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Durability issues with motors


#1

Hi,

I built a small balancing robot. For the motors I used the 50:1 6V high-power micro metal gearmotors with encoders. While I had lots of fun with them and was impressed with how much power they pack in those small packages, after a few got broken and replaced (broken brushes and/or gears) I am thinking that they might not be durable enough for what I want. After all (correct me if I’m wrong), not only a balancing robot can exert a lot of mechanical load on motors when braking, accelerating etc. but crashes can happen and motors can get stuck while on full power.

I know I always have the option to go to medium or low power micro motors, but I prefer having at least the same performance of the motors I used in terms of torque and speed. Therefore, I thought of choosing a bigger diameter motor which is either low or medium power having the same or a little bit higher performance than what I have. I thought of the following alternatives (all with encoders):

  1. 20D 25:1 6V
  2. 25D 9.7:1 6V low power
  3. 25D 9.7:1 12V medium power (would probably need a step-up converter for this one)

A few specs of my robot that I think are relevant:

  • Motor voltage: between 7.2 to 8.4 volts
  • Robot weight: Approximately between 750g and 1000g
  • Motor drivers: DRV8838
  • Wheels: Pololu 90x10mm wheels

The questions I want to ask are the following:

  • Are my alternatives the right direction towards dealing with durability issues while maintaining performance?
  • All of the alternatives above require me to have a driver with a higher current or voltage rating. What is the closest feasible alternative to the DRV8838 having the right current and voltage rating?
  • Is there any chance that the motors got broken not due to being not fit for the task and if so, is there any possibility that I will not need a different type of motors? (For example setting a current limit)

Thanks a lot,

Yinon


#2

Hello, Yinon.

You are probably correct about the micro metal gearmotors not being the right fit for your robot. A 750g-1000g load is likely too much for them, especially in an application where switching directions many times at high speed is required, so increasing the motor size might help. All three motors you mentioned would have similar speed and torque capabilities as the 50:1 6V HP micro-metal gearmotor. I recommend using 2 of the TB9051FTG Single Brushed DC Motor Driver Carrier or one of our dual versions of the TB9051FTG driver, which are in the form factor of an Arduino shield and Raspberry Pi expansion board.

However, please note that the TB9051FTG does not have the same 2-pin interface as the DRV8838. I recommend reading through the product pages to see if any of them would be a good choice for your application.

-Rocky


#3

Thanks Rocky! That was very helpful to me.

Since my robot is technically a 11x21 cm rectangle with two wheels on one 11cm edge I reached the conclusion that any of the 25D motors will be too bulky for it since they are about 6cm in length not including the encoders and two of them weigh about 200g which is pretty heavy for a robot weighing a few hundreds of grams. Therefore I am thinking that the 20D motors which weigh about half and pack the required punch will do the job.

The TB9501FTG driver you suggested seems to provide up to 2.6A while the 6V 20D 25:1 motor’s stall current is 2.9A. How significant is that difference?
I can also try the 12V 20D 25:1 which has a stall torque of only 1.6A. Is it okay to use a step up converter from 7.2-8.4V to 12V for this purpose?


#4

The TB9501FTG driver carrier should be fine since it can handle 5 A peaks for a few seconds, The motors should not be constantly running at that high of a current anyways, as it will damage and reduce the lifetime of the motor significantly.

I think trying to use a boost regulator is unlikely to be worth it (it will be expensive to get one with sufficient power, and it will still probably be more of a bottleneck for your system than the driver is by itself).

-Rocky


#5

That’s all I needed to know. Thanks Rocky, you rock!