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Intec
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SCXI-1600 and SCXI-1121, use for strain gage conditioning

 

To NI engineers:

 

 

 

I am having trouble getting the SCXI-1121 to work with SCXI-1600 (The Chasis is SCXI-1001).  Originally, this chasis is setup with SCXI-1600, SCXI-1100, and SCXI-1520 together; however, once I plug in the SCXI-1121 onto the chasis, I could not get any strain signal output out of the 4 channels of SCXI-1121.  Basically, I kept getting this -200077 error on the DAQ assistance.  I've checked both the MAX and the jumper setting inside the SCXI-1121, and they both match.   

 

Anyway, I am hoping if you guys can answer the following questions and offer some insights to my current setup.

 

1) Is it possible to mix SCXI-1520 modules with SCXI-1121 modules on the same chasis?

 

2) Does the SCXI-1600 digitizer work with SCXI-1121?

 

3) Any particular SCXI-1121 jumper settings that must be done first in order to get the module to work with SCXI-1600 digitizer?

 

I attached the screen shot of the MAX setup.

 

Thanks in advance for this matter!

  

 

Kin

 

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Brooks_C
Posts: 357
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Re: SCXI-1600 and SCXI-1121, use for strain gage conditioning

Hello Intec,

 

NI DAQmx is generally pretty helpful when it comes to providing feedback on the appropriate scaled range.  It will take the gain that you have configured on the module properties tab into consideration along with the specified gauge parameters and scaling equation and limit the AI Min and AI Max properties to the values that will max out your hardware input range.

 

With that in mind, you probably need to decrease the gain setting on the SCXI-1121 module so that your sensor's full scale output (or at least the output at 20000 uStrain) is within the SCXI-1121 modules range.  If you're unsure what range to use, take ±5V and divide it by the gain you've chosen for the channel.  If that is smaller than your sensors full scale output then you need to decrease the gain.

 

If you've already checked this out, please post back with the gain setting you're using.

 

Cheers,

Brooks
Member
Intec
Posts: 7
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Re: SCXI-1600 and SCXI-1121, use for strain gage conditioning

Dear Brook.

 

Thanks for responding to my post!   The jumper & MAX setting on SCXI-1121 module is as followed:

 

Excitation voltage = 3.33v

Filter = 10k

Gain = 200

Full scale strain = 20,000 microstrain

Strain Gage factor = 2.105

 

Now, is the gain too high for this strain gage?  What will be the optimum jumper setting on SCXI-1121?

 

Thanks!

 

Kin

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Brooks_C
Posts: 357
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Re: SCXI-1600 and SCXI-1121, use for strain gage conditioning

Hello Kin,

 

The SCXI-1520 Universal Strain Gauge Input module has a great Theory of Operation section on Strain Gauges if you're interested in learning the math.  (Page 53 has the quarter bridge I information).

 

Basically, ignoring the error due to lead resistances, that is, assuming that the resistance of your leads is << the gauge resistance, here's the scaling equation for a Quarter Bridge I strain gauge:

 

Strain = (-4 * Vr) / (GF * (1 + 2 * Vr))

where GF is the gauge factor (2.105 for you) and Vr is the ratio of Vmeasured/Vexcitation.

 

Since you want to know the optimal gain setting for your sensor, we will solve for Vr given that you want to measure 20000 micro strain (0.02 strain).  Then, solving for Vr (I'll spare you the details):

 

Vr = [GF*Strain/-4] / (1 - 2 * [GF*Strain/-4])

 

and substituting in the values of GF and Strain:

 

Vr = [2.105*0.02/-4] / (1 - 2* [2.105*0.02/-4]) = -0.01031

 

Then since we know Vr = Vmeasured / Vexcitation, we can solve for Vmeasured (which is the full scale output of the gauge), Vr * Vexcitation = -0.01031 * 3.3 V = -0.03402 V.

 

This means that you need an input range that can measure at least +/- 0.03402 Volts to measure +/- 20000 ustrain.  Since the SCXI-1121 has a 5V range, when you set the input gain to 200, the input range becomes 5 V / 200 = 0.025 V which is not big enough to measure +/- 20000 ustrain.  This means you can either not use the full scale range of your sensor (i.e. if you only want to measure ~14000 ustrain you can just reduce the max and min range) or you can decrease the gain to the next lowest setting, 100 and get a range of 5 V / 100 = 0.05 V which is large enough to measure the +/- 0.03402 V required for the full scale range of your gauge.

 

Best Regards,

 

 

Brooks
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Reeves77
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Re: SCXI-1600 and SCXI-1121, use for strain gage conditioning

Hello, I work in the Mining school of Paris, in the geomechanical center, to study soils and rocks mechanics.

I have the same problem whith a 25 kN load cell, whith an excitation of 10 V and a sensibility of 2 mV/V. So I choose a total gain of 200, in order to mesure the noise under the two low-pass filters. The SCXI-1121 is mounted whith the SCXI-1600, in the SCXI-1000 chassis. An USB cable link the PC to the chassis. I use DAQmx 4.5 to read the virtual task, and the driver is correctly installed. Also, I have tested my sensor in the MTS press (www.mts.com) to verify its sensibility and the range (the press can exerce a maximum force of 750 kN, and it totally automated, whith a LCD display to control the force), but the signal don't display corectly the tension in DAQmx. The SCXI-1121 is linked whith the SCXI bus. I have probably make a mistake when I configured the w33, w32, w38 and w45 jumpers. I have the SCXI-1121 manual, but this part of configuration is dark. The SCXI-1121 is associated whith SCXI-1321.

May I have a answer ?

This load cell is tested in order to reduce and study the noise of the tests of drilling pastilles, in petroleum applications.