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grounding and fluctuation

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#1 basheera

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Posted 16 April 2012 - 11:59 PM

I am using Danfoss MBS 3100 Pressure transmitter 0-4 bar as analog input with VERSAMAX PLC, i have used a AWG 26 wire to connect the pressure transducer with the PLC. While monitoring the analog data ,there is continuous fluctuation in analog data.Bits (0-32767) continuously fluctuate.First ,the fluctuation was higher, but after using the same power source(the source used to power the PLC, then using the 24 VDC from PLC to power my transducer) the fluctuation has been significantly diminished,it fluctuates between 0 and 8 (at no pressure).I want to get rid of that fluctuation.What should i do?
what kind of grounding have i done?
i have grounded the PLC to my power supply, and then grounded the pressure transducer to the same power supply... there is one confusion though, in the MBS 3100 literature it says that ground the transducer wire to the enclosure of the transducer, i dont know what this means, there are 4 connections in the MBS 3100 available out of which 3 are to used, one of them being ground, so am i to take this wire and wrap it around the enclosure???

#2 Mickey

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Posted 17 April 2012 - 12:19 AM

I am using Danfoss MBS 3100 Pressure transmitter 0-4 bar as analog input with VERSAMAX PLC, i have used a AWG 26 wire to connect the pressure transducer with the PLC. While monitoring the analog data ,there is continuous fluctuation in analog data.Bits (0-32767) continuously fluctuate.First ,the fluctuation was higher, but after using the same power source(the source used to power the PLC, then using the 24 VDC from PLC to power my transducer) the fluctuation has been significantly diminished,it fluctuates between 0 and 8 (at no pressure).I want to get rid of that fluctuation.What should i do?
what kind of grounding have i done?
i have grounded the PLC to my power supply, and then grounded the pressure transducer to the same power supply... there is one confusion though, in the MBS 3100 literature it says that ground the transducer wire to the enclosure of the transducer, i dont know what this means, there are 4 connections in the MBS 3100 available out of which 3 are to used, one of them being ground, so am i to take this wire and wrap it around the enclosure???



That is about 1/1000 of a bar. Not something I would worry about. That is 0-8 on a 0-32767 scale?

But, is the analog wiring shielded?



#3 basheera

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Posted 17 April 2012 - 03:02 AM


I am using Danfoss MBS 3100 Pressure transmitter 0-4 bar as analog input with VERSAMAX PLC, i have used a AWG 26 wire to connect the pressure transducer with the PLC. While monitoring the analog data ,there is continuous fluctuation in analog data.Bits (0-32767) continuously fluctuate.First ,the fluctuation was higher, but after using the same power source(the source used to power the PLC, then using the 24 VDC from PLC to power my transducer) the fluctuation has been significantly diminished,it fluctuates between 0 and 8 (at no pressure).I want to get rid of that fluctuation.What should i do?
what kind of grounding have i done?
i have grounded the PLC to my power supply, and then grounded the pressure transducer to the same power supply... there is one confusion though, in the MBS 3100 literature it says that ground the transducer wire to the enclosure of the transducer, i dont know what this means, there are 4 connections in the MBS 3100 available out of which 3 are to used, one of them being ground, so am i to take this wire and wrap it around the enclosure???



That is about 1/1000 of a bar. Not something I would worry about. That is 0-8 on a 0-32767 scale?

But, is the analog wiring shielded?

yes the analog wiring is shielded.is it possible to completely vanish that fluctuation

#4 BobB

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Posted 17 April 2012 - 04:33 AM

What is the analogue signal - 4-20ma? 0-10V? 4-20ma is much better for noise rejection - I will not use voltage analogues at all!
Is the 24VDC supply negative grounded? That is always a no no as far as I am concerned - do not ground the negative at all but let it float - ground noise introduced.
I expect the 24VDC supply is from a switch mode power supply. If so what brand? There is some really cheap junky stuff out there.
Is the analogue cable only grounded at one end? It should be grounded at the source only.

Edited by BobB, 17 April 2012 - 04:34 AM.

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#5 Mickey

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Posted 17 April 2012 - 11:33 AM

What is the analogue signal - 4-20ma? 0-10V? 4-20ma is much better for noise rejection - I will not use voltage analogues at all!
Is the 24VDC supply negative grounded? That is always a no no as far as I am concerned - do not ground the negative at all but let it float - ground noise introduced.
I expect the 24VDC supply is from a switch mode power supply. If so what brand? There is some really cheap junky stuff out there.
Is the analogue cable only grounded at one end? It should be grounded at the source only.


If you can't resolve your problem with the above advice then you could filter the signal. I am not familiar with your PLC but if it supports math function then see filter below.


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#6 basheera

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Posted 17 April 2012 - 02:06 PM


What is the analogue signal - 4-20ma? 0-10V? 4-20ma is much better for noise rejection - I will not use voltage analogues at all!
Is the 24VDC supply negative grounded? That is always a no no as far as I am concerned - do not ground the negative at all but let it float - ground noise introduced.
I expect the 24VDC supply is from a switch mode power supply. If so what brand? There is some really cheap junky stuff out there.
Is the analogue cable only grounded at one end? It should be grounded at the source only.


If you can't resolve your problem with the above advice then you could filter the signal. I am not familiar with your PLC but if it supports math function then see filter below.

i have done a lot f experimentation regarding the ground of the power supply and transducer, the fluctuations due to different ground levels of the power supplies are signaficantly higher.
what i noticed was that when my PLC was disconnected from its power source, and while i was unscrewing the analgue input screws i felt a bit of charge/current(because i got a minor shock.)
now the only reason i could think of was that this charge was comming through the programming cable which was connected to my PLC and PC
the cable i made myslf and where it said ground i just let those wires be free
now my question to you all is is it possible that the fluctuations are being caused by the programing cable.
and what to do with the ground wire ? am i to connect them with each other?

#7 DanW

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Posted 18 April 2012 - 11:21 AM

The accuracy spec for the transmitter is 0.5%, including non-linearity, hysteresis and repeatability. 0.5% is 1 part in 200, right?

Jitter consisting of 8 counts out of ~32,000 counts is 0.025% of span, 1 part in 4,000, more than an order of magnitude less than the accuracy spec of the device (1/20 of the accuracy spec, to be exact).

Just because you can 'see' it doesn't mean it's real or useable.



Edited by DanW, 18 April 2012 - 11:35 AM.



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