Among the different types of motors widely in use today, we would like to draw your attention to another special motor called ultrasonic motor. This ultrasonic motor is essentially an electric motor. The difference lies in the fact that it is powered by the ultrasonic vibration of a component. This component may be a stator which is placed against another component called the rotor. The second component may be a rotor or a slider depending upon the scheme of operation used.
The scheme of operation may be either rotational scheme or linear translational scheme. These ultrasonic motors and piezoelectric actuators use some type of piezoelectric material, lead zirconate, lithium niobate or other single crystal materials. But there are many differences between the two. The most prominent difference that perceive is that in ultrasonic motors, resonance is used to amplify the vibration of the stator which is in contact with the rotor. Also the rotation and sliding distances of the ultrasonic motors are large when compared to the piezoelectric actuators. For the piezoelectric actuators, it is limited by the static strain which may be induced in the piezoelectric element. The common application of these ultrasonic motors is in camera lenses. In camera lenses, it acts as part of the autofocus system to move the lens elements. The conventional micro motor which was much slower and noisier was replaced by the ultrasonic motor.
In the mechanism of the ultrasonic motor, the ultrasonic vibration induced in the stator is used to impart motion to the rotor and for the modulation of the frictional forces that is present at the surface. And in the contact quite often dry friction is used. I tis the modulation of the friction that allows the bulk motion of the rotor. In the absence of this modulation, the ultrasonic motors cannot operate. There are two ways of vibration available to control the friction at the stator- rotor interface and they are travelling wave vibration and standing wave vibration.
The peak vibration that can be induced in the structures occur at a constant vibration velocity inspite of the frequency. This vibration velocity is essentially the time derivative of the vibration displacement. And it is not at all related to the wave propagation speed within the structure. Most of the engineering materials that are liable to vibration allow a peak vibration velocity of 1m/s. If this vibration velocity of 1m/s is used in lower frequencies, the provided displacement will be of about 10 nm and as the frequency increases, the displacement also increases.