Re: [CAD_CAM_EDM_DRO] Synchronizing Two Servo Motors

Mattison wrote:

>I'm building a gantry router with dual lead screws to drive the gantry, and
>I need a way to make sure they never get far enough out of sync to rack the
>gantry out of shape. I thought about mechanical linkages but I want to use
>two motors for the extra speed and power, and I already have the motors.
>Does anyone know of a circuit that can take two encoders as input, plus the
>step and direction inputs, and output a fault signal if the two encoders get
>out of step? There are several ways I could build such a gizmo, but figured
>I'd check first to see if I can avoid reinventing the wheel.
>
>

As Alan said, you can do this with EMC2, but homing is a little more
complex.

The easiest thing to do is just run two step-to-servo drives (like
Geckos) off one set of step/dir signals. You can figure out some way of
mechanically homing, like by pushing the gantry against some end stops
(but not too hard).

One person using step-driven drives made a small logic board that would
prevent any steps from going to the drive if the home switch on that
side was closed and the direction was toward the switch. The two
switches (one on each side) also connected to the PC as a single axis
home input, so when both were closed, EMC would detect it and stop
moving. This way, whichever one closed first would stop, and they'd get
"in rack" as part of the homing process. Some of this can be done in
software as well - you can connect a switch to an input bit, then use
that as part of the home signal and also to disable step output from the
software step generator.

This can also work for servomotors with analog or PWM drive, but it's a
little more complicated to set up.

I think it's fair to say that there's no turnkey solution for this, if
you want to detect an out-of-rack condition while in motion. You can
set it up with EMC2, but it's not turnkey.

Note that Jarrett's setup (with Mach) can't detect if a motor dies until
a drive faults or a limit is hit. If you use steppers, then the drive
will never fault since there's no feedback. With servos you could get a
drive fault which would stop Mach.

- Steve