The cascade loop is one of the most common multi-loop control systems. The application of such a system can greatly decrease deviations of the primary variable and increase “line out” speed after a process disturbance has occurred.
Why cascade control loop ?
- Under control by the simple feedback control loop, the process variable is slow in responding to system disturbances and equally as slow in establishing a corrected output. This time consumption results in undesirable large deviations that optimum controller tuning cannot eliminate.
- A change in the condition of the process causes serious upsets in the controlled variable.
- The value of the variable other than the controlled one is being affected by the disturbance and there is a definite relationship between its value and the controlled variable.
- The secondary variable is one that can be controlled. It responds swiftly to process disturbances and adjustments of the final control element. The value to which it is controlled must be dictated by the condition of the primary variable.
Consider following point for Implementing Cascade control
- In going to cascade control, at least two items of instrumentation must be added: A transmitter and sensor to measure the secondary variable and another controller.
- The additional controller is a remote setpoint unit. This feature may call for a device that is slightly higher in cost as compared to a standard controller.
- The secondary controller must be capable of controlling the secondary variable independently. In the previous example, the purpose of the secondary controller was to keep the flow at a desired level. Flow alone, as a major controlled variable, cannot be used because its value would not respond to PV load changes or changes in the setpoint. The controlled variable (temperature) is still of primary concern: The secondary is important only because its value affects the primary.