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Old February 20th, 2006, 08:18 AM   #1
TimothyMoulder
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Some great advice from somebody else - not me

I stumbled across this today while looking up the resistor combination to turn NPN inputs in PNP inputs. Kudos to whoever first posted the link.

We've had alot of excellent posts lately about good design standards and practices, so I thought I'd dust this off and swing it back around.


http://www.mrplc.com/kb/index.php?pa..._v2&id=44&c=38
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Old February 20th, 2006, 09:28 AM   #2
Mickey
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WOW, that is some good stuff. Thanks
Although I must admit I am not a fan of grounding 24VDC power supplies.
But I cannot argue with his explaination for doing so. I will rethink my methods.
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Old February 20th, 2006, 09:45 AM   #3
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Interesting...I've never seen a design done like that. You'd think the authors point about the input activating when grounded would have been obvious to the designer.
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Old February 20th, 2006, 09:50 AM   #4
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Great article. Im printing it out now to pass out to all the guys. Unfortunately we get most of our equipment from Japan and it all has NPN wiring. Anything that I can spec out and have built in the US has PNP.
thanks,
Bob

Last edited by bob1371; February 20th, 2006 at 09:53 AM. Reason: Additional info
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Old February 20th, 2006, 09:53 AM   #5
Thomas Sullens
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Timothy I`m glad you brought this up. We just got through building a new machine and used what we thought was the right input card, it was an AB card and this happened.
Quote:
You can't go by the terms "sourcing" and "sinking" when choosing I/O modules. Different manufacturers use the terms in opposite ways. You must check their drawings before you can be sure of what you are getting. A sourcing device is one that supplies positive charges to another.
because we (assumed) source was source and sink was sink. NOT SO!and ended up with a exact same dangerous problem as your link explained. Until this thread jogged my OLD BRAIN i didn`t realize what we had done. It will be fixed!!
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Old February 20th, 2006, 11:57 AM   #6
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A great example Of why I love this site. Quick, clear, and to the point. I also printed and passed it to others.
Thanks
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Old February 20th, 2006, 01:10 PM   #7
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Quote:
21)Don't be too quick to use palm actuators and similar devices that use electronics, proximity triggers and other complex methods to issue start signals. They've been implicated in more than one accident. Some of the models that I have looked at, were low-grade devices that have no place in most industrial applications. They were single-channel, non-redundant, and non-self-checking (can you say non-acceptable?). Their focus was less on safety than on cashing in on the fears of repetitive strain lawsuits. Regardless, I wouldn't use even the best ones except in truly non-hazardous applications (where they might actually be quite useful). The addition of a "safety relay" does not miraculously change the situation either, unless your sales rep bears more than a passing resemblance to Jesus.
I cant say enough about this one. I was called by one of our suppliers to help in determining the cause of an accident. A small press with two hand start controls had inexplicably activated while a female operator was adjusting the tooling. The accident broke fingers on both of her hands. I made the fourty mile trip to the supplier and inspected the machine.

The shop was a small manufacturing plant, about a dozen employees, who were mostly twenty-someting single males - a slightly rought crowd. The victim was the only female employee.

The press was a narrow hydraulic punch press designed to be operated by someone of average male height standing in front of the press. It had capacitance type soft-touch buttons placed about shoulder distance apart facing forward on the front of the machine. The buttons had shrouds.

I inspected the machine and the safety system and I could not find any kind of a malfunction or evidence of tampering. I spoke with the opeartor and she was not very forthcoming about exactly what had happened - there was something that she was reluctant to tell me. It was on the way home that I realzied what had happened - and why she was reluctant to give the details. She was young, petite, about 5'3, and she was very large breasted. I surmised that as she was adjusting the machine, she leaned forward and contacted the soft-touch buttons with her breasts. I called her employer with my hypothesis and he later confirmed that after he had confidentally talked with her she had reluctantly admitted that was in fact what had happened. She would not come out and say what happened for fear of being teased by the men she worked with. I advised the owner to relocate the buttons - one to each side of the machine so that the buttons were facing in the opposite direction. I suggested he also notify the machin manufacturer.

I've told this story before, but I'll tell it again and again. You can never be too careful in thinking through a safe design. I'm sure that whoever designed the machine never imagined that such an accident could occur. When thinking of large breasted females, he was not likely thinking about safety. Few of us would be. But it is no laughing matter. In this case she escaped with a few broken fingers on each hand. It could have been much much worse.
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Old February 20th, 2006, 02:50 PM   #8
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If those buttons were 20" apart, she must have been well-endowed indeed. Maybe a lock-out procedure when making press adjustments would have helped too.
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Old February 20th, 2006, 02:57 PM   #9
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Incidentally, to give credit where it's due, the original poster for this link was Trojan Goat. Not sure if he's around, but if so, thanks!

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Old April 6th, 2006, 05:18 PM   #10
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jstolaruk is right. standard says that buttons must be at least some 12" appart (nothing wrong if distance is greater) and if mounted in same plane they must be covered (to prevent misuse like palm and elbow of same arm to activate both buttons).
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Old April 7th, 2006, 04:35 AM   #11
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I have ALWAYS avoided NPN inputs around machinery for the reason used in the article. NO NO!!!!!

I tend to use ungrounded 24VDC though. I use switch mode power supplies and once you ground the negative, you lose your isolation - plain and simple.

For some reason the Japanese love NPN (sinking) inputs. I fail to see why. No problem with sinking outputs though - use those all the time.

The confusion with AB is that they appear to call sinking and sourcing the reverse to almost evyone else. Once again, do not know why. The first time I used an AB PLC I wondered why things did not work - was used to the Japanese way of describing sinking and sourcing.
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Old April 7th, 2006, 05:04 AM   #12
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panic modejstolaruk is right. standard says that buttons must be at least some 12" appart (nothing wrong if distance is greater) and if mounted in same plane they must be covered (to prevent misuse like palm and elbow of same arm to activate both buttons).



In the USA, the standard is more like 20". 12" allows a operator to run the machine using their elbow and hand. This allows their other hand to be unprotected.
OSHA 1910.217 (b)(6) or ANSI B11.19 4.2.4 both talk about palm button use and design, however, 12" or 20" is missing. If 12" is used it's up to the employer to ensure two hands must be used to
trip the machine. The only times I have seen 12" is when the palms are mounted verticaly, or if they are covered with a guard. If they are covered, the operator must put their hand under the guard and than depress the palms.
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Old April 7th, 2006, 05:25 AM   #13
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Agreed, approx. 20" apart is what I see mostly in customer's specs and about 40" off the ground.
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Old April 7th, 2006, 01:05 PM   #14
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I agree with Bob the Japanese like NPN outputs and most of the system I made is with Omron and AD.
PNP in NPN out.
Grounding the (-) would not help it, might make it worst.
I looking for groung detector to avoid this problem.
Any one has saw or use it before?
Or I have to design it by my self.
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Old April 7th, 2006, 02:51 PM   #15
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I am also a PNP fan, just makes more sense, and if indeed the negative side is grounded, can make fault finding easier.

I worked on a large mail sortation center a few years back, all the control wiring was NPN, it was an american system by the way, I think it was to do with the small DC motor controllers that were installed ( 6000+ ). The power supplies were supplied with the negative side grounded, so we had to go to all the power supplies and remove all the ground links, a tedious task for those involved, especially with the access issues as the project progressed.


Quote:
Don't make Emergency Stop buttons do things when pressed. Make them stop things when pressed. I once almost chopped a client's finger off because of this. I noticed he had his hand inside of a swaging machine adjusting it while it was running. Being a considerate guy, I punched the E-Stop so he wouldn't get hurt. The machine instantly retracted an air cylinder under full power in order to get back to its rest position. He pulled his hand out just in time with only slight bruising of one finger. I feel very stupid for doing that. Just the same, I'd love to get my hands on the idiot that designed it. Remember, they are called "Emergency" buttons for a reason.

I say this is a major issue, something that needs to be considered when designing the pneumatic system.

The article very informative, but NPN is out there, and we all come across it somewhere, and after the initial disbelief that it is wired this way, attempt to diagnose the problem.

I came across a module on a project that I was upgrading a year ago that had a NPN module, and I has wired my additions for PNP, I ended up re-working that module to PNP, so it was standardised.
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