Cascade PID control is an advanced application of PID control that can be deployed to enhance the performance of a process control system.
Sometimes a process can be critical such that disturbances which causes significant deviation from the process set point is not permissible. For some of these systems, a single loop PID controller might not be sufficient to keep the system in the permissible operating conditions, a secondary loop called the slave loop is introduced.
In general, in Cascade PID Controls, there are two or more loops namely, the primary or Master Loop and secondary or Slave Loop(s). The output of the Master Loop is usually the setpoint of the slave loop.
The process variable of the slave loop is usually one with a faster dynamic(usually three to five times). It quickly detects a disturbance in the system and reacts in a way to minimize the effect of the disturbance to the master loop.
Consider a sample application whose P &ID diagram is shown above. There two process variables namely Level and Flow. The Level control is the Master Loop, the Flow Control the slave loop and a Flow Control Valve the Control Variable.
The Flow transmitter detects a change in the flow rate which eventually could disrupt and deviate the process Level from the set point. The Flow transmitter quickly reacts and its loop causes the Flow Control Valve to act and tries to minimize the effect of the disturbance to the Process Level.
Another application which might require the use of a Cascade PID control, is one where there are two or more process variables but only one Control Variable is available.
In the video below, we would be demonstrating a Cascade PID Loop with a flow transmitter as the master loop process variable and a pressure transmitter as the slave loop process variable. The control variable is variable frequency drive whose speed is varied to maintain the flow transmitter at it’s set point.