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delay in PWM pulse (LaserMode)
« on: January 14, 2013, 03:18:00 PM »
hi, i've made new machine: Laser CNC
but have a problem with pwm, it's running in LaserMode and PWM spindle control(at500hz), i've made a circuite with microcontroller to convert it's frequency to 30khz(as mentioned in PSU chineeeeeeees  DATA*********) and using e1p1-e1p0 to firing that (as Tweakie.CNC noted in other threads) i'm using versiong v3.043.066 (my older version had  problem with instant on/off of output1), there is no problem and duty cycle changes with speed, but at the begin of a cut it has delay that cause the shape can't be cut at  about 14mm of the start of that, in otherwords the head moves (and output1 is ON without no problem) but PWM signal doesn't act until passing about 14mm, is there any problem with mach or my settings??


thanks of any reply ;)

Offline Tweakie.CNC

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Re: delay in PWM pulse (LaserMode)
« Reply #1 on: January 15, 2013, 02:29:29 AM »
Hi bdn,

In Config / Ports & Pins / Spindle Setup - check that your Spin Up / Down Delays are all set to zero.

(I have found a number of problems using the Mach3 generated PWM for laser power control so no longer use it).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: delay in PWM pulse (LaserMode)
« Reply #2 on: January 15, 2013, 01:54:58 PM »
so much thanks of ur reply
you are right, i'll try it ,but i have found another issue which may help others.
it is with psu (match with my 80w reci tube), as user guide says it operate at 29ma for maximum 80watt optical output, so with a 10% duty cycle there is about 2.5mA and there should be about 10% of output power, but the tube doesn't act at all before reaching to more than 3mA so the machine doesn't cut some first mms of any shape.
in laser mode there is always outgoing pulse of computer with very low duty cycle i thaught with a 10% minimum PWM dutycycle in configuration will force it to be 10% , but it doesn't change even if i put it to be 90%  ???

Offline Tweakie.CNC

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Re: delay in PWM pulse (LaserMode)
« Reply #3 on: January 16, 2013, 02:20:38 AM »
Hi bdn,

Please ignore what I said about the Spin-Up / Down delays, it will make no difference – My head was still full of another post I had made regarding over-burn with the use of the M3 / M5 command set.  :-\

I think you have identified the cause. My tube requires a minimum of 4mA to fire instantly at it’s first trigger (this would translate to 20% PWM). Thereafter it will fire at 2mA if the next pulse is within approx 100mS of the previous pulse.
The RF tubes have a slightly larger threshold so they overcome this problem by providing the 5kHz ‘tickle pulse’ before and in-between triggers.

I don’t fully understand the physics behind this but it appears that the various gasses within the tube need to be mixed / agitated in order to keep the tube in a state or readiness so you could perhaps try modifying your GCode to include an initial pulse or two (outside the work area) in order to prepare the tube before starting the cut.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: delay in PWM pulse (LaserMode)
« Reply #4 on: January 16, 2013, 05:03:02 AM »
hi Tweakie
so it can be general issue, but i think it's not the tickle pulse problem (bec tickles are so short about us) i can't imagine away to change gcode for a minimum DC, but instead it's easy to change my Ccodes in AVR micro to do that, hope soon reply the resualt.
what about the minimum DC in mach config, it has no effect on the outgoing pulse of com, i've set it to 50% and the worse resualt, it doesn't pulse untill to reach 50% of feed rate and at that point starts with 50% DC, so more uncutted path of the shape! :-[

Offline Tweakie.CNC

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Re: delay in PWM pulse (LaserMode)
« Reply #5 on: January 16, 2013, 11:39:23 AM »
Quote
what about the minimum DC in mach config, it has no effect on the outgoing pulse of com, i've set it to 50% and the worse resualt, it doesn't pulse untill to reach 50% of feed rate and at that point starts with 50% DC, so more uncutted path of the shape!

Hi bdn,

Sorry, I can't be of much help with that one as I don't use Mach3 to control my laser output power via the PWM pulse train.

Tweakie.

 
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.

Offline Tweakie.CNC

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Re: delay in PWM pulse (LaserMode)
« Reply #6 on: January 16, 2013, 12:30:27 PM »
Hi bdn,

It would be helpful if you could zip and attach a simple GCode program which demonstrates the problem.

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.
Re: delay in PWM pulse (LaserMode)
« Reply #7 on: January 16, 2013, 03:11:32 PM »
hi tweakie

It would be helpful if you could zip and attach a simple GCode program which demonstrates the problem.

not Gcode but Ccode
program for AVR micro!
Re: delay in PWM pulse (LaserMode)
« Reply #8 on: January 20, 2013, 02:41:59 AM »
i done it,set minimum DC (in micro) 10% and for jobs with S command less than 10(maximum S=100,maximum power) it works fine
but with big S (for example90)the problem exist again, i have very shallow cut at 10mm of start then the depth changes to desired amount it next milimeters.
so there should be a way to rise DC much more faster than the Feedrate, i've not seen such in mach .
where i can find some diagrams about out power of a co2 laser by current? ( it shouldn't be linear, and it will be usefull to program micro to solve the pro.) ::)

Offline Tweakie.CNC

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Re: delay in PWM pulse (LaserMode)
« Reply #9 on: January 20, 2013, 03:36:15 AM »
Not sure if it helps but this is Synrad’s approximation of the relationship between tube output power and PWM % (this curve may alter dependant on the PRF).

Tweakie.
Success consists of going from failure to failure without loss of enthusiasm.  Winston Churchill.