Re: Driving gantry with two paired stepper motors

rumancik@... wrote:

> From: rumancik@...
>
>
> I have a question concerning the use of stepper
> motors to drive a gantry on an xyz table. One
> problem that sometimes arises is that if the gantry is
> driven on one side only, it exhibits a slip-stick
> phenomena, especially if there is a cutting load
> furthest away from the driven side. There are a few
> solutions that I have seen. --

Richard,My machine is dual ballscrew to move the 800 lb gantry. You are exactly right about the problemsof driving with only one
screw.(Or at least that is what my finite element analysis led me to conclude)

I think there could be a real problem with running two steppers from the same pulse source.
Most ballscrews Have significant pitch errors. For example I am using C7 accuracy class
preloaded screws from Hiwin. E300 for these is .003". It is possible to develop .02"+ error
in nut position difference over the length of the screws. On my machine the resultant cocking of the
gantry would put unreasonable forces on the structure. Even if you pay a lot more for
high accuracy ground screws the error can be many thousandths over several feet.

What can you do?

1) use two steppers or servos with separate controllers. Error map out the pitch difference
in software. So for your steppers step rate would be _slightly different for the two during
a move.

2) Allow limited articulation of one end of the gantry in the xy plane with a bearing system.
This lets it develop cocking errors without big reaction forces or a loss of stiffness.
Map out the error with software.

3) Just make the structure flexible. This would be no good for me as most of
the structure has a design stiffness of .001"/100lb.

4) Buy matched ballscrews. They are available at a big price premium.

I am doing #1. It does require two axes for the gantry motion but is
the easiest and quickest to implement IMHO.

Leslie M.Watts
Illinois Tool Works
Corporate Advanced Technology group
(847) 657-4559
http://www.netcom.com/~leswatts/leswp.html