System Calibration vs Device Calibration

Question

Hi Guys,
Looking for some experiences/opinions on an approach we are looking to take on a measurement system.
We are using compactRIOs which are buit into a system which is installed in a quite restrictive environment and we are concerned will calibration. To retrieve the measurement cards and replace them will be a costly exercise.
We are considering a concept of using a portable calibration unit to perform a system calibration on site. This would involve putting in known values and notifying the software which can calculate new scaling coefficients to use going foward.
I believe this should account for everything that calibrating the devices would (and more). Assuming this is done at regular intervals and the devices calibration drift is roughly linear there should be no need to calibrate them seperately.
Have you had experience of this method? Would you be confident in this for new systems?
Cheers,
James

Answer 1

OK, so I've dug a bit deeper and believe that the Nonlinear System Solver is the appropriate starting point, as found in the example file Equation Explorer.vi.
In that example, I can define a system of nonlinear equations and the vi should spit out the solutions (zeros) of the variables.  I got it to work with some arbitrary test data generated through excel.
Now, I'm moving on to the next level of complexity in the model, and have derailed m'self again..
The simulated device:
a three axis accelerometer, each of which would generate an output of the form Voltage = Scale * Acceleration + Offset. 
In a perfect world, the only accleration I'll be seeing is gravity (9.8 m/s^2), and I set that gravity in a random direction (indicating the accelerometer is at an arbitrary orientation).  Knowing that orientation, I then calculated what the components of the acceleration would be along all three axes, and then applied values for Scale and Offet to deterine what the equivalent voltage reading off the accelerometers would be.
So now I start the long way home again.  From a Labview perspective, I know that I have three voltage outputs which would feed into a known relationship
((Vx - OffsetX)/ScaleX)^2 + ((Vy - OffsetY)/ScaleY)^2 + ((Vz - OffsetZ)/ScaleZ)^2 - 96.04 = 0
where the 96.04 comes from g^2.  This is my nonlinear equation with six unknowns in it and three constants.
Since I have six unknowns, I need six equations, which I can get by rerunning my simulation, right? 
I put six generated "voltages" (Vx, Vy, Vz) into this form using my expected values for Scale and Offset, and run the vi. However, the vi returns nothing.  I know there should be a solution vecotr, because that's what I used to created the simulated data.  Is there something I'm overlooking?
Attachments:
Calibration.png ‏74 KB