Imagine you want to maintain a tank level at 60%.
But the actual level changes because the outlet flow changes.
How does the system automatically correct the level?
Feedback control!
What is a Feedback Control Loop?
A feedback control loop is a control system in which the actual process output is measured and fed back to the controller, which compares it with the desired setpoint and adjusts the manipulated variable to reduce the error.
In simple terms:
Measure → Compare → Correct → Measure Again
Basic Feedback Loop
Setpoint (SP)
Controller
Final Control Element
Process
Sensor / Transmitter
Feedback to Controller
The controller continuously compares:
Setpoint − Process Variable = Error
The controller then uses this error to determine the required control action.
Example: Tank Level Control
Suppose:
Setpoint = 60%
Actual level:
PV = 50%
Therefore:
Error = 10%
The controller may increase the inlet valve opening:
50% → 65% → 75%
More liquid enters the tank.
The level rises:
50% → 55% → 59% → 60%
The controller continuously adjusts the valve based on the measured level.
Main Components
A typical feedback control loop contains:
Setpoint (SP)
The desired process condition.
Measurement / Transmitter
Measures the actual process variable.
Controller
Compares SP and PV and calculates the control action.
Final Control Element
Acts on the process—for example, a control valve or VFD.
Process
The physical system being controlled.
Why Is Feedback Important?
Without feedback, the controller does not know the actual process result.
With feedback:
Process changes
Measurement detects change
Controller calculates error
Control action changes
Process responds
Measurement is repeated
This allows the system to automatically compensate for disturbances and process changes.
Open Loop vs Feedback Control
Open Loop:
No feedback
Controller does not measure the actual output for correction
Feedback Control:
Feedback is present
Actual output is measured
Controller corrects the error
Interview Question
What is a feedback control loop?
A feedback control loop measures the actual process output, compares it with the desired setpoint, and uses the resulting error to adjust the manipulated variable so that the process moves toward the desired condition.
Remember:
SP → Compare → Controller → FCE → Process → Measurement → Feedback
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