Question:
In cascade control, where is the PID controller typically used?
Options:
A) Master only
B) Slave only
C) Both Master & Slave
D) Output stage
View Answer
Correct Answer: C) Both Master & Slave
Detailed Explanation
In a cascade control system, there are two control loops:
- Master (Primary) Controller
- Slave (Secondary) Controller
Both loops typically use PID control.
Master Controller
- Controls the primary process variable
- Sends its output as the setpoint to the slave controller
Slave Controller
- Controls a faster inner variable
- Directly manipulates the final control element
Using PID in both loops improves:
- Disturbance rejection
- Response speed
- Stability
Example
Temperature control of a heat exchanger:
- Master PID → Controls temperature
- Slave PID → Controls steam flow
The inner (slave) loop reacts faster to disturbances before they affect the primary variable.
Why Other Options Are Incorrect
A) Master only → Slave loop also requires control logic.
B) Slave only → Master loop still performs primary regulation.
D) Output stage → PID is implemented in controllers, not in output hardware.