Problems With Stepper Motor

Discussion in 'The Projects Forum' started by H2OS, May 7, 2010.

  1. H2OS

    Thread Starter New Member

    May 7, 2010
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    Alright, I'll start with what im using it for.
    For my culminating project in my computer engineering class, I got the old anemometer off the roof of the school and decided to digitize the signal so it could be read by the newer computers. What I did is I opened the housing where the magnet and the coil used to be and am now using a stepper motor as a switch. Everything works great except for this one thing that has stumped the teacher and myself for a couple days now :mad:.

    When I take the stepper motor and turn the shaft with my fingers, it generates the signal perfectly, showing up on the led display. Now, here's the issue, when I put the 3-cup wind catcher onto the stepper motor and spin it, I get nothing.I have made completely sure that the wind catcher is indeed spinning the shaft and everything is wired exactly the same as when I spin the shaft with my hand. I have tried different motors all to the same affect, I have also tried spinning the shaft with my hand while touching the shaft to the 3-cup catcher to rule out the possibility of the metal doing something to it. I am completely stumped here guys, I can't see any reason at all why it won't work, but maybey I'm just ignorant of the situation. Hopefully you can help me out, many thanks.:)

    -Ryan Watters
    --H2OS
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
    282
    A stepper motor is not a switch. Using a magnet to operate a reed switch is a simple and effective way to make a digital input.

    What were you expecting the stepper motor to do?
     
  3. SgtWookie

    Expert

    Jul 17, 2007
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    The stepper motor will generate voltage pulses as it passes the poles. However, the voltage output will vary a good deal depending on the speed of the shaft.

    You've added a lot of additional resistance. The original magnet/coil arrangement would have rather low friction, and the pulses could be made into TTL level by using a voltage comparator.

    A reed switch will also work, however that's an electromechanical solution. Switch contacts do eventually wear out.

    You might look into using Hall-effect sensors. They're available quite inexpensively in a variety of configurations.

    If you want something you can hack, grab a brushless computer fan that uses ball bearings. They have Hall-effect sensors matched to a magnetic hub. Chop off the feed to the transistors, and add an extra return wire to get the output from the Hall-effect sensor. You'll get 2 output pulses per revolution at whatever voltage you feed to the fan power wires.
     
  4. BMorse

    Senior Member

    Sep 26, 2009
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    Using a stepper motor as you have stated makes the stepper act more like a "generator" depending on how many wires/poles it has, basically you will be getting unrectified DC out of it.

    but if only using 1 pair of wires to detect the pulsing dc on it and use that as an input, you may not get the results you need spinning it any faster than what you can by hand, since the output at any other speed would appear continuous, not off and on as you would expect (I use small steppers as generators in a few of my projects...)......

    for an anemometer, you would have been better off retaining the magnet and switch (or replace the electromechanical reed switch with a hall effect sensor) assembly in it and just make changes to the interface between that the PC, since the original setup was most likely matched with the cup size......

    My .02

    Edit:
    I see we are cross posting sarge, sorry....
    B. Morse
     
  5. SgtWookie

    Expert

    Jul 17, 2007
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    Brent,
    No need for an apology; x-posting happens all the time - and a variety of ideas gives the OP's more things to think about. ;)
     
  6. rjenkins

    AAC Fanatic!

    Nov 6, 2005
    1,015
    69
    Are you sure you are trying a stepper motor?

    They usually show quite a bit of magnetic 'cogging' as the rotor aligns with the poles and do not turn easily at low speed.

    If it is a stepper or a permanent maget motor, what may be happening when you turn it by hand you are getting a bit of a wind-up-and-flick effect due to the cogging and springiness of your fingers, causing higher voltage pulses.

    With a rigid rotor fitted the inertia of that means you get a smoother, lower output.

    How many wires does the motor have and are there any markings on it?
     
  7. H2OS

    Thread Starter New Member

    May 7, 2010
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    0
    The motor has 6 wires, it is indeed a stepper motor, and to the other posts the original magnet/coil that was in there is an analog output, to witch i do not have the money for conversion. The only problem with the project right now, is that the 3 cup catcher doesn't work, while my fingers do, otherwise it would be perfect. Also something i did not mention, i am not using only two of the wires, i am using 3, one power (green) and two others that i tested with a continuity meter (blue/white). I'd really like to figure out why it wasn't working with the catcher, i know there are other solutions to use as a switch, but i posted this on here cause i wanted to know why my solution wasn't. Although i appreciate the other ideas, I have already thought of them and will probably have to use something else, either way I still wanted to figure out why mine wont work as it is bugging me a fair amount. Lastly ( im looking over the posts as i type) when i spin the shaft with my fingers and with the 3-cup, i do so at the same speed going back and forth (i have tried just forward also) so i don't think were getting any cogging / wind up affect, but thanks for the idea.

    Edit - The original switch wasn't a magnet - reed type, it was a magnet on top of a copper coil, the thing has been up there since the 60's, I'm trying to modernize (and in the process make it work at all) the whole thing.

    Once again, many thanks

    -Ryan Watters
    --H2OS
     
    Last edited: May 8, 2010
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Try coupling the stepper motor to the shaft of the "3 cup catcher" using a piece of automotive type 1/4" ID fuel line and a couple of hose clamps to secure it. That will make the coupling a bit springy. You'll have to experiment to get the length of the rubber coupling right. You might try starting with a piece about 2" long.
     
  9. H2OS

    Thread Starter New Member

    May 7, 2010
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    Forgive my ignorance, but what would be the point of making it "springy" wouldn't that just add another factor to calibrate out?
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    You say it works when you turn it with your fingers, but not with the 3-cup rotor.

    Your fingers allow the motor to "cog" along, as Robert mentioned. The fixed coupling you have to the 3-cup rotor eliminates this "cogging" action due to the added weight, producing a flywheel effect. You should not have to worry about any additional calibration, as the stepper will still be physically coupled to the 3-cup rotor. It will simply have a bit of "wind-up".
     
  11. H2OS

    Thread Starter New Member

    May 7, 2010
    5
    0
    Alright, thx wookie I shall try that idea tomorrow and post the results after my class (sometime around 1:30 PM (-5:00 GST Eastern)) Thanks for all the help, hope this works.
     
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