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Legal servos and power sources for articulated arm?

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  • Legal servos and power sources for articulated arm?

    My team has designed and built from scratch an articulated gripper arm, run by five servos of various sizes. The arm's "shoulder" is bolted to the robot's platform, and can traverse right and left (azimuth servo) and raise up and down (elevation servo). There is an elbow that can also raise up and down (elbow servo). There is a wrist joint (wrist servo) that can raise the gripper hand up and down, and a mechanical gripper that can open and close to grasp things (gripper servo).

    Thing is, the elevation and elbow servos are very powerful 6-turn servos (sailboat servos) geared down for even more power, and the azimuth servo is pretty hefty too. Sometimes when the arm moves, the servos draw enough current that the voltage out of the MR servo controller module drops, and all the servos become unhappy and start oscillating. Seems the total current output of the servo control module is only around 2 amps, not nearly enough for an arm with these powerful servos.

    If it were a model airplane, I'd get a large 20A switching BEC (with a fuse of course) and use that to supply power to the servos, while the control signals came directly from the MR servo controller module. (A BEC is a circuit that takes the 12V of the main battery and produces a 6VDC output to power the servos, with a total rated current of up to 20A in this case.)

    Do you know if such a BEC would be legal at an FTC contest? The Handbook says that servos must be controlled solely by one or more module such as the MR servo controller module. These big servos are being controlled by that module, but aren't being POWERED by it. Has anybody tried such a thing?


  • #2
    You're not supposed to exceed the 5 amp limit of the servo controller assuming all the servos are stalled drawing maximum current, but you can have as many servo controllers as needed. So you can't use all the ports available on a servo controller when using high power servos.

    UNTIL this year. See part e. of RE07 below. The current game manual also addresses what you can connect a servo to (page 33 below).

    page 32 of the game manual
    https://www.firstinspires.org/sites/...ual-part-1.pdf

    RE07

    d. Any quantity of Core Motor, Legacy TETRIX DC Motor, Core Servo, and Legacy TETRIX Servo Controllers are allowed.
    e. Any quantity of REV Servo Power Modules is allowed

    page 33
    Servos -
    A maximum of twelve (12) servos are allowed. Any servo that is compatible with the attached servo controller is allowed. Servos may only be controlled and powered by an allowed Servo Controller, REV Expansion Hub or REV Servo Power Module (when used with an allowed Servo Controller or REV Expansion Hub). Servos may be rotary or linear but are limited to 6V or less and must have the three-wire servo connector.

    Also on page 33
    Teams should be prepared during Robot inspection to show documentation confirming that the servos individually and together on the same servo controller do not exceed the manufacturer specifications for the controller.

    REV servo power module
    http://www.revrobotics.com/rev-11-1144/

    Reading the game manual is highly recommended.
    Last edited by 3805Mentor; 07-31-2017, 03:44 AM. Reason: bulletted copy from other documents get erased when posting

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    • #3
      Originally posted by 3805Mentor View Post
      Great! I didn't know that module existed. Sounds like exactly what the team needs. Thank you!

      P.S. I've read about twelve manuals of varying thickness over the summer, trying to solve more problems than I thought were possible for a relatively straightforward program like FTC.

      Sorry I missed one.

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