Tonight I installed and tested flywheels on my stepper motors that drive my
sixteen inch scope.  The flywheels make a significant improvement in max
speed slewing without stalling the motors.  I can slew at 2.5 deg/second,
and might be able to slew faster, but I'm afraid to slew faster with such a
long tube!  ;-)

Based on the reading I've done in the last few days, I think almost all
amateur scopes driven with Mel's stepper system can benefit from addition of
flywheels.  Why?  The worm/coupling/whatever that's directly coupled to the
stepper motors in amateur drive systems has a very low rotational
inertia...far lower than the inertia of the stepper rotor itself...roughly
the equivalent of the stepper running under no load.  According to the
literature I've reviewed (links from Mel's page) in some (many?) cases
steppers don't perform well at the higher speeds under no-load
conditions...resonance freq's are up high, near the speeds you want to slew
at and the motor stalls.

Adding a flywheel (or adding friction, or adding viscous damping) can lower
the motor's resonant frequency, and improve high speed performance.

My flywheels are very simple - 1/4 inch plate steel, cut into 2.25 inch
squares by hacksaw, with the center hole drilled with a 1/4 inch drill.
Balancing this thing?  Put the flywheel on the drill shank and tap the
shank...the flywheel will fall/settle with the heavy portion lowest...grind
some of that away and test balance again until the flywheel won't settle
into one position over any other.  (Balance is not all that critical
here...your steppers won't spin faster than about 10 times per second at max

The only tradeoff?  You may have to ramp up/down the steppers a bit slower
now, and perhaps start slewing at a slower speed, and perhaps lock the
rotors after a slew for another .1 second or so.

There may be another flywheel improvement in some cases:  it might smooth
out microstepping motion.  I say that because my motors are making less
noise while microstepping now that I've added the flywheels.  That's not
proof that there's smoother motion, but perhaps an indication.

More good news from experimenting tonight.  Now that I have a flywheel in
place, I can use a smaller series resistor with my steppers (6V driven at
24V).  This allows a bit more current...allowing even higher max slew

There seems to be a synergy between:  over voltage driving of steppers,
flywheels to control resonance (lower the fundamental frequency), and
resistors to keep current within motor ratings so that resonance is
controlled at lower speeds.

Before the flywheels I had to use 30 ohms series resistance to keep the
motors from vibrating so much (they would stall at even the slowest speeds).
With the flywheels I need only 10 ohms series resistance.  I haven't even
had to slow the ramp up/down speed...which means I can probably use an even
heavier flywheel...which may mean a further increase in slew speed.  Yikes!
I'm already up to about 2.75 - 3 deg/sec!  For now I'm satisfied with slew
performance.  Later I may experiment with heavier flywheels.

Tom Krajci