**3. Review on piezoelectric ultrasonic motors**

This section provides a brief summary of the articles published since last 5 years since 2015 [2–298]. The articles are taken from eclectic resources like conference

#### *Piezoelectric Actuators - Principles, Design, Experiments and Applications*

proceedings, high impact journals, patents & masters, Ph.D. thesis from reputed universities. After reviewing the articles, we found that most of the articles on ultrasonic motors found were stressing upon research areas like new design, modeling & optimization, motion control, friction & wear, piezoelectric materials used in USM, thermal performance, applications of USM & review papers. [2] We carried out statistical analysis for the research articles published on USM during this period. We plotted graph for number of publications with research areas (**Figure 3**), with year of publication (**Figure 4**), by country of research (**Figure 5**) & journal publication (**Figure 6**). From **Figure 3** one can see that majority of the articles published on USM stressed upon new design & modeling & optimization. **Figure 4** shows that gradual increase in the trend of number of publications. **Figure 5** indicates that countries like China & Japan are the leaders in doing research on USM. **Figure 6** shows journal "Ultrasonics" & "Sensors & Actuators A: Physical are the favorite among researchers & publishers for publishing articles on USM. In the following section, we are going

**Figure 3.** *Summary of number of publications in each research areas of USM in the years of 2015–2020.*

**Figure 4.** *Summary of number publications per year on USM from 2015 to 2020.*

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

**Figure 5.** *Summary of USM publication numbers of author's country and region distribution.*

**Figure 6.** *Summary of USM publication numbers in the top ten journals.*

to briefly discuss the achievements, challenges, & opportunities about those publications made in the areas identified above.

### **3.1 New design**

In this section, we summarize the novel ideas proposed by the researchers for designing & developing ultrasonic motors as well as optimizing their designs in order to improve their efficiency & performance. The major objectives are to make UTMs with small size, high torque, and high-power density. Among them, miniaturization of the motor is the key challenge to achieve.

#### *3.1.1 New developments for miniaturization*

Since 2015 various research articles emphasized upon miniaturization, micro USM, scaling to sub millimeter range. Tomoaki Mashimo et al., a research group at Japan, develop serial of micro ultrasonic motor (μ-UTMs) with volume scale of a few cubic millimeters to submillimeter [3–10]. Those μ-UTMs include rotary and linear

types for different applications. Don L. DeVoe et al. developed one of the smallest bulk PZT TRUMs capable of bidirectional motion with PZT stator of diameter 4.12 mmand 323 mN preload force. The motor stator was fabricated using micro powder blasting of homogeneous PZT sheet. It achieved a maximum speed of 30 rpm & stall torque of 501 mN-mm [11]. Yingxiang Liu et al. carried out an overall weight of 8.5 g longitudinal–bending hybrid linear USM, whichis able to achieve 487 mm/s no-load speed, the maximum output force of 2.3 N, and & weight of the prototype obtained, and respectively [12]. Qiquan Quan et al. developed U shaped piezoelectric ultrasonic motor that mainly focused on miniaturization and high-power density [13]. Fulin Wang et al. developed a miniature spherical ultrasonic motor using wire stators for directional adjustment of a vascular endoscopic camera [14]. Ho et al., proposed a miniaturized simple shear vibration piezoelectric screw-driven structure USM to drive the high precision linear motor [15]. Zhou et al., developed a novel a radius of 2 mm three-dimensional contact model of piezoelectric TRUM utilizing MEMS fabrication technology [16].
