Vibrational devices feed off motion produced as a by-product in order to generate power, and so are natural AC power sources. Because they are AC, they require rectifying and regulatory circuits. Sources include the human gait, trains, motors, engines, and radio frequencies.
The two most common methods to generate power are that of electromagnetic induction and piezoelectricity.
Electromagnetic induction is harvested using motors, with the difference being that the magnet inside the coils moves back and forth instead of just spinning.
Piezoelectric materials produce voltages under pressure changes brought about by sound waves or touch or mechanical strain. Crystals made from this substance form diaphragms or are linked to them, and they can function as speakers or microphones. For energy harvesting, piezoelectric generators are usually cantilevers with masses attached to a free end.
Vibrational harvesting devices are useful monitoring equipment and machinery in industrial environments without the use of batteries or cables. However, this method is not good to use in devices where mobility is a requirement, due to concerns with stability and interference that will impact features such as velocity and noise. They are also best used where the input is at a consistent, predictable frequency.