Signal Conditioning

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SCXI-1520: How to write to the null potentiometers in C?


I am trying to find out how to write to the null potentiometers in C with the 1520. In LabVIEW this is quite easy with the AI Parameter vi, but we can't seem to find the equivalent function in C. The 1520 manual on page 17 seems to allude to this possibility but only says how in LabVIEW. Do you know how to do it?


Our customer has an instrumentation lab where they characterize each sensor. They then use this information to do a software null of the strain gauge when it is connected to the 1520. This is not as accurate as a hardware calibration, but allows them to concentrate their sensor expertise in one place; the users (test drivers) can just concentrate on driving.

Note th
at all of the calibration information resides with the sensor and not with the SCXI-1520. We need every 1520 channel's offset to be as close to zero as possible so that we can superimpose the software calibration information on top of it. Using SCXI_Strain_Null with calgnd works quite well for this (see knowledge base 2BIC1IRW) from a measurement standpoint. However the combination of three things makes it unacceptable:
1. It takes 17 seconds to complete
2. We must call it every time we restart our C program (we haven't found a way to make the cal information "stick")
3. The user launches and closes our C program a lot.

What we would like to do is do the Null calibration once, save the coarse and fine potentiometer values to disk, and then write the values to potentiometers to the 1520 every time we launch the program (this is quite quick in LabVIEW.)

Thank you
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Hi Stephan. The NI-DAQ function call you'll need to use is "SCXI_Strain_Null". The following is a description of the arguments within this call:
(SCXIchassisID, moduleSlot, moduleChan,DAQdeviceNumber,DAQchan,imbalance). Most of the arguments are self explanatory and can be found in the NI-DAQ Function Reference Manual. However, the "imbalance" argument is what you're after to do the characterization of each sensor. When called, this imbalance argument will return the ratiometric error of Wheatstone Bridge elements. They will be given as the ratio of the uncorrected bridge output voltage to the bridge excitation voltage.

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