Oh Bollox

[The_Doctor]

Would you believe, I have no idea what you are talking about?

No problem, I either went into full depth or mentioned it in passing whenever some thread's context warranted it, and linked to the original post. I never got enough feedback till now to figure out how to re-pitch my question.

I also posted in existing threads instead of shouting from the rooftops with a new one every time I wanted to bring this up, but I'd hoped someone might understand what they saw enough to be interested in it. This could affect anyone trying for fine scan detail in either graphics or beam shows. (Beam shows to a much lesser extent, but it DOES affect them too if you're using straight lines and sharp angles.)

Zoof gets it though. Thankyou, that's exactly what I was looking for. Pictures aren't needed, anyone who actually does it will know what I mean, and will be able to understand the responses of anyone else who tries it.

I imagine all scanners show it to a limited extent, the question was how much? While I can blank the return draw so the line is formed by only one, the real purpose is to reveal what effects hysteresis has on small scale drawing, and the large reach in the other axis in this rotating line test just amplifies this in a way that makes it easy for us to gauge by eye. The rotation is important too, it shows a dynamic result, not just a static one, it shows at what points the hysteresis begins and ends, and even gives it shape so it might be possible to figure out exact causes. A lot of the time it won't be possible to hide it by blanking, it will affect the fine details of many things we want to show.

Here's the first post I made:
http://www.photonlexicon.com/forums/...0862#post20862
View the AVI file if nothing else, it will show the problem, and you'll see what's needed to explore it yourselves.

What I really want to see is a comparison of several scanner types, so we can pool information about this. Some scanners might be better by far than others for fine detail, and there might even be nice suprises. I suspect some of the cheaper ones might score high. No way to know if no-one tries. No special skills needed, beyond the ability to use whatever scanners and software you've got.

[Lampy]

I'll try this with my new DT-40 pros once the new projector is finished. I used to notice this with the old LCII catweazles when projecting waves with a re-trace to save blanking and jumping straight back to the start. It was never by more than the thickness of the line though. The increase in brightness for not blanking the laser is quite suprising though.

[The_Doctor]

Nice. Thankyou. Looking forward to that, the DT-40's are one of the scanners I'm most interested in seeing a result for. I suspect they might be very good for fine detail, judging by some pics here.

WideMoves might be rare in being that bad, as that AVI shows, it's a LOT more than a line width.

The CT's ought to be flawless, but are they? No-one's shown anything like this test as far as I know, so I think they need to be tested for this.

Trying at full scan angle is good, but also, trying at maybe an eighth of it is important because fine detail is obviously a small part of any image.

What's special about the line test is it shows that you can't eradicate the problems of small scale hysteresis just by using the full scan angle and hoping it will pale into insignificance. It won't.

[stanwax]

Hi Doc

here is a vid of my ct6800s doing it. Its not using your audio file but just a line set to rotate with mambs. I will try it with your audio file when I can rig up the sound output to the scanners.

vid can be downloaded from
http://www.stanwax.plus.com/laser/rotline.wmv


Rob

[The_Doctor]

Thanks, that looks great. I think the CT's pass well. No need to try the audio unless you want to, I think your rotation rate is good, not sure what your scan angle and line widths were, but looks like fairly narrow angle. If so, trying a wide one might reveal more. Looks like there won't be much hysteresis to reveal though. Could be worth trying at various scan speeds too, independent of line rotation speed.

Anyone want to buy some WideMoves?

[stanwax]

scan speed is 30k @15 degrees I can open them up some more and try again but im certain it will be good. I will try the audio just cos im interested - after all I have followed your problem on and off since the start and I like you would be interested to know where the problem lies - ok probably in the scanner but what in the scanner is causing it?

Rob

[The_Doctor]

My guess is it's due to either friction, or a dip in the distribution of a magnetic field. I first thought it was a flat on a bearing formed during long off-times, but as the mirrors can turn full circles freely it's easy to turn them a couple of times to prove that the new resting place for a flat won't likely be in the centre resting position, yet the error is still centred after turning the shafts and reapplying power.

I've looked at the paper sheets that came with them, and the PDF brochure, and the MediaLas page for them, and not found a word about how WideMoves do their position detection. It might be a magnetic sensor, I vaguely remember something like that being written somewhere. I don't think it's capacitative like CT used, or optical.

I made a similar test of a small circle. I won't post pictures unless someone really needs them, but what they showed is a circle generated by a quadrature oscillator pair built from Wein Bridges, nice pure sine waves. Oscillograph of signal output shows extremely clean round circle, laser scan and oscillograph of scan driver feedback show equal flattening of sides and top, so it's clear the sensor IS working fine, but the scan drive can't push the scanners into overcoming the physical cause.

I guess I might overcome it to some extent by increasing gain and damping, so the motor torque is increased, but that could overstress WideMoves badly, and they don't have large heatsink blocks to help them either. Still, I'd settle for slower overall speed if this trick can be used to increase their accuracy.

Comments welcome, I'm not sure enough of this to tweak recklessly. Florian Rotter more than made good, he helped me immensely with some laser parts, and these WideMoves are way out of guarantee period so I don't have to leave then untouched to preserve the condition I got them in, but as they are still in exactly new condition, adjusted as they left the factory, I don't want to disturb such a clean start without a better idea of what should be done.

[stanwax]

Ok heres something.

I realised that this test - the Mamba one was done with blanking. So I repeated with no blanking and guess what I get similar results to you. Same scan speed and angle just red laser as the vid camera seemed to overecxpose the green making it difficult to see. First vid shows red scan complete then shows a close up.
second vid - and heres the thing shows the scope trace set to XY and it displays a similar shape to that being projected. This shows that the scanners cant track accurately with this input.

http://www.stanwax.plus.com/laser/rednoblank.wmv

http://www.stanwax.plus.com/laser/scope.wmv

The thing I find most interesting though - and maybe someone has an explanation for this - is that when I lower the scanspeed to 1000pps so the intermediate points are shown more clearly, the return seems to have as many points as the scanned line. I would have expected several points as the object is drawn with a quick return to the starting point without any intermediate points. I am guessing this is needed for better return accuracy but dont know.I expected the scope trace of YT to show a sawtooth wave but it in fact is a triangle wave.

Rob

[Lampy]

Rob, have you checked the output settings in mamba? You can change the interpolation for blanking as well as adding more blanking points.

At risk of hijacking this thread, when I was trying this out earlier, I noticed my Mamba has a bug where the "Don't return to centre" option doesn't work properly anymore. When selected, the galvos make an awful noise and the image is distorted and when I click "Show blanking lines" you can see it is still returning the beam to centre but with no interpolated points and with only maybe one point at the centre sor the scanners screem! If I un-check the "Don't return to centre" box, the scanners return to centre after the frame as expected with interpolated points.

Sorry for the large slightly off-topic post but I wasn't sure whether to start a new one or not.

Cheers,

Matt.

[carmangary]

Please explain this "Don't return to center" option and what it is for.

Thanks,
Gary