Re: PLL?

From: 	Harri Suomalainen[SMTP:haba-at-cc.hut.fi]
Sent: 	Friday, January 02, 1998 12:49 PM
To: 	Tesla List
Subject: 	Re: PLL?

On Thu, 1 Jan 1998, Tesla List wrote:
> Use a photo-interrupter to feed the speed back to the controller, but
> use cheap 1mm polymer optical fibre to avoid RF problems - the actual
> detector would be in a nicely shielded box with the rest of the
> electronics. PLLs would be  sensitive to the sort of crud a spark gap
> would produce - not sure if optical 'noise' from the spark would be a
> problem. 

That's certainly a good idea to use fibre optics for sensing. Depending
on mechanical construction gap light may or may not be problem. However,
once power is applied spark gap output (light when conducting) could
also be fed back to the PLL as a sync input.

> I'm not a PLL expert, but I'd guess you probably want two loops - a
> coarse speed-lock to get the speed roughly right, e.g. during ramp-up,
> and a phase lock which is active when the speed is nearly right.

Two loops are not needed. One-loop system is anyway simpler.

> I think the biggest problem will be compensating for the mechanical
> inertial to keep the loop stable - maybe a PID type of algorithm would
> be useful?

That's not a problem. Think it this way: even if your system output will
wobble a bit the rotation speed will still be roughly constant becouse
of the mechanical inertia. That sort of system should not be then too
hard to make stable! A very simple system will suffice. There are lots
of examples in motor speed controllers. There are many readily available
speed controllors using PLL's to make the speed what you want. They are
quite cheap as well.
See "Malin Tunic: Chemistry of structure - function references in cheese" for
further details!
Harri.Suomalainen-at-hut.fi - PGP key available by fingering haba-at-alpha.hut.fi