Input/output devices are the interface between the controller and the controlled system. Input devices convert physical quantities to electrical signals, while output devices allow the controller to act on the system.

The electronic brains in control systems depend on a network of input/output devices. These devices tie the controller to the outside world by supplying it with physical data and allowing it to act on the controlled system. Smart sensors and smart power ICs do more than just provide access, they also share processing and decision-making tasks as well.

Input devices include transducers, sensors, and switches. Sensors and transducers convert physical data such as speed, position, temperature, acceleration, and pressure into electrical signals that are recognized by the controller. Switches allow operators to supply information that supplements or overrides input data.

Output devices include power semiconductors and ICs, relays, and circuit breakers. By providing control over electrical power subsystems that drive motors and solenoids, they allow the controller to initiate, halt, or modify action in the controlled system.

Position and velocity sensors are common in industrial and automotive applications. Position sensors span a broad spectrum, and virtually all closed-loop motion-control systems use one. Velocity sensors or tachometers also are widely used. Other sensors measure pressure, temperature, and acceleration.

Factory automation, in particular, requires linear and rotational position transducers. These demands and a need for digital interfacing are being met by synchros, resolvers, optical shaft encoders, linear-variable differential transformers (LVDTs), and potentiometers.