When the BOOL input parameter MAN_ON of the S7-300 PID control function block FB41 is in the 1 state, the controller operates in manual mode. Conversely, when it is in the 0 state, the controller is in automatic mode. In manual mode, the output value of the PID controller is no longer calculated based on the process variable but is instead directly controlled by the manual input value, referred to 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
However, when the controller is in manual mode, the integral component is automatically reset to LMN - LMN_P - DISV, and 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 mechanism ensures a smooth transition between manual and automatic modes, preventing any sudden changes in the output value. This bumpless switching is crucial in industrial control systems where abrupt changes could lead to instability or damage to equipment.
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