I hope you didn't buy any scanners yet...
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Originally Posted by
skuupi
So theoretically one point-to-point movement could be the whole 40 degree span (this would result in the 50 microsecond movement) ?
No, not with any non-resonant scanner currently available. You're going to end up at least an order of magnitude too slow, and probably two orders of magnitude or more too slow. Here's why:
30Kpps scanners have a maximum small step bandwidth of around 2.5hz at 8 degrees optical scan angle. (8 degrees optical means the maximum deflection of the scanners is only 4 degrees, since the beam bounces off the mirror, effectively doubling the angle) Of that 4 degrees of actual scanner travel, a small step equates to around 1 degree (or 2 degrees optical). Remember too that even at such (relatively) slow speeds and small angles, you'll still have significant distortion.
If you step up to the Cambridge 60Kpps units (The HC6215's), then you can expect to get close to 5Khz at the same angle. Still, that's nowhere close to your goal of 20Khz, and if you really need the full 40 degrees of travel, you're going to be slower still. (Probably another order of magnitude slower, if not more.)
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I'm just wondering because it seems a bit much to expect for the motor to be able to make either a really small movement (say 1/100th of a degree or so) or 4000 times larger movement in the same time.
Your gut instincts have served you well! For you are absolutely correct. :)
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I think the piece of glass will be lighter than the mirror it replaces.
The rotor mass is the limiting factor. The mirror weight is small enough that it doesn't play a major role. Furthermore, a very thin mirror can actually resonate, introducing more distortion.
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Nevertheless if it's even roughly near to the theoretical value of 50 microseconds it would work for me.
It will be closer to 500 microseconds, and that's only if you can work with just 1 degree of travel. For a full 40 degrees of travel, you're probably looking at something near 5 milliseconds, if not more.
I don't know what the speed limits are for resonant scanners, as I've not had much experience with them, but I do believe you could get much better performance using one of them instead of a closed-loop scanner that was designed for laser show applications. Note, however, that you may still have trouble meeting the 40 degree travel specification.
For the other people reading this who wonder where the above numbers come from, here is a brief breakdown of what "30Kpps" really means: It means you can properly scan the ILDA test pattern at 8 degrees optical with the center circle just touching the inner square. But what is that center circle? If you look at the test pattern in an ILDA frame editor, you'll see it's actually made up of 12 points that are all positioned well outside the center square. (It's a decagon *EDIT* dodecagon, and it's scanned 3 times in succession.)
Think about that for a moment: It takes 12 points to make it around the "circle". 30,000 points per second divided by 12 points yields a bandwidth of 2500 Hz for each trip (or cycle) around the circle. There's your small-step bandwidth.
Now consider the fact that those points are seriously distorted. The beam never makes it outside the square, much less far enough out to actually hit any of the 12 points that make up the decagon. Those points are "rounded out" as the scanners race to catch up with the next point. The scanners are said to be ballistic (that is, under maximum continuous acceleration) while drawing that circle. Thus, this is the fastest they can move...
But also remember that the circle is not as wide as the entire test pattern. It's much smaller. So while the entire test pattern is being scanned at 8 degrees optical (which represents only 4 degrees of total mirror movement), the center circle is much less than half the total size of the pattern. So the circle is being scanned at around 2.5 degrees optical, which means the scanners are only moving a little over 1 degree total when they are scanning that circle.
Adam
