Balanced PID tuning application to series cascade control systems

This communication provides an approach for the application of PID controllers within a cascade control system configuration. Based on considerations about the expected operating modes of both controllers, the tuning of both inner and outer loop controllers are selected accordingly. This fact motivates the use of a tuning that for the secondary controller that provides a balanced set-point / load-disturbance performance. A new approach is also provided for the assimilation of the inner closed-loop transfer function to a suitable form for tuning of the outer controller. Due to the fact that this inevitably introduces unmodelled dynamics into the design of the primary controller, a robust tuning is needed. The introduction and use of an additional sensor that allows for a separation of the fast and slow dynamics of the process results in a nested loop configuration as it is shown in figure (1). Each loop has associated its corresponding PID controller. The controller of the inner loop is called the secondary controller whereas the controller of the outer loop as the primary controller, being the output of the primary loop the variable of interest. The rationale behind this configuration is that the fast dynamics of the inner loop will provide faster disturbance attenuationandminimizethepossibleeffectdisturbancebeforetheyaffecttheprimaryoutput. Thissetupinvolves two controllers. It is therefore needed to tune both PIDs. The usual approach involves the tuning of the secondary controllerwhilesettingtheprimarycontrollerinmanualmode. Onasecondstep,theprimarycontrolleristunedby considering the secondary controller acting on the inner loop. It is therefore a more complicated design procedure than that of a standard single-loop based PID control system. In this paper a design issue that has not been addressed is considered: the tradeoff between the performance for set-point and load-disturbance response. When a load -disturbance occurs at the primary loop, the global load-disturbance depends on the set-point tracking performance of the secondary loop. In addition, good load- disturbance performance is expected for the secondary controller in order to attenuate disturbances that enter directly at the secondary loop. Also, it is well known that when the controller is optimally tuned for set-point response, the load-disturbance performance can be very poor (1). Based on this observation this paper proposes the use of a balanced performance tuning (2) for the secondary loop. Furthermore, an approximation procedure is provided in order to assimilate the dynamics seen by the primary controller to a First-Order-Plus-Time-Delay model such that usual tuning rules for PID control can be applied. However here a robust tuning is suggested, because,theprimarycontrollerwillneedtofacewithunmodelleddynamicscomingfromthemodelapproximation used for the secondary loop. Note that this kind of approximation is always needed if simple-model based tuning rules are to be applied. The rest of the paper is organized as follows. Next section presents the cascade control configuration and control setup to be used. Section 3 provides the main contribution of the paper as the design approach involving tuning of the controllers and approximation method. Section 4 presents an application example whereas section 5 ends with some conclusions and suggestions for further research.