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I believe the advantages of this drive system are the stability of a
tangent drive with the accuracy of a direct drive. The drive is the
only part of this project that is somewhat unique so I'll describe it first.
I used a threaded rod as is commonly used in tangent drive systems.
See Chuck Shaw's page at http://www.ghgcorp.com/cshaw/platform.htm
I like the idea of a threaded rod to drive the platform because it is very
stable and a variety of scopes, tripods, etc... can be placed on the platform
without haveing to worry too much about weight and balance. The problem
is the ever changeing drive rate that is required. It's not really
that big a deal to vary the drive rate now and then while observing, I
did it for several years with my split ring equatorial mount on which I
used a tangent drive. But when designing the platform I decided to
try to avoid the need to vary the drive rate if possible. One way
to do this is with a direct drive system. See another excellent page
by Jan Van Gastel http://www.geocities.com/janvangastel/Astronomy/
But this system is prone to slipping if not very well balanced. I
don't know this from personal experience but it seems as though it would
be the case. Also, I don't know anything about computers or stepper
motors. In order to avoid the problem of slipping a piece of a tape
meassure is used to attach the drive nut to the north sector. I got
the idea of a tape drive at http://faculty.washington.edu/quarn/stpmtr.html
The tape is attached with 2 clamps and turns through a little more
than 180 degrees on each of the main bearings. One clamp attaches
the tape to the drive nut (actually it attaches the tape to a plastice
plate which in turn is attached to the drive nut as well as a rod to keep
the plate from rotating. The plate used to carry the print head in
an old dot matrix printer. The rod is also from the printer.
I will add more about this at a later date.) The other clamp
attaches the tape to the north sector. As can be seen in one of the
pictures each end of the tape is attached to the clamp with a screw.
So when the tape is clamped to the north sector the tape and sector move
as one unit.
The sector actually rides on the tape as it goes around the bearings. The tape follows the curve of the sector between the bearings and point of attachment to the sector. This is what allows a constant drive rate to drive the platform even though it is not a direct drive system.
When the other side of the tape is clamped to the drive nut a very tight, rigid, and best of all, simple, drive linkage results. Unless the drive nut is moved by rotating the threaded rod, there is absolutely no movement of the platform. The clamps are quite inexpensive and were purchased at the local hardware store. They are sold in the same section as the squares and are intended to be clamped to a square to speed up the process of making repetitive measurements. They work great! The top of the platform can be removed in a matter of seconds if need be simply by loosening the thumb screw on the clamp. The tape can also be removed very easily.
The scope I use has a very low center of gravity so I can run the drive for 90 minutes without resetting and don't need to worry about the scope falling off at the beginning or end of a run. I attached a coupling nut to the west side of the threaded rod and use a power screwdriver to reset. Resetting takes about one minute.
