Bolton Instrumentation and Control Systems

2. Auflage 2015
ISBN: 978-0-08-100621-4
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 360 Seiten

Instrumentation and Control Systems addresses the basic principles of modern instrumentation and control systems, including examples of the latest devices, techniques and applications in a clear and readable style. Unlike the majority of books in this field, only a minimal prior knowledge of mathematical methods is assumed. The book focuses on providing a comprehensive introduction to the subject, with Laplace presented in a simple and easily accessible form, complimented by an outline of the mathematics that would be required to progress to more advanced levels of study. Taking a highly practical approach, the author combines underpinning theory with numerous case studies and applications throughout, to enable the reader to apply the content directly to real-world engineering contexts. Coverage includes smart instrumentation, DAQ, crucial health and safety considerations, and practical issues such as noise reduction, maintenance and testing. PLCs and ladder programming is incorporated in the text, as well as new information introducing the various software programs used for simulation. The overall approach of this book makes it an ideal text for all introductory level undergraduate courses in control engineering and instrumentation. It is fully in line with latest syllabus requirements, and also covers, in full, the requirements of the Instrumentation & Control Principles and Control Systems & Automation units of the new Higher National Engineering syllabus from Edexcel. - Completely updated - Assumes minimal prior mathematical knowledge - Highly accessible student-centred text - Includes an extensive collection of problems, case studies and applications, with a full set of answers at the back of the book - Helps placing theory in real-world engineering contexts

Former Lecturer at Buckingham Chilterns University College, High Wycombe, UK, and now retired, William Bolton has worked in industry and academia as a senior lecturer in a college of technology, a member of the Nuffield Advanced Physics team, an adviser to a British government aid project in Brazil on technical education, as a UNESCO consultant in Argentina and Thailand, and as Head of Research and Development at the Business and Technician Education Council. He has written many engineering textbooks, including Mechatronics, 4th ed., Engineering Science, 5th ed., Higher Engineering Science, 2nd ed., Mechanical Science, 3rd ed., and Instrumentation and Control Systems.

Zielgruppe

Academic/professional/technical: Research and professional

Autoren/Hrsg.

Bolton, William

Leseproben

Chapter 1

Measurement Systems

This chapter introduces the basic constituent elements of measurement systems and the performance terminology used in the specification of instrumentation, i.e. terms associated with resolution, accuracy, errors, range, precision, repeatability, reproducibility, sensitivity, dynamic characteristics, and reliability. The idea of national standards and traceability of measurements is introduced.

Keywords

Instrumentation terminology; national standards

Outline

1.1 Introduction 1

1.1.1 Systems 1

1.2 Instrumentation Systems 2

1.2.1 The Constituent Elements of an Instrumentation System 2

1.3 Performance Terms 4

1.3.1 Resolution, Accuracy, and Error 4

1.3.2 Range 6

1.3.3 Precision, Repeatability, and Reproducibility 7

1.3.4 Sensitivity 7

1.3.5 Stability 8

1.3.6 Dynamic Characteristics 9

1.4 Reliability 9

1.5 Requirements 10

1.5.1 Calibration 11

1.5.2 Safety Systems 12

Problems 13

1.1 Introduction

This chapter is an introduction to the instrumentation systems used for making measurements and deals with the basic elements of such systems and the terminology used to describe their performance in use.

1.1.1 Systems

The term will be freely used throughout this book, and so here is a brief explanation of what is meant by a system and how we can represent systems.

If you want to use an amplifier then you might not be interested in the internal working of the amplifier but what output you can obtain for a particular input. In such a situation we can talk of the amplifier being a system and describe it by means of specifying how the output is related to the input. With an engineering system an engineer is often more interested in the inputs and outputs of a system than the internal workings of the component elements of that system.

A can be defined as an arrangement of parts within some boundary which work together to provide some form of output from a specified input or inputs. The boundary divides the system from the environment and the system interacts with the environment by means of signals crossing the boundary from the environment to the system, i.e. inputs, and signals crossing the boundary from the system to the environment, i.e. outputs (Figure 1.1).

Figure 1.1 A system.

A useful way of representing a system is as a . Within the boundary described by the box outline is the system, and inputs to the system are shown by arrows entering the box and outputs by arrows leaving the box. Figure 1.2 illustrates this for an electric motor system; there is an input of electrical energy and an output of mechanical energy, though you might consider there is also an output of waste heat. The interest is in the relationship between the output and the input rather than the internal science of the motor and how it operates. It is convenient to think of the system in the box operating on the input to produce the output. Thus, in the case of an amplifier system (Figure 1.3) we can think of the system multiplying the input by some factor , i.e. the amplifier gain, to give the output .

Figure 1.2 Electric motor system.

Figure 1.3 Amplifier system.

Often we are concerned with a number of linked systems. For example, we might have a CD player system linked to an amplifier system, which, in turn, is linked to a loudspeaker system. We can then draw this as three interconnected boxes (Figure 1.4) with the output from one system becoming the input to the next system. In drawing a system as a series of interconnected blocks, it is necessary to recognise that the lines drawn to connect boxes indicate a flow of information in the direction indicated by the arrow and not necessarily physical connections.

Figure 1.4 Interconnected systems.

1.2 Instrumentation Systems

The purpose of an used for making measurements is to give the user a numerical value corresponding to the variable being measured. Thus a thermometer may be used to give a numerical value for the temperature of a liquid. We must, however, recognise that, for a variety of reasons, this numerical value may not actually be the true value of the variable. Thus, in the case of the thermometer, there may be errors due to the limited accuracy in the scale calibration, or reading errors due to the reading falling between two scale markings, or perhaps errors due to the insertion of a cold thermometer into a hot liquid, lowering the temperature of the liquid and so altering the temperature being measured. We thus consider a measurement system to have an input of the true value of the variable being measured and an output of the measured value of that variable (Figure 1.5). Figure 1.6 shows some examples of such instrumentation systems.

Figure 1.5 An instrumentation/measurement system.

Figure 1.6 Examples of instrumentation systems: (A) pressure measurement, (B) speedometer, (C) flow rate measurement.

An for making measurements has an input of the true value of the variable being measured and an output of the measured value. This output might be then used in a control system to control the variable to some set value.

1.2.1 The Constituent Elements of an Instrumentation System

An instrumentation system for making measurements consists of several elements which are used to carry out particular functions. These functional elements are:

1. 
This is the element of the system which is effectively in contact with the process for which a variable is being measured and gives an output which depends in some way on the value of the variable and which can be used by the rest of the measurement system to give a value to it. For example, a thermocouple is a sensor which has an input of temperature and an output of a small e.m.f. (Figure 1.7A) which in the rest of the measurement system might be amplified to give a reading on a meter. Another example of a sensor is a resistance thermometer element which has an input of temperature and an output of a resistance change (Figure 1.7B).

2. 
This element takes the output from the sensor and converts it into a form which is suitable for display or onward transmission in some control system. In the case of the thermocouple this may be an amplifier to make the e.m.f. big enough to register on a meter (Figure 1.8B). There often may be more than an item, perhaps an element which puts the output from the sensor into a suitable condition for further processing and then an element which processes the signal so that it can be displayed. The term is used for an element which converts the output of a sensor into a suitable form for further processing. Thus in the case of the resistance thermometer there might be a signal conditioner, such as a Wheatstone bridge, which transforms the resistance change into a voltage change, then an amplifier to make the voltage big enough for display (Figure 1.8B) or for use in a system used to control the temperature.

3. 
This presents the measured value in a form which enables an observer to recognise it (Figure 1.9). This may be via a display, e.g. a pointer moving across the scale of a meter or perhaps information on a visual display unit (VDU). Alternatively, or additionally, the signal may be recorded, e.g. in a computer memory, or transmitted to some other system such as a control system.

Figure 1.7 Sensors: (A) thermocouple, (B) resistance thermometer.

Figure 1.8 Examples of signal processing.

Figure 1.9 A data presentation element.

Figure 1.10 shows how these basic functional elements form a measurement system.

Figure 1.10 Measurement system elements.

The term is often used in relation to measurement systems. Transducers are defined as an element that converts a change in some physical variable into a related change in some other physical variable. It is generally used for an element that converts a change in some physical variable into an electrical signal change. Thus sensors can be transducers. However, a measurement system may use transducers, in addition to the sensor, in other parts of the system to convert signals in one form to another form.

Example

With a resistance thermometer, element A takes the temperature signal and transforms it into resistance signal, element B transforms the resistance signal into a current signal, element C transforms the current signal into a display of a movement of a pointer across a scale. Which of these elements is (a) the sensor, (b) the signal processor, (c) the data presentation?

The...