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basheera

grounding and fluctuation

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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???
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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?

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

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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. Filtering.txt
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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?

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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

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HI  BOB, 

"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.  "

 

Based on red coloured statement how can i do wiring ? Something as below pic?

 

 

capture.png

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Correct! Don't ground the negative unless you are installing the system in an automobile! 

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the best is to use small dedicated PSU with low ripple. also check local regulations as this will likely need to be double insulated etc. if not grounded.

connecting one PSU terminal to GND with shared (high frequency) loads can have negative effect on noise immunity, specially if analog inputs are high impedance. inverters and drives (welders, VFDs and Servos) usually operate at 8-16kHz and are capable of producing plenty of noise with harmonics going to very high frequencies. when such devices are not installed properly this can be nightmare to troubleshoot. at those clocks, it does not take much to create incuctive or capacitive coupling. but if you do follow manufactures guidelines (separation, shielding, grounding etc.) of those devices, analog are not fussy. i had voltage analog inputs withe very long cables (20-30m) and wired to same cabinet with bunch of drives.

don't remove much of shield, bring it close to terminations. use cable that is not just shielded but conductors are twisted pairs.

 

 

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you think it is fustrating to see some decimal places fluctiate in the HMI? imagine how these guys felt when interferrence is significant and tied to some output (robot)? in this very educational video robot works perfectly before welder is activated. but once the welder is turned on, robot feedback is compromised and robot goes wild (runaway axes). solution - always consider manufacturers guidelines for the application. make sure that grounding and shielding is fine.
https://www.youtube.com/watch?v=7FwdMjYUyKM
 

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Hi All ,

Thanks for all the reply and help. 

let me try as what u said.

And thanks for sharing the video.

 

 

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