RE: [CAD_CAM_EDM_DRO] Re: Synchronizing Two Servo Motors

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Frank



From: CAD_CAM_EDM_DRO@yahoogroups.com
[mailto:CAD_CAM_EDM_DRO@yahoogroups.com] On Behalf Of Yahoo
Sent: Wednesday, March 11, 2009 1:07 PM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: [CAD_CAM_EDM_DRO] Re: Synchronizing Two Servo Motors



Jon,
Thanks, to you and a couple of other guys, for the ideas on big machines,
but this is a small one, only 24" travel X & Y. I've thought of many
different mechanical solutions, but every one ends up being too complicated.
The only practical one would be to slave the two screws by a chain or belt,
but it seems like this could cause the motors to fight, leading to potential
oscillation problems or excess current when idle. Setting the controller to
stop everything when any channel faults, and slaving two channels for the
dual lead probably would work, except in case one of the motor couplers
failed. This failure also would not be covered by any electrical solution
that relies on the motor encoder signals, as they are mounted on the motor,
not the lead screw. The only really fail safe solution would be a
differential switch controlled by a timing belt at the far end of the
screws. If only I could find a belt long enough. It would need to be roughly
6' and anything too thick would eat up a lot of torque just in friction.
Longest I've seen in the 3/8" width range is about 4'.
--
Phil Mattison

----- Original Message -----
From: Jon Elson <elson@... <mailto:elson%40pico-systems.com> >
To: <CAD_CAM_EDM_DRO@yahoogroups.com
<mailto:CAD_CAM_EDM_DRO%40yahoogroups.com> >
Sent: Tuesday, March 10, 2009 7:50 PM
Subject: Re: [CAD_CAM_EDM_DRO] Re: Synchronizing Two Servo Motors

> Yahoo wrote:
> > I thought about sharing a belt but haven't been able to find a belt long
> > enough. Seems like it would be sort of a kludge anyway.
> >
> > I guess if this has been discussed ad naseum and nobody has a ready
> > solution, then it probably has never gotten beyond the talking phase.
> > Apparently I'll have to build something from scratch (sigh).
> >
> No, I think we DO have a solution. There have been several schemes,
> depending on whether the gantry is rigid or flexible.
> In big commercial machines, the beam across the gantry sides has a pivot
> on one side and a slotted bearing on the other.
> This allows the gantry sides to get a certain amount out of sync without
> any binding. They probably put an angle limit switch on the pivot so if
> the angle exceeds a certain amount, it E-stops the machine. Then, they
> have a scheme for homing the two sides of the gantry simultaneously.
> The two sides run at the same (servo controlled) rate. Each one hits
> its own home switch and completes the precision homing sequence by
> itself. After that, the tandem axis stays in sync as well as the servo
> can, which will usually be quite close. So, that is how the "pros" do
> it on $100K plus machines like gantry bed mills and vertical boring mills.
>
> I think you could do the same on a smaller machine. It would also allow
> you to adjust the orthogonality of the machine by just offsetting the
> home position in software until a square came out square. But, this
> would require buidling in the joints in the gantry.
>
> Another scheme I've heard, but not sure it has been used, is to home one
> side while allowing the other side to just coast with the servo disabled.
> When the "master" side is homed, then the slave side servo is enabled.
>
> I know EMC could easily be set up to independently home the two sides of
> a tandem axis, as described in my first paragraph. It would take very
> little special HAL coding to do this.
>
> Jon
>
>





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