Question:
What happens to offset if only PD control is used?
Options:
A) Offset remains
B) Offset removed
C) Offset increases
D) Offset decreases
View Answer
Correct Answer: A) Offset remains
Detailed Explanation
PD control includes:
- Proportional (P) → Responds to present error
- Derivative (D) → Responds to rate of change of error
Neither term accumulates error over time.
To eliminate steady-state offset, the controller must continuously adjust output until error becomes zero. That function is performed only by Integral (I) action.
With PD control:
- A non-zero error is required to maintain output
- Therefore, steady-state offset remains
Practical Understanding
P → Reacts immediately
D → Predicts future trend
I → Eliminates offset
Without integral action, perfect steady-state accuracy cannot be achieved.
Why Other Options Are Incorrect
B) Offset removed → Only integral action removes offset.
C) Offset increases → It may remain, but not necessarily increase.
D) Offset decreases → PD does not guarantee reduction to zero.