Lasercad Um/Pulse or Pulse Unit Calculation Help

Discussions and help on this commercial controller.

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Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby techsavvy34 » Sun Sep 14, 2014 5:29 am

I'm trying to figure out how to convert steps per inch or mm to pulse unit. Does anybody know the equation?
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby Techgraphix » Mon Sep 15, 2014 9:13 pm

You have to know all about your driven axis.
Say, you have used a beltdriven carriage. On the stepper that drives this belt is a wheel with 20 teeth and the belt has a teeth-to-teeth-distance of 5mm.
One rotation of the steppermotor moves the carriage 20 teeth x 5mm = 100mm
This steppermotor needs 200 pulses per rev. but you also have a stepperdriver installed to drive your steppermotor. Say you have selected a resolution of 1/8 microsteps. That means you have to apply 8 microstep x 200 steps/rev = 1600 pulses per rev. So to travel 100mm you need 1600 pulses.. the distance per pulse is 100/1600 = 0,0625 mm or 62.5 um/pulse

Clear???

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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby cvoinescu » Mon Sep 15, 2014 10:39 pm

It's very easy if you spell out the units for each parameter. Then you can just look at the units and figure out whether you need to multiply or divide. :)

Using Techgraphix's example:
Belt pitch: 5 mm/tooth
Drive pulley: 20 tooth/rev
Stepper motor: 200 step/rev
Driver microstepping: 8 pulse/step

We want microns, and there are 1000 um per mm.

Now just chain them together so that the units cancel out:

1000 um/mm * 5 mm/tooth * 20 tooth/rev * 1/200 rev/step * 1/8 step/pulse = 62.5 um/pulse

Here's another example: your axis is screw-driven with a 12 tpi, four-start screw. You use a 0.9°/step motor. Yeah, absurd for a laser (or maybe it's your Z table), but bear with me, it's just an example to show how the units work.
Screw pitch: 12 thread/inch
Screw starts: 4 thread/rev
Motor: 0.9 degree/step
Microstepping: 8 pulse/step

We want microns, but the length unit we have is inch, so we start with the conversion factor:

25,400 um/inch * 1/12 inch/thread * 4 thread/rev * ...

What now? We don't have revs, but we have degrees, and one rev is 360 of those. That's 360 degree/rev. Keep going:

... * 1/360 rev/degree * 0.9 degree/step * 1/8 step/pulse = 2.645833 um/pulse. Ta-da!
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby techsavvy34 » Tue Sep 16, 2014 6:11 am

alright thanks guys. I figured out what my X & Y axis should be but i'm having trouble with my Z-axis. I'm using a buildlog 2.x laser cutter so my z-axis is a 1.8degree stepper motor with a 20 tooth mxl pulley attached to the shaft. The stepper motor pulley is connected to a 48 tooth mxl pulley with a mxl belt. The 48 tooth pulley is connected to a 1/4-20 threaded rod that drives the table. The only thing different about mine from the plans on this site is at the bottom of my 1/4-20 threaded rods I have GT2 20 tooth pulleys connected with a GT2 belt since I couldn't find a MXL belt that was the right length. If i'm correct the pulleys and belt on the bottom that connect the the threaded rods won't be part of the equation right? Also my stepper drive is set to 1/10 or 2000 microsteps but I might change it to 1/4 or 800 microsteps. So if you could help me figure out the Um/Pulse for both stepper drive settings I would really appreciate it. Thanks in advance.
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby cvoinescu » Tue Sep 16, 2014 1:04 pm

You are correct, the GT2 pulleys and belt are just making sure all theaded rods turn at the same time.

Input gear: 20 tooth/motor rev
Output gear: 48 tooth/threaded rod rev (not the same 'rev' as the input gear, because it's a different shaft, but the same 'tooth' because they're connected by the MXL belt)
Thread: 20 thread/in
Starts: 1 thread/threaded rod rev
Motor: 200 steps/motor rev
Driver: 1/10 step/pulse
Conversion: 25,400 um/in

25,400 um/in * 1/20 in/thread * 1 thread/threaded rod rev * 1/48 threaded rod rew/tooth * 20 tooth/motor rev * 1/200 motor rev/step * 1/10 step/pulse = 0.26458333333 um/pulse
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby techsavvy34 » Tue Sep 16, 2014 3:20 pm

Thanks cvoinescu. Could you tell me what my Um/pulse would be if my stepper driver was 1/4 step/pulse?
I'm having problems trying to figure out how you got 0.264583 Um/pulse. If I multiply 25,400 x 20 x 1 x 48 x 20 x 200 x 10 it doesn't come out to 0.264583.
I know i'm doing something wrong. Don't I have to divide some numbers too?
Thanks for your help.
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby cvoinescu » Tue Sep 16, 2014 3:29 pm

techsavvy34 wrote:Thanks cvoinescu. Could you tell me what my Um/pulse would be if my stepper driver was 1/4 step/pulse?

Plug 1/4 step/pulse instead of 1/10.

Or, to save you some work: smaller microstepping factor means fewer pulses per step; in other words, your axis moves more per pulse. How much more? 10/4 times as much: 0.66145833333 um/pulse.

techsavvy34 wrote:I'm having problems trying to figure out how you got 0.264583 Um/pulse. If I multiply 25,400 x 20 x 1 x 48 x 20 x 200 x 10 it doesn't come out to 0.264583.
I know i'm doing something wrong. Don't I have to divide some numbers too?

Of course you do. Look carefully at my calculation. You have to do:

25,400 * (1/20) * 1 * (1/48) * 20 * (1/200) * (1/10).

That is, 25,400 / 20 * 1 / 48 * 20 / 200 / 10 = 0.26458333333.

That's the whole point of the unit "trick", to figure out whether you need to multiply or divide by each parameter.

techsavvy34 wrote:Thanks for your help.

You're welcome.
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Re: Lasercad Um/Pulse or Pulse Unit Calculation Help

Postby techsavvy34 » Tue Sep 16, 2014 3:42 pm

Alright I figured it out. Thanks.
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