oss engineering control ver 2 1 pn

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Draft version 2.1 23 April 2012

Engineering Control Copyright Outsource Services

DIPLOMA OF ENGINEERING

Engineering Control Study Guide


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Version trackingVersion Date Changes Changes authored by Status

.01 29/11/2011 Basic r&d phase Peter Newnham Initial design

.11 13/2/2012 Content writing 70% complete Peter Newnham Under development

.21 20/3/2012 90% complete Peter Newnham Ongoing development

.22 23/3/2012 Editing, and building assessment Peter Newnham Almost at review stage

.23 27/3/2012 Matching logic to experiment Peter Newnham Almost at review stage

.25 27/3/2012 Finalising assessment Peter Newnham Almost at review stage

.31 28/3/2012 More finalising assessment Peter Newnham Almost at review stage1.0 29/3/2012 Draft 1 Peter Newnham Beta 1

1.1 29/3/2012 Update image p 52;update TOC/TOI Peter Newnham Beta 2

1.2 3/4/2012 Update TOC to include assessments Peter Newnham Beta 31.3 4/4/2012 Correction to spelling in KQ 3 Peter Newnham Beta 4

1.4 5/4/2012 Addendum: SI system and prefixes Peter Newnham Beta 5

1.5 5/4/2012 KQ 3 modified according to above

change

Peter Newnham Beta 6

1.6 16/4/2012 New hyperlink page 3 Peter Newnham Beta 7

2.0 16/4/2012 Release version Peter Newnham Beta 8

2.1 23/4/2012 Minor edit to add content on power

calculation

Peter Newnham Beta 9

Table of ContentsVersion tracking ...................................................................................................................... i

Table of Contents .................................................................................................................... i

Table of Figures ..................................................................................................................... iii

About Outsource Services .................................................................................................... iv

The Engineering Course Structure (Phase 4 from January 2012) ......................................... vi

Self-paced learning and other information to assist you with your course ......................... vii

Preamble to this learning guide ........................................................................................... viii

Recommended reference books............................................................................................ix


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Assessment strategy ..............................................................................................................ix

1. Introduction ................................................................................................................... 1

Knowledge Quiz 1 review of the introduction ........................................................................... 6

2. Control system techniques and hardware ..................................................................... 8

2.1 Electromagnetic devices ................................................................................................ 13

Knowledge Quiz 2 review of the fundamentals of control systems hardware ........................ 16

2.2 Sensors ........................................................................................................................... 18

2.2.1 Strain Gauges ..................................................................................................................... 18

2.2.2 Thermistors ........................................................................................................................ 19

2.2.3 The pressure transducer .................................................................................................... 23

2.2.4 Ultrasonic sensors .............................................................................................................. 26

2.2.5 Differential sensors ............................................................................................................ 27

2.2.6 Position sensing ................................................................................................................. 28

2.2.7 Differential sensing ............................................................................................................ 29

2.3 Controlling position ....................................................................................................... 30

2.3.1 Stepper motors .................................................................................................................. 30

2.4 Measurement techniques .............................................................................................. 32

2.4.1 Calibration .......................................................................................................................... 32

2.4.2 Tuning ................................................................................................................................ 33

2.4.3 Correcting for non-linearity ............................................................................................... 34

2.4.4 Other calibration factors .................................................................................................... 35

2.4.5 Common sources of error in measurement ...................................................................... 36

2.4.6 Instrument errors ............................................................................................................... 37

2.4.7 Using the measuring instrument ....................................................................................... 38

2.4.8 Universal measuring devices ............................................................................................. 39

Knowledge Quiz 3 review of sensors and measurement ......................................................... 403. Controlling a Process ................................................................................................... 42

3.1 Relay and Float switch control .............................................................................................. 42

Knowledge Quiz 4 : Observations from practical exercise ......................................................... 45

3.2 Fundamentals of logic in control circuits .............................................................................. 46

3.3 Microcontrollers: principles and programming .................................................................... 50

3.4 Ladder Programming ............................................................................................................ 53

3.5 Computer programming ....................................................................................................... 56

Knowledge Quiz 5 review of controllers and programming .................................................... 60


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4. Communicating between devices ................................................................................ 61

4.1 Proprietary ............................................................................................................................ 62

4.2 IEEE Standards ...................................................................................................................... 63

4.3 Serial RS232 standard ........................................................................................................... 63

4.4 Ethernet ................................................................................................................................ 64

4.5 USB ........................................................................................................................................ 65

4.6 Firewire ................................................................................................................................. 65

4.7 GSM ....................................................................................................................................... 65

Knowledge Quiz 6: Communications .......................................................................................... 67

Final Assessment/RPL Checklist. ................................................................................................. 70

Table of FiguresFigure 1 RJ connectors (Image BLINN.EDU) .......................................................................................... 2

Figure 2 Generator equivalent circuit ................................................................................................... 8

Figure 3 Strain Gauge sensor patches ................................................................................................ 18

Figure 4 Wheatstone Bridge ............................................................................................................... 19

Figure 5 Ratio and degrading graphs .................................................................................................. 20

Figure 6 Ratio chart ............................................................................................................................. 21

Figure 7 Bridge sensor connection ..................................................................................................... 22

Figure 8 U tube manometer ............................................................................................................... 24

Figure 9 Pressure transducer, basic .................................................................................................... 24

Figure 10 Ultrasonic T-R ...................................................................................................................... 26

Figure 11 Ultrasonic sensing ............................................................................................................... 27

Figure 12 Differential pressure sensor ............................................................................................... 27

Figure 13 Position sensing, linear and angular resistance types ........................................................ 28

Figure 14 LVDT .................................................................................................................................... 29

Figure 15 Selection of stepper motors ............................................................................................... 30

Figure 16 Linear actuators .................................................................................................................. 31

Figure 17 Carbon resistor colour coding ............................................................................................. 36

Figure 18 SMD resistors (surface mount devices) .............................................................................. 36

Figure 19 Time domain and frequency domain representations ....................................................... 37

Figure 20 Tank control system during basic setup ............................................................................. 42

Figure 21Gauger Ultrasonic transducer .............................................................................................. 44

Figure 22 Control and power board (IDEC PLC to be fitted) ............................................................... 44

Figure 23 Bottom tank with float switch C and pump (internal) ........................................................ 44

Figure 24 IDEC Microscan PLC example .............................................................................................. 44

Figure 25 Top tank with float switches installed and Microscan ultrasonic height measurement

transducer ........................................................................................................................................... 44

Figure 26 Logic circuit families (TTL) ................................................................................................... 47Figure 27 Simple TTL version of relay controller ................................................................................ 50
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Figure 28 Microcontroller Block Diagram ........................................................................................... 51

Figure 29 Ladder logic and TTL equivalent (From EC&M) ................................................................. 55

Figure 30 Bus depiction - controller system ....................................................................................... 61

Figure 31 Backplane Controller (Source Wikipedia) ........................................................................... 62

About Outsource Services

Our Business

Outsource Servicesis a Registered Training Organisation (RTO) delivering management and

engineering programs to some of Australias largest engineering, mining, oil and gas, LNG and

public and private sector organizations. The unique nature of our flexible and high quality training

and assessment services allows us to quickly design, implement and deliver training solutions that

not only meet organizational compliance and performance expectations, but attracts significant

state and federal funding support to our client base.

Our Team

We have the most experienced team of managers and training consultants in the industry.From

engineers that have designed internationally recognized engine components to management

trainers holding PhDs and years of management experience in leading Australian and international

organizations. We not only understand training, we understand industry!

Outsource Services is a business-to-business training services provider that delivers nationally

recognized qualifications and training to medium and large employers to assist them with,

employee retention, skills enhancement, skill shortages, compliance requirements and general

human resource development. Because we deliver our services in a flexible and customer focused

manner our clients are able to reduce their training costs and production disruptions normally

associated with large scale training and development projects.

Our Competition:

Our competitors offer quality training but in a traditional classroom focused manner. Outsource

Services is able to deliver onsite, online or in a blended format in a contextualized manner that

meets client and individual needs.

OurFinancials:Outsource Services has grown between 80-100% in turnover each year since 2003. This growth has

been driven by a Queensland and national skills shortage, federal and state government policies

that support the skilling of our national workforce and Outsource Services ability to meet client and

individual needs using our flexible and customized training approaches. We deliver what you want,

where you want, when you want!

Our Future
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Through the introduction of additional Engineering, Construction and Manufacturing related

qualifications Outsource Services will grow its market share in the training and development of

para-professional engineering candidates. We currently have Cert III in Engineering Technical

(CAD), Cert IV in Engineering (Fluid Power) and (Certified Welding), Diploma of Engineering

(Mechanical) and Advanced Diploma of Engineering (Mechanical). The Diploma and Advanced

Diploma both have university pathways reducing a Bachelor of Engineering Mechanical by up to 18

months.

Other areas of Outsource Services growth will be generated by the ongoing and expanding need for

qualified OH&S practitioners in all enterprises due to the harmonization of OH&S legislation

nationally. Outsource Services is currently registered to deliver the Cert IV, Diploma and Advanced

Diploma of OH&S becoming one of the few providers in the state of Queensland.

Outsource Services also has Productivity Placement Program funding through the following

Industry Skills Councils and directly with DET under the Userchoice provisions using their preferred

supplier offering:

Manufacturing Skills Qld ISC $500K

Construction Skills Qld ISC $280

Community and Allied Health ISC $50K

Local Government ISC $50K

Skilling Solutions Qld (Recognition of Prior Learning Funds) $100K

Userchoice Dept of Education and Training (DET) Unlimited

Co-provider with TAFE Qld 70,000 SCH $410K

All the above preferred-supplier contracts have been held by Outsource Services for a number ofyears and annually reviewed and increased to meet the demand of each industry.


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The Engineering Course Structure (Phase 4 from January 2012)

Engineering Subject ClusterAdv.Dip ENG

UNITSDescription

Advance

Dip

Dip

Cert3

Engineering Mathematics

MEM12024A Perform computations

MEM12025A Use graphical techniques and perform simple statistical computations

MEM30012A Apply mathematical techniques in a manufacturing, engineering or related environment

Computers & Communication in Engineering

MEM16006A Organise and communicate information

MEM16008A Interact with computing technology

Engineering Material ScienceMEM30007A Select common engineering materials

MEM23061A Select and test mechanical engineering material

Engineering ControlMEM23003A operate and program computers and/or controllers in engineering situations

MEM23051AApply basic electro and control scientific principles and techniques in mechanical and manufacturing

engineering situations

Engineering Mechanics of SolidsMEM30005A Calculate force systems within simple beam structures

MEM30006A Calculate stresses in simple beam structures

Engineering Drawing 2D & 3D CAD with

Solidworks

MEM30001A Use Computer aided drafting systems to produce basic engineering drawings

MEM30002A Produce basic engineering graphics

MEM30003A Produce detailed engineering drawings

MEM30004A Use CAD to create and display 3D models

Environmental SustainabilityMSAENV272B Participate in environmentally sustainable work practices

MEM22007A Manage environmental effects of engineering activities

Mechanical Engineering Design

MEM30009A Contribute to the design of basic mechanical systems

MEM23041A Apply basic scientific principles and techniques in mechanical engineering situations

MEM23071A Select and apply mechanical engineering methods, processes and construction techniques

Advanced Engineering Project Management

MEM22001A Perform engineering activities

MEM22002A Manage self in the engineering environment

MEM22004A Manage engineering projects

Advanced Mechanical Engineering Design CalculationsMEM23081A Apply scientific principles and techniques in mechanical engineering situations

MEM23091A Apply mechanical system design principles and techniques in mechanical engineering situations

Advanced Design in ManufacturingMEM23093A Apply plant and process design principles and techniques in engineering situations

MEM14081A Apply mechanical engineering fundamentals to support designs and development of projects

Advanced Mechanical Engineering Design

MEM09141A Represent mechanical engineering designs

MEM09151A Apply computer aided modeling and data management techniques to mechanical engineering designs

MEM14061A Plan and design mechanical engineering projects


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Self-paced learning and other information to assist you with your

courseOutsource Services offers a unique clustering of course units to assist you to get the most out of

your training in the most effective way, without duplication. Our courses are developed to fullymeet the national training requirements of the diploma and advanced diploma of engineering.

They are also developed by us to eliminate, as much as possible, you covering the same ground

twice. The intent is that you study in a self-paced mode, and we support you with tutorials at our

premises at Murarrie, or by arrangement with regional partners, at their premises.

Our tutorials are highly concentrated, designed to get you through the difficult spots, to help you

with your motivation and to keep you on track to complete the units in the shortest timeframe

possible. This is important for you to understand, because we have found that the best learning

comes from this immersion process. You will see rapid results and you will reach your qualification

goals earlier than by the traditional route. But you need to devote yourself to the process and workhard to keep up with the tutorial programme.

We will provide additional telephone or email support to get you over any hurdles you encounter,

providing you are putting in your own effort.

The commitment you have made by enrolling in the course is towards you career aspirations. We

are there to support those aspirations. You will need to make a concerted effort to work though

this guide, and to complete the knowledge sheets, assessment items and practical exercises as

effectively as you can in the time frame provided for the cluster and in keeping with the tutorial

process. Following this guidance will see you graduate earlier than through any other type of

participation, and give you the best opportunity for university articulation should you want to

advance through to professional engineering qualifications.


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Preamble to this learning guideWelcome to the exciting word of process control and instrumentation. This is probably the most

universal technology throughout the industrial world. The sciences and mathematics surrounding

and within every manufacturing process, every plant, building, advanced motor vehicle and aircraft

have lead to control technologies that permeate the manufacturing process as well as the

operation of the product, and give additional safety and environmental protection.

By completing this course, you will gain skills and knowledge about the types of devices available,

the conditions under which devices will function correctly, and programming principles. These

skills will enable you to participate in design, commissioning and tuning of fundamental control

systems in industrial settings.

Success in this cluster addresses the requirements of the following manufacturing skills trainingunits:

MEM23003A Operate and program computers and/or controllers in engineering situations

MEM23051A Apply basic electro and control scientific principles and techniques in mechanical and

manufacturing engineering situations

Training for this cluster involves both theory and application using a basic control system and

application of relevant mathematical concepts as covered in the mathematics cluster of units; the

maths cluster will precede and develop relevant concepts directly applicable to the control cluster.

Your training will involve you in development of a control task with a number of variations, several

control approaches including computer and Programmable Logic Controller, software design

elements, tuning and adjustment, calibration, selection of transducers and control elements such

as pumps and valves, safety procedures for working range and de-energising of the system prior to

disassembly of any component, low voltage and AC control considerations and lock-out techniques.

Team work will be essential and the nature of the tasks will have direct relevance to real world

engineering applications.

Attention will focus on design and fine tuning, and documenting the development and any changes

made to the design.

The object is not to demonstrate the skills of an electrical/instrumentation fitter, but rather the

ability to work within a project team, contributing to the outcomes and problem solving by the

demonstration of underpinning scientific principles, mathematical skills, research, understanding of

programming principles, options for reliability and accuracy, and delivering a result based on client

requirements.

In this guide, many drawings come from Wikipedia, and there are numerous hyperlinks to

additional information found in Wikipedia and other sources. These are fabulous resources where

the best information can be easily found. Dont be afraid to use the information commons that are


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increasing in the internet research area and obviously there is, ongoing, a building of reliable

information and a reduction in unreliable information as experts run their eyes over the content

that is found there.

Recommended reference booksThese books are on hand at our training rooms at Murarrie:

Fundamentals of Industrial Instrumentation and Process Control, William C. Dunn, McGraw Hill,

2005. ISBN 0 07 145735 6

Industrial Control Handbook 3rd edition, E.A.Parr, Newnes, 1998. ISBN 0 7506 3934 2

Programmable Logic Controllers 5th Edition, W. Bolton, Newnes, 2009, ISBN 978 1 85617 751 1

Assessment strategy

Assessment will comprise

1. Successful completion of each of the embedded knowledge quizzes, and wherever there is

an identified gap, that is corrected through additional research, support or study.2. Completion of a practical control exercise using both a non-programmed method and aprogrammed method. For those attending tutorials, the equipment is made available and a

specific exercise is to be undertaken. For those not attending, evidence of the ability to

assemble systems and components, run the control system and make calibration and

tuning adjustments will be required to be gathered and certified by a colleague or

supervisor qualified in the field.3. Demonstration of the ability to identify requirements, plan solutions to meet client

requirements, research available solutions and components needed, and work as part of a

team to implement the solution, understanding the testing and safety requirements. This

will be recorded by observation and a checklist maintained by the course coordinator. The

arrangement for those not attending will need to be addressed as above in (2).

4. A final assessment will be conducted to assess the level of skill and knowledge evident. The

assessment instrument used will be common with the RPL process used for these units.


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1. IntroductionControl of tasks by computer or programmable microcontroller requires information (signals or

data representing conditions at any time during the execution sequence), instructions (a program

of sequenced events), devices to acquire information and devices to perform the functions

required. These latter are called IO (input-output) devices, a generalisation because there is a hugevariety of devices to measure, and similarly a huge variety of actuators and valves, rams, electrical

switchgear and so forth to do the work. Some are off the shelf, but many are designed and built to

suit a specific task. Every industry uses some form of automation, and the same concepts are found

in buildings, homes, cars, medicine, aircraft, and the list goes on.

When we consider how to control a task (that has to be repeated, ongoing, precise, reliable and

safe) we need to consider many factors. But in reality the factors come down to four categories:

1. Techniques we can use in the building of logical circuits

Techniques include the selection of a group of devices, such as relays, or logical gates, or the clocksand counters and maths units such as shift registers that are available to us. The choice depends on

the complexity of the element to be controlled. Once we get past simple functional control, we

soon realise that dedicated controllers (PLCs, PLAs) will offer greater overall simplicity and

reliability. Computer control using a PC in a rack or on a desk is also feasible as the logical system is

very flexible, by means of programming, and multi-tasking is available as a feature.

Many systems will commence life as a ground-up design, and developed to perform a specific

function. The designer needs to consider reliability, the effects of transient spikes on the system,

human safety, environmental safety, and maintenance-safe design using lock-out devices, system

power bleeding, safe reservoir capacity, and other factors that may lead to the design rapidlybecoming more complex than originally intended. Therefore fixing ones ideas on a particular

approach too early may lead to shortcomings in commissioning. Careful understanding of the client

requirement and high-quality planning will get the design moving in the right direction and lead to

a better outcome with less lead time to construction.

2. I/O Devices

Sensors are devices for measuring temperature, height, mass, pressure, level, distance, direction,

altitude (really the same as pressure) and anything else that can be measured, and are many and

varied. What the designer needs to know is the range over which the device needs to operate inthe task to be undertaken, and the conditions in which it has to operate. A thermal sensor for

room temperature will not be useful in a blast furnace; something designed for water may not be

suitable for acid, and so on. Precision, accuracy and linearity need to be understood in selecting

these devices.

Actuators - devices for doing the control functions - are also numerous, and again the designer has

to understand the function in order to choose a device that will actually do the job. Many are

electro-mechanical devices, such as solenoid operated valves, pumps, relays, motors for conveyor

belts. Some are purely mechanical, transferring energy from one point to another (now rarely

used) and some use available energy such as thermal, solar-thermal, gas pressure or hydro energy


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to perform a function. These are highly specialised and we will not attempt to deal with these in

this course; however you may encounter something unusual in the field and awareness that these

systems can be used will also alert you to the inherent dangers of not being able to shut down the

energy source in some circumstances.

Transducers are devices that convert energy from one form to another. Most sensors aretransducers, and so are many actuators. The term transducer is more commonly applied to

sensors.

3. Programming

Programming is the technology to bring measurement and control functions under one roof using a

digital controller. There are industry standards in the PLC environment, and software tools to

accompany commercially used controllers. However, a computer may be used to act as the

controller and designing a program may be quite unique to a particular system, such as a banks

high technology security and monitoring system.

When a program, of whatever type, is developed, obviously it needs to work with all of the inputs,

over their range of signal both expected and unexpected (leading to over- range corrections etc); it

needs to be able to read changes in information from feedback signals; it needs to understand

sequences; it needs to know when certain elements of the task are complete so that others can

begin or stop; and it needs to provide alerts to situations that are appearing to trend abnormally. It

should also understand all of the limits on all of the devices and provide fail-safe responses.

Overall, the program will read inputs, and make adjustments as needed to outputs. This is not

unlike a spreadsheet used to calculate an end result (or many outputs) from a number of inputs.

Many of the design principles share common ground with spreadsheeting.

We describe the parts of a computer program where we describe all of the real-world steps to do

something in very simple controller steps, as algorithms. This term came about as computing began

but the technique has existed in mathematics for centuries, in the more complex reasoning

problems faced by scientists and mathematicians. When we develop an algorithm, we identify all of

the miniscule elements of performing a task. Algorithms assist programmers to construct programs

in high level computer languages such as FORTRAN or Visual Basic or HTML. After that, the program

has to be compiled into something the processor chip can use, i.e converted to the instruction set

used by the chip. When designing a controlled process, designers need to develop algorithms to

convert the problem into steps that can be programmed.

4. Communications

Communications is the means of signalling to and from devices that are

deployed in, for example, a large plant, and transmitting to remote

points of control or monitoring. By adopting a signals standard, the

designer works on a common platform and technical installations will

be fool proofed. Historically there have been a number of platforms

(protocols, industry standard ISO xyz) used in the field, such as

MODBUS and C-BUS, which are still in use, but progression to moreFigure 1 RJ connectors (ImageBLINN.EDU
http://www.google.com.au/imgres?imgurl=http://www.blinn.edu/acadtech/resnet/rj11-45.gif&imgrefurl=http://www.blinn.edu/acadtech/resnet/cable.htm&h=582&w=749&sz=90&tbnid=hc9O_5zDcKImUM:&tbnh=90&tbnw=116&prev=/search?q=rj+connectors&tbm=isch&tbo=u&zoom=1&q=rj+connectors&docid=v4bFAKUeWNuGQM&hl=en&sa=X&ei=0gxMT_HxKajSmAW-_eniDw&sqi=2&ved=0CHQQ9QEwCQ&dur=118


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universal concepts across the technologies has led to increasing ETHERNET -based communications

technology to be deployed. This allows use of wired and wireless circuits and means that the

majority of future digital control devices will come out of the box with an RJ network connector

and behind it an Ethernet interface to communicate with the processor inside, for data in and data

out. This is indeed very good news for all of us - including the technical installers and repairers who

will eventually require only Ethernet testers for testing the status of a controller element. Perhaps

the most common serial communications standards between controller and device are the RS 232

serial bus standard and the 4-20 mA current loop.

The 4 to 20 milliamp (4-20mA) two- and four- wireloopis one of the most widely-used sensor

interfaces, in use for many decades and continuing to be favoured because it is relatively free from

interference from noise (electrical transient pulses from high current equipment switching is a

common cause) in industrial plant sites, and immune from voltage drop over a long cable run. This

low current analog loop enables simplicity of installation in noisy situations where shielded wiring

would otherwise be needed, and allows a current variation in the loop over the range indicated,

with 4 being the lowest current (but not zero, so it is self testing) and up to 20mA when the device

controlling the loop is at maximum range. And it doesnt matter how long the loop is, because it is

not concerned by voltage drop over the circuit. These loops were once run over thousands of

kilometres (including the Transatlantic Cable) using conventional telephone wire, in the early days

of teletype; and teletype machines were used as the first computer printers, using the same analog

loop in the age of the digital computer.

Digital sensing is still less convenient than analog sensing, particularly in some heavy plant sites,

due to the corruption of small digital signals by plant noise. However, digital control equipment is

universal today, so there needs to be a conversion of the 4-20mA signal using an analog to digital

converter. Many sensors provide an analog signal (i.e. an analogy, in electrical energy, of the

original which may have been mechanical or another energy form). You should always consider the

use of a current loop for these transducers if the environment where they will be installed has

issues as outlined above.

This introduction has been designed to cover the essential principles of measurement and control,

and although we havent yet embarked on the specifics of design and selection of IO devices,

writing a program or establishing communications, it is absolutely essential that you have

understood everything to this point. So you will now review your knowledge about the

introductory information. Please advise your tutor of any difficulty and ensure you go on with the

course with confidence. Rule #1: if you dont understand something, probably no-one in the course

does. So dont be afraid to ask.

We suggest you refer to the three reference books listed before this introduction when you

undertake the first of the Knowledge Tests to help you to get familiar with research, which will be

an important skill for you to work in the control and measurement field. Also make use of websites

such as Applied Solid Technologies http://www.solidat.com/content.asp?id=15

Additional notes:
http://sound.westhost.com/appnotes/an011.htmhttp://sound.westhost.com/appnotes/an011.htmhttp://sound.westhost.com/appnotes/an011.htmhttp://www.solidat.com/content.asp?id=15http://www.solidat.com/content.asp?id=15http://www.solidat.com/content.asp?id=15http://sound.westhost.com/appnotes/an011.htm


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Knowledge Quiz 1 review of the introduction

1.1 Define analog

1.2 Digital sensing is less convenient than analog sensing - explain this.

1.3 What sort of device is a transducer?

1.4 Write a brief algorithm for making a cup of tea

1.5 What is a 4-20mA loop?

1.6 What is an actuator? List two types and explain what the transducer action is for each:

1.7 Make a checklist of things to do before commencing the design of a controlled process:

1.8 Go to a website and find one sensor that senses the level or height of a water head in a tank.

What are its basic principles as a transducer, and what would be the best way to connect it back to

the controller in an industrial plant?


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2. Control system techniques and hardwareIn this section we will look at the devices you can use to read signals, and control outputs. This

includes IO devices and examines transducer principles. Since we are mainly concerned about

electrical analogscoming from the sensors we use, lets start with a review of some basic electrical,

electromagnetic and electromechanical principles.

EMF is electromotive force which is also energy. The unit of measurement is the volt. Sources of

EMF include AC generators (alternators), DC storage cells and generators, photovoltaic cells and

even quartz crystals. You probably start your barbeque with a button striker that involves a quartz

crystal. When struck it converts some of the mechanical energy to noise and some to electrical

energy in the form of a spark to ignite the gas supply. This is a good demonstration of important

work of Sir Isaac Newton who pondered that the total amount of energy out of a system is equal to

the energy put in. For centuries this formed the basis of maths and science study and Albert

Einstein was foremost in proving the case under isolated situations, leaving open some key pointsabout whether it applied across the entire universe. Energy may convert to differing forms such as

frictional heat, noise and the requirement you intended, but you have to energise your system with

enough to meet the total output.

Similarly, any generator of EMF has internal resistance made up of heat energy given off, losses in

insulation material inside it, and eddy currents circulating in its metallic structure or core that rob

energy. And so we can think of every source of EMF as having a circuit like this:

(Source: Wikipedia)

The losses inside the generator are all grouped into RG, which isa resistor equivalent to the open circuit voltage VG (which can

be measured by a high impedance1

voltmeter) without the

usual load RL being connected, divided by the maximum current

the generator can make. This is the current that would flow in a

short circuit. So you place a current meter (Ammeter) across

the voltage terminals (in theory; there are less dangerous ways to do this with some circuits

knowledge) and read the number of amperes out of the generator. Heres whereGeorg Simon

Ohmcomes in. The ohm () is theSIunit ofelectrical resistanceof any conducting material,whether it is designed to be a resistor or a

conductor. This definition is worthy of thought,

because so much depends on you having a good understanding of resistance and what it can do to

create unanticipated errors in your design: The ohm is defined as a resistance between two points

of a conductor when a constant potential difference of 1 volt, applied to these points, produces in

the conductor a current of 1 ampere. (Note that the conductor in this definition cannot be the

source of any electromotive force.) Have a look athttp://en.wikipedia.org/wiki/Ohmfor more

information, but as a basic rule the definition results in the formula that resistance equals voltage

applieddivided by the current that flows because of it, and this applies to both AC and DC current

sources in general. In AC circuit theory, impedance replaces resistance but has the same definition

1 At least 10 times higher impedance than the size of RG so as not to create a meaningful error in reading

Figure 2 Generator equivalent circuit
http://en.wikipedia.org/wiki/Georg_Ohmhttp://en.wikipedia.org/wiki/Georg_Ohmhttp://en.wikipedia.org/wiki/Georg_Ohmhttp://en.wikipedia.org/wiki/Georg_Ohmhttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Ohmhttp://en.wikipedia.org/wiki/Electromotive_forcehttp://en.wikipedia.org/wiki/Amperehttp://en.wikipedia.org/wiki/Volthttp://en.wikipedia.org/wiki/Electrical_resistancehttp://en.wikipedia.org/wiki/International_System_of_Unitshttp://en.wikipedia.org/wiki/Georg_Ohmhttp://en.wikipedia.org/wiki/Georg_Ohm


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at low AC frequencies - such as 50 Hz, and holds fairly true over minor changes in environmental

factors such as temperature.

Resistors; Photo by P.J. Newnham

You will recall that direct current (DC) flows from one terminal of the source, through the load, and

back into the other terminal, as a one-way flow like a stream flowing from a lake. In the case of

water from a lake, the equivalent of EMF is the mass of the water and its elevation, as stored

energy. Nothing flows if there is a closed sluice gate, (an off-switch) and nothing would flow if the

source and the end terminal were at the same energy levels (you dont have currents flowing in a

lake unless water is leaving it, going downhill and causing energy differences). So when the sluice

opens, down the stream goes the water, limited by the size of the conducting path and the height

of the point to where it flows. DC has the same behaviour.

AC exhibits similar behaviour, except that it is dynamic current coming from a source such as analternator, swinging through sinusoidal varying peaks, zeros, negative peaks, zeros and positive

peaks, flowing in the circuit and able to pass through resistors unchanged in nature (but amplitude

will lessen as EMF is converted to heat in the resistor), and reactive components (capacitors and

coils or inductors) where it changes behaviour by current leading voltage or current lagging voltage.

More later.

AC is cyclical. A bike wheel behaves the same way. Paint a spot on a bike wheel and move the bike

along a path, and the spot will rise and fall around the axle level as the wheel travels forward,

making a similar locus to the time-based AC sine wave. The repetitive period of AC current

generated in the Australian energy grid system is 50 revolutions per second, so the bike wheel

could do that easily if the rider was pedalling along at about 1000 KPH. Other sources of AC currentinclude microphones because they produce an electrical analog of voice or music, made up of

many sine wave electrical currents but at very low energy levels. Any shape of sound signal is made

up of multiple sine waves with amplitudes and timing (and phases) all adding up to create the

shape we see on an oscilloscope; so a display may not look sinusoidal, but it is the instantaneous

sum of sine wave signal amplitudes. Microphones are sensors, and also transducers (Im sure you

understand that now). If the diaphragm of the microphone receives a single frequency movement

of air (a clean note from a guitar struck about the centre of the string will produce hardly any

harmonic tone) it will output a sine wave at the same frequency as an electrical signal with

amplitude proportional to the energy of the source.


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When a wire moves through a magnetic field it creates a flow of electrons in the wire, if something

is connected that enables the flow (a circuit). If the magnetic field reverses as it cuts the wire, the

current flow will reverse and go through the circuit the other way.

This simple device uses both poles contributing to generation of current at once, because the wire

is looped. The device is converting rotary energy (from a handle,the wind, a motor or a steam turbine to electrical current flow,

which will be sine wave shaped, and the amplitude and

frequency are both dependent on the energy source (This is an

energy in = energy out + energy lostsituation). In industrial use

alternators are frequently more complex that something like

this, but something almost this simple could be used as a

tachometer, to sense rotary speed from a motor of some type.

(source Wikipedia)

Now take a look at Wikipedia for a model of a three phasealternator using, not fixed magnets, but electromagnets, which

is the common way of providing the magnetic field. We are about to review electromagnetism in

the next section of this workbook.http://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gif

Additional notes:
http://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gifhttp://en.wikipedia.org/wiki/File:3phase-rmf-noadd-60f-airopt.gif


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Lets examine a simple circuit using a 12 volt battery (or DC supply - our source of EMF) and a

variable resistor, so that the current flow can be varied. We will monitor the current flow in the

circuit, and discuss the basic principles of what is going on. Then we will replace the variable

resistor with a sensor that responds to changes in temperature.

Calculating power

Power is consumed by losses and the load (the lamp in the above example). The load is where the

work is done in the real world, and the source provides the energy to deliver the function at the

load. To calculate power, simply measure the voltage at, and the current in, the load, and multiply

these together.

So if we have 12 volts at the load, and measure 0.5 amps in the circuit, the lamp above will beconsuming 6 watts.

Notes:

Amps

Variable Resistor 0-50 ohm

12 VoltsLamp


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2.1 Electromagnetic devicesElectromagnetism plays an important role in sensing and control devices. The underlying principle

of electromagnetism is the formation of a magnetic field around a current-carrying wire. The

inverse is also true. Moving a magnet past a wire will cause

current to flow. This latter effect requires a dynamic change inthe field as it intersects, gives us the principle of the dynamo and

explains why transformers require AC current to work.

A way to determine the polarisation of the field formed by a

current in a wire is to use the right hand grip rule which is true

for conventional current flow - from positive terminal to

negative terminal. The field resulting is in the direction B. Again, this is the conventional depiction

whereby in a permanent magnet, the field direction is from North pole to South pole.

(All images sourced from Wikipedia)

By making the wire into a solenoid (coil), the field can be concentrated

through the centre of the coil. The diagram below shows a slice

view.

The dot in the cross section of the wire represents the head of

an arrow while the cross is the tail feather of the arrow, so

current is flowing towards your face at the top of the coil and away from you at the bottom. If you

curl your fingers of your right hand in the same way so that the current flows out of the fingertips,

your thumb indicates the direction of the North pole. If the current in the coil is AC, the field will

collapse to nothing then reverse on each successive half cycle of the sine wave of current. The

current will be found to lag behind the EMF, due to the time factor of building the field. Interesting

point, and the higher the frequency the more the time lag, so keep this in mind with

electromagnetic transducers.

Adding a core of iron or ferrite material assists in drawing the field into a higher concentration of

magnetic flux, or energy. This helps to make electromechanical devices work more efficiently. The

relay is one such device, but so is the electric motor.
http://upload.wikimedia.org/wikipedia/commons/4/45/Solenoid-1.pnghttp://upload.wikimedia.org/wikipedia/commons/0/0d/VFPt_Solenoid_correct2.svg


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PCB relay Miniature relay in circuit

Automotive relay

(Source: Photos by P.J. Newnham)

Relays were invented in early telegraphy when words were encoded into pulses using a code such

as Morse code, keyed and interpreted by humans. As the voltage dropped across the miles of wire

cable, the signal weakened until it was refreshed by a relay. The relay needs only a small activation

current, and at the next signal relay station along the route, this small residual signal switched a

battery pack thereby replicating the signal at full strength. We know that the current loop would

have achieved a simpler result but the genius of that idea had not yet been uncovered. Relay based

systems formed the basis of automation and signal switching up until the late 1970s, providing

industry with the fundamental logical building block, the gate. Very complex systems needed large

numbers of relays, and reliability was an issue. The semiconductor and the emergence of digital

semiconductor-based logical devices and controllers provided the miniaturisation of these systems

and reduced the power wasted by the massive relay banks in heat alone, although, for a time, the

relay endured in the interface between low voltage controllers and the high current AC or DC

circuits they controlled.

Despite excellent isolation between the activating source and the controlled source, relays are

rarely designed into electrical circuits any more, fading away even in the car industry, and

increasingly replaced in DC systems by transistor and SCR devices and in AC systems by TRIACS.

Semiconductor switches compete favourably in reliability, cost and operational effectiveness; they

can provide virtually the same isolation between input signal and output signal as a relay (opto-

isolators in particular),can handle large currents, have no moving parts and no mechanical contacts

that may burn and erode. Essentially, a relay is a solenoid2

device; there are many contactors,

actuators, one- and two-way solenoid valves, and other solenoid activated switch systems found in

any plant where they control large amounts of energy such as high current supplies, fluid power

systems, water mains, storage hoppers and so on. Links to manufacturer information are easily

2 Solenoid = electromagnetic actuator


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found, an example being MGA controls athttp://www.mgacontrols.com/2011/02/24/mm-

international-solenoid-valves/

The semiconductor is the future. Many solenoid devices, increasingly, are being replaced by

semiconductor devices. In the electric train speed controller circuit, QRs electric trains use massive

TRIACS with water cooling. Though semiconductor theory is not required here, you may want todiscuss or read up on the basics. In a nutshell: conventional junction diodes conduct one way only;

zener diodes have designed reverse breakdown points that enable them to control voltage; a

junction transistor operates by injection of minute current into the base-emitter junction,

controlling a large current in the emitter-collector circuit; a field-effect transistor (FET and

MOSFET) uses a voltage on the gate (no current flows) to control large current in the drain circuit;

SCRs (also called thyristors) use a small gate current to switch on current flow from cathode to

anode, whereby it will remain on until the power is cut; the silicon controlled switch (SCS) can gate

the current on and off; TRIACs are bilateral gate-controlled switches for AC; once the control

current into the gate is set, the device operates on each successive half cycle to control its current

from cathode to anode, which obviously also interchange.

Additional notes:
http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/http://www.mgacontrols.com/2011/02/24/mm-international-solenoid-valves/


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Knowledge Quiz 2 review of the fundamentals of control systems hardware

2.1 Define EMF

2.2 Name one important source of EMF, and the principle of energy conversion associated with

this. Bonus points for finding a source not yet discussed. Bonus points also for discussing losses

and the conservation of energy principle.

2.3 Ohms law enables us to determine the current that flows in a circuit when energised by an

EMF. What limitations are associated with this law?

2.4 Calculate the current that flows in the following circuit and the voltage between A and B:

12 Volts

330 ohm

870 ohm

A

B


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2.5 What is electromagnetism?

2.6 What applications of electromagnetism can you list? (At least four please)

2.7 What is meant by the equivalent circuit of a generating device?


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The concept that makes this, and many other sensors, work best is as an arm of a very clever

bridge comprising four elements in a rest-state balanced configuration like so:

If all resistors in this circuit are identical, the voltage at the two points

B and D will be exactly the same (and this hold true if R1=R2, and

R3=RX). Therefore the voltage VG will be zero. If RX is our strain gauge,installed at the test site, and its resistance before being deformed is

the same as R3, (R1 and R2 also being identical) the circuit is perfectly

balanced and the output of the feedback signal VG is zero. This is a

very nice arrangement because VG will become dependent upon the

deformation of the beam to which RX is attached, and will be a positive (B more positive than D) if

the strain deformation stretches the gauge and a negative (B more negative than D) it the

deformation compresses the gauge.

2.2.2 Thermistors

Rx is a positive temperature coefficientthermistor(a

PTC) for sensing temperature inside a refrigeration

plant. It has a nominal resistance of 470 ohms @

25oC and senses from -55oC to +125oC at +/- 1%

tolerance.

Q: What would be the expected reading on the voltmeter VG at -40oC, -20

oC, -10

oC and 0

oC?

To understand how this device varies with temperature, you will need a data sheet. One

manufacturer of close tolerance PTCs is Vishay Dale (Vishay type number TFPT0603L4700FV). Other

sources are Digikey and AllDatasheet.com but when selecting, you need to always visit the

manufacturers website and recheck. Data and products develop and improve, or get replaced, and

as a result the key information changes; 2nd and 3rd party vendor links are often out of date.

http://www.vishay.com/docs/33017/tfpt.pdfor33017

This will get you the TFPT0603 family PDF data sheet. Have a look at the data and find tables or

graphs of temperature vs. resistance and calculate the nominal resistance at each of the above

temperatures.

TemperatureoC -40 -20 -10 0 +25

Multiplier .75 .823 .861 .9 1

Resistance 470 +/- 1%

330 330

330

25 volts

Figure 4 Wheatstone Bridge
http://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermistorhttp://en.wikipedia.org/wiki/Thermistorhttp://www.vishay.com/docs/33017/tfpt.pdfhttp://www.vishay.com/docs/33017/tfpt.pdfhttp://www.vishay.com/doc?33017http://www.vishay.com/doc?33017http://www.vishay.com/doc?33017http://www.google.com.au/imgres?imgurl=http://upload.wikimedia.org/wikipedia/commons/thumb/9/93/Wheatstonebridge.svg/300px-Wheatstonebridge.svg.png&imgrefurl=http://en.wikipedia.org/wiki/Wheatstone_bridge&h=231&w=300&sz=8&tbnid=MSKArGdLkORHIM:&tbnh=84&tbnw=109&prev=/search?q=wheatstone+bridge&tbm=isch&tbo=u&zoom=1&q=wheatstone+bridge&docid=FjYGnIRDIaG0SM&sa=X&ei=fL_aTteGCKWSiAfT-bTvDQ&ved=0CDoQ9QEwAg&dur=1113http://www.vishay.com/doc?33017http://www.vishay.com/docs/33017/tfpt.pdfhttp://en.wikipedia.org/wiki/Thermistor


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Figure 6 Ratio chart

Tables and graph reproduced with permission from Vishay Dale


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(Further Training information on these devices may be found at

http://dkc1.digikey.com/au/EN/tod/VishayDale/CRCW-HP/CRCW-HP.html )

Refer back to the circuit, which your knowledge tells you is a Wheatstone Bridge; if all resistors are

equal, there will be no potential difference between points D and B

Refer to

http://en.wikipedia.org/wiki/Wheatstone_bridge

or a textbook for the derivation of the formula

below:

VG

Substituting each value of resistance from the

table we created enables us to calculate the value

ofVG and ultimately to calibrate the voltmeterscale to indicate temperature.

TemperatureoC -40 -20 -10 0 +25

Resistance 354 388 405 423 470 +/- 1%

Voltage VG

Complete the table using Excel or another spreadsheet for your calculation. A sample is attached

below:

According to its scale, the meter to the right reads from 10 to 16 volts. That means it swings over a

range of 0 volts (pointer at the left) to 6 volts (pointer at the right). This is a good illustration of the

card in the meter display representing something different to the actuating force. We could

make a new card and slip in inside the cover, with the corresponding temperatures such as 25o

C

Vs TempoC Ratio Rx VG

25 25 1 470 2.1875

R1=R2=R3

0 0.9 423 1.543825

-10 0.862 405.14 1.277648

-20 0.825 387.75 1.005747

330 -40 0.753 353.91 0.437009

Figure 7 Bridge sensor connection
http://dkc1.digikey.com/au/EN/tod/VishayDale/CRCW-HP/CRCW-HP.htmlhttp://dkc1.digikey.com/au/EN/tod/VishayDale/CRCW-HP/CRCW-HP.htmlhttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://en.wikipedia.org/wiki/Wheatstone_bridgehttp://dkc1.digikey.com/au/EN/tod/VishayDale/CRCW-HP/CRCW-HP.html


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just above the current 12, -10oC just above the 11, and so on. This would not give much precision

but the principle is illustrated. Or we could use a converter to make a digitised version of the

analog signal (AD Converter) and drive a seven segment display panel containing whatever number

of digits we require for precision.

2.2.3 The pressure transducerPressure metrology requires transducing pressure into an electrical analog, varying around a

nominal value (reference value, usually indicated by zero volts) (perhaps that could represent the

zero pressure of a vacuum 0 PSI or 0 kPa but it may be anything, e.g. in a barometer, sea level

pressure: 14.7 PSI or 101.325kPa). There are a number of ways to do this, including a diaphragm

with strain gauges bonded to it acting as resistive elements as described above. Pressure

transducers can be used to read changes in air pressure, gas pressure, and head of fluid in a

container and so the analog signal can be calibrated for volume or weight. Often these are found in

applications controlling the level of water in a vessel. The types of pressure gauges found in

industry are based on a number of technologies

Piezoresistive Strain Gage: Uses the piezoresistive effect of bonded or formed strain gauges to

detect strain due to applied pressure.

Capacitive: Uses a diaphragm and pressure cavity to create a variable capacitor to detect strain due

to applied pressure.

Magnetic: Measures the displacement of a diaphragm by means of changes in inductance

(reluctance), LVDT, Hall Effect, or by eddy current principal.

Piezoelectric: Uses the piezoelectric effect in certain materials such as quartz to measure the strain

upon the sensing mechanism due to pressure.

Optical: Uses the physical change of an optical fiber to detect strain due applied pressure.

Potentiometric: Uses the motion of a wiper along a resistive mechanism to detect the strain caused

by applied pressure.

Resonant: Uses the changes in resonant frequency in a sensing mechanism to measure stress, or

changes in gas density, caused by applied pressure.

Historically, pressure transducers converted the pressure signal into mechanical energy and then

some means had to be found to convert that to electrical energy. For example, expanding orshrinking bellows systems, the U Tube manometer and the Bourdon tube have been used for

scientific and industrial measurement and developed into highly accurate analog instruments; but

in the digital age there are other means to achieve the outcome, as above.
http://en.wikipedia.org/wiki/Piezoresistivehttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Inductancehttp://en.wikipedia.org/wiki/LVDThttp://en.wikipedia.org/wiki/Hall_Effecthttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Resonant_frequencyhttp://en.wikipedia.org/wiki/Piezoelectrichttp://en.wikipedia.org/wiki/Eddy_currenthttp://en.wikipedia.org/wiki/Hall_Effecthttp://en.wikipedia.org/wiki/LVDThttp://en.wikipedia.org/wiki/Inductancehttp://en.wikipedia.org/wiki/Capacitorhttp://en.wikipedia.org/wiki/Piezoresistive


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Figure 8 U tube manometer

There are five types of references used for pressure sensing:

1. Absolute: reference is 0 pressure where there is no atmospheric weight (space)

2. Vacuum: something less than atmospheric pressure as a reference point, a man-made

vacuum

3. Differential: the pressure drop across a system, i.e. one point becomes the reference, and

the reading at the monitoring point is relative to that pressure

4. Gauge: the reference point is the atmospheric pressure at the point of measurement

5. Sealed: A reference pressure is sealed into the sensor unit at manufacture.

Low cost pressure sensors such as the one below are now generally a silicon wafer whose

conductivity is altered by pressure in the top port, compared with atmospheric pressure in a small

breather port on the reverse side.

(Source: Photo by P.J. Newnham; diagram from Dunn W.C. Fundamentals of Industrial Instrumentation and Process Control)

Notes:

Figure 9 Pressure transducer, basic


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2.2.4 Ultrasonic sensors

Used across many industries and in numerous applications, ultrasonics (in medicine, ultrasound)

utilise high frequency sound waves (i.e. above the limit of human hearing) generated by a signal

source and converted to mechanical sound waves by a piezo crystal transducer (which is very

similar to the device in a piezo tweeter in hi fi audio systems). Quartzcrystals are capable of very rapid physical change when a voltage is applied

across the crystal, one element of the Piezo Effect. (Another element of

the PE is that if quickly distorted by a mechanical impact, the crystal

produces a high voltage - a discovery attributed to the Curies during the

C19th). Driven by a high frequency ac voltage, the crystal oscillates at its

natural resonant frequency and its walls radiate sound energy - emit

pressure waves - similar to the way a speaker cone moves the air, except

very much faster. Depending on the application, frequencies anywhere between 21KHz and 4MHz

are used. There are applications in the hearing range (around 3KHz) where pulses of sound

(SONAR) are transmitted and the echo delay is measured, for range finding, usually in water. This is

used in defence and survey applications. Modern medicine incorporates SONAR and ultrasound

principles using piezo and other transmitting devices for human tissue analysis, etc.

Ultrasonic devices (transceivers when they both send and receive) are designed for a variety of

industrial conditions and are robust units, compact, cost effective, and very reliable.

In general, ultrasonic measurement relies on receiving an altered or delayed signal reflected from

the system under measurement. Some means of interpreting the amount of alteration (or the

error result) is part of the modular unit, as is a means to calibrate it to develop an accurate

indication of the measurement being taken. An industrial ultrasonic transducer includes all of the

system elements, but calibration can be external to the module. For distance measurement using a

pulsed signal, the mathematics is to halve the time between the sent pulse and its echo - as it

travels both to and back from the object at the same speed (caution: Doppler effect can come into

play here if the object is moving to or away from the source) and to multiply the speed of sound

in the medium by the time for the one-way trip.

The speed of a sound wave depends on the medium in which it travels. Sound pressure waves

travel by elastic vibrations of the particles forming the medium through which it is travelling, be it

air, a fluid, a gas or a solid like wood or steel. In air, at 20oC, median pressure and relative humidity

(moisture content) of 5% the velocity of sound is 343.2 metres per second (1,126 ft/s; 1235.5 KPH

for Mach 1 at low altitude, rising as temperature decreases with altitude but falling with a

reduction in air density). In ultrasonics, calculations need to compensate for the type of medium,

the altitude, and other factors relevant to the specific measurement.

Ultrasonics is often used in distance measurement, motion and speed detection in robotics, level of

fluid in a tank, flow rate, weight, strain and pressure and there have been many other applications

deployed across industry and in medicine. The isolation of the transducer from direct contact with

the system under measurement (acids, fuels and so on) means added safety and longevity, and

inherent immunity to electrostatic discharge energies. (Cautionary note: any components that are

(Source: edaboard.com)Figure 10 Ultrasonic T-R


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electrically connected to the transceiver, including a 4-20mA loop driver, may reduce this immunity

if such circuits are not isolated from ESD). Generally, pressure or other reading losses (errors) due

to an ultrasonic measuring system can be ignored or contribute a small fixed measuring error,

whereas many other types of measurement impact on the system under measurement, and error

tables need to be developed.

In applications such as controlling the pump-out level in a vessel, of all the choices of transducer

available, the right ultrasonic transducer

will provide the most precise

measurement and flexibility in calibration

for the controller to adjust pump speeds

and to control valves.

(Diagram Source: fromArticle supplied by HawkerElectronics Ltd - www.hawker-electronics.co.uk)

Some ultrasonic transceivers in high precision work use multiple beams and digital signal

processing in, or associated with, the transceiver. For these, DSP is essential to interpret the

information from the beams and to produce a coherent result out of the device. Such transceivers

are associated with flow rate meters and turbulence situations (including human body blood flow

associated with the heart chambers and so on). From laboratory studies, calibration algorithms

have been developed that are embedded in the DSP software. These are generally quite high-cost

systems, but industrial flow rate meters using three beams and on-board signal processing are used

in large plant situations and are cost-effective for the accuracy of their output signals.

2.2.5 Differential sensors

We briefly alluded to a differential sensor when discussing pressure sensing. This type of sensor

produces an output relative to another fixed reference point. This is very useful if we are

attempting to control a system around a reference value within the system. Pressure in a gas

storage vessel may be many times higher than the atmospheric pressure reference and controlling

the peak and low pressurisation points is difficult when comparing to something so different.

Establishing a reference point at maximum pressure would be much more useful and would

provide a safety alarm trigger point.

Figure 11 Ultrasonic sensing

Figure 12 Differential pressure

sensor
http://www.hawker-electronics.co.uk/http://www.hawker-electronics.co.uk/


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http://www.potentiometers.com/select_honeywell.cfm

2.2.7 Differential sensing

There are numerous applications where differential sensors are needed. One very important use iswhen measuring a change in position of a component relative to a fixed point which may sit in a

moving system such as an aircraft wing flap system, a precision NCmilling machine3, an

earthmoving or mining machine control component, a vane in a wind turbine, etc.

Linear Variable Differential Transformers or LVDTs are very accurate analog devices for sensing

mechanical change into or away from a reference point. The system inside a basic LVDT uses three

coils fed by an input ac signal (A), with a moving core which alters the coupling and hence the

output signal (B) relative to the input. It is a transformer where the core is adjustable.

(Image below sourced from Wikipedia, at right from RDP Group)

Figure 14 LVDT

Apart from a myriad applications in sensing linear motion and

distance, these devices have been adapted to many types of

sensing including pressure, flow, wind speed, rotary motion

and so on. They have the advantage of isolation from the

system under measurement and so are widely used in industry.

Digital signal conversion is now built into many units.

For additional explanation, see

http://en.wikipedia.org/wiki/Linear_variable_differential_trans

former

http://www.rdpe.com/displacement/lvdt/lvdt-principles.htm,

and for images and more information and data sheets, seehttp://www.rdpe.com/ex/d5-d6.htm

Other types of position sensors involve hybrid analog and digital incorporating devices

incorporating a combination of technologies such as ultrasonic, optical, LVDT, Hall Effect, anddigital. (Seehttp://www.positek.com/rotary.htm?gclid=CJrN2LuQoa4CFQFLpgodR0dJ4A for

examples of digital sensors and for use in a variety of applications).

Other useful information for sensors may be sourced from

http://www.sensortechnics.com/

Notes:

3NC=Numerical Control, such as a programmed machine for complex machining operations involving

multiple shaping operations and tooling changes all done by programming
http://www.potentiometers.com/select_honeywell.cfmhttp://www.potentiometers.com/select_honeywell.cfmhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://www.rdpe.com/displacement/lvdt/lvdt-principles.htmhttp://www.rdpe.com/displacement/lvdt/lvdt-principles.htmhttp://www.rdpe.com/ex/d5-d6.htmhttp://www.rdpe.com/ex/d5-d6.htmhttp://www.rdpe.com/ex/d5-d6.htmhttp://www.positek.com/rotary.htm?gclid=CJrN2LuQoa4CFQFLpgodR0dJ4Ahttp://www.positek.com/rotary.htm?gclid=CJrN2LuQoa4CFQFLpgodR0dJ4Ahttp://www.positek.com/rotary.htm?gclid=CJrN2LuQoa4CFQFLpgodR0dJ4Ahttp://www.sensortechnics.com/google-pressure?direct_call=sensor&ge=b263851a2e87ccdca2e72392017e7d5ahttp://www.sensortechnics.com/google-pressure?direct_call=sensor&ge=b263851a2e87ccdca2e72392017e7d5ahttp://www.sensortechnics.com/google-pressure?direct_call=sensor&ge=b263851a2e87ccdca2e72392017e7d5ahttp://www.positek.com/rotary.htm?gclid=CJrN2LuQoa4CFQFLpgodR0dJ4Ahttp://www.rdpe.com/ex/d5-d6.htmhttp://www.rdpe.com/displacement/lvdt/lvdt-principles.htmhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://en.wikipedia.org/wiki/Linear_variable_differential_transformerhttp://www.potentiometers.com/select_honeywell.cfm


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2.3 Controlling positionIn practice, the majority of motion control is achieved by controlling energy either stored or

available through a reticulated power source. For example, a system may energise/pressurise fluid,

gaseous or mechanical energy which is then used in a mechanically enabled circuit, controlled using

a controller of some type. A car is such an example, where the engine is managed by a controller

circuit with human input to adjust how the energy of the engine is deployed to the wheels. We

have also discussed how minute motion can be created using the properties of quartz crystals, and

this finds use in microcontrol applications. But there is also a requirement in many specialised

situations for converting an electrical signal directly into motion, which requires some form of

actuator and many of these operate on electromagnetism. A relay is just such an example, as is the

electric motor.

2.3.1 Stepper motors

T he digital age has given us a relatively new type of motor called the stepper motor, and this is the

device that gives the laser