Analysis of the Controlled System

An automatic control system is made up of four blocks: (1) process, (21 transducer, (3) controller, and (4) final element.

The process block produces the required control parameter. The transducer block converts this signal and transmits it to the controller block, which sends the signal to the final element where it is translated into a usable parameter of the proper type to modify the process. Hence, the process block can be described as being derived from the complete physical system.

In the same manner, the transducer block can be defined as a signal selector-converter. The main function is that of interpreting a signal to the controller. The controller block compares this signal to a reference signal called the set point, and issues a corrected signal to the final element. The final element physically changes the process as directed by the controller block. The determination of the final element depends upon the control parameter selected. The elements may be suction throttle valves, discharge bleed valves, governor speed changers, inlet guide vanes, slide valves, variable volume pockets, movable stator vanes, or other compressor elements.

These basic blocks may be arranged to control various parameters as shown in Figure 8-40. Actual applications of the various block arrangements should help in understanding them.

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