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Why No DC Motor Filter Caps in Servos? — Parallax Forums

Why No DC Motor Filter Caps in Servos?

ercoerco Posts: 20,256
edited 2015-10-12 02:43 in General Discussion
In the middle of modding my umpteenth hobby servo for continuous rotation, I realized there was no motor filter cap on this or any other servo I recall. I usually put three 0.1 uF caps on any DC motor in a delta arrangement in my projects, otherwise there is lots of noise to reset a micro.

So why aren't they used/required in servos?

Comments

  • Keith YoungKeith Young Posts: 569
    I have no idea, but could it have something to do with the circuitry? A motor is stand alone but the servo will have a chip in there maybe with caps or diodes? I don't have the electronics background some of the guys here do but maybe that's part of it?

    I answered your question with 3 questions? I'm gunna have to google that a bit. Sounds interesting.
  • Phil Pilgrim (PhiPi)Phil Pilgrim (PhiPi) Posts: 23,514
    An interesting experiment would be to add the caps (if there's room) and see if they have a positive effect on performance.

    -Phil
  • Beau SchwabeBeau Schwabe Posts: 6,566
    edited 2015-10-12 05:58
    My guess is that the motor controller is using MOSFETS over bipolar transistors. The mosfets have a builtin reverse biasing diode and would effectively perform the same task as placing a reverse biased diode across a relay. With the MOSfets, the diodes come FREE, where as with bipolars, the caps might be used as a cheaper alternative (or less real-estate) to using 4 diodes, or a single bridge rectifier with the AC terminals going to the motor, and the +/- terminals going to the respective supply and GND terminals.
  • ercoerco Posts: 20,256
    I emailed that question to HiTec and 'Mike' replied, "All the filtering is done in the I.C."

    When I asked for more detail, Mike replied, "Sorry no, our circuit is proprietary."

    My mystery remains.
  • GordonMcCombGordonMcComb Posts: 3,366
    edited 2016-03-16 16:02
    I used to see caps in older servos, the ones where they used 100% analog components. The latest servos are really digital devices,, and I'm sure the chip is designed to eliminate the expense of A ) the cost of the cap itself and B ) the cost of soldering it to the motor.

    This may not apply to the much more expensive true-digital servos that carry the much higher pricetags. You might expect they'd engineer better filtering in an $80 servo for an R/C helicopter.
  • ercoerco Posts: 20,256
    Good to know, thanks. I view DC motors as pure RFI generators after experiencing firsthand the "miracle" of how adding 1-3 filter caps stops mystery glitches and it's hard to imagine effective filtering from a distant IC.

    I thought it had something to do about the motor reversing rapidly (growling) in position, polarity rapidly reversing, and the cap having some detrimental resonance effect. I have a weird mind.
  • Beau SchwabeBeau Schwabe Posts: 6,566
    I answered this already .... the transistors used to drive the motor is mosfet rather than older versions that used bipolar transistors. mosfets have an internal reverse biased diode that effectively does the exact same thing as placing a reversed biased diode across a relay coil. With a mosfet device there is no need for a filter/snubbing cap across the motor terminals.

    The guy from HiTec probably didn't answer, because he didn't know.
  • ercoerco Posts: 20,256
    Since Beau is doubly sure, then I'm sold. :)
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