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Locate a servo or similar device

  1. Jan 17, 2010 #1
    Hey guys...i'm trying to locate a servo or similar device that has just three basic stipulations...

    It needs to:

    -Be able to turn a thin 6 inch diameter plastic wheel (negligible weight, no resistance as it's not attached to anything but the servo)

    -Complete at least a half revolution in either direction or 1 full revolution

    -Be $3 or under...

    Sound possible?
  2. jcsd
  3. Jan 17, 2010 #2


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    Re: Servo's!

    You can buy a micro-motor powered by a battery for about a dollar frim any electronics surplus store.

    (I'm going to predict that further stipulations are forthcoming...)
  4. Jan 18, 2010 #3
    Re: Servo's!

    As you wish...another stipulation is that it has to have a little precision as the wheels orientation is of significance to us. So basically we need to be able to tell it how many degrees to rotate...
  5. Jan 18, 2010 #4


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    Re: Servo's!

    Very different animal.

    You are talking about a stepper motor.

    You should go visit your local electronics surplus store. They will have what you need.
  6. Jan 18, 2010 #5
    Re: Servo's!

    Look for a pair of selsyns in electronic surplus stores. See
    I used to use these before ~1965.
    Bob S
  7. Jan 19, 2010 #6
    Re: Servo's!

    Most servo motors are not stepper motors, but instead use some kind of absolute rotary encoder (a potentiometer, for example) to provide feedback which is used to correct the output of an ordinary DC motor. Never seen a $3 servo motor, though, particularly not one capable of a full rotation.

    Some other DC motors have optical or magnetic encoders built in. These are often quadrature encoders that are only useful for measuring speed and relative motion, you would need something else (a limit switch, photointerrupter, etc) to establish a reference point to measure absolute position. You would also need something to decode the quadrature signal and control the motor. A DC motor with gear reducing and encoder is also unlikely to be had for $3, and the decoder and control circuitry won't be free.

    Stepper motors are also relative positioning, requiring a reference point to set a zero against, and also require special control circuitry. You can get some used/surplus steppers in that price range that might be suitable, but the control electronics will increase the overall cost.

    Sources...look at bgmicro.com, jameco.com, allelectronics.com, e-Bay...really, just Google for some lists of places to get robotics parts. You also haven't mentioned what kind of quantity you want these motors in...you can probably get a box of motors for considerably less than $3/motor off e-Bay with luck and patience, or one or two $2-3 steppers from somewhere else, but reliable supply is another matter unless you're doing huge quantities.
  8. Jan 19, 2010 #7


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    Re: Servo's!

    Yes, it was pretty obvious from the OP that we were going to need much more detail about what use it's being put to. There's all sorts of criteria to consider.

    This is like playing 20 questions. I'm hoping the OP will volunteer to elaborate.
  9. Jan 21, 2010 #8
    Re: Servo's!

    Sorry guys, you are dealing with a software guy trying to do hardware stuff he is in no way qualified for! But if you have the patience, i'll try to give more detail, i appreciate the help :)

    Basically what I'm trying to accomplish is having 4 plastic circular platters or turn tables if you will (very thin and about 6 inches in diameter) rotate behind a small window. Around the brim of these platters will be cutouts of different images or symbols. The stepper motor will rotate the platters, thus showing a different symbol through the window...much like a slot machine would but they rotate around the z-axis rather than the x-axis.

    The accuracy doesn't have to be exact, just enough so that one symbol can line up inside the window. I've read that the stepper motor's error % is not cumulative to each step possibly making this feasible?

    The cheapness of the motor comes by the fact that I need to move 4 platters, but maybe if I get frisky with it I can rig it so that 1 motor can control all 4? But i'd like to just get one working right now.
    Last edited: Jan 21, 2010
  10. Jan 21, 2010 #9


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    Re: Servo's!

    Is this the basic idea?

    Attached Files:

  11. Jan 22, 2010 #10
    You got it...am I in over my head or is this fairly simple?
  12. Jan 22, 2010 #11
    Re: Servo's!

    It sounds easier to do in software with an LCD display...but you probably know that.
    The per-step error of stepper motors is not cumulative, but if they miss a step they will become out of sync with the software controlling them. This is also a problem if they get moved or if things gets shut down in an unknown state. You could notch or mark the disk and use an IR LED/phototransistor pair to determine the orientation, or provide some way to adjust the disk positions to reset them to a zero position. Servos will not have this problem, you can tell them to go to an absolute position and they will do so.

    Servos are more expensive, but stepper motors will require more drive electronics, and more work overall...specialized driver chips, or microcontrollers with sufficient output pins and power transistors/H-bridges, and possibly some kind of feedback. Your desired price per motor makes me suspect either option will cost more than you want to spend.

    Driving multiple disks from one servo/stepper/gearmotor/whatever will add a great deal of mechanical complexity. Unless you're well equipped to design and fabricate whatever custom mechanical parts you need, I would not recommend it.
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