Position control of DC brushed motor.

Discussion in 'General Electronics Chat' started by BillM, Apr 3, 2009.

  1. BillM

    Thread Starter New Member

    Feb 9, 2009
    6
    0
    Hello all,

    I have a brushed DC motor that works nearly good enough. I've been using a timer to control when to start/stop the motor. However, after a period of time, the error accumulates and becomes large.

    I'd really just like a third wire on my brushed DC motor that pulsed everytime it did 1 revolution. I could then count the pulses on my microcontroller to determine when to stop.

    The problem I have is the cheapest method I currently have found are continuous servo's, which run around 10 dollars, and I have to send commands to send it in different directions.

    Does anyone know of a place that sells cheap brushed DC motors that has a built-in hall effect sensor or something?

    Thanks!

    Bill
     
  2. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    What type of motor is it -Brand/model ?

    does the shaft stick out a ways on both ends?

    An optical encoder disk with an opto-interrupter is all that comes to mind for a permanent magnet, brushed DC motor.

    What are the torque and RPM requirements? Sounds like you are on a budget, would something like a stepper motor out of an old printer work for you? Lots of torque and good control, feedback is available if one gets fancy with the driver. I believe there is a stepper driver IC that uses back EMF for position verificaton as well.
     
  3. BillM

    Thread Starter New Member

    Feb 9, 2009
    6
    0
    Right now my DC brushed motor is this DC gearmotor from solorbotics.com

    http://www.solarbotics.com/products/gm3/

    It has just the right amount of torque, is cheap, and can operate at around 3.5V.

    My only problem is I can't detect where it's at :(

    I think a stepper would work, as long as I could ensure it had enough torque to move my load. I also haven't seen any that were cheap.

    I need something that can spin 7 or so revolutions one direction, then come back 7 revolutions the other direction and be in the same place. Originally I was using a timer... but it kept getting way out of wack :(

    Thanks for the help.

    Bill
     
  4. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    If you're looking for cheap steppers, here's a possibility for you:
    http://www.jameco.com/webapp/wcs/st...ype=mfgr&ddkey=http:StoreCatalogDrillDownView
    $1.95/ea when you buy 10. The holes in the mounting tabs are about 1.75" apart. They're unipolar, which are easier to drive than bipolar. They're rated for 7v, but you can run them on 5v with reduced torque. The shaft is 2mm diameter. They have a 10-tooth gear on one side which you could remove, and a shaft about 8mm long sticking out the other side.

    No, they don't have a rotary encoder or Hall-effect sensor built in, but at those prices you have more of a budget for adding such features. You could even use an opto-interruptor and a disk to count revolutions if you wanted.
     
  5. BillM

    Thread Starter New Member

    Feb 9, 2009
    6
    0
    Thanks for the help guys,

    Do you think that stepper motor will be able to turn my load? It seems like 20 mNm holding torque isn't near the 45 oz*In of the solarbotic GM3.

    Should I look for a geared stepper? Or do you think I should just give it a try? :)

    Bill
     
  6. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It isn't; it's around 2.83 oz. in. You'd need about a 16:1 gear reduction to get the same torque; but that doesn't count the friction loss in the gear or belt drive. Since you're looking for a precision amount of movement per step, you would need either gears, chain or a toothed belt over sprockets.

    Well, you might consider adapting a Hall-effect sensor or photointerruptor to what you have already. A photointerruptor has an IR LED on one side, and a phototransistor on the other side of a slot. When the slot isn't blocked, the transistor is turned on by the IR emitter. Digikey carries quite an assortment of them. Here's an example: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=H22A1-ND

    There are also reflective-type photointerruptors.
     
  7. BillM

    Thread Starter New Member

    Feb 9, 2009
    6
    0
    I think the Nippon PF35T-48L4 may work well. It seems like a gear reduction head would still keep the cost below $5 total. Any idea who might sell the gear heads? It's such a pain ordering samples from the manufacturer (I see that they do make gear heads).

    Bill
     
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