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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 FTC6155
    Subject: Servo Legality

    Question: Can you confirm that the Gobilda 2000-0025-0002 servos are legal for this season 2019-2020 game ?

    Answer: There is no way for us to confirm/deny the legality of individual servos in any reasonable way. <RE10> allows any servo that is compatible with the attached servo controller to be used. In general, this implies that the servo operates at 6v, uses a three-wire PWM type signalling for control, and requires power that fits within the power limits of the servo controller.


    • #3
      Originally posted by FTC11477

      Subject: GoBilda Motor/Gearbox Combos

      Question: Are the GoBilda 5202 Series Yellow Jacket Planetary Motors legal for Skystone 2019-2020. GoBilda is listing them as legal but it doesn't specifically say that they are legal in the manual.

      Answer: Technically, this is an assembly of a motor from MATRIX/Modern Robotics and a gearbox from GoBilda. The motor is allowed, as are single-degree-of-freedom gearboxes.


      • #4
        Originally posted by FTC5110
        Subject: Removing PreInstalled Encoder

        Question: AndyMark NeveRest motors have an encoder attached. It is possible to completely remove this encoder leaving just the 2 motor terminals exposed which is exactly how the approved Tetrix MAX DC motor W39530 is supplied.

        Q: Provided we insulate wires connected to the motor terminals to ensure safety, is removal of the NeveRest encoder permitted by <RE15>?

        Answer: Yes.


        • #5
          Originally posted by FTC7172
          Subject: Payload Release Servo
          Question: We're wanting to know if the EFLA405 "payload release" servo is legal. ( )

          Even though it's advertised as "servoless", it clearly has a servo-like motor inside and interfaces with a standard servo port, so we believe it meets the requirements of <RE10> as a linear servo.

          However, the EFLA405 also appears to use an integrated spring-loaded pin for the payload release (see video at ), and so we wonder if collectively the servo and its integrated release mechanism could violate <RM02> (e.g., more than one degree of freedom for the assembly).

          Any guidance on the legality of this device is greatly appreciated!

          FTC7172 - Technical Difficulties

          Answer: There is nothing in the rules that currently would prohibit this servo from legal use. It would appear from the very limited documentation that the "daisy chain" port/connection is simply a signal pass-through.


          • #6
            Originally posted by FTC16147
            Subject: Rule RE15 - modifying a servo for continuous rotation


            If the purpose of this rule is safety, then changing a servo to a continuous rotation servo is clearly not violated. The change is entirely mechanical, not electrical, and has no impact to the safety of the motor. The change only makes the servo behave like any other legal off-the-shelf CRServo motor.

            In reviewing posts from earlier years, however, there seems to be a consensus that this is illegal. Unless this was explicitly prohibited in earlier years, it must have been considered illegal due to the words "may not be modified internally". But focusing on those few words ignores the emphasis of the rule which is on safety, and as that relates to electrical modifications. It also ignores that mechanical changes are allowed.

            Even considering the change to be illegal because the change is "internal" is not valid because it is simple to remove the external casing of a servo. Doing so would make all changes "external" modifications.

            For reference, the changes required to make a servo continuous rotation are:
            1. removing a plastic stopper on one of the gears ("changing gear" is allowed).
            2. shortening a portion of the protruding shaft so the encoder does not detect changes in the current position ("shortening motor shafts" is allowed),
            3. gluing the shortened shaft so it will not move, so the encoder never detects a change in position (tiny drop of glue easily applied)

            Answer: Modifying a servo to alter it from limited rotation to continuous rotation would fall into the "internal" category of modifications and is not allowed per RE15.


            • #7
              Originally posted by FTC3708
              Subject: Servo Power Limits

              Question: We don't really understand the rules on servos.
              Rule <RE10> states "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.
              This is further expanded upon in Appendix B - Robot Inspection Checklist: "Servos ...must be compatible with the attached REV Expansion Hub, REV Servo Power Module, or servo controller and not exceed the manufacturer specifications for the controller."
              So our question concerns servos like the Savox SA1230SG ( This servo has a voltage range of 4.8-6.0 V, and a rated stall current of 4.2A (at 4.8V) and 5.3A (at 6V). It seems like the purpose of these rules is to avoid driving a servo at an unsafe voltage or current. So IF the servo controller is capable of generating more than 5.3A of current to a servo motor being held in stall position, then this could clearly be a problem (i.e., if you continued to stall the servo, and the controller would continue to dump current into the servo up to, say, 9A, then you could damage the servo and potentially create a fire hazard).
              But the REV Expansion Hub has overcurrent protection (, section 1.3), which limits current output to each pair of servo channels to just 2A (section 1.3). So you could put the biggest servo available on the REV Expansion Hub, and it wouldn't be an issue for either the Expansion Hub OR the servo, because the 2A being provided at 5V (10 W) would still be FAR lower than the servo's power capability.
              It appears that this rule <RE10> is being interpreted as, "it is not legal to plug any servo that can handle more than 10 W of power into the REV Expansion Hub," but this doesn't make any sense! If a servo (such as the Savox SA1230SG referenced above) is plugged into the REV Expansion Hub, it will be inherently underpowered, and thus not a safety issue in any way. Why would FIRST care if we bought an expensive servo and used it at a fraction of its potential power?
              Now if we WANTED to use the Savox servo at closer to its full potential, we would need to plug it into the REV Servo Power Module, which ups the available voltage from 5V to 6V, and from 2A max to 15A max (assuming that only a single servo is plugged into the REV SPM). But this would never happen because the Savox servo won't try to DRAW more than its max stall current of 5.3A. This would increase the power available to the Savox servo from 10 W (2A @ 5V) to 31.8 W (5.3A @ 6V).
              Therefore, we would argue that the Savox servo IS in fact compatible with the REV Expansion Hub (as it is capable of operating at 5V and 2A, and has a standard 3-wire connector), and it does NOT exceed the manufacturer specifications for the controller.
              Are we correct, and if not, why not?

              Answer: No. Usages of servos that are designed to rely on the over-current protection within the controllers is unwise and unsafe. Robot mechanisms that suddenly become unpowered can collapse/move in unexpected ways.


              • #8
                Originally posted by FTC16072
                Subject: Servo with Additional Analog Position Output

                Question: There are some servos that bring out the potentiometer pin with a fourth wire so that you can have closed loop feedback (ie, you know what position they are currently at)

                Are these legal? (Here is an example: )

                They are compatible, they are limited to 6V or less, and they do have the 3 wire servo controller so it seems to me that they are legal. But they do have an extra wire coming out of them.

                While this season is suspended, this might be useful to have clarification in game manual 1 next season as well about these.

                Thanks in advance,

                FTC 16072

                Answer: If the sensor output is provided by the manufacturer, then the use of the signal is allowed.