43.3.1. Signal Coding
All but the most elementary control and instrumentation systems involve several units, such as transducers, amplifiers, recorders, and actuators, which are connected together to perform the task required. An important feature of the system is the manner in which the signals passed between units are coded.
All early systems used analog coding in which the signal which represents a system variable has a magnitude proportional to it. For example, an oil lamp invented by Philon in about 250 BC used a float regulator in which the movement of the regulator was proportional to the level of oil.
One of the earliest applications of an analog controller for industrial purposes was the rotating ball governor used ...