| variable quantity, which is not influenced by the control but is introduced into the control system from outside with the aim of making the final controlled variable follow it in a given relationship SEE: Figures 1 and 2. Note 1 to entry: This entry was numbered 351-27-09 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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