Adding second X stepper to XYZ router

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22 Feb 2011 02:03 #7364 by macmarty15221
I have a homemade gantry router rig. Control is based on a Geckodrive G540, which was easy to set up with StepConf.

I've noticed some yaw problems on the X axis, so I have obtained all of the hardware needed to install a second identical leadscrew assembly. One leadscrew will drive the near-side (Y=0) side of the gantry, the other will drive the far side of the gantry. (Y=30") To my newbie eyes, It looks like this is referred to as a XYZX configuration. The G540 will make the hardware a snap.

Today my question is "where do I start" on the EMC configuration side? I see a few threads here relating to debugging XYZX gantries, but nothing that helps me understand the jumping-off point, some general context in 25 words or less. :P I have a good software background, but I'm a total newb in this neighborhood. If someone can grab me by the shoulders and get me started in the right direction, it would be enormously appreciated. I really hate crafting square wheels when I suspect that perfectly round ones already exist.

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22 Feb 2011 09:08 #7367 by goaran
if you add a second stepper to the axis you dont have to configure anything.
Just connect the dir / step pin from your PC to both stepper controllers.

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22 Feb 2011 11:28 #7371 by BigJohnT
The first thought I had was gantrykins. There is an example but it includes a THC so you would have to strip that out. ATM, I don't know anything about gantrykins and what it might add to the functionality of a gantry vs just tying both step and direction pins to the same output and or tying them together in hal.

The next thought that crossed my mind was homing... are you using home switches? If not then you could physically push the gantry to a square stop and turn on the machine power then home the Y axis.

I solved this on my plasma table by using a jack shaft to drive both sides of the gantry from one motor but this is not always a good solution depending on the design.

John

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22 Feb 2011 12:44 #7379 by Rick G
I use the same basic set up that John spoke of.
Without a seperate homming switch for the motors I just push them aginst their stops before turning EMC on.
I used a standard set up and just edited the hal file to send the signals to two sets of parallel port pins, one for each Y motor, you can also invert the signal to one pin if needed.
There is more on this posted elsewhere on the forum.

Rick G

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22 Feb 2011 17:40 - 22 Feb 2011 18:09 #7381 by BigJohnT
It seems like EMC can home gantries to two separate home switches on the gantry now since 2.4.x. The stepper-gantry config shows how that is done. You just have to remove the THC part for a normal gantry. It won't home a gantry to a index pulse from an encoder but from normal home switches.

More info, it homes two gantry joints at once and each one does a full homing sequence to its own switch.

I assume you just need to make sure axis 1 and axis 3 (for a Y axis gantry) have the same homing values and homing sequence.

John
Last edit: 22 Feb 2011 18:09 by BigJohnT.

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26 Feb 2011 17:52 #7475 by macmarty15221
Thanks, guys. Your varied solutions provided just enough oomph to get me past my blind spot. The solution was so simple that I need to give myself a dope-slap.

The GeckoDrive G540 can control four steppers, right? X is on pins 2 and 3 of the parallel port. Y is 4 and 5, and Z is on 6 and 7. The fourth controller is on pins 8 and 9. I just set 8 and 9 to ALSO drive the X axis, let's call that X'. Works like a charm! DUH! I'm guessing it's all handled by the HAL. YAY!

Now back to the basement to bolt the assembled X' stepper assembly in place to see what we get.

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26 Feb 2011 19:22 #7476 by BigJohnT
Glad you got it sorted out. Post a photo of your machine if you get a chance.

John

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27 Feb 2011 17:06 #7494 by andypugh
Gantrykins adds some complexity to this scenario, but also some functionality.
It might be worth trying it to see if you like it.

Make a copy of your existing config directory, and rename it to create a cloned config.

You need to edit the new HAL file so that the line

loadrt trivkins

instead reads

loadrt gantrykins coordinates=XYZX

This creates a system where the G-code X position commands are sent to joints 0 _and_ 3. (in this context, joints == motors). You need to make changes to the HAL file so that the new joint position commands appear on a P-port pin pair, and INI file changes so that the scale and direction of the new motor are correct.

The advantages are that the two motors can home independently, (auto-squaring) and it also becomes easier to have the motors running in different directions or with different gear ratios (or, perhaps more usefully, different leadscrew compensations)

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27 Feb 2011 18:26 #7496 by macmarty15221
AH, now THAT is the exact info I was looking for. Thanks, Andy!

Seems like Sunday is EMC day. I just now got the second X joint to work, using the simpler approach I mentioned in my last post. I'm tickled pink with THAT small accomplishment. I have a short-deadline project back on the rails now, one that only requires woodworking-level tolerances.

Homing and limits are definitely in order soon, I certainly don't like sound of steppers pushing against a hard limit.

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27 Feb 2011 19:58 #7497 by andypugh
macmarty15221 wrote:

Homing and limits are definitely in order soon, I certainly don't like sound of steppers pushing against a hard limit.


Actually, this is not a real problem, as long as the rest of the system is strong enough.

Also, I forgot to mention that you could look in stepper_gantry in the sample configs to see how to set up gantrykins.

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