How to read CompactRIO 9-35V system input voltage?

[rbardsley]

Hello,

I would like to measure the voltage of the two 9-35V DC inputs going to the CompactRIO RT controller. I am using the 4-slot chassis due to space considerations and do not have a 24V analog input module on-board to be able to do this via a module. On the 9205 analog input module there is a hook for "chassis temperature" that works perfectly. I am trying to find a similar hook to read the voltage that the CompactRIO is running on.

I will be running this CompactRIO on two 24V batteries and wanted to be able to detect when the battery levels were too low to continue operation. When this state is reached, or hopefully prior to, I want to offload the data I have collected before running out of power to do so.

Any ideas?

Thanks,

Ryan

[Jason_Whaylen]

Hi Ryan,

Thank you for contacting National Instruments. 

From the situation you were describing in this post, I understand why you would want to do this.  However, the features that are available that include chassis temperature are all listed, and there is nothing that has been inherently provided for reading this voltage.  It does seem that you are aware that the other way to achieve these readings and behaviors would be to use a 24V analog input module and measure the batteries.

I hope this helps!  I do agree that this might be a useful function to have.  I would suggest filing a product suggestion here, and hopefully we can look to add something like this in the future.  Thanks!

Jason W.

[Bassett_Hound]

Hi Ryan,

Jason is correct that they is not a way to get an analog voltage value back from the Power  Supply.  However, you can get a digital  value back that  indicates which power supply is being used.  From there you could set your system up to detect when it switchs inputs and close down appropriately.  You can find a VI for the 900x controller here:

Programmatically Determine the Usage of Backup Power on a cRIO-900x Controllers [broken link removed]

Regards,

Bassett

[rbardsley]

Hi Jason,

Yes, I have the power source VI running in the system already. But as it is switching between two 24V batteries, it would be nice to know what the voltages of those batteries are. I'll follow up on the feature request like you suggested, but that sounds like more of a long-term solution.

Thanks,

Ryan

[Jason_Whaylen]

Hey Ryan,

I understand the problem you are facing here.  Unfortunately, this type of operation is not inherent to the system, and the only method that is evident to me is to try to use a module to achieve this behavior.  The power source VI should be somewhat useful, but isn't going to provide this specific functionality.  I apologize that we don't have any specific suggestions for the very short term. 

Regards,

Jason W.

 

[rbardsley]

Ok, we build a voltage divider and now have the battery voltage down from 28V to a max of 7. We have connected the + and - wires coming out of the voltage divider to the COM pin and AI04 of a 9205 module. But... it doesn't seem to be reading the voltage at all. I even tried hooking up our DC power supply and changing the voltage manually... no response from the CompactRIO or the 9205.

Am I wiring it wrong? We have 2 pressure sensors hooked up to the same 9205 module, so I know that works.

Thanks!

Ryan

[Jason_Whaylen]

Hey Ryan,

It looks like you are going about this in the right way.  In referencing the 9205 manual, are you connecting the known voltage '+' to pin 5 for AI4, and the '-' to pin 17 for COM?  Have you tried any of the other channels, specifically the ones that were working with the pressure sensors to determine if the wiring is the problem?  Are you testing this from within Measurement and Automation Explorer or LabVIEW?  It is always a good idea to try testing at the lowest level and building from there.  Hope this is helpful.  Have a great day!

Jason W.