The interest in fractional-order (FO) control can be traced back to the late nineteenth century. The growing tendency towards using fractional-order proportional-integral-derivative (FOPID) control has been fueled mainly by the fact that these controllers have additional "tuning knobs" that allow coherent adjustment of the dynamics of control systems. For instance, in certain cases, the capacity for additional frequency response shaping gives rise to the generation of control laws that lead to superior performance of control loops. These fractional-order control laws may allow fulfilling intricate control performance requirements that are otherwise not in the span of conventional integer-order control systems. However, there are underpinning points that are rarely addressed in the literature: (1) What are the particular advantages (in concrete figures) of FOPID controllers versus conventional, integer-order (IO) PID controllers in light of the complexities arising in the implementation of the former? (2) For real-time implementation of FOPID controllers, approximations are used that are indeed equivalent to high-order linear controllers. What, then, is the benefit of using FOPID controllers? Finally, (3) What advantages are to be had from having a near-ideal fractional-order behavior in control practice? In the present paper, we attempt to address these issues by reviewing a large portion of relevant publications in the fastgrowing FO control literature, outline the milestones and drawbacks, and present future perspectives for industrialization of fractional-order control. Furthermore, we comment on FOPID controller tuning methods from the perspective of seeking globally optimal tuning parameter sets and how this approach can benefit designers of industrial FOPID control. We also review some CACSD (computer-aided control system design) software toolboxes used for the design and implementation of FOPID controllers. Finally, we draw conclusions and formulate suggestions for future research.

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Control loop performance assessment (CLPA) techniques are crucial for optimizing any plant or machine. They can bring huge energy and material savings and increase product quality. In this paper, the employment of running discrete Fourier transform (RDFT) in CLPA field is discussed. The first part of the paper documents the development of new RDFT function block which is suitable for CLPA. The paper focuses on implementation aspects whose aim is to minimize the number of arithmetic operations and to avoid numerical errors which are cumulated in many algorithms when running over longer time period. Then three RDFT applications are introduced. They are mostly dedicated to CLPA area: The changes in RDFT output help to detect increasing valve stiction or reveal a cause of oscillations in the loop. RDFT can be also used for continuous monitoring of process changes at particular frequencies. The most advanced problem presented is the estimation of special performance indices. More specifically, key samples of sensitivity function are gained and compared to the reference ones. Inspired by the model free design techniques, only a minimum a priori information about the process is assumed. The authors believe that the presented ideas will be suitable for both academic and industrial sphere.

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