1520 shunt cal

[Greg_Reynolds]

As shown here, the problem has to do with my effort to select shunt cal resistors SCA/SCB. If I take those channel property nodes out of the VI, then it works fine, as shown below.

With shunt cal enable (T) and shunt cal select (SCA), the shunt cal switch is lef closed. The strain reading (no load) is 571 micro-strain, which is the simulated strain with the 100kshunt cal resistor and 120 ohm gages.

Now, if I select shunt cal enable (F), the unloaded strain reading after shunt cal is very small, as expected.

Then loaded to 2000 micro-strain, it reads 1978 micro-strain (slightly low as expected with my physical setup).

As noted above, the shunt cal VI's use SCA, and as shown here, its best not to mess with the associated properties when using the built-in routines for shunt cal.  I'd like to know, however, if there is a way to use the SCB shunt calibration on the 1520/1314? 

[Greg_Reynolds]

Ok, its clear that the ai.bridge.shunt.cal.enable just opens (F) and closes (T) the SCAx switch.  I put the property node into the read data loop and I can open and close the SCA switch and see the appropriate strain value change on my chart.

Shunt.Cal.Enable (F), I see almost zero:

Shunt.Cal.Enable (T), I see 571 micro-strain:

This does not work if SCB is selected (11,880), which should produce 5000 micro-strain:

[Kyle_A.]

Hey Greg,

You should be able to use SCB to do shunt calibration.  It is strange to see that it is not possible.

Can you try installing your 11880 Ohm resistor in SCB, take out the resistor in SCA (leave it open), and running your calibration while selecting SCB as your shunt circuit?  One thing in programming you can add for testing purposes is the AI.Bridge.ShuntCal.Select property in the DAQmx Channel property node and do a read from that property on the right side of your Shunt Cal vi.  This is to check and see if SCB is actually getting selected or not.  If you can post screenshots of the selections and any errors you receive I would appreciate it.

Another thing, be sure you are setting the Shunt Resistance to 11880 when you are using that particular resistor.  The DAQmx driver uses this value for its calculations and adjustments, so having this value at the default 100k will affect the reading you do. 

[Greg_Reynolds]

Hi Kyle, It's pretty clear that the SCB is not closing through the ShuntCal.Select and ShuntCal.Enable selections.  This is shown below, where the null and shunt cal are completed using SCA (used exclusively by the built-in shunt cal), followed by the read data loop, to which I've added the channel property node to select:

ShuntCal.Enable (T) and ShuntCal.Select (A) with an 100Kohm resistor in SCA. I open and close the ShuntCal.Enable Boolean to produce the output shown... when (T), the simulated strain is 571microstrain as expected.  The switch is closing as expected.

Below ShuntCal.Enable (T) and ShuntCal.Select (B) with an 11,880 resistor in SCB. I open and close the ShuntCal.Enable Boolean to produce the output shown... when (F) OR (T), the strain is zero (should be 5000 micro-strain)

Below is the code segment there the property nodes are set in the data read loop:

With this approach, SCB is clearly not closing...

[Greg_Reynolds]

Here's the missing second figure from the post above, with ShuntCal.Select (B) and ShuntCal.Enable toggled (T) and (F).  These settings have no effect on the bridge output.  The SCB switch must not be closing.

[Kyle_A.]

Greg,

I have worked with our R&D department on this issue, and we have determined this is a bug in the Shunt Calibration vi.   I have attached a .llb file with an older version of the Shunt Calibration which should work for you.  If you replace the DAQmx Perform Shunt Calibration (Strain) vi with the top level vi in this library, subSCXI Shunt Cal Channels.vi, then you should be able to select SCB and use it for your calibration.  You will also have to change some of your controls to match the types of the subSCXI vi.  

Let me know if this works for you, thank you.

[Greg_Reynolds]

Hi Kyle, I cannot use the Shunt Cal Channels.llb since I'm running LV 8.6.1 with DAQmx 9.0
Regarding a bug issue with the DAQmx Perform Shunt Calibration (Strain).vi, once I realized that SCA is assumed, I have not had problems with the VI's functionality.  It would be nice if this VI had an input terminal for selecting SCA/SCB.
The bug issue however, seems to be inability of the channel property node to close SCB,

[muks]

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Kyle A. wrote:

Greg,

I have worked with our R&D department on this issue, and we have determined this is a bug in the Shunt Calibration vi.   I have attached a .llb file with an older version of the Shunt Calibration which should work for you.  If you replace the DAQmx Perform Shunt Calibration (Strain) vi with the top level vi in this library, subSCXI Shunt Cal Channels.vi, then you should be able to select SCB and use it for your calibration.  You will also have to change some of your controls to match the types of the subSCXI vi.  

Let me know if this works for you, thank you.

---

Can you post the CAR#?

[RavensFan]

What if you swap the order of the ShuntCalEnable with the ShuntCalSelect?  Property nodes execute in order from top to bottom.  In a property node like working with a graph cursor, you need to use the Active Channel property before you set the particular properties you want to change.  I'm wondering if you need to select the channel to be B before you enable it.  (How do you like those 3 B's in a row?)

[Greg_Reynolds]

I'd be glad to... if I knew what a CAR# was