PXIe 6341 and SCB-68: problem with multiple inputs.... *URGENT*

[ap8888]

Hello,

I have an urgent problem that I'd really appreciate some help with.

I have a terminal block (SCB-68) connected to a multifunctional DAQ (6341). There are 4 inputs in the terminal block:

1. Laser displacement sensor, coming from a controller as an analog-out voltage signal ("1V" and "0V" on the controller). Connected to AI 0 and AI GND.

2. Accelerometer (ICP, coming from a signal conditioner so as voltage) connected to AI 10 and AI GND.

3. Load cells (ICP, coming from a signal conditioner so as voltage) connected to AI 11, 12, 13 and 14, each connected to a AI GND pin nearby.

Problem:

Via MAX, if I read in the above 6 channels from my DAQ (6341), my displacement sensor (#1) and accelerometer (#2) behave fine. However, two of the load cell channels (AI 11 and AI 12) shift in voltage, as if they're following the displacement sensor voltage (#1). For example, there's typically a -800 mV shift in AI 11 voltage.

This problem totally disappears when I don't request the laser displacement (AI 0, #1) reading. i.e. as soon as I remove that channel from my task, the load cell and accelerometer readings go back to normal (base voltage ~0V). Again if I re-request the laser channel, I see a vertical shift in 2 of the load cell readings (the shift varies with the current voltage in AI 0).

Attempts at fixing:

-tried changing the laser sensor to differential by using AI 0 and AI 9

-tried removing the ground from the laser 120V power supply

Any ideas at all would be greatly appreciated!

SCB-68:

https://www.ni.com/en-us/support/model.scb-68.html

PXIe-6341:

https://www.ni.com/en-us/shop/model/pxie-6341.html

[Neil_S.]

I'm not entirely sure, but to me it sounds like the ICP channel has high impedance which is causing the channel "ghosting". I would recommend taking a look at this document for suggestions of some things you might try:
https://knowledge.ni.com/KnowledgeArticleDetails?id=kA03q000000YHy6CAG&l=en-US

[ap8888]

Interesting Neil! I have a feeling you're right.

I plan to read the article tonight, but I'd like to ask:

Do you think the problem will be solved if I use more than 1 signal conditioner for my 4 cells? e.g. 2 cells in 1 signal conditioner and 2 cells in another. Not sure why/how that would solve the problem, but that looks like 1 of the things that would be simple to implement quickly.

[ap8888]

The article Neil linked to states that changing the order of the channels to avoid ghosting only works if I am "not sampling in round-robin mode." What does this mean?

If I understand correctly what's happening is my 1st low-impedance displacement sensor is "ghosted" in the high-impedance 2nd and 3rd load cell readings. To avoid this, can I put an empty short-circuited channel between the displacement sensor and the load cells?

displacement sensor: AI0/GRND
empty short-circuited: AI1/GRND <<<<< absorbs the discharge from multiplexer charge
load cells: AI9/GRND
etc.

or

load cells: AI0/GRND
displacement sensor: AI9/GRND

But I'm not sure if this helps anything...again the article (linked again below) says this only helps if I'm not sampling in round-robin mode.

Thanks for the help!

[johnsold · Accepted solution]

That is the general idea.  If you must use sources with high impedance and you have enough channels available, placing a grounded-input channel between each of the others usually helps.  You must scan those grounded channels.  Just grounding them is not enough.

Example: Sensor A = AI0, Ground = AI1, Sensor B = AI2, Ground = AI3, Sensor C = AI4, Ground = AI5.  Read AI0:5.

Lynn

[ap8888]

Makes sense Lynn.

And "ground" is equivalent to shorting the analog input with a ground.

So I can connect the following and read in data in the following order:

Sensor A = AI0/GRND
Shorted = AI1/GRND
Sensor B (high impedance) = AI10/GRND
Sensor C (high impedance) = AI11/GRND
Sensor D....
Sensor E....
Sensor F....
Shorted = AI7/GRND

Seem like a remedy?

[johnsold]

I would put a grounded input between each of the high impedance sensors.

AI0 = Sensor A

AI1 = Ground

AI2 = sensor B

AI3 = Ground

AI4 = Sensor C

AI5 = Ground

AI6 = Sensor D

AI7 = Ground

AI8 = Sensor E

AI9 = Ground

AI10 = Sensor F

AI11 = Ground

Common or return side of all sensors connected to ground.

Set up the read task to read AI0:11. Then discard the odd numbered channels (the grounded inputs).

Lynn