So far the focus has been on open-loop systems, that is how a system naturally behaves before any intelligence is added to it. Now simple signal manipulations and rerouting, including feedback, will be considered. The ‘base layer’ control scheme for a processing plant is generally constructed using signal interconnections between a number of standard elements. Configuration software for SCADA and distributed control systems allow one to do this graphically. This chapter focuses on the standard functions made available in these schemes.
Measurement signals can be altered at three points: