Question:
Why does proportional-only control typically result in offset?
Options:
A) No reset action
B) Needs error to act
C) No D action
D) SP drives output
View Answer
Correct Answer: B) Needs error to act
Detailed Explanation
In proportional-only control, the output is given by:
For the controller to produce output, error must exist.
At steady state:
- If error becomes zero → Proportional output becomes zero
- But the process usually requires some non-zero output to maintain equilibrium
Therefore, a small error must remain to maintain the required output.
This remaining error is called steady-state offset.
Why Integral Removes Offset
Integral action accumulates error over time and continues adjusting output until error becomes zero.
Proportional action alone cannot eliminate it.
Why Other Options Are Incorrect
A) No reset action → This is technically true (integral is called reset), but the fundamental reason is that proportional action requires error to produce output.
C) No D action → Derivative does not remove offset.
D) SP drives output → Output depends on error (SP − PV), not SP alone.