2 - Input—output devices

Abstract

This chapter presents a brief overview of typical input and output devices used with a programmable logic controller (PLC). The input devices considered include digital and analogue devices such as mechanical switches for position detection, proximity switches, photoelectric switches, encoders, temperature and pressure switches, potentiometers, linear variable differential transformers, strain gauges, thermistors, thermotransistors, and thermocouples. Output devices considered include relays, contactors, solenoid valves, and motors. The term sensor is used for an input device that provides a usable output in response to a specified physical input. The term transducer is generally used for a device that converts a signal from one form into a different physical form. Thus sensors are often transducers, but other devices can also be transducers; for example, a motor that converts an electrical input into rotation. Sensors that give digital/discrete (that is, on–off) outputs can be easily connected to the input ports of PLCs. Sensors that give analogue signals have to be converted into digital signals before inputting them to PLC ports.