Mirror galvanometers were originally developed in the 17th century to precisely measure very small changes in current. Unlike other instruments of the day, a mirror galvanometer could clearly show minute current variations by translating tiny movements of the mirror into large movements of the light reflected off of the mirror. Before clean electrical amplification became possible, this was the best means of measuring tiny differences in current. True mirror galvanometers are very sensitive instruments, but hobby servos can be used as a low-fidelity alternative, like with this project on Hackaday.io created by [robives].
Using a mirror galvanometer is by far the most common technique for laser projection shows – it’s really the only way to move the laser’s beam quickly enough to create the visual illusion of a solid line in real time. A mirror galvanometer works by using coils to attract magnets attached to the mirror, allowing the angle of the mirror to change when current is applied to the coils. This movement is extremely small, but is amplified by the distance to the projection surface, meaning the laser’s beam can move huge distances in an instance. If you’ve ever seen a laser show, it almost certainly used this technique. But driving galvos requires a beefy DAC, so we can’t blame [robives] for wanting to keep it digital.
[robives’s] project side-steps the need for galvanometers by using glow-in-the-dark vinyl and a UV laser. The result is a laser beam trail which lasts much longer, which means that solid lines are visible without the need for high-speed galvos. A build like this lets you experiment with laser projections without dealing with sensitive mirror galvos, and instead use components that you probably already have sitting on your workbench.
One tiny nit to pick:
“A mirror galvanometer works by using coils to attract magnets attached to the mirror…”
Maybe not every one, but the vast majority have the coil mounted on the mirror while the magnet(s) are stationary (because the coil is lighter than a magnet, and a lower mass means faster acceleration, which in turn allows more lines per second). This is also why speakers have the coil on the cone.
Really? I always thought there are servo motors in speakers to move the membrane.
He compares coils to (heavy) magnets in the speaker analogy, and you look silly if you don’t get that.
Probably one of the most remarkable, because of its tiny size, electromagnetic controlled mirrors that direct a laser light in a consumer device is the PicoP engine used in the Microvision ShowWX pocket projectors.
I have a couple of them and I’m suitably impressed at how quickly and accurately they managed to control that tiny (less than 1mm square) mirror.
It displays the projected image much like a CRT, scanning the 480p image line by line, first red then green then blue, then onto the next line and so on until the whole image is displayed in 1/60 second, meaning the mirror ‘scans’ 86400 times a second, and I managed to force those little projectors to run at 72hz so they display 24p footage really smooth, meaning 103680 ‘scans’ per second.
I’ve had them running for hours on end without any major issue, though I did stick a heatsink on top because they can get almost too hot to touch otherwise after running for long periods.
I must investigate the new PicoPro / PicoAir versions sometime as they’re 720p laser projectors….
Oooooo. Sounds like the perfect display device for LASER MAME for all the vector scan games.
I think that would be difficult. AFAIK this mirror is a single mirror resonating in x and y direction. I think this is not suitable for vector scan.
How did you change the frequency of the projector?
Using PowerStrip on Windows I forced them to do 72hz (also 75hz works) in their native 848×480 resolution.
Interesting, I always assumed they had some way of shining a laser into a standard DLP chip. I knew it pulsed r/g/b. Always thought it was cool how they didn’t need focusing and could project onto curved walls.
Nearly 100% of the galvos used for laser scanning applications (both for entertainment and industrial applications) have switched to moving magnet based designs because it is much easier to cool the windings when they are stationary, and with modern Nd magnets you can get away with using a physically tiny magnet and still get a reasonable torque constant.
I stand corrected. Now I wonder if anyone is making more powerful speakers with the permanent magnet on the cone and (e.g.) water-cooled windings…
I suspect the mass would be an issue there, since speaker cones are still extremely lightweight. If you want lots of power, use a rotary woofer.
If you put a magnet vertically on bearing in a stationary setup you of course avoid a weight issue too.
So the question is if those moving magnet types are also the choice for devices in motion.
Contrary to that, many (or all?) Galvos for Laser Display use a moving magnet design. And have a feedback – these days mostly optical – for closed loop control.
Just this close to a raster scan (or even old-school video) display…
Should use surface silvered mirrors; you’re going to get blurry/double imaging from glass reflections in addition to refracted silver reflections…
Or mirrors with UV reflective coating. Glass is very nonlinear W/R/T wavelength.
“originally developed in the 17th century”
sure??
This caught my attention also.
Wikipedia seems to think they were invented in 1936. I’m curious what source said they were invented in the 1600s, long before batteries or even Leyden jars.
Very very sceptical of 17th century. Possibly in the 1700s but very unlikely 1600s. What would they have used it for? – there was nothing to measure! The earliest reference I can find is 1820.
Might be an error based on this:
” The term “galvanometer,” in common use by 1836, was derived from the surname of Italian electricity researcher Luigi Galvani, who in 1791 discovered that electric current would make a dead frog’s leg jerk”
Incidentally, you can’t have something named after the guy before his birth either of course :)
Sine wave on the video ? Well my mathematic is all fucked up because that looks like a really weird sine wave.
Anyway, people usually make cheap galvos with speakers. See http://www.instructables.com/id/Arduino-Laser-Show-with-Full-XY-Control/ for example.
I thought the same thing but realized that the servos are probably just hitting their limits.
I work with and service industrial 3D laser scanning optics. They’re used for welding with up to 10KW of IR. Those mirrors, which are pretty substantial, are driven by stepper motors. They have a resolution on the order of microns over a field of up to 250mm. And they can move very very fast.
Very cool! This reminded me of this post from ~2 years ago where a similar concept was done on a Scribbler robot.
http://forums.parallax.com/discussion/153404/parallax-scribbler-s2-robot-ultra-violet-laser-xbee-logo-part-2-of-5