Re: Do you think NI can fix my PLL? -- Details
- From: Joerg <notthisjoergsch@xxxxxxxxxxxxxxxxxxxxx>
- Date: Sat, 03 Jun 2006 20:49:16 GMT
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 haven't either. But I must confess that I also used the 121 in the old TTL days. Not anymore, and I hope that can be chalked up to teenage sins :-)
I was thinking of using LM2907 with one phase of the 500pp/rev encoder signal directly. This should be easy enough.
But mind it's linearity error. +/-1% isn't exactly something to write home about.
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.
Sorry, I meant integrator plus S&H. A lowpass will still let substantial ripple pass. Ideally you'd want nice steps where each level corresponds to the velocity, like the output of a DAC. That makes filtering a lot easier. So an integrator would start integrating, then hold and store when the one-shot time is up. Now it is reset and waits for the next pulse. You will be up to one pulse behind in phase when doing that, something that may matter in a clutch application.
... 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
Sweet. Even has selectable filters.
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.
A good sample and hold helps.
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
Man, you guys must have a very comfy R&D budget over there.
--
Regards, Joerg
http://www.analogconsultants.com
.
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