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Pololu 37D Oscillation Issue

Hi

I purchased 3 Pololu 37D 70:1 motors with encoder, initially, I heard 2 of them were making a “chopping” sound but I thought it was not much of an issue. Today, I ran some tests using a microcontroller using PWM to control motor speed along with a Pololu TB67H420FTG motor driver. I then realised that there seems to be an issue with those 2 motors as an oscillation appeared when using 2 of the motors. Here is a plot that shows this.
myplot3
The 3rd motor works perfectly, is there possibly a defect with the motors?

The chopping sound (oscillation) still persists even when plugged directly into the power supply. It is not an issue with the PWM control, which works perfectly for the 3rd motor. I also tested without the motor gears and the issue still persists.

Hello.

Can you specify what specific gearmotor you are using (we have a 12V and 24V version of the 70:1 37D gearmotor)?

Brushed DC gearmotors typically have a fair amount of unit-to-unit variation, so two of your motors behaving slightly differently than the third doesn’t necessarily mean something is wrong. I do not understand your graph; can you provide more context for it? What are the units for the x- and y-axis? Were the motors under load when you recorded that data (or prior to that)? How are you controlling the motor when that data is being taken? If you are not doing closed-loop speed control, some variation in the output speed is expected.

Brandon