| difference between the reference variable and the feedback variable SEE: Figure 2. Note 1 to entry: As the variable meant at this item is equivalent to the negative of the quantity designated error of measurement or error as used in metrology (see item 3.10 of International Vocabulary of Basic and General Terms in Metrology (VIM), 3rd Edition 2007) and of the quantity designated absolute error as used as well in metrology (see entry 311-01-05 of IEC 60050-300:2001), the term error variable is deprecated in control technology (see IEC 60027-6:2006-12). Note 2 to entry: This entry was numbered 351-27-04 in IEC 60050-351:2006. 
Figure 2 – Functional diagram indicating typical elements of an elementary closed-loop control system
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