Newbie questions regarding Galvos (controls, basic properties)

[skuupi]

Hello

I have a few questions regarding some glossary and specs on galvos and also about their control.

I'm not building exactly a laser show system but a simpler setup. My aim is to just simply rapidly rotate a thin piece of glass in a repetitive motion over a certain angle range. The idea is to pass a laser beam through the glass.

I've been trying to look for different solutions and a single galvo (or a resonating scanner) would seem the easiest option to do this. However there are a few things that were a bit unclear to me. And even though my own stuff doesn't exactly belong under this topic, I though that the know-how I need can still be found here

About my setup: the repetition rate would need to be something >60 Hz with a mechanical angle ranging from the zero position to about +/- 20 degrees. So basically the galvo would need to move the whole total 40 degree range in about 17 milliseconds or less. Is this unrealistic?

Many of the galvos are specified with their speed in Kpps and with some specified "optical angle". What is this optical angle exactly and how is it related to the mechanical movement angle of a galvo? Is it just the same thing but a different term used with the galvo people?

Also, I have no clue how the Kpps relates to the possible rotating speed of the motor?

As I'm aiming for a low-budget solution, would a 20 Kpps and 20 degree system like this (http://cgi.ebay.com/20Kpps-HightSpee...-/250701784619) work for me, any thoughts? Or would a resonating scanner be a cheaper/easier solution for me?

Lastly about controlling the galvo. How is it done? Do you use some computer software to control the drivers and what kind of a connector is used with it? Or could you possibly just use a signal generator? A simple motion needed for my application probably won't be that hard to make?

I haven't done much of DIY electronics and I'm really unfamiliar with this topic otherwise so try to bare with the overly simple questions

Thanks in advance!

[Solonar]

The kpps rating is for the system as a whole. It says that the 2 galvos can translate to ~20,000 different points per second. This is sort of an abitrary rating though. In theory a 20kpps galvo could transit the 40 degree span in 50 microseconds, probably somewhat slower in practice though. Also keep in mind that galvos are limited by the mass they are moving, i.e. the heavier the glass, the slower the movement. Normal galvo mirrors are very small and very light. adding even a small amount of weight would slow the galvo down significantly.

[allthatwhichis]

Sounds like you need a set of 60k Cambrige scanners and an abstract generator which could be hardware or software... not really a low-budget solution.

[flecom]

you could use a function generator or a simple tunable oscillator (like a VCO) as your signal source... no need for software

[laserist]

Have you considered a rotating polygon mirror?

[flecom]

Have you considered a rotating polygon mirror?

if you just need to scan it that would be a much simpler solution... depends if he needs it to oscillate or not

[skuupi]

Thank you for all the replies!

Yes I'm aiming for an oscillating movement. Perhaps you could get the same effect with enough fast rotating movement, but I'd rather just aim for the correct movement rather than risk it.

So theoretically one point-to-point movement could be the whole 40 degree span (this would result in the 50 microsecond movement) ? 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 But I guess this is what you were talking about when you talked about practice.

I think the piece of glass will be lighter than the mirror it replaces. But I guess that probably doesn't make it much faster since it's designed to work with a certain weight.

Nevertheless if it's even roughly near to the theoretical value of 50 microseconds it would work for me. There are still three orders of magnitude before it starts to get close to my limit..

Hopefully I'll get the control working with the function generator. Don't think it should be that hard though.

[taggalucci]

I think the piece of glass will be lighter than the mirror it replaces. But I guess that probably doesn't make it much faster since it's designed to work with a certain weight.

I'm no expert on this, but I believe it is likely to be faster with a lighter weight because of there's less inertia to deal with when reversing direction. The designed for weight is actually set through the tuning of the galvos. For example, I changed mirror types on my Cambridge CT6800HPs and needed to compensate for the weight/inertial difference through re-tuning them.

[skuupi]

I'm no expert on this, but I believe it is likely to be faster with a lighter weight because of there's less inertia to deal with when reversing direction. The designed for weight is actually set through the tuning of the galvos. For example, I changed mirror types on my Cambridge CT6800HPs and needed to compensate for the weight/inertial difference through re-tuning them.

Well ok, but if it just moves faster, it won't hurt me much. Could the "faster than expected" movement damage the galvos, in other words is the compensation necessary? Of course if something is changed in an already well-designed setup it probably won't be better after changing a parameter, but how dramatic consequences could this change have?

[taggalucci]

If you change the mirrors for glass, it would be wise to tune the scanners so that you get optimal performance for the weight. This shouldn't cause any issues, in fact, the stress should be less. But again, I'm not an expert, so welcome others input to confirm this.