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Thread: I need to move a stepper a step (or a solenoid).

  1. #1
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    Default I need to move a stepper a step (or a solenoid).

    I want to build a beam positioner. All it needs to do is insert a mirror into my beam so that it shoots it off at 90 degrees. I figure I could use one of those $10 steppers on ebay with the mirror mounted on it and just position it so that I can move it one step to insert the mirror. I also have some solenoids that I could use to rig something up.

    I know how to rig up a transistor for TTL control but I am worried about noise from the motor or solenoid windings from causing noise (spikes) throughout my system when I change the position. So, do any of you know how I could rig this up and isolate everything from any noise it might produce? I'd prefer not to have to use a seperate P/S since I am running out of room in the box.

  2. #2

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    A solenoid needs a diode reverse biased across it, something like 1N4007, it has to withstand a PIV of up to 1000 volts. I also recommend a capacitor across it, for two reasons, one is to further reduce noise, the other is to soft start the drive current to reduce shocks to the mounted mirror. You also need to add another diode reverse biased across the switching device, to protect it from residual charge in that capacitor. That's less important actually, because the cap is wired directly across a coil, but it's a cheap way to prevent cumulative erosion of a semiconductor junction. If your input signal is 5V or more, you might do ok with a single enhancement mode MOSFET like an IRF630. That has inbuilt diode protection, and might be the only part needed other than the solenoid, and it's associated capacitor and diode. That cap should be a few microfarads and rated at least 5V higher than whatever you're supplying the solenoid with.

    Stepper motors are a lot harder to contol. Definitely overkill in this case. While they might seem to offer better physical stability, it's easier to devise a solenoid driven system than to control a stepper motor well. Just make sure the mirror is moved on axis with the solenoid if possible, as levers complicate things, are noisy, and probably compromise stability. Arrange for a cushioned stop to motion in each direction too, or you'll have a terrible shock to a mounted mirror.

  3. #3
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    Thanks for the advice on the solenoid and stepper. I was thinking something a lot less complicated for the stepper though. I was just thinking of keeping one pair of control wires held at some voltage. And then just changing the other pair from off to on. Basically, that would give one step and I'd ge a stepper motor that had something like 20 degree steps or more. The idea would be just to turn the mirror into the path enough to reflect the laser to some other angle. During normal use, the laser beam would be parallel to the mirror but not hitting it. So only slight rotation of the mirror would place it in the beam path. Seems like all the same issues as the solenoid would apply since it just another winding. Am I right?

  4. #4
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    A 20 degree/ step might be hard to find, I dunno. Maybe a small DC brush motor with a spring attached..? Or, Ive used disassembled relays w/ mirrors mounted to the lever for angular deflection. You definitely need the diode w/ this as the Dr said.

  5. #5

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    The stepper has a problem... They can easily overshoot. If you can apply enough energy to quickly launch it one notch, it takes very little energy to keep it moving over the magnetic (bumps) and end up pointing wildly off in a new direction. The only way to overcome that is to use a driver designed to handle it.

    Steve's relay idea is nice. That might easily be the cheapest way to get reliable action at moderate speed.

    Edit: I might be wrong about that stepper, at least if the coil remains powered. It's years since I played with them. 15° per step might be had, from the cheaper types used in old printers, but the neat small ones almost all have a fine step size. The only way to get good repeatable step positioning is to use those, so you'll ether get big crude movements or fine ones that are too small to use easily. A relay is nice because it usually has one fixed (closed contact) state that is repeatable for years of use.
    Last edited by The_Doctor; 05-09-2007 at 06:23.

  6. #6
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    I think those cheap ebay galvo steppers would have been perfect. Unfortunately, I don't see them listed anymore.

  7. #7
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    What speed do you need?
    Somebody here at PL may have a single slow open-loop one they might part with if you post inthe wanted/ needed/ for sale area..

  8. #8
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    I just need to change beam 90 degrees in 0.1secs or so. So basically, speed is not an issue.

    All it is for is to switch my beam from the xy scanner path to a lumia wheel path. Nothing fancy.

  9. #9
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    Oh I thought you said 20 degrees. Multiple of 20 degrees

    For 90 degrees, a small DC motor with stops and current reduction- (foldback) maybe..

  10. #10
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    Sounds complicated. I think I like the stepper motor idea better since it will move back and forth by simply energizing and deenergizing the coil.

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