PSN-L Email List Message

Subject: Re: Not so simple photoelectrics, or are they?
From: Thomas Leiper twleiper@........
Date: Sun, 3 Jun 2001 00:50:17 -0400 (EDT)

Yeah, I must have missed an order of magnitude in my mental
guestimation as well, because 2.6 ns is out of the question.
So it's back to the old coil. I wouldn't have wasted my time
anyway. I suppose you could just slow down your sample rate, but
then you would have the flutter factor to deal with. There must
be SOME extremely complicated way we can replace the magnet and
coil that work so well. How about a force-balance scheme where
we just use some silver (or gold) contacts and keep them barely
touching by PW modulating the coil with a high frequency square wave...


------Original Message------
From: "Charles R. Patton" 
To: psn-l@..............
Sent: June 1, 2001 8:44:42 PM GMT
Subject: Re: Not so simple photoelectrics, or are they?

Thomas Leiper wrote:
> Not sure you get it, Charlie. You don't have to
> resolve the pulse width, only the timing of the
> relationship between the reference pulse and the
> detector pulse.=20


I did make a mistake in my math by thinking in magnitudes rather than
powers of two.  But the point I was trying to make has just been made
very well by Chris Chapman who has just posted, "... 1 arc second is
1/60*60*360 of a revolution - 1/1,296,000, so at 30 RPS, 0.1 arc sec
represents ~2.6 nano sec=85"  His observation about diffraction errors is
probably quite accurate.

Although parts are available at 300 MHz, they generally cannot be used
with simple perf board techniques.  They require ground plane techniques
and careful attention to lead lengths and so forth - not a construction
area kind to the beginner.
And as Chris's post and other posts have mentioned, it still doesn't
solve the need for a small dot in order to get that 2.6 ns edge.  Other
factors are the speed and position variations one will find in the
scanner mechanism.  One can expect at best perhaps random 5 uin position
errors at the motor mirror if the best bearings are used (not likely),
and more likely much worse due to imperfections in the ball bearings.=20
Those errors can be overcome with hydrostatic bearings, but you just
left the few dollars territory and headed into kilobuck land.  Some of
these position errors will translate as angle errors and thereby timing
that is somewhat random with respect to the timing photodiode.=20
Additionally there will be speed variations greater than and perhaps
much greater than 10 us per revolution.  Some of it will be cancelled by
the timing pulse, but unless the timing pulse and the measurement pulse
are co-located, there will be that inaccuracy to add in.

Charles R. Patton

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Larry Cochrane <cochrane@..............>