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Thread: Scanner mirror design question

  1. #1
    Join Date
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    Default Scanner mirror design question

    So after breaking a couple scanner mirrors, having a bad experience with cheap eBay replacements and having a difficult time finding an easy source for relatively quality mirrors that don't come with scanners attached I thought it might be fun to model up my own and see what kind of price we're talking for having a run made. I compared info from what I could find about DT's, PT's and the 506's as well as bought Bill Benner's book on scanners specifically to read the chapter on mirror sizing. Now, assuming my understanding of the math is correct (it doesn't seem that difficult), it seems to me like the 7.4x8mm X axis mirror that Phenixtechnology has listed for their PT-A40's doesn't fit the claim of being designed for a 5mm beam. Either way, I came up with a couple drawings of my own and am curious what other's input on them are.

    Keep in mind that the style is generic, I've only changed some dimensions that appear to fit Mr. Benner's formulas better. I calculated the numbers based on a 5mm beam and an 8.5mm distance between the X and Y scanners which is comparable to both the PT-A40's and the 506's according to their technical drawings.

    I guess I'm just curious to know how significantly the increased mass of larger mirrors will affect the scanner performance. According to 506 data it specifies rotor inertia as .014g/cm^2 but it doesn't appear to indicate what mirrors were used for the calculation. When I calculated the approximate inertial force of my drawn mirrors I got .114g/cm^2. That seems significantly more than the 506's but I'm not sure what I'm comparing it to. I'm just hoping someone can chime in with some more info. Thanks.

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    Last edited by ZeroPoint; 01-28-2017 at 02:47. Reason: Trying to figure out the attachments.....
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  2. #2
    Join Date
    Oct 2014
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    Orlando, FL
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    Default

    Certainly a 7.4mm wide mirror can't really move a 5mm beam. It barely fits on the mirror at rest!

    The 0.014g * cm^2 is the inertia of C506 rotor (Magnet, Output and PD shaft, and two bearings) with no mirrors.

    For approximation, our mirror inertias are (roughly) as follows.... of course the substrate changes this somewhat

    3mm 60 degrees - 0.0027 g*cm^2
    4mm 40 Degrees - 0.007 g*cm^2
    5mm 60 Degrees - 0.013 g*cm^2
    6mm 50 Degrees - 0.022 g*cm^2
    8mm I cant remember off the top of my head. Maybe around 0.100 g*cm^2?
    10mm 40 degrees - 0.160 g*cm^2

    I also have some spreadsheets I made that I can calc theoretical scanner performance, based on load applied. Basically frequency vs angle, for a sinusoidal waveform. This is represented in graph form on the backs of all SM datasheets.
    Sincerely,
    Ryan Smith
    ScannerMAX Mechanical Engineer
    ryan {at} scannermax.com

  3. #3
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    Default

    Ok, that makes more sense. so The rotor inertia would be added to the mirror inertia for the total inertia of the system. My .114 g/cm^2 still seems high, however it was a raw calculation without taking into account the chamfered corners and using the heavier BK7 as opposed to fused silica. Perhaps with a more accurate estimate of total mass it would be more comparable. Also, I know some mirrors use chamfers on the rear face in addition to the corners. Is this practice geared more towards mirrors over 1mm thick? I personally have never encountered such a mirror but all the ones I am familiar with are generally between .7-1.0mm thick.
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  4. #4
    Join Date
    Oct 2014
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    Orlando, FL
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    Default

    I just use SW to calculate all my inertias

    I only see back chamfers on thicker mirrors, usually 2mm or thicker

    The largest mirrors we stock at this time are around 1.1mm thick total.
    Sincerely,
    Ryan Smith
    ScannerMAX Mechanical Engineer
    ryan {at} scannermax.com

  5. #5
    Join Date
    Oct 2014
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    Orlando, FL
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    Default

    I think youre off by an order of magnitude.

    I got 0.0128 g*cm^2 for your X mirror using glass substrate (ironically exactly what our C506 5mm mirror is

    your Y mirror is right around 0.007 g*cm^2

    Ideally for a mirror set the X and Y mirrors will have matched inertia, even if this means adding useless mass to one mirror (usually X mirror). Better for tuning and raster scanning

    - - - Updated - - -

    I think youre off by an order of magnitude.

    I got 0.0128 g*cm^2 for your X mirror using glass substrate (ironically exactly what our C506 5mm mirror is

    your Y mirror is right around 0.007 g*cm^2

    Ideally for a mirror set the X and Y mirrors will have matched inertia, even if this means adding useless mass to one mirror (usually X mirror). Better for tuning and raster scanning
    Sincerely,
    Ryan Smith
    ScannerMAX Mechanical Engineer
    ryan {at} scannermax.com

  6. #6
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    Syracuse, NY
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    Default

    Well that's reassuring. Both mirrors can be reworked in order to get them to match. However, I still can't quite figure out the math. I'm using a density of 2.46 g/cm^3 which gives me a mass of .197g. This then gives me an inertia of .128 g/cm^2. If I move the decimal place of the density from 2.46 to .246 then I get the same .0128 as you but I don't understand why I would do this. Additionally, I'm not getting anything close to your result for the Y mirror. Another thing that confuses me is which dimension is considered the width. My initial assumption was that the width would be perpendicular to the rotational axis but in order to get the .0128 result I had to use the dimension that was parallel to the shaft.

    I'm using the formulas right out of the book, i.e. Mass(g) = density(g/cm^3) * length(cm) * width(cm) * thickness(cm) and then mirror inertia(g/cm^2) = mass(g) * (width(cm)^2 + thickness(cm)^2). If you (or anyone else) could confirm this as well as confirm which dimension is the width that would be excellent. If it's also not too much trouble to just run through how you got your numbers that should clear it up for me. I'm clearly missing something but I can't figure out what it is. Thanks again.

    Edit: just noticed your comment about using software for inertia but if there's any bit of info you have that might clear it up for me i would still appreciate it.

    Luke
    Last edited by ZeroPoint; 01-31-2017 at 21:37.
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