Strain Range Error when reading Displacement Gage

[lokisspydr]

I have an NI9237 and Labview 2010 and I am attempting to read an MTS Displacement Gage with it. I have read strain gages and have no difficulty doing that. As the 632.06H Displacement gage is essentially a half-bridge this is my configuration. I have 2 sets of known calibration values that I am trying to use and I keep getting error 200077 AI.Max (or AI.Min) is out of range. I adjust to the range LabView specifies and I get another error that lowers the range again, and again, and again, etc. Any input would be appreciated. My Gage factor is: 2.82. Resistance: 350. Vex: 5. Initial V should be 0. Calibrated range is 19.24m to -19.37m Thanks

[Sean_F]

Hi Loki,

How are you setting up your task in MAX? The range for the 9237 is +/- 25mV. If you create a Bridge Task (V/V) do you still get this error? If yes, can you attach a screenshot of your configuration and the error?

Thanks,

[lokisspydr]

Sean,

I had Drew here from National Instruments on Friday and he took a lookl at the configuration and couldn't find any issues (although we didn't get a chance to dive into it due to time constrictions.) Yes my limits are set below 25mv. The exact configuration is: max: 19.28mV Min:-19.37 Vex:5V Gage Factor:2.82 Resistance:350 and after disecting an old one we found that it is a full bridge and the half in the connection is for shunt cal. Yes I have also tried it in Bridge and gotten the same errors. I am working on another project and will load screen shots when I get a free moment to do so. Thanks.

[lokisspydr]

Sean,

I had Drew here from National Instruments on Friday and he took a lookl at the configuration and couldn't find any issues (although we didn't get a chance to dive into it due to time constrictions.) Yes my limits are set below 25mv. The exact configuration is: max: 19.28mV Min:-19.37 Vex:5V Gage Factor:2.82 Resistance:350 and after disecting an old one we found that it is a full bridge and the half in the connection is for shunt cal. Yes I have also tried it in Bridge and gotten the same errors. I am working on another project and will load screen shots when I get a free moment to do so. Thanks.

[DrewPierce]

Joining this thread to track the troubleshooting/solution...

[lokisspydr]

Sean,

Attached are the requested photos. The first set is with known calibration values and the second is putting in the values that LabVIEW suggested. As Drew can attest, this is an endless cycle. I change to the values it recommends and then I get another error and so on and so on. Thanks.

[lokisspydr]

Second set.

[Sean_F]

Hi Loki,

If you look next to your signal input range you'll see that the units are actually in strain, not volts. To convert between units of strain and voltage, check out this dev zone article. You should be able to set your strain limit to range from  -1m to 1m. Shouldn't your bridge also be configured to be half-bridge rather than full bridge? 

[lokisspydr]

Sean,

Correct me if I am wrong but a Displacement gage reads strain not volts and it is absolutely a full bridge. When I leave the default settings of 1m to -1m I am able to complete calibration but it does not read anything (Drew witnessed that attempt as well). Any suggestions.

Thanks.

[Sean_F]

Hi Loki,

Here's what I'm thinking. Your displacement gage is going to produce a voltage that you should be able to scale to displacement. What we should do is to create a Bridge (V/V) Task. The input limits here are specified as V/V and are, by default, 25m and - 25m. This is explained in more detail in this KB

To test things are working correctly you could create a simple voltage divider circuit as shown in Figure 1. Vout = R2/(R1+R2)*Vin and should be connected to your input (CH+ and CH-), Vin is your excitation voltage, and R1 and R2 should be picked such that R2/(R1+R2) < 0.025. 

Figure 1- Voltage Divider Circuit

With your signal returned in volts, you can create a custom scale to convert from volts to displacement. This KB details the process. 

Hope this helps.