Re: [Phys-l] Discrepant amplitude measurement

[brian whatcott <betwys1@sbcglobal.net>]

On 8/3/2011 4:40 PM, chuck britton wrote:
> A 13 track encoder would give a resolution of 0.0439 degrees (if I
> understand it correctly)
> Wonder how this compares with the current 'hands-off' methods?
> .
> At 2:08 PM -0700 8/3/11, John Denker wrote:
> > If I were instrumenting a clock, I'd start by putting a Gray-
> > coded optical encoder on it (not just a single photogate).
> _
While an encoder that can resolve 360 degrees to 8192 values does indeed 
distinguish rotations of
40 millidegrees or so, what is needed is a binary sense: is the critical 
amplitude of 5 degrees or whatever,
 exceeded or not?    This takes but one photogate..
The drive is then applied at mid swing.  Or not. This takes another 
binary sensor.

It would be well to know that the drive is only applied at the velocity 
peak  immediately after the amplitude determination,
but not also on the second half cycle;   so that could give rise to a 
third sensor to decide the pendulum's direction.
One moving flag would provide less incremental drag than an encoder disk 
no doubt - though that would be rather academic for an evacuated chamber.

Brian W