Siemens SIMATIC S5 Manual

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Notes on Controller Settings4–129IP244C79000–B8576–C860–02The operating point y o is the value of the output signal at which the signal deviation becomes zero.The proportional band X p and the proportional coefficient K p have the following relationship: Within the Xp band, the output variable and input variable are directly proportional, i.e. the changein the output variable = proportional coefficient x change in input variable;as a formula: From the formula, it is clear that a change in the input variable , e.g. by a disturbance variable,causes a change in the output variable by a factor of Kp.In a static state, a change in the input variable means that the controller controls the system to adifferent value than before the occurance of the disturbance variable. This characteristic is commonto all proportional controllers. This deviation is known as the proportional offset or proportional error.The proportional error cannot exceed the proportional band X p.