How to determine and compensate DAQ board + SCC-SG24 error

[Oleks]

Hello everybody,

I am kind of confused regarding how should I treat/compensate device error. I am using PXI 6221 + scc SG24 to operate a load cell that measures Fx,Fy,Mz.
For Fx/Fy it has ~0.5mV/V with the range +/-115N and for Mz ~ 0.4mV/V with the range +/-3Nm

For the DAQ board, <<CALIBRATION PROCEDURE B/E/M/S/X Series For NI-DAQ™mx>>, on page 58, there are some data for calibrating the board with different Voltage
range, I am using +/- 1V. At each range there are two values for "0 Volt" load and "Maximum Volt" load.

By using them and considering the amplification
factor of 1000 (10V excitation and 100 gain) on the strain gages, I have found the corresponding value
for the torque noise due to the DAQ board. But it is not clear for me that what is the error in
this device.

On page 6 of the "SCC Configuration Guide", on the specifications for the SG24, there are two parameters of gain error and
offset error. I just wonder that can I use them for the output error that actually causes the noise? Also if I can, which of those two should I use and what will be the best way to compensate that error?

I appreciate any help.

Thank you in advance

[jschwartz]

Hi Oleks,

It sounds like you are trying to do a system level calibration.  Typically, we would recommend calibrating each component seperately.   The gain and offset error are specified for calibrated hardware and and won't be able to be compensated for.  To calibrate your system together, you will have to have a strain guage with a known strain and zero out the errors associated with that particular strain guage.  

Message Edited by jschwartz on 11-12-2009 03:44 PM

[Oleks]

Hi Jim,

Thank you for the response. I am not really trying to perform system level calibration. I should of being more clear about my problem. When all components are connected together, DAQ board, SCC units and a load cell, the only tool I was able to use to "zero out" signal from the load cell channels, was the resistors on SG24 units. However, even at "zero" level, I do have a noise, for instance Fx,Fy is ~+/-0.2N, while Mz ~ +/-0.002N with no load. Ideally I would like to get rid of that noise as much as possible, as it does not allow me to measure forces, say less then 2N if I want to have an error ~ 10% for Fx,Fy axis. So, what I am trying to figure out is if the part of that noise is actually come from gain and offset errors, and whether I can "software compensate" it.

If I understood you correctly, the only way to do that is by working on the loadcell hardware level. Is there an alternative?

[jschwartz]

Hi Oleks,

If you have a constant offset, you can easily zero this out in software.  This Community Example shows one way you could do this.  If you have  noise on the system, Field Wiring and Noise Considerations for Analog Signals is a good resource for ways to reduce noise.  You aren't going to be able to compensate for noise unless you are seeing a constant error - compensation can only add a constant offset, which will make no difference for noise. If you believe that the noise that you are seeing is out of spec, please let me know.