Temperature control is a process in which the temperature of an object is measured and the passage of heat energy into or out of the object is adjusted to achieve a desired temperature.

A thermostat is a simple example for a closed control loop: It constantly measures the current temperature and controls the heater's valve setting to increase or decrease the room temperature according the user-defined setting. A simple method switches the heater or cooler either completely on, or completely off, and an overshoot and undershoot of the controlled temperature must be expected. A more expensive method varies the amount of heat or cooling provided by the heater or cooler depending on the difference between the required temperature (the "setpoint") and the actual temperature. This minimizes over/undershoot. This process is called PID and is implemented using a PID Controller

A proportional-integral-derivative controller (PID controller) is a common feedback loop component in industrial control systems (see also control theory).

The controller takes a measured value from a process or other apparatus and compares it with a reference setpoint value. The difference (or "error" signal) is then used to adjust some input to the process in order to bring the process' measured value to its desired setpoint. Unlike simpler controllers, the PID can adjust process outputs based on the history and rate of change of the error signal, which gives more accurate and stable control. In contrast to more complex algorithms such as optimal control theory, PID controllers can often be adjusted without advanced mathematics. However, pushing robustness and performance to the limits requires a good understanding of the theory and controlled process.

Control loop basics
The purpose of a control loop is to automate what an intelligent operator with a gauge and a control knob would do. The operator would read a gauge showing the output measurement of a process, and use the knob to adjust the input of the process (the "action") until the process's output measurement stabilizes at the desired value on the gauge. The position of the needle on the gauge is a "measurement" of the "process value" or "process variable". The desired value on the gauge is called a "setpoint" (also called "set value"). The difference between the gauge's needle and the setpoint is the "error".

A control loop consists of three parts:

Measurement by a sensor connected to the process (or the "plant"),
Decision in a controller element,
Action through an output device ("actuator") such as a control valve.
As the controller reads a sensor, it subtracts this measurement from the "setpoint" to determine the "error". It then uses the error to calculate a correction to the process's input variable (the "action") so that this correction will remove the error from the process's output measurement.

For example: suppose a water tank is used to supply water for use in several parts of a plant, and it is necessary to keep the water level constant. A sensor would measure the height of water in the tank, producing the measurement, and continuously feed this data to the controller. The controller would have a setpoint of (for example) half full. The controller would have its output (the action) connected to a valve controlling the water feed. The controller would use the measurement of the level to calculate how to manipulate the control valve to maintain the desired level.

A controller can be used to control any measurable variable which can be affected by manipulating some other process variable. For example, it can be used to control temperature, pressure, flow rate, chemical composition, speed, or other variables. Automobile cruise control is an example of a process outside of industry which utilizes control.