> > What you are saying is that the lightbulb to optocoupler is not a linear
> > system, which I totally agree with.  However, that doesn't mean it can't
> > be controlled with a PID controller.  The simple method is to set the
> > damping so that it still works with the worst case light bulb delay.
> > Another more sophisticated approach would be to add some extra logic to
> > the PID controller.
>
> No. I am saying that the time constant associated with the lamp depends
> heavily on its filament temperature. A lamp will cool down MUCH faster
> when it is very bright than when it is just barely lit and the opposite
> effect (brightening) slows down with increasing filament temperature. This
> makes for some assymetrical 'rounded' triangle shapes that change in shape
> for dL/dV at any given V. Compensating for this in a tightly designed PID
> loop is usually a no-no so it needs to be overdamped. Unless you run the
> lamp very bright (not a good idea).

I think we are both saying the same thing and both agreeing that the simple
solution is extra damping.  All the characteristics you describes make the
system non-linear.  Linear simply means that scaling the input by a constant
will also scale the output by the same constant.  As you pointed out, this
is definitely not the case with a light bulb.


********************************************************************
Olin Lathrop, embedded systems consultant in Littleton Massachusetts
(978) 742-9014, olin@embedinc.com, http://www.embedinc.com

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