| functional unit forming part of the controlled system and arranged at its input, driven by the manipulated variable and manipulating the mass flow or energy flow SEE: Figures 1 and 2. Note 1 to entry: If the final controlling element is mechanically actuated, an additional actuator (positioner) is used in some cases. Note 2 to entry: The output variable of the final controlling equipment is usually not free from feedback. The interface between the actuator and the final controlling element should therefore be selected in such a way that the manipulated variable is not affected by feedback from the final controlling element. Note 3 to entry: This entry was numbered 351-28-08 in IEC 60050-351:2006. 
Figure 1 – Functional diagram indicating typical elements of an elementary open-loop control system 
Figure 2 – Functional diagram indicating typical elements of an elementary closed-loop control system
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