TABLE OF CONTENTS This chapter is a brief consideration of typical input and output devices used with PLCs. 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. For example, a thermocouple is a sensor which converts a temperature difference into an electrical output. The term transducer is generally used for a device that converts a signal from one form to a different physical form. Thus sensors are often transducers, but also other devices can be transducers, e.g. a motor which converts an electrical input into rotation.
Sensors which give digital/discrete, i.e. on-off, outputs can be easily connected to the input ports of PLCs. Sensors which give analogue signals have to be converted to digital signals before inputting them to PLC ports. The following are some of the more common terms used to define the performance of sensors.
Accuracy is the extent to which the value indicated by a measurement system or element might be wrong. For example, a temperature sensor might have an accuracy of 0.1 C. The error of a measurement is the difference between the result of the measurement and the true value of the quantity being...
