In Figure 1 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