Demistifying the PWM Myth

[r691175002]

The point of PWM and the number is that you are supposed to have discrete flashes from the laser. This is more efficient, repeatable, and control is more linear. Also pulsing reduces charring on some materials. This is because you get the advantage of high power instantly vaporizing the material, but since the laser turns off there is no excess radiant heat to brown the surrounding material.

I'm sure that TTL PWM control of discrete pulses is much more linear in average watts but that's fairly meaningless in terms of cutting and engraving. With the analog power control I can draw a brown line on a piece of cardstock. With analog control set for 40W even a 1/20000th second pulse will punch a hole completely through the card. That's why controllers vary both the analog power and use pulsed cutting.

I've got pulsed cutting working with EMC2 and I can cut 5mm plywood with 25% duty cycle and 40W power which produces a much different cut than 100% duty cycle (continuous wave) at 10W. Similarly I can engrave at low power on cardstock but it's impractical to use full power because I can't move the X stage fast enough to avoid burning completely through.

I am running at 100% current and only vary the PWM and can engrave very lightly on paper no problem. My lowest power won't mark the paper at 100% speed. I suspect EMC doesn't run fast enough to truly deliver PWM at 20Khz. Mach3 even on a fast machine will run at around 60Khz which leaves exactly 3 power levels if you try to do a 20Khz signal.

The EMC2 webpage also states that PWM is low frequency:
http://www.linuxcnc.org/docview/html/ma ... gen.9.html

That being said, I still play with the current for rastering since it is harder to fit PWM pulses in when the laser is already pulsing.