4 ms·

The main difficulty with this circuit is actually dealing with the galvo itself. Feedback alone is not sufficient to linearize the thing to get an excellent ima

The main difficulty with this circuit is actually dealing with the galvo itself. Feedback alone is not sufficient to linearize the thing to get an excellent image. You need to get position feedback from the galvo and then drive an error amp with it.

dreamcompiler 6y ago
Yes but... Galvos have predictable responses. They have measurable slew rates and measurable resonant frequencies and Q factors. If you can characterize them along those dimensions (which might have already been done and described in the galvo data sheet) you can build a software model of them and use the model for feedback. It's not as perfect as your suggestion but much easier than putting shaft encoders on galvos. Another idea is to use a high-speed video camera to watch the flying dot on the screen as the galvos move it and take feedback from that. You don't even need to do this during performance; you could use this setup only once to get a very accurate characterization for the galvo model.
- exmadscientist 6y ago
  My "suggestion" is not quite a suggestion. It is actually how galvos for big laser projectors are driven! I'm told that the position feedback is not really something you can get away without, though I haven't tried myself. Galvos are not quite predictable enough devices that you can usefully model them. The resonant frequencies to be filtered out have to be tuned individually for each one. (At least, for the projector I'm familiar with, which is one of the very big, very nice, very expensive ones. They're low-volume and hand-tuned.)
  - dreamcompiler 6y ago
    Didn't know this about the prepackaged projectors. My experience is with low-end, cobble-it-together-yourself galvos where the highest quality output is not a factor. Thanks.
  - Animats 6y ago
    Yes, here's what optical bench quality galvos look like and cost. [1] My comment on the drive electronics was simply about how to fix the drive circuit cheaply so it had enough current capacity to not overheat and burn out. [1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3770 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=37...

dreamcompiler 6y ago

Yes but... Galvos have predictable responses. They have measurable slew rates and measurable resonant frequencies and Q factors. If you can characterize them along those dimensions (which might have already been done and described in the galvo data sheet) you can build a software model of them and use the model for feedback. It's not as perfect as your suggestion but much easier than putting shaft encoders on galvos. Another idea is to use a high-speed video camera to watch the flying dot on the screen as the galvos move it and take feedback from that. You don't even need to do this during performance; you could use this setup only once to get a very accurate characterization for the galvo model.

exmadscientist 6y ago
My "suggestion" is not quite a suggestion. It is actually how galvos for big laser projectors are driven! I'm told that the position feedback is not really something you can get away without, though I haven't tried myself. Galvos are not quite predictable enough devices that you can usefully model them. The resonant frequencies to be filtered out have to be tuned individually for each one. (At least, for the projector I'm familiar with, which is one of the very big, very nice, very expensive ones. They're low-volume and hand-tuned.)
- dreamcompiler 6y ago
  Didn't know this about the prepackaged projectors. My experience is with low-end, cobble-it-together-yourself galvos where the highest quality output is not a factor. Thanks.
- Animats 6y ago
  Yes, here's what optical bench quality galvos look like and cost. [1] My comment on the drive electronics was simply about how to fix the drive circuit cheaply so it had enough current capacity to not overheat and burn out. [1] https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=3770 https://www.thorlabs.com/newgrouppage9.cfm?objectgroup_id=37...