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Thread: Can someone explain the "4 mA preionization" engraving mode?

  1. #1

    Can someone explain the "4 mA preionization" engraving mode?

    I'll admit this question is mostly academic, but I keep reading that tubes will engrave at their best when in a mode of preionization at a level of 4 or 5 mA. What does this mean exactly? I'm taking it to mean the laser is kept at 4 mA during the entire engrave, with pulses being fired at 10 or 15 mA (whatever the normal firing current is). Is that correct?

    If so, is this implemented by specific power supplies or by the controller? Does there need to be a higher initial current command to "strike" the tube first? I've noticed I can't get my 80W tube to fire very reliabnly at low power levels, leading me to assume the 4 mA is actually the laser being "off" at that level.

    Anyway I did some Google research but apparently I used the wrong keywords. All I could find were the sellers descriptions of their tubes and scientific papers on lasers in dermatology

    I have an older Full Spectrum controller with an upgraded tube and power supply, and I don't think they implement any preionization wizardry. If it's not terribly complex of a controls problem I'd like to try to add it to get better engraves, but I'm having a hard time finding good info on what voltage/current profile to feed into the power supply command.

    Last, I assume this doesn't affect vector cutting, correct?
    Last edited by Bert McMahan; 01-09-2018 at 1:29 PM. Reason: typos

  2. #2
    Great question - I'll add that I'm supposing it's a separate setting in the controller somewhere... to make it happen like that for engraving, as opposed to cutting... but I currently have no idea.
    Best wishes,
    Ian



    ULS M-300, 55w made 2002 with rotary. Goldenlaser 130 watt, 1300x700 made 2011.
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  3. #3
    Dave will have an answer for this - but my 'best laymans guess' concerning preionization is that lasers need to be at or above the PI level to fire consistently/reliably, below this level is hit or miss. And I assume these levels increase as tube wattage ratings increase.

    All lasers are different, so one's results will so-called identical equipment will likely be different from another's. My 80 RECI will consistently 'scuff' black Rowmark all day at 8% 'entered' power (which is closer to 10.5% actual), never breaking thru the black surface, but many other 80w tubes may fire erratically at the same power level.

    Also, I don't use a refrigerated chiller with my machine, and in the summer months the tube's output noticeably decreases as water temps rise, and this will surely affect low-power operation. With cool water black Romark engraves nicely at 14-15% power, but as water temp gets to around 24-ish, I'm needing 17%...

    DC glass tubes, IMO, are a crap shoot, too many variables affect their operation...
    ========================================
    ELEVEN - rotary cutter tool machines
    FOUR - CO2 lasers
    THREE - fiber lasers
    ONE - vinyl cutter
    CASmate, Corel, Gravostyle


  4. #4
    FSL has made the claim that their RetinaEngrave card is more stable than Chinese controllers at low vector power.

    FWIW, I'm at 1mA @ 1% raster, 2mA @ 1% vector, and 26mA @ 90% vector with an 80W tube.

  5. #5
    I can get the power much lower using the "vector current" slider rather than doing super low PWM cycles. I don't think I've been able to get it to reliably fire at 1% raster but I haven't really checked- maybe my beam just wasn't marking anything. Matt which FSL controller are you using? The new web-based one, the older Beaglebone one, or the even older green-board gen 5?

  6. #6
    Quote Originally Posted by Bert McMahan View Post
    I can get the power much lower using the "vector current" slider rather than doing super low PWM cycles. I don't think I've been able to get it to reliably fire at 1% raster but I haven't really checked- maybe my beam just wasn't marking anything. Matt which FSL controller are you using? The new web-based one, the older Beaglebone one, or the even older green-board gen 5?

    Weike with 2013 XMOS green-board card.

  7. #7
    Think of it like boiling water

    at 100 degrees C water boils (the laser tubes ignites)

    If your water is room temp (say 20 degrees C) you have to pump an almighty amount of heat into it to get it to boil instantly

    If however you have a heater under it that keeps it at 75 degrees it is not boiling but it will also take a lot less power to make it boil (the extra 25 degrees worth)

    In decreases the time to rise (from when the current is applied to when the tube sends out a beam) and you also have less of a spike from the trigger current (causing a deep spike in the materials when the laser triggers)

    All of this is separate to both PWM and PRF though, they are properties of the laser once it is firing rather than before it triggers

    Kev's right, there is a base line of current that a tube needs to fire reliably and that varies a LOT from tube to tube depending on the physical make up of the tube, the gas mix, the voltage applied and the gas temperature, the idea of the 4mA trickle charge is to keep it just under that point as much of the time as possible when the lasers isn't actually triggered
    You did what !

  8. #8
    Thanks Dave. So how exactly does one command a Chinese supply to "idle" at 4 mA? I'm still a touch confused on the precise voltage levels and signal patterns of the various connectors. It seems that you'd have to have the controller change its output (either a 5V PWM signal or a 0-5V analog signal) automatically in sync with the "fire the laser" (TL, I think?) connection. Is that correct?

    It seems like you could add this feature to a controller that didn't have it normally by just using a couple of transistors tied to the "fire the laser" input, which would switch the power control signal between either the main controller's output or a second output.

    As an aside- the whole "0-5V or PWM" signal thing confuses me a bit... can you do a combo of both, and do something like a 4V PWM signal, with its "max power" corresponding to a 4V constant analog signal? The Chinese manuals for these lasers aren't the most helpful
    Last edited by Bert McMahan; 01-09-2018 at 10:48 PM.

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