Control System is that means by which any quantity of interest in a machine, mechanism or other equipment is maintained or altered in accordance with a desired manner.
1. Closed loop control: The information about the instantaneous state of the output is feedback to the input and is used to modify it in such a manner as to achieve the desired output.
2. Open loop control: Any physical system which does not automatically correct for variation in its output.
In modern usage the term servomechanism or servo is restricted to feedback control systems in which the controlled variable is mechanical position or time derivative of position. Example: velocity and acceleration.
A servo system is used to position a load shaft in which driving motor is geared to load to be moved.
Historical Development of Automatic Control
The first automatic control system, the fly ball governor, to control the speed of steam engines, was invented by James Watt in 1770.
About hundred years later Maxwell analyzed the dynamics of fly ball governor.
In early 1920, Minor Sky performed the classic work on the automatic Steering of ships and positioning of guns on shipboards.
The word 'servo' has originated with Hazen's work in 1934.
Sampled Data and Digital Control Systems
Sampled data and digital systems differ from these continuous data systems in that the signals at one or more points of the system are either in form of a pulse train or digital code.
The term discrete data control system is often used to describe these systems.
The important features of non-linear system performance may be completely overlooked if they are analyzed and designed through linear techniques as such.