On 6 Mar 2010 22:03:04 GMT, "DoN. Nichols"
wrote:

On 2010-03-06, Don Foreman wrote:
On Thu, 4 Mar 2010 14:31:33 -0500, "Buerste"
wrote:

Damn inertia!

[ ... ]

I would consider a low-inertia servo motor driving wire-drive
capstans. Recall the action on reel-to-reel tape digital data storage
years ago? Those fairly large reels started and stopped very quickly.

But -- not quickly enough. The tape typically ran through a
vacuum column one each side to provide a very low inertia, buffer. The
The capstan can be pretty quick, but there is a lot of rotational
inertia in a full tape reel.

I used the example to keep my post simple, but I really did do the
math here. I guessed at 4" dia capstan with a mass of 1 lb,
accelerating and decelerating to have peripheral travel of 3" in 0.25
seconds starting and stopping at zero velocity. Peak torque was
trivial compared to the capability of even a small servomotor driving
direct -- no gears or anything. Torque required was 0.056
Newton-meters, trivial for even a very small coreless servomotor.
Accurate design calculation? Of course not, it's a quick sanity check
that clearly shows feasibility.