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Related to Micrometal Gearmotors


#1

Hi,

I had bought Pololu 30:1 HP Micrometal Gearmotors last year in November. Recently a strange behaviour has been noticed in a motor mounted on my robot. The motor ran perfectly for a short period of time (about 15 minutes with regular breaks of 2-3 minutes each time the two motors heated), but after that it got jammed. I detached the gearbox and found out that the small gear situated behind the gearbox was not rotating. Instinctively I slowly pulled it outwards marginally, and it started rotating again. I reassembled the motor and continued my work, but the same problem was observed, and after a similar period of time, in the other motor also.

The only change I have incorporated in the PD based code for my line follower robot is a functionality of reversing the appropriate motor when the error becomes too high or too low.

I would appreciate a solution, and would also like to know about the reason for this problem.

Thank you.

Anmol


#2

Hello, Anmol.

Can you tell me a little more about your setup and how you were using the gearmotors? In particular, I am concerned that the change to your code could have led to rapid changes in motor direction if your constants were not well tuned. Which gear in the gearbox was the one that you found was not rotating? Can you post a picture or video showing your setup and which gear was giving you a problem?

Grant


#3

Hello, Grant.

Thank you for your response!

I’ve been using the gearmotors for line following at speeds upwards of 400 rpm. The robot also utilizes the Arduino Uno, a Pololu QTR-8RC array, a TB6612FNG motor driver and 42 x 19 mm wheels.

I’m pretty sure that my constants were not badly tuned. I also noticed during various runs that the motors didn’t actually reverse, because the power difference didn’t increase to that extent.

I’m attaching some images below. The gear encircled in the first image is the one that isn’t rotating.

Idea%20note_20180815_194935_01(2)

20180815_194639

Anmol


#4

It sounds like it’s the motors themselves that are damaged, not the gearboxes. Can you tell me specifically which gearmotors you have and what you are using as your power supply? Also, can post some pictures of your robot?

If you apply power to the motors now without the gearboxes, what happens?

Grant


#5

Hello, Grant.

I’m using 30:1 HP Micrometal Gearmotors operating at 6V, controlled using a Toshiba TB6612FNG motor shield purchased from Pololu; the shield is powered using a 3S 1500 mAh Lithium Polymer battery. Incidentally, the motor shield also got burnt today, due to mishandling by one of my teammates.

One motor has stopped working completely. The relevant gear of this motor has stopped rotating, despite applying voltage. The other one is working fine presently.

I’ve just placed an order for a Pololu DRV8833 motor driver for use with these motors.

Thanks,

Anmol


#6

A 3S LiPo has a nominal voltage of 11.1 V and is about 12.6V when fully charged. Are you regulating down to a lower voltage between the battery and the motor driver? If not, it is possible that your motors and driver were damaged by the voltage. Running motors at a higher voltage increases their torque which puts more stress on the gears and though 12.6V is within the maximum the TB6612FNG can handle, being that close makes spikes more of an issue.

Also, if you are not regulating the battery voltage, the DRV8833 is not appropriate either since it has a maximum input voltage of 10.8 V.

Grant


#7

Hello, Grant.

Thanks for your reply!

You’re right, I wasn’t regulating the voltage between the TB6612FNG and the LiPo. I presume the motor shield blew because one of my teammates held the wheels too firmly while the motors were trying to rotate when he was operating the robot in my absence.

Therefore, for the DRV8833 as well, is it a good idea to buy a 2S LiPo instead? Also, would using a 2S LiPo affect the performance of the 30:1 HP 6V micrometal gearmotors ie max RPM? Please note that I would like to operate my motors at around 700-800 RPM in my competitions.

If the 2S option is fine, please advise about the recommended ‘mAh’ rating of the battery (1000 or 2200 mAh) for my RPM requirements stated above?

Alternatively, can I use a 3S LiPo with a LM2596 step-down voltage regulator, adjusted to give an output of 7.5V (with 3A constant current output as stated in the datasheet)?

Anmol


#8

A 2S LiPo would be better as it has a nominal voltage of 7.4 V. Increasing the voltage applied to the gearmotors increases the performance of the gearmotor (including the speed), but as I mentioned previously, it also puts more stress on the gearmotor. The speed of the motor depends on the load you have on your gearmotor, but at 6 V, you should be fine if you run the gearmotor at 30% of the stall current. You can find additional information in the performance graphs on the datasheet for the micro metal gearmotors.

The mAh rating on the battery rating is about capacity, and does not have a significant affect on the speed of the gearmotors. If want to know more about battery ratings, you might find this blog post about battery capacity helpful. You should select the battery based on the how much battery capacity you need.

You can also use a voltage regulator to step down the voltage from a 3s LiPo to a more appropriate voltage, but you should make sure the regulator can handle the stall current of both gearmotors (2 A each at 7.5 V). From a quick look, it seems like the regulator you mentioned is a constant voltage regulator, not constant current, so the amount of current it will be able to handle will be based on the input voltage and other factors like heat.

Grant


Tackling line breaks in line following arenas
#9

Hello, Grant.

Thank you for the information.

I was hoping to finalize the regulator I need to purchase. I am considering the XL4015 since you mentioned that the LM2596 would not ensure 2A supply to each motor. The XL4015 has a 5A rating, though not sure if it would affect performance of the motors since it is a 180 KHz fixed frequency PWM buck (step-down) DC/DC converter.

I would currently prefer to continue using the 3S LiPo along with the TB6612FNG - have just received a second one ordered from Pololu’s dealer here. Will aim to shift to the 2S LiPo for my next project.

In case you feel that the XL4015 is not suitable, would appreciate if you could suggest any other one that would give me the desired results i.e. rpm and torque.

One last point - in the datasheet of the Micro metal gearmotors, I could not find the graph showing current as a function of voltage (e.g. you mentioned that 7.5 V would lead to stall currents of 2A each, and Brandon had previously mentioned that 11.1V LiPo would lead to currents of ~3A each Click here for the post

Anmol


#10

You can still use your 3S LiPo if you are regulating the motor voltage to an appropriate level. From a quick glance, that regulator looks like it should be fine.

The graph that applies to that motor is on page 21 of the datasheet and is only for the rated voltage, but the current and torque of a brushed DC motor have a linear relationship with the applied voltage. For example, the stall current is 1.6 A at 6 V and goes to 2.4 A at 9 V.

Grant


#11

Hello, Grant.

Thank you very much for your help!

Will get in touch if I need any further assistance.

Anmol