Cheap Galvos

[chuck]

What degree of rotation is considered normal for a laser galvo?

I have had an idea using inexpensive steppers from a surplus place, and using two adjacent phases to generate the motion. Conceivably could even use ones from scrapped 3.5" floppies, as their strength is almost immaterial. Partially energizing two adjacent phases, simultaneously, would definitely give me "microstepping," which would give me full positional control.

The problem is that even in "low quality" steppers with relatively few steps, there are 50-100 steps per revolution. That's not much rotary movement...

So, I'm asking the question...How much rotational movement is considered normal for galvos? 45 degrees? 90 degrees? 180 degrees?

-- Chuck Knight

 

[allthatwhichis]

Depends on the points per second you want to draw. The ILDA test pattern is the standard for testing this I think. My scanpro30s off eBay are spec'd at 8° at 30Kpps... I think 8° is average, but you can get others that have a higher scan angle and speed.

Hope that helps.

 

[chuck]

8° of movement, or 8° of reflection angle? As we all know, angle of reflection equals angle of incidence, so it effectively doubles...4° of rotation would result in 8° of reflection angle...

I'm trying to design this thing on the cheap -- frankly, I'm shocked that the angle available is so small. I would have thought 30-45° at least.

Let's see -- a 200 step stepper is 1.8° per step. A 100 step stepper is 3.6° per step. A 50 step stepper is 7.2° per step. This might just work!

So, other than angles...do you think that this idea has merit? I'm big on theory...practical hardware design is a totally different field. This is what I have in mind. Take a stepper, apply equal current to 2 adjacent windings, and "lock" the position into a central location. By varying the current to either winding, I would get a + or - movement, relative to the change in current.

Sound about right, or am I barking up the wrong tree?

-- Chuck Knight

 

[Daedal]

Stepper motors are fun... Considering the big stack I have lying around (anywhere from .72° to 7.2°) I have no Idea what you're talking about. But the best thing to do is to get a stepper motor driver and send the signals off through some kind of I/O module. That is what I do. It works miracles!

This is what I am using for my CNC router.

GL;
DDL

 

[allthatwhichis]

8° of movement, or 8° of reflection angle? As we all know, angle of reflection equals angle of incidence, so it effectively doubles...4° of rotation would result in 8° of reflection angle...

-- Chuck Knight

Um...

Draw angle... :huh2: You may want to go over to photonlexicon and ask your question there. Much wiser folk there on this type of subject... Well, wiser than I at least.

 

[chuck]

This shows it as clearly as I can. By rotating the mirror 45 degrees, you get a beam deflection of 90 degrees. Deflection = 2x rotation.

Does this make sense?

-- Chuck Knight

 

[Daedal]

That is true. A mirror rotation of x will cause a beam deflection of 2x. When translated to stepper motors, a motor step of 2° will result in a beam deflection of 4°.

My previous comment was in regards to the way you wanted to operate the stepper motors. My personal suggestion is, after having worked with stepper motors for quite some time now, is to use a stepper motor driver, if even home built. The reason being is that the issues you would face trying to step the current at a useful rate would end up needing that you create a simple motor driver. Schematics should be available online somewhere, but I haven't looked. Unless, of course, you are attempting to 'lock' the stepper motor in one location and simply taking it one step back and one step forward. This would greatly limit your motor, but would actually be sufficient for the application.

After re-reading the post again, this is actually what you are attempting to do, and yes, this would work perfectly. Quite interesting since I had the tools and never thought of doing it myself even. Good thinking

I hope I clarified more than I confused;
DDL

 

[chuck]

What speed can I expect from a stepper? They always seem to rotate so slowly, even when driven from my parallel port.

That's why I was thinking of using only 1 or 2 steps...less motion, faster response.

Do you run your steppers at increased voltage to increase their response rate? A trick I read about on CNCZone.

-- Chuck Knight

 

[allthatwhichis]

I think it is 8° of reflection angle... Here is the link I was shown to calculate the angle of my projection on my wall...

http://www.pangolin.com/userhelp/scanangles.htm

 

[chad123]

HI, Just a couple of comments.

Using a standard stepper driver will not get you much, but you can cobble something together.
People have made stepper scanner systems before. The only trick is they have to be analog and not "digital" like a standard stepper driver. In other words a differential voltage to swing + and - . You will have to figure out a way to keep the stepper (here by known as "scanner") centered mechanically. The performance will be really bad but for something really cheap you could get some scribbles out of them.

Here is a schematic from a Old laser system that used steppers for scanners. The only thing that is relevant is the scanner "driver" and then it was made for a mono polar supply. It will take some tweaking but it would be a place to start.

Chad
http://picasaweb.google.com/moki123/LaserHead/photo#5063201651586144386/http://picasaweb.google.com/moki123/LaserHead/photo#5063201651586144386

 

[Daedal]

What speed can I expect from a stepper? They always seem to rotate so slowly, even when driven from my parallel port.

That's why I was thinking of using only 1 or 2 steps...less motion, faster response.

Do you run your steppers at increased voltage to increase their response rate? A trick I read about on CNCZone.

-- Chuck Knight

Steppers are not slow to step at all, as far as I know, it's the time it takes them to go from one setp to the next that makes them slow, and that usually depends on the driver board that you are using.

I would suggest trying this out, you can use several ways to make the voltage step back and forth across the 2 terminals.. motors, boards, simple logic. I tried it last night by simply hooking up the 'lock' terminal and then quickly running the other wire back and forth on the other 2 terminals to make it step, and it did exactly what you though it would. Just make sure you give the motor what it needs to be operational. Don't try to push the motor too much. It should be fine at whatever voltage it's set up for. The voltage and current limitations in a stepper are due to the kind of cabling used inside, so I wouldn't suggest pushing it hard.

GL;
DDL