The major advantages of USMs are: 1. Compact, lightweight, flexible and robust. 2. High positioning accuracy. 3. High low-speed torque and holding torque. 4. Unaffected by external electric or magnetic fields. 5. Quiet drive system. 6. Hard brake with no backlash. 7. Variable stroke. 8. Quick response.

One thing all USMs have in common is their use of piezoelectric material to transform electrical energy to mechanical energy. USMs typically use ceramics derived from lead-zirconate titanate (abbreviated PZT). After the PZT ceramic is shaped and fired, it is then electric field polarized. This allows the material to deform with a changing electric field.

Here we will discuss the operation of a ring type USM. Like traditional motors, USMs have a stator and a rotor Some USMs use a toothed stator to increase the holding torque. Other motors simply rely on frictional forces. As shown in the illustration on the right, the bottom layer of the stator is composed of the PZT material mentioned earlier.

Two electrical signals with orthogonal modes (like sin(wt) and cos(wt)) are introduced in the stator material. If a constant phase difference exists between the two modes a traveling wave is created in the stator. Otherwise the wave is standing.

The repeated rolling motion of the stator creates microscopic orbit of the stator’s surface particles (much like water drops in a water wave. These small movements move the rotor forward. Thus, the traveling wave in the flexural stator material moves in the opposite direction of the rotor spin.

The stator may drive the rotor using tiny teeth or simply the force of friction. While the angular velocity of the rotor is proportional to the frequency of the traveling wave, that does not mean they are equal. The traveling wave may pass through the stator several times for a single rotation of the rotor.

Linear USMs, sometimes called “tube” or “rod” USMs, also use piezoelectric metals or ceramics for actuation. Show here is a picture of New Scale Technologies tiny “Squiggle” motor. The Squiggle motor weighs about 30gm and boasts a stall force of 10N. Micro deformations also give resolutions as high as 1nm, and max speeds of 15mm/sec

The Piezo LEGS motor, developed by MicroMo Electronics Inc., illustrates one popular technique for linear USM actuation. Like other USMs, the LEGS motor generates motion in discrete steps. 4 bimetallic metal/ceramic “legs” are positioned around a single nut on a threaded rod.

Applying voltage to a PZT leg causes it to change shape. This strain in the leg causes the nut to bend and shift on the threaded rod. By synchronizing the 4 legs an elliptical force pattern moves the rod in either the forward or reverse direction. Because deformations are small, several thousand pulses/sec are needed.

Spherical USMs may be employed when more than one degree of freedom is needed. Potential applications include surgical robots or robotic eyes The concept of a spherical USM is simple. Three separate ring USMs control actuation in the x, y and z directions. Thus, the sphere can be given any orientation in 3 space

USMs have lots of potential for use in medical applications. One very promising research area is in medical diagnostic instruments. The Robotics Institute of Carnegie Mellon University and the Division of Cardiac Surgery at the University of Pittsburgh are teaming up to create a tiny robot called the HeartLander

The Heart Lander is a tiny robot that surgeons could insert into a patient’s chest cavity through a minimally invasive incision. This tiny robot could then move along the surface of the heart and perform interventions. The surgeon would be able to control every movement via a controller and monitor external to the patients body.

Canon. (2007). Using Ultrasonic Vibrations to Drive Focus and Zoom Lenses. Retrieved March 2, 2009, from Canon: Carnegie Mellon University. (2007). HeartLander. Retrieved February 22, 2009, from CMU: Toyama, D. S. (2008, April). Sherical ultrasonic motor, piezoelectric actuator, spherical sensing system. Retrieved March