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Makeshift DC Motor Radiating EMF

  1. Jun 5, 2016 #1
    My brother is a highschool student tasked with making a DC motor. It is finished and works however I thought for the sake of learning and the additional cool factor (I'm relatively new to this field), I could make an optical RPM sensor which I could tack on.

    So I made this:
    http://www.instructables.com/id/Measure-RPM-DIY-Portable-Digital-Tachometer/

    It works perfectly and I have tested it on an un-powered motor (using a drill). The problem arises the second the motor starts turning with electricity as the power source. I'm guessing the sparking commutators?

    Either way, it causes the arduino to go haywire if it is within 50cm of the motor, and eventually corrupts forcing me to hard reset it. This happens almost instantly.

    Something to note, the motor and the arduino are powered seperately.

    I tried shielding the components with aluminium and had no luck. This project is due in a few days so I have to try fix this ASAP with what I have on hand. Worst case scenario, he submits it without the extra electronics.

    Hopefully shielding isn't the only way. Is there an electrical engineering solution I can apply to the sensor above or the motor itself to dissipate this noise? I have a large range of capacitors/inductors/resistors/transistors on hand.

    Also some things I have tried to no avail:
    • I have tried 2x 10nF ceramic capacitors attached to the motors commutators.
    • I have tried using a 33uH inductor in series with the VCC of the motor.
    • I have tried connecting the iron core holding the magnets to the ground of the arduino and the ground of the motor power supply. The motor is running on 6xAA batteries.

    As requested, pictures:
    Motor not turned on: utwifWP.jpg

    Here's a video of the issue:


    Cheers
     
    Last edited: Jun 5, 2016
  2. jcsd
  3. Jun 5, 2016 #2

    Nidum

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    Please do .
     
  4. Jun 5, 2016 #3
    Added
     
  5. Jun 5, 2016 #4

    Nidum

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    Do you know about grounding/earthing of electrical equipment ? This is something easy to do that should help .

    Try searching on ' grounding electronic circuits and equipment '
    and ' interference suppression in electronic circuits '

    Some things that often work :

    Linking the 0 Volts lines of motor and electronics .

    Shielding commutator of motor and/or electronics with metal boxes connected to 0 Volts line .

    nb: Ideally ferrous metal for shielding - thin steel sheet from bean cans is quite adequate .

    Eliminating any open connections in the electronics that might pick up interference and generate spurious inputs to processor .
     
    Last edited: Jun 5, 2016
  6. Jun 5, 2016 #5
    A schematic would be useful. Are you using a flyback diode?
     
  7. Jun 5, 2016 #6
    The schematic is exactly as seen in the instructables thread posted in my opening post. Please refer to step 3 for the identical circuit I have used. (but to answer your question, not sure what a flyback diode is actually, or where to install it, having said that, I have zener diodes and generic diodes (0.7v drop) on hand..)

    Also, I have linked the metal casing holding the magnets, to the negative line on the arduino battery and to the negative line of the motor battery. Is my application of grounding correct?
     
  8. Jun 5, 2016 #7
    Added a video to the original post, please refer to it :)
     
  9. Jun 5, 2016 #8

    jim hardy

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    Sparking at the brushes makes radio waves emanate from the sparks there

    likewise the rapidly changing current in the wires feeding your motor radiates EM waves just as in any antenna
    Place a capacitor across your wires as close as possible to the brushes . That'll reduce radiation from your wires by short circuiting the antenna wires..

    Might be enough to do the job - interesting experiment......

    When car radios were invented in 1920's the engineers had to handle brush sparking.

    condenser_generator.jpg
     
  10. Jun 5, 2016 #9

    rbelli1

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    I would also make sure that all of the unused GPIO pins are set to output. Set them to the pulled state if any pullup/pulldown resistors are present. Ground if none. Any input pins such as reset and button pins should be pulled strongly (5K or less if possible) to the inactive state and have capacitors to ground fitted. Functional inputs should also be pulled as hard as possible and still be functional with capacitors fitted if possible. Add more capacitors to the power rails.

    BoB
     
  11. Jun 5, 2016 #10
    I'll make the GPIO pin adjustments, and set them to ground. As for the sparking commutators, what value ceramic capacitor should I use? 10nF? I have large range of values so thats not an issue. Should I use more than one?

    Thanks
     
  12. Jun 5, 2016 #11

    Baluncore

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    You have a huge leakage of rotating external magnetic flux and electrical commutator noise.

    Use a longer screened pair, or a coaxial cable, for the optical sensor. That is critical to reliable operation.

    Avoid loops of wire both for motor power and for the optical sensor.
    Identify circuits, pull wires close and parallel to their return circuit wire. Minimise loop area.
    Tape the pairs of circuit wires together so the radiated fields cancel and they do not work like an antenna.
     
  13. Jun 5, 2016 #12

    jim hardy

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    Do you have a couple hundred nf ? I'm guessing between 0.1 to 1,0 uf ought to do

    Balun's advice 'minimize area' is excellent, remember your flux formula : area is the term over which you have most control.
     
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