Running discrete Fourier transform and its applications in control loop performance assessment

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.