problem reading 9236 Strain guage as a golbal variable

[RS1973]

Dear All

Hi

I'm new in this ,

My problem  description : 

I use NI cDAQ -9138 Chassis with LabVIEW Real-Time, I have multiple cards such as

NI 9205 and NI9235 strain gauge on the system.

My configuration is so simple, using signal express software, I need to read and capture online data from these cards.

When I define a NI-DAQ task  for NI 9205(analog voltage) and change it to virtual global channel , I can easily read the data both on NI-DAQ task and also SignalExpress 2013.

 But with NI9235 card ( 120 ohm Quarter bridge) the situation is different, when I define a new task on my remote system, I can read  the strain values in continues mode, but when I change any strain channel to a global virtual variable ( to be able to use it with signal express software)  the sample rate  decrease and almost stops, the value is not update on task window and on signal express the value stays zero all the time.

To me it’s like when any strain channel on (NI9235) is change to a global virtual channel the communication between software ( NI MAX or Signal express ) is interrupted.

I need to rectify this problem , so I really appreciate if anybody helps me with this situation.

 Regards

Ron

[DangerDave]

Hi Ron,

That does sound odd.  Though it wouldn't explain a drop in sample rate, you might want to check the scale of your graphs.  The 9205 reads in voltages from +/-10V while the 9235 will very likely be outputting something on the order of mV depending on the source.

Another thing to note is that the NI 9235 requires external excitation in order to operate.  Do you have external excitation wired?

I got those specifications from:

https://www.ni.com/en-us/shop/model/ni-9205.html

https://www.ni.com/en-us/shop/model/ni-9236.html

There are also some very helpful manuals on those pages!

Best,

David R.

Applications Engineering

National Instruments

[RS1973]

Dear Dave

Thank you for your support , as far as I know and it is mentioned in spec sheet of 9235 there is a  Built-in 2 VDC  excitation for quarter-bridge sensors.

Also, I do use 9235 card alone when doing measurment and still I get no data from any channel .

As I mentioend when I use Tasks on remote Cdaq I can read and watch the changes in strain , but when I change this task to a virtual global channel 

then data sampling rate reduces and I can not ready any change in intial value. 

I have read these manuals and could not find any mismatch in manuals and my setup which is very simple

I would be very helpfull if I get any tips from you m to fix this a.s.a.p

Regards

Ron

[DangerDave]

Hi Ron,

You’re right.  Sorry about that!

Let me make sure I fully understand your situation.

When you said that you change a strain channel to a global virtual variable, did you mean you created a global virtual channel?  And in regards to the slower sample rate, are you referring to how quickly your value updates within MAX while running the global virtual channel?  If that’s the case, how can you determine a sample rate if the values are not updating?

Perhaps, could you share some screenshots?  That might makes things more clear.

In general, virtual channels do not include timing or triggering properties; those are specific to tasks.  However, you should still be able to set the desired timing once you copy the global channel into SignalExpress.

Regardless, I’m going to try to get a hold of a cDAQ 9138 chassis and 9235 module and see if I observe any strange behavior.  I’ll keep you posted on my progress!

[DangerDave]

Hi Ron,

I got a hold of a cDAQ 9139 (close to 9138) and 9235 module and wanted to give you an update.  When I make a task and global virtual channel, the only option is to make them on the data neighborhood of the cDAQ, which prevents me from seeing it in signal express on my desktop.  What Real Time OS are you using on your cDAQ?  Is it windows embedded standard by chance?

Please do still answer my previous questions, as they are still very important for understanding the issue.

Best,

David

[RS1973]

Dear Dave

Sorry for later reply .

I do try to send you some screen shots , I need to get back to the lab for screen shots.

I connect an quarter bridge strain gauge which is installed(attached) on a rod and connects it's wires to 9235 accroding to the wiring diagram recommnded by NI,  and using MAX task when I impose  force to the sample which has strain gauges on it the value is changed .

But when I change this task (transform it) to a virtual global variable, then  the same action

 (applying force) returns no change in the value, displayed  value is fixed , fixed on offset value which is strain value when no force is applied.

and with force applied there is no change in this value , with SignalExpress is the same situation except here  the value is fixed on ZERO .

Let me remind you again that same configuration is used for a voltage channel on a 9205 card , in this  case values are updates with the set sample rate both as a task , as a global virtual variable , and using the same global virtual  variable in SignalExpress I can log and see the values with no problem.

I am going to provide you screen shots and also version of  Real Time OS , come back to you soon.

Regards

Ron

[RS1973]

Dear Dave 

Attached is a zip file containing pictures  of the  setup and  problem, also the OS specification.

Pitcures are :

strain-task and strain task2 which show the value and change in the strain value when the force is applied .

strain-virtual-Gl-Var is the value of the same setup when the task is changed to a global Var. as you can see now the value is fixed and 

no change can be seen when the force is applied .

setting on signalexp shows the the screen for selection of  strain Var  to be monitored with signal express.

and signal express value shows that the same signal which was 1.7m strain on MAX now is 0 in signal express.

OS picture has the info related to RT OS and other specifications .

Hope this gives you better understanding of the situation.

Regards

Ron

[DangerDave]

Hi Ron,

Thanks for the screenshots!  I'm able to replicate your issue with signal express not updating values at all for the global virtual channel.

In terms of unresponsiveness to strain within MAX, I'm now in the process of acquiring and setting up a quarter bridge strain gauge so I can see if I observe the same behavior.

I'll keep you posted!

David

[DangerDave]

Also, just as an aside, have you tried implementing this using LabVIEW instead of SignalExpress?

[RS1973]

Dear Dave 

I don't have licence for labview , this setup was supposed to be a simple setup for some users with no experience in coding and Labview,to just use signal epxress for simple measurement tasks .

Awaintg for your response.

Ron