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Programmable Logic Controllers von Bolton, W. (eBook)

  • Verlag: Elsevier Reference Monographs
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Programmable Logic Controllers

This is the introduction to PLCs for which baffled students, technicians and managers have been waiting. In this straightforward, easy-to-read guide, Bill Bolton has kept the jargon to a minimum, considered all the programming methods in the standard IEC 1131-3 - in particular ladder programming, and presented the subject in a way that is not device specific to ensure maximum applicability to courses in electronics and control systems. Now in its fourth edition, this best-selling text has been expanded with increased coverage of industrial systems and PLCs and more consideration has been given to IEC 1131-3 and all the programming methods in the standard. The new edition brings the book fully up to date with the current developments in PLCs, describing new and important applications such as PLC use in communications (e.g. Ethernet - an extremely popular system), and safety - in particular proprietary emergency stop relays (now appearing in practically every PLC based system). The coverage of commonly used PLCs has been increased, including the ever popular Allen Bradley PLCs, making this book an essential source of information both for professionals wishing to update their knowledge, as well as students who require a straight forward introduction to this area of control engineering. Having read this book, readers will be able to: Identify the main design characteristics and internal architecture of PLCs Describe and identify the characteristics of commonly used input and output devices Explain the processing of inputs and outputs of PLCs Describe communication links involved with control systems Develop ladder programs for the logic functions AND, OR, NOT, NAND, NOT and XOR Develop functional block, instruction list, structured text and sequential function chart programs Develop programs using internal relays, timers, counters, shift registers, sequencers and data handling Identify safety issues with PLC systems Identify methods used for fault diagnosis, testing and debugging programs Fully matched to the requirements of BTEC Higher Nationals, students are able to check their learning and understanding as they work through the text using the Problems section at the end of each chapter. Complete answers are provided in the back of the book. Thoroughly practical introduction to PLC use and application - not device specific, ensuring relevance to a wide range of courses New edition expanded with increased coverage of IEC 1131-3, industrial control scenarios and communications - an important aspect of PLC use Problems included at the end of each chapter, with a complete set of answers given at the back of the book


    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 304
    Sprache: Englisch
    ISBN: 9780080462950
    Verlag: Elsevier Reference Monographs
    Größe: 12653 kBytes
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Programmable Logic Controllers

2 Input-output devices

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.
2.1 Input devices

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.

1. 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 measured errors can arise in a number of ways, e.g. the term non-linearity error is used for the error that occurs as a result of assuming a linear relationship between the input and output over the working range, i.e. a graph of output plotted against input is assumed to give a straight line. Few systems or elements, however, have a truly linear relationship and thus errors occur as a result of the assumption of linearity ( Figure 2.1(a) ). The term hysteresis error ( Figure 2.1(b) ) is used for the difference in outputs given from the same value of quantity being measured according to whether that value has been reached by a continuously increasing change or a continuously decreasing change. Thus, you might obtain a different value from a thermometer used to measure the same temperature of a liquid if it is reached by the liquid warming up to the measured temperature or it is reached by the liquid cooling down to the measured temperature.
Figure 2.1 Some sources of error: (a) non-linearity, (b) hysteresis
2. The range of variable of system is the limits between which the input can vary. For example, a resistance temperature sensor might be quoted as having a range of -200 to +800°C. 3. When the input value to a sensor changes, it will take some time to reach and settle down to the steady-state value ( Figure 2.2 ). The response time is the time which elapses after the input to a system or element is abruptly increased from zero to a constant value up to the point at which the system or element gives an output corresponding to some specified percentage, e.g. 95%, of the value of the input. The rise time is the time taken for the output to rise to some specified percentage of the steady-state output. Often the rise time refers to the time taken for the output to rise from 10% of the steady-state value to 90 or 95% of the steady-state value. The settling time is the time taken for the output to settle to within some percentage, e.g. 2%, of the steady-state value.
Figure 2.2 Response of a sensor or measurement system to a sudden input. You can easily see suc

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