*3.1.2 New developments for high power densities*

In addition to miniaturization some researchers focused on high torque and power density aspects of piezo motors, Mizuno et al., developed a hybrid torsional/ bending (T/B) modes USM to provide high driving force, large driving distance, and low weight, resulting into high torque density and high-power density [17]. It is constructured with rod-shaped transducer operating in torsional/bending (T/B) modes and excited two elliptical motions on its bilateral ends to drive the rotor orthogonally pressed onto the transducer. Chang et al., developed a ring-shaped traveling wave ultrasonic motor with a suspension stator for improving output power density [17]. The maximum stator vibration amplitude of 4.25 μm, which is nearly 4.7 times of that without suspension, with speed of 62 rpm and a stall torque of 49.5 mNm was observed, under a driving signal of 30 Vpp when the mass block was 0.30 g. Fan et al. developed a miniaturized ultrasonic motor with a high thrust–weight ratio by using the first order bending vibration mode (B1 mode) and second order bending vibration mode (B2 mode) to realize bidirectional movement through a single-phase driving signal [19]. Li et al., constructed screw-type USM with a three-wavelength exciting mode to achieve a high-output thrust [20].

In order to meet the requirement of large thrust & maximum output few research articles on the ultrasonic motors were based upon typical shapes, such as U-shape, V-shape, L-shaped, and Π-type. For instance, the structure of the linear ultrasonic motor with a laminated stator, which was made of two identically single U-shaped stators, was proposed by Sun et al., [36]. The testing results showed that the maximum output force of the laminated motors increases by 40% than that of single layer U-shaped motor, while the maximum velocity increased by 38%. Yao et al. proposed a novel large thrust-weight ratio V-shaped linear USM with a flexible joint operated in the coupled longitudinal-bending mode. The motor had a compact size and a simple structure with a large thrust-weight ratio (0.75 N/g) [37]. Furthermore, they also proposed a novel large thrust L-shaped linear USM utilizing the antisymmetric and symmetric modes of the L-shaped stator operating in a single resonance mode to realize the bidirectional motion of the slider [38]. In order to meet the demand of a linear ultrasonic motor with large thrust in narrow space, a novel Π-type linear ultrasonic motor with double driving feet was constructed [39]. The motor had structural stability and high dynamic performance, such as a no-load speed of 273 mm/s and 238 mm/s in two directions, corresponding to a maximal thrust of 80 N and 110 N. Wang et al., constructed a V-type motor having two driving feet and a simple structure, which torque applied to the motor was converted into a normal

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

preload between the driving feet and the mover to avoid the use of a large preloading mechanism [40]. The maximum no-load velocities of the motor moving to the right and left are 85.2 mm/s and 76 mm/s, respectively, and the maximum output force is 1.96 N.

Light weight, high torque & desired output performance are some of the important features required in USMs. Niu et al., developed a light arch shaped, four legged linear & hollow USMs, which a light arc-shaped USM with the firstorder longitudinal vibration mode and the second order bending vibration mode were superimposed in the stator plane, in order to meet the requirements [45]. The output torque of the USM under the single-stator configuration reached up to 2.6 × 10−2 N·m, and the double-stator which was 1.5 times greater than that under of single stator configuration.

To realize applications involving low speed and high torque in the high-performance actuator industry, especially in the aerospace field, a novel 70H (Hollow) TRUM with an outer diameter of 70 mm and an aperture ratio of 53% (the ratio between the aperture and outer diameter) with a mass of 210 g was developed [46]. The TRUM, The torque density of 11.43 N·m/kg, maximum no-load speed of 50 rpm, and the maximum stall torque 2.4 N·m were achieved.

The influence of the vibration mode of the stator and the structural dimensions of the metal elastomer and piezoelectric ceramic ring on the effective electromechanical coupling coefficient (EMCC) was analyzed by Niu et al. [47]. The efficiency of a hollow USM was improved by optimizing the stator's effective electromechanical coupling coefficient. In addition, a four-legged linear ultrasonic motor with a new structure which is the in-plane first-order longitudinal vibration mode and the out-ofplane anti-symmetric vibration mode superimposed to produce linear motion [48]. The USM consists of a stator and four groups of eight piezoelectric ceramic sheets. The experimental results for a prototype 600 × 160 mm showed the maximum translational speed could reach 135 mm/s and the maximum thrust of 3.6 N with a 200 V driving voltage. The USM had the advantages of simple structure and high output efficiency, which made it suitable for precision systems and industrial applications.

Izuhara et al. proposed a linear piezoelectric motor using a hollow rectangular stator that can translate a load placed inside it by a direct drive [49]. This stator structure enabled a quick response and high resolution by few components for controlling autofocus and zoom mechanisms in imaging devices.

## *3.1.3 Multiple degrees of freedom piezoelectric ultrasonic motor (multi-DOF-USM)*

Shi et al., constructed a new type of multiple-degree-of-freedom (Multi-DOF) compact structure USM to achieve high output torque [33]. It consisted of a ring type composite stator with four driving feet uniformly arranged in the inner circumference of the ring stator. The stator employs two orthogonal axial bending modes and a radial bending mode, by exciting two of them simultaneously, to generate elliptic trajectories on driving feet tips and to push sphere rotor around x, y and z axis respectively. Su et al. improved the performance of a non-resonant piezoelectric motor, which is a symmetric piezoelectric linear motor driven by three-phase square-triangular waves signal and four-phase sine waves signal of peak to peak value 100 V at 100 Hz with 50 V offset [34]. The speeds of prototype reached 733 μm/s and 667 μm/s and the maximum thrust is 8.34 N and 6.31 N respectively. Similarly, a non-resonant linear ultrasonic motor utilizing longitudinal traveling waves was proposed by Liang Wang et al. [35]. The stator system was modeled by utilizing the transfer matrix method (TMM). The motor prototype achieved a maximum mean velocity of 115 mm/s and a maximum load of 0.25 N.

Li et al. proposed electromagnetic-piezoelectric hybrid driven three-degreeof-Freedom USM which is hybrid driven electromagnetic filed and electrical field [41]. In one of their design, a novel ball-type spherical multi-DOF USM, composed of three built-in stators and a hollow spherical rotor was developed and tested for the design of a compact multi-degree-of-freedom (multi-DOF) piezoelectric driven actuator [42]. The rotational speeds of X-axis, Y-axis and Z-axis can reach 29 r/min, 17 r/min and 16 r/min, respectively, when the frequency matches, which verifies the feasibility and rationality of the multi-DOF movement of the motor. They also proposed a multi-DOF spherical USM with built-in traveling wave stators, in which each traveling wave stator could be controlled independently and the spatial arrangement of the support structures [42]. The maximum speed achieved 45.6 rad/min with output torque of 1.265 Nm when an excitation voltage of 400 V with the preload of 100 N. The motor had the advantages of large output force and adjustable preload.

Kazokaitis et al., developed a novel design of a multi-DOF USM, which is combined the magnetic sphere type rotor and two oppositely placed ring-shaped piezoelectric actuators into one mechanism [44]. Such a structure increases impact force and allows rotation of the sphere with higher torque useful for attitude control systems used in small satellites.

#### *3.1.4 Preload effect study*

Contact mechanism between the stator & rotor is one of the important factors responsible for the efficient performance of the USMs. The studies of contact surface, contact mechanism, preloading method of USMs has been one of the prominent topics in the USM research field. Zhang et al., proposed a solution to reduce the radial sliding by optimizing the stator comb-teeth of a TRUM [50]. They further developed a 3D finite element model for longitudinal torsional USM by ADINA in order to study the mechanical simulation and contact analyses [51]. A novel hollow type USM, which the preload was applied from the bottom of the stator through a wave spring, was proposed, [52] It could not only enhance the anti-overload ability but also extended the working life of the motor.

Wang et al., analyzed the characteristics of a TRUM with considering the structural stiffness of the preload structure [53]. It demonstrated that the prepressure on the rotor was not a constant value because of the structural stiffness of the preload structure. In addition, it explained the driving mechanism of the TRUM under unsteady pre-pressure and deduced a dynamic model considering the stiffness of the preload structure.

In addition, contact force analysis by Hertz contact theory popped up in few research articles. Dong et al., carried out design and performance analysis of a TRUM with double vibrators [54]. The analytical model of double-vibrator motor was established based on elliptical distribution rule of surface point velocity, linear superposition of motions and contact force analysis under Hertz contact theory. Pan et al., focused on the coupling relationship between the flywheel vibration and the gimbal rotation through the variable stiffness of the bearing [54].

#### *3.1.5 Multivibrators*

Some research articles emphasized on novel idea of constructing USMs by using multivibrators. Yang et al. illustrated that TRUM with double vibrators can improve the output performance effectively [56]. Inheriting the concept of two traveling waves propagating in the stator and rotor, a dual traveling wave rotary ultrasonic motor (DTRUM) energized only in the stator was proposed. The experimental

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

results showed that the performance of dual traveling wave TRUM was superior to the TRUM with single traveling wave. The no load speed was 60 rpm and the stalling torque was 0.85 Nm. They further presented, an optimal design of a doublevibrator USM using combination methods of finite element method, sensitivity analysis and adaptive genetic algorithm [57]. The measured results showed that this method was effective for the optimal design of ultrasonic motors. Lu et al. proposed a new idea for constructing the motor with the stator containing several vibrators fabricated by bonding piezoelectric ceramics (PZTs) to a metal base [58]. The longitudinal and bending modes were excited in the vibrators by two alternating current (AC) voltages with a 90° phase difference were applied. The bending vibrations of the vibrators were stacked to form the torsional vibration of the stator, ultimately generating longitudinal-torsional composite vibration. Mohammed & Zakariyya proposed an idea on the development of a new type of a linear USM with double cantilever vibrators [59]. The resonance frequencies of the vibrators were 21.33 kHz, and this was also the frequency in which the two vibrators were driven to determine the output parameter such as driving force and velocity.

Multi-vibration mode USMs and sandwich type USMs were designed & analyzed in some of the research articles. Zhou et al., developed a novel multi-mode differential USM with two sandwich-type transducers, which utilized the diverse combination of four vibration modes: symmetrical and anti-symmetrical first longitudinal modes, symmetrical and antisymmetric second bending modes, which it could realize three-step speed regulation with different speed-thrust force characteristics by switching the operation mode [65]. They also presented a novel 2-DOF planar linear USM which the stator of the motor was divided as two transducers and two isosceles triangular beams [66]. The operating principle of the USM and the formation of the elliptical trajectory of the driving foot were analyzed, and the variable mode excitation method was illustrated. This motor can gain a maximum speed of 211.3 mm/s with thrust force of 3.15 N under an exciting voltage of 400 VP − P. A new sandwich type ultrasonic motor using combination of the first symmetrical and anti-symmetrical longitudinal modes was presented by them [67]. The working principle of the motor and the elliptical trajectory formation of the driving foot were analyzed. A new linear USM using hybrid mode of the first symmetric and anti-symmetric longitudinal modes was described [68]. The stator was constructed by two Langevin transducers in combination with two isosceles triangular beams. Zhou et al., constructed a rotary USM with rotationally symmetrical structure, which the stator consists of four connected sandwich-type transducers and eight driving feet [69]. With the driving frequency of 50.93 kHz and voltage 300 VP-P, the motor gave a maximal no-load speed of 157.9 r/min and a maximal output torque of 11.76 mNm.

Lu et al., proposed a single-modal linear motor based on multi vibration modes which contained two kinds of PZT ceramics [70]. The linear motor works by exciting the transverse vibration mode of the PZT ceramic on the upper surface of stator elastomer and the shear vibration mode of PZT ceramics at two ends simultaneously. The no-load velocity and the maximum output force reach 169.4 mm/s and 1.1 N, respectively. Mizuno et al., developed a high-torque sandwich-type MDOF-Spherical USM using a new annular vibrating stator with a strong excitation structure [70]. The maximum torques of rotation around the X(Y)-axis and Z-axis were measured as 1.48 N·m and 2.05 N·m respectively. Moreover, the values for torque per unit weight of the stator were obtained as 0.87 N·m/kg for the X(Y)-axis and 1.20 N·m/kg for the Z-axis, separately. Ma et al., developed a compact motor in which the stator composed of two piezoelectric plates attached to a T-shaped steel body [72]. Two orthogonal bending modes were excited by driving one piezoelectric plate and the reversed motion of the rotor could be obtained by driving the

piezoelectric plate on the opposite side. Maximum power of 2.3 mW and efficiency of 9% with a load of 0.8 mN m at a rotation speed of 27 rpm were obtained for a prototype stator with a size of 15 mm × 2.44 mm × 2 mm, operated at 44.8 kHz.

Ceponis et al., presented a numerical and experimental investigations of a multimodal TRUM, which is driven by four electric signals with phase difference of π/2., being able to generate up to 115 RPM rotation speed at constant preload force [81]. They further proposed a new flat cross-shaped USM, which operation principle based on the first in-plane bending mode of the cross-shaped stators driven by four harmonic signals with phase difference of π/2 [82]. The advantages of the motor were high rotation speed, simple and scalable design, and the small space required for motor mounting wherein it can be directly mounted on the printed circuit board. Prototype achieved a maximum rotation speed of 972.62 RPM at 200 Vp-p when the preload force of 22.65 mN was applied.

Tanoue et al., designed a novel ultrasonic linear motor equipped with a quadruped stator that used the first longitudinal mode and the first and second bending modes [85]. A maximum driving speed of 148 mm/s and a maximum thrust of 294 mN were achieved for a device with a total length of 20 mm and a weight of 5 g. One more linear USM that drives a slider rod inside the quadruped stator to realize a compact linear motion system was proposed by them [86]. Maximum no-load speed of 258 mm s-1 and maximum thrust of 490 mN were obtained with total length of the stator transducer of 20 mm and its weight of 4.9 g. Cheon et al., proposed a new type of ultrasonic rotary motor that could replace existing ultrasonic motors for driving camera zoom lenses and investigated experimentally [87]. Peng et al., presented a new kind of the rotary USM with a longitudinal vibration model of the Langevin transducer acting as the stator, while the rotor consisted of a shaft and spiral fins, the spiral fins working as an elastic coupling component by which it cannot change its direction because the spiral fins' incline direction was fixed [88]. This motor can be used when one directional motion was required. Romlay et al., proposed an improved stator design of TWUSM using the comb-teeth structure which was expected to increase the overall efficiency [89]. Le et al., proposed a novel design methodology to optimize actuator configuration for linear USMs by considering the dynamic behavior of the stator in its operating environment, where it interacts mechanically with the moving stage and other peripheral components [90]. This helped to evaluate the actuator output performance parameters for design optimization. Pan et al., developed a novel low-friction type piezoelectric rotary motor based on centrifugal force with high speed, high power, and high efficiency output, novel low-friction type piezoelectric rotary motor [91]. Yang et al., proposed a dual-rotor hybrid USM with four side panels without using the torsional piezoelectric ceramics, which was indirectly excited by four uniformly distributed side panels along the circumference of stator cylinder [92]. The stalling torque of the prototype is 8 mNm and the no-load speed is 140 r/min was obtained at 44.7 kHz for a prototype with the size 27.2 mm x 27.2 mm x 70 mm, while the outer diameter of the stator cylinder was 20 mm. The experimental results indicate that the motor could operate in the first longitudinal and the second torsional coupled vibration modes transformed from the first longitudinal and the first bending vibration modes of four side panels.

Jiu et al., proposed a modal independent USM with dual stator based on optimizing the location of a rotor and two stators which excited at the same mode [93]. Modal test showed the disparity between the modal frequencies of the stators was 0.78%. The rotary speed of the USM is 75 revolutions per minute (clockwise) and 65.8 revolutions per minute (anti-clockwise) with the maximum torque of 8.4 N. mm at the voltage of 400 Vp-p. Li et al., proposed a traveling wave ultrasonic motor with a metal/polymer-matrix material compound stator which the stator was

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

composed of a metal ring and polymer-matrix teeth [94]. The main merits of the proposed ultrasonic motor were low cost, light weight, high processing efficiency and long life. Sanikhani proposed a new linear ultrasonic motor based on the orthogonal vibration modes of an elliptical shaped [94]. Based on the experimental results, the prototype has a no-load speed of 40 mm/s and maximum thrust force of 1.55 N under excitation voltage of 70 Vp and preload of 12 N. Sun et al., developed a novel cylindrical ultrasonic motor easy to be fixed [96]. Two orthogonal B03 bending vibration modes of the stator were generated with temporal shift of 90 to produce elliptical movement on the driving surface. The weight of the proposed stator and motor was only 2.56 and 4.1 g, respectively & it achieved a maximum speed of 170 r/min under working frequency of 31.6 kHz [96].

In order to reduce the driving voltage and gain better output characteristics of piezoelectric actuators, an eight-zonal piezoelectric tube-type threaded ultrasonic motor based on two second order bending modes was analyzed by Chu et al. [97]. The USM could output a stall force of about 5.0 N and a linear velocity of 4.9 mm/s with no load at the driving voltage of 40 Vpp. This USM with a compact structure and screw drive mechanism showed fine velocity controllability and had great application in micro-positioning systems. Borodinas et al., described a USM that used the radial mode of excitation of the double ring's stator [98]. The main goal of the proposed design was to increase motor performance using d33 ceramic polarization working in the radial mode. The motor could be driven by a simple harmonic signal and used for standard piezoceramic rings [98]. An ultrasonic linear motor with dual piezoelectric (PZT) actuators which a traveling wave motion was generated on the stator by a double-sided excitation of the stator of the USM, was developed by Yang et al. [99]. The simulation results showed the differences to the characteristics that are achieved by adjusting the critical parameters, such as the PZT boned positions, the excitation frequency and the preload, in order to derive the best design [99]. Aoyagi et al., analyzed an application of noncontact transportation utilizing the near-field acoustic levitation phenomenon, which is a rotary-type noncontactsynchronous ultrasonic motor using acoustic viscous force [100]. Xu et al., proposed a novel rotary ultrasonic motor with two longitudinal transducers [101]. Only first order longitudinal vibration mode was used in the ultrasonic motor, which avoided the frequency degeneration of modal coupling ultrasonic motors. Mechanical performances showed that the motor can obtain rotary speed of 350 r/min and the maximum torque is 186 N·mm under the voltage of 300 Vp − p. Wu et al., fabricated & investigated a ring-shaped alumina/PZT vibrator to form a traveling-wave USM. The rotation speed of the alumina/PZT motor was larger than that of the stainlesssteel/PZT motor, meanwhile, it exhibited superior maximal-torque-to-voltage and maximal-output-power-to voltage ratios [102].

Stable operation is one of the most crucial requirements for resonators in vibratory gyroscopes and ultrasonic motors, but eigenvalue splitting can deteriorate operation stability. Wang et al., proposed the estimation and elimination of eigenvalue splitting and vibration instability of resonators arranged in a fashion of ringshaped periodic structures [103]. To simplify the driving power of the inertia drive USM, a low-frequency USM driven by a 50 Hz sine wave was proposed. Wang et al., proposed a standing-wave trapezoidal ultrasonic linear motor, which consisted of a trapezoidal piezoceramic plate with slanted sidewalls and a clip fastener to achieve bidirectional linear motion [105]. A trapezoidal piezoceramic plate sized 22 x 8 x 1.5 mm3 and provided a travel distance of 10.10 mm and an output force of 12.151 g at a driving voltage of 10 V was useful for compact products.

Patents were filed on by various inventors. For instance, 1) YANG et al., files a patent which described a multi-spoke-type ultrasonic motor, to increase output performance of the ultrasonic motor, prolong service life, and reduce

manufacturing costs; [106] 2) Rosenkranz et al., for U -shaped piezo motors; [107] 3) YANG et al., for ultrasonic linear actuation device includes a mover and a plurality of stator sets; [108].

Shi et al., proposed a deep-sea linear ultrasonic motor, which took in-plane expansion mode as the working mode [109]. The influences of static seal and the pressures of water on the performance of the ultrasonic motor were studied. Performance of prototype whose velocity was measured at 214 mm/s while the water pressure was 8 MPa and the voltage signal with a frequency of 72 kHz and a voltage magnitude of 200 V. Nakajima et al., proposed a MDOF-USM consisting of a spherical stator and a rotor of various shapes [110]. Chen et al., presented a hollow, linear, nut-type USM based on two degenerate, 3rd-order bending modes in the section plane of cylinders [111]. The motor with four PZT plates reached an upward speed of 0.95 mm/s when the load force was 3 g, and the maximum thrust force was 0.35 N.

#### *3.1.6 Improvements on linear USMs*

Linear USMs are one of the most commonly used USMs among all because of the less complex design & effective driving method. Zheng et al. proposed a novel single-phase standing wave linear ultrasonic motor, which was made of a single PZT ceramic square plate with a circular hole in the center [60]. The driving mechanism of the motor was based on the combining the in plane expanding and bending modes to generate bidirectional linear motion [60]. They also proposed a miniature, ring-shaped, linear piezoelectric ultrasonic motor based on multimodal coupling operating in a single, in-plane mode [61]. This motor can produce a maximum driving force of 2.7 N, a no-load moving speed of 56 mm/s, and a high positioning resolution of 0.1 μm in open-loop control. It had advantages of simple structure, controllable micrometer-scale displacement, and large bidirectional working stroke indicated that the proposed linear motor had great potential for industrial applications for precise actuations [61]. Further a novel ring-shaped linear ultrasonic motor operating in orthogonal mode was proposed by them [62]. The motor was fabricated using a self-made high-performance PSN-PMS-PZT ceramic with the optimal composition, which had a high vibration velocity of 0.86 m/s. It exhibited a faster moving speed of 248 mm/s, a relatively large driving force of 2.6 N, and a high positioning precision of 0.2 m in open-loop control, indicating that the proposed linear motor based on self-made PSN-PMS-PZT ceramic had a great potential application for precise actuations [62].

Bai et al., proposed a two-way self-moving linear USM, which composed of a diamond-shaped metal elastic body, a piezoelectric ceramic piece and a parallel guide rail [63]. By exciting the piezoelectric ceramic sheets on both sides of the elastic body, the first order bending vibration mode was excited to realize the bidirectional movement of the motor. Under the excitation of 200Vpp, the forward and reverse frequency of the ultrasonic motor is 18.18KHz and 18.07KHz, and the forward and reverse no-load speed was 43.76 mm/s and 43.14 mm/s, respectfully. Takemura et al., developed a prototype of linear ultrasonic motor with an embedded preload mechanism [64]. The motor was driven bidirectionally by selective excitation of the second and third resonant vibration modes of the stator. The maximum velocity, thrust and power of the motor are 62.5 mm/s, 0.12 N and 1.01 mW respectively [63].

#### *3.1.7 Energy harvesting type USMs*

Wang et al., proposed an energy harvesting type ultrasonic motor in which two PZT rings were adopted in the new motor, one was bonded on the bottom surface

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

of the stator metal body to generate the traveling wave in the stator, and the other one was bonded on the outside top surface of the stator metal body to harvest and convert into the vibration-induced energy of the stator into electric energy [83]. They further developed a novel multifunctional composite device by using one single PZT ring, in which a piezoelectric actuator, a sensor and an energy harvester are embedded [84]. The piezoelectric ceramic ring was polarized into three regions to produce the actuating, sensing and energy harvesting functions.

#### *3.1.8 Comprehensive approaches*

Liu's research group made a remarkable contribution in the development of novel ultrasonic motors in last 5 years [21–32]. their research manly focused on bonded type structure, USM with nanometer resolutions, symmetric & asymmetric structure, multi degree of freedom motors & hybrid excitation. Some of his work from year 2015 to 2020 are described below: 1) A cylindrical traveling wave ultrasonic motor using bonded-type composite beam was proposed, a new exciting mode for L-B (longitudinal-bending) hybrid vibrations using bonded-type was adopted, which requires only two pieces of PZT ceramic plates and a single metal beam; [21] 2) A crossbeam ultrasonic motor with miniature size was developed, which used a bonded PZT ceramics to excite two first bending vibration modes that are orthogonal in space. The symmetrical crossbeam assured that two vibrations have the same resonance frequency, which solved the problem of mode frequencies degeneracy; [22] 3) A new-type linear ultrasonic motor which combined two orthogonal bending vibration modes & eight pieces of PZT ceramic plates and a metal beam that includes two cone-shaped horns and a cylindrical driving foot was developed & the maximal velocity of the achieved by this motor was 735 mm/s and the maximal thrust 1.1 N; [23] 4) A ultrasonic motors having three degree of freedom using four piezoelectric ceramic plates in bonded-type structure was proposed. It took advantage of a longitudinal mode and two bending modes, different hybrids of which can realize three-DOF actuation [24]. Because of symmetric structure the resonance frequencies of the two bending modes were identical; 5) A novel single-mode linear piezoelectric ultrasonic motor based on asymmetric structure was proposed [25]. The motor adopts the combination of the first longitudinal vibration and the asymmetric mechanical structure to produce the oblique movement on the driving foot which resulted in linear output motion under the friction coupling between the driving foot and the runner; 6) A two-degrees-of-freedom ultrasonic motor, which could generate linear motions with two DOF by using only one longitudinal–bending hybrid sandwich transducer, was proposed [26]. The results indicate that the maximum no-load velocities of the motor in horizontal and vertical directions are 572 and 543 mm/s under the preload of 100 N and the voltage of 300Vp − p respectively. The maximum output forces in horizontal and vertical directions are 24 and 22 N when the preload was 200 N; 7) A cantilever ultrasonic motor with nanometer resolution was designed, fabricated and tested, & it achieved an output speed of 344.35 mm/s when the frequency and voltage were 22.7 kHz and 200 Vp-p respectively [27]. The maximum output force was 8 N under the voltage and preload of 100 Vp-p and 50 N & high displacement resolution of 48 nm under the resonant working state was achieved; [8] a novel spherical stator multi-DOF ultrasonic motor using in-plane non-axisymmetric mode was proposed [28]. The mechanical output characteristics around X, Y and Z axes were measured under different excitations, pre-tightening forces and loading conditions. The no-load rotary velocities of the prototype were 200 r/min, 198 r/min and 250 r/min and the maximum load torques were 10.8 Nm, 11.0 Nm and 12.3 Nm around X, Y and Z axes, respectively was achieved; [28] 9) a novel rotary stack

having advantages of high precision, high stiffness, high dynamic range, and simple structure, which was not only suitable for generating the precise rotary motion, but also for exciting high-frequency rotary vibration was proposed this was required in the applications of micro–nano manipulations; [29] 10) A novel bending-bending piezoelectric actuator driven by a single-phase signal was proposed, in which the two-dimensional 8-shaped trajectory of the driving tip moved the runner [30]. This prototype could rotate a pulley (22 mm in diameter) at the maximum speed of 1373 rpm forward and 1350 rpm backward under a preload of 9 N, respectively; 11) A new method to reduce the volume of the traveling wave USM with ring-shape stator and improve its output speed, torque density, efficiency, and power density [31]. The USM obtained an output speed of 53.86 rpm under a preload of 0.69 N when the frequency and voltage were 24.86 kHz and 250 Vp-p, the maximum stall torque was tested as about 0.11 Nm. under the preload of 3.14 N. 12) A sandwichtype multi-degree-of-freedom (MDOF) ultrasonic motor with hybrid excitation was proposed [32]. The prototype achieves no-load speeds of 109.8 r/min, 107.9 r/ min, and 290.8 r/min in the YOZ, XOZ, and XOY driving modes, respectively. The proposed motor employs only four pieces of lead zirconate titanate ceramics to achieve the MDOF rotations of a spherical rotor.

USMs are known for their precise motion & position control. Fen, et al., developed a novel integral terminal sliding-mode-based adaptive integral backstepping control (ITSMAIBC) to accommodate the impacts of inherent friction, hysteresis nonlinearity, model uncertainties and retain high tracking precision [73]. Chen et al., presented a new butterfly-shaped linear piezoelectric motor for linear motion [74]. In the closed loop condition the positioning accuracy of plus or minus <0.5 μm was experimentally obtained for the stage propelled by the piezoelectric motor. Xu et al., presented a novel standing wave ultrasonic stepping motor operated in radial vibration mode [75]. Metal blades of the stator and grooves of the rotor were designed for precise positioning. In order to improve the torque, the rotor was pushed by blades of the stator directly without any friction material.

Sarhan et al., proposed a tubular USM operating in single phase, with rectangular plate having in plane out of plane vibration [79]. The maximum speed and torque of the tubular USM motor was 59 rpm and 0.28 mNm at 80 Vpp of applied voltage. It can be used where accurate control and high resolution at low speed is required. [They further proposed a motor working with coupled in-plane and outof-plane vibration modes of rectangular plate provided a large contact area between stator and rotor of motor which can reduce wear and enhance motor lifetime [80]. Overall dimension of prototype was 49x14 x2 mm, working frequency of motor was 49.6 kHz, no-load speed and stall force of motor are 122 rpm and 0.32 mN m at 50 V, respectively.

Mustafa et al., proposed extremum seeking control (ESC) as an adaptive seeking technique with fast convergence and high robustness to optimize the USM performance by tracking maximum efficiency states [243]. The application of a non-sinusoidal periodic excitation voltage to induce a near-square-wave driving tip trajectory in linear ultrasonic motors (LUSMs) was proposed by Le et al. [244]. This would reduce lost power in the periodic driving tip motion, thereby, increasing the output force and power of the LUSM. A high-efficiency Pseudo-Full-Bridge inverter with the aid of the soft-switching technology, which was accomplished by the resonance of the in-series inductance with the snubber capacitance was presented by shi et al. [245]. The efficiency of the whole drive increases by a factor of 1.25 after replacing the traditional inverter with the proposed one. A method for adjusting difference between the longitudinal and bending mode frequencies of the laminated composite stator was proposed by Li et al. [246]. The frequency adjustment method was realized by changing the applied magnetic field which affected

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

the effective elastic modulus of the composite stator. The sensitivities of motor performances on the pre-pressure were analyzed and a targeted optimization method was discussed by Chen et al. [247]. A simulation model with power dissipation and an integrated experimental facility with the preload adjustment device was adopted to analyze the laws from multiple perspectives. Peng et al., presented a new kind of the rotary ultrasonic motor with a longitudinal vibration model of the Langevin transducer acting as the stator, while the rotor consisted of a shaft and spiral fins [248]. A high-efficiency compensation method of the dead zone with the aid of the adaptive dither for the ultrasonic motor was proposed by Shi et al. [249]. The method not only could effectively compensate the dead zone and conveniently control the velocity, but also superior to the existing phase-difference-based method in terms of improving the efficiency of the ultrasonic motor. Zhu, et al., presented a novel linear piezoelectric motor suitable for rapid ultra-precision positioning [251]. By changing the input signal, the motor could work in the fast-driving mode as well as in the precision positioning mode. In the fast-driving mode, the motor achieved maximum no-load speed of 181.2 mm/s and maximum thrust of 1.7 N at 200 Vp-p. & in precision positioning mode, the motor acted as a flexible hinge piezoelectric actuator producing motion in the range of 8 μm. Li et al., proposed a novel dualfrequency asymmetric excitation method for motors which can operate under traditional single-phase asymmetric or two-phase symmetric excitation modes [252]. The motor demonstrated acceptable temperature characteristics and operating stability under the proposed excitation method with calculated optimal frequencies. Zhou et al., presented a novel linear ultrasonic motor with two operation modes wherein the stator of the motor was divided into two transducers and two isosceles triangular beams [253]. Dong et al., proposed a new equivalent circuit of a piezoelectric ring in radial vibration mode considering three types of fundamental losses, i.e., dielectric, elastic, and piezoelectric. Prototype achieved a maximum torque of 270 Nm [254].
