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open-hardware transparent polygon scanner

laser scanner which uses transparent instead of reflective polygon

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An open-hardware laser scanner suited for Printed Circuit Board (PCB) manufacturing. The laser scanner uses a transparent prism instead of a reflective polygon with a f-theta lens.

The goal of this project is to develop a PCB machine which can get an accuracy smaller than 100 micrometers, i.e. 4 mil. The PCB machine should be fast and have a writing speed greater than 1 meter per second. The machine uses a transparent instead of a reflective polygon to move a laser beam. The main advantage is that an expensive f-theta lens is not needed and the projection is tele-centric.

Specifications

The current version of the Hexastorm has the following specifications:

  • wavelength: 405 nm
  • rotation frequency:  up to 21000 RPM, current 2400 RPM
  • line speed: up to 34 meters per min @ 21000 RPM
  • spot size: elliptical, 35 (short axis) x 68 (long axis)* micrometers
  • cross scanner error: 0 micrometers (only one out of four facets is used)
  • laser driver frequency: 2.6 MHz
  • maximum scan line length: 24 mm
  • typical scan line length: 8 mm
  • optical power: 500 mW
  • facets: prism has 4 facets, 1 is used

* most likely smaller, used fit ellipse of Opencv as measure, i.e. not the correct
1/e2 measure

Electronics

  • Beaglebone green
  • Firestarter cape

Status

An image can be uploaded to the scanner and exposed on a substrate.
The spot quality of the laser needs to be analyzed.


An exposure goes as follows (for the result see above):

Software

Hexastorm fork of LDGraphy
Optical design

Links

Reprap article
Official website
Failed kickstarter campaign

firestarter2.PDF

bom of firestarter and overview of layout

Adobe Portable Document Format - 912.66 kB - 05/07/2018 at 21:30

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output files.zip

gerber files GBL -> Gerber Bottom Layer GTL -> Gerber Top Layer GTO -> Top Overlay GBO -> Bottom Overlay (empty) GTS -> Top Solder GBS -> Bottom Solder GTP -> Top past GBP -> Bottom Past GKO -> KeepOut Layer DRR is the Drill Layer with APR that is the tool if you wanted to make the cards ... send all the files

Zip Archive - 258.00 kB - 05/07/2018 at 20:59

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basf.pdf

presentation given to various companies on the potential applications of the Hexastorm

Adobe Portable Document Format - 5.37 MB - 11/03/2017 at 12:42

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  • 1 × Quartz optical window, 2mm thick, 30x30 mm 40 euros, faces 60/40 < 5 arc min, chamfers 0.10 – 0.30mm, edges Polished 60/40, top bottom polished 60/40,
  • 1 × Beaglebone Green 45 euros, kiwi electronics
  • 1 × Firestarter cape 50 euro, made by hand
  • 1 × Ricoh Aficio AF-1027/270 polygon mirror motor 20 euros, ali express
  • 1 × Front surface mirror 25x25 mm, 1.2 mm thick 9.10 euro’s, uqg optics

View all 8 components

  • Polygon scanner cross-scan (wobble) error

    Hexastorm11/05/2018 at 23:16 0 comments

    A crucial feature is that the resolution of the scanner should be better than 100 micrometers. I measured the prism today and the lines generated by different facets do not overlay each other. Although, I thought I had solved this. Luckily a business contact clarified it, link.
    I quote;

    "One of the characteristics of a polygon scanner is cross-scan error which is a deviation perpendicular to the scan line.  This is also referred to as wobble or dynamic track error.  There is non-repeatable wobble error from the motor bearings.  There is also repeatable error from polygon facet to datum error.  There is no such thing as a perfect polygon scanner.  There is always some wobble error"

    Zeller stated that he also had this issue with reflective polygons, see link.
    "but also different mirror faces are ever so slightly tilted up/down resulting in a forward/backward movement of the project lines in some mirrors"


    There are several ways to mitigate this;
      optics
        collimate laser bundle with aspherical lens
        focus laser into one direction with cylindrical lens
        refract through prism
        focus laser into other direction with cylindrical lens

      This would fix a lot of issues;
               - beam is more round
               - wobble error is gone           

      software
         only use one facet or correct on facet basis
         This has several disadvantages;
               - cannot reduce non-repeatable motor bearing errors
               - speed is clipped, in case of one facet

    A multi facet exposure is shown below, The different facets are visible.


    This is fixed in the software by enabling only one out of four facets.  For the current application one facet is fine. At the moment, the UV laser direct has a speed of 166 mm/s at 120 mW. With one facet and 2400 RPM Hexastorm has an effective writing speed of 1 m/s at 300 mW.

  • First exposure

    Hexastorm10/29/2018 at 23:15 2 comments

    Succeeded in making my first exposure today. At the moment, I am working with cyanotype paper. This is pretty cheap, 50-70 eurocent per sheet, and easy to develop with water.
    The dosage was not optimized.  The polygon speed was 2400 RPM. The laser driver was set at 50% of max power.  Each line was exposed 10 times, so the speed of the scan head was something like 0.2 mm per second. The line speed of the scanner was 1 m/s.  The laser frequency is 400.000 Hz.
    Besides the not perfect dosage, there are several other issues. The laser goes through and over the prism. So lines are visible in the end result. The text in the final image is flipped,

  • Prism alignment

    Hexastorm04/26/2018 at 11:27 0 comments

    I recently received a question in my mail on the alignment of the prism. When i made the technical presentation, I did not know how to align the prisms. I have improved the mounting procedure. Basically what i do is, i take the top of with a Dremel ( a drilling machine). I can then remove the reflective polygon. At the center of the reflective polygon there is an axis. With my Dremel I grind this axis away until i have a small hole at the center of the top plateau of the polygon basis. I put a bit of UV glue in the hole i created and place the prism on top. Then by rotating the prism with respect to a reference point I am able to center it. I then use a UV flash light to fix the prism. Afterwards, I measure how planar the prism is with a displacement meter. The current prism is planar within 5 microns. I later figured out that the facets, however, do not overlay each other.

  • firestarter pcb

    Hexastorm04/25/2018 at 22:34 0 comments

    I used an FPGA in my original design but it has proven to be complicated and expensive. The Xula LX25 is well documented but produced in low volume and seems to be no longer for sale.
    Luckily, Henner Zeller made a polygon laser scanner, the LDgraphy, with a beagle bone black / green. With the LDgraphy firmware a laser scanner can have a clock speed of 2.7 MHz. This is more than sufficient for a transparent polygon scanner. I looked at Zeller's code and it is actually very accessible. With a couple of minor changes it should be suited for the Hexastorm.
    I also had the luck of finding an electrical engineer, Salvatore Puglisi, who could design me a board. Result is below see figure 1. I have added the schematics of the board as pdf to the project repository, see firestarter.pdf. I also added the gerber files in  a zip.
    I use a simple voltage divider to detect the signal of the photodiode. This worked with the Xula LX25.
    Zeller uses a Schmitt-Trigger to have a clean signal for the BeagleBone. I already tested the firestarter board and this is not needed.
    Zeller also didn't mount the laser driver on the cape and made his own laser diode driver.
    I decided to go for the IC-HKB from IC-haus. It is not seen in the picture and needs to be mounted on the board at U2. Maybe in a future version, I will make one PCB per laser scan head and one central board with stepper drivers.

View all 4 project logs

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Discussions

Paul wrote 11/05/2017 at 03:49 point

You've certainly done your homework very thoroughly, and I see that in your application with very small optical cone angles (large focal ratio), the optical aberrations and field curvature appear to be tolerable.  That's great.

One question: You say that previous scan techniques require a large (and therefore expensive, you argue) f-theta lens, which must have one dimension at least as wide as the scan line.  In your approach, your polygons must be larger than the scan line length, but in *two* dimensions, making the volume of your optical element much larger.  Since either shape would use identical materials and fabrication processes in volume production (i.e. injection molding), one would naively expect the smaller element (the f-theta lens) to be cheaper.   How is this line of reasoning flawed?

  Are you sure? yes | no

Hexastorm wrote 11/05/2017 at 10:51 point

A telecentric f-theta lens requires one dimension at least as wide as the scan line. A non-telecentric f-theta lens does not require that.  In my approach, the polygon must have one dimension longer than the scan line. The second dimension is 2 mm. An f-theta lens consists out of multiple lens elements, e.g. a 3 element f-theta lens. These elements have curved surfaces. The prism consists out of a single element with a flat surface. The prisms now costs 40 euro's per piece with a minimum batch size of 10. They are not injection molded. You can't make the f-theta lens out of plastic, so you would have to injection mold quartz. I am unfamiliar with the prizes for that. Besides most likely a higher price and the fact you have to use multiple elements, you also need to worry about patents. Envisiontec patented the usage of reflective polygons; US 9079355 B2 . Finally, you need a thick reflective polygon as thick polygons can deflect collimated bundles with a large diameter and these can be focused to smaller spots.  A transparent prism uses a focused bundle and therefore typically can be thinner, which keeps the price of the bearing lower.

  Are you sure? yes | no

Paul wrote 11/05/2017 at 15:55 point

You have clearly thought about this a great deal, and have your arguments worked out well.

I wasn't considering that you might be requiring quartz.  I would have guessed even BK7 would be overkill for this application.

I mention injection molding because I look around my offices and see several laser printers.  Each of them contain a laser scanning unit with a rotating (reflective) polygon and a f-theta lens arrangement, with the final element being very large (>200 mm long).  All the optical elements of the ones I have inspected appear to be injection-molded PMMA or similar plastic.  The entire laser scanning unit must cost considerably less than $50, given the prices of the printers (all less than $200, two less than $70).  Granted, these are produced in huge volumes, but they serve as existence proof that these scanning systems with large optical elements are not intrinsically costly.

  Are you sure? yes | no

Hexastorm wrote 11/05/2017 at 16:52 point

PMMA absorbs light at 405 nm. The laser printers at your office use 800 nm and low power lasers. I use 405 nm and high power lasers. So yes; there is concrete proof that PMMA injection molded systems are not intrinsically costly. There is no proof that quartz systems are not intrinsically costly.  It's unclear what the prices of these systems would be .

  Are you sure? yes | no

Paul wrote 11/03/2017 at 13:21 point

A good variation on the usual way.  I build a few instruments based on this and similar methods between 1985-1989.  A couple of important notes:  

1. The scanned field is NOT flat: The optical distance to the target plane *increases* as the polygon rotates away from normal.  Not only is it geometrically longer, some of the increased  path length is in the high-index polygon too, increasing the path length even more.  The result is a curved focal or scan plane.  One of my instruments actually depended on this: a rotating polygon was used to tune the optical path length inside an optical resonator cavity, to adjust a tunable laser that was phase-locked to that cavity. 

2. You get some significant spherical aberration when you focus through a thick window like that, for similar reasons: the light rays at the periphery of the optical cone take a longer path to the target plane than rays going through the middle of the cone, with the result that they focus at a different depth.  For a laser at f/50 (or whatever it is), this probably isn't significant, but in an imaging system at f/2, it seriously degrades focus. 

  Are you sure? yes | no

Hexastorm wrote 11/04/2017 at 22:09 point

Paul, thank you for your reply!
A transparent polygon scanner with a single laser bundle was first patented by Lindberg in 1962.  The scan field is not flat and you can get some significant optical aberrations. A full numerical model is available here https://github.com/hexastorm/opticaldesign . A description of this model is available in the technical presentation. The result shows that in practice the scan field is flat and the optical aberrations are not significant.

  Are you sure? yes | no

Hexastorm wrote 11/03/2017 at 11:58 point

Well this is the first comment! If Hackers are interested in transparent polygon scanning let me know :P ..

  Are you sure? yes | no

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