Instrumentation Interview Question : Brief idea about Cascade Control


#1

Cascade Control

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 ?

  1. 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.
  2. A change in the condition of the process causes serious upsets in the
    controlled variable.
  3. 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.
  4. 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

  1. 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.
  2. 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.
  3. 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.