[m_c]

Is it possible and what's needed to sychronize 2 steppers or servos to control the one axis?

The reason is that for a plasma table i'm designing, I require the gantry to be powered on both sides of the x-axis, and was considering using 2 seperate motors instead off a using a drive shaft to both sides.

[ger21]

Mach 2 has a slave feature to use two motors for 1 axis.

[metlmunchr]

Mach2 control software supports slaving. You would just set up the a-axis slaved to x. Other software based controls may offer this option too, but I don't know.

[tekno]

I have been thinking of a drive.. I think you would have to have a box of steppers or servos and test them all to get a matched set. But Im new at this so you should see what the experts have to say.

[m_c]

Thanks for the quick replies ger21 + metlmunchr. I was planning on using Mach 2 for it's Torch Height Control support anyway.

My plans for the x-axis are coming along quite nicely.
Just need to figure out how to use TurboCad to make some proper plans now.

Thanks
Moray

[Al_The_Man]

Both the Galil and Acroloop motion cards have a the ability to slave one axis off of another for parallel servo control as in a gantry system.
Al

[sol]

I once talked to Mariss (GeckoDrive) about this and he said the motors will always be fighting each other and it is better not done if at all possible. Use belts and one larger motor. He was quite adamant about this. Just because the software can send the orders does not mean the hardware can respond as intended.

[Al_The_Man]

This has not been my experience with the Acroloop card, this feature works great, also I know of two major US plasma table manufacturers that use the card this way in their products.
Al

[m_c]

I have been thinking about this some more, and was wondering what would happen if one of the 2 motors ran slightly faster than the other one. Would this lead to the gantry going off-square on long movements, or would the software/hardware detect this and correct it, before it reached the end of the move?
I was planning on using either rutex or gecko drives, with Mach 2.

[ynneb]

Moray, I have considered this too for my next machine. I descided the expence of another servo and controller card was more than just running a drive shaft across the table. Depending on how you are going to drive the gantry, you could fix the drive shaft to the gantry itself. To me there is something more positive using the drive shaft method, and less chance for electrical errors. I can just imagine what would happen if you had a run away servo on one side. It would be very hard to make a limit switch method to avoid a twisted gantry.

[metlmunchr]

I've seen Mariss's response to this as well, but only when the question involved someone putting two motors at opposite ends of one screw to try to make use of motors that were too small to work effectively by themselves. Possibly he has the same opinion when the application involves driving opposite sides of a gantry with slaved motors, but I've never read a post from him indicating this was verboten. This two motor setup is used extensively on commercial plasma cutters today. The difference between hard coupling two motors via a shaft or screw and slaving them to drive opposites sides of a gantry lies in the compliance of the gantry. Hard coupling two steppers, for example, brings the inaccuracies of their mechanical construction into play. If a partiuclar full step in one of the motors is 1.82 degrees, and the corresponding step in the other happens to be 1.78 degrees, they're obviously going to be fighting one another when hard coupled, since each will be attempting to come to rest at a different postion and will be trying to wind up the shaft to do so. The angular alignment of the motors to have both at full step position when they are attached to the common shaft also becomes critical. Spread this difference out across a gantry, and the resultant racking would be far less than what might be expected to occur from lead errors in a pair of screws driven by the same motor. Obviously, only Mariss can clairfy his own position on this particular arrangement. In using a cross shaft to drive both sides from one motor, attention must be paid to the torsional stiffness of the shaft, and to its being of sufficient diameter to prevent whipping at speed. A tube generally addresses both these issues far more effectively than a solid shaft of increasing size. The tube can have a very thin wall and yet exhibit high torsional stiffness, approaching that of a solid shaft of equal diameter, while at the same time having far less rotatiing mass to induce whip at operational speeds.

[Kookaburra]

It is always a good Idea if you are dual driving the gantry to allow the homing of both sides. While some pieces of software will allow you to slave one axis to another, they do not allow a homing limit input for the slave side. I have heard that a procuct is soon to be released that will externally slave two motors (thus not tying up one of your software axes) and allow the independant homing of both sides. As soon as I find out more about it I will let you all know.

[Bent]

We have a big flamecutter at work and there are one motor at each rail. It is controlled by 2 encoders directly on the rails not on the motors. Would this not eliminate the problem of driving the motors evenly

Bent

[metlmunchr]

Independent homing is evidently possible in Mach2. Tom Caudle (father of the THC hardware) mentioned in a recent DIY-CNC post how his plasma rig is set up to independently home each side and re-square the bridge in the process. Independent homing would be a definite asset to allow for easy correction for that inevitable time when a burnout pops up and causes one or the other of the motors to lose a few counts. An external slaving device would allow a 3 axis control to run a machine like this, whereas a minimum of 4 axis control is required now.

[m_c]

I think some research into the independant homing will have to be done, although the gantry could probably be easily trued by giving the motors a quick turn by hand.

The idea of using running a drive shaft for both sides would be hard to implement on my planned design (will post some skecthes in the plasma forum hopefully tomorroe), as it would require transmitting drive through 90deg at each side of the gantry.

I've been looking at the gecko drives, and see that they fault if the posistion error exceeds +/- 128 steps. This would mean that the most the gantry could be off square is 256 encoder steps, before the gecko drive would register a fault. Obviously the amount the gantry would be off square at that point will depend on the encoders + gearing.

The only thing that concerns me just now is regarding the mach 2 software. If 3 axis are needed for the x-axis + y-axis, would I still be able to add the Torch Height Control at a later date?

[DSL PWR]

I think the thc has it's own driver and runs off of a second port on the pc.