Need Help!- pwm control of laser - slightly confused about what it means

[geekinesis]

I am fitting my laser with a tb6560 board (ebay) and linux emc

At the moment it is set up to switch the laser on and off with the spindle relay and using the spindle on off commands within g code

I dont think I can use this particular board to control the laser with pwm although the laser power supply allows this option.

The laser power supply allows the option to control the laser power either with the existing potentiometer or with pwm. Does this mean the pwm signal just tells the power supply to set the power at a certain level - similar to the pot - or is the laser tube itself being switched on and off rapidly to adjust the power?

Anyway its not a major issue as I can still use the potentiometer as before, it would just be more convenient to set different power levels for different parts of the drawing using g code

[Tweakie]

Don't know anything about emc because I use Mach3 but I found that the spindle control from GCode was not fast enough to turn the laser off without producing a small dot at the end of each line. Sometimes this was not noticable but it always seemed to show when I didn't want it to.

PWM is Pulse Width Modulation which is essentially variable duty cycle - a 50% PWM is on for 1/2 the time and off for 1/2 the time repeated at a rate of say 5kHz (This represents a 5Khz square wave with 50% duty cycle). On most controllers the frequency (PRF - Pulse Repetition Frequency) is also adjustable from say 500Hz to 20 kHz.

After a lot of experimentation I have found no need to adjust laser power from within a GCode program, so I am happy for manual setting of the PWM.

I am currently working on a (low tech) controller / interface for a Synrad RF laser to be controlled from Mach3 and details of my (slow) progress can be found here *RF Laser

Hope this helps.

Tweakie.

[twehr]

I don't use/know EMC, but as Tweakie said, the spindle controller will not provide you with what you want of firing the laser.

Over at MachSupport, I wrote a detailed explanation of how to fire the laser cleanly using Mach3. It may give you some ideas on how to approach the same thing in EMC.

As for the PWM, you replace the variable resistor controlled 0-5v power control at your laser power supply with the PWM signal. The internal circuitry on the ps will interpret the PWM signal (actually may just be smoothing) to provide the correct 0-5v level.

tim

[geekinesis]

Hi thanks for the info. That makes sense.

I will probably stick with the analog control as well for now.

I went with emc because the spindle on off isn't delayed and also because I find the interface is much less cluttered (= less scary)

Plus it is free and can be installed along with ubuntu as a complete package.

[geekinesis]

Ok thanks. It's good to know there is a mach solution as well.

[twehr]

Originally Posted by geekinesis Hi thanks for the info. That makes sense. I will probably stick with the analog control as well for now. I went with emc because the spindle on off isn't delayed and also because I find the interface is much less cluttered (= less scary) Plus it is free and can be installed along with ubuntu as a complete package.

Unless you have a volt meter in your circuit for measuring what you are actually feeding the ps, the pwm will be much more accurate. You will always know that 50% = 2.5v for example. It is more repeatable than setting a vr without the additional v meter.

FYI, 50% (example) does not necessarily mean 50% power but if you know that % gives you the results you want, then it should always give you the same results. If using a vr and ma meter, it is much harder to precisely set the power next time to match previous experiences.

[geekinesis]

Yes the accuracy is one thing that would prompt me to try pwm

So I just need to work out how to interface my board to the laser pwm input and then work out in emc how to send the pwm signal?

I can send pwm spindle speed signals but the board only has a relay for off on. I imagine I would have to bypass a couple of unused pins on the parallel port to send the pwm signal directly from the pc?

I am not an electrician so don't want to experiment too much.

[methuso]

Not hijacking or stealing Your thread.. .but....

Im at the exact same stage... being electrically challenged i need to connect my chinese-ebay-psu to my breaktout board too... Also using the EMC2 package.

I've got this answer in another forum:
"what you can do is connect the laser PWM input to the step pin of an unused axis, then set Mach3 to use that as the spindle motor". this answer is from a person using Mach (oviously).

then How about testing?? can one just connect a voltmeter to the 25kVolt output from the PSU without connecting the tube?? or will that too let out some magic smoke ??

Hope its ok to ask here since inte already brought up and if one get an answer in this thread, we all get an answer
/Ulf

[Tweakie]

then How about testing?? can one just connect a voltmeter to the 25kVolt output from the PSU without connecting the tube?? or will that too let out some magic smoke ??

Please don't connect anything to the 25kV output of your PSU except the tube. (the current limiting should prevent death but there are dangerous voltage here).

Tweakie.

[methuso]

hmm ok. i was thinking of connecting it before plugging in the mains. but i wil not.
any other ways of testing it without connecting the tube first ??

Off to work.. will check back in the morning
/Ulf

[DaveDoesIT]

Hi Tweakie,

A bit of a thread hijack, but I was just looking at your PWM project. What software do you use to generate the GCode for the isolation routing?

Thanks

Dave

[Tweakie]

Originally Posted by DaveDoesIT Hi Tweakie, A bit of a thread hijack, but I was just looking at your PWM project. What software do you use to generate the GCode for the isolation routing? Thanks Dave

Hi Dave,

I usually use Roland vinyl sign cutting software to create the outline then convert the HPGL to GCode. The programs SignGo or FrontDesigner would work just as well.

Tweakie.