load cell and scxi-1001

[Adzel]

Dear LabVIEW Team,

Overview:
I am a relative newcomer to LabVIEW 7 but have many years of experience in Mechanical Test. I am currently working with a pair of PCB Piezotronics 3-component load cells. I would like to use LabVIEW 7 as a data acquisition system with these load cells.

To do so I have an SCXI-1001 12 module chassis. I am connecting a load cell to an SCXI-1121 block/module. The device is set at 10VDC excitiation, which is recommended by the manufacturer.

Global Channels with key load cell data:
To begin, I initialize three Global Channels with the MAX. These channels are for Custom Voltage with Excitation. In each channel I use the “Table” feature to record and set up a tabl;e based on the manufacturer’s calibration info. This cal info is for each of the three strain gages that make up the load cell. I input the data as follows:

mV/V Load (Lbs)
0 0
0.001662 10
…
0.008334 50

50 being the maximum load. These are created as three Global Channels (Fx, Fy, and Mz)

VI Development:
To create a new vi and use the DAQ-Assistant as follows:

1. Select “New” on LabVIEW 7 entry screen
2. activate DAQ Assistant
3. select Analog input
4. select Custom Voltage with Excitation
5. select the three Global Channels (Fx, Fy, and Mz)
6. Hit test button to run and look at data

The test data is confusing. If I use the “excitation for scaling” it tends to place the reading for each gage at its maximum. As the gages are unloaded I find this surprising.

In addition, if I opt to step away from any scale and consider the system a full bridge, the readings will all be 0.005 volt. This is not bad for a strain gage, however, when I load the cells, the voltage readings do not budge.

As I am new to the world of SCXI, I was very careful to check jumpers for proper excitation and set up the system. The load cells worked fine this morning on an INSTRON Series 9 software and DAQ system, but not with the LabVIEW.

My question is: Has anyone on the LabVIEW team supported this combination of SCXI 1001 chassis with SCXI-1321 block module with a load cell before? Is it appropriate to use load cells with the SCXI-1001 (I don’t see any reason why not).

On paper the set-up would seem very straightforward. I simply cannot get any voltage output from these load cells with LabVIEW.

Please advise.

Thanks,

Adzel

[Ayman K]

Hello Adzel,

Thank you very much for contacting National Instruments. It is apparent that you followed the instructions in the manual to setup your system. I would like to clarify very few concepts before I proceed to explore the reason for the behavior you are seeing now. The DAQ assistant is a feature that was incorporated in LabVIEW to ease the development of Data Acquisition applications. You could actually create a DAQmx Task in MAX and it will do the same thing the DAQ assistant is doing (Right Click on Data Neighborhood -> DAQmx Task and then follow the instructions). Once the task is created you could then test it.

Now, one reason for the data to saturate at the maximum is if your input range is specified for the raw values (Unscaled values) Which seems to be the case here. The input range need to be specified for the scaled values (minimum of 0 and maximum of 50 in your case. Please change the input range for your global channels to correspond to the scaled values and let me know whether this fixes the problem.

There should not be a problem with using the SCXI-1121 with its terminal block SCXI-1321 inside SCXI-1001 chassis. This is indeed the intended use of these SCXI modules and this chassis is generic to all the rest of the modules.

I hope this helps.

[Adzel]

Ayman,

I attempted to use the scaled values as input and this did not seem to eliminate the problem. The vi readings still saturate at the max limit.

Assuming this load cell is full bridge I will attempt to simply collect the voltages and then use the linear scaling from the calibration data in the vi to see if I can get some type of read-out.

Thanks for the help and note about the SCXI-1121 and SCXI-1321 combination. As I am new at this that was a point of concern but I am glad that they will work together with this unit.

I will update as progress occurs (hopefully)

Adzel

[Adzel]

Other notes:

NOTE 1:
I have attempted to use the Input Range values as the scaled values. For this load cell these correspond to

Fx= -50 to +50

Fy= -50 to +50

Mz = -200 to +200

When I input these LabVIEW (or at least the DAQ Assistant application) gives me an error code (-200007) that indicates "AI.Min" is incorrect. It then gives me a suggested maximum and minimum value for the input range. For the above these suggestions are as follows:

Fx = -30 to +30 (approx)

Fy = -29 to +29 (approx)

Mz = -36 to + 36 (approx)

I find this puzzling and do not know where or how "AI.Min" comes to these values.

NOTE 2:
Assuming the load cell is full bridge (and this has been confirmed with the vendor) I go in and simply set up DAQ Assistant as a simple Analog Voltage input. In the DAQ Assistant slot I select the three physical channels and when I go to run test I receive the same error code as in NOTE 1, above, but this time the Input Range values are suggested to be -0.005 to +0.005 volts. Again, I am puzzled as to how the "AI.Min" vi comes to this value.

NOTE 3: The values in NOTE 2 are especially puzzling as the system requires a 10 VDC excitation. I pre-set all appropriate jumpers on the SCXI board to meet this, per the SCXI User's manual.

NOTE 4: When I change the Input Range to the suggested range I get three readouts from each channel: all are set at the maximum of that range and do not change when I load or jog the load cell.

NOTE 5: An interesting day in the lab!!

Adzel

Message Edited by Adzel on 05-25-2005 12:43 PM

[Adzel]

I worked with the load cell vendor today and they were very helpful.

One concern I have is that I noted that when I load the load cell and probe the terminals on the SCXI-1321 the voltage measurements are in line with the calibration data (as noted above).

After doing this I left the weights hanging on the load cell and ran a quick check using the DAQ Assistant. Again, the vi returned the maximum value based on the max value that I place on the Input Range.

Is this something unique to the SCXI-1321? The block has only 4-channels and the load cell must receive a 10-VDC excitation. I am only using three channels. Should the 4th unused channel be grounded?

Thanks.

[Marni]

Hello Adzel. When you are measuring the data from the load cells, are you reading in data from the DAQ device or the SCXI module? To read from the module, add it as a DAQmx device by right clicking on NI-DAQmx devices and selecting Create New NI-DAQmx Device>>NI-DAQmx SCXI Chassis.

Also, when you use the DAQ Assistant to read in your data, you lose the data that you used to calibrate your global channel. Instead of using the DAQ Assistant, right click on the block diagram and select All Functions>>NI Measurements>>DAQmx-Data Acquisition>>DAQmx Global Channel Constant. Place this on the block diagram and right click on it and select Generate Code>>Example. This will keep the calibration data. Please let me know if you have any questions. Have a great day!

Marni S.
National Instruments

[Adzel]

Marni,

Thanks very much for your reply.

1. In response to your question: are you reading in data from the DAQ device or the SCXI module? I am reading from the SCXI module and I have gone in and checked the settings. So I think I am okay there.

2. Also, I was totally unaware that when you use the DAQ Assistant to read data, I would lose the data that was used to calibrate my 3 global channels!!!!! Thanks for this info!

3. I have opened a new vi and on the block diagram I placed the DAQmx Global Channel Constant icon.

4. One problem now exists with step 3, however, when I right click on the DAQmx Global Channel Constant icon I get the standard selections, but I am not able to select Generate Code and then Example. Am I missing a step here?

Please advise and thanks again for your help (I actually feel closer to solving this)

Adzel

[Adzel]

Marni,

I have built the DAQmx Global Channel Constant via the front panel and I have also generated the example code. Thanks, again!

I am still reading the MAX voltages from the input range, however.

I will now try to reset the SCXI chassis and see if this helps. Perhaps I am reading the device (card) and not the chassis, alsthough I am puzzled why this should be so.

Thanks again.

[Adzel]

Marni,

Well, it finally works!

I erased the Chassis via MAX and then re-created it and its modules and all of the load cell channels are registering and giving the correct readings.

I am a happy LabVIEW user today!

Thanks again!

[Marni]

Dear Adzel,

I'm glad to hear that everything is working. Have a great day!

Marni S.
National Instruments