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[DPRG] Re: The quadrature blues, or- The Tale of two sensors

Subject: [DPRG] Re: The quadrature blues, or- The Tale of two sensors
From: R. Bickle rbickle at swbell.net
Date: Tue Aug 28 13:21:18 CDT 2001

Hey dave,

I haven't kept up with your entire thread with the encoders, but here's
something that might be helpful to you. Rodent tells me you all are using
reflective type optointerrupters. If this is the case, you can simply mount
the two opto's side by side and make your encoder with as many divisions as
you like. Mount the opto's together so that they can be rotated and once you
have the assembly done, turn the encoder and connect the opto's to two
channels of a scope. Then rotate the sensors with respect to the encoder
direction of rotation such that your opto outputs are at a 90 degree phase.

This is essentially what I did with Model 1 as I was using quadrature
transmissive optointerrupters with an encoder that was not designed for
them. i.e. the spacing was not correct. I mounted the opto's on a swing arm
which could be rotated against the encoder.

When designing encoders, you generally want to keep the number of periods
(i.e. opens + closes) at 1/4 multiple of the distance between the encoders.
This will insure your 90 degree phase shift between channels. For most
applications, you can use a multiple of 1/4, 3/4, 1-1/4 and so on. The
calculation for the encoder then becomes:

Pitch radius = (distance between optos / 1/4 multiple) x (pulses per
revolution) x 1/2 x (pi)

The pitch radius is the encoder radius to the center line of your encoder.
Not to the edge.

Also, you should keep the opens to closes at a 2/3 ratio. If you want the
period of the encoder to be say 0.050", then make the open .020" and the
close 0.030".

Hope this helps...
Rick Bickle

-----Original Message-----
>From: dprglist-admin at dprg.org [mailto:dprglist-admin at dprg.org]On Behalf
Of David P. Anderson
Sent: Tuesday, August 28, 2001 12:34 PM
To: dprglist at dprg.org
Subject: [DPRG] Re: The quadrature blues, or- The Tale of two sensors



Howdy

I've been thinking about this problem a bit.  A single PCB spacing can be
made that will allow any power of 2 number of encoder divisions without
having to change the sensor spacing.  One sensor placed to find edges
that pass thru the 12 oclock position, and one mounted centered on the
3rd or 4th quadrant (4:30 or 8:30 oclock).  The second sensor will be
in the proper spacing for 2,4,8,16, or 32 segments.  That might be enough
to work with.  If other divisions are needed then an adjustable second
sensor might be needed, which complicates things considerably.  Or perhaps
a custom PCB can be made for a particular number of division.

dpa


> Might as well make one adjustable, as you will have to compensate for
shaft
> / encoder disk wobble, etc...
>
> ----- Original Message -----
>
> >     After some head scratching I came to the realization that the
optimal
> > separation of the two sensors was a function of the number of divisions
of
> > the encoder disk.  This is assuming that you want on sensor smack in the
> > middle of one division while the other sensor is in a transition area.
In
> > other words you want the sensors 1/2 a division away from each other.
Even
> > these tiny Hamamatsu sensors are to big for such close mounting so we
must
> > move the sensors (1/2 division) + n*divisions from each other, where n
is
> > any number of divisions up to 180 deg away.  I came up with the
following
> > equation to solve for the number of degrees that should (could) separate
> > the two sensors, it's been a number of years since algebra class
> though.....
>
> >     I'm currently thinking that a pcb made for this application would
need
> > to have a fixed location for one sensor and several choices (for
different
> > number of divisions on encoder disk) for the other, if you were wanting
a
> > quadrature encoder.  Does all this sound right? or have I gone of the
deep
> > end?

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