Re: [CAD_CAM_EDM_DRO] Encoder/Servo question

On Fri Jul 16 16:29:23 2004 Leslie Watts wrote:

>Hi Bill
>A dividing head or rotary stage might have a lot of backlash and friction
>(such as a typical worm drive rotary table) or it might have very little.
>I am having to design one now for a company that has no backlash.
>
>Backlash and stick/slip cause some difficulties with servo systems. That
>is because they are non-linear. I am using the word non-linear in the
>engineering sense...That means that the motions cannot be described by a
>linear differential equation. It's a bit different from describing circular
>motion as non-linear.
>
>That being said... servo systems operate routinely with a good bit of
>stick slip and other non-linearities. They can partially correct for some of
>them,
>even though phase margin (stability) might be a little lower.
>
>Are you trying to build a servo driven rotary axis?

It's on the, "todo", list. At the moment, though, I'm just trying to put
together basic info on things. I finally saved up enough pennies to get a 7X14"
Minilathe, and hopefully, will soon be looking at a Minimill. Ya gotta start
somewhere. I'll be trying to do a manual rotary first, and once the bugs are
worked through, a servo job. That being said, I'm trying to figure out some
basic ballpark stuff, like what degree of accuracy I can get out of one, hence
the encoder line count vs % of rotation question. Assuming a 360:1 reduction,
a 1000 line encoder would theoretically give you one thousandth of a degree
accuracy, yes?

On another tack, there's the question of what thread to use. How would a
28/29/29.5/30 degree. (whichever), acme thread compare to the 40 degree gear
type thread for the worm/wormwheel? Seems like it should have better torque
transmission, but whether they would be smoother acting or even more or less
suitable for cnc-ing, one over the other, is something I haven't seen discussed
in this context.

Bill


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