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?
The current in the motor will vary much less and the sound will be reduced.
To reduce the sound even further you can put an additional coil to flatten out the variations in the motor current. The coil in the motor is in fact already a component that can be used to filter.
Idea 1 is correct, go to ultrasonic PWM frequency.
Idea 2 is a no-no, with power electronics we stay in the switch-mode domain. Otherwise you start to need heatsinks, large power resistors and worry about thermal management.
Which motor drivers are you using?
By the way the link to the data on those motors is nonsens - the stall current is listed as being less than the no-load current!! Ignore that product unless you want to have to characterize it yourself...
If you want quiet motors you need brushless (with Hall sensors) and use no gears at all. There are some high quality brushed motors with low brush noise, but they are typically expensive (I have a lovely Maxon motor I got 2nd hand that's nice as quiet for instance, list price $100 or so though!).
If you avoid gears you have the issue that torque depends on the size of the motor.