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Old 07-31-2012, 12:54 PM #1
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Default Looking for some conformation.

OK so new to laser scanners and controllers so just wanted to ask for some conformation.

From what I understand most laser displays work by using a scanning process

Each laser is swept back and forth using Galvo's at a set speed. By knowing how fast the laser is scanning and timing the laser beams on an off you can display graphics and other effects. so in some ways it is simular to how a CRT TV works.

I want some thing a bit different.

I have a grid of X,Y coordinates not graphics, and I only want a single (green) laser.

So my thoughts is I need to achieve the steps below. Please correct me if I am wrong or if this would not work.
  1. For each of the coordinates I have, feed in to a DAC and translate to an analog voltage, I was thinking a 4 channel 10/12bit DAC that would give me 1024/4096 out put levels
  2. Do the same with the Brightness level for each point.
  3. Pass the results through an amp to scale them to the Galvo / laser driver tolerances
  4. send the X/Y voltage to the Galvo's and laser brightness.

One question is how can I tell if the laser beam has reached the correct point before switching on/of the laser? Do they feed back or is it all done on timing.

Secondly a 20K galvo for example does that mean it can point to 20K random point per second? or are there different limits depending how it is set to scan?

My thought that while Scanning works well for moving graphics and shows, absolute points would look crisper for static points.

Any thoughts would be welcome, participialy around creating a 4 channel 12bit DAC.

What I am not sure about is how to interface the DAC to a Raspberry Pi, this has GPIO pins that I wanted to use to send the X,Y and Brightness in binary through a serial interface to the DAC. As this gives me lots of control via an easy to code interface.

Cheers

Aaron


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Old 07-31-2012, 05:12 PM #2
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Default Re: Looking for some conformation.

A galvo-based laser display system has more in common with the old vector graphics arcade machines than CRT displays, which are effectively raster displays. Vector graphics render the image as a set of lines, not as the pixels in a raster-based system. You'll be hard-pressed to even emulate a raster display with galvos because they are simply not fast enough to render a raster image reasonably.

CRTs are able to achieve their high speeds because they use magnetic fields to steer the beam. Galvos are mechanical devices, and are severely limited in their ability to deflect the laser beam.

To put this into perspective, an analog CRT television utilizes video bandwidth on the order of 4-5Mhz. This allows the display to produce 525 or 625 lines of vertical resolution, with 720px of horizontal resolution, all at about 25-30fps (progressive).

Compare this to a galvo-based system, where even the best galvos can push maybe 50khz worth of "points" which are not actually pixels, but just points of curves. Many points are needed to reasonably represent sharp angles. Even if the galvo could operate at its full 50khz to produce pixels, you'd be looking at a producing an image of 220px x 220px per second. You'd need some means to retain the brightness over that entire time much like the phosphors on the CRT screen. Then there are the limitations of the lasers. Some lasers such as DPSS lasers (green) cannot be reasonably/consistently modulated above a few tens of khz.

If you really want to do laser-based raster scanning you'll need to use other methods that don't rely on galvos. Maybe look at the Kuntman scanner. It is not a galvo-based scanner, but rather relies on an oscillating beam deflector that operates at a fixed rate. Even then, the green channel of the image cannot be modulated sufficiently to provide good color reproduction.

Overall, you should use the galvo system as a vector drawing system, not a raster system. If you want raster graphics, look into a computer projector.
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Old 07-31-2012, 06:56 PM #3
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Default Re: Looking for some conformation.

Got you, I see what you mean, in vector you plot points along a line and the galvos trace along it while you modulate the laser.

you are right when you say you could only produce an image of 220X220 but that assumes you at times want to reproduce all 50,000 points at once. However if I only want to have 1000 points in any one frame, then in terms of the Galvos the resolution is not an issues as I could plot these points in a 100X100 or 10,000X 10,000 grid.

But this still does not answer the laser modulation issues, as to get 30fps with 1000 points would need 30Khz refresh rate.

Cheers for the tips and comments. Thinking maybe a projector is the way to go on this. But having read around a lot I would at some point like to build a scanner from start to finish using the standard vector methods.
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Old 07-31-2012, 08:22 PM #4
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Default Re: Looking for some conformation.

It's not really too useful to draw something without it depicting the image as a single frame. Even CRTs rely on the fade time of phosphors and the persistence of vision to construct a full image; even then one can still see flicker. It's like having a camera that measures light with only a tiny spot over a period of time, rather than a focal plane (though that is possible).
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Old 07-31-2012, 09:02 PM #5
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Default Re: Looking for some conformation.

yes you need to produce the image as a frame and at a fast enough refresh rate to fool the eye in to seeing a single image.

I was saying if each complete frame has no more than 1000 points then you could use a raster method to display it.

not saying its a good idea just possible.
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Old 07-31-2012, 10:31 PM #6
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Default Re: Looking for some conformation.

Pushing "possible".
Definitely not "plausible".
Projector = Scanner for the most part.

So from your questions :

The image is NOT drawn by galvos sweeping back and forth at a set speed. That is raster drawing as explained above. Laser scanners and/or projectors DO NOT work like a TV does ! (although there are some who are trying to do this.)

To define "Graphics" = the complex line drawing of an image. Usually created and stored as a "frame" element in a show.
So your grid of X,Y coordinates are in fact a "graphic" image.
That image is converted by software and fed into the DAC which turns the image into analogue waveforms.
Those waveforms are what controls the Galvos.
One of the outputs from the DAC will be the X axis (with positive and negative values) galvo and one for the Y axis galvo.
This makes the two galvos work in unison to move the mirrors so that they draw out a line drawing.
A truly fast set-up will have so many points that you might be fooled into thinking that it isn't just one line with many angles being drawn out.

Brightness : Unless you have a very expensive analogue controlled laser you have the option of either having your laser on or off.(digital control)
Now you can fool your eyes into thinking that it is being dimmed by controlling the time that it is on versus the time it is off at a rate that is faster than the brain can "see" (Pulse Width Modulation) but that needs to include calculations on the speed at which your galvos are moving as well so that the lower brightness doesn't come out as a dotted line being drawn out where a solid one should be.

The other channels on the DAC are used to control the different lasers(colors) and safety issues as well. For only one color you wouldn't need to use them all but you might find that incorporating them might allow for some redundancy or even slightly nicer control of the single laser. It would be totally subjective.

Amplifying the signal for the galvos is desired as it allows tuning the response size maximum. Most projectors have that built in.

Yes, it is all done on a matter of precise timing.

20K galvo means that at a predetermined(limited) maximum angle of motion that 20K points are possible. Most are rated at an angle of something like 9 degrees or so. So if you plan on drawing an image that moves the galvos more than 9 degrees off center then your number of possible points per second go down. And they can go down quite quickly. Better galvos have a larger rated angle and more points per second possible.

Hope that helps clear it up for you.

And by the way, The title of the thread should read "confirmation" : as in you would like someone to help confirm the presuppositions you put forth.
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