Re: [ATM] Flex Rocker Question

["matt" <electro_optic@bellsouth.net>]

Hi John,
Sorry to disappoint you but it doesn't quite work the way you described it.
While it's true that there are reflective encoders in use , and they do work
as you described, the USDigital are not reflective.

The US Digital and Hewlett Packard/Agilent compatible encoders are
transmissive and are based on light passing through the mylar tape from one
side to the other . The active area is not the bulls eye area but the fine
darker strip at the bottom , just under the index mark in their drawing.

Why is it important that they're transmissive and not reflective ? Because
you must have the strip free on both sides , it really works the way you'd
use a credit card reader , where the card has to go through the reader slot.
The IR emitters and receivers are assembled in a solid piece of plastic, the
Agilent version is called HEDSxxx and the USDigital is the EM1xxx , they
both are interchangeable and have a slot in the middle for the optical strip
or disk . So you'd have to figure out how to attach the strip and allow it a
free edge, plus protect that free edge along the whole strip length .

Agilent makes some reflective transmitters and receivers but they do not
work with the optical strips from USDigital, infact they do not work with
any optical strip . You'd have to make a custom strip for them , that's the
AEDR series .

If you want a reliable solution, you're close with the strip linear encoder
, _in principle_ . Large observatory scopes use exactly such encoders, there
are several good suppliers and models . Renishaw has an optical tape encoder
with a reflective read head , Heidenhain and Sony both have precision linear
magnetic encoders . However, for a robust tape encoder expect to pay more
than for a premium 20" mirror .

Before flaming me, I must add that I'm only sharing here some of my
experience as an electronics design engineer in the industrial and
automation field for a quarter century .

best regards,
matt tudor


-----Original Message-----
From: john sherman <atm@mail.johnspics.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Tuesday, March 29, 2005 3:33 PM
Subject: Re: [ATM] Flex Rocker Question


>
>Hi Chuck,
>
>>What is a strip reader and encoder strip?
>
>I went to US Digital <www.usdigital.com> and did a search on "strip". Here
is one of the pages I got to http://www.usdigital.com/products/lin/  It
shows a picture of a strip which appears to be a series of bulls-eyes. US
Digital makes the encoders used by some of the commercial DSC units, so
these things should be immediately compatible (you'd have to adjust the
number in the DSC unit).
>
>A strip is a long skinny piece of plastic with alternating reflectors on
one side (red and white, for example). The reader shines a light on the
strip, and as you move the scope the light "flashes". Each flash is read as
a certain amount of movement. I think the regular encoders have the
reflectors lined up on the outside of a disc, with the reader built in. So
imagine separating the two functions into separate units.
>
>In our case, I'd want to line the inside of the groundring with a strip,
and mount the reader to the underside of the rocker (to measure AZ). This
arrangement should allow you to mount your scope arbitrarily close to the
ground. If you're not careful, your mirror box can be made so low as to hit
the ground.
>
>If you go this route, please post your experience. I'd really like to know
how it works out. Thanks,
>
>John
>
>
>
>
>
>
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>

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