Cascade control may be applied when a process is composed of two or more (sub)processes in series. To apply cascade control, the output of each of these processes must be measured. The input to the first process is directly manipulated. Since the output of each process in the series is the primary input of the next process in the series, any change in the manipulated input to the first process in the series will impact the output of the other processes. The output of each process in the series is the controlled parameter of the PID associated with that process. An example of cascade control for a process made up of two (sub)processes in series is illustrated below.
One reason for implementing cascade control is that the PID at each point in the cascade can react quickly to disturbance inputs to its associated process. Also, cascade control may be implemented in some cases to compensate for the non-linear installed characteristic of a regulating valve. For example, the slave loop of a cascade might be associated with a flow process where the installed characteristic of the valve is non-linear. Since the flow process is capable of very fast changes in flow rate, the slave loop could be tuned to quickly adjust the valve to achieve the flow rate setpoint requested by the master loop. The non-linear installed characteristic of the valve would have no impact on the tuning or response of the master loop.
The implementation of cascade control is illustrated below where the PID, analog input, and analog output blocks are based on Fieldbus Foundation blocks.
Through the back calculation connection, the status that is communicated indicates high and low limit conditions. When a limit condition is indicated, the PID integral calculation is automatically modified to prevent windup if the calculated change in output would drive the process further toward the limit. If the PID block is designed to support external reset feedback, the performance of cascade control loop may be improved by enabling this option in the PIDs that make up the cascade control strategy. Also, the control option Use PV for Back Calculation should be selected in the slave loop to provide better dynamic response.
Several examples of cascade control applications are detailed in chapter 13 of Control Loop Foundation – Batch and Continuous Processes. Also, the cascade control workshop included in that chapter provides several exercises that may be used to further explore cascade control. For example, as part of the exercise the control performance of a PID with external reset enabled is compared to the performance achieve with external reset disable. By accessing the book’s web site, you may complete this cascade control workshop using your web browser. The viewer below may be used to see the solution to this exercise.