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Thread: What Makes Galvo's Faster?

  1. #1
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    Default What Makes Galvo's Faster?

    The differences between my 20KPPS and 50KPPS scanners seem pretty minimal in appearance. (Definitely a difference in performance )The biggest difference I notice is larger transistors and more pots on the drivers and slightly longer armatures on the galvos. That can't be all their is to it given the large price difference. I notice the Cambridge scanners have very long armatures compared to the Chinese made galvos. What is it? Maybe the tolerance between the moving magnet and the armature? Or, is it the precision quality of the feedback end of the galvo? Or both? Is it mostly in the drivers? Your comments would be greatly appreciated.
    Thanks!
    Adam

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    What Makes Galvo's Faster?

    $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$......

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    I'm gonna' go way out onna' limb here and totally speculate....

    1. Quality of the bearings holding the shaft.

    2. Quality of the magnetic to electrical to physical match of the motor system. Meaning, just how close did this design get to the exact correct diameter of copper (or silver, or super conductor) wire wound onto an armature of what precision of physical dimension, suspended in what Gaussian flux of magnetic field, polarized in which way?

    3. How good is the amplification and feedback mechanism driving the unit (WOW! now that's a whole thread on its own.).

    That leaves out a whole bunch of other important stuff though...

    At the end of the day, it's purely power over mass. How much power can you get into the system, before it burns up, to control the mass in motion as fast and accurately as possible.

    But, I just made all that up. So don't rely on it.

    James.
    Last edited by James Lehman; 06-19-2008 at 20:16.

  4. #4
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    Quote Originally Posted by James Lehman View Post
    I'm gonna' go way out onna' limb here and totally speculate....

    1. Quality of the bearings holding the shaft.

    2. Quality of the magnetic to electrical to physical match of the motor system. Meaning, just how close did this design get to the exact correct diameter of copper (or silver, or super conductor) wire wound onto an armature of what precision of physical dimension, suspended in what Gaussian flux of magnet field, polarized in which way?

    3. How good is the amplification and feedback mechanism driving the unit (WOW! now that's a whole thread on its own.).

    That leaves out a whole bunch of other important stuff though...

    At the end of the day, it's purely power over mass. How much power can you get into the system before it burns up to control the mass in motion as fast and accurately as possible.

    But, I just made all that up. So don't rely on it.

    James.
    Hey James,
    Good points, I thought about the bearings as well. Maybe the high speed units use some type of ceramic, sapphire or even ruby bearings. If not, they probably should.

    Winding material and thickness would also most likely play an important part, also impedance. Just like a quality speaker voicecoil. Though, I couldn't imagine that adding any exorbitant cost's

    I also believe their is a lot to say about the feedback arrangement. I imagine their would be a benefit by switching the LED in the feedback circuit to a LD and using a micro prisim on the shaft. Just a thought.

    As for your last statement, I wonder if TE cooling the galvo's would allow the gain to be tuned up higher? Thinking out loud again...

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    I think galvos would just laugh at a TEC.

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    Cool

    Part of the problem is getting the heat out of the coil. In an earlier post, Bill Benner mentioned a specific trick that Cambridge came up with that allowed them to drive their scanners to really high speeds (thus the 6215's that will run at 60K), but evidently none of the Chinese copies have managed to duplicate this trick. Without it the coil gets too hot, and that limits your scan speed...

    While excessive heat can be a deal-breaker, I'm sure bearings, drivers, position sensors, and even the mirrors all play a significant role as well. It's not as if one single part can be upgraded to gain speed. You've got to improve everything.

    To borrow an automotive analogy, scanners are like grand prix race cars. They need to accelerate quickly, stop quickly, and turn quickly, all while minimizing vibration and maintaining perfect accuracy through the narrow track layout. If you increase the speed of the car without increasing the grip of the tires, you spin out and hit the wall. Same thing with galvos - run them too fast and they "miss" the points of the pattern. Screw up the damping control, and the same thing happens. Heck, even if the driver and scanner are both perfect, you can still have harmonic vibrations in the mirror itself that will mess up your pattern.

    Bottom line, there's a big difference between 15 K scanners and 60 K scanners, and those differences are reflected in the prices.

    Adam

  7. #7
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    Quote Originally Posted by Pitts View Post
    What Makes Galvo's Faster?

    $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$......

    hahaha! That and magic....

  8. #8
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    I've tried rubbing dollar bills on my scanners and I can't get them to go any faster... WTF?

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    You also have to keep in mind that there is already an established dollar to performance ratio. It doesn't really matter what the material cost of the scanner is. If it is faster than model X it should cost more. Right?

    James.

    What happens when an irresistible force collides with an immovable object?

    There is probably a point on a curve somewhere that indicates that no matter how much power you apply, you will never overcome the inertial reluctance of the mass to change direction at and above some rate.
    Last edited by James Lehman; 06-20-2008 at 11:31.

  10. #10
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    One problem with this discussion is the assumption that the scanners that supposedly go 50K, are actually going 50K!

    Remember, for the longest while, there were people who were selling scanners that look like the Cambridge 6800 (almost every Chinese scanner I have ever seen looks like the Cambridge 6800), and selling them as "40K" scanners. After a year-long education (through forum posts, private emails, etc.), now they are tuning their scanners to 30K (but maybe keeping the "40" in the part number)...

    Another problem is this. While I was on my three-week-long trip to China, I visited four separate manufacturers of scanners. One of these companies showed me all of the different models of scanners that they make (and boy do they make a lot of different models)... Long story short, for some of their models, they are selling what are really 30K scanners, as something more than 30K... I told them words to the effect that "hey, this is only a 30K scanner", and they said words to the effect that "I know, but we have to say that they are faster so that people will buy our scanners instead of someone else's"...

    So once again we can get into the topic of "specmanship", which means, exactly how much do you exaggerate your specs (much less, understand them in the first place), in order to try to gain sales?

    Here are a few cold, hard, facts: The Cambridge model 6800 was designed to be a 30K scanner (actually 24K, but improvements in around 1995 allowed it to go up to 30K). Yes, it is possible to tune the servo to go faster than 30K -- as Pangolin was the very first company to publicly show faster-than-30K scanning at the LaserFX meeting in (I think it was) 1998. So yes, you can tune the servo to go faster but, certain sacrifices will be made when you do that -- namely heat and scan angle. And we can say that there is a "square" relationship between the tuning and the heat and angle. So, if you tune the scanners to 40K, you will be putting (40/30)^2 = 1.8 times the heat into the scanner, and you will reduce the angle to 60% of what you would have at 30K.

    Lets talk about heat for a minute, because that plays a big role in the true answer to 300EVIL's question. The Cambridge model 6800 scanner -- when made properly, can only dissipate 15 watts of heat, period. This number is not subject to negotiation. So if you tune for faster-than-30K, and pump more heat into the scanner, the scanner won't be able to dissipate more than 15 watts. This means "power limiting" will happen (a phenomenon where the servo is supposed to reduce the size of the scanned image to reduce the heat). So sure, you can tune for faster-than-30K with a 6800, but you won't be doing yourself, or your customers any favor.

    Normally people need a pretty decent scan angle out of these things -- at least 30 degrees, and 50 degrees is more common. At Disney's Epcot Center, they project I think 1800 feet from a laser projector onto the "Spaceship Earth" ball. So the scan angle is around 2 degrees. For them, they can tune the scanners faster, because they have a tiny angle. But nobody else does... So for that reason, we would NEVER recommend that anyone using 6800, or 6800-like scanners, tune for faster than 30K.

    There's also that pesky matter of the standard... remember that the ILDA standard is 30K? If you want to go faster than 30K, then the ILDA Technical Committee has established an interim recommendation that you scan at 60K. So there are two standards -- 30K and 60K. 40K isn't a standard. 50K isn't a standard. Only 30K and 60K are standards...

    Anyway, let's get back to 300EVIL's question, and the real answers. And we will use a practical example for this.

    I was sent a set of, what was claimed to be, 20K scanners. (There are lots of people re-selling these scanners, and I would honestly bet that very few people know who the real manufacturer of these scanners are. But that's beside the point...) When I tuned them correctly, they could only really do around 8K, and didn't really even look very good at that. I did a mod on the scanner amp to make them go around 18K when tuned correctly, and they actually looked pretty good. BUT! the cheap construction of the scanner (mostly related to how -- or even "if" the coil was bonded to the stator) made these scanners such that they really couldn't dissipate 15 watts like real Cambridge scanners can. On top of that, the scanner amps didn't have any kind of "power limiting" type protection I mentioned above. So, the scanner wasn't able to dissipate the heat, and the amp wasn't able to detect how much heat dissipation was needed. The result? Eventual destruction of the scanner.

    Basically, the Chinese company made what was in essence, a not-very-good scanner. Then they put purposeful limits into the scanner amp, so that they would never be driven very hard (rather than put true heat detection into the amp). THAT'S a "20K" scanner. The scanners that are claimed to go faster are built (a little bit) better, and are (hopefully) able to dissipate more heat. PLUS, for these scanners, they sell you a more expensive scanner amp, that actually has the "power limiting" circuit onboard. THAT'S their alternative, and more expensive scanner.

    So, in reality, the scanners may be made at the same factory and using the same techniques. I have also seen one company just "build scanners", and then do what IC manufacturers call a "bin-out" in the end, which basically means that the ones that turn out the best are put into a different "bin". So it could be that it is their entire intention to create "30K" (or so-called "50K") scanners, but a lot of them simply don't work all that well, due to poor machining tolerances. They pair these with lower-cost amps, and sell them as "20K".

    I hope this answers your question 300!

    Best regards,

    William Benner

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