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Old April 5th, 2021, 08:17 PM   #1
Ones_Zeros
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PLC Analog Input signal Bouncing

Hello
I have a analog input signal that is bouncing
a bit. The PLC card is a 1769-IF8

I changed the input card filter setting
from 60hz to 50hz & the bouncing has greatly
decreased.

My question is does this affect the input card scaling
or accuracy by changing this filter setting?

Thanks
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Old April 6th, 2021, 08:33 AM   #2
danw
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Not on any brand/model that I’ve worked with.
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Old April 6th, 2021, 08:56 AM   #3
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Some brands are ridiculous, I have not worked with a 1769-IF8 but have installed a 50ish 1769-IF4XOF2 and they are not bad, you can build your own filter but by doing so there is a little lag, most of the filters I have built are just time based so just slows down the update with some sort of scaling
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Old April 6th, 2021, 02:10 PM   #4
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i have had this issue before.

is the analog cable shielded and the drain wire grounded at the analog input card only?
check ALL wire terminations, from the source terminals to the input card itself.
check the terminals themselves, we had a bad terminal (broke inside the terminal) and had to replace it.
try to put a dummy signal at the device terminals and see f the input jumps around, the device may be bad or have bad terminations in the device.
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Old April 6th, 2021, 09:17 PM   #5
Ones_Zeros
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Thanks for the guidance
I appreciate it

I will definitely check all the terminations & cable shields.

Thanks
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Old April 7th, 2021, 08:55 AM   #6
mylespetro
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How much bouncing are you talking about? Depends on the scaling and resolution, but if it's scaled 0-100% and it's bouncing from 20-30% that's not great, but if it's scaled 0-5 and it's bouncing from 1.28494-1.31843 I would consider that acceptable.
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Old April 7th, 2021, 06:03 PM   #7
Ones_Zeros
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I notice the bouncing when the process is down and no
Water is flowing through the flow meter.

Since this data has to be reported and when the process
is down the flow meter should read zero. It’s bouncing up to a positive number
and logging flow data when it shouldn’t.

I changed the filter setting on this particular analog input to
50hz and reduced the bouncing.

I appreciate all the help
Thanks
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Old April 7th, 2021, 06:17 PM   #8
James Mcquade
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when you say it bonces, just how much?
just because the process is down does not mean that a water valve may not open and allow water flow, that's why i asked how much.
it may be a small valve supplying coolant water, filling a process tank, or just to allow the water to flow and prevent materials from settling to the bottom of pipe(s). i have seen this occur at several plants.
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Old April 7th, 2021, 06:23 PM   #9
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My advice isn't the solution you need, but for future readers, Keyence makes a flow meter that you can attach directly outside the pipe, and will measure the flow (not the most accurate), but if you just need to see it above a certain GPM and get an input back, it works great, but definitely $$$. Keyence may have a way to get you an analog reading back, but I'm unsure about it. It may be over EIP, so it's not analog, but integer/decimal format into PLC. But you probably using SLC, so may be worth more trouble than you looking for.
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Old April 7th, 2021, 07:01 PM   #10
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Quote:
Originally Posted by Ones_Zeros View Post
I notice the bouncing when the process is down and no
Water is flowing through the flow meter.

Since this data has to be reported and when the process
is down the flow meter should read zero. It’s bouncing up to a positive number
and logging flow data when it shouldn’t.

I changed the filter setting on this particular analog input to
50hz and reduced the bouncing.

I appreciate all the help
Thanks
I am curious as to why changing the notch filter to 50Hz improved the noise rejection ?

Don't you guys over the pond use 60Hz mains frequency anymore ?

In class I always told my delegates to set the notch filter frequency to 50Hz for the UK, "for best immunity to noise generated by the mains wiring".

Anyway, don't rule out that the signal may be coming from the flow-meter itself, so you could test for this by substituting the flow-meter with a 4-20mA current source and running the process. If you still get bouncing you'll know it's wiring/cabling issues. If you don't, refer back to the flow-meter manufacturer.

EDIT : Are you confident that the flow-meter is installed correctly, to the manufacturers specifications ?

Most flow-meters I have used prefer to be in upwards vertical flow, and greater than a minimum distance, related to the pipe diameter, from a preceding bend.

Is there any entrained air or gas in the fluid ?

Have you actually proved there is no flow when the process is idle ?
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Last edited by daba; April 7th, 2021 at 07:19 PM.
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Old April 8th, 2021, 10:20 AM   #11
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Quote:
Originally Posted by Ones_Zeros View Post
I notice the bouncing when the process is down and no
Water is flowing through the flow meter.

Since this data has to be reported and when the process
is down the flow meter should read zero. It’s bouncing up to a positive number
and logging flow data when it shouldn’t.

I changed the filter setting on this particular analog input to
50hz and reduced the bouncing.

I appreciate all the help
Thanks
What are the details of the flow meter? Most magmeters have a setting to adjust how they cope with zero or low flow and some have settings that can be fine tuned for when there is air in the pipe (empty pipe threshold). Most electronic meters also have output filtering for their analog signals.

If it is just a simple prop meter with no programmability, I would be inclined to put a filter in the PLC code where you can trend and adjust the filter to optimize it.
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Old April 8th, 2021, 11:22 AM   #12
Mickey
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It's very common for any type of flow meter to have a "low flow cutoff". This can be a feature configurable in the flow meter itself or you can program it in your PLC.
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Old April 12th, 2021, 10:53 AM   #13
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When there is "no flow" is there water stagnant inside the pipe or is it dry during the now flow condition. In the past I have noticed that slight vibrations can cause stagnant liquid to register a small flow. Also if you have any AC power 120V+ (wires for a pump perhaps) run adjacent to the signal wire the expanding and collapsing magnetic fields from these wires can induce voltage in your signal wire and create noise. if this is the case you can invest in shielding for your cable or perhaps enquire from the manufacturer about a filter to reduce this noise.

Also, What sort of cable is being used to connect the device with the plc?
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Old April 12th, 2021, 11:36 AM   #14
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I also noticed that if the flowmeter is pressurized, even with no flow the values will change slightly.
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Old April 12th, 2021, 11:42 AM   #15
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I would build a averaging logic

Quote:
Originally Posted by Ones_Zeros View Post
I notice the bouncing when the process is down and no
Water is flowing through the flow meter.

Since this data has to be reported and when the process
is down the flow meter should read zero. It’s bouncing up to a positive number
and logging flow data when it shouldn’t.

I changed the filter setting on this particular analog input to
50hz and reduced the bouncing.

I appreciate all the help
Thanks
If your main problem is that it is Recording a sec or two of data and you don't need to have a fast response then I would build a averaging logic.
Make a Clock one shot that you can change how fast it is, then use that to store about 5 to 10 samples, then average those to give you a just the value that is Turning On the Record request.
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