RA Drive Problem

[Tom Droege <droege@wwa.com>]

Well, I found a problem with the RA drive.  I have "fixed" it, and now
things look much better.  I have sent Michael a few images to put up on the
home page.  The sky was not good enough last night to make all the right
adjustments, so the jury is still out on whether or not I have fixed the
problem.

Let me explain.  

Picture how the RA drive works.  Assume a wheel with a couple of holes in
it 30 degrees apart.  Looking down the RA axis there is a right hole and a
left hole.  A string coming out of the right hole wraps around the wheel to
the left where it attaches to block 1 on a lead screw.  The string coming
out of the left hole wraps around the wheel to the right where it attaches
to a block 2 on the lead screw.  The lead screw moves the blocks together,
so one string wraps on the wheel and the other unwraps, translating linear
to rotational motion.  There is a turnbuckle that adjusts the tension on
the strings.  (Actually stainless steel cable.)  A stepping motor drives
the lead screw.  If everything is well designed, then there is no change in
tension of the wire as the RA drive moves, and there is good translation
from linear to rotational motion.  To improve the drive precision, the lead
screw engages many threads through the blocks.  The lead screw is a rolled
stainless steel screw.  By the manufacturing process, the rolled thread
tends to have a constant pitch.  Note that as the turnbuckle is tightened,
the string tension pulls the two blocks together.  This tends to take any
backlash out of the system.  

The last change I made was to replace the fairly rigid coupling between the
motor and the lead screw with a piece of soft rubber tubing.  This really
reduced the noise that the stepping motor was making.  In the process, I
told Dan to tighten the turnbuckle "as tight as possible".  

The result was that the two blocks were pulled tightly together.  They now
tended to stick.  So the motor would wind up on the soft coupling, then
break loose.  This resulted in a jerky drive motion.  Just backing off on
the turnbuckle tension caused everything run smoothly.  

So now I need a clear night to do fine adjustments and see if the problem
is fixed.  The pictures I hope Michael is putting up show the progress.  

Tom Droege