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Topics - jtodd

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General Mach Discussion / 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

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Hi -
  Sorry if this isn't the right place to put this, but it seems like the only place.

  I've got a CNC plasma table that I've imported from China, with a "built-in" computer.  It is not ideal.  The mechanics of the table are GREAT, but the computer needs to be replaced and this all needs to be connected to Mach3 directly.  I need someone who can:

 - tell me which of the Chinese parallel interface boards are best
 - wire up one of those interface boards on a test rig
 - wire up a board for Z control (it's not a stepper; it's just a motor)
 - create a wizard for Z control, if necessary
 - source a number of parts for building RF-shielded enclosures (DB-25 interfaces)

I'm smart enough to bodge something together myself, but I'm looking for someone that has done this kind of thing before and can deliver a "product" instead of a one-off hack.  Of course, I'll pay for all the supplies and will ship the head unit (just the control box) to you for testing.  Location is unimportant if you have some references, but anyone in the Pacific Northwest will get priority since I can meet you.

Contact me at jtodd@loligo.com.

JT

keywords: consulting, consultant

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