When the BOOL input parameter MAN_ON of the S7-300 PID control function block FB41 is set to 1, the controller operates in manual mode. Conversely, when it is set to 0, the controller is in automatic mode. In manual mode, the output value of the PID controller is replaced by the manually entered value, known as MAN.
The PID output consists of three main components: proportional (P), integral (I), and derivative (D), along with a disturbance component called DISV. The formula for the PID output is:
PID Output = LMN_P + LMN_I + LMN_D + DISV
In manual mode, the integral component of the controller is automatically adjusted to (LMN - LMN_P - DISV), while the derivative component is set to zero. As a result, the PID output becomes:
PID Output = LMN_P + (LMN - LMN_P - DISV) + 0 + DISV = LMN
This ensures a bumpless transition between manual and automatic modes. It guarantees that the output value of the PID controller remains stable and does not experience any sudden changes during the switch, maintaining consistent performance throughout the process.
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