Quote:
Originally Posted by mental4astro
Cripes, Gary, thanks.
The reason for my asking is a mate has his azimuth bearing made out of a lazy susan. I've thought of fitting an encoder's shaft with a little wheel, and set the wheel to run parallel to the tangent of the pivot point. Depending on the distance out from the centre of the pivot, the effective number of encoder revolutions can be quite large.
No backlash with this direct drive method either.
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Hi Alex,
Rubber friction rollers have the advantage of zero backlash. Many professional
telescopes such as the Keck Observatory 10m and the Lick Observatory Shane
3m employ incremental encoders that are mounted off-axis and coupled via
a friction roller.
However, even on professional telescopes, great care has to be taken to avoid
slippage. Any slippage will result in a pointing degradation. On amateur telescopes,
even a well engineered roller solution can prove challenging. Even very tiny
angular misalignments can result in significant slippage.
For the types of telescopes you are talking about, I recommend to avoid friction rollers.
One can spend countless hours trying to get them to work reliably and for any
scope that is portable, the challenges of maintaining alignment of a roller is made
even more difficult whenever the scope is transported.
Rather than a friction roller, I recommend you consider a timing belt and pulley.
For example, take a look at this image on our web site -
http://www.wildcard-innovations.com....es/hector4.jpg
This is a 20" Dob and you will notice a black strip of MXL timing belt has been
glued to the inside of the Alt trunnion. An encoder with a small aluminium
timing pulley fitted on its shaft is spring loaded so that the pulley pushes down
on the belt. The shaft spins many times as the mount rotates around the Alt axis.
The encoder employed had a very low resolution but the effective step count is in the
order of 56,000 steps.
This particular timing belt arrangement never slips and this solution has proven very reliable.
However, determining the effective step count on the Alt axis using this
arrangement can be challenging, made all the more difficult because the Alt axis cannot
be rotated through a full 360 degrees.
On the Az axis, rather than exploit the diameter of the lazy suzan bearing, it
is generally advisable to run a timing pulley and belt off the main Az pivot bolt instead.
When a roller runs on top of the lazy suzan like a needle around one revolution of an old vinyl record,
it is extremely difficult to avoid slippage. Even when one tries to run a wheel on the outside edge
of the lazy suzan, one has to ensure that the lazy suzan is perfectly circular and again
great care has to be taken to avoid slippage.
Best Regards
Gary Kopff
Managing Director
Wildcard Innovations Pty. Ltd.
20 Kilmory Place, Mount Kuring-Gai
NSW. 2080. Australia
Phone +61-2-9457-9049
Phone +61-2-9457-9593
sales@wildcard-innovations.com.au
http://www.wildcard-innovations.com.au