strain measurements using SCXi 1314

[Jarrod_B.]

Hello sfjd,

Have you tried connecting the same load cell to different physical channels of the SCXI-1520?  If it works on other channels, I might suspect a hardware problem.  If it doesn't work on other channels, it is probably still a configuration issue. 

Also, looking at the screenshots you sent, I was wondering what value you were expecting to read when you took that picture of the test panel.  Was it not supposed to be 27?  If you notice, the y-scale of that graph is extremely narrow because you have the "Autoscale Y-Axix" option checked.  Does the signal continue to drift upwards?  How high does it get?

Finally, please include a screenshot of the Settings tab in MAX for the task you are creating. 

Best regards,

[sfjd]

Dear Jarrod,

The settings tab screen shot is included in my earlier Loadcell_error.doc. It is in the second page. The latest  scenario is as follows:

( Created a custom analog voltage measurement in MAX. I have load cell which can measure a maximum of 200 kg(f))

1. Created a map range with

        prescale values as -0.002 and 0.002

        Scaled values as 200 & -200

    2. Under the settings tab:

            Max = 200 and Min -200

    3. Excitation voltage : 2 ( not 2m as shown in the screen shots)

    4. Full bridge differential

    5. Click Test

    6. A constant DC signal with value 200.

    7. Added 10 kg weight

    8. Output no variation ; remained at 200.

I checked the P+ and P- with multimeter it gave the reading 2 and S+ and S- showed zero

I added 10 kg weight; P+ and P- showed 2 and S+ and S- showed zero.

I have also tried to connect it to other channels as well

Regards,

Rajeevan

[Jarrod_B.]

Hello Rajeevan,

That's strange that you got a reading of 0 between S+ and S- for both no load and 10 kg tests.  Shouldn't your load cell have been outputting a positive voltage with the 10 kg load?  I'm a little suspicious of that.

It doesn't look like you are doing anything wrong in the configuration of this task.  It's possible that there could be a problem with the module, but I would really want to test that by trying another 1520.  Do you have a second SCXI-1520 module that we could compare to?  If not, it might be possible to for you to get a loaner, but for that, you will need to create a service request on the web and speak with an Applications Engineer.  Once you have a service request open, if an RMA is necessary, we can also help you with that. 

I definitely want to make sure that you're successful, but it seems like we may have reached the end of what we can troubleshoot on the forums.  Please let me know if there's anything else I can do to help.

Best regards,

[sfjd]

Dear Jarrod,

The details of hardware as follows:
Chasis PXI-1052
1520 - three numbers (total of 24 strain measurement possible?)
1314 - three
1540 - one
1315 - one
The three 1314 are connected to each 1520.(?)
The 1315 is connected to 1540.

I have also tried to connect the loadcell to the other channels available in the other two 1314 terminal blocks of other two 1520. Still of no use.
I  contacted your local service representative and the previous mail was the result of this consultation. But the problem is tha teven after repeated requests from our side, no local representative (Madras) is visiting our lab and this kind of online query and phone call is going on and on. And we are struck with our testing here. Please do some help to sort it out. Actually in our real testing environment we have 5 strain gauges (fixed on steel bar), 6 LVDT and 8 load cells simutaneously connected and data needs to be monitored. And here we are not able to read even reading from one load cell? I think you got the seriousness of the problem?

Thanks,
Rajeevan

[sfjd]

Dear Jarrod,

Today morning I have changed the channel to some other 1520 module in the 1052 chasis. The screenshots are attached. It seems that there is no output. Also the signal is random than a DC type.

Regards,

Rajeevan

[sfjd]

Dear Jarrod,

I connected one drycell with 1.5 volts to the S+ and S- and clicked the test button. Still the same random signal of same value as shown in the previous attachement. This I have done with different 1520 and 1314.

Rajeevan

[Jarrod_B.]

Hello Rajeevan,

What happens if you connect a DC voltage from a power supply or a battery to one of the channels (+S and -S)?  Do you see the same signal as in your attachment?  Try this on every channel of every 1520. 

Also, what DAQ board are you using to digitize the analog signals from the SCXI modules?  Do you have a PXI DAQ board in the right-most PXI slot of your PXI-1052 combo chassis?  Are you connecting to the 1520's in multiplexed or parallel mode?  If you're seeing the same problem on multiple 1520's, it's probably not a hardware problem with the modules.  It is possible that there could be a problem with a channel on the DAQ board.  With this in mind I would recommend the following:

First, try running the DAQ Diagnostic Utility on the board and see if all of the analog input channels pass the test.  Next, if you are connecting in multiplexed mode to the 1520's, try switching to parallel mode and see if you can get a better reading on any of the channels. 

Let me know how these tests go, and we'll take it from there.

Best regards,

[sfjd]

Dear Jarrod,
This seems a big task for me. As a Civil Engineering my knowledge to such testing very limited. Still I have earlier done a testing with a battery of 1.5 V (currently showing 1.4 volts using a multimeter). This is what I did:

1. Remove the S+ and S- cables from the 1314 and connect it to the + and - of the battery.
2. Click test in the MAX.
3. A constant DC signal of value 200.(This is absurd since the P+ and P- still shows the voltage as 2 which I entered in the excitation text box under the settings tab).
4. I also connected the measuring proble of the multimeter simultaneously to the positive and negative of the batttery. It showed zero in the display.
5. I think the DAQ is PXI 6331 (correct? I don't remember since the system is housed in the structural laboratory and on Sat and Sun I have no access to it. This number I have told to Mr. Akhil of NI from INDIA).

Meanwhile I will send this instructions to Mr. Akhil. Since the remaining tasks I may not be able to do with my very little knowledge about the system.

Answers to some of the queries:

1. The DAQ 6331 is connected to the rightmost  but just left of the first 1520.
2. This follows the three 1520's and the next slot one 1540. Remaining (three ) slots are free.
3. "multplexed or parallel" - I don't have any idea about this.
4. DAQ diagnostic utility also I fear I may not be able to do it.

Regards,
Rajeevan

[Neil_S.]

Rajeevan,

Sorry this issue has been so frustrating.  Before I attempt to answer your question let me see if I understand what is going on.  You have connected your load cell up to a 1314 that is connected to the front of the SCXI-1520.  The SCXI-1520 is located in the left-most slot of the SCXI portion of your PXI-1052 chassis.  The SCXI-1520 is controlled by the DAQ card in the right most slot of the PXI section of the chassis.  You have created a task in Measurement and Automation using the following parameters:

Type of Task:          Custom voltage with excitation
Range:                    200 to -200 kg
Prescaled Range:   0.002 to -0.002
Excitation:                2V

When you read the channel it returns 200.  This means the channel is railed, or in other words the voltage present at the channel is greater than the max prescaled value or 0.002.  The driver will clip any value that exceeds the specified limit.  If you use a battery as you have mentioned the voltage will far exceed the 0.002 V limit.  You cannot use your task to test the battery.  You should just use the test panel.  You can get the test panel by expanding the Devices and Interfaces folder under My System.  Then expand the NI-DAQmx Devices folder.  You should see the SCXI portion of your PXI chassis listed.  Expand the chassis to find the 1520 in slot 1.  Right click the 1520 module and select Test Panels...  Specify a Max and min voltage greater than the voltage of the battery.  Apply the battery across S+ and S- with the load cell disconnected. 

The DAQ diagnostic utility is actually very useful and simple way to diagnose possible problems.  It is simply an executable that you run on your system that automatically checks your card.  You do not need to connect external signals or do anything complicated.  Just run the program.  You can get the DAQ diagnostic utility at:

http://digital.ni.com/softlib.nsf/websearch/C8D5A739965EF222862571D900780D3C

Download the zip file and run the diagnostic utility.  Make sure nothing is connected to the front of the DAQ card before running it.  The utility will generate a report and will tell you if the card is functioning properly.

What is the specific load cell you are using?  Could you give me the manufacturer and module so I can take a look at the specs? Different load cells have different excitation requirements and this could also be a potential problem.  I also want to confirm that the correct range and scale were specified.

How did you specifically wire the load cell?  Which wires did you attach to each pin on the 1314 connector block?

You also made mention that the multimeter read zero volts when nothing was applied and also for when 10 kg was applied.  What range did you have selected on your multimeter.  The top of your range is only 2 mV and will register as 0 V if you set your DMM to a large range.

Let me know if I misunderstood some part of your application.  Also let me know if you have questions about the things that I mentioned.  Good luck with your application.

Regards,

Neil S.
Applications Engineer
National Instruments

[sfjd]

Hi,

"When you read the channel it returns 200.  This means the channel is railed, or in other words the voltage present at the channel is greater than the max prescaled value or 0.002.  The driver will clip any value that exceeds the specified limit.  If you use a battery as you have mentioned the voltage will far exceed the 0.002 V limit.  You cannot use your task to test the battery.  You should just use the test panel.  You can get the test panel by expanding the Devices and Interfaces folder under My System.  Then expand the NI-DAQmx Devices folder.  You should see the SCXI portion of your PXI chassis listed.  Expand the chassis to find the 1520 in slot 1.  Right click the 1520 module and select Test Panels...  Specify a Max and min voltage greater than the voltage of the battery.  Apply the battery across S+ and S- with the load cell disconnected."


I have done the above step today but found that the same random signal output is generated with no meaningful data. I have also tried with your support group from Bangalore with a SCXI 1000 chasis but with no good result has come. The problem now continues for almost ten months (though we started using it recently), but still no good outcome has come. We expect that such problem if exists will soon be rectified rather than prologing at the client side to test the system by himself for almost one month. Hoping to get a better solution soon...

Regards,
Rajeevan