Re: Driving gantry with two paired stepper motors

rumancik@... wrote:

> 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. One is to use dual
> leadscrews and mechanically couple the two
> together, so that torque (and therefore force) is
> applied to both sides of the gantry. Another is to
> use two stepper motors. SuperTech uses this
> technique with their XYYZ 3-axis driver boards.
> They have two completely separate stepper driver
> channels for the y axis.
>
>
> I have seen someone suggest that you can couple 2
> stepper motors in parallel on a one-axis drive. I
> have tried this with a simple L297/L298 driver
> board. The two motors appear to be staying in
> phase with each other. I realize that you can't get
> the same torque from each motor (as you would if
> driven individually) but if you have a mechanical
> transmission, you also split the torque.
>
> My question: are there any potential problems with
> this technique, electrical or otherwise?

This is called a tandem gantry. All the large machines,
like Vertical Boring mills and gantry bed mills use this
approach. They have two completely separate servo
systems, with the same position commands sent to both
at all times, except when homing. The home sensors are
set such that the beam between the gantry towers is
orthogonal to the towers, when everything is homed.
They apparently put bearings on one end of the beam,
and a sliding connection at the other, so the beam and
gantry will not bind when out of coordination.

If you can come up with a scheme to home the two
steppers such that they are not just synchronized to
the same step phase, but actually have the beam level,
it should work. You might have a home position where
some slotted discs allow light to pass sensors when
the motor is at exactly the right position, and relays
could engage or disengage the 2nd motor. Then, just
a little jogging on one motor could get the pair
completely synched. Then, you put both back on,
and as long as no motor loses steps, it stays in sync.

Jon