## 4 Conclusions  
Structural control applications of an SMA actuator were investigated and discussed in this review article. Two main applications were vibration control of flexible structures and morphing wing application in the field of aerospace engineering. The conclusions of this review paper can be summarized as follows. (1) In this review paper, 52 reference papers on the vibration control issue of flexible structures were investigated and evaluated. It has been confirmed that the vibration control system using SMA can be applied to various types of structures such as simple beams, flat plates, cylinders, and shell structures. It has also been known from the evaluation of five references that SMA actuators can be applied to control the vibration of a structure or building in the event of an earthquake. The use of SMA actuating technology is being increased in civil engineering fields to protect bridges and buildings from damages from severe environments. (2) The main application of SMA actuators in the field of aerospace engineering includes the morphing wing field that transforms the wing shape of an airplane. By investigating 31 articles on the morphing wing field, it has been confirmed that vibration controllability and flying stability can be enhanced by adjusting or controlling the wing shape. In addition, a review was conducted on four representative reference papers in the field of aerospace showing that the research on the self-deployed solar sail field using SMA actuators is newly and actively conducted. (3) For vibration control of flexible structures, various types of controllers have been proposed and used, from simple PID controllers to optimal controllers such as LQR to robust controllers such as sliding mode controllers. It has been found that both vibration and motion controls are well accomplished in diverse types of structural systems by employing SMA actuators. In applications in the field of aerospace engineering, simple PID controllers or passive controllers are mostly  
Figure 10: Folding configuration of self-deployed solar sail [123] Reproduced with permission from MDPI under STM Permissions Guidelines. used rather than mathematically complex controllers. Using these simple controllers or passive controllers, the morphing wing has been successfully controlled through both computation and wind tunnel experiments. (4) An iterative learning control technique is a representative competitive algorithm that has recently been newly proposed for the control of systems using shape memory alloys [124; 125]. The proposed method is characterized by using only errors and previous control laws instead of using little information about the system. It is noted here that although the SMA actuator has some disadvantages such as slow response time compared with other actuators such as piezoelectric actuators, it has relatively higher control force or torque. In addition, control of multi-degree-of-system can be easily accomplished by combining several different types of SMA actuators. However, there are still many works to be conducted for successful realizations. For example, the development of more sophisticated constitutive models, adaptable compensator design for the large hysteresis loop, efficient control strategies at wide operating temperatures, and fast computation of control input associated with deep learning method are to be explored in the future. Conceptualization, S.-B.C. ; writing--original draft preparation, J.W.S., D.-G.Y. and J.S.R. ; writing--review and editing, J.W.S. and S.-B.C. ; supervision, S.-B.C. All authors have read and agreed to the published version of the manuscript. This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (2020R11A3074547).