In the figure below a schematic of a system with a PID controller is shown. The PID controller compares the measured process value y with a reference setpoint value, y0. The difference or error, e, is then processed to calculate a new process input, u. This input will try to adjust the measured process value back to the desired setpoint.
The alternative to a closed loop control scheme such as the PID controller is an open loop controller. Open loop control (no feedback) is in many cases not satisfactory, and is often impossible due to the system properties. By adding feedback from the system output, performance can be improved.
Unlike simple control algorithms, the PID controller is capable of manipulating the process inputs based on the history and rate of change of the signal. This gives a more accurate and stable control method.
The basic idea is that the controller reads the system state by a sensor. Then it subtracts the measurement from a desired reference to generate the error value. The error will be managed in three ways; to handle the present through the proportional term, recover from the past using the integral term, and to anticipate the future through the derivate term.
The figure below shows the PID controller schematics, where Tp, Ti, and Td denote the time constants of the proportional, integral, and derivative terms respectively.
