Measurements from PXI 1052 jumping around

[Muyopumping]

I am currently logging 8 analog inputs with the NI SCXI 1102 (via1300 terminal block.) on a PXI1052 Chassis. I noticed that the measurments  equally fluctuate,This is more pronounced(sharp jumps) at the slightest contact with any part of the rack. Please see attached  pictues for futher details. it is important to note that the red plot in all 3 photos had a real amplitude value of 5.

I have tried using the PXI 6259 (via an SCC 68 terminal block)  but this problem persists. Please help!!!

[Eden_S]

Hi Muyo,

Can you please post your code so I can see what is going on? Also, what is the 'amplitude' (y axis) measured in? When you say it produces sharp jumps at the slightest contact with the rack - do you mean when you touch the rack yourself? This could indicate a wiring problem which is backed up by the fact that you also have the same issues on a different board.

Are you sure the components are properly grounded? You may find this document useful for checking your wiring: http://www.ni.com/white-paper/5362/en

Many thanks,

[Muyopumping]

Hello Eden,

Thanks for your reply,

Find attached my code. the amplitude is in celsius. ( i have 4- 20 mA PT100 transmitters which have a range of -50 to 150 celsius. The equation of the scale i am using is Y=12500X + (-100) ). 

Yes i mean when I touch the rack myself.

What components  are you refering to? The rack (which is fitted with a monitor and keyboard) was installed by NI personel

the output from the  transimitters are diffrential. Do i need to ground the 1300 terminal block?

Thank you

[NEWKlear]

Hey Muyo,

Can you show us the setup of the task in MAX?  Your configuration will give more insight on the issue.

Thanks!

[Muyopumping]

Unfortunately, I can not get a screen shot of this until sometime next week as I am snowed in. 🙂 

however these are the Values for the configurations

Channel Settings - 8 analog current input from 1300 terminal block - channels 10 - 17 - channels 10 is from a CMF 025 Coriolis flow meter, while channels 11-17 are from PT100 4-20 mA transmitters.  

for Channel 10 The signal input range is Max = 130, Min = 0  while the Scaled unit = G/s (this cannnot be changed as this is the unit of my scale)

for Channel 11- 17 The signal input range is Max = 100, Min = 0  while the Scaled unit = Cels (this cannnot be changed as this is the unit of my scale)

The terminal configuration for the 8 channels are 'diffrential' ( this is the only option that i can choose for the SCXI 1102 )

The resistor Value is 250

My custom scale is selected for channel 10  and 11- 17  

The aquisiton mode is  1sample on demand  - my VI has a time delay of 0.5 secs. I have also tried using the continious mode but still encounter the same problem. 

i have calibrated the flow metre and transmitters seperately (also tried running it uncalibrated but yielded the same results)

I have also tried setting the Auto zero mode to 'once' and 'none' 

I hope this is adequate information for my configuration.

Many Thanks

Muyo

[Muyopumping]

Find attached screeen shots of the MAX  configuration

[NEWKlear]

Hey Muyo,

Thanks for sending in the screenshots.

So we can determine the source of the noise, can you hook up a constant DC supply to the 1300 terminal block and take a reading in the MAX test panels to see if the noise is attributed to the sensors or the module?

If you have a nine volt battery lying around this may work, or the output from a power supply as long as it is under 10 V, will allow us to see if the noise is inherent to the sensors or the chassis.

Also what other modules are on the SCXI side of the chassis?  Are they receiving the same noisey readings?

Thanks!

[Muyopumping]

Hello J Newk

Thanks for your reply – it was very helpful

Firstly, i tightened all the screws on the modules and chassis and this eliminated the jumps due to physical contact,

I used a constant voltage source as you suggested and I discovered that the amplitude drifted each time I  changed the pump speed of my circulating bath.  However, when the flow meter was turned off, changing the pump speed didn’t have any effect. This implies that it is the flow meter distorting all measurements on the board. Any thoughts on this?  see attached screen shots plot 8 (pink) is the constant voltage source

The board has two 1100, a 1102 , a 6259, 2587 and 8360, I observed the same thing when I used the 6259 via the scc68

Many thanks

[NEWKlear]

Hey Muyo,

Thanks for the update, there are few things that could be happening to cause the crosstalk on the channels.  Are the sensors all sharing a common ground?  This could be creating a ground loop that will show measurements from each sensor on each input line.

The SCXI side of the chassis multiplexes all of the inputs so there will be some internal crosstalk due to the internal multiplexing.  If we unscale the data and read in the raw voltages how many volts of noisie will we see?  This will be much more indicative of multiplexed nosie issues.

One other thing to try is to remove the flow meter from the task and take readings from the RTDs only to be certain that the flow meter is the source of the noise.

I hope that helps!