Control units are used to achieve a certain output variable despite disturbances and interferences in the input variable. Control units and factors are a constant part of our lives, whereas they appear in nature or are employed by humans. Already in ancient times people used irrigation systems with drains and inflows to ensure a constant water level. The use of a steam engine with a centrifugal governor by James Watt appears to be the origin of control system theory as part of engineering science. This centrifugal governor was connected to the drive of the steam engine (output) and the faster the engine was operating the more the governor closed a valve that was feeding the machine (input). Therefore, the output of the engine could be kept on a constant level without outside assistance.