catching an intermittent

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new guy
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catching an intermittent

Post by new guy » Tue Jan 23, 2007 5:12 pm

I have an escalator safety circuit that consists of switches, many relay contacts (of different relays), and some internal PLC contacts that are all in series. If any link in this chain is broken it affects other links and the machine will shut down within one second. My problem is finding what part of the circuit loses power first. Is there a stopwatch type of device or devices that I can hook in parallel at various points along the string to catch where the break occurs first. (the reading on that clock will have stopped a split second before all the others)?

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dacflyer
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Post by dacflyer » Tue Jan 23, 2007 5:31 pm

i delt with something simmelar before.. if your able to. connect a tester that will sound an alarm once the switch opens up..
i was using this idea on the interlock system of a home type old style elevator. and i had 1st connected the open/ closed switch detector to the control bd. 1st to isolate the few systems it had...then once i got to the culpret, i went and tested each switch in that area..

i do not know what type of voltages your messing with on the system
but you can use a probe of some sort..
unless its micro controlled,,then you will have fun.

good luck

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Crowbar
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Post by Crowbar » Tue Jan 23, 2007 6:12 pm

Not knowing what your budget is, all I can suggest is what I use at a forging mill...http://www.protectioncontrolsinc.com/teleflt.htm. These are cheap compared to full blown annunciators, but still run around $800. They do offer first out indication and can be cascaded for more monitoring points. I'm sure other units are out there, they can be priceless when the application is critical.

{edited to add...}

I don't know why I didn't think of this 2 minutes ago...you mention a plc, is there any unused inputs than can used to monitor the contacts in question? The internal's can be monitored with a simple sub-routine in the program as can the external's. I did this on our cooling water system to log what failed, and how many times with simple latches and counters with an unused input for a reset, if only the plc I'm using could time stamp it would be a dream come true.
Keep Prying...

Crowbar

Colinr
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Post by Colinr » Wed Jan 24, 2007 5:37 am

The following suggestion may need some though as it may inpari the safety of the system.

if you can determin what current is normally flowing in the switch / relay contacts and it is of a meaningfull level (several hundred miliamps) how about putting very low value fuses in parallell with each of the switches.

The fuse should be rated at 1/100 or less than the normal current flowing through the switches to ensure that it blows quickly.

with all switches made no current flows through any of the fuses

if a switch opens the fuse in parallell with it will take all of the current and blow. this will then cause the machine to stop.

it is then a simple task of checking each fuse to find which fuse is open circuit.


another option is to use a latching circuit to detect the first switch opening something like this was described in an old issue of nuts&volts as a indicator for a Quiz game.

Colin

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haklesup
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Post by haklesup » Wed Jan 24, 2007 12:06 pm

Does this stopping happen often enough you can be there to man the equipment or do you need something to capture a rare event when noone is around?

My first thought was just use an oscilloscope with a very slow sweep (1sec/div +/-) but with 2 or 4 channels, you couldn't look at too many relays at once. With slow enough sweep you would only need to trigger on one relay and the others should end up on the screen assuming all the relays trip within a second or 2.

A logic analyzer connected to each relay coil through a big resistor would give you plenty of channels but these may not run slow enough for you to capture the event you need.

A PC based data acquisition system can sit and monitor all the relays for days and then capture the events on as many channels as you care to configure. A simple 16 channel system can be had for under $1k
http://www.dataqstore.com/mm5/merchant. ... Code=MI710

Any of these can be rented if you don't have one.

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jollyrgr
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Post by jollyrgr » Wed Jan 24, 2007 3:23 pm

How about using a game show type buzzer arrangement to see who buzzes in first? Refer to the schematic at the link below. Replace the push buttons with relays. Connect the coil of the relay to the motor or whatever is being powered. Connect a set of normally closed contacts to where the push button would go and hit the reset. Since all the relays will be getting power the buttons will be "open". When a cicuit experiences a power loss that relay should de-energize first and "buzz in" first.

See this schematic site:
http://www.techlib.com/electronics/games.html
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Chris Smith
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Post by Chris Smith » Wed Jan 24, 2007 5:06 pm

See if you can rent or borrow a data [event] logger.

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Lenp
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Post by Lenp » Thu Jan 25, 2007 2:39 pm

I like the fuses! Data logging, interfacing, voltages, way too much complicaton to fix one problem. How about the old divide and conquer theory. Bridge a group of switches with one fuse, It reduces the fuse count, and cost, but will take a few tries to close in on the errant device.
Len

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Crowbar
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Post by Crowbar » Thu Jan 25, 2007 6:18 pm

Installing first out indicators costs money initially, but reaps returns throughout its lifetime everytime someone needs to fix it, I'm all for going cheap, but if your fixing these things for a living there's no time to guess.
Keep Prying...

Crowbar

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