Fractional order PID controller design for an SOPDT model by online tuning method

Abstract

A relay based online tuning of a series fractional order proportional integral and derivative (FOPID) controller is proposed in this paper. A relay with hysteresis is connected in parallel to the FOPID controller inorder to obtain the limit cycle oscillation. The information provided by this limit cycle can be expressed by using a describing function which helps in determining the process parameters of the unknown plant which is further modeled into an equivalent second order plus dead time (SOPDT) plant. The identification of these parameters is a much needed necessity because the re-tuning of the FOPID controller is based on these model parameters. The proposed method highlights a simpler yet potent identification method which in turn leads to the design of a powerful controller. Also the controller always stays in loop which is not only cost-effective but also negates the harmful effects to the on-off replacement of the controller with the relay and vice versa. This method is then verified through the simulations of certain process models.