**6. Conclusions**

Fractional-order controllers have emerged as a generalization of the standard PID, allowing for greater flexibility and improved performance and robustness. The tuning of these FO-PIDs is not an easy task, since the complexity of the design increases along with the number of tuning parameters. Several tuning methods have been developed, but the majority of them require a process transfer function. In some cases, obtaining an accurate mathematical model of the process is time consuming and tedious, especially in the industrial sector. To cope with this issue, autotuning methods for FO-PIDs have emerged. In this paper, a survey of the existing autotuning methods for FO-PIDs is presented. Several autotuning approaches are compared for lag-dominant and delaydominant FOPDT processes, for higher order systems, for integrative time-delay processes or poorly damped ones.

For each type of process, the autotuning methods are compared in terms of closed-loop performance regarding reference tracking and disturbance rejection. Robustness was not considered as a means for comparison, since some of the reviewed methods do not address directly this issue, while others do. This aspect would have led to unfair comparisons and possibly different remarks on the opportunity of using one autotuning method, instead of another.

Some of these autotuning methods have also been validated experimentally. Research in this area is still under way and the current autotuning methods stand as the premises for further innovation in this area. Further research regarding the robustness of the autotuning methods will be considered.

**Author Contributions:** Conceptualization, C.I.M. and I.B.; methodology, C.I.M.; software, C.I.M. and I.B.; validation, C.I. and R.D.K.; formal analysis, E.H.D.; writing—original draft preparation, C.I.M. and I.B.; writing—review and editing, E.H.D.; supervision, C.I. and R.D.K.; funding acquisition, C.I.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by a grant of the Romanian Ministry of Education and Research, CNCS-UEFISCDI, project number PN-III-P1-1.1-TE-2019- 0745, within PNCDI III. This research was also supported by Research Foundation Flanders (FWO) under grant 1S04719N.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
