1. The Mechanism of Proportional (P) Control: In a P-controller, the output ($P$) is related to the error ($e$) by $P = K_p e + P_0$, where $P_0$ is the bias or manual reset value. For the controller to produce an output different from its bias to counter a load change, an error
must exist.
2. Why Offset Occurs: If a disturbance requires more output from the controller than the bias provides, the only way the controller can generate that extra output is by having a non-zero error. Thus, the system settles at a point slightly away from the setpoint. This remaining error is the
offset.
3. Eliminating Offset: To remove this offset, an
Integral action is required. The integral term continues to change the controller output as long as any error exists, eventually driving the error to zero and returning the system exactly to the setpoint.