What is Cascade Control System ?
• Cascade control system is a method which improves the performance of control strategies, reducing both maximum deviation and the integral error for disturbance responses.
• It uses an additional measurement of a process variable to assist in control system.
• In cascade control system, there is one manipulated variable and more than one measurement.
Cascade Control System Block Diagram
• Fig. shows a block diagram representation of cascade temperature control system.
• The cascade control system employs two feedback controllers, with the output of the primary controller changing the set-point of secondary controller.
• The output of exit temperature controller adjusts the set-point of flow controller in the cascade structure i.e. the secondary controller set-point is equal to the primary controller output.
• Thus, the secondary flow control loop is essentially the manipulated variable for the primary temperature controller.
• The objective of controller here is to provide tight control of exit temperature.
• The conventional feedback controller attempts to maintain the exit temperature of the stirred tank near its set-point in response of all disturbances.
• Cascade control system considers the likely disturbances and tailors the control system to the disturbances that strongly degrades the performance.
• It uses additional secondary measured process input variable that has the important characteristics of indicating occurrence of the key disturbance.
Cascade Control System Features
The important features of cascade control system are:
1. The flow controller is much faster than the temperature controller due to much shorter dead time in the secondary loop than in the original single loop.
2. The temperature controller with an integral mode remains in the design to ensure zero offset for all disturbance sources.
Advantages of Cascade Control System
Advantages of cascade control system are:
1. Improves the performance of control system.
2. Reduces both maximum deviation and the integral error.
3. It eliminates the effects of some disturbances and improves the dynamic performance of the control loop.