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Motors and Servos

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  • Motors and Servos

    Answers to questions about Motors and Servos.

  • #2
    Originally posted by FTC9999
    Subject: Servo Power Module Device Limits

    Question: Section 7.2.1 [Robot Technology Definitions] of Game Manual Part 1 defines a REV Servo Power Module as
    "An electronic device that boosts the power supplied to 3-wire servos. A REV Servo Power Module has 6 input servo ports and 6 matching output ports. It draws power from a 12V source and provides 6V power to each output servo port. A REV Servo Power Module can provide up to 15A of current across all output servo ports for a total of 90 Watts of power per module."

    The stall current of the VEX EDR 393 is rated as 3.6 amps at 7.2 volts (or 3 amps at 6 volts), but when connected to the VEX "Motor Controller 29" the stall current is limited to 3 amps at 8.5 volts (or 2.2 amps at 6 volts).

    The stall current of the goBILDA 2000-0025-0002 (25-2) servo is rated as 3 amps at 7.4 volts and 2.5 amps at 6 volts. Thus, teams are allowed to power six (6) goBILDA 25-2 servos from a single REV Servo Power Module (SPM).

    Even though teams are allowed to power six (6) goBILDA 25-2 servos per SPM, and even though the VEX 29/393 draws less current at stall than the goBILDA 25-2, teams are only allowed to power two (2) VEX 29/393s per SPM.

    Like most teams, after the season is over our competition bot becomes an outreach bot. Since the SPM can safely power six (6) VEX 29/393s we reduce the number of SPMs on the bot from the 5 to 7 required for competition to at most 2 SPMs as allowed by the specifications. We use the 4 to 5 SPMs recovered from the previous season's bot on next season's competition bot so that the team need not have to purchase any more SPMs than necessary in the long run.

    Nevertheless, in the interests of reducing congestion, debugging complexity, and points of failure on a competition bot, we request that the restriction of two (2) VEX 29/393s per SPM be removed. As with any other servo, the SPM's over-current shutdown feature will safely inform teams in the event the team miscalculates the max current draw of the mix of servos, VEX and non-VEX, connected to any given SPM.

    Answer: Thank you for the thoughtful analysis. We do not plan to make any changes to the Servo Power Module limits for this season.


    • #3
      Originally posted by FTC14433
      Subject: Servo Legality?

      Question: Can we use this servo It has feedback from a potentiometer located on the final shaft of the Servo Gearbox so that the rotation is not limited or effected by the ratio chosen. The servo has input from potentiometer. When we buy it, the servo already has that input. We do not modify anything.

      Answer: No. The servo used in this gearbox assembly has been modified, post-manufacturing. As such, it violates <RE16>


      • #4
        Originally posted by FTC12611
        Subject: Servo/Gearbox Legality

        Question: 1.
        These servo gearboxes have the goBILDA servo altered by the manufacturer.

        These servo gearboxes don't have any modifications on the servo.

        Are any of these servo gearboxes legal?

        Answer: In general, it is not possible for us to make rulings on all servos.

        For Servo #1 - this servo is not a legal servo. It has been modified from its original configuration to add an additional potentiometer output

        For Servo #2 - these appear to be legal servo combinations. The servos appear to be digital servos that have been programmed to control their range of motion.


        • #5
          Originally posted by FTC12533
          Subject: Servo Power

          Question: Is powering a servo with a stall current above 2A at 4.8V, via the 4.8V ports on the rev hub legal if a 2A fuse is added by the manufacturer or by a team?

          Answer: No. Servos may only be connected to/powered by the servo ports on a REV Expansion Hub, the servo ports on a REV Control Hub or the servo outputs on a REV Servo Power Module