Question:
How is error calculated in a PID controller?
Options:
A) PV − MV
B) SP + PV
C) SP − PV
D) Output − PV
View Answer
Correct Answer: C) SP − PV
Detailed Explanation
In a standard PID controller, error is calculated as:
Error = SP - PV
Where:
- SP (Setpoint) = Desired value
- PV (Process Variable) = Measured value
This error represents the deviation between where the process is and where it should be.
What the Sign Means
- If PV < SP → Error is positive → Controller increases output (reverse acting case)
- If PV > SP → Error is negative → Controller reduces output
The sign determines the direction of correction.
Why Other Options Are Incorrect
A) PV − MV → MV (manipulated variable) is controller output, not part of error formula.
B) SP + PV → Error is a difference, not a sum.
D) Output − PV → Controller output is not used to calculate error.
Key Point for Interviews
Error calculation is the foundation of PID control.
All three terms (P, I, D) act on this error signal.