I'm creating a kinetic sculpture with about 18 vertical-axis, spinning, 3d-printed doo-dads. The sculpture must be very quiet, and the speed of each spinning motor needs to be adjustable, but somewhat slow: 50-100RPM. The motors must be small, less than 1" diameter. From early experiments I learned I simply cannot find gearmotors that run quietly enough. I tried maybe a dozen from various sources. Then I got my hands on these little cuties from DigiKey.
They were the lowest RPM motor I could find without a gearbox. When run from a variable power supply, they stay very quiet, and spin at a great speed when given about 2.2VDC. BUT when I plug them into a MEGA 2560 with motor controllers and give them analogWrite signals, that dreaded 490Hz signal is audible and the motor becomes very loud at the same speeds it's happily quiet on receiving straight analog voltage.
My two ideas so far:
Raise the frequency of the PWM up to a range outside of human hearing. Having some issues with this- the motor controller doesn't seem to understand the 25kHz signal - it's either all on or all off. Even though it works perfectly fine with the 490Hz standard PWM signal. Can anyone point me towards a dc brushed motor control breakout / module / shield that will place nicely with this higher frequency signal?
Squash the PWM signal with a low-pass filter. It's been a few years since engineering school so I may need some assistance calculating resistor and capacitor values to accomplish this.
Am I on the right track? Or is there a better way?