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.
Sometimes, the capstan pinch roller would be engaged/disengaged
to start and top the tape, and the servos on the reels would be used to
keep the proper amount of tape in the buffers.
Of course, there were densities of up to 5600 BPI, so the tape
motion needed to be controlled to about 0.0002" to stop on a bit,
(normally, the stops were in an IRG (Inter Record Gap) whose length
would have to be determined by the worst case drive. :-)
For the slower ones -- the buffer would be a cats-cradle of tape
spooled between the fingers of a "dancing arm".
However, given the likely stiffness of the flat wire being fed,
I somehow doubt whether the air column would do anything useful, and
even the dancing arms would have to have pretty widely spaced rollers to
avoid too sharp a bend.
In your case, the diameter and mass of the drive capstans (gears if
you like) should be somewhat matched to the servomotors or vicey
versey. A servomotor is essentially a current-to-torque transducer
but you must factor in its own moment of inertia into torque
calculations. That torque can be braking torque as well as
accelerating torque.
Yes -- how much precision is needed in the feed again? Were we
told, or did I just forget it?
If the wire comes from a large spool with significant moment of
inertia, then a secondary mechanism would be in order that feeds a
"wire buffer" like on a coil winding machine so that all the feed
mechanism need do is accelerate and decelerate a few feet of wire and
overcome the mild resistance of the springs or weights in the buffer.
The "dancing arms" -- which with the wire in question, would
have to be rather large and at the same time rather low inertia
themselves.
The spool would have its own servo drive that would run at fairly
constant speed with the buffer handling the reciprocating difference
in slack. If you haven't seen a coil winding machine, visit a company
that makes transformers.
I'm sure that he has -- or other "dancing arm" mechanisms.
This is all moot if you're determined to Rube it with a completely
mechanical solution.
:-)
Enjoy,
DoN.
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