Strange wavy-line distortion (resonance?) in DT-40 Pro scanners at specific scan speeds...

Discussion in 'ScannerMAX' started by buffo, Jul 4, 2018.

  1. buffo

    buffo Member

    Hey Everyone;

    I know this question doesn't pertain to the Scanner-Max scanners, but Bill Benner asked me to post my question here anyway because he has some specific experience with the scanners in question. I suspect that posting here may also lead to further discussion about *why* this problem happened. Here is the original message I sent:

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    Hi Bill! We ran into an interesting problem at the recent Florida Laser Enthusiast's Meeting, and I'm hoping you can shed some light on the issue.

    We had a set of DT-40 pro scanners set up, and we noticed that at a very narrow range of scan speeds (from about 17,300 points/sec to about 18,500 points/sec) there was a noticeable "wobble" in the scanned lines, as if the scanners were trying to draw a wavy line - like a sine wave. It didn't always appear though. It was most prevalent on the laser media test pattern and the "TOP" pattern, but it showed up on other frames as well, but not nearly as often.

    The effect was subtle... The wobble was maybe a half centimeter (peak-to-peak) on a pattern that was a meter and a half wide on the wall. And it was most prominent on long diagonal lines, although some of the long horizontal and vertical lines would also exhibit the effect.

    We tried several different controllers (including one of my QM2000s and one of my FB3s), and the problem was consistent across controllers, so we're sure it's related to the scanners. Then we tried different scanner power supplies, and even hooked up a second set of scanners outside of the projector (just sitting on the table with a bare green module pointed at them) and the problem was still evident.

    I'm pretty convinced that this is some kind of resonance with the scanner, either in the rotor itself, or more likely in the mirrors. Also, these DT-40 pros had the original style scanner amps that had both low and high frequency damping pots and the notch-filter daughter board. I remember that you did a bunch of work with Jian to improve the design of this amp, and for a while they were using it, but it appears that they've gone back to the old design again.

    Anyway, the problem only happens at a narrow range of scan speeds, so it's not really a huge problem, but to eliminate it my thought was to add a drop of epoxy to the back of the mirror. That would both stiffen the mirror and also add a bit of mass to the entire rotor, which should change the resonance...

    But then I wondered if it might be better to try adjusting the notch filter first, to see if that would help. However, I've never actually messed with that daughter board, and if I remember correctly, you had said that the purpose of that notch filter was to correct for a problem with the early position sensors, and since no one uses that old position sensor design any more, the notch filter is actually useless.

    So before I recommend anything be done to these scanners, I thought it would be best to contact you first and see what your recommendation would be. There are a total of 3 identical sets of these DT-40 pro scanners, and they all exhibit the same wavy-line resonance problem.
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    After I sent the above message to Bill, I received a picture from the guy who I've been working with to troubleshoot these scanners. I've attached that picture to this message to help illustrate the problem. Note that in this shot it's hard to see the wavy-line effect on the vertical lines, but it is present. It's much more visible on the horizontal and diagonal lines though.

    Adam dt40proScannerDistortion.jpg
     
  2. Pangolin

    Pangolin Staff Member

    HI Adam,

    You raise some very interesting and important questions. To aid in understanding, see the screen shot found here.

    http://www.pangolin.com/_Files/Resonances_PL_Discussion.jpg

    This particular screen shot is from an analysis we did in October of 2013. That's when we first started using Ansys.

    (For those who don't know, Ansys is among the most powerful, most capable, and most expensive analysis packages in the world...)

    This screen shot is of a 6800-like scanner we were analyzing, while trying to understand our competitive advantages. The screen shot shows the resonance "modes", and also particularly what is doing the "bending" and at what frequencies. Since the DT40 is a kind of copy of the original Cambridge model 6215, this particular screen shot / analysis should be relevant.

    It shows that the lowest frequency is the first bending-mode resonance (frequency of 5017.1 Hz). This first bending mode also shows that it is almost exclusively the mirror itself that does the bending.

    It shows the next-highest frequency is the second bending-mode resonance (frequency of 8036.1 Hz). It shows that in this particular mode, the mirror itself doesn't bend much, and it is mostly the bending of the magnet, shafts, and shaft interfaces (i.e the "glue").

    It also shows the torsional resonance (which in this case is the eighth-highest resonance) at 24003Hz. It also shows how the mirror and position sensor vane are "twisting" at that frequency.

    (By the way, this analysis was done when we first started using the program. Since then we have refined our material properties and done other "calibration" to the computer model. I can tell you from experience that all of these numbers are pretty optimistic for a Cambridge model 6800. Nevertheless, the screen shot and numbers can still be instructive...)

    Now, as it relates to "wavy lines", here's the deal. The wavy lines are most certainly caused by resonances in the system. Sometimes it is hard to tell whether the wavy lines are caused by the "bending mode" resonances, or the "torsional" resonances. However, I do believe that the particular picture you are showing there is because of "torsional" resonances.

    Torsional resonances would be caused by the mirror -- or the shaft being too thin. You mentioning that this was a DT40 scanner, they probably use a 0.85mm thick mirror (or maybe even 0.8 or 0.75), and definitely use only a 2mm thick shaft.

    As I have discussed in my videos, and also in my book, stiffness of a shaft goes as the FOURTH POWER of diameter. So if we compare DT40 2mm shaft to the shaft used on a Cambridge model 6800 (and 6210 and 6215), the comparison is (2mm / 2.38mm) ^ 4 = the DT40 shaft is LESS THAN HALF THE STIFFNESS of a Cambridge model 6800. Since Frequency is a square-law (not linear law), the half stiffness does not mean torsional resonant frequency is half as high, but rather 70% as high, or around 16.8kHz according to this analysis (but as I said, I think these numbers started out being optimistic in the first place).

    In a similar vane, longitudinal (i.e. "bending" stiffness of a 2mm shaft -- and 0.85mm thick mirror) are also much lower than the 2.38mm thick shaft and 1mm thick mirror used by Cambridge.

    Torsional resonances can be "notched" using a notch filter. But this adds at least one more adjustment to the servo driver (usually two more adjustments). Moreover, to PROPERLY adjust the notch filter, it requires a special piece of test equipment called a Dynamic Signal Analyzer, which surely the DT folks do not have. Therefore even if they happened to have a notch filter on the driver board itself, the likelihood of it being adjusted properly is... well... I'll let you do the math on that one ;-)

    If the resonance is torsional in nature, then those wavy lines would be exacerbated when the point output rate of the software EQUALS the torsional resonance of the scanner. So for example, in this particular screen shot, it shows the torsional resonant frequency is around 24K. So if things were really bad in the system, those wavy lines would be most exaggerated if our point output rate is also 24K. You might notice that even small changes above or below that point output rate will get rid of these particular wavy lines.

    My guess is that you probably had your point output rate set to 16-18K. Is that right?

    By the way, sadly I do not have particular images that would be a good visual aid for my next topic, but you can do some experimentations there and capture images. The image that you will be looking for is just the normal ILDA test pattern. Bring that up and look at the CENTRAL VERTICAL FEATURES (for example, the lines at the top of the pattern used for blanking, lines at the top/middle, and lines at the bottom/middle). While displaying that pattern increase and decrease your point output rate. Of course the "circle in the square" will distort, but for the purposes of this discussion, ignore that. What you're looking for is for those CENTRAL VERTICAL FEATURES to either remain straight (at all point output rates) or all of the sudden become bent / distorted / or wavy. When you see THIS KIND of wavy line, it is because of the First Bending Mode frequency of the system (i.e. the mirror resonance).

    I've forgotten what those numbers are for Cambridge model 6800, but for the Cambridge model 6215 with a 1mm-thick 3mm Y-axis mirror, the first bending-mode resonance happens at around 3.75kHz (which is around 45K PPS on the ILDA test pattern) and their first torsional resonance is around 24K. For sure I've seen many Cambridge 6215 where the central vertical features of the ILDA test pattern becomes highly distorted when the ILDA Test Pattern is projected at 45K and above.

    For comparison, using the same mirror our Saturn 1 (which has the smallest / "weakest" rotor in the ScannerMAX repertoire) has a first bending-mode resonance above 5kHz (above 60K) and first torsional resonance is around 65kHz (around 2.7 times the industry average). Because of this, we rarely have "wavy line" problems. We also are very careful during mirror mounting to make sure everything remains balanced, in which case the bending-mode resonances won't even be excited.

    Anyway, I hope that the screen shot along with this brief explanation is instructive. Questions are welcome.

    Best regards,

    William Benner
     
  3. buffo

    buffo Member

    Thanks for the detailed reply, Bill! :)

    I remember you talking about shaft resonance at one of the Laser Enthusiast's Meetings years ago, but I had forgotten about the fact that you had also noticed that many Chinese galvos used smaller diameter rotor shafts (as well as thinner mirrors) vs the original Cambridge design. That certainly makes sense though, give what we saw.

    Even if this *could* be compensated for by careful adjustment of the notch filter, I'm starting to think it might not be worth the trouble. The guy who had these in his projector admitted that he rarely if ever needed to run at exactly 18K. And I'm worried that he could make things much worse if he doesn't get the notch filter exactly right.

    Longer term, he plans to replace the galvos with Compact 506 scanners anyway. (He already has a few pair of those and he loves them.)

    Adam