Anyone who's ever worked on VEX, or turned on a camera flash, has dealt with the microphony effect. This is when a high-frequency changing magnetic field is induced in a wire due to an electrical wave, and the field opposes itself, causing the whole coil to vibrate. It's what makes that high-pitch whine when you turn a VEX motor on half-power, or charge a camera flash. So, knowing this effect exists, I was wondering if you could turn a motor into a speaker using PWM sound encoding.
The most common way of producing an analog signal on a microcontroller is using a PWM signal pulsed through a low-pass filter. Basically, an analog signal is represented by the duty-cycle of a high-frequency PWM wave (usually around 40 kHz), and the low-pass filter takes out the high-frequency, leaving a fairly good approximation of an analog voltage. Motors are inductive, and inductors were the first low-pass filters, and continue to be used for such purposes as RF chokes and some high-current audio equalizers. So what if you just used a motor as the low-pass filter, and encoded an audio signal into a PWM wave, say, through a motor controller. Would the motor make music, or would it be too high a load and cause too much interference?
Just food for thought. I don't think this would be possible through a servo signal motor controller, the updates to servos (and VEX motors) only happen once every 20 ms or so, so I think this would have to be done with a hardwired motor on a CCP port on the VEX if that were the platform being used. Maybe I'll try that if Santa brings an Arduino
