Chapter 2 - Input/Output Devices

Abstract

This chapter is a brief consideration of typical input and output devices used with PLCs. The input devices considered include digital and analog 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. A mechanical switch generates an on/off signal or signals as a result of some mechanical input causing the switch to open or close. Proximity switches are used to detect the presence of an item without making contact with it. There are a number of forms of such switches, some being suitable only for metallic objects. Output devices considered include relays, contactors, solenoid valves, and motors. When a current passes through a solenoid, a magnetic field is produced; this can then attract ferrous metal components in its vicinity. With the relay, this attraction is used to operate a switch. Relays can thus be used to control a larger current or voltage and, additionally, to isolate the power used to initiate the switching action from that of the controlled power.