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Re: Gray code shaft encoder photo master



Hello, all-

A very viable alternative to the Gray code encoder is an
ordinary quadrature encoder.  They are decoded with a
single D-type flop, and provide both direction and pulses
which can indicate degeees of shaft rotation.  Pulleys can
modify mast vs shaft gearing to provide any desired number
of pulses per degree of mast rotation.  There are not many
ham antennas which need better than 1/2 degree pointing
accuracy (1/2 degree implies in the neighborhood of 30 dB
gain) thus 720 steps/mast revolution should be plenty for
most antenna pointing.

The only drawback to the quadrature encoder is that it is
not an 'absolute' encoder- you can't turn off the power
to your circuit and power up later and instantly get your
position information.   I've solved this problem on several
projects by driving the shaft to a known position and then
resetting the up/down counter to a known state.  Ordinary
floppy disk drives use this technique to recalibrate the
seek counters when the stepper is suspected of having
slipped a step or two.  Another way to solve the problem
is to leave power on your decoder/counter circuits all the
time.

The good news is that quadrature encoders are pretty easy
to find at hamfests for a buck or five.  With gearing or
belting (which you might need with ANY encoder) it dosn't
make much difference how many steps/rev the encoder you
find has, you can fix it with gear/pulley ratios.

The Gray code encoder is great, but requires a bunch of
external decoding to extract the positional information.
Unless you guys have come up with a better scheme, you'll
need a big old ROM or at least a PIC to decode it.

Best regards,  John  W5EME

--------------------------------------------------------

Clifford Buttschardt wrote:
> 
> Of course Bdale, never thought of a Google search!  Cliff K7RR
> 
> On 14 Jan 2001, Bdale Garbee wrote:
> 
> > cbuttsch@slonet.org (Clifford Buttschardt) writes:
> >
> > > Right Ross, we are all in one the "simple" positioner!  I have seen a
> > > photo master of the gray code shaft encoder about three inches in diameter
> > > SOMEWHERE in the last ten years.  It is just a matter of time before one
> > > of us finds it.  Then all we have to do is put three photo interrupters
> > > on the film along with some basic electronics.  Cliff K7RR
> >
> > I did a quick Google search ("gray code shaft encoder").  Try this, it was
> > the first hit, and looks pretty cool.
> >
> >         http://www.phesk.demon.co.uk/encoder/
> >
> > 73 - Bdale, KB0G
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