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