**2. Piezoelectric ultrasonic motors**

Development of Piezoelectric USMs has been in progress since 1980. Piezoelectric motors worked on the principle of reverse piezoelectric effect i.e., electrical energy applied to piezoelectric substrate is converted into mechanical actuation or motion in this case it refers to vibration. These motors are known as "Ultrasonic Motors" since the frequency of vibration of the piezoelectric element inside the motor is in the range of ultrasonic frequency band i.e., greater than 20 kHz. The chronology of events in the development of ultrasonic motors is summarized in **Table 1** [1].

#### **2.1 Classification**

Ultrasonic Motors do not have a uniform methodology for classification because of the design flexibility & structural diversity [1]. For application point of view, USMs can be classified as rotary and linear type motors; from vibration shape, USMs can be classified as rod shape, π-shape, ring shape, and cylinder shape, from vibration characteristics, USMs can be classified as standing wave and propagating wave types; etc. More details of classification are done by viewing angle as illustrated in **Table 2** [1]. Classification is also done based upon the vibration type i.e., Longitudinal Vibration, Longitudinal Bending Vibration, Longitudinal-torsional vibration, Bending Vibrations & In-Plane vibrations [1].

#### **2.2 Operating mechanism of USM**

The most common USMs is the Traveling Wave Ultrasonic Motors (TRUMs) because of their simple constructions and broad applications. One typical TRUM is mainly composed with a stator, rotor, shell, bearing, spring, friction linear, PZT, base, etc. Piezoelectric ceramic is affixed to stator while rotor is affixed by friction liner [1]. Friction liner is bonded to a rotor, which contacts with stator through axial pressure. The traveling wave is formed by superposition of two mode responses with equal amplitude and phase difference π/2 both in time and space. If pre-pressure is applied to the rotor, then the vibration with micro amplitude of points on stator surface will be transformed to rotary motion of the rotor through frictional force. A structural diagram of TRUM is shown in **Figure 1** and the working principle of traveling wave, which is formed inside of the USM leading to the motion of the rotor, is presented in **Figure 2**. For successful development of the traveling wave, it is necessary that the two resonant modes with an identical frequency and mode shapes (standing waves) in an elastic body have π/2 phase difference both in


#### **Table 1.**

*Summary of the development of ultrasonic motors [1].*


#### **Table 2.**

*The classification of UTM methodologies [1].*

space and time [1] & quarter wavelength difference i.e., λ/4 [2]. Consider a point P on the stator the traveling process of point P is illustrated in **Figure 2a**–**d**. At time t = 0 point P is at initial position as shown in **Figure 2a**, at t = T/4, **Figure 2b** the wave propagates right the wave peak reaches point P further at t = T/2, **Figure 2c** wave & Point P moves λ/4 forward then at *t* = 3 T/4 **Figure 2d** wave valley reaches point P & at t = T point P reaches initial position [1].

*The Roles of Piezoelectric Ultrasonic Motors in Industry 4.0 Era: Opportunities & Challenges DOI: http://dx.doi.org/10.5772/intechopen.100560*

#### **Figure 1.** *Structural diagram of TRUM [1].*

#### **Figure 2.**

*Traveling wave mechanism of TRUM [1].*

In order to achieve traveling wave, selection of right piezoelectric materials is required. Most commonly used piezoelectric materials are Barium Titanate BaTiO3, Lead Zirconate-lead titanate (PZT), relaxor ferroelectrics of Pb(MgxNb1-xO3)-PbTiO3(PMN-PT) and Pb(ZnxNb1-x)O3-PbTiO3(PZN-PT) single crystals [1].
