DT 40 Pro set or......

I have had the wide move 9024 scanner set for a couple of years now. I have tuned on these things too long and too many times. I am just not going to get them to do great text with nice square corners. Looking at ZOOF's logo....I couldn't get it that nice and crisp. I realised that these would not be ...well....what I wanted in some future time. Now it is approaching fast. Any suggestions? I have seen good reports on the DT sets. Can't do the big Cambridge thing at all. Just looking for better text and graphic definitions. In the mean time I will look into some of the specs on different scan sets. Waiting to see how much damage the tax man is going to do. It may depleat savings to a point where it will just have to wait a while.

Zoofs aren't dt-40's

they are the raytrack35+ from jmlaser

Hi MechEng3,

My first choice for scanners is Cambridge. But if you can't afford Cambridge, then the DT-40 PRO is not a bad choice. From what I can tell, the scanner amp is nearly identical (if not identical) to the Cambridge SMT amp design (which is a good thing because, even though it's a copy, it's a copy of something that is really good...). The scanner itself is basically a copy of the original (circa 1992) 6800, with some differences. There are things about the DT-40 that I don't like, but Jian is constantly improving them.

I will soon be receiving some scanners, amps, and other "parts" from PR-Laser. They are sending me a complete kit for evaluation, including internal parts (so I don't have to destroy a scanner to test the internal parts). I am not sure if it is the case right now, but my intuition is that PR should be the best scanner manufacturer in China. I say this because the principal (Bill Lu) worked for 10 years in what was essentially the Chinese division of General Scanning. I keep saying that scanners is a business of "details" and you really can't get the details just by copying what other people have done from the outside in. The details must come from trade secrets which Bill was no doubt privy to. Also, Bill Lu is engineering oriented and as far as I can remember, he received his Masters degree. I will let you know how the testing goes...

Best regards,

William Benner

We have been selling(and using) the PR scanners for over a year now. I will be interest to know your findings Bill :)

I hope http://www.pr-laser.net/ will come in english website also ;)
Har to understand Chinese...

It looks as if they have a model BL-50. 50 kpps? Would be nice to get english info on these. Mirrors show >90% reflection @45 Deg. Nice if it was a better mirror....say >97%. It would be a pain in the a$$ to make an acurate fixture to just do one set of mirrors....swapping out theirs for real high quality units. Wonder what kind of price they have.....always in the dark on the money side of manu's prices. They seem to have a strange way of offering prices via direct contact. Maybe someone will invite them here for the lowdown on what they have to offer........

Yep, they are Raytrack 35 plus from JMlasers. The galvos are the DT40 pros but the driver board is different. Set comes tuned to 35K ilda.

Dave,

You have over a years exeprience with selling PR-Laser galvos. What has your experience been by way of feedback... any known issues ?? Mirror quality ??
Thanks
Ray NZ

mirror quality isn't the greatest(~90%)

Apart from that, we haven't had any issues in our hire lasers, and we have nearly 10 sets in use..

Quote:

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Originally Posted by Pangolin

but my intuition is that PR should be the best scanner manufacturer in China.

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Hi bill,
Do you remember that projector of PR-laser scanner in ILDA conference?
I think you already saw PR-laser scanner of lasershow.
Why intuition ?

You will know which is best in future.:-)

Jian

Jian-
Just trying to clear something up- what is the 400-660nm reflectivity on your present mirrors? >90%? >95%?

Quote:

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Originally Posted by steve-o

Jian-
Just trying to clear something up- what is the 400-660nm reflectivity on your present mirrors? >90%? >95%?

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Hi Steve,
Sorry,I haven't laser power meter on hand.
I think most people have this misconceive in lasershow system.
http://www.photonlexicon.com/forums/...?t=2154&page=4

Seem no anyone discuss how many loss at the laser of the modulation!!!

Even the mirror loss 10%,but is that your lasershow always runing at 90% power output???
Maybe,if you cut the modulation.

At fact when the laser of the modulation at 2Khz ,your laser output almost just 30% output,40% is the very good performance laser.

That loss 60%-70%!!!!!!!!!!!!!!!!!

e.g.
You buy one 1W green laser for your laser system.
hypothesis the galvo's mirror reflectivity 99.9%.
When the modulation up to 2Khz,the laser output about 300-400mw only!!!
If you buy the 10Khz modulation laser,maybe at 2Khz just 500-600mw output!!!

So DT40pro scanner using dielectric mirrors.
Anyway that better than aluminium mirrors ,maybe better than silver-gilt mirror.

Jian

Thanks Jian, I think I understand.
I plan on buying a set of yours when money allows..:)
Steve

Quote:

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Originally Posted by jian725

Hi bill,
Do you remember that projector of PR-laser scanner in ILDA conference?
I think you already saw PR-laser scanner of lasershow.

---

Actually no. That projector did not have PR scanners in it as far as I know. Plus, one thing we must keep in mind is exactly WHO's software was running that projector... :D

Quote:

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Originally Posted by jian725

Why intuition ?

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I already explained my intiution. When someone has his masters degree and dozens of published papers, and spends 10 years working for one of the top scanner companies in the world, thereby having access to trade secrets of how to make scanners, and when the vast majority of scanners used by Chinese companies are made by PR, well, it just makes sense that they would be the best. That's my intuition. I have not yet received scanners and parts that they are sending, so I can test them and back-up my intuition. But until then, that's going to be my intuition.

By the way, I seem to recall that you were going to send me another scanner, or at least another rotor to replace the one that was shattered during testing of DT scanners. Have you sent it yet?

Quote:

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Originally Posted by jian725

You will know which is best in future.:-)

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Sure. The answer to that question is simple. Whoever has access to the best knowledge base of the details of scanners, plus access to the best machines and test equipment to manufacture and test scanners, plus access to the best actual parts (magnets, bearings, winding jigs, etc.), and finally whoever has the best quality control in place, will, by definition, be the best. :D


Bill

Quote:

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Originally Posted by Pangolin

Actually no. That projector did not have PR scanners in it as far as I know. Plus, one thing we must keep in mind is exactly WHO's software was running that projector... :D

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Oh,I sure that is PR-laser scanner .They using phoenix software and CNI blue laser.
You care what software they using.I care what scanner they using:-)

Quote:

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Originally Posted by Pangolin

so I can test them and back-up my intuition. But until then, that's going to be my intuition.

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Ahhh,let's see~~~~~~~

Quote:

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Originally Posted by Pangolin

By the way, I seem to recall that you were going to send me another scanner, or at least another rotor to replace the one that was shattered during testing of DT scanners. Have you sent it yet?

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I make the bigger order for the parts,(300 shafts,450PCB and 250 glavo's housing)so everything slow and the parts not enough.
That will get better at next week,everything enough.send to you ASAP.
Last at this month-end.

Quote:

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Originally Posted by Pangolin

Sure. The answer to that question is simple. Whoever has access to the best knowledge base of the details of scanners, plus access to the best machines and test equipment to manufacture and test scanners, plus access to the best actual parts (magnets, bearings, winding jigs, etc.), and finally whoever has they best quality control in place, will, by definition, be the best. :D

Bill

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All right.
BUT make an exception,you will find soon.:-)
Jian's company not a company,just a team.
No oscillograph etc...
But our team can creation the details :-)

Jian

Hi All,

I have done a bit more testing of the DT-40 PRO scanner. Some people might recall that David Zurcher was kind enough to send me his pair, since he also had an older DT-40 set he was using. During some initial testing, I actually shattered the rotor in one of the DT-40 PRO scanners (first time I ever shattered a rotor in my life!!), so I stopped testing right then.

Since I recently received some scanners from PR-Laser, I wanted to do some testing of DT-40 just for comparison purposes, so I used the remaining scanner for this purpose, being more careful this time not to shatter the rotor. I have produced a write-up with some of the results, which can be downloaded here:

http://www.pangolin.com/DT40Testing.pdf

Basically, the reader's digest of my initial testing results is that I like the output shaft thickness and mirror thickness of the PR-Laser scanners better than the DT-40 scanners. Since stiffness goes as the fourth power of a solid cylinder, and at least as the square of the thickness of a solid plank, the larger output shaft and thicker mirror is far better from a mechanical resonance standpoint. Also, PR-Laser scanners are tuned for 30K "from the factory" while DT scanners are tuned "higher". But as we have learned, the "higher" tuning does not do end-users any favors, as discussed in the past, and as seen in person by Photon Lexicon members at the last FLEM (and no doubt many meetings to come)...

I think Jian can improve his scanner by making the output shaft diameter thicker, and mirrors thicker, as discussed in several previous postings. Jian would also do his users good (some of whom are also Pangolin users) by a) getting an oscilloscope; b) learning how to use it to tune scanners; and c) tuning scanners to 30K.

If you want to go faster than 30K, it requires a different scanner design, which is why Cambridge has the model 6215 and PR has their model BL-40 (which may some day become the BL-60).

There are some aspects of DT scanners which I like better than PR-Laser scanners. I think Jian's X-Y mount is better, and certain electrical-connection-related things are better. But this will not be the case for long. PR-Laser has specifically sent me all of their scanners, and even pieces parts, and asked me to give them feedback. They will no doubt take my advice and quickly improve their products. The company who takes these recommendations and the recommendations of other users to heart, will no doubt produce the best scanners. If Jian and PR both take the advice from users, then both will produce very good scanners and everyone wins, which is of course the point of this whole exercise...

Best regards,

William Benner

Interesting stuff Bill :)

Tanks Bill, that is interesting.. I'm just curious now how Cambridge achieves that low rotor inertia with the ticker shaft... Or should the difference be accounted to the difference in calculated and specified inertia?

Hi Bill,

Quote:

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Originally Posted by Pangolin

Hi All,
I think Jian can improve his scanner by making the output shaft diameter thicker, and mirrors thicker

William Benner

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In fact,I was finished all parts,but I still not send to you,because waiting the 0.8mm mirror.Now the output shaft diameter is 1.95mm,and the gap is 0.8mm.
So I will send the rotor and the galvo when I get the 0.8mm mirror.

Greeting
Jian

Hi Jian,

Sounds good, but you don't have to "turn down" the output shaft to 1.95mm. The perfect shaft size for the inner journal of the bearing is 2mm. By "turning down" the shaft to 1.95 mm, you add an extra machining step, thereby increasing your costs of manufacture, and also involve a "tolerance" into the equation, which could possibly cause you to throw away parts that don't meet tolerance.

What all of the big scanner companies do these days is quite easy -- don't turn it down!! Just come straight out of the bearing journal with the full shaft size, unmodified. No extra machining steps, no extra tolerance, no wasted parts, no additional cost.

But what's more important than all of this is the increase in stiffness. Stiffness goes as the fourth power of the diameter for a solid cylinder. So it might seem that going from 1.95mm to 2mm wont do much (it's only a 2.6% increase). BUT, the stiffness is increased by (2 / 1.95) ^ 4 = 11% !! THAT is my point :D.

Best regards,

William Benner

Hi Bill,

Quote:

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Originally Posted by Pangolin

Hi Jian,

What all of the big scanner companies do these days is quite easy -- don't turn it down!! Just come straight out of the bearing journal with the full shaft size, unmodified. No extra machining steps, no extra tolerance, no wasted parts, no additional cost.

---

For me also,that is easy!!! Don't need additional any cost.
Small ship easy to turn around:D (Chinese idiom)

Quote:

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BUT, the stiffness is increased by (2 / 1.95) ^ 4 = 11% !! THAT is my point :D.

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Oh,I real don't know that proportion.Now I know how to do it.
Thanks
Jian

Bill,

I think Jian turns down the shaft, because a shaft will never fit in the bearing when at exactly 2mm. Bearings normally have an inner tolerance of H6. If you use a shaft of H7, for example, you could just move the bearing along the shaft by using very high power. This can damage the bearing.
If you use g6 for the shaft (means about a few 1/100mm less diameter), you can move the bearing to its final mounting position.
The part where the bearing is located, is thicker, so the bearing can be pressed on the shaft.
I am quite shure that also CT and others do this.

About your tests:
Quite interesting! It is the same result, I also had. I also slowered down the scanners a bit to get a nicer looking result of the image.
I also did some tests with notch filters.
The tuning of the filter is not that difficult. The filter has a center frequency of about 10KHz and a bandwidth of about 10KHz. So it starts limiting the signal from 5KHz. There "frequency" adjustment of the filter nearly has no effect. You can replace it by a fixed value. Because of the large bandwidth, it makes no big change if the center frequency is at 9500 or 10500 Hz.
The gain adjustment is the only important one. There is only a small tuning area where the thing is stable.
Naturally, when increasing filter gain, all other parameters of the driver are changed, because of the increasing delay (phase) of the signal.
For this reason, the filter works properly in a small tuning gap only.
The driver first should be tuned without filter to get the basic settings.
Then the filter is added and tuned to the optimal value.
It works.
But as you told: It is not necessary to use the filter in any case.
If there is no mirror resonance at all, it is better to go without the filter.

Joachim

Quote:

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Originally Posted by JoJo

I think Jian turns down the shaft, because a shaft will never fit in the bearing when at exactly 2mm.

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Nope, that's not the case. Or at least, that's not my point... Jian (and a few other Chinese manufacturers) come through the bearing with a full-size shaft, and then turn down the shaft after they are through the bearing. Cambridge did that on the original (circa 1992) 6800, but then later learned that it is smarter to just come out of the bearing and leave the shaft alone.

There is an intuition that smaller shafts are better because they are "lighter". There is also an intuition that thinner mirrors are better because they are "lighter". Unfortunately, these are instances where intuition leads you down the wrong path... Dynamic stiffness is often far more important than "light"-ness...

Quote:

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Originally Posted by JoJo

I also did some tests with notch filters. The tuning of the filter is not that difficult.

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From reading your entire post above, the only thing that I can say is that your own tests with notch filters must be flawed.

A few points:

First, it's not difficult to tune "wrong"... I will post a few pictures of the right way and wrong way to tune a notch filter. The wrong way is to adjust notch depth for maximum effectiveness. This can be done "not that difficult" with a scope. But unfortunately this will be wrong in most cases...

A proper notch filter for a servo system has both frequency and notch depth adjustments. The notch depth adjustment is put there to match the amplitude of the resonance. If the notch frequency and depth are set correctly, then there is no "bump" in servo response, both in terms of phase and gain. As far as I have been able to figure out for myself, a notch filter can only be adjusted *properly* with a dynamic signal analyzer.

Second, in your example above, the notch does very little at 5kHz. The "bandwidth" you speak about is the -3dB bandwidth. BUT, if the resonance is 60dB, then -3dB doesn't buy you much...

Also, you are considering "bandwidth" above, but really you should be considering the "Q" of the filter... (similar, but different way of looking at it) The Q is set relatively low (typically between 1 and 4) to avoid phase loss inside the servo loop. Also, ideally the Q is set to roughly match the resonance properties of the rotor/mirror system, including both frequency and Q. If the notch frequency was off by the amount you suggest above, it would be, essentially, ineffective. Again, I will post a few pictures to illustrate these facts.

Third, Cambridge wouldn't put both a frequency and notch depth adjustment on a notch filter module, if both were not absolutely necessary to tune. So with this being the case, intuition should lead you to believe that smart people would not put adjustable controls where they are not needed. Cambridge are smart people... (and this is one case where intuition WOULD lead you down the right path...)

Quote:

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Originally Posted by JoJo

But as you told: It is not necessary to use the filter in any case.
If there is no mirror resonance at all, it is better to go without the filter.

---

Well, there is always a mirror resonance. Most often, this is a longitudinal resonance which is well within the bandwidth of the system. In addition to this, there is always an anti-resonance which occurs at a frequency related to the shaft thickness and mirror inertia, and a resonance which is related to the shaft thickness, mirror inertia and rotor inertia (which is why it is important to make the shaft as thick as practical). There may also be other resonances related to the position sensor and other rotor components. If you look closely at the plots in the document I posted earlier (http://www.pangolin.com/DT40Testing.pdf), you will see that this scanning system has an anti-resonance, then a resonance, then another resonance, then another anti-resonance. These resonances are ALWAYS there. The question is, will they erode gain margin yes or no? For scanners that were designed to operate at 30K, and then tuned to 30K, the amplitude of these resonances will not erode gain margin, so you don't need a notch filter.

But that's one of my key points! Some people try take a scanner that was designed to go 30K (6800 and its derivatives) and make them go faster than 30K. This is never a good idea for a variety of reasons. As proof, I fall back on the fact that all people at FLEM reported BETTER looking images after their scanners were re-tuned (down) to 30K.

Best regards,

William Benner

This is an interesting conversation--I have a semi-related question for you experts. What does 30K equivalate to in Hz? That is; if you put a square wave into the amplifier to obtain full angular deflection, what is the maximum frequency that the galvos would be able to handle before catastrophic damage (mirror or mechanical)
From your oscilloscope pictures it looks to be around 5 KHz.

A square wave, though used to form the inner square on the ILDA test pattern, is probably not the best wave to try to equate to a frequency, because it involves very sharp transients. For scanning, only the lines that are blanked while reaching new draws will be traced with a transient that sharp. I'm not sure what regular wave is best for testing to a maximum frequncy to equate to a point rate in some way, but I think the triangle might be it, because that way you can test linearity and overshoot. Sine waves might be useful, but not as useful as triangles.

Edit: Even with triangle waves, you can get steeper sides if you add dwell time at each peak, making trapezoid waves. What this means that maximum safe scan frequency of a periodic wave is very dependent on the waveshape, which is why specifying in Hz is not as practical as it sounds. It took me some time (and attempts to program scan patterns) to understand that. I also saw that it depends on the scanners. WideMoves have a smaller variation in step speed per unit angle that most other types, so the dwell time could be made constant for most angular changes of drawn lines, only changing for speed or overall scan angle. Scanners with more step speed change per angle change would complicate any attempt to convert point rate to a frequency.

Well, let's just say that you wanted to put a wave into a scanner to see how fast it'll go before coming apart. Any ideas?

Start slow. Keep scan angle constant, and speed it up gently. If you have a synthesizer or wave generator, either will probably do it. Look for small errors growing in the scan shape. Just take care, cos if you use a sine, you can go a LOT faster than with other waves, so if you change the wave but forget to reset slow, you might really damage something. Listen closely to the scanners too. Their sound can be a cue to upper thresholds for safety, but only as a secondary guide, and only if you can hear them...

More.. If you have Sound Forge or Cool Edit or GoldWave, draw a trapezoid wave (four steps, slope up, hold, slope down, hold, each step equal duration) at around 10 Hz in one channel, do same in the other channel, 90 degrees phase shifted. This will scan a square. Duplicate both waveshapes to form a long continuation to about three minutes worth, then put an upward pitch bend on the whole thing to around 5 KHz (up 9 octaves from 10 Hz). Play it and pause the wave the moment you see anything start to warp, then look at the editor to work out the frequency. This test won't give you a simple useable answer, but once you try it with a few waveshapes it will give you a strong sense of what scanners do with different inputs. It's also cool what happens if you put a very small detune between each channel before or after the pitch bend.

Mis -communication i think. I'm not testing scanners, I'm just curious. (I'm fooling around with very small magnets and coils in different configurations) Just wondering how fast those things go (in Hz instead of points.)

You just said you were testing scanners, to see how fast they could go before something fell apart. Not sure how else I can interpret a question like that. I answered all I can. It depends on lots of stuff. I described something you can do. If you try it, you'll soon figure out why there's no simple answer, no matter mow much one might be wanted.

I understand, perhaps I wasn't clear. Just as a motor has a max RPM, I thought scanners on a set single input wave would have a maximum scan back-and-forth rate. I guess not. Apples and oranges I suppose.

Quote:

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Originally Posted by steve-o

I understand, perhaps I wasn't clear. Just as a motor has a max RPM, I thought scanners on a set single input wave would have a maximum scan back-and-forth rate. I guess not. Apples and oranges I suppose.

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Ok, you could try the simplest form, two sine waves in quadrature (90 degrees phase shift). That draws a circle, and if you speed that up gently, you'll see the circle break up with erratic rings accompanied by overall distortion of shape, and hear the scanners start to scream. Do at your own risk. >:) That will show a direct upper frequency, but it's still highly dependent on scan angle, and is harder to derive an equivalent point rate from than the trapezoids I described earlier. Those would have a much slower top speed too.

OK I'm going to be honest here, I'm not testing scanners- I dont even own a set:o I build most of my own stuff and I was just curious about the scan speed of pro-made galvos. I'm probably gonna do it again and diy. Sorry.
No, I'm not sorry. That's my hobby and my preference. It's my party and I can build if i want to:) -and yes I realize they wont even be 1/10 as good as the 'big boyz' but I'm not pro, only do one show a year and dont have Pangolin/ DT40s/ at the top of the to-do list. (blasphemy-I know) I just like lasers. always have. My first he-ne was 20 years ago, and I still love piddlin' around with 'em the best I can.

Hey Steve-o;

I don't know about your comment that "home made galvos won't be 1/10th as good as the big boys..." I remember the home-built scanners that this guy built, and from the pics on his website they look to be about on par with the 15kpps galvos that Dave and Aijii sell on E-bay. (In fact, the guy said he can run his home-built scanners at 18Kpps....)

Personally I'd rather not attempt to build my own scanners, but I don't doubt that it is possible to build something that works quite well. Sure, you might not get "top" performance, but if you get 75 percent of what the "big boys" have, and you've done all the work yourself, I'd say that's a *huge* accomplishment. So go for it!

Don't ever apologize for having the drive to do something yourself. Hell, if it wasn't for Patrick Murphy and Bill Benner building a new laser show hardware controller and writing the custom software to go with it, we'd never have Pangolin! (And I promise you they never apologized for doing it themselves...)

Finally, for what it's worth - I have always wanted a rule-of-thumb conversion for Kpps speed to Hz for the scanner when moving at a given scan angle. And, like the Doctor posted, the answer is always a fuzzy "it depends"... Now, I understand the issues that he raised in his posts, and they're valid, but you can control for nearly all of them.

Thus the question remains: Assuming you are running a set of galvos within their normal operating range (that is, they're not screaming, the pattern is not distorting, the coils are not overheating, and you are not in danger of damaging the scanners), if you set your scan speed at 30 Kpps, what is the *maximum* speed that the scanner will move (either in degrees per second, or in Hz), assuming 8 degrees optical scan angle? The answer will be specific to a single scanner, I know, but it would be enough to make some comparative calculations. It would be useful information for some of the other discussions we've had here on PhotonLexicon. (A second question would be, what is the galvo speed with the same conditions above if you are scanning at the maximum scan angle for the galvos - say 60 degrees, instead of only 8 degrees.)

Adam

Hi guys,

A few quick points. Some may be related to Steve's questions, and some may not.

First, you really can't equate squarewave performance to "K" rating. This is because a squarewave requires maximum acceleration, and then maximum deceleration from the scanner. If the servo and scanning system is designed properly, you can produce squarewaves at any scan angle up to the scan angle rating of the scanner. The frequency limitation of the squarewave will have essentially nothing to do with "RPM" or similar factors, but will probably have everything to do with the heat limitations of the system. When the scanner is producing maximum acceleration and maximum deceleration, heat will be generated for those brief intervals. If they happen often enough (i.e. high enough frequency), then you will reach (or exceed) the heat dissipation capability of the system. Also, it might be obvious, but you will be able to scan a smaller image faster than you can scan a larger image, before the heat dissipation capability is reached.

So unfortunately, from a squarewave perspective, we really can't give a difinitive frequency or size that will apply to all scanning systems. A General Scanning model G120 can handle scanning squarewaves at 1kHz or more, and at its full rated angle, but moving magnet scanners could never do this because of the way heat is generated and dissipated in these systems. Also, a Cambridge model 6800 can scan squarewaves wider and faster than Chinese scanners can, even though the Chinese scanners are theoretically copies of the 6800, because of the way the coil is manufactured and then bonded to the stator. So we can't even say that a particular style of scanner will do the same job...

Really, a squarewave is not a good way of rating a scanner, other than to say "will it produce a critically damped step yes or no". Regardless of the size or type of scanner, I usually use a 50Hz squarewave to test scanners, just to calibrate critical damping. (If you look at the picture I posted earlier, you will notice that the function generator is set for 50Hz.)

Also, while on the topic of "K", you find the small signal bandwidth of the system by dividing the "K" by 12. For example, 30K equates to a small signal bandwidth of 2.5kHz. This is because there are 12 points in the circle portion of the ILDA test pattern, AND because the square is 70% of the ORIGINAL size of the circle. So if the circle is inside the square, it has been reduced to 70% of it's original size, or 0.707, which constitutes the -3dB (power) point, or -6db (gain) point. Eyes might be starting to glaze over about now... so just suffice it to say that 30K is a small signal bandwidth of 2.5kHz, and 60K is a small signal bandwidth of 5kHz.

However, just because you have your scanners tuned for 30K, and thus, you have a small signal bandwidth of 2.5kHz, doesn't mean that the scanning system will produce a 2.5kHz circle at any angle. There is also an angular limitation which is related to heat and to the power supply capability of the servo driver. That's why when using the ILDA test pattern, you are supposed to only use it at a scan angle of about 10 degrees. Scanning the pattern wider will require more power from the servo, and greater heat dissipation from the entire system. Eventually the servo will "run out of power supply" and the circle will stop getting larger, and will deform.

There is a related question / factor, which is "well exactly how large is this small signal". That's a point I am always making to scanner companies. At Pangolin, we have some scanners that can scan a 15mm beam, and have a small signal bandwidth of 5kHz. Yes, that's right, a scanning system capable of moving a 15mm beam at "60 K". BUT, at 5kHz that "small signal" is pretty small... It can only produce a 5kHz sinewave at 1 degree or less...

One more thing while on the topic of "K" and bandwidth. Just because a scanning system is tuned for 30K, and has a small signal bandwidth of 2.5kHz, doesn't mean it won't scan any faster than 2.5kHz. The mirror will still produce some motion out to around 7.5kHz for systems tuned to 30K, and out to around 10kHz for systems tuned to 60K. So this relates to the "highest reproducible frequency".

Anyway, there are a number of ways we rate scanners, but for lightshow folks, there are only two meaningful measures. One is "small signal bandwidth". This is probably the most meaningful measure and the ILDA test pattern can help to determine this. The second is "heat dissipation capability". We have a test pattern which can help determine that as well, but it is a bit more abstract in the sense of what it buys you in terms of imagery.

Best regards,

William Benner

Thanks William!
Very informative. Exactly the information that I was looking for!
Steve

I don't think anyone advocated square waves. I set out by saying that they were probably NOT the best source, as my first sentence in this thread. I'm beginning to wonder if there is really a flaw in my input here, or if it's just coming from the wrong person.

I really don't know how to answer that Doc. All I wanted to see is some numbers in Hertz instead of 'points' for a ballpark scanning frequency. I know that X-Y scanning is a lot more complicated than that. I didn't see any "flaws" anywhere.

Ok. The main concern I had was that my input might be seen as somehow wrong, corrected or contradicted by Bill's. If you review it though, you'll see a lot in common. While Bill links the small signal bandwidth to the circle's 12-point form in the ILDA pattern, he says that the size of this small signal depends on scanners (available power, inertia, heat dissipation), so it can be difficult to pin down. He's given you some specific numbers, but even that /12 might be considered as a convention, because the circle is run fast enough to smooth those points into a round form, not the 12 sided polygon you see in a chart of the ILDA pattern. I think that's why he says that the small signal bandwidth is the most meaningful, because point rate is a derivation of the choice of 12 points in that circle. There's no reason why a slightly different number of points (anywhere between 8 and 24) might not have been used, so long as one circle's worth was drawn in the same time. 12 is a good compromise, as less might mean that at slow speeds you'll see more polygon than circle, and more would not help it look any more circular.

I like Buffo's point about being able to contol for nearly all the variables. The more things are the same, the easier you can test the changes. If you start with the maximum speed at which you can safely draw a small circle of a chosen scan angle, you can then take the perimeter distance, divide by twelve (ILDA convention), then you know roughly how many points you can make and how fast, and how far apart you can make them. Even this isn't easy to predict because some scans reduce the point separation to navigate awkward turns, but it should give you a working average.

Thank you Doctor,
Helpful info also. I'm now getting a good grasp on the fundamentals and intricacies of scanning principles.

If you can find the money to get some WideMoves, you can pick up loads. I should know. :) I thrashed mine, only partly by accident, and they work as well as ever. They deserve their rep for taking a beating well. I'm glad I learned from those and not from a pair of new-and-subsequently-destroyed CT scanners. I still don't know how WideMoves do take it so well, given that they're not the best at getting heat out. Might have something to do with the thin mirrors and mounts. While not ideal, their pliability makes a weak point that bends rather than breaks, so people tend to back off well before serious hammering occurs.

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If you can find the money

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Ahhh, yes, the money. mmm. If only i was born rich instead of so talented;) hehe

Yeah, was a big decision for me, getting them.. Glad I did though, despite their shortcomings with small movements. I got my money's worth. Could be worth putting on the eBay watch list...

BINGO!

Bill, if you were a dancer at a strip club, I would have thrown you a twenty for that last post of yours! "Small Signal Bandwidth..." That is *EXACTLY* the information I was looking for. And though I hadn't heard the term mentioned here before, I instantly understood what you were talking about when you said it.

In retrospect, the answer really was right in front of us. I knew that at maximum acceleration the galvos would be ballistic (just like they are when scanning the circle in the ILDA test pattern), but I didn't think about how the circle in the ILDA test pattern is actually made of just 12 points... If I had made that connection the conversion would have been obvious.

Either way though, thanks for clearing that up!

Adam

Uhhh- I hope he takes that 1st sentence as a compliment..;) otherwise he might not be coming back here 00
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I think Bill knows I didn't mean it offensively. ;) Seriously though, that question has been nagging me for the better part of a year now. It's come up here on the forums several times, and I've never gotten a decent answer before now.

Adam

Hi guys,

Just a few quick points :)

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Originally Posted by The_Doctor

There's no reason why a slightly different number of points (anywhere between 8 and 24) might not have been used, so long as one circle's worth was drawn in the same time.

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The main thing to understand is that when the servo system is tuned properly, it behaves as a second-order Bessel low pass filter. The points in a circle attempt to present the scanner with a sine-wave. 12-points ordinarily wouldn't make a very nice sine-wave, but due to the scanner's low pass filter response, the scanner will produce a circular image.

What Doc said above is somewhat true that it doesn't matter how many points a circle has, but there are a few elements to keep in mind. First, the fewer the number of points, the greater the harmonic distortion. If the number of points is too low, it comes back to haunt you -- the servo has to work extra hard to filter out the out-of-band signals. Second, this may be obvious by now, but you can't just re-do the circle part of the ILDA test pattern and expect it to still work at 30K. That pattern requires a certain ratio of number of points to K. If you re-do the test pattern with 24 points in the circle, you will need to run the pattern at 60K even though the small signal bandwidth being tested would still be only 2.5K.

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Originally Posted by The_Doctor

but I didn't think about how the circle in the ILDA test pattern is actually made of just 12 points...

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Actually the circle portion of the ILDA test patern has multiple iterations of a circle. There are probably around 36 or 40 points in that area. But there are only 12 points PER circle (the circle is drawn over and over again to get the scanner up to speed before turning on the beam, and keep it going the same speed after the beam is turned off).

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Originally Posted by Buffo

if you were a dancer at a strip club, I would have thrown you a twenty for that last post of yours

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Ooooo. Big tipper!! But how is it that this was the first thing to come to your mind? :D


Bill

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Originally Posted by Pangolin

Second, this may be obvious by now, but you can't just re-do the circle part of the ILDA test pattern and expect it to still work at 30K. That pattern requires a certain ratio of number of points to K. If you re-do the test pattern with 24 points in the circle, you will need to run the pattern at 60K even though the small signal bandwidth being tested would still be only 2.5K.

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True, I tend to overlook that because I was working with wave data, with the sample rate rather than a point rate. If I chose two points closer together in space, they'd also be closer in time (and if on a linear gradient I'd not consider them as separate points until the gradient changed). The thing about that is, it can interpolate within the same timeframe which is another way of reducing unwanted harmonics, as what stepped gradient changes there are can be smaller on curves, within the allowance set by the sample rate. I'll still use a stepped change though, as calculating nice curves takes so much time, and the scanner's inertia is part of a filter that has no CPU overhead.

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Actually the circle portion of the ILDA test patern has multiple iterations of a circle. There are probably around 36 or 40 points in that area. But there are only 12 points PER circle (the circle is drawn over and over again to get the scanner up to speed before turning on the beam, and keep it going the same speed after the beam is turned off).

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I should have said twelve points per turn.. I noticed from a file that James Lehman made for me to test with, that there were 3.25 turns, I think. I got the feeling that this total could have been made smaller with the WideMoves, but I guess it's chosen to allow time for all scanners that have been tested with it.

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Originally Posted by Pangolin

Ooooo. Big tipper!! But how is it that this was the first thing to come to your mind? :D

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Ha! You got me there Bill... :o I honestly don't know either. I can count up the total number of strip clubs I've ever been in on the fingers of one hand. I can't imagine why that thought popped into my head. :rolleyes: (And no, I'm not ready for a Freudian analysis of this!) :D

But I did appreicate the information. We've kicked that topic around several times here on PL without ever really answering it. And the number has real meaning, knowing how fast (or slow) the galvos actually move has huge implications for other parts of the projector system. (Laser blanking speed, for one!)

Adam

Hi buffs,

I feel realy stupid asking the following qeustion after reading all the posts in this thread, but can some one explain to me how to propperly tune scanners to ILDA 30k?

I think I have PR scanners, got them from Lumina, after reading about ILDA 30k test patern the first time I tried to reset the scanners in hope of better performance and since then the scanners have never been the same again!!

Another stupid qeustion, Lets assume your scanners are set/ tunned to ILDA 30k, how do you ensure that your software controlling the scanners are doing the right thing/ sending the right signals to match the tuned state of the scanners??

Hoping to get this ILDA test patern and setting under the knee once and for all....

(thanks for not laughfing)

Hi Werner;

Actually, your question isn't the least bit stupid. It's actually a damn good one. It's a bit hard to answer, even.

Tuning galvo amps is as much an art as it is a science. There are several different ways to procede. This link is a good place to start reading about the procedure. Here is a more step-by-step approach. Both links have a lot of background information on what various areas of the standard ilda test pattern are designed to test.

Having said all that, when I watched Bill Benner adjust the galvos at our gathering in Florida (FLEM), I noticed that the first thing he did was to turn off the blanking, so you saw the entire pattern, not just the lines in the picture but all the re-trace lines too.

Then he adjusted the gain first, which is different than the procedure I linked to above. Basically he first made sure the x and y gain were about equal (the circle was round and not elliptical), and then he started tweaking the x and y gain up and up until the circle stopped getting larger. That's when you know the scanners are at max acceleration (ballistic), which is how they should be for the circle. Then he proceded to adjust the damping; often looking at different parts of the pattern to get it right. There is a lot of trial and error involved, and it's a time-consuming process. (Bill is a pro, and he still spent at least a good 30 minutes doing it.)

Hopefully Bill will chime in here with some of his other tricks. I remember he was using one of the re-trace lines near the bottom of the pattern to adjust something, (high frequency dampinig, maybe?) but I can't remember it now. (This is why the next time I see Bill adjusting a set of scanners, I'm going to videotape the whole damn event and study it!)

Normally, once you get them dialed in you don't have to mess with it again. But getting them dialed in can be a pain - especially if you've never done it before.

As far as knowing that your software is right - well, that's harder to tell. If you're running Pangolin, you can check the setting, and as long as it's set for 30K you should be fine. Same thing with Mamba Black, LDS, Full Auto, etc... However, there are some laser show packages out there (like the Alphalite) that either have slight variations in the timing, or that need to be calibrated for the specific computer you are useing before it will work right. So the answer to your second question is hard to say without knowing what hardware and software you are using.

Either way though, good questions! ;)

Adam