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Old January 26th, 2006, 09:03 AM   #1
elhagan
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Problem with air compressor

Hello friends

I have new project

Air compressor the company want to upgrade it to work with PLC and HMI

The compressor work with micro processor and the micro processor

Is damaged the problem in 2 solenoid open the air gate and if the solenoid coil is broken the micro processor stop the machine and make alarm

The solenoid coil is normal only 2 wire 24v dc

How the micro processor feel the broken coil and if the wire is cut

And the owner need in the upgrade this feature

How can I make the plc feel the coil if broken or damaged

I will use plc s7 200 with out put relay

I think the micro processor measure the coil resistance

But in the plc ? what we will use ?

Sorry about my little English

And I am wait to answers me
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Old January 26th, 2006, 12:15 PM   #2
TConnolly
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Some PLCs manufacturers have output cards that will detect an open circuit on the output. I have only seen this feature with solid state outputs, never with relay outputs. The PLC output circuit applies a very low voltage limited current to the output terminal. This signal is not strong enough to activate a solenoid coil but it indicates to the controller if there is a completed circuit.
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Old January 26th, 2006, 01:05 PM   #3
keithkyll
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Wire the output as sinking. Wire an input directly to the output.
Input will be on when output is off, if the coil has continuity.
Input will be off when output is on.
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Old January 26th, 2006, 01:17 PM   #4
10baseT
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You could also play the same trick with the input - have it high all the time , and the coil will pull it down . Touble is , that even if the coil fails , there is a chance that leakage alone will be enough to give spurious inputs . How about using a change over relay (actually not necessary) , and wacking a voltage over the coil with a ballast resistor (high impedance), you could then use an analogue voltage input to measure the voltage across the coil , which would change if the coil failed .
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Old January 26th, 2006, 01:18 PM   #5
stasis
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an alternate method would be to use a pressure switch tied to a timer. If in X time the pressure switch doesn't activate/deactivate...
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Old January 26th, 2006, 01:53 PM   #6
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Like that one even better stasis , now that is the proper way to go .
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Old January 26th, 2006, 04:15 PM   #7
TConnolly
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Try a current sensing relay:

http://www.newarkinone.com/NewarkWeb...KU=90F3320&N=0

or a leakage monitoring relay
http://library.abb.com/GLOBAL/SCOT/S...04C0203_02.pdf
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Old January 26th, 2006, 07:30 PM   #8
elhagan
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dear Alaric

thank you very much can you tell me what the kind for this plc

and any mark and i see current sensing and leakage monitoring relay

its very good from you this is what i need but if i found the good plc will detect an open circuit on the output its will be the good idea i wait for your replay i need the plc manufacturers

and data sheet about it


dear keithkyll

i cant understand who is the input i have one solnied and need to feel it if the coil is damaged or broken the machine will stop


dear 10baseT

thank you for your help


dear stasis

big story to use pressure switch tied to a timer stasis another circle man

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Last edited by elhagan; January 26th, 2006 at 07:32 PM.
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Old January 27th, 2006, 04:39 AM   #9
keithkyll
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Output is relay. Solenoid has +24DC. Relay is controlling the zero side of the solenoid.

Think backwards. When the relay is off, +24 volts will pass through the solenoid coil to the relay contact. Connect a PLC input to this point.
I assume input is on with 24 DC input - off with zero.

If the PLC input has a 4ma load, then there will always be 4ma on the solenoid. This shouldn't be a problem, unless the solenoid is very small.
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Old January 27th, 2006, 06:53 AM   #10
elhagan
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Thumbs up

Quote:
Originally Posted by keithkyll
Output is relay. Solenoid has +24DC. Relay is controlling the zero side of the solenoid.

Think backwards. When the relay is off, +24 volts will pass through the solenoid coil to the relay contact. Connect a PLC input to this point.
I assume input is on with 24 DC input - off with zero.

If the PLC input has a 4ma load, then there will always be 4ma on the solenoid. This shouldn't be a problem, unless the solenoid is very small.
dear keithkyll
can you draw this steps for me
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Old January 27th, 2006, 07:16 AM   #11
keithkyll
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Wire the output normally. Add a jumper from the output terminal to an input terminal.
Someone help me out here with a drawing.

elhagen, think of using a meter to troubleshoot the output. Red lead on Output terminal, black lead on ground. When the output is on, meter will read zero volts. When the output is off, meter will read 24 volts if the solenoid is not open. If the solenoid is open, you will always read zero volts.

Use a PLC input as a meter.

Last edited by keithkyll; January 27th, 2006 at 07:26 AM. Reason: More info
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Old January 27th, 2006, 09:47 AM   #12
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Hope the attached sketch helps.....

Regards


Rob.
Attached Files
File Type: pdf Current Sense.pdf (7.3 KB, 45 views)
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Old January 28th, 2006, 07:24 AM   #13
elhagan
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dear sir Alaric
can you answer me please
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