1520 shunt cal

[Greg_Reynolds]

Pardon the previous post, it went up prematurely. 

I'm using the SCXI 1520/1314 with a quarter-bridge configuration to do a simple strain measurement with shunt calibration.  I have a the standard 100K resistor installed at SCA0 and a 11,880ohm resistor installed at SCB0.  When I select SCA or SCB and specify the respective installed shunt resistance, I get the error 200007 regarding shunt calibration gain adjust being out of bounds.  I can only get the VI to work by selecting SCB and specifying 100Kohm shunt cal resistance (the resistance installed on SCA).  With those settings, I obtain a valid gain value (around 1.003) and I can apply known strain to the gage and read reasonable values.  The reverse does not work either (using SCA with 11,880ohm specified). I've checked my wiring and all seems to be correct. Is there something wrong with my VI???

[Kyle_A.]

Hi Greg,

I do not see anything sticking out as wrong with your VI, but I was unable to reciprocate the error that you are getting.  In LabVIEW, the error is explained as something different, so I'm wondering if you possibly mistyped the error code, or if you can provide a screenshot of the window of the error that you are receiving.  Are you using a Data Acquisition card to read from your chassis or are you using a SCXI-1600?  Also might be worth finding out what driver version you are using with DAQmx.

[Greg_Reynolds]

Hi Kyle, I've attached the error that I'm having when I try to do the shunt calibration.  I'm running DAQmx 9.0 with the SCXI 1520/1314 in a PXI 1050 crate.  The DAQ is a PXI 6221.  I cannot find a single example VI that shows how to use the shunt cal swithces SCA/SCB.  Any suggestions there?

Greg

[Kyle_A.]

Hi Greg,

I think part of the issue lies in the 11.8KOhm resistor that you are using.  Is there a reason you are choosing this resistor rather than using the internal resistors on the 1314 block?  Per the SCXI-1314 Terminal Block Calibration Procedure, there are two 100K high precision resistors installed to be used for shunt calibration procedures.  You can also read this on page 4-27 and 4-28 of the SCXI-1520 User Manual and Specifications.  Is there a particular reason you are choosing to use the lower resistance for your calibration?

Another thing you can try is the Cont Acq Strain Samples (with Calibration) - SCXI 1521.vi in Example Finder, found under the folder Hardware Input and Output » DAQmx » Analog Measurements » Strain folder.  Using this with the standard resistors, see if you can get valid measurements on your strain gauge.

Let me know, thank you.

[Greg_Reynolds]

Hi Kyle, yes the 1314 comes stock with 100K resistors in both locations SCA and SCB.  This works pretty well for 350ohm gages, producing a simulated micro-strain of 1744.  But we're using 120ohm gages, and the 100K shunt resistor only produces 599 microstrain.  We're testing at around 2000 micro-strain, so the 599 micro-strain is not a very good calibration point.  The 1314 manual clearly indicates that the shunt cal resistors are in sockets so that the user can select a desired shunt calibration. Using the 11,800ohm resistor produces a calibration point at 5000 micro-strain.

I used "Cont Acq Strain Samples (with Calibration) - SCXI 1521.vi" as a template to write the VI attached in the original post.  But that template VI does not implement the selection of SCA or SCB switches.  I presume the default is to always use SCA.  Anyway, I cannot find a single example VI that selects these switches...

Greg

[Kyle_A.]

Hi Greg,

Try installing the 11880 Ohm resistor in SCA, because the built in shunt calibration uses that switch.  Also, be sure that you have wired up the SCA input to the correct nodes on your strain gauge, on either side of R3.  Sometimes this error is received when the circuit is not configured as expected by the device.

[Greg_Reynolds]

Thanks Kyle, I'll try that. 

I actually got the shunt calibration to work using the SCA and the stock 100Kohm resistor.  This occurred when I switched to using a different computer/scxi system.  Then I switched to SCB with the 11,880ohm resistor, and got the same error discussed previously. I then came back to the SCA shunt cal selection in the VI (that previouslyworked) and the VI failed with the same error.  In short, once the shuntcal.gainadjust error occurred once, no shunt cal operations would work.  I tried resetting the card and the crate, to no avail. 

Greg

[Greg_Reynolds]

I have found some answers… and have some more questions.

First, some of my problems with shunt cal were due to poor connections in the 1314. In particular, the SCA/SCB jumper connections are paired up with another wire (here at the QTR terminal, see Fig 2-1 on pg 2-2 of the SCXI 1520 User Manual). As noted in the manual, the method of just putting two wires into the terminal w/o joining them together can produce an unreliable connection. I soldered these wires and presto, the shunt cal began to operate w/o the error mentioned above. Then I had other problems.

Second problem is that it appears the SCA is always assumed in the DAQmx Perform Shunt Calibration (Strain).vi. To show this, I set up SCA(11,880) and SCB(no resistor), selected SCB with a DAQmx Channel property node, and specified the Shunt Resistance to be 11,880 (the resistor on SCA) using the DAQmx Shunt Cal VI. The shunt cal completed without errors, which clearly would not happen if SCB were actually used. So it seems that the AI.Bridge.ShuntCal.Select is never used by the above VI.

Third problem is that after shunt calibration, there is a large bridge offset. After some investigation, I realized that the bridge offset was the same as the shunt cal offset. It appears that the SCA switch is closed during shunt cal, but never opened afterward. I’m going to try to use the AI Parameters.VI to force the SCA to open after calibration, but my lab computer build does not currently include the “Data Acquisition” module. I’ve attached my final VI.

[Brandon_B]

Hi Greg,

In regards to your second problem, as Kyle mentioned earlier, the built in shunt calibration uses SCA, which is why you want to place the resistor there.  Also, we will investigate the third problem but let us know how the AI Parameters.vi works out.

Regards,

[Greg_Reynolds]

No, using the AI Parameter VI did not work.  It produced the following error,

This is what I did:

Message Edited by Greg Reynolds on 04-02-2010 12:27 PM