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  • Motor Durability

    We have burned out two DC12 motors recently. Is it more likely an age, load, or wiring issue? Why didn't our fuse blow before the motor burned out if it was a load issue? Would using a smaller amp fuse be advisable to prevent this from happening again?
    Thanks for your thoughts.

  • #2
    Under what conditions did your motors burn out? Were they moving something heavy or light?
    Also, how is your wiring set up?
    Last edited by Sparbots; 11-22-2012, 05:37 PM.

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    • #3
      Originally posted by Sparbots View Post
      Under what conditions did your motors burn out? Were they moving something heavy or light?
      Also, how is your wiring set up?
      Turning the lower joints of our arm. I think of it as a heavy load, but the motors are set to run at 10 and have no problem moving the joints, well, unless they are smoking.

      I believe they are wired correctly. We are using the encoders sold by Tetrix attached to the Tetrix Motor Controller.

      Comment


      • #4
        Originally posted by FTC6298 View Post
        ...the motors are set to run at 10 and have no problem moving the joints...
        Found your problem: reducing a motor's speed by sending a smaller value within the program also reduces the motor's turning force, or torque. A motor burns out when its torque is insufficient to move a load, but current continues is still applied. In your case, the HiTechnic motor controller is sending power to the motor from the battery, but the motor cannot move. The electrical energy, having "nowhere to go," is converted into heat, which causes lubricant inside the motor to evaporate (that's the smoke). Smell your motor; if it's got an oily odor, some of that fluid has evaporated. Though the motor is not directly affected, it will not perform as well as other motors.

        If you want your arm to move more slowly, put a small gear on the motor axle and a large gear on the joint of the arm, and supply the motor with 100% power. In general, it's not good to send motors values less than 15 when they have to bear a load, especially one that is changing (when the arm is horizontal, the load on the motor is greater than when it is vertical, and fully supported by the motor).

        Hope that helps!

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        • #5
          Thanks....unfortunately I know about that issue but if we use anything like a speed of even 20 the joint moves too fast to be controllable. We do have is geared way down. We actually have it geared down from a 40 to 120 then a 40 to 120 again, or 1 to 6 overall.

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          • #6
            Originally posted by FTC6298 View Post
            Thanks....unfortunately I know about that issue but if we use anything like a speed of even 20 the joint moves too fast to be controllable. We do have is geared way down. We actually have it geared down from a 40 to 120 then a 40 to 120 again, or 1 to 6 overall.
            Hi,

            First, I think 40 to 120 and 40 to 120 again results in 1 to 9, not 1 to 6.

            Second, if you have it geared down 1:9 and it's still way too fast, have you considered using a continuous servo instead?

            The Tetrix DC motor has 300 oz-in stall torque (at 0 rpm and 12V, more at 14V) and unloaded speed of 152 rpm (at 0 oz-in torque).

            [Side note: to achieve the peak power output, your roughly halve each of these (i.e., around 150 oz-in at around 76 rpm).]

            I can't find a spec for the Tetrix continuous servo, but according to Appendix 2 in the second link below the HT-475HB 180deg servo has a stall torque of around 60-to-75 oz-in and an unloaded speed of around 48 rpm. If this is the case, the servo has 1/4 - 1/5 the torque capability and 1/3 the speed capability of the DC motor (or 1/12 - 1/15 the peak power capability, if I'm doing my math/physics right. others please correct me if I'm messing something up).

            If the continuous servo is anything close to the 180deg servo, then it sounds like the continuous servo may be all you need and may solve your DC motor burnout problems. If you've geared the DC motor down by a factor of 9 and it's still too fast (i.e., it has little torque applied and thus is running faster than you want), then a servo might be sufficient.

            The links below have good info on running motors to achieve peak power, etc.

            http://www.chiefdelphi.com/forums/sh...ad.php?t=89072

            http://www.ptc.com/WCMS/files/103895...C_Robotics.pdf (check out appendix 2 and 3 in particular for motor specs)
            Last edited by FTC6124; 11-23-2012, 11:20 AM.

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            • #7
              Doh...and I teach AP Calculus...of course it is 1:9.
              Anyhow, much of our code relies on encoder values. I've seen discussions on setting the DC motors to a float state. Do you think we would still be able to use encoder values while using the continuous rotation servo to also turn a motor axel in the float state? The continuos rotation servos give no feed back.

              Comment


              • #8
                Originally posted by FTC6298 View Post
                Doh...and I teach AP Calculus...of course it is 1:9.
                Anyhow, much of our code relies on encoder values. I've seen discussions on setting the DC motors to a float state. Do you think we would still be able to use encoder values while using the continuous rotation servo to also turn a motor axel in the float state? The continuos rotation servos give no feed back.
                If you wouldn't mind posting the code (please include the #pragmas), that might help.

                It's perfectly legal to use a command like motor[motorD] = 100; even without engaging PID control. The PID algorithm implemented in RobotC alters the actual power sent to a motor based on the feedback from encoder values (with past, current, and projected error taken into account). We usually don't use the RobotC PID at all; if we have to use encoders, we write our own, simplified speed regulation system. This gives us much more control.

                You're right; there is no encoder sold specifically for the continuous rotation servo, and the servo has no position feedback. There is, however, a HiTechnic angle sensor that plugs into the NXT sensor port (if you're willing to spend an additional $55), but that probably over-complicates things.

                Putting a motor in a float state and turning it with a continuous rotation servo is probably not a good idea. Even a gearbox (detached from the DC motor) is difficult to turn by hand, and servos aren't very powerful to begin with. I'm surprised that you're having so much trouble with a 1:9 gear ratio. Perhaps you should power the motor for the arm based on an analog joystick value, rather than a pre-coded value. Again, it would help to look at the code.

                The "float" state is activated by using the command bFloatDuringInactiveMotorPWM = true; Turning the motor with an external force would still require a bit of power; this simply prevents the motors from applying the electronic braking system.

                But, if you really don't want to use the DC motor, I'd suggest trying a Lego NXT motor instead (the orange and white ones). They have built-in encoders, accurate to 1 degree. I'm not sure how the torque rating of a Lego motor compares to that of a continuous rotation servo, but you may have better luck.
                Last edited by Sparbots; 11-23-2012, 02:02 PM.

                Comment


                • #9
                  Lubricating motors - ideas?

                  Originally posted by Sparbots View Post
                  ... The electrical energy, having "nowhere to go," is converted into heat, which causes lubricant inside the motor to evaporate (that's the smoke). Smell your motor; if it's got an oily odor, some of that fluid has evaporated. Though the motor is not directly affected, it will not perform as well as other motors.
                  Do you know how to restore the fluid? I've partially disassembled a motor (separated the gearcase from the rest of the motor), but I wouldn't know what type of lubricant to use. Any tips on what to use, how to apply it, etc. would be greatly appreciated! Also, if anybody has an opinion (yes, I know there's a forum for official questions) on whether this is legal for competition, I'd love to hear it. The relevant rules are "<R03>(s): Motors, sensors, controllers, and any other electrical components may not be altered from their original state in ANY way unless specifically allowed by the Robot rules" and "<R06> Any type of COTS lubricant is allowed, provided that it doesn't contaminate the playing field, game elements, other Robots, etc."
                  John McDonnell
                  Co-Mentor, Team 5873
                  https://www.facebook.com/Team5873

                  Comment


                  • #10
                    Originally posted by Sparbots View Post
                    But, if you really don't want to use the DC motor, I'd suggest trying a Lego NXT motor instead (the orange and white ones). They have built-in encoders, accurate to 1 degree. I'm not sure how the torque rating of a Lego motor compares to that of a continuous rotation servo, but you may have better luck.
                    Team Unlimited's site at http://www.ftc0001.org/2012-13season...ips%202012.pdf says:

                    * TETRIX DC motors – 152 rpm (full rotation), 300 oz-in. of torque
                    * TETRIX (non-continuous) Servomotors rotate 180°, 76 oz-in of torque
                    * LEGO motors – 11 RPM (full rotation), 24 oz-in of torque
                    * Continuous rotation servomotors (from http://www.tetrixrobotics.com/Buildi...t.aspx?aid=112 ):
                    ** Stall Torque : 2.8kg.cm (38.88 oz-in)
                    ** Stall Current : 640ma
                    ** Operating Speed : 43.4 rpm +/- 10%
                    John McDonnell
                    Co-Mentor, Team 5873
                    https://www.facebook.com/Team5873

                    Comment


                    • #11
                      Originally posted by JohnMcDonnell View Post
                      Team Unlimited's site at http://www.ftc0001.org/2012-13season...ips%202012.pdf says:

                      * TETRIX (non-continuous) Servomotors rotate 180°, 76 oz-in of torque
                      * LEGO motors – 11 RPM (full rotation), 24 oz-in of torque
                      * Continuous rotation servomotors (from http://www.tetrixrobotics.com/Buildi...t.aspx?aid=112 ):
                      ** Stall Torque : 2.8kg.cm (38.88 oz-in)
                      ** Stall Current : 640ma
                      ** Operating Speed : 43.4 rpm +/- 10%
                      Interesting! I didn't realize that the non-continuous servos were so much stronger than the continuous ones! And it turns out that the continuous rotation servos have much higher rotation speeds than the Lego motors, and with more torque.

                      Comment


                      • #12
                        Originally posted by JohnMcDonnell View Post
                        Do you know how to restore the fluid? I've partially disassembled a motor (separated the gearcase from the rest of the motor), but I wouldn't know what type of lubricant to use. Any tips on what to use, how to apply it, etc. would be greatly appreciated! Also, if anybody has an opinion (yes, I know there's a forum for official questions) on whether this is legal for competition, I'd love to hear it. The relevant rules are "<R03>(s): Motors, sensors, controllers, and any other electrical components may not be altered from their original state in ANY way unless specifically allowed by the Robot rules" and "<R06> Any type of COTS lubricant is allowed, provided that it doesn't contaminate the playing field, game elements, other Robots, etc."
                        We've burned out quite a few motors over the past few years. Until Bowled Over, we didn't really know what to do with them. As it sounds you have, we also took gearboxes off of broken motors and moved them to functional ones, especially when the motors had encoders attached (we still haven't yet figured out a good way to remove the Tetrix encoders without damaging the optical sensor or disk).

                        I heard at the Georgia state championship last year that some teams send their burned motors off to be repaired. I apologize for the lack of clarity; I have no idea how this works, and I only got word of it third-hand. If you have a burned motor that you're willing to sacrifice for R&D, and risk permanently breaking, you might try taking it apart, but we couldn't figure out how to open up the motor itself.

                        I'm not sure that repairing motors is legal within the Ring It Up rules. I can post a question on the Q&A forum from our team account if you'd like (since it applies to everyone, and we really need to do something with our pile of not-quite-dead motors).

                        Also, I'm not an expert on electronics or any of this stuff, I'm just (to some extent) spitting back things I've been told. I've read on Chief Delphi that burning a motor occurs when ceramic insulation on the internal wiring melts, but I had always thought it was lubricant.

                        Hope that helps!

                        Comment


                        • #13
                          Originally posted by JohnMcDonnell View Post
                          Do you know how to restore the fluid? I've partially disassembled a motor (separated the gearcase from the rest of the motor), but I wouldn't know what type of lubricant to use. Any tips on what to use, how to apply it, etc. would be greatly appreciated! Also, if anybody has an opinion (yes, I know there's a forum for official questions) on whether this is legal for competition, I'd love to hear it. The relevant rules are "<R03>(s): Motors, sensors, controllers, and any other electrical components may not be altered from their original state in ANY way unless specifically allowed by the Robot rules" and "<R06> Any type of COTS lubricant is allowed, provided that it doesn't contaminate the playing field, game elements, other Robots, etc."
                          This is a grey area question in my mind so I'd advise posting on the official forum. The question being, Is "rebuilding" the same or different from "modifying?"

                          Comment


                          • #14
                            Update on our motors burning out....
                            We do have our joints a being controlled by our joysticks but we had the maximum "speed" set at 10 so a 127/2 reading form the joystick was only sending a "speed" of 5. We changed the max speed to 50 and we are having success and obviously a much more diverse range of speeds available to the driver.
                            Thanks for all the comments and suggestions.

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                            • #15
                              Glad you got that worked out! The burned motors will still work, but with lower performance, so you can still use them for lighter jobs.

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