I believe that most ultrasonic cleaners work by having a large piezoelectric plate attached to the outside of the metal basin. when the plate is excited it vibrates and transfers the ultrasonic vibration to the liquid in the basin.
large amounts of ultrasonic vibration in the liquid causes "cavitation" at the surface of objects that are placed in it, essentially little temporary 'bubbles' at the surface that are actually a vacuum, which quickly collapse, and work to suck dirt and grime off the surface.
tek, you need to explain what you are actually trying to find out. ultrasonic cleaners do not involve sensors. different piezoelectric devices can be used as ultrasonic transducers and sensors, but those devices would be much smaller and completely differently designed from the large, high-power piezoelectric devices used for ultrasonic cleaners, so there is not much direct relation.