I need help regarding plc

muhammadomar.khan

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• Oct 13, 2011

• #1

Dear Guys I need help regarding PLC.

I am stuck for some days now and problem is strange. Machine PLC has two encoder modules when machine is powered inputs on Encoder module 1 starts flickering with 2-3 volt presence and in ladder logic one can see degrees malfunction as status of bits are continuously changing and this behavior also effects second module as in ladder logic its bits starts to fluctuate.

I have changed encoder module, power supply, cpu but still same behavior.

Some times problem get fixed and there is no flickering for some time lets say 30 minutes and than again it starts.

When this problem appears 24 V drops to 22 V and when ever this problem disappears it again comes back to 24V

 

Tharon

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• Oct 13, 2011

• #2

Is something on the field drawing down your 24V power supply? What parts of the machine are running on this power supply? Only the encoders, or other things as well?

 

muhammadomar.khan

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• Oct 13, 2011

• #3

DC supply is common for all machine instruments I have connected separate encoder directly to encoder module and also deattach module from PLC but flickering remains its only on those encoder inputs which are not active

 

sthompson

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• Oct 13, 2011

• #4

I assume you are using the specified Cable for the Encoder with a good shield grounded at one end, and kept them well away from all Power Circuits? If you Power Supply is ok, this sounds like a noise issue to me. Do you have an Oscilloscope you can put on the Encoder wires to check for noise?

Stu....

 

muhammadomar.khan

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• Oct 13, 2011

• #5

no its not shielded and Even if disconnect this encoder module' from main rack the second encoder module keeps on flickering like if its degree is 250 Octal it will show 250 and than 0 250 and than 0
255
0
255

The status of removed Encoder module is 000. If I remove the second encoder module from main PLC rack both show status 000

If I put first encoder module in rack it will show flickering even
I connect Encoder directly on module

 

iant

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• Oct 14, 2011

• #6

have you tried putting PULL UP resistor from either the 0V or 24V rail
It sounds as though you are not giving a voltage input to the card.

Are you using PNP or NPN as the encoder is probably capable of both connections - You need to set up that wiring

 

muhammadomar.khan

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• Oct 14, 2011

• #7

any body online

 

iant

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• Oct 14, 2011

• #8

i am now but your not

 

Lancie1

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• Oct 14, 2011

• #9

When machine is powered inputs on Encoder module 1 starts flickering with 2-3 volt presence.

Click to expand...

If you Power Supply is ok, this sounds like a noise issue to me.

Click to expand...

No, its not shielded, and even if I disconnect this encoder module from main rack, then the second encoder module keeps on flickering like if its degree is 250 Octal, it will show 250 and than 0 250 and than 0....

Click to expand...

Exactly, these are classic symptoms of outside influences on your encoder. Encoders are sensitive to induced voltages from any source in the plant. You must do diligent shielding and effective grounding. The "noise", or induced voltage on your signal circuits, can come from anywhere. Any unshielded signal into the PLC can be a source for the noise. Even the main power supply to the PLC can cause noise to show up in the PLC modules. You must start with the encoder inputs and change those to shielded cables, then if the problem is still there, make sure all your other input and output signal circuits are shielded. Then if still present, add a filtering devcie to your PLC power supply circuit.

 

JesperMP

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• Oct 14, 2011

• #10

Also add a separate regulated PSU for only the more sensitive parts of the machine, including the encoders.

 

muhammadomar.khan

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• Oct 14, 2011

• #11

Today I gave external 24V supply to all output modules because when ever I stop PLC progamme flickering disappears.

Even after this flickering stays when I run PLC programme.

Question 1:

I have two encoder modules. E01 and E02.
Flickering is on E01 but it also disturbs E02 Degrees when I see in Logic Sector Out puts.
Even if disconnect module E01 from Rack the flickering is still evident on hanging E01 Module.
Even if i disconnect encoder wires from E01 and put external encoder directly on E01 module flickering still present even it is out of Rack

I can not understand logic i need help

 

muhammadomar.khan

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• Oct 14, 2011

• #12

i am online can i discuss with anyone who has experience in these problems

 

Lancie1

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• Oct 14, 2011

• #13

Muhammadomar,

Flickering can be caused by external induced voltage. Induced voltage can be transmitted to almost any device capable of acting as an antenna. Wire makes a good antenna. Steps to take, in order: (1)Shield your encoder wires, (2) shield your devices, (3)filter your power to the encoder, (4)filter power to everything!