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Author Topic: Twin Steppers on slave axis  (Read 9736 times)

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Offline Katoh

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Twin Steppers on slave axis
« on: September 18, 2008, 01:44:54 AM »
Hi To All
This is my first post so please excuse me if its in the wrong area.
If have built a 6' x 10, 3 axis router based on the design by Tom (cadcut). The trouble I'm running into is I have run twin steppers on both X and Y axis, The Xaxis is rack and pinion style drive both motors are connected to a drive shaft via bully and belts, the Y axis is a ballscrew and again both motors are connected to either side of the ballsrew by pulley and belts again, Both X and Y axis are configured in mach3 as Master and Slave All Fine! When the machine works it tends to get a lot of motor lockup and tuning and speed is way down to what it should be. If I disconnect one of the belts from either motor and run that axis I can achieve 5x the speed without loosing steps, I have come to the conclusion that somewere the motors are fighting each other or they just are not getting the same signal.
Has anyone outhere got any suggestions?
Specs on the Machine.
Steppers Nema 34 900oz/in
Gecko 201 drives (one per motor)
Power 48v 20 amp
On both X and Y axis one motor CW the other CCW
Cheers
Katoh

Offline Greolt

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Re: Twin Steppers on slave axis
« Reply #1 on: September 18, 2008, 01:54:31 AM »
Get rid of the connecting shaft between the X axis motors.  Let them each drive one side.

Also ditch the second motor on the Y axis.  One should be fine.

Well you did ask for suggestions.   ;D

Greg

Offline Katoh

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Re: Twin Steppers on slave axis
« Reply #2 on: September 18, 2008, 01:59:52 AM »
Thanks for the suggestion I dont realy want to go to that extreme yet.
Cheers
Katoh

Offline ger21

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Re: Twin Steppers on slave axis
« Reply #3 on: September 18, 2008, 11:16:05 AM »
I'd call putting two motors on one screw going to extremes. :) I don't think I've ever seen anyone mount two motors on one screw like that. But I have read posts advising against it, for the reasons your seeing. The twio steppers will always be fighting each other. One thing you can try is loosening the pulleys, power up the steppers, then tighten the pulleys. That will get them aligned as close as possible, but it still might not work correctly.
Gerry

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Re: Twin Steppers on slave axis
« Reply #4 on: September 18, 2008, 12:23:48 PM »
Hello Katoh,
    You didn't mention if there was a reduction ratio with your pulleys. Either way, you could reduce the current ratio by 2:1 and use 1 motor. It would then have the same torque as two motors at 1:1 but of course the speed would be cut by half. You may then be able to increase the speed in motor tuning, upping the kernel speed or getting yourself a SmoothStepper.
Or go to a single larger motor......like a COOL AC servo. 
RC

Offline Katoh

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Re: Twin Steppers on slave axis
« Reply #5 on: September 18, 2008, 09:05:31 PM »
Thanks for the Info
for Overloaded my ballscrew only has a 5mm movement per turn so my ratio is actually 1:1.3 this gives me about 7mm per turn.
I suppose I could live with having one motor on the Y. My problem now lays with the X if I break the driveshaft as Greg suggested whats going to stop the whole thing from binding. Just a Thought could it be I am using two two breakout boards C10 from CNC4PC that Ive taken the master from BOB1 and the Slave from BOB2?
Cheers
Katoh

Offline Greolt

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Re: Twin Steppers on slave axis
« Reply #6 on: September 18, 2008, 09:36:59 PM »
One thing you could try is have the two drives share one step and direction signal.

Not suggesting as a permanent fix but would be easy to try and interesting to see the result.

If needed to reverse one motor then reverse one winding connection.

Greg

Offline Katoh

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Re: Twin Steppers on slave axis
« Reply #7 on: September 18, 2008, 10:10:48 PM »
Hi Greg
Tried Doing that run probably a little better, tried Even taken both signals from the one board really made no difference. all I can see is I'm going to have to run the Y on one motor. The X its a good idea to cut the shaft in halve but I still have my reservations about racking and binding. Looks to me that if one motor looses a step the the other motor jams up is it ties to correct it. I wonder if it would be any different if both motors were turning the same way?
Cheers
Katoh

Offline Greolt

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Re: Twin Steppers on slave axis
« Reply #8 on: September 18, 2008, 10:41:54 PM »
"Looks to me that if one motor looses a step the the other motor jams up is it ties to correct it"

That concern is shared by a lot of builders but not those who have tried it.

Many machines run independent slaved drives.  Mine included.  Mach handles it well.

Stepper driven machines need to operate within their performance envelope. 

If steppers are loosing steps then you need to retune or redesign.

My way of seeing it is one motor driving both pinions via a linking shaft is a legitimate way of doing it.

Independent slaved drives definitely works.   Two motors hard linked is not the way to go.   IMO of course.

Greg
« Last Edit: September 18, 2008, 10:50:07 PM by Greolt »
Re: Twin Steppers on slave axis
« Reply #9 on: September 18, 2008, 10:52:04 PM »
Katoh,
If you are using 2 BoB's, you should probably have your X Master and X Slave on ONE BoB, and the Y Master and Y Slave on the OTHER BoB.
The way it is, it sounds like you have them mixed between ports....Probably not good.
RC