tattle tale

[milldrone]

OkiePC, Or anyone else.

Can you explain to my feeble mind why this won't work? Or what is wrong with this?

I have been watching this thread with some interest. And have read in other threads about the dangers of monitoring E stop strings. What I'm looking for is some feedback on what is wrong with this.

[geniusintraining]

Paul,

Ours are close to the same thing but you are using the PLC, but the wiring is the same as per their drawing

We had a conversation last time about this and my only concern then was... the speed, what would be faster? the PLC or micro processor/board

In my thinking they would all go false in a very short time (due to the latch droping out), so you would need something very fast to read this??? or am I wrong

Have you tried yours? I never had the chance... by the time the thing showed up in the mail we resolved our issue

Edit: Milldrone posted while I was typing, but if you wired it that way then that would resolve loosing your power on all points, I don't really see anything wrong with it (its just weird to see it like that) but it may work... good out of the box thinking

[OkiePC]

Originally Posted by milldrone

OkiePC, Or anyone else.

Can you explain to my feeble mind why this won't work? Or what is wrong with this?

I have been watching this thread with some interest. And have read in other threads about the dangers of monitoring E stop strings. What I'm looking for is some feedback on what is wrong with this.

Now that's the way I would have wired the circuit if it were up to me. Then it is a piece of cake to troubleshoot, but unfortunately, most of the "run logic" or e-stop strings I have dealt with put the seal and reset button at the beginning of the line.

EDIT: And the calendar line I worked on was modified to be like this one when we hooked up the tattle tale relays. It was later, that I made one with a Micrologix just for chits and giggles in the office and ended up using it several times.

[OkiePC]

Originally Posted by geniusintraining

Paul,

Ours are close to the same thing but you are using the PLC, but the wiring is the same as per their drawing

We had a conversation last time about this and my only concern then was... the speed, what would be faster? the PLC or micro processor/board

In my thinking they would all go false in a very short time (due to the latch droping out), so you would need something very fast to read this??? or am I wrong

Have you tried yours? I never had the chance... by the time the thing showed up in the mail we resolved our issue

Edit: Milldrone posted while I was typing, but if you wired it that way then that would resolve loosing your power on all points, I don't really see anything wrong with it (its just weird to see it like that) but it may work... good out of the box thinking

Yes, I used mine at goodyear about 10 times in 7 years, and it worked about 8 of ten with immediate accurate results.

One or two times, the relay was too fast for the micrologix, even with my honed little program, which is probably not perfect still.

I am sure a micro-controller would be much much faster. That is an area I need to learn more about. Christmas is coming, maybe I can get someone to buy be a begninner's microcontroler kit. . .Any suggestions?

I am constantly beeching about the firmware in gas pumps, cash registers, consumer electronics and cellphones, maybe I need to inject some of my perspectives into that arena of the world of controls. . .and I have always wanted to roll my own home controller. . .

PiEaCe!

[spifldorf]

Look up First Out Indicators, I think this guy is in N.M. I've used them many times. Very fast, monitors up to 16 inputs I think. 12/12 Designs is the companys name. KEG

[milldrone]

Originally Posted by OkiePC

Now that's the way I would have wired the circuit if it were up to me.

Is there a mandate I have overlooked that prevents this method?

[allscott]

Originally Posted by milldrone

Is there a mandate I have overlooked that prevents this method?

I certainly hope not, I have been doing this for years. This is exactly how I do E-stop circuits. I have also been using the latched relay method of tracking down what is dropping out a series interlock circuit and until this thread started I never worried about the safety implications of my "tattle tale relay".

Time to think.........

[MajorFault]

here's a link to the ones we use. I just purchsed dome from the guy last month. http://triptrapsales.com/

[OkiePC]

Originally Posted by milldrone

Is there a mandate I have overlooked that prevents this method?

No, only red tape at the mega corporation I used to work for. They made me put it back after we fixed the switches, and were supposed to contact corporate engineering to approve any changes.

Since the design was 1978 era and grandfathered in to current osha approval, any changes to the controls would require bringing them up to modern safety standards. Instead, they wrote test porcedures, LOTO policies, and taught mill and calendar rescue procedures to minimize the limb loss when accidents occurred.

They were afraid to do anything to this extremely dangerous machinery. I was not about to do anything without upper level approval, so I complied, until I could go to work for a better company with some technical common sense.

The smaller company I work for now is much more progressive. So, now I get to trouble shoot dual channel solid state safety relays with pulsed input strings, output contactor monitoring, and monitored pushbutton reset. But it is way better than seing people at work with missing fingers, broken arms, etc.

I should start another thread about the Allen_Bradley Guardmaster MSR38DP. . .

[Ron Beaufort]

Greetings to all ...

folks I hate to be the harbinger of despair, but the technique that you’re using for your “PLC Tattle Tale” is fundamentally flawed ... in simple terms, you can’t count on it to give consistently reliable results ... this means that (even though this idea might be handy and useful in some cases) the method that you’re using is NOT 100% reliable ...


but don’t just take my word for it ... let’s run an experiment to prove the point ...



suppose that I use a MicroLogix 1000 with ten 120VAC inputs ... suppose that I jumper all ten of those inputs together ... suppose that I connect just one field device (a NC pushbutton for example) to those ten inputs ... obviously when the button is NOT pressed, the PLC will see ALL ten inputs as electrically ON - and the input data table will have ten ONES all in a row ... so far so good ...



quick question: what will happen to the inputs at the instant that we press the button? ...



softball answer: ALL ten inputs will electrically turn OFF - and ALL ten of the bits on the PLC’s input data table will SIMULTANEOUSLY turn to ZEROS ...



hardball answer: ALL ten inputs will electrically turn OFF - and SOME of the bits on the PLC’s input data table may turn to ZEROS - but SOME bits on the PLC’s input data table may stay as ONES ... in other words, it’s a RANDOM thing ...



oops! ...



I just ran a quick experiment using a “trap the difference” program similar to the one which my distinguished colleague OkiePC attached to his post #15 ... (I’ve attached my program below if anyone is interested in looking at it) ... actually ANY program which is capable of recording a difference from ONE SCAN TO THE NEXT will prove the point that I’m trying to make ...



back to my experiment ... I ran the test 100 times ... at the instant that I pressed the NC button, ALL ten bits SIMULTANEOUSLY turned to ZEROS - EXCEPT on the tests shown below:

test  7 ... results = 00 1111 1111 

test 40 ... results = 01 0000 0000

test 60 ... results = 01 0000 0000 

test 67 ... results = 11 0000 0000 

test 70 ... results = 11 1101 1111

test 75 ... results = 01 0000 0010 

test 95 ... results = 01 0000 0010 

 

all other tests ... = 00 0000 0000

now you’ve got to admit, that’s pretty weird ... but when you really think about it, it does make sense ... the PLC’s “inputs” are ten separate electronic devices ... in a perfect world, each of these devices SHOULD have exactly the same electrical characteristics ... in reality there is a certain amount of “fudge factor” (or “range”) in the voltage levels which each device considers to be an ON signal or an OFF signal ...



and so my primary point is this ... the “PLC Tattle Tale” idea is fundamentally flawed DUE TO THE IMPRECISE RESPONSE OF THE INPUTS ...



now in most cases, we’re not concerned if one particular input happens to take one or two scans longer to change state than another particular input ... but in the program you guys are dealing with we ARE concerned ... you’re actually trying to monitor a string of inputs which are WIRED IN SERIES - and trap the SPECIFIC input which flaked out first ...



let’s shift back into softball mode for a second ...







in the figure above, if Input C opens, the PLC will see Input C - and all of the DOWNSTREAM inputs D-E-F-and-G as being electrically turned OFF ... so bits C through G should all turn to ZEROS on the PLC’s input data table ... if we trap and store that “changed” status, we should see that Inputs C through G all SIMULTANEOUSLY changed to ZEROS ... we can now point - with certain assurance - to Input C as being the “first” input which opened ... so apparently we’ve just pinpointed our problem ...

[note: do NOT miss the point that what I've said above is what most people EXPECT to see ... it is NOT always correct] ...

but before we take a deep bow and accept a firm pat on the back, let’s look at this same scenario from the hardball viewpoint ...



in the figure above, if Input C opens, the PLC will see Input C - and all of the DOWNSTREAM inputs D-E-F-and-G as being electrically turned OFF ... but (and here’s the point) bits C through G MAY or MAY NOT all instantly turn to ZEROS on the PLC’s input data table ... remember it’s a RANDOM thing ... so if we trap and store the “changed” bit status, we MAY or MAY NOT see Inputs C through G in a ZERO state ... we could then point to the WRONG input as being the “first” input which opened ...



that’s not going to make the bo$$ happy - especially after we have confidently assured him that his pesky intermittent shut down problem has finally been fixed ...



and so ... this “PLC Tattle Tale” idea COULD be a handy little troubleshooting tool ... BUT ... just a word of advice ... personally I might bet my lunch money on its results - but I would NOT bet the rent ...



I’ve covered this same “non-simultaneous reaction of inputs” issue before ... if anyone is interested, you can see more here and here ...



side trip: personally, I’d sure like to know how one of those “ready made” Tattle Tale input monitoring devices would react to this same “all inputs jumpered together test” ... I’ll bet (pocket change only) that they’ll also give a wrong report from time to time ... anyone want to try one out and post the results? ...



hope this helps ... party on ...

[spifldorf]

Ron,
As I stated above the FOI from 12-12 designs is accurate and fairly cheap. I have used it many times, for these problems and my results made my Boss eat crow. KEG

[Ron Beaufort]

Greetings spifldorf ...

if it's microprocessor based, I'm just wondering if it would be able to nail down the same "all inputs jumpered together" test ...

I have no reason to doubt that it's accurate - but I'd like to know for sure ... can you run a test for us some day? ...

all I'm saying right now is that the "PLC Tattle Tale" only had a 93% accurate response in my tests this morning ... that's good - but it's not 100% reliable ... the gadget you're talking about might be able to beat that percentage ... then on the other hand - I sure would like to see a horserace ...

[Lancie1]

Yes, I used mine at goodyear about 10 times in 7 years, and it worked about 8 of ten with immediate accurate results.

I believe that Okie is an honest fellow, and I also have used this method as a First Out indicator for a gas-fired burner. It worked well enough to determine which of the interlock switches was not staying closed during light-off.

folks I hate to be the harbinger of despair, but the technique that you’re using for your “PLC Tattle Tale” is fundamentally flawed ... in simple terms, you can’t count on it to give consistently reliable results ...

As your failure analysis is also slightly flawed. You were using a pushbuton, but the circuit in question is using a MCR physical relay. The relay will not drop out for several scans after a switch contact opens. This is usually enough time for the PLC to detect which switch opened. Not perfect, but a lot better than sending an electrician out without ANY clue which switch opened. Think fuzzy logic instead of precision logic. If there is a bad switch in a string of 10, and we don't want to have to check all 10, then which of the 10 is the most likely to be causing the problem? That is all we need, a place to start checking.

Sometimes we have to use the tools at hand, even though they do not give 100% success. 100% sucess is something achievable only in the classroom "theoretical" setting. Engineering is all about replacing the "theoretically possible" with the "real-world achievable".

Besides that, if someone has developed a more reliable PLC First Out method, I will be one of the first to adopt it. Anyone got one that is better than 93%?

[spifldorf]

Not sure what you mean by all inputs jumpered together test is. But after I wired the FOI into this gas train and tested ALL 10
switches 3 times each I was ready to serve dinner to my Boss.
Also I might add these switches varied from solid state to vacuum to ice cube relays. We bought 5 more. Our electricians love them, they say NO MORE GUESSING.

[Lancie1]

...all inputs jumpered together" test ...

I assume you mean that the interlock test switches are wired in series, and that a separate PLC input is taken from each switch?

Literally "jumpering all inputs together" (into ONE input) would short out an interlocked series switch string, bypassing it, and making the circuit completely useless.