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Author Topic: Gravity vs. DC motor on Z-axis - gravity wins.  (Read 2970 times)

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Gravity vs. DC motor on Z-axis - gravity wins.
« on: October 10, 2011, 01:32:18 AM »

I'm working on a plasma table that has a Z-axis that is managed by a DC motor.  Not a servo, not an encoded motor, not a stepper: a DC motor.  Give it +/- and it goes up.  Give it -/+ and it goes down.  No power means it stays put.

For various reasons, I can't get rid of this setup- I have to stick with the motor I've got, and can't physically retrofit to something more useful nor can I really do things like add springs or counterweights.  Currently, I've got Z-axis fed into a chip that turns the square wave into DC power at 18V, and inverts the polarity when the direction bit gets toggled on the parallel port.  So far, so good.  The problem is that due to gravity, the motor and arm moves down faster than it moves up.  This leads to all sorts of big problems - it's around 33% faster down than up, so very quickly the head starts dragging on the metal as I go through various lifts/lowers for doing pierces.

Is there a way (brains? macros?) to somehow tune Z-up movements so they have a longer energized time "per inch" than Z-down movements?   I'll move to the appropriate forum if someone can give me the hint.  I'll pay for an open-source software solution if someone has one...

JT
Re: Gravity vs. DC motor on Z-axis - gravity wins.
« Reply #1 on: October 10, 2011, 01:52:29 AM »
No reason you couldn't fit an encoder to it, and use a DC Servo drive, like a Gecko G320.....  That would allow you to slow down the up moves to the same speed as the down moves, and ensure it was always at the desired position.

Regards,
Ray L.
Regards,
Ray L.
Re: Gravity vs. DC motor on Z-axis - gravity wins.
« Reply #2 on: October 10, 2011, 02:00:04 AM »
No reason you couldn't fit an encoder to it, and use a DC Servo drive, like a Gecko G320.....  That would allow you to slow down the up moves to the same speed as the down moves, and ensure it was always at the desired position.

Regards,
Ray L.

I'm really not able to do that at this point, for various reasons.  I can't modify the equipment at all for various reasons outside of technical issues, so it has to be via software.

JT
Re: Gravity vs. DC motor on Z-axis - gravity wins.
« Reply #3 on: October 10, 2011, 09:16:37 AM »
In that case, your only reasonable option would be to modify the G-code.  Since your hardware sounds like it simply turns motor power on whenever there are step pulses coming out, just make the up feed slower than the down feed, which will make the up moves take longer than the down moves.  That should do what you need.

Regards,
Ray L.
Regards,
Ray L.

Offline BR549

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Re: Gravity vs. DC motor on Z-axis - gravity wins.
« Reply #4 on: October 10, 2011, 10:25:21 AM »
To answer your question NO it cannot be done with mach3.

You have been over the net talking about the bad hacks this and that BUT so far nothing has compared to the original BAD hack to the Z axis. (not even CNC related)

MAN up and just fix the thing correctly.

The only crutch I see is to use a counterweight pulley an cable system. That way the weight will reamain constant across the range of movement. Adjust the weight until you have comped for the  gravity effect.

Still does not solve your positioning problems.

(;-) TP
« Last Edit: October 10, 2011, 10:36:40 AM by BR549 »
Re: Gravity vs. DC motor on Z-axis - gravity wins.
« Reply #5 on: October 10, 2011, 01:17:44 PM »
HI JT,

Could you explain why you can't do any "physical" mods?, such a spring counter weight.

Maybe could do this electrically,  have 2 voltages, a low voltage for Z down and a higher voltage for  Z up, to balance the speeds.
However, the longest way is always the shortest...in the end.

ATB
Derek.
« Last Edit: October 10, 2011, 01:20:14 PM by derekbpcnc »
You can "chop it off" but can't "chop it on"