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Author Topic: Need to build a rotating mirror like this  (Read 1706 times)
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http://web.media.mit.edu/~stefanm/TinyProjector/popup.html?TinyProjector_files/TinyProjector2_pics_62.JPG

So the advice I need is what type of motor/specific model should I use for the rotation. I need one that can handle precise, quick movement. I'll be firing a laser at the rotating mirror 40-100 times per second and it needs to move each time.

Also, where do you wonderful people get your mirrors cut when you need them?
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I need one that can handle precise, quick movement. I'll be firing a laser at the rotating mirror 40-100 times per second and it needs to move each time.

I've never worked with lasers, but I had the impression that in rotating mirror/laser experiments typically the mirror rotated at a uniform speed and the laser pulse was timed to occur when the mirror was in the correct position. If you're trying to do it the other way round then (a) are you sure that's the best approach? and (b) what type of movement (speed, acceleration and positional accuracy) do you need to achieve?
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Cutting the mirror:  Glazier.
Motor:  Well the picture doesn't really show enough, but I would guess that it is a general one.

You will probably have to get the motor/mirror thing going and then fire your laser at it and see what happens.

If it "fails" as in the "image" - which I am guessing is what will be made - is wrong, you may have to adjust the speed of the motor, or the timing of the laser being fired to sync them.

This could be done by having a detector system on the mirror and when it passes the sensor, it tells the arduino that the mirror is at "point x" and so they both sync that way.

Good luck with it.
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Both great replies. I'm a little embarrassed that I didn't realize the motor rotates it's own way and it's the laser timing I need to work out. That sounds easier, so thats good smiley-wink
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Any advice on a motor to get? I can only find fat ones, the one pictured looks very low profile comparatively.
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Look for a hobby motor, and the easiest thing to do the sensing will probably be a rotary encoder.  Good luck.
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Try a google search for pager motor, that is what is used in the link you supplied.

Pager motors are used in mobil phones, or pagers, to make them vibrate
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Ok cool, the parts are on the way. Question: I understand that the mirror rotates a constant speed and the onice is on the lasers as far as timing goes, but don't I still need something that detects when the mirror is starting a new rotation?
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Yes you do to allow to syncronise the laser and the mirror. Normally you would use an opto reflective switch or an opto slot detector.
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I believe a lot of commercial laser imaging devices which project a laser beam in X and Y axis use a 45 deg mirror attached to an electromagnet like a speaker coil, then they drive the coil with an analogue voltage to move the mirror as required. If the mirror is very thin and light, your movement will be very quick and precise.
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Anyone know of places to buy this kind of thing in NYC? I'm sick of shipping stuff from China ;p
I love the optical sensor idea... have an opaque tab pass by it once per mirror rotation, yeah? Maybe I can source one from a discarded object. I believe old mouse wheels use this... what else?
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I believe a lot of commercial laser imaging devices which project a laser beam in X and Y axis use a 45 deg mirror attached to an electromagnet like a speaker coil, then they drive the coil with an analogue voltage to move the mirror as required. If the mirror is very thin and light, your movement will be very quick and precise.

No, they don't.

What are used are known as "galvanometers" (or "galvos" colloquially) - at least, that is what they are called. Basically it's a really lightweight first-surface mirror attached to a very simple (and lightweight) air-core "motor" (that is, it is essentially a coil - no metal armature like in a regular motor) on a shaft with high-precision bearings. It is made to be as lightweight and high-speed with rapid changes as possible. It is very similar to the device that moves the needle in old analog voltmeters and multimeters, in fact.

But moving the mirror is only half the problem - you also need to sense the mirror's position. There are a variety of methods to sense the position via feedback (which is absolutely required if you want to have any hope of a stable image); a couple of common ones are capacitive sensing and optical sensing. In a capacitive device, the shaft on which the mirror is mounted also has a disk of thin PCB material (or similar) on which "plates" are etched; a similar plate is placed underneath or over the first - it is held stationary. As the shaft (and plate) rotates, the capacitance between the two changes (similar to old-school analog capacitance tuners in old radios). You can sense that capacitance, and use it to gauge the rotation angle. Lightweight, and precise.

Another method uses optical means to measure an amount of light; one way is with a slot detector, but placing two pieces of linear polarized film between; hold one stationary, and put the other on the shaft, then measure the amount of light that passes between them. As the shaft rotates (rotating one of the pieces of film), the amount of light allowed to pass will change from almost 100 percent (it will never be 100 percent, as the films aren't clear to begin with), to pretty much zero percent. Basically a simple light-based potentiometer in a way.

Ultimately, homebrewing such galvs - while possible - isn't for the faint of heart. Here's a link to one setup:

http://elm-chan.org/works/vlp/report_e.html

Google "homemade laser galvanometer" for more interesting ideas...
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Anyone know of places to buy this kind of thing in NYC? I'm sick of shipping stuff from China ;p
I love the optical sensor idea... have an opaque tab pass by it once per mirror rotation, yeah? Maybe I can source one from a discarded object. I believe old mouse wheels use this... what else?

If you want to detect the angle of the motor (and hence, the mirror) - you'll need to do a couple of things to be more precise.

First - you need to monitor the current and/or voltage of the motor as it is running. Motors can speed up/slow down over time - you'll want to compensate for this, to make the motor run at a very constant rate. Monitoring the rotation of the mirror will get you most of the way there, though (if your timing pulse is fast, drop the PWM; if too slow, raise the PWM). But if you can add the other monitoring, you can make it even better (to a limit, of course).

Now - as far as monitoring the mirror when it passes:

Thing about if you have a laser shining on the mirror. As the mirror rotates, it will sweep the laser in a line, but at one point - the laser will be aimed directly back at the laser (depending on how accurate you are with aiming the laser). At another point (ie - 90 degrees later), the laser will "shoot past" the mirror to the opposite side of the mirror...

If you position an optical detector (like a photo-transistor) at either of these points (and make sure the detection bandwidth of the detector is sufficient for the wavelength of the laser) - you will be able to detect the pulse when the mirror is at one of those positions. All you have to do then is time the period between pulses. With that information, you can calculate (to a certain precision) what angle the mirror should be in based on the amount of time that has passed between pulses.
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I think you would be better off synchronising on each revolution as the motor can easily drift off.
This sort of switch is often used in flat bead scanners to detect the end of travel.
There are lots here:-
http://www.digikey.co.uk/product-search/en/sensors-transducers/optical-photointerrupters-slot-type-transistor-output/1967054
Sorry don't know an shops in NYC, but then you will live out in the sticks  smiley-wink
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If the plane of your laser is slightly above the horizontal plane of the mirror then the laser pulse will pass over when the mirror is in the horizontal position.

If you place a detector in this line then it will receive two pulses per revolution as the mirror rotates through the horizontal and out of the laser sight line. One when it's mirror side up, the other when mirror side down.

You can use this to sync your laser pulse to certain angles of the mirror in the position when the shiny face is where you need it, dependent on the incident angle you want the laser to reflect of for direction.

You can also use this feedback to compensate in software for the laser pulse timing, to keep in sync with any slight variations in motor speed. If the motor speed dropped 50% then pulsing the laser at half the original speed will mean it still hits the mirror in the same spots, but your refresh frequency is halved.

It'll always be slightly lagging on the positional information, but that is why it's called feedBACK and not feedFORWARD. I wish I knew how to do the latter so that I could get this coming Saturdays Lotto results . :-P
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