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notes about the new calibration functions

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David Schleef 2002-06-12 22:44:44 +00:00
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comedi_calibrate/README
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@ -133,6 +133,11 @@ This is one way that STATUS_SOME is useful, because you can calibrate
the zero offset, then get a much better idea which other channels
are likely to be for the voltage reference.
Note that we haven't done anything with caldac[3]. It clearly
does something useful, but until we attempt a coarse calibration,
it's not certain what it does. It turns out to be a fine postgain
adjustment.
In this example, there doesn't appear to be a caldac that affects
unipolar zero offset, so it will not be used in the final function:
@ -144,3 +149,29 @@ unipolar zero offset, so it will not be used in the final function:
cal1(ni_reference_low, 1);
}
There are a number of functions that are useful for optimizing a
given caldac, each optimized for different cases. The inconsistently
named postgain_cal() and cal1() measure the observable(s) at a
number of points throughout the entire caldac range, and then do a
linear fit to determine the optimum value for caldac. These functions
are good if the caldac dependence is strictly linear. They are also
useful if the target value for the observable is at the endpoint of
the measurable range, as when measuring unipolar zero offset, since
the functions automatically compensate for bad input values.
The function cal_fine() is useful for fine-tuning of the results of
cal1(), especially if the dependence is close, but not quite linear.
The goodness of the linear fit is quantified by the S_min value in the
log -- an S_min value that is approximately the same (within a factor
of 2 or 3) as dof (degrees of freedom) indicates a good fit. An S_min
value that is about 10 times dof indicates that fine tuning is probably
necessary. An S_min value that is many orders of magnitude larger than
dof indicates that linear fitting should not be used.
The functions cal_binary() and cal_postgain_binary() are used when
the caldac dependence is highly non-linear. It does a binary search
in the range of the caldac to find a decent value. Once a decent
value is found, cal_fine() should be used, since the caldac dependence
should be relatively linear in a small range around that value.