Re: Do you think NI can fix my PLL?

Chris Carlen wrote:
My assertion has been that PLL stabilization is not something that can
be done the way most controls are handled around here (usually PID)
where you can use heuristic algorithms to getting it to work). Rather a
PLL must be computed via classical analysis and servo loop design
methods, ie, do the math.

"Doing the math" is great if you've got a precise model of the behavior
of the system at each VCO/Control voltage.

Over small changes in load/frequency you probably only need two or
three numbers to characterize the system over the small linear regime
and you can "do the math" classically.

But when the system (in your case, not just electronic but
electromechanical) is not truly linear but may be nonlinear over
typical range of operation, and ESPECIALLY if you've got noise into
your phase detector, then the classical math is no longer so easy.

The classical PLL math is still relevant to understanding the system
and explaining why it's screwed up at certain regimes.

It is not necessary to do all the classical PLL math to come up with a
working PLL. You'd be surprised at how many were done by
seat-of-the-pants hey-lets-change-parts-until-it-works methods :-).

Since the LabVIEW guy has no such experience the only possible way this
could work using the NI approach is if:

1. NI has PLL tuning algorithms that can "autotune" successfully.
2. OR NI will also provide us with contracted design assistance to
have one of their experts tune and set up the digital filtering LabVIEW
code.
4. AND I am wrong about the need for analysis to solve this PLL's loop
stabilization requirements.
3. AND I am wrong about the non-linearities being the root of
difficulties. Although, a digital platform might actually help in this
regard if some sort of linearizing function must be applied. I suspect
however, that this would be better solved by a simpler minor-loop
synthesis approach. Ie, run the motor in speed servo mode using the
drive and a speed feedback sensor (which should then be highly linear),
and build the PLL around that.

One thing that going digital allows is easy variablee tuning of filters
(in the case of a PLL usually low-pass filter) to the
averaging/windowing parameter that is relevant.

Tim.

.