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External Servo Power — Parallax Forums

External Servo Power

jawnlooijawnlooi Posts: 23
edited 2006-10-05 14:01 in Robotics
hi dudes,
im using the board of education (BOE) to control 4 servos, but powering the servos eat up too much battery power,
so i tried to power the servo using a Futaba receiver (model R1490P), and i snipped the white wire and plugged the
white wire at P15.
the servo does have the correct response when its plugged into the servo port (15) on the BOE, but not when the black
and white are in the receiver and the white at P15. any idea why this is happening?
(ps it does work when all 3 are plugged into the receiver, but the signal i want should come from P15)

Comments

  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-10-03 13:12
    jawnlooi -

    The grounds need to be common for a setup like that to work correctly.

    Regards,

    Bruce Bates

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  • Tom WalkerTom Walker Posts: 509
    edited 2006-10-03 13:15
    In order for the servo to understand what the signal wire is "saying", it has to understand how high above "ground" the signal is. Since your servo is getting its power and ground from the receiver, it has no way to "reference" the signal line to ground...there is no connection. If you atttach a wire from your BOE's Vss to your servo's ground (typically black) wire, then things should be closer to what you are looking for.

    This is referred to as having a "common ground" and shows up in these forums quite a bit.

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  • jawnlooijawnlooi Posts: 23
    edited 2006-10-04 04:36
    thanks for the replies and sorry for asking a repeated question.
    searched for common ground but couldnt get any results.
    my white (signal) is in P15, my black is at VSS, and my red is connected
    to the receiver, but i'm still not getting any results (other than the click-click
    sound from the servo, which i also get when all the servo wires are plugged
    into the BOE's servo slots and the BOE is set at 1 (no power to servo))

    do i need to parallel my ground to the receiver as well?

    my electronics knowledge is quite inadequate, would you mind explaining
    how having a red wire in the receiver and the black in a BOE would allow
    power from the receiver battery to go to the BOE? i thought a circuit had
    had to be looped around to the same battery? (also, since VSS isnt really
    'grounded', wouldnt all the spent current by the servo go to the battery?)
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-10-04 05:57
    jawnlooi -

    You asked: Do I need to parallel my ground to the receiver as well?

    The answer is YES, and that's what provides the "common ground"!

    Regards,

    Bruce Bates

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  • jawnlooijawnlooi Posts: 23
    edited 2006-10-04 07:52
    OH haha alright, i thought you were referring to connecting the ground to the VSS and
    not having a parallel circuit to it. will try!
  • jawnlooijawnlooi Posts: 23
    edited 2006-10-04 08:07
    excellent! it worked, thanks a bunch guys, could you also explain why
    it works the way it does? ie how common grounding would help the servo
    to have a reference. i have a rough idea how a servo works, but didnt
    know it needed a ground reference point (i thought 0v was 0v haha)
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-10-04 08:59
    jawnlooi -

    I'll let you answer your own question, by providing the following questions about your project:

    Who "owns" and provides the POWER for the servo(s)?

    Which part of this project supplies the SIGNAL to the servo?

    Who "owns" the SIGNAL line?

    When the SIGNAL comes from one source and the POWER comes from another, there must be a method to ensure that BOTH elements are using the same zero reference (aka common ground), since once they do, all other voltages, signals, etc. will naturally fall iinto place. Does that make more sense now?

    Regards,

    Bruce Bates

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  • jawnlooijawnlooi Posts: 23
    edited 2006-10-05 13:33
    ahh excellent, thank you very much! i assume this means that when anything refers to 'common' it means a common ground? (eg.. a voltage regulator with input output and common)
  • Bruce BatesBruce Bates Posts: 3,045
    edited 2006-10-05 14:01
    jawnlooi -

    That is a VERY invalid presumption. In electrical terms, "COMMON" ONLY means that generally, more than one wire is conected to it, or the terminal is connected to more than one wire, or it (the COMMON item) is a part of more than one circuit. A typical example would be a single pole double throw (SPDT) switch. Often the connection markings may read as follows:

    circuit 1 common circuit 2

    There is NO ground on an ordinary SPDT switch, thus that proves your presumtion is invalid. In this example the COMMON is a COMMON POSITIVE connection. Thus, COMMON GROUND is just that, COMMON POSITIVE is just that, and COMMON anything-else is just that. The text of the data sheet or documentation should indicate how or why it is "COMMON".

    Regards,

    Bruce Bates

    Post Edited (Bruce Bates) : 10/5/2006 5:04:46 PM GMT
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