Re: Do you think NI can fix my PLL? -- Details

Joerg wrote:
Hello Chris,


Of course there needs to be a speed feedback path for #3 to work. As we know, there are issues with that. For my fast wheel, there is an encoder available, which would need an F-to-V converter. ...


That would be pretty easy, basically a precise pulse generator (_not_ the usual 555 thingie) whose pulse length is set to the shortest possible interval at the highest motor speed (minus some margin), and a non-leaking integrator.

Oddly, I haven't touched a 555 since I was a kid. I use a few HC221 and HC123 when I need a reliable one-shot and can tolerate a few % variation in timing mostly due to cap tempco. Never had a problem with these.

I was thinking of using LM2907 with one phase of the 500pp/rev encoder signal directly. This should be easy enough.

Oh I see, by setting a constant pulse width for each encoder tick, then high speed->high duty cycle / low speed->low duty. So low-pass filtering that gives a smooth result.

I don't get why an integrator would be desired. That would accumulate V(t)*dt until saturation.

A low-pass filter != an integrator.

... For the slow wheel, there would only be the possibility of getting 3 pulses/rev from the halls.


How about measuring the time between two consecutive pulses and outputting a DC voltage proportional to that? Could be done via a counter and a DAC. The DAC would update at every pulse and the counter would be reset at that same time, to clock up the next episode. Or you could do it analog.

Actually I have a good digital F->V converter. In case you ever need an off-the shelf instrument, this is a really nice little gadget:

http://www.ectron.com/prods/441.htm

It has been indispensable in measuring the AC response of the motor. I can scope frequency vs. control voltage. Something like an LM2907 isn't good for this because to get the ripple down you need to kill the BW.

Perhaps the Ectron F->V should be fed 3pp/rev from the halls. I am also curious how close to 50% duty the hall signals are. It is remotely possible that I could get 6 pp/rev fairly evenly spaced.

Another nice F->V converter on the market:

http://www.onosokki.co.jp/English/hp_e/products/keisoku/revo/fv1300.htm



Thanks as always for the input.


Good day!


--
_____________________
Christopher R. Carlen
crobc@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx
SuSE 9.1 Linux 2.6.5
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