Noise in Digital Inputs - ABB PLC

Dear All,
We have made the age old mistake of laying all cables (DI/DO/AI/AO/ VFD Power & Control/ DOL Power & Control) in one cable trey. I know it's a sin but now it will be hard to undone. The simplest way is to re-lay the cables properly separated by distance between Power/ Control & Signal cable treys but the plant is up & running so it will be hard to do it.
The problem I am facing is only with 05 nos of Digital inputs (24VDC). These inputs are close feedback of Pneumatic Actuator Valves. When the valve is open the open feedback is rock steady but when they are close the close feedback fluctuates on & off. Giving feedback loss alarm. On IO module the voltage is steady 23.4Volts & LED Glows full amber & there is no fluctuation but when we monitor it in PLC Program it shows the bit getting On/ Off. Rest of the plant is running fine even analog signals in which shield is grounded panel side are ok.
Is there any way or any device which can reduce the noise in the close feedback cable.
I have heard of Ferrite Cores but have not seen anyone actually using it in IO cables of PLC.
If any one of you have used them or any other device to filter noise kindly guide me.
The PLC I am using is ABB PM581 & Input Module is DI524
Regards,
Neeraj

Hi!

I totally agree that your cables should be rearranged. As is, it can cause a lot of mysterious trouble, what you experience now is just the beginning...
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Did you try to check the input signals with a o-scope?

Did you check if your input card has the possibility to change the signal delay time setting?

As a quick and dirty fix, you could make your own ripple filter by using timers TON+TOF on the trouble signals.

Kalle

Sometimes just a couple of inches of added separation is enough. Are these cables in a large wireway in which you can physically move the VFD and I/O wires to opposite sides?

Also, using shielded VFD cables on the motors may allow you to get away with routing them near low level signals and get away with it. Still, just barely working correctly is not a great answer, just might buy you some time to make better and more reliable improvements.

Dear Friends,
Thanks for quick answers. I agree the final & permanent solution is shifting cables but I want to know has anybody used ferrite bead or any other device to filter out noise so that cable rearranging can be prevented.
Awaiting your responses
Regards,
Neeraj

the ferrite should be on the cables giving the noise.
Use a capacitor of 100u on your input.
You can also use a resistor of 1kohm this will defelct your noise.
Even both can be used (snubber circuit)
Put the meter not in DC but in AC and measure again. The DC is filtered.
If in AC you see more as 5 volts you found your problem.
Most sensors will respond good when there is some current flowing.
the leds are lighting up very short due to the AC on the line.

Thanks shooter for your guidance & thanks to Kalle/ Okie for valuable responses. I will try to do as guided & will reply the result.
Regards,
Neeraj

Might be grounding problem rather than noise

One thing you might consider is a ground problem (like a ground loop) rather than signal noise. The PLCs have a fairly long input filter so any signal noise needs to be there for a long time to flicker on and off like you mentioned. Seems somewhat unlikely.

If the inputs are changing when the solenoids are closed, that's when you are drawing the most current. So your ground might be moving on those inputs, messing them up. Have a close look at your ground structure.

Putting an oscilloscope on your signals will help answer your questions. Beats guessing your way to an answer...