In feedback control terminology, what is “offset error” in steady operation of a control loop?

Introduction / Context:
In closed-loop control, “offset” refers to a steady-state deviation between the desired setpoint and the measured process value. Recognizing offset is essential for diagnosing controller tuning and deciding when to introduce integral action to eliminate persistent error.

Given Data / Assumptions:

• We consider steady operation after transients have decayed.
• A proportional-only controller can leave nonzero steady-state error for load or setpoint changes.
• Integral action is commonly used to drive steady-state error to approximately zero.

Concept / Approach:
Offset error = setpoint − measured value (with sign convention as defined). If the plant experiences a disturbance or requires a control effort not producible at zero error with proportional action, the loop settles with a bias. Integral action accumulates error and adjusts output until the difference is reduced, ideally removing offset.

Step-by-Step Solution:

Verification / Alternative check:
Standard control texts illustrate that proportional-only loops on nonintegrating processes exhibit residual error after step disturbances; integral action is introduced to eliminate it, confirming the definition.

Why Other Options Are Wrong:

Common Pitfalls:
Confusing sensor bias with control offset; neglecting actuator saturation or deadband, which can also cause residual error even with integral action.

Final Answer:
The difference between the control setpoint and the actual (measured) process value