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DAQ readings in Veristand don't match the readings in MAX

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I have three accelerometers hooked up to a SCB-68 connector block which is connected to a PXI-6221 DAQ on an RT system. I've created a task for each accelerometer in MAX and have tested each one to see that it works and is giving appropriate readings. In Veristand I've created a workspace file to test the accelerometers and I have the DAQ created in the System Explorer. After deploying the system definition and running the workspace, I created three simple graphs and connected the appropriate channels to each graph. I scaled the voltages in the exact same way I scaled them in MAX but for some reason one of the accelerometers is giving voltages that are way off from where they should be. The other two accelerometers work perfectly and the one that isn't working works perfectly in MAX. So I don't think there is a problem with the accelerometer, it just doesn't want to work in Veristand for some reason. I'm wondering if there is anybody that may at least have an idea about why something like this would be happening. I have all three accelerometers connected to the same connector block and I'm wondering if putting too much into one block would cause something like this.

 

Any help would be appreciated.

 

Thanks

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Hello GriffE,

 

This does sound like an interesting problem.  If all three channels read fine in MAX then I wouldn't expect there to be an issue with your connector block.  I agree that the issue is likely on the VeriStand side of things.  Would you be able to post a screenshot of your System Definition?  I am particularly interested in looking at how the DAQ channels are configured and if you are doing any scaling. 

Also, could you define the signal that is acting incorrectly in a little more detail?  It might give some good clues as to where the problem is.  Thanks!

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 The first screenshot is of system explorer with MAX in the background showing how the DAQ is configured. I changed the names of the channels so that the graphs in Veristand would label which channels were showing data from which axis and which accelerometer. X-Axis1 is connected to channel AI13, Y-Axis1 is AI14, Z-Axis1 is AI15, X-Axis2 is AI5, Y-Axis2 is AI6, Z-Axis2 is AI7, X-Axis3 is AI10, Y-Axis3 is AI11, and Z-Axis3 is AI12. The second screenshot is of the channels and scaling. I scaled each axis to give units of g, and the one that's giving me trouble is X-Axis2/Y-Axis2/Z-Axis2. The votages it's recording for the second accelerometer are not the same as the voltages for the 1st and 3rd. The other channels that were calibrated were for something else that's no longer connected and can be ignored. I also wanted to mention that each accelerometer is identical. There is nothing different about them or the way they are connected. Now I am new to Veristand and all of these instruments, so I wouldn't be surprised if there's something I'm doing wrong. Hopefully this will be enough to see what's wrong.

 

Thank You for you help.

 

system definitions.jpg

 

Scaling.jpg

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What kind of signal conditioning are you using between the accelerometer and the PXI data acquisition board?

 

The only time I see something like this is if someone forgot to turn on the charge amplifier for the accelerometer, or its batteries are going dead.

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The accelerometers are conditioned internally, other than that I don't know much about them. They don't run on batteries just the +5V power source inside the connector block. I do have the 1st and 2nd (the one with the funny readings) accelerometers connected to the same +5V connector since there are only two of them in the block and I wanted to connect the three accelerometers. The 3rd accelerometer is connected to the other +5V connector. I was told I could connect as many things to one of the +5V connectors as I wanted as long as the current didn't exceed the rated capacity of the supply. I don't believe that I'm anywhere close to that capacity, but I wouldn't be suprised if this was the source of the problem. But even if it was then why would it work in MAX?
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The only thing I can think of right now is your channel settings may be different for X2 and Y2 than the rest of the channels. The screenshot only shows the configuration X1 which doesn't have a problem, so I couldn't tell.

 

It might also be worthwhile to try deleting and reading the channels, or using different channels to try to narrow it down. Swap your #1 and #2 accelerometer physical wiring and see if the problem moves with the accelerometer or if it stays with the NI VeriStand channels...

Stephen B
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I've played around with the channels a bit and I'm pretty confident that is where the problem is. I swaped the 1st and 2nd accelerometer and the problem moved to the 1st accelerometer. I also noticed that because the 1st and 2nd accelerometers are set up on adjacent terminals, whenever I would excite the accelerometer connected to the higher number channels (AI13-15) the accelerometer connected to the lower number channels (AI5-7) would record an excitation. I guess I can't have them connected to adjacent terminals even though I'm still confused to why it would work in MAX and not Veristand. I guess I'll have to figure out how to wire it properly.

 

Thanks for all your help.

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Solution
Accepted by topic author GriffE

For anybody that may care I did figure out what's wrong. The default setting in Veristand read the channels as differential measurements, so anything wired to channels 8 or below will be mapped to both channels A(n) and A(n+8) where n is the channel number. For me to use the accelerometers in the way that I configured them I just needed to switch to Reference Single Ended (RSE) measurements in the System Explorer.

 

Again thanks for all your help.

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