Dry friction is often used in contact, and the ultrasonic vibration induced in the stator is used both to impart motion to the rotor and to modulate the frictional forces present at the interface. The friction modulation allows bulk motion of the rotor (i.e., for farther than one vibration cycle); without this modulation, ultrasonic motors would fail to operate. Two different ways are generally available to control the friction along the stator-rotor contact interface,
traveling-wave vibration and
standing-wave vibration. Some of the earliest versions of practical motors in the 1970s, by Sashida, for example, used standing-wave vibration in combination with fins placed at an angle to the contact surface to form a motor, albeit one that rotated in a single direction. Later designs by Sashida and researchers at
Matsushita, ALPS, and
Canon made use of traveling-wave vibration to obtain bi-directional motion, and found that this arrangement offered better efficiency and less contact interface wear. An exceptionally high-torque 'hybrid transducer' ultrasonic motor uses circumferentially-poled and axially-poled piezoelectric elements together to combine axial and torsional vibration along the contact interface, representing a driving technique that lies somewhere between the standing and traveling-wave driving methods. A key observation in the study of ultrasonic motors is that the peak vibration that may be induced in structures occurs at a relatively constant
vibration velocity regardless of frequency. The vibration velocity is simply the
time derivative of the vibration displacement in a structure, and is not (directly) related to the speed of the
wave propagation within a structure. Many engineering materials suitable for vibration permit a peak vibration velocity of around 1 m/s. At low frequencies — 50 Hz, say — a vibration velocity of 1 m/s in a
woofer would give displacements of about 10 mm, which is visible. As the frequency is increased, the displacement decreases, and the acceleration increases. As the vibration becomes inaudible at 20 kHz or so, the vibration displacements are in the tens of micrometers, and motors have been built that operate using 50 MHz
surface acoustic wave (SAW) that have vibrations of only a few nanometers in magnitude. Such devices require care in construction to meet the necessary precision to make use of these motions within the stator. More generally, there are two types of motors, contact and non-contact, the latter of which is rare and requires a working fluid to transmit the ultrasonic vibrations of the stator toward the rotor. Most versions use air, such as some of the earliest versions by Hu Junhui. Research in this area continues, particularly in
near-field acoustic levitation for this sort of application. (This is different from
far-field acoustic levitation, which suspends the object at half to several wavelengths away from the vibrating object.) ==Applications==