Control engineering is the design of systems capable of accurately controlling a physical device. Control engineering is traditionally considered a sub-field of electrical engineering.
The role of a control system is to translate a high level command into one or more low level commands to achieve a desired system performance. In response to a desired state (or command), a controller calculates a control signal which affects the operation of some actuator, which in turn affects the state of the system (or plant). Open loop control systems calculate the control signal using only the command, and perhaps an assumption about the state of the system. In contrast, a closed loop control system takes some measurements to obtain some information on the present state of the plant and these measurements are used by the controller to calculate the control signals. This feedback of information about the state of the plant into the controller allows a control system to better control the plant.
Examples of control engineering
- Toaster. A human operator wants to make toast. The human operator inserts the toast and pushes a start button. The toaster heats the toast until a timer expires. However, the toaster cannot guarantee the color of the toast---only that this period of time usually gets the desired color, but this method will fail (perhaps) if the weather is cold or if the toast is thicker than usual. This is an example of an open loop control system. Some toasters use a bimetallic strip to detect the temperature inside some part of the toaster and close a circuit which automatically shuts off the toaster and pops out the toast, providing an example of a closed loop control system. If the toaster could observe toast color, a better control system could be designed to achieve ideal toast color.
- Thermostat. A human operator specifes a desired room temperature. The thermostat measures the actual room temperature and calculates an error signal in temperature. The thermostat then calculates a control signal using the error in temperature, as well as the history of the error signal and the control signal. The control signal connects to the heating and cooling hardware to effect this change. This is an example of a closed loop control system, because it incorporates feedback in the form of temperature measurement.
- Cruise Control. A human operator specifies a desired velocity for her automobile. The cruise control system measures current velocity and acceleration, and computes an amount of gas to inject into the engine (the control signal) to achieve the desired velocity. This is another example of a closed loop control system.
- Manufacturing. A robotic arm must pick up a part at one end of a factory, and carry it to the other end. The arm is mounted on a linear rail, and is capable of moving in either direction along that rail. At the two positions of interest, limit switches are installed. These switches produce a boolean signal (either true or false) which indicates that the arm is at one of those two positions or somewhere in between. The controller is a simple finite state machine, with a certain operation defined if the arm is at the first position, the second, or somewhere in between going towards one or the other. The controller sends signals to cause motors to operate which move the arm in one direction or the other or cause it to making grasping or releasing motions.