Approximate analysis of direct controller tuning methods

This paper analyzes the properties of controllers which are tuned by using direct controller tuning methods such as fictitious reference iterative tuning (FRIT) and virtual reference feedback tuning (VRFT). As a result, it is shown that due to disturbance, the tuned controllers depend on the reference signal, initial controller, and reference model. Moreover, it is shown that the controllers tuned by iterative FRIT and VRFT converge to a controller under some assumptions and approximations. A numerical example is provided to illustrate the effectiveness of the analysis results.