control//feedback control//control loop//controller

The controller is the part of a control loop that decides: it reads the measurement and the setpoint and computes the command for the actuator, many times a second, by a rule called the **control law**. In a factory it is a function block inside a PLC or a DCS; in a car or a drone it is a few lines of code on an embedded processor; in an old thermostat it was a bimetal strip. What matters is the rule and how often it runs, not the box.

The controller is the part of a control loop that decides: it reads the measurement and the setpoint and computes the command for the actuator, many times a second, by a rule called the control law. In a factory it is a function block inside a PLC or a DCS; in a car or a drone it is a few lines of code on an embedded processor; in an old thermostat it was a bimetal strip. What matters is the rule and how often it runs, not the box.

The simplest rule is on-off control: full power below the setpoint, nothing above it. A domestic thermostat or a fridge works this way, and the result is a temperature that cycles around the setpoint, because the plant keeps moving after each switch. A dead band between switching on and switching off trades a wider swing for fewer cycles, which saves the relay and the compressor. When the actuator can be modulated, a proportional rule replaces the switch, and its extensions make the PID controller, the default of industry.

Control laws form a ladder of effort. Above PID sits state feedback, which uses every state instead of one error (feedback control), and above that optimal control such as LQR and LQG, which trades error against effort explicitly. Each step up needs a better model of the plant, and that is the real price.

A digital controller runs in discrete time. It samples, computes and holds its output until the next sample, so the sample time is part of the design rather than a detail of the code (PID implementation).

An industrial controller has modes. In manual an operator sets the output directly; in automatic the control law does. Switching between them without a jump in the output, bumpless transfer, is a standard feature of industrial controllers and a classic source of bugs in home-made ones.