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Thread: what exaaaaactly does 30000 pps mean...?

  1. #1
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    Default what exaaaaactly does 30000 pps mean...?

    i know it's a speed for how fast your galvos can move.

    i know it stands for pixels per second.

    i'm slowly coming to terms with point optimization so your galvos aren't spending all their time just freaking out - maximum segment length, dwell and stuff.

    Buuut 30000 pps... what EXACTLY is meant by a pixel? EtherDream coordinates are usually in the range of 0..65535 (or -12768..12767, whatever) for x and y.

    But I am pretty dang sure my laser doing 30000pps will take WAY less than a second to draw a line along one of borders (32768 pixels, right??)

    So what exactly is a "pixel" in pps ?

    And is there any GOOD SOLID fully formed docs on point optimization so my dang squares quit lookin like circles?
    i (also) get that you wanna split super long lines up into a maximum segment length. I got the trig for that done. I get there should be multiple points at the and of a line so the galvo can stabilize. But not so many that the point outshines the line...

    But I really could use some help with that optimization stuff. Any links round here you guys got?

    Thanks muchooo.

    ...Steve

  2. #2
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    30000pps means NOTHING.

    Projector with 30000pps @ ILDA 8Deg which is something like a Standart means he can scan the ILDA 30K Testpicture with 30000 Points per Second.

    But also if a Chinese Projektor is sold with 30Kpps ( Kilo Points per Second) this dont need to be the truth :-)


    The ILDA Testpattern are availible for free but it is important to scan them as they are. No additional Interpolation, Corner Repeat or what ever….

    https://www.ilda.com/resources/Stand...n95_rev002.pdf
    Greetings
    Guido

    www.mylaserpage.de

  3. #3
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    Quote Originally Posted by Guido View Post
    30000pps means NOTHING.

    Hrm. Ok, that part is comin' back to me.

    But you DO send pps to the DAC. And it is doing SOMEthing with that dang number.

    So what does it mean to the DAC? Does it simply pass it along to the galvo firmware that kinda takes whatever the max your projector is rated as and slows down the galvos X percent or somethin ?

    Is there any good way to TRULY test your laser's galvo speed?
    Like put up a plain old border rectangle and count the millisecs for the frame to complete?

    Hrm.

  4. #4
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    Hi,

    to compara Galvos please use the ILDA Testpattern like linked above.


    The DAC is sending "X" Points per Second the Galvo Driver Circuit which send Current through Galvo´s Coils to reach the wished Position.

    But if the Galvo is too slow the next Point from the DAC is comming before this Position is reached. So the Output dont look as expected.


    The ILDA Testpattern is exactly what you Need to get the true Galvospeed measured at a defined Angle.
    Start with 15kpps. In the middle of the Figure you see a blue Circuit outside the Square. No increase Scanspeed. The Blue Circle gets smaller and smaller.

    When he is exactly inside the Square and still round you can say "My Galvos can do XX Kpps @ILDA 8 Degree" . When the Blue Circle is an oval the Scanspeed ( Combination of Gain/Damping of Driver, adjustable by Resistors )
    isnt equal. But when you never adjust Galvodrivers by yourself dont try: Normally the Result after the first Try is bader than before :-)


    Hope you understand what i was writing.. my Motherlanguage is German :-)
    Greetings
    Guido

    www.mylaserpage.de

  5. #5
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    I'm thinking of it as a low pass filter, the slower the galvos the lower the cutoff frequency. If you try and draw a square too fast, corners get clipped as the signal is clipped and so the filter has prevented the galvos from ever defining a sharp corner. Even though it's a DC signal to the galvo amps, any sort of output is going to have an AC frequency that generates that pattern. The mass of the galvos combined with the electrical impedance of the coils combined with the maximum available current sets the max speed of the set.

    You don't send PPS to a DAC, you send an analog AC signal. If the frequency of the analog signal (scan speed) is higher than the bandwidth of the galvo set you get round corners! The only thing can be done is slowing down the scan speed, sending a slower frequency that allows the galvos time to dwell and reach the commanded position. Course you can always reduce the scan angle. As I understand this will lower the DAC output frequency and magnitude to something the scan set can handle, assuming the frame is just a simple square.

    Also, in case anyone else more experienced is here can confirm for me, a circle drawn at full frame width should be a perfect sine wave on the X and Y signals? At 30KPPS, you'd expect the sine wave to be ~30KHz, at 20KPPS 20KHz? If you leave all else the same, a circle drawn at half frame width, 30KPPS, would be 15KHz input signals?

    You can also tune galvo sets, since galvos have position sensors they know if they are in the desired position or not. If they aren't, the tuning settings configure the maximum allowable current to drive those galvos into position. These settings need to stay set for a defined scan speed and output frame size, or scan angle. Tuning a galvo set isn't something a newbie needs to be taking on, so it would be best for you to try displaying ilda test frame at multiple scan speeds and angles untill you find where that circle just touches the square. Keep scanning at those speeds and angles, and your squares should stay square. Good luck!!

  6. #6
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    What nobody seems to understand is the circle in the square in the ILDA TEST PATTERN measures the approximate "3 dB Down" point of the galvo frequency response at a specified test condition.
    ~
    The test pattern is a unit of measurement set up so artists who are non-techies and techies who are non-artists can get a figure of merit both sides agree on for a given galvo pair under standard conditions.
    It gives us a common reference to interchange vector artwork and to tune the galvo servo loops, without needing a table top full of test gear. . 30Kpps is roughly a 2.4 KHz galvo response bandwidth for very short jumps. As that bandwidth available changes with angle, decreasing with larger jumps, the angle of the standard test pattern projection is specified and the sample rate for its reconstruction is specified.
    ~
    There is no easy formula to convert KPPS to a galvo frequency response , because the equation approximating the motion of the galvo under closed loop control is a very complex polynomial.

    1/KPPS (ie..the reciprocal) is the time in seconds between DAC output updates.

    ~
    KPPS in terms of the DAC and Software is the update rate per point in the image without any image optimization or vector processing applied. Then you can assume the galvo acts as a complex, non-linear low pass filter with a integrated memory of its last positions and some resonances if driven out of its ballistic response region. There are whole books on Galvo dynamics, it is not an easy subject to understand.
    ~
    Closed Loop Galvos have a frequency response of DC to low audio.
    ~
    Steve
    Last edited by mixedgas; 01-21-2019 at 07:58.
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  7. #7
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    well i DEFINITELY appreciate the responses.

    All i send to the dac in a frame is a list of x,y,r,g,b.
    All i send with the frame is reps and pps.

    IS IT TRUE that the DAC takes that pps of 30,000 and goes...

    OK! My ONE job is to send these x,y,r,g,b things to the laser ONE at a time in analog.
    So every 1/30000 of a second I'm gonna pop a new x,y,r,g,b in analog voltages onto my output wires.

    That -absolutely- means that every 1/30 of a millisec, a new pixel is given to the galvo circuitry.

    Which means that for a given frame, I can NOT adjust that duration per pixel. All pixels are sent at the SAME speed.
    And the only way to adjust the speed is sending the SAME pixel more than once for multiples of that duration OR
    to split lines into shorter lines by adding points in between.

    So if things are nice for me, the galvos ARE at the last point and stable. Say at top left corner.
    And I want those galvos to go to, say, the bottom right corner.
    1/30 of a millisec is probably not long enough to get there? and the galvos will be freakin when they DO get there.
    But I kinda need to know that max speed.
    Then I kinda need to know a minimum and maximum "freak recovery" speed.
    minimum will be if the next dot is in the direction we were already going.
    maximum will be if the next dot is the opposite direction (or maybe 45 degrees off opposite?)

    Couldn't those 3 parameters get you what you need as far as optimizations with
    points between and dwell point repetition?

    I understand we wanna get the artists and the techies talking. But that test pattern...
    It's not sinking in for me...

  8. #8
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    Missing Basic, when you do a Jump in one Step from upper left to lower Right and repeats this several Time your Galvos will answer with Smoke..
    The Galvodriver is designed for fast , small Steps..such big Jumps need to be interpolated.
    In case of direction Change ( a.e. at Square´s Corner ) you need to add angle depending Point Repeats and so on...
    Greetings
    Guido

    www.mylaserpage.de

  9. #9
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    Here is a layman's description of "pps" which could stand for "pixels-per-second" or "points-per-second". A picture pixel or point has a precise location described on an X (horizontal) and Y (vertical) grid system. In theory and practice the grid system could be 256 "X" columns by 256 "Y" rows (256,256) or higher multiples such as 512,512 or 1024,1024 or 32768,32768.

    Remember the connect-the-dot pictures we did a kids. Each dot had a number from 1 to 50 or 80 or more. Each dot had a precise location (X,Y coordinate) on the page, although what they were did not matter is this example. The picture revealed itself when all the dots were connected starting at dot "1" connecting it with a pen or pencil to "2" then "3" then "4". The closer the dots were together the faster you could draw a line between them and the further apart adjacent dots were the longer it took you to draw a line between them.

    If you timed yourself on how long it took you to draw a line segment, in numeric sequence, from start to finish this would be described as your drawing at "50 dots-per-minute" for example. So dots=points=pixels of a picture or image.

    A laser picture or image is comprised is just the same way as the connect-the-dot pictures, where each point or pixel has a "numbered position" in the image (being the 1st point, 2nd point, 3rd point, etc. until the last point of the image) except that the laser image also has additional color information for each pixel or point as well. The pixel or point of the image is defined by its X coordinate value and its Y coordinate value. But what determines how fast a laser image can be drawn is dependent on a number of factors.

    When laser imaging software is "spitting" out an image to the laser XY galvanometers or scanners it is send each point's XY number value to the X and Y DAC in rapid succession until it reaches the end of the image then it repeats the image until instructed to display a new image or stop. And of course the X DAC takes the digital X number and converts it to a proportional X signal voltage to send to the X galvo amp that drives the X mirror to a new position that deflects the laser beam moving in a similar fashion, and ditto for the Y DAC.

    1. How many total points there are in the image
    2. How close and/or how far away each adjacent point is from each other
    3. How quickly the laser XY galvos can respond to changing voltage values

    The cheaper the XY galvos are the slower they tend to be. 20-pps do not cost near as much as 40 or 60 pps galvos.

    The speed at which a computer can spit out the image XY values to the DACs and the XY mirror deflecting galvos is far, far faster than the galvos can respond, so the are ways of making sure the computer software sends the XY point sequence of an image at speeds the frequency the galvos are capable of following accurately such that the images shape stay true to what it is supposed to look like.

    Like mixedgas said, the ILDA test pattern visually helps show whether the galvos are setup correctly or are capable of rendering the ILDA image accurately speed and size-wise.
    ________________________________
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  10. #10
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    ok.

    But is it true that if pps is 30000 that THIS MEANS that
    the DAC delivers the analog x,y,r,g,b to the laser for exactly 1/30 millisec before giving it the next point?

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