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Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Lately I've been playing around with stepper motors a lot lately and I ordered 15 small 7.5 degree step angle unipolar 8-wire stepper motors from ebay.
I have a couple high-power op-amps (10A max current I believe ) and I was thinking about using these to drive the stepper motor. I could hold one winding on at a constant current, and vary another with the opamp and get 3.75 degrees of motion, with positive and negative rails, I could get a full 7.5 degrees then too, correct?
Would a stepper motor with a small piece of mirror on the shaft even work well as a galvanometer like this? Or would the frequency response be poor? For initial experimentation, I planned to drive the X and Y with my soundcard, Possibly later, using a PIC microprocessor and couple DACs. I'm guessing it would be better to hook the op-amps up in current-mode instead of voltage-mode?
Registered Member #65
Joined: Thu Feb 09 2006, 06:43AM
Location:
Posts: 1155
no and no, You can buy pre-made XY scanning assemblies that include optical encoded position feedback. However, most DIY units I have seen use PWM driven H-bridges and custom made coils.
Most stepper motors are not good for these types of applications....
Your output amps may be of some use if you can use a polarizing wheel with an optical sensor as a feedback to somewhat automatically correct the position (although expect a non-linear response.) The best unit I have seen so far was a modified projection TV unit (old 3-lenses kind with a impressive blue laser.)
There are great sites that include the ARM code and software. Cheers,
Registered Member #193
Joined: Fri Feb 17 2006, 07:04AM
Location: sheffield
Posts: 1022
If you get a couple of these things and take the elecr=tronics out, just leaving the coil, magnet and armature, then remove the "grill" at the front and glue in a small mirror you can have a laser galvo (with a rather limited scan angle and poor linearity) very cheaply.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
Ok, it depends on what you want to do.
If all you want is to make sweet geometric patterns, you just need 2 motors (driven normally) with a flat mirror on the shaft, positioned 90degrees apart. Vary the speed of each motor to change the shapes.
If you want to make the beam dance to music, then your motors should work ok. You energise one winding at a constant value which acts as a spring to return the shaft to 0degrees. Then you use another winding to deflect the shaft.
If you want to just have crude arbitrary patterns (some have managed crude letters...) then you can take a speaker, cur out most of the cone (just leave enough supports to keep the voice coil separated) and then mount a lever to the cage, with a mirror mounted on it.
But if you want a 'real' scanner that will to 30kpps, you need to either buy a real set of salvos/drivers (~$500) or it is possible to DIY them.
Registered Member #509
Joined: Sat Feb 10 2007, 07:02AM
Location:
Posts: 329
Ok thats what I was afraid of, too much mass to move the stepper rapidly enough. Ive run across both pages ... linked to on my own, but I couldnt really find anything using stepper motors, and I was wondering if that was for a reason or not .
I'm not really looking to make a scanner, more or less looking for things to do with the steppers besides the normal robotics stuff. I think I'll end up using one as a crank-generator for a homebrew flashlight or something.
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Making a laser Galvanometer
) and I was thinking about using these to drive the stepper motor. I could hold one winding on at a constant current, and vary another with the opamp and get 3.75 degrees of motion, with positive and negative rails, I could get a full 7.5 degrees then too, correct? 
