| functional unit that generates in closed-loop control the reference variable at its output from the command variable at its input SEE: Figure 2. EXAMPLE 1: Electronic ramp-function generator the output signal of which follows variations of the input signal only up to a maximum permissible rate of change. If the rate of change of the input signal exceeds the permissible limit, the output signal follows only at a preset maximum rate of change. EXAMPLE 2: A turbine wall temperature measuring device ensures that the turbine output power is varied at such a rate that the temperature difference between the various zones of the turbine wall does not exceed permissible values. Note 1 to entry: Reference-variable generating elements are in many cases used to prevent critical limit values of the reference variable, its derivatives or other process variables being exceeded. Note 2 to entry: This entry was numbered 351-28-10 in IEC 60050-351:2006. 
Figure 2 – Functional diagram indicating typical elements of an elementary closed-loop control system
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