicates "below high level" with an N.C. contact.

Argumentation:
Assuming that overfilling will be a worse scenario than the tank goes empty.
If power supply is lost to both sensors, the PLC will see it as simultanously below low level and above high level. This is impossible, so it should generate an alarm, and halt the pump.
If power is lost to the low level sensor, it will start the pump but only until the high level sensor is reached.
If power is lost to the high level sensor, it will stop the pump completely.
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Last edited by JesperMP; December 13th, 2013 at 03:56 AM.
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Old December 13th, 2013, 07:41 AM   #25
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JesperMP makes a very good point. Since we in the US are about 20 years behind Europe in safety I see this quite often in safety systems over here. Designers will throw "safety rated" components on a machine but not use the safe design concepts the devices were designed to support. But they'll call it safe because of the components that are used.

The same applies to "failsafe" systems as JesperMP points out.

Keith
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