Mechatronics Engineering (PDF) - Menoufia University - 2024/2025

Menoufia University FACULTY OF APPLIED HEALTH SCIENCE TECHNOLOGY Mechatronics Engineering Video link about the course Prof. Hamdy Awad Dr. Mohamed Abdo Dr. Essam Gomah Dr. Ramy Farid Dr. Alaa Khalifa Dr. Mohamed Salah Dr. Khalil Ramadan 2024/2025 ‫رؤية ورسالة كلية‬ ‫تكنولوجيا العلوم الصحية التطبيقية‬ ‫ رؤية الكلية ‪:‬‬ ‫كلية رائدة تتميز بتخريج تكنولوجي صحى ذو كفاءة عالية قادر على المنافسة في سوق العمل‬ ‫محليا وإقليميا لالرتقاء بمستوى الخدمات الصحية‪.‬‬ ‫ رسالة الكلية‬ ‫تخريج تكنولوجي صحى‪ ,‬ملتزم باالخالقيات المهنية متقن للمهارات التقنية المتخصصة التى‬ ‫تؤهله للمساعدة فى تقديم رعاية صحية متميزة وأمنة مبنية على االدله والبراهين تخدم مختلف‬ ‫المؤسسات الصحية‪ ,‬وأن يكون الخريج قادر على المشاركة في البحث العلمى وخدمه المجتمع‬ ‫ومهيأ للتعليم المستمر والعمل الجماعى متمتعا بمهارات االدارة من خالل تقديم برامج متنوعة‬ ‫تلتزم بمعايير الجودة التعليمية‪.‬‬ ‫‪1‬‬ ‫‪Table of Contents‬‬ Contents Table of Contents Course Specification.......................................................................................................................................... 5 Chapter 1 Introduction to Mechatronics........................................................................................................... 9 1.1 What is mechatronics?............................................................................................................................ 9 1.2 The design process................................................................................................................................ 11 1.3 Systems.................................................................................................................................................. 13 1.3.1 Modelling systems.............................................................................................................................. 14 1.3.2 Connected systems............................................................................................................................. 15 1.4 Measurement systems.......................................................................................................................... 16 1.5 Control systems..................................................................................................................................... 17 1.5.1 Feedback......................................................................................................................................... 18 1.5.2 Open- and closed-loop systems...................................................................................................... 20 1.5.3 Basic elements of a closed-loop system......................................................................................... 22 1.5.4 Analogue and digital control systems............................................................................................. 25 1.5.5 Sequential controllers..................................................................................................................... 29 Problems...................................................................................................................................................... 29 Chapter 2 Sensors and Transducers................................................................................................................ 30 2.1 Introduction........................................................................................................................................... 31 2.2 Introduction to Sensors and Transducers............................................................................................... 33 2.2.1 Sensor Classification...................................................................................................................... 35 2.2.2 Parameter Measurement in Sensors and Transducers.................................................................... 37 2.2.3 Quality Parameters......................................................................................................................... 40 2.2.4 Errors and Uncertainties in Mechatronic Modeling Parameters..................................................... 43 2.3 Sensors for Motion and Position Measurement..................................................................................... 44 2.3.1 Resistance Transducers................................................................................................................... 47 2.4 Force Sensors......................................................................................................................................... 49 2.4.1 Strain Gauges.................................................................................................................................. 50 2 Table of Contents 2.5 Sensors for Flow Measurement............................................................................................................. 54 2.5.1 Solid Flow...................................................................................................................................... 55 2.5.2 Liquid Flow.................................................................................................................................... 56 2.6 Temperature Sensing Devices............................................................................................................... 57 2.6.1 Thermistors..................................................................................................................................... 59 2.6.2 Thermocouples............................................................................................................................... 61 2.6.3 Radiative Temperature Sensing...................................................................................................... 63 Problems...................................................................................................................................................... 64 Chapter 3 Signal Conditioning and Real Time Interfacing............................................................................... 66 3.1 Introduction........................................................................................................................................... 66 3.2 Elements of a Data Acquisition and Control System............................................................................ 67 3.2.1 Overview of the I/O Process........................................................................................................... 69 3.2.2 General Purpose I/O Card (GPIO).................................................................................................. 72 3.3 Transducers and Signal Conditioning.................................................................................................... 73 3.4 Data Conversion Process....................................................................................................................... 77 3.4.1 The Analog-to-Digital Converter................................................................................................... 79 Chapter 4 Electrical Actuation Systems........................................................................................................... 83 4.1 Electrical Systems................................................................................................................................. 83 4.2 Mechanical Switches............................................................................................................................. 84 4.2.1 Relays............................................................................................................................................. 84 4.3 Solid-State Switches.............................................................................................................................. 87 4.4 Solenoids............................................................................................................................................... 87 4.5 Direct Current Motors........................................................................................................................... 92 4.5.1 Brush-type d.c. Motor.................................................................................................................... 92 4.5.2 Brush-type d.c. Motors with Field Coils.......................................................................................... 96 4.5.3 Control of Brush-type d.c. Motors.................................................................................................. 98 4.5.4 Brushless Permanent Magnet d.c. Motors................................................................................... 102 4.6 Alternating Current Motors................................................................................................................. 104 4.7 Stepper Motors.................................................................................................................................... 108 3 Table of Contents 4.7.1 Stepper motor specifications......................................................................................................... 110 3.7.2 Stepper Motor Control.................................................................................................................. 112 4.7.3 Selection of a stepper motor......................................................................................................... 117 Problems.................................................................................................................................................... 120 References..................................................................................................................................................... 122 Multimedia 4 Table of Contents Course Specification 1. Course basic information : Course Code: OFRME200 Course Title: Academic year:2024/2025 Mechatronics Engineering Level (2 ) – Semester : 1 Teaching hours: Lecture - Tutorial - Lab 2- Course objectives 1. To define the basic principles of mechatronic systems. 2. To demonstrate the function of the different components of a mechatronic system 3. To study some applications of mechatronic systems 3- Intended Learning Outcomes: (ARS) Course (ILOs) a4) Explain Principles of mechatronics a4-1) Explain principles of mechatronics Understa A- Knowled system. system. ge and nding a8) Describe Current technologies of a a8-1) Describe the current technologies of mechatronics. electrical, pneumatics, hydraulics drives. b5) Assess and evaluate the b5-1 )Assess and evaluate the characteristics of eB. Intellectual characteristics and performance mechatronics control machine. of components, systems and b5-2 )Assess and evaluate the characteristics of processes. mechatronics computer numerical Skills control machine. c24) Apply modern techniques, skills c24-1) Apply modern engineering tools and engineering tools. related to Mechatronics C- Professional Skills 5 Table of Contents d3) Communicate effectively. d3-1) Communicate effectively d7) Search for information and d7-1) Search for information about new engage in life-long self learning technologies of Mechatronics. D- General Skills discipline. d8) Acquire entrepreneurial skills. d8-1) Make contacts with the industrial companies to acquire entrepreneurial skills 4- Course Contents Introduction to Mechatronics - Fundamentals of Mechatronics - Mechatronics Components - Elictrical Drives -Mechatronics Applications - Case Study. 5- Teaching and Learning - Lectures Methods - Tutorials - Research assignments 6- Teaching and Learning Assign a portion of the office hours for those students. Methods for disable Arrange meetings for more discussion and declaration. students Give them specific tasks. Repeat the explanation of some of the material and tutorials. 7- Student Assessment a-Assessment - Weekly sheet exercises at class room Methods - Quizzes - Mid term, and final exams b- Assessment Schedule - Exercise sheet : Weekly - Quizz-1: Week no 5 - Mid-Term exam: Week no 8 - Quizz-2: Week no 10 - Final – term examination: Week no 14 6 Table of Contents c- Weighting of - Class tutorial and quizzes : 10 % Assessment - Mid-term examination: 10 % - Case study and/or practical exam: 40 % - Final – term examination: 40 % Total 100 % 8- List of text books and references: a- Course notes Lectures notes prepared in the form of a book authorized by the department b- Text books Clarence W. de Silva" Mechatronics: A Foundation" , McGraw-Hill, Jun 4, 2010 Musa Jouaneh "Fundamentals of Mechatronics' , Wiley and Sons, Jan 1, 2012 c- Recommended books W. Bolton Mechatronics " Electronic control systems in mechanical and electrical engineering", Taylor and Francis Group, (5th Edition) Feb 3, 2013 d- Periodicals, Web sites http://www.mechatronics.com/ ………etc 9-Course contents - ILOs Matrix Content Topics Week A- B- C- D- General Knowledge & Intellectual Professional and Understandin skills and practical transferable g skills skills Introduction to 1 a4 Mechatronics. Fundamentals of 2 a4 Mechatronics. Mechatronics 3 a4 c24 d3 Components. 7 Table of Contents Sensors 6 a8 d8 Signal Conditioner 7 a8 b5 d3 Actuators 9 a8 b5 d7 Ac and DC Motors 10 a8 c24 Stepper Motor 11 a8 b5 c24 d7 Mechatronics Control b5 Machine. 12-13 a8 d3 Case Study. 14 a8 b5 c24 d3 10-Teaching and Learning Methods - ILOs Matrix Teaching and A- Knowledge B- Intellectual C- Professional D- General and Learning Methods & skills and practical transferable Understanding skills skills Lectures a4, a8, b5 d3 Tutorials. a4, a8, b5 c24 Exercises b5 c24 Labs and/or case studies Reports and a4, a8 b5 c24 d3, d7, d8 assignments 11-Assessment Methods - ILOs Matrix Assessment Methods A- Knowledge B- Intellectual C- Professional D- General and & skills and practical transferable Understanding skills skills Weekly sheet a4, a8 b5 c24 exercises Reports c24 d3, d7, d8 Quizzes a4, a8 b5 Laboratory exam Midterm, and Final a4, a8 b5 Written exams 8 Table of Contents Chapter 1 Video link Introduction to Mechatronics Objectives: The objectives of this chapter are that, after studying it, the reader should be able to: Explain what is meant by mechatronics and appreciate its relevance in engineering design. Explain what is meant by a system and define the elements of measurement systems. Describe the various forms and elements of open-loop and closed-loop control systems. 1.1 What is mechatronics? The term mechatronics was ‘invented’ by a Japanese engineer in 1969, as a combination of ‘mecha’ from mechanisms and ‘tronics’ from electronics. The word now has a wider meaning, being used to describe a philosophy in engineering technology in which there is a co-ordinated, and concurrently developed, integration of mechanical engineering with electronics and intelligent computer control in the design and manufacture of products and processes. As a result, mechatronic products have many mechanical functions replaced with electronic ones. This results in much greater flexibility, easy redesign and reprogramming, and the ability to carry out automated data collection and reporting. A mechatronic system is not just a marriage of electrical and mechanical systems and is more than just a control system; it is a complete integration of all of them in which there is a concurrent approach to the design. The integration across the traditional boundaries of mechanical engineering, electrical engineering, electronics and control engineering has to occur at the earliest stages of the design process if cheaper, more reliable, more flexible 9 Table of Contents systems are to be developed. Mechatronics has to involve a concurrent approach to these disciplines rather than a sequential approach of developing, say, a mechanical system, then designing the electrical part and the microprocessor part. Thus mechatronics is a design philosophy, an integrating approach to engineering. The key elements of the mechatronics approach are presented in Figure 1.1. Even though the literature often adopts this concise representation, a clearer but more complex representation is shown in Figure 1.2. Mechatronics is the result of applying information systems to physical systems. The physical system (the rightmost dotted block of Figure 1.2) consists of mechanical, electrical, and computer systems as well as actuators, sensors, and real-time interfacing. Figure 1.1: Mechatronics constituents. 10 Table of Contents Figure 1.2: Mechatronics key elements. 1.2 The design process The design process for any system can be considered as involving a number of stages. 1. The need The design process begins with a need from, perhaps, a customer or client. This may be identified by market research being used to establish the needs of potential customers. 2. Analysis of the problem The first stage in developing a design is to find out the true nature of the problem, i.e. analysing it. This is an important stage in that not defining the problem accurately can lead to wasted time on designs that will not fulfil the need. 3. Preparation of a specification Following the analysis, a specification of the requirements can be prepared. This will state the problem, any constraints placed on the solution, and the criteria which may be used to judge the quality of the design. In stating the problem, all the functions required of the design, together with any desirable features, should be specified. Thus there might be a statement of mass, dimensions, types and range of motion required, 11 Table of Contents accuracy, input and output requirements of elements, interfaces, power requirements, operating environment, relevant standards and codes of practice, etc. 4. Generation of possible solutions This is often termed the conceptual stage. Outline solutions are prepared which are worked out in sufficient detail to indicate the means of obtaining each of the required functions, e.g. approximate sizes, shapes, materials and costs. It also means finding out what has been done before for similar problems; there is no sense in reinventing the wheel. 5. Selections of a suitable solution The various solutions are evaluated and the most suitable one selected. Evaluation will often involve the representation of a system by a model and then simulation to establish how it might react to inputs. 6. Production of a detailed design The detail of the selected design has now to be worked out. This might require the production of prototypes or mock-ups in order to determine the optimum details of a design. 7. Production of working drawings The selected design is then translated into working drawings, circuit diagrams, etc., so that the item can be made. It should not be considered that each stage of the design process just flows on stage by stage. There will often be the need to return to an earlier stage and give it further consideration. Thus when at the stage of generating possible solutions there might be a need to go back and reconsider the analysis of the problem. 12 Table of Contents 1.3 Systems In designing mechatronic systems, one of the steps involved is the creation of a model of the system so that predictions can be made regarding its behaviour when inputs occur. Such models involve drawing block diagrams to represent systems. A system can be thought of as a box or block diagram goes on inside the box but with only the relationship between the output and the input. The term modelling is used when we represent the behavior of a real system by mathematical equations, such equations representing the relationship between the inputs and outputs from the system. For example, a spring can be considered as a system to have an input of a force F and an output of an extension x (Figure 1.3(a)). The equation used to model the relationship between the input and output might be F = k x , where k is a constant. As another example, a motor may be thought of as a system which has as its input electric power and as output the rotation of a shaft (Figure 1.3(b)). A measurement system can be thought of as a box which is used for making measurements. It has as its input the quantity being measured and its output the value of that quantity. For example, a temperature measurement system, i.e. a thermometer, has an input of temperature and an output of a number on a scale (Figure 1.3(c)). 13 Table of Contents Figure 1.3: Examples of systems: (a) spring, (b) motor, (c) thermometer. 1.3.1 Modelling systems The response of any system to an input is not instantaneous. For example, for the spring system described by Figure 1.3(a), though the relationship between the input, force F , and output, extension x , was given as F = k x , this only describes the relationship when steady-state conditions occur. When the force is applied it is likely that oscillations will occur before the spring settles down to its steady-state extension value (Figure 1.4). The responses of systems are functions of time. Thus, in order to know how systems behave when there are inputs to them, we need to devise models for systems which relate the output to the input so that we can work out, for a given input, how the output will vary with time and what it will settle down to. 14 Table of Contents Figure 1.4: The response to an input for a spring. As another example, if you switch on a kettle it takes some time for the water in the kettle to reach boiling point (Figure 1.5). Likewise, when a microprocessor controller gives a signal to, say, move the lens for focusing in an automatic camera then it takes time before the lens reaches its position for correct focusing. Figure 1.5: The response to an input for a kettle system. 1.3.2 Connected systems In other than the simplest system, it is generally useful to consider it as a series of interconnected blocks, each such block having a specific function. We then have the output from one block becoming the input to the next block in the system. In drawing a system in 15 Table of Contents this way, it is necessary to recognize that lines drawn to connect boxes indicate a flow of information in the direction indicated by an arrow and not necessarily physical connections. An example of such a connected system is a CD player. We can think of there being three interconnected blocks: the CD deck which has an input of a CD and an output of electrical signals; an amplifier which has an input of these electrical signals, and an output of bigger electrical signals; and a speaker which has an input of the electrical signals and an output of sound (Figure 1.6). Figure 1.6: A CD player. 1.4 Measurement systems Of particular importance in any discussion of mechatronics are measurement systems. Measurement systems can, in general, be considered to be made up of three basic elements (as illustrated in Figure 1.7). 1. A sensor responds to the quantity being measured by giving as its output a signal which is related to the quantity. For example, a thermocouple is a temperature sensor. The input to the sensor is a temperature and the output is an e.m.f., which is related to the temperature value. 2. A signal conditioner takes the signal from the sensor and manipulates it into a condition which is suitable either for display or, in the case of a control system, for use to exercise control. Thus, for example, the output from a 16 Table of Contents thermocouple is a rather small e.m.f. and might be fed through an amplifier to obtain a bigger signal. The amplifier is the signal conditioner. 3. A display system displays the output from the signal conditioner. This might, for example, be a pointer moving across a scale or a digital readout. Figure 1.7: A measurement system and its constituent elements. As an example, consider a digital thermometer (Figure 1.8). This has an input of temperature to a sensor, probably a semiconductor diode. The potential difference across the sensor is, at constant current, a measure of the temperature. This potential difference is then amplified by an operational amplifier to give a voltage which can directly drive a display. The sensor and operational amplifier may be incorporated on the same silicon chip. Figure 1.8: A digital thermometer system. 1.5 Control systems A control system can be thought of as a system which can be used to: 1. control some variable to some particular value, e.g. a central heating system where the temperature is controlled to a particular value; 17 Table of Contents 2. control the sequence of events, e.g. a washing machine where when the dials are set to, say, ‘white’ and the machine is then controlled to a particular washing cycle, i.e. sequence of events, appropriate to that type of clothing; 3. control whether an event occurs or not, e.g. a safety lock on a machine where it cannot be operated until a guard is in position. 1.5.1 Feedback Consider an example of a control system with which we are all individually involved. Your body temperature, unless you are ill, remains almost constant regardless of whether you are in a cold or hot environment. To maintain this constancy your body has a temperature control system. If your temperature begins to increase above the normal you sweat, if it decreases you shiver. Both these are mechanisms which are used to restore the body temperature back to its normal value. The control system is maintaining constancy of temperature. The system has an input from sensors which tell it what the temperature is and then compare this data with what the temperature should be and provide the appropriate response in order to obtain the required temperature. This is an example of feedback control: signals are fed back from the output, i.e. the actual temperature, in order to modify the reaction of the body to enable it to restore the temperature to the ‘normal’ value. Feedback control is exercised by the control system comparing the fed-back actual output of the system with what is required and adjusting its output accordingly. Figure 1.9(a) illustrates this feedback control system. One way to control the temperature of a centrally heated house is for a human to stand near the furnace on/off switch with a thermometer and switch the furnace on or off according to the thermometer reading. That is a crude form of feedback control using a human as a control element. The term feedback is used because signals are fed back from the output in order to modify the input. The more usual feedback control system has a 18 Table of Contents thermostat or controller which automatically switches the furnace on or off according to the difference between the set temperature and the actual temperature (Figure 1.9(b)). This control system is maintaining constancy of temperature. Figure 1.9: Feedback control: (a) human body temperature, (b) room temperature with central heating, (c) picking up a pencil. If you go to pick up a pencil from a bench there is a need for you to use a control system to ensure that your hand actually ends up at the pencil. This is done by your observing the position of your hand relative to the pencil and making adjustments in its position as it moves towards the pencil. There is a feedback of information about your actual hand position so that you can modify your reactions to give the required hand position and 19 Table of Contents movement (Figure 1.9(c)). This control system is controlling the positioning and movement of your hand. Feedback control systems are widespread, not only in nature and the home but also in industry. There are many industrial processes and machines where control, whether by humans or automatically, is required. For example, there is process control where such things as temperature, liquid level, fluid flow, pressure, etc., are maintained constant. Thus in a chemical process there may be a need to maintain the level of a liquid in a tank to a particular level or to a particular temperature. There are also control systems which involve consistently and accurately positioning a moving part or maintaining a constant speed. This might be, for example, a motor designed to run at a constant speed or perhaps a machining operation in which the position, speed and operation of a tool are automatically controlled. 1.5.2 Open- and closed-loop systems There are two basic forms of control system, one being called open loop and the other closed loop. The difference between these can be illustrated by a simple example. Consider an electric fire which has a selection switch which allows a 1 kW or a 2 kW heating element to be selected. If a person used the heating element to heat a room, he or she might just switch on the 1 kW element if the room is not required to be at too high a temperature. The room will heat up and reach a temperature which is only determined by the fact that the 1 kW element was switched on and not the 2 kW element. If there are changes in the conditions, perhaps someone opening a window, there is no way the heat output is adjusted to compensate. This is an example of open loop control in that there is no information fed back to the element to adjust it and maintain a constant temperature. The heating system with the heating element could be made a closed-loop system if the person has a thermometer and switches the 1 kW and 2 kW elements on or off, according 20 Table of Contents to the difference between the actual temperature and the required temperature, to maintain the temperature of the room constant. In this situation there is feedback, the input to the system being adjusted according to whether its output is the required temperature. This means that the input to the switch depends on the deviation of the actual temperature from the required temperature, the difference between them being determined by a comparison element – the person in this case. Figure 1.10 illustrates these two types of system. Figure 1.10: Heating a room: (a) an open-loop system, (b) a closed-loop system. An example of an everyday open-loop control system is the domestic toaster. Control is exercised by setting a timer which determines the length of time for which the bread is toasted. The brownness of the resulting toast is determined solely by this preset time. There is no feedback to control the degree of browning to a required brownness. To illustrate further the differences between open- and closed-loop systems, consider a motor. With an open-loop system the speed of rotation of the shaft might be determined solely by the initial setting of a knob which affects the voltage applied to the motor. Any 21 Table of Contents changes in the supply voltage, the characteristics of the motor as a result of temperature changes, or the shaft load will change the shaft speed but not be compensated for. There is no feedback loop. With a closed-loop system, however, the initial setting of the control knob will be for a particular shaft speed and this will be maintained by feedback, regardless of any changes in supply voltage, motor characteristics or load. In an open-loop control system the output from the system has no effect on the input signal. In a closed-loop control system the output does have an effect on the input signal, modifying it to maintain an output signal at the required value. Open-loop systems have the advantage of being relatively simple and consequently low cost with generally good reliability. However, they are often inaccurate since there is no correction for error. Closed-loop systems have the advantage of being relatively accurate in matching the actual to the required values. They are, however, more complex and so more costly with a greater chance of breakdown as a consequence of the greater number of components. 1.5.3 Basic elements of a closed-loop system Figure 1.11 shows the general form of a basic closed-loop system. It consists of five elements. 1. Comparison element This compares the required or reference value of the variable condition being controlled with the measured value of what is being achieved and produces an error signal. It can be regarded as adding the reference signal, which is positive, to the measured value signal, which is negative in this case: error signal = reference value signal - measured value signal 22 Table of Contents The symbol used, in general, for an element at which signals are summed is a segmented circle, inputs going into segments. The inputs are all added, hence the feedback input is marked as negative and the reference signal positive so that the sum gives the difference between the signals. A feedback loop is a means whereby a signal related to the actual condition being achieved is fed back to modify the input signal to a process. The feedback is said to be negative feedback when the signal which is fed back subtracts from the input value. It is negative feedback that is required to control a system. Positive feedback occurs when the signal fed back adds to the input signal. Figure 1.11: The elements of a closed-loop control system. 2. Control element This decides what action to take when it receives an error signal. It may be, for example, a signal to operate a switch or open a valve. The control plan being used by the element may be just to supply a signal which switches on or off when there is an error, as in a room thermostat, or perhaps a signal which proportionally opens or closes a valve according to the size of the error. Control plans may be hard- wired systems in which the control plan is permanently fixed by the way the 23 Table of Contents elements are connected together, or programmable systems where the control plan is stored within a memory unit and may be altered by reprogramming it. 3. Correction element The correction element produces a change in the process to correct or change the controlled condition. Thus it might be a switch which switches on a heater and so increases the temperature of the process or a valve which opens and allows more liquid to enter the process. The term actuator is used for the element of a correction unit that provides the power to carry out the control action. 4. Process element The process is what is being controlled. It could be a room in a house with its temperature being controlled or a tank of water with its level being controlled. 5. Measurement element The measurement element produces a signal related to the variable condition of the process that is being controlled. It might be, for example, a switch which is switched on when a particular position is reached or a thermocouple which gives an e.m.f. related to the temperature. With the closed-loop system illustrated in Figure 1.11 for a person controlling the temperature of a room, the various elements are: Controlled variable – the room temperature Reference value – the required room temperature Comparison element – the person comparing the measured value with the required value of temperature 24 Table of Contents Error signal – the difference between the measured and required temperatures Control unit – the person Correction unit – the switch on the fire Process – the heating by the fire Measuring device – a thermometer An automatic control system for the control of the room temperature could involve a thermostatic element which is sensitive to temperature and switches on when the temperature falls below the set value and off when it reaches it (Figure 1.12). This temperature-sensitive switch is then used to switch on the heater. The thermostatic element has the combined functions of comparing the required temperature value with that occurring and then controlling the operation of a switch. It is often the case that elements in control systems are able to combine a number of functions. Figure 1.12: Heating a room: a closed-loop system. 1.5.4 Analogue and digital control systems Analogue systems are ones where all the signals are continuous functions of time and it is the size of the signal which is a measure of the variable. The examples so far discussed 25 Table of Contents in this chapter are such systems. Digital signals can be considered to be a sequence of on/off signals, the value of the variable being represented by the sequence of on/off pulses. Where a digital signal is used to represent a continuous analogue signal, the analogue signal is sampled at particular instants of time and the sample values each then converted into effectively a digital number, i.e. a particular sequence of digital signals. Because most of the situations being controlled are analogue in nature and it is these that are the inputs and outputs of control systems, e.g. an input of temperature and an output from a heater, a necessary feature of a digital control system is that the real-world analogue inputs have to be converted to digital forms and the digital outputs back to real-world analogue forms. This involves the uses of analogue-to-digital converters (ADC) for inputs and digital-to-analogue converters (DAC) for the outputs. Figure 1.13(a) shows the basic elements of a digital closed-loop control system; compare it with the analogue closed-loop system in Figure 1.11. The reference value, or set point, might be an input from a keyboard. Analogue to- digital (ADC) and digital-to- analogue (DAC) elements are included in the loop in order that the digital controller can be supplied with digital signals from analogue measurement systems and its output of digital signals can be converted to analogue form to operate the correction units. It might seem to be adding a degree of complexity to the control system to have this ADC and DAC, but there are some very important advantages: digital operations can be controlled by a program, i.e. a set of stored instructions; information storage is easier, accuracy can be greater; digital circuits are less affected by noise and also are generally easier to design. The digital controller could be a digital computer which is running a program, i.e. a piece of software, to implement the required actions. The term control algorithm is used to 26 Table of Contents describe the sequence of steps needed to solve the control problem. The control algorithm that might be used for digital control could be described by the following steps: Read the reference value, i.e. the desired value. Read the actual plant output from the ADC. Calculate the error signal. Calculate the required controller output. Send the controller output to the DAC. Wait for the next sampling interval. 27 Table of Contents Figure 1.13: (a) The basic elements of a digital closed-loop control system, (b) a microcontroller control system. However, many applications do not need the expense of a computer and just a microchip will suffice. Thus, in mechatronics applications a microcontroller is often used for digital control. A microcontroller is a microprocessor with added integrated elements such as memory and ADC and DAC converters; these can be connected directly to the plant being controlled so the arrangement could be as shown in Figure 1.13(b). The control algorithm then might be: Read the reference value, i.e. the desired value. Read the actual plant output to its ADC input port. Calculate the error signal. Calculate the required controller output. Send the controller output to its DAC output port. Wait for the next sampling interval. 28 Table of Contents An example of a digital control system might be an automatic control system for the control of the room temperature involving a temperature sensor giving an analogue signal which, after suitable signal conditioning to make it a digital signal, is inputted to the digital controller where it is compared with the set value and an error signal generated. This is then acted on by the digital controller to give at its output a digital signal which, after suitable signal conditioning to give an analogue equivalent, might be used to control a heater and hence the room temperature. Such a system can readily be programmed to give different temperatures at different times of the day. 1.5.5 Sequential controllers There are many situations where control is exercised by items being switched on or off at particular preset times or values in order to control processes and give a step sequence of operations. For example, after step 1 is complete then step 2 starts. When step 2 is complete then step 3 starts, etc. The term sequential control is used when control is such that actions are strictly ordered in a time- or event-driven sequence. Such control could be obtained by an electric circuit with sets of relays or cam-operated switches which are wired up in such a way as to give the required sequence. Such hard-wired circuits are now more likely to have been replaced by a microprocessor-controlled system, with the sequencing being controlled by means of a software program. Problems 1. Identify the sensor, signal conditioner and display elements in the measurement systems. 2. What are the key elements of mechatronic systems. 29 Table of Contents 3. Explain the difference between open- and closed-loop control. 4. Identify the various elements that might be present in a control system involving a thermostatically controlled electric heater. 5. Explain what is meant by sequential control. Chapter 2 Video link Sensors and Transducers Objectives: 30 Table of Contents The objectives of this chapter are that, after studying it, the reader should be able to: Describe the performance of commonly used sensors. Describe the sensor classification. Evaluate sensors used in the measurement of motion, position, force, flow and temperature. 2.1 Introduction Instrumentation plays a key role in the modern technological world. An essential component in mechatronic systems which is integrally linked to instrumentation is the sensor, whose function is to provide a mechanism for collecting different types of information about a particular process. Sensors are used to inspect work, evaluate the conditions of work under progress, and facilitate the higher-level monitoring of the manufacturing operation by the main computing system. They can be used during pre- process, in-process, and post-process operations. In some situations, sensors are used to translate a physical phenomenon into an acceptable signal that can be analyzed for decision making. Intelligent systems use sensors to monitor situations influenced by a changing environment and to control them with corrective actions. In virtually every application, sensors transform real-world data into electrical signals. A sensor is defined as: a device that produces an output signal for the purpose of sensing of a physical phenomenon. Sensors are also referred to as transducers. They cover a broader range of activities, which provide them with the ability to identify environmental inputs that can extend beyond the human senses. A transducer is defined as: a device that converts a signal from one physical form to a corresponding signal, which has a different physical form. In a transducer, the quantities at the input level and the output level are 31 Table of Contents different. A typical input signal could be electrical, mechanical, thermal, and optical. Signal detection is normally handled by electrical transducers in manufacturing industries involving certain process automation. A transducer is an element or device used to convert information from one form to another. The change in information is measured easily. A spring is a simple example of a transducer. When a certain force is applied to a spring, it stretches, and the force information is translated to displacement information, as shown in Figure 2-1. Different quantities of force produce differential movements, which are a measure of the force. Displacement y is proportional to force F, which can be expressed as Figure (2.1) : A mass-spring system. 𝑭 = 𝑲. 𝒚 (3.1) where 𝑲 is constant, 𝑭 = applied force, 𝒚 = deflection 32 Table of Contents 2.2 Introduction to Sensors and Transducers The extent to which sensors and transducers are used is dependent upon the level of automation and the complexity of the control system. The modeling requirements of the complex control systems have introduced a need for fast, sensitive, and precise measuring devices. Due to these demands, sensors are being miniaturized and implemented in a microscale by combining several sensors and the entire unit can be contained in a silicon chip of the size less than 0.5 × 0.5 mm. Selection of a sensor or a transducer depends on Variables measured and application. The nature of precision and the sensitivity required for the measurement. Dynamic range. Level of automation. Complexity of the control system and modeling requirements. Cost, size, usage, and ease of maintenance. Two important components in modern control systems (whether electrical, optical, mechanical, or fluid) are the system’s sensors and transducers. The sensor elements detect measurands (variables to be measured) and convert them into acceptable form, generally as electrical signals. The maximum accuracy of the total system is controlled by the sensitivity of the individual sensors and the internally generated noise of the sensor itself. In a control system used for measurement and control, any parameter changes either in measured (variables to be measured) or in signal conditioning, has a direct effect on the sensitivity of the model. Figure 2-2 shows elements of a sensor-based measurement system. The function of the sensor is to sense the information of interest and to convert this information into an acceptable form by a signal conditioner. The function of the signal conditioner is to accept 33 Table of Contents the signal from the detector and to modify in a way acceptable to the display unit. The function of the display-readout is to accept the signal from the signal conditioner and to present it in a displayable fashion. The output can be in the form of an output display, or a printer, or it may be passed on to a controller. It also can be manipulated and fed back to the source from which the original signal was measured. Figure (2.2) : Elements of sensor-based measurement system. Figure 2-3 presents the components of an instrumentation system used for a general sensing application. A typical system consists of primary elements that sense and convert information into a more suitable form to be handled by the measurement system, signal conditioning stage for processing and modifying the information, an input/output stage for interface, and control with the external processes. 34 Table of Contents Figure (2.3): Compenenets of instrumentation system. 2.2.1 Sensor Classification In the design of a mechatronics system, selection of a suitable sensor is very important. Table 2-1 summarizes some general sensor classifications. Sensors are classified into two categories based on the output signal, power supply, operating mode and the variables being measured. Analog sensors: Analog is a term used to convey the meaning of a continuous, uninterrupted, and unbroken series of events. Analog sensors typically have an output, which is proportional to the variable being measured. The output changes in a continuous way, and this information is obtained based on amplitude. The output is normally supplied to the computer using an analog-to-digital converter. Digital sensors: Digital refers to a sequence of discrete events. Each event is separate from the previous and next events. The sensors are digital if their logic-level outputs are of a digital nature. Digital sensors are known for their accuracy and precision, and do not require any converters when interfaced with a computer monitoring system. 35 Table of Contents Another form of classification, active or passive, is based on the power supply. Active sensors: Active sensors require external power for their operation. The external signal is modified by the sensor to produce the output signal. Typical examples of devices requiring an auxiliary energy source are strain gauges and resistance thermometers. Passive sensors: In a passive sensor, the output is produced from the input parameters. The passive sensors (self-generating) produce an electrical signal in response to an external stimulus. Examples of passive types of sensors include piezoelectric, thermoelectric, and radioactive. Table (2.1) Some general sensor classifications. 36 Table of Contents Based on the operating and display mode of an instrumentation system, sensors are classified as deflection type or null type. Deflection sensors: Deflection sensors are used in a physical setup where the output is proportional to the measured quantity that is displayed. Null sensors: In null-type sensing, any deflection due to the measured quantity is balanced by the opposing calibrated force so that any imbalance is detected. A final classification of sensors is based on the subject of measurement. Such subjects include acoustic, biological, chemical, electric, magnetic, mechanical, optical, radiation, thermal, and others. 2.2.2 Parameter Measurement in Sensors and Transducers Let us examine the instrumentation system model from the viewpoint of its functional elements in a generalized way. The elements contribute to the sensing and measurement of an instrumentation system and influence the quality of the device. Figure 2-4 shows a block diagram of elements of a typical instrumentation system. The basic subsystems include the following modules. Sensing module Conversion module Variable manipulation module Data transmission Presentation module 37 Table of Contents Figure (2.4) : Block diagram of elements of a typical instrumentation system. The integrated effect of all the functional modules results in a useful measurement system. A description of each module is given here. 1) Sensing Module: The first element to receive a signal from the measured medium and produces an output depending on the measured quantity. During the process of sensing, some energy gets extracted from the measured medium. In fact, the measured quantity gets disturbed by the act of measurement, making a perfect measurement theoretically impossible. Good instruments are normally designed to minimize the error of measurement. 2) Conversion Module: Converts one physical variable to another. It is also known as a transducing element. In certain cases, the transduction of the input signal may take place progressively in stages, such as primary, secondary, and tertiary transduction. 3) Variable Manipulation Module: Usually, this involves signal conditioning. Some examples of variable manipulation element are amplifiers, linkage mechanisms, gearboxes, magnifiers, etc. An electronic amplifier accepts a small voltage signal as an input signal and generates a signal that is many times, larger than the input signal. 38 Table of Contents 4) Data Transmission Module: This sends a signal from one point to another point. For example, the transmission element could be a simple device such as a shaft and bearing assembly or could be a complicated device, such as a telemetry system for transmitting signals from ground to satellites. 5) Data Display Module: Produces information about the measured quantity in a form that can be recognized by one of the human senses. Example 1: Home Heating System The functional elements of a typical home heating system are shown in Figure 2-5. Solution The block diagram represents the six major system components and their interconnections. The interconnections completely define the inputs and outputs for each of the six major blocks. For instance, the thermostat block processes two input signals (a room temperature and a temperature set point,) to produce one output signal, which is sent to a mechanical relay switch. The thermostat acts as a primary sensor and transducer. Figure (2.5) Functional elements of a typical home heating system. Example 2: Pressure Sensor An example of a pressure sensor in the form of a spring-loaded piston and a display mechanism is shown in Figure 2-6. This pressure sensing instrument can be broken down into functional elements. The source is connected to a pneumatic cylinder. The pressure acts 39 Table of Contents on the piston and spring mechanism. The spring works as a primary sensor and variable conversion element. The deflection of the spring is transferred to the display as a movement of the dial indicator. Figure (2.6) An example of a pressure sensor. 2.2.3 Quality Parameters Sensors and transducers are often used under different environmental conditions. Like human beings, they are sensitive to environmental inputs such as pressure, motion, temperature, radiation, and magnetic fields. Sensor characteristics are described in terms of seven properties discussed and illustrated in the following subsections. Sensitivity Resolution Accuracy Precision Backlash Repeatability Linearity 1) Sensitivity: Sensitivity is the property of the measuring instrument to respond to changes in the measured quantity. It also can be expressed as the ratio of change of output to change of input as shown in Figures 2-7 and 2-8. Sensitivity is measured by: 40 Table of Contents ∆𝑂 𝑺= (2.2) ∆𝐼 where S is the sensitivity, ∆𝑂 represents change in output, and ∆𝐼 represents the change in input. For example, in an electrical measuring instrument if a movement of 0.001 mm causes an output voltage change of 0.02 V, the sensitivity of the measuring instrument is 0.02 𝑆= = 20 𝑉/𝑚𝑚 0.001 Figure (3.7) Figure (3.8) ‫ناحتما‬ 2) Resolution: Resolution is defined as the smallest increment in the measured value that can be detected. It is also known as the degree of fineness with which measurements can be made. For example, if a micrometer with a minimum graduation of 1 mm is used to measure to the nearest 0.5mm, then by interpolation, the resolution is estimated as 0.5 mm. 41 Table of Contents 3) Accuracy: Accuracy is a measure of the difference between the measured value and actual value. Accuracy depends on the inherent instrument limitations. An experiment is said to be accurate if it is unaffected by experimental error. An accuracy of ± 0.001 means that the measured value is within 0.001 units of actual value. In practice, the accuracy is defined as a percentage of the true value. 𝑚𝑒𝑎𝑠𝑢𝑟𝑒𝑑 𝑣𝑎𝑙𝑢𝑒 − 𝑡𝑟𝑢𝑒 𝑣𝑎𝑙𝑢𝑒 𝑝𝑒𝑟𝑒𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑡𝑟𝑢𝑒 𝑣𝑎𝑙𝑢𝑒 = × 100% (𝟐. 𝟑) 𝑡𝑟𝑢𝑒 𝑣𝑎𝑙𝑢𝑒 If a precision balance reads 1 g with error of 0.001 g, then the accuracy of the instrument is specified as 0.1%. The difference between the measured value and true value is called bias (error). 4) Precision: Precision is the ability of an instrument to reproduce a certain set of readings within a given accuracy. Precision is dependent on the reliability of the instrument. Example 3: Target Shooting Figure 2-9 presents an illustration of degree of accuracy and precision in a typical target-shooting example. Solution The “high precision, poor accuracy” situation occurs when the person hits all the bullets on a target plate on the outer circle and misses the bull’s eye. In the second case, “high accuracy, high precision”, all the bullets hit the bull’s eye and are spaced closely enough. In the third example, “good accuracy, poor precision”, the bullet hits are placed symmetrically with respect to the bull’s eye but are spaced apart. In the last case, “poor accuracy, poor precision”, the bullets hit the target in a random order. 42 Table of Contents Figure (2.9) Degree of accuracy and precision in a typical target-shooting. 5) Backlash: Backlash is defined as the maximum distance or angle through which any part of a mechanical system can be moved in one direction without causing any motion of the attached part. Backlash is an undesirable phenomenon and is important in the precision design of gear trains. 6) Repeatability: Repeatability is the ability to reproduce the output signal exactly when the same measured is applied repeatedly under the same environmental conditions. 7) Linearity: The characteristics of precision instruments are that the output is a linear function of the input. However, linearity is never completely achieved, and the deviations from the ideal are termed linearity tolerances. The linearity is expressed as the percentage of departure from the linear value (i.e., maximum deviation of the output curve from the best-fit straight line during a calibration cycle). The nonlinearity is normally caused by nonlinear elements, such as mechanical hysteresis, viscous flow or creep, and electronic amplifiers. 2.2.4 Errors and Uncertainties in Mechatronic Modeling Parameters Modern mechatronic technology relies heavily on the use of sensors and measurement technology. The control of industrial processes and automated systems would 43 Table of Contents be very difficult without accurate sensors and measurement systems. The economical production of a mechatronic instrument requires the proper choice of sensors, material, and hardware and software design. To a large degree, the final choice of an instrument for any particular application depends upon the accuracy desired. If a low degree of accuracy is acceptable, it is not economical to use expensive sensors and precise sensing components. If, however, the instrument is used for high-precision applications, the design tolerances must be small. Any system which relies on a measurement system will involve some amount of uncertainty. The uncertainty may be caused by the individual inaccuracy of sensors, random variations in measurands, or environmental conditions. The accuracy of the total system depends on the interaction of the components and their individual accuracy. This is true for measurement instruments as well as production systems, which depend on many subsystems and components. A typical instrument may consist of many components that have complex interrelations, and each component may contribute to the overall error. The errors and inaccuracies in each of these components can have a large cumulative effect. 2.3 Sensors for Motion and Position Measurement An integrated manufacturing environment typically consists of Machining centers/manufacturing cells Inspection stations Material handling Devices Packaging centers Areas where the raw material and finished products are handled The integrated system monitors the environment to understand the progress of the product in the production scheme. The sensors interact with the controllers and provide a 44 Table of Contents detailed account of status of the process as well as environmental conditions. The controller sends signals to the actuators, which respond according to the functions. Sensor-based manufacturing systems consist of data measurement by a plurality of sensors, sensor integration, signal processing, and pattern recognition. Motion measurement (especially the measure of displacement, position, and velocity of physical objects) is essential for many feedback control applications (especially those used in robotics, process, and automotive industries). Motion transducers are a class of transducers used for the measurement of mechanical quantities that include: Displacement Force Pressure Flow rate Temperature Primary and Secondary Transducers Sometimes the transducer measures one phenomenon to measure another variable. The primary transducer senses the preliminary data and converts it into another form, which is again converted into some usable form by a secondary transducer. As an example, measurement of force is performed using a spring element, and the resulting displacement of the spring is measured using another electrical transducer. The force causes the spring to extend, and the mechanical displacement is proportional to the force. The spring is the primary transducer, which converts force into displacement. The end of the spring is connected to another electrical transducer, which senses its displacement and transmits it as an electrical signal. This electrical transducer is called a secondary transducer. In most measurement systems, it is common to have such combinations of transducer elements in which a primary transducer is the mechanical element, and an electrical transducer (acting in the secondary stage) is the secondary unit. 45 Table of Contents Selection Criteria for a Transducer The range of the measurement Suitability of the transducer for such measurement Required resolution Material of the measured object Available space Environmental conditions Power available for sensing Cost Production volume Transducers of the electrical, electromechanical, optical, pneumatic, and piezoelectric types are commonly used in motion measurement. Transducer Classification Based on the Principle of Transduction Potentiometric: Potentiometric transducers apply the principle of change in resistance of material in the sensor. Capacitance: Capacitance transducers apply the principle of capacitance variation between a set of plate assemblies. Inductance: Inductance transducers are based on the principle of variation of inductance by the insertion of core material into an inductor. Inductance variations serve as a measure of displacement. Piezoelectric: Piezoelectric transducers are based on the principle of charge generation. 46 Table of Contents Whenever certain piezoelectric crystals are subjected to mechanical motion, an electric voltage is induced. This effect can be reversed by applying an electric voltage and deforming the crystal. 2.3.1 Resistance Transducers Potentiometric Principle: A displacement transducer using variable resistance transduction principle can be manufactured with a rotary or linear potentiometer. A potentiometer is a transducer in which a rotation or displacement is converted into a potential difference. As shown in Figure 2-10, the displacement of the wiper of a potentiometer causes the output potential difference obtained between one end of the resistance and the slider. This device converts linear or angular motion into changing resistance, which may be converted directly to a voltage or current signal. The position of the slider along the resistance element determines the magnitude of the electrical potential. The voltage across the wiper of linear potentiometer is measured in terms of the displacement, d, and given by the relationship: 𝑑 𝑉=𝐸𝐿 (𝟐. 𝟒) Here E is the voltage across the potentiometer, and L is the full-scale displacement of the potentiometer. 47 Table of Contents Figure (2.10): Potentiometer transducer principle. If the movement of the slider is in a circular path along a resistance element, then rotational information is converted into information in the form of a potential difference. The output of the rotary transducer is proportional to the angular movement. If there is any loading effect from the output terminal, the linear relationship between the wiper position and the output voltage will change. The error, which is called the loading error, is caused by the input impedance of the output devices. To reduce the loading error, a voltage source, which is not seriously affected by load variations (e.g., stabilized power source) and signal- conditioning circuitry with high-input impedance should be used. It is also advisable to isolate the wiper of the potentiometer from the sensing shaft. The disadvantage of the potentiometric transducer is its slow dynamic performance, low resolution, and susceptibility to vibration and noise. However, displacement transducers with a relatively small traverse length have been designed using strain-gauge-type resistance transducers. Features 48 Table of Contents Linear potentiometers are often considered when an electrical signal proportional to displacement is required, but also where cost should be kept low and high accuracy is not critical. Typical rotary potentiometers have a range of_170°. Their linearity varies from 0.01 to 1.5%. Applications Used for position monitoring of products on assembly lines and checking dimensions of the product in quality control systems. Rotary potentiometers are used in applications involving rotational measurement for applications ranging from machine tools to aircraft. 2.4 Force Sensors Mechatronic systems in automated manufacturing environments require extensive environmental information to make intelligent decisions. Such information relates to the tasks of material handling, machining, inspection, assembly, painting, etc. Assembly tasks and automated handling tasks require controlled operations like grasping, turning, inserting, aligning, orienting, and screwing. Every situation has somewhat different sensing requirements. This section discusses some of the techniques used for force and torque sensing. A precise measurement of strain is an important consideration in measurement. Strain measurement is used as a secondary step in the measurement of many process variables, including flow, pressure, weight, and acceleration. Electrical-resistance strain gauges are widely used to measure strains due to force or torque. When a force is applied to a structure, it undergoes deformation. The gauge, which is bonded to the structure, is deformed by strain, and its electrical-resistance changes in a nearly linear fashion. If a piece of metal wire 49 Table of Contents is stretched, not only does it get longer and thinner, but its resistance increases. The greater the strain experienced by the wire, the greater is the change in resistance. The resistance, R, of a resistance wire depends on its area, length, and electrical resistivity. 𝒍 𝑹=𝝆 (2.5) 𝑨 where 𝝆 = resistivity, Ω m 𝑹 = sample resistance, Ω 𝒍 = length, m 𝑨 = cross-sectional area, 𝑚2 Sensitivity or gauge factor, 𝐺𝑓 is defined as the ratio of unit change in resistance to unit change in length. 𝚫𝑹/𝑹 𝑮𝒇 = (2.6) 𝚫𝒍/𝒍 2.4.1 Strain Gauges A resistance strain gauge consists of a grid of fine resistance wire of about 20 m in diameter. The elements are formed on a backing film of electrically insulating material. Current strain gauges are manufactured from constantan foil, a copper-nickel alloy, or single-crystal semiconductor materials. The gauges are formed either mechanically or by photochemical etching. Strain-gauge transducers are of two types: unbonded and bonded. Unbonded Strain Gauges: In an unbonded strain gauge (Figure 2-11(a)), the resistance wire is stressed between the two frames. The first frame is called the fixed frame, and the second is called a moving frame. The wires in the unbonded gauges are connected such that the 50 Table of Contents input motion of one frame stretches one set of wires and compresses another set of wires. As an example, a 20 µm diameter wire is wound between insulated pins with one attached to a stationary frame and the other to a movable frame. For a particular stress input, the winding experiences either an increase or decrease in stress, resulting in a change in resistance. The output is connected to a Wheatstone bridge for measurement. With this type of strain gauge, measurement of small motions as small as a few microns can be made. Bonded Strain Gauge Bonded strain-gauge transducers are widely used for measuring strain, force, torque, pressure, and vibration. The gauges have a backing material. Bonded strain gauges (Figure 2-11(b)) are made of metallic or semiconductor materials in the form of a wire gauge or thin metal foil. When the gauges are bonded to the surface, they undergo the same strain as that of the member surface. The coefficient of thermal expansion of the backing material should be matched to that of the wire. Strain gauges are sensitive devices and are used with an electronic measuring unit. The strain gauge is normally made part of a Wheatstone bridge, so the change in its resistance due to strain either can be measured or used to produce an output, which can be displayed. Strains as low as a fraction of a micron can be measured using strain gauges. Table 2-3 presents characteristics of bonded strain gauges. For precise measurement, the strain gauges should have the following properties. A high gauge factor increases the sensitivity and causes a larger change in resistance for a particular strain. The gauge characteristics are chosen so that the variation in resistance is a linear function of strain. If the gauges are used for dynamic measurements, the linearity should be maintained over the desired frequency range. High resistance of the strain gauge minimizes the effect of resistance variation in the signal-processing circuitry. 51 Table of Contents Strain gauges have a low temperature coefficient and absence of the hysteresis effect. Figure (2.11) Strain Gauge. Table (2.3) Characteristics of bonded strain gauges. 52 Table of Contents Bridge Circuit Arrangement: The Wheatstone bridge circuit is used to measure the small changes in resistance that result in most strain-gauge applications. The change in resistance either can be measured or provided as an output that is processed by the computer. Figure 2-12 shows an arrangement of a bridge circuit. In the balanced bridge arrangement, strain- gauge resistance, 𝑅1 , forms one arm of the Wheatstone bridge, while the remaining arms have resistances 𝑅2 , 𝑅3 , and 𝑅4. Between the points A and C of the bridge, there is a power supply; between points B and D, there is a precision galvanometer. The galvanometer gives an indication of the presence of current through that leg. For zero current to flow through the galvanometer, the points B and D must be at the same potential. The bridge is excited by the direct current source with voltage, V and 𝑅𝑞 is the resistance in the galvanometer. The bridge is said to be balanced when there is no current flowing through the galvanometer. The condition of balance is: 𝑅1 𝑅2 = (2.7) 𝑅4 𝑅3 If 𝑅1 changes due to strain, the bridge (which is initially in the balanced condition) becomes unbalanced. This may be balanced by changing 𝑅4 or 𝑅2. The change can be measured and used to indicate the change in 𝑅1. This procedure is useful for measuring static strains. 53 Table of Contents Figure (2.12): Bridge circuit with strain gauge. Features of strain Gauge Strain gauges should have the following features: A high gauge factor increases its sensitivity and causes a larger change in resistance for a particular strain. High resistance of the strain gauge minimizes the effect of resistance variation in the signal processing circuitry. Choose gauge characteristics such that resistance is a linear function of strain. For dynamic measurements, the linearity should be maintained over the desired frequency range. Low temperature coefficient and absence of the hysteresis effect add to the precision. Applications of strain gauge Strain-gauge transducers are used for measuring strain, force, torque, pressure, and vibration. In some applications, strain gauges are used as a primary or secondary sensor in combination with other sensors. 2.5 Sensors for Flow Measurement Flow sensing for measurement and control is one of the most critical areas in the modern industrial process industry. Regardless of the state of the fluid, gas, or liquid, accurate flow measurements are critical. In some situations, optimum performance of a machine is dependent on the correct mix of definite proportions of liquids. The continuous manufacturing process relies on accurate monitoring and inspections involving raw materials, products, and waste throughout the process. 54 Table of Contents 2.5.1 Solid Flow While monitoring the bulk of solid materials in transit, it is necessary to weigh the quantity of material for some fixed length of the conveyor system. A flow transducer in a solid measurement is the assembly of a conveyor, hopper opening, and weighing platform. Small, crushed particles of a solid material are carried by conveyor belt or through pipes in a slurry which is pumped through the pipes. As can be observed from Figure 2-12, the flow is measured as the necessary weight of the quantity of material on a fixed length of the conveyor system. Figure (2.13) In this situation, the flow measurement becomes weight measurement. The material on the platform displaces a transducer, usually a load cell, which is calibrated to provide an electrical output proportional to the weight of the solid flow. Weight is usually measured by a load cell, which is calibrated to give an indication of the solid flow. 𝑊𝑅 𝑄= (2.8) 𝐿 where Q : flow (kg/min) W : weight of material on section of length L 55 Table of Contents R : conveyor speed (m/min) L : length of weighing platform (m) 2.5.2 Liquid Flow The basic continuity equation in flow calculations is the continuity equation which states that if the overall flow rate in the system is not changing with time then the flow rate past any section is constant. The continuity equation in the simplest form can be expressed as 𝑉 = 𝑄/𝐴 (2.9) where: 𝑉 = flow velocity 𝑄 = volume flow rate Volume flow rate is expressed as a volume delivered per unit time. The common units are cubic meters per hour and liters per hour. Mass flow rate or mass of flow per unit time is expressed in kg/hr. Figure 2-14 illustrates the fluid flow phenomenon through varying cross-sectional areas. Figure 2.14 56 Table of Contents 2.6 Temperature Sensing Devices Temperature is one of the most familiar engineering variables. Its measurement and control are one of the earliest known metrological achievements. Temperature measurement is based on one of the following principles. 1. Material expansion based on change in length, volume, or pressure. 2. Based on the change in electrical resistance. 3. Based on contact voltage between two dissimilar metals. 4. Based on changes in radiated energy. An RTD is a length of wire whose resistance is a function of temperature. The design consists of a wire that is wound in the shape of a coil to achieve small size and improve thermal conductivity. In many cases the coil is protected from the environment by a protecting tube which inevitably increases response time, however, this enclosure is essential when RTDs are used in hostile environments. Resistance relationships of most metals over a wide range of temperature are given by quadratic equations. A quadratic approximation to the R–T curve is a more accurate representation of the resistance variation over a span of temperatures. It includes both a linear term and a term that varies as the square of the temperature. An analytical approximation is represented as, 𝑅 = 𝑅𝑜 (1 + 𝛼(𝑇 − 𝑇𝑜 ) + 𝛽(𝑇 − 𝑇𝑜 )2 +... ) (2.10) Here 𝑅𝑜 is the resistance at absolute temperature T and 𝛼 and 𝛽 are material constants which, dependent on the purity of material used. An examination of the resistance versus temperature curves of Figure 2-15 shows that the curves are quite linear in short ranges. This observation is employed to develop approximate analytical equations for resistance versus temperature of a particular metal. Over a small temperature range of 0°C to 100°C, the linear relationship is written as, 57 Table of Contents 𝑅𝑇 = 𝑅𝑜 (1 + 𝛼(𝑇 − 𝑇𝑜 )) (2.11) Here 𝛼 is the temperature coefficient of resistivity. Typical values of 𝛼 for three materials are Cu = 0.0043 /°C; Pt= 0.0039 /°C; Ni = 0.0068/°C. An estimation of RTD sensitivity can be calculated from typical values of the linear fractional change in resistance with temperature, as shown in Figure 2-15. The sensitivity for platinum is 0.004/°C and for nickel is 0.005/°C. Usually, a specification provides calibration information, either as a graph of resistance versus temperature or as a table of values from which the sensitivity can be determined. Figure (2.15) An RTD has a response time of 0.5 to 5 seconds or more. The speed of the response is governed by its thermal conductivity which governs the time required to bring the device into thermal equilibrium with its environment. The operating range of an RTD depends on the type of wire used as the active element, for example, a typical platinum RTD has an 58 Table of Contents operating range between -100 to 650°C, and an RTD constructed from nickel has a range in the vicinity of -180°C to 300°C. Variation of the resistance in a sensing element is measured using some form of electrical bridge circuit. Such a circuit may employ either the deflection mode of operation or the null mode. Resistance variations in a typical RTD tend to be quite small—in the vicinity of 0.4%. Because of these small fractional resistance changes with temperature, process-control applications require the use of a bridge circuit in which the null condition is accurately detected. 2.6.1 Thermistors A thermistor is a temperature transducer whose operation relies on the principle of change in semiconductor resistance with change in temperature. The particular semiconductor materials used in a thermistor vary widely to accommodate temperature ranges, sensitivity, resistance ranges, and other factors. The characteristics depend on the peculiar behavior of semiconductor resistance versus temperature. When the temperature of the material is increased, the molecules begin to vibrate. Further increases in temperature cause the vibrations to increase, which in turn increase the volume occupied by the atoms in the metal lattice. Electron flow through the lattice becomes increasingly difficult, which causes electrons in the semiconductor to detach resulting in increased conductance. In summary, an increase in temperature decreases electrical resistance by improving conductance. The semiconductor becomes a better conductor of current as its temperature is increased. This behavior is just the opposite of a metal. An important distinction, however, is that the change in semiconductor resistance with respect to temperature is highly nonlinear. Individual thermistor curves are approximated by the following nonlinear equation, 59 Table of Contents 1 = 𝐴 + 𝐵 𝐿𝑛 𝑅 + 𝐶(𝐿𝑛 𝑅)3 (2.12) 𝑇 where T = temperature in kelvins R = resistance of thermistor A, B, C = curve fitting constants The temperature range measured with a typical thermistor is between -250°C and 650°C. The high sensitivity of the thermistor is one of its significant advantages. Changes in resistance of 10% per degree Celsius are not uncommon. Because a thermistor exhibits such a large change in resistance with respect to temperature, there are many possible circuits which can be used for their measurement. A bridge circuit with null detection is most frequently used because the nonlinear behavior of the thermistor makes it difficult to use as a primary measurement device. Thermistors using null detecting bridge circuits and proper signal conditioning provide extremely sensitive temperature measurements. Since the thermistor is a bulk semiconductor, it can be fabricated in many forms including discs, beads, and rods varying in size from a bead of one millimeter in diameter to a disc several centimeters in diameter and several centimeters thick. By varying the manufacturing process and using different semiconducting materials, a manufacturer can provide a wide range of resistance values at any temperature. The response time of a thermistor depends primarily on the quality and quantity of material present as well as the environment. When encapsulated for protection against a hostile environment, the time response is increased due to the protection from the environment. 60 Table of Contents 2.6.2 Thermocouples When two conductors of dissimilar material are joined to form a circuit, the following effect is observed. When the two junctions are at different temperatures, 𝜃1 and 𝜃2 , small emf, 𝑒1 and 𝑒2 , are produced at the junctions and the algebraic sum of these causes a current. This effect is known as the Seebeck effect. The Peltier effect is the inverse of the Seebeck effect and described as follows. When the two dissimilar conductors which are joined together have a current passed through them, the junction changes its temperature as heat is absorbed or generated. Another effect, called the Thomson effect, predicts that, in addition to the Peltier emf, another emf occurs in each material of a thermocouple which is due to the longitudinal temperature gradient between its ends when it forms part of a conductor. When a thermocouple is used to measure an unknown temperature, the temperature of the thermo-junction, called the reference junction, must be known by some independent means and maintained at constant temperature. Figure 2-16 shows a typical thermocouple circuit using a chromel constantan thermocouple, reference junction, and a potentiometric circuit to monitor the output voltage. Calibration of the thermocouple is performed by knowing the relationship between the output emf and the temperature of the measuring junction. The standards to produce thermocouples are provided by The National Institute of Standards and Technology (NIST). Table 3-4 presents standard thermocouple characteristics. 61 Table of Contents Figure (2.16) Table (2.4) Chromel is an alloy of nickel and chromium, alumel is an alloy of nickel, aluminum is an alloy of nickel, and constantan is an alloy of copper. Thermocouple materials are divided into two categories: base metal types and rare metal types using platinum, rhodium, and iridium. The general requirements for industrial thermal transducers are High output electromotive force. Resistance to the chemical changes when it comes in the contact with the fluids. Stability of voltage developed. 62 Table of Contents Mechanical strength in their temperature range. Linearity characteristics. The resultant emf of a particular transducer may be increased by multiplying the number of hot and reference junctions. If there are three measuring junctions, the emf is enhanced appropriately. If the thermocouples in this arrangement are at different temperatures, the resultant emf is a measure of the mean value. Susceptibility to interference is an important consideration in any measurement application. Temperatures measured in hostile environments; in the presence of strong electrical, magnetic, or electromagnetic fields; or near high voltages are susceptible to interference. Susceptibility can be reduced by using non-contact methods of temperature detection. 2.6.3 Radiative Temperature Sensing Bodies at any temperature emit radiation and absorb radiation from other bodies. A body at a temperature greater than 0°K radiates electromagnetic energy in an amount that depends on its temperature and physical properties. A sensor for thermal radiation need not be in contact with the surface to be measured. Since the radiation emitted by an object is proportional to the fourth power of its temperature, the following relationship exists. 𝑊 = 𝜎𝑇 4 (2.13) Here W is the flux of energy radiated from an ideal surface and 𝜎 is the Stefan-Boltzman constant. Commercial radiation thermometers or radiometers vary in their complexity and accuracy. A schematic of a basic radiometer is shown in Figure 2-17 schematic of thermocouple circuit. 63 Table of Contents Figure (2.17)

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