CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Line I: Install Control Circuits and Devices LEARNING GUIDE I-5 INSTALL PROCESS CONTROLS

Transcription

1 I-5 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM Level 4 Line I: Install Control Circuits and Devices LEARNING GUIDE I-5 INSTALL PROCESS CONTROLS

2

3 Foreword The Industry Training Authority (ITA) is pleased to release this major update of learning resources to support the delivery of the BC Electrician Apprenticeship Program. It was made possible by the dedicated efforts of the Electrical Articulation Committee of BC (EAC). The EAC is a working group of electrical instructors from institutions across the province and is one of the key stakeholder groups that supports and strengthens industry training in BC. It was the driving force behind the update of the Electrician Apprenticeship Program Learning Guides, supplying the specialized expertise required to incorporate technological, procedural and industry-driven changes. The EAC plays an important role in the province s post-secondary public institutions. As discipline specialists the committee s members share information and engage in discussions of curriculum matters, particularly those affecting student mobility. ITA would also like to acknowledge the Construction Industry Training Organization (CITO) which provides direction for improving industry training in the construction sector. CITO is responsible for organizing industry and instructor representatives within BC to consult and provide changes related to the BC Construction Electrician Training Program. We are grateful to EAC for their contributions to the ongoing development of BC Construction Electrician Training Program Learning Guides (materials whose ownership and copyright are maintained by the Province of British Columbia through ITA). Industry Training Authority January 2011 Disclaimer The materials in these Learning Guides are for use by students and instructional staff and have been compiled from sources believed to be reliable and to represent best current opinions on these subjects. These manuals are intended to serve as a starting point for good practices and may not specify all minimum legal standards. No warranty, guarantee or representation is made by the British Columbia Electrical Articulation Committee, the British Columbia Industry Training Authority or the Queen s Printer of British Columbia as to the accuracy or sufficiency of the information contained in these publications. These manuals are intended to provide basic guidelines for electrical trade practices. Do not assume, therefore, that all necessary warnings and safety precautionary measures are contained in this module and that other or additional measures may not be required.

4 Acknowledgements and Copyright Copyright 2011, 2014 Industry Training Authority All rights reserved. No part of this publication may be reproduced or transmitted in any form or by any means, electronic or digital, without written permission from Industry Training Authority (ITA). Reproducing passages from this publication by photographic, electrostatic, mechanical, or digital means without permission is an infringement of copyright law. The issuing/publishing body is: Crown Publications, Queen s Printer, Ministry of Citizens Services The Industry Training Authority of British Columbia would like to acknowledge the Electrical Articulation Committee and Open School BC, the Ministry of Education, as well as the following individuals and organizations for their contributions in updating the Electrician Apprenticeship Program Learning Guides: Electrical Articulation Committee (EAC) Curriculum Subcommittee Peter Poeschek (Thompson Rivers University) Ken Holland (Camosun College) Alain Lavoie (College of New Caledonia) Don Gillingham (North Island University) Jim Gamble (Okanagan College) John Todrick (University of the Fraser Valley) Ted Simmons (British Columbia Institute of Technology) Members of the Curriculum Subcommittee have assumed roles as writers, reviewers, and subject matter experts throughout the development and revision of materials for the Electrician Apprenticeship Program. Open School BC Open School BC provided project management and design expertise in updating the Electrician Apprenticeship Program print materials: Adrian Hill, Project Manager Eleanor Liddy, Director/Supervisor Beverly Carstensen, Dennis Evans, Laurie Lozoway, Production Technician (print layout, graphics) Christine Ramkeesoon, Graphics Media Coordinator Keith Learmonth, Editor Margaret Kernaghan, Graphic Artist Publishing Services, Queen s Printer Sherry Brown, Director of QP Publishing Services Intellectual Property Program Ilona Ugro, Copyright Officer, Ministry of Citizens Services, Province of British Columbia To order copies of any of the Electrician Apprenticeship Program Learning Guide, please contact us: Crown Publications, Queen s Printer PO Box 9452 Stn Prov Govt 563 Superior Street 2nd Flr Victoria, BC V8W 9V7 Phone: Toll Free: Fax: crownpub@gov.bc.ca Website: Version 1 Corrected, March 2016 Corrected, September 2015 Revised, April 2014 New, October 2012

5 LEVEL 4, LEARNING GUIDE I-5: INSTALL PROCESS CONTROLS Learning Objectives Learning Task 1: Describe the components of an automatic control system Self-Test Learning Task 2: Describe common types of sensors and transducers Self-Test Learning Task 3: Describe the action of the controller in automatic control systems Self-Test Learning Task 4: Describe common types of electrical actuators Self-Test Answer Key CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 5

6 6 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

7 LEARNING OBJECTIVES I-5 Learning Objectives The learner will be able to describe the operating principles of process control. The learner will be able to connect and maintain process control systems. Activities Read and study the topics of Learning Guide I-5: Install Process Controls. Complete Self-Tests 1 through 4. Check your answers with the Answer Key provided at the end of this Learning Guide. Resources All resources are provided in this Learning Guide. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 7

8 LEARNING OBJECTIVES I-5 BC Trades Modules We want your feedback! Please go the BC Trades Modules website to enter comments about specific section(s) that require correction or modification. All submissions will be reviewed and considered for inclusion in the next revision. SAFETY ADVISORY Be advised that references to the Workers Compensation Board of British Columbia safety regulations contained within these materials do not/may not reflect the most recent Occupational Health and Safety Regulation. The current Standards and Regulation in BC can be obtained at the following website: Please note that it is always the responsibility of any person using these materials to inform him/herself about the Occupational Health and Safety Regulation pertaining to his/her area of work. Industry Training Authority January CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

9 Learning Task 1: Describe the components of an automatic control system In every control system, there are parameters, or processes, that need to be controlled. These parameters are called process, or control variables, and may be: Motor speed Position of a workpiece for machining Fluid pressure Liquid level Room temperature Liquid flow In order to understand automated control systems, you need to know the following terms: Process variable The variable that is controlled (e.g., temperature of the oven) Sensor The sensor measures and senses the process variable to indicate its actual value, and provides feedback to the controller (e.g., thermostat temperature indicator) Actuator/controlled device The actuator alters the process variable (e.g., solenoid valve) Set point The set point provides the information to the controller about the desired value of the process variable. Error (signal) The error is the difference between the set-point signal and the feedback signal. It is used by the actuator to control the process variable. Controlling device/controller The controlling device controls the actuator. It compares the feedback signal from the monitor with the set-point signal to generate an error signal. Control systems are categorized by how the process variables are controlled. There are two types of systems: open-loop and closed-loop. In an open-loop control system, only one process variable is controlled. An indicator and an operator are necessary to keep the process variable at a constant value. In a closed-loop system, control of the process variable is automatic. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 9

10 LEARNING TASK 1 I-5 Open-loop control system An open-loop control system is controlled manually. It requires an operator to monitor the process variable and to operate the controlling devices. The operator requires a monitor to measure the value of the process variable. The controlling devices alter the actuator, which affects the value of the process variable. To better understand these terms, refer to Figure 1, which shows a basic, open-loop temperature control system. Figure 1 Basic, open-loop temperature control sytem An operator monitors room temperature with a thermometer, and turns the heater on and off with a switch to maintain the desired temperature. Figure 1 can be described in control system terms, as follows: Process variable = room temperature Monitor = thermometer Controlling device = heater switch Actuator = heater An open-loop system is not self-correcting; the process variable has no effect on the operation of the controlling device (i.e., the rising or falling temperature in the room has no effect on the operation of the switch). 10 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

11 LEARNING TASK 1 I-5 System elements To understand a control system, you must understand the interaction between various components of the system. Functional blocks do not have to be identified with physical components or with the hardware of a system. Figure 2 shows the block diagram of the basic, open-loop temperature control system of Figure 1. Actuator Figure 2 Block diagram of the basic, open-loop temperature control system Closed-loop control system In an open-loop control system, an indicator and an operator are required to keep the process variable at a constant value. To overcome this disadvantage, closed-loop systems are used. The following explanation refers to the basic, closed-loop temperature control system shown in Figure 3. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 11

12 LEARNING TASK 1 I-5 Figure 3 Basic, closed-loop temperature control system The position of the set screw is adjusted in the thermostat so that at the desired temperature, the contacts are just touching. This adjustment is called set-point adjustment. If the bimetallic strip senses a rise in temperature, it bends to separate the contacts, opening the heater circuit and turning off the heater. As the room temperature falls, the bimetallic strip straightens, reducing the distance between the contacts. At the desired temperature, the contacts close, turning on the heater. Once the desired temperature is achieved, the control system tries to automatically maintain it within a narrow range. The following observations can be made about this type of system: Process control (room temperature) is automatic. The sensor senses the process variable (temperature sensing by bimetallic strip position). The value of the process variable is compared with the desired temperature set point (contact position adjusted with the set screw). The error (the distance between the contacts) is the difference between the set point and the process variable. The output of the controller (thermostat) is used to control the actuator (heater) to bring the process variable closer to the desired value. This type of system is discontinuous, since the circuit is opened and closed. The process variable is constantly sensed and compared with the desired value. The error is used by the actuator to bring the process variable closer to the desired value. 12 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

13 LEARNING TASK 1 I-5 Circuit elements Figure 4 is a block diagram of a closed-loop system and the related circuit elements. Figure 4 Block diagram of a closed-loop system The elements in Figure 4 are interconnected to form a closed loop. (Each process variable generally requires one loop of control.) Since an automatic control system requires feedback from the process variable to form a closed loop, it is also called a closed-loop feedback control system. This represents the basis of automatic control of any process. Notice that in the basic roomtemperature control system in Figure 3, the sensor and the set point are part of the controller hardware. However, in the block diagram of Figure 4, they are separate. The monitor element of the block diagram does not take an active part in the control of the process variable, and is, therefore, not an essential element. It merely shows how close the controlled process variable is kept to a desired value. Discontinuous and continuous systems Open-loop and closed-loop systems can be further categorized as discontinuous or continuous systems. In discontinuous control systems, full power is applied or removed to modify the process variable. The temperature control system in Figure 1 is an open-loop discontinuous control system, where the circuit to the heater is opened or closed, supplying no power or full power. In continuous control systems, power supplied to modify the process variable is controlled from very low power to full power. Figures 5 and 6 show examples of open-loop continuous control systems. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 13

14 LEARNING TASK 1 I-5 Figure 5 Continuous, open-loop temperature control system Figure 6 Continuous, open-loop speed control system The systems shown in Figures 5 and 6 have different process variables, yet both are continuous, open-loop control systems, with no feedback control. Applied power is controlled by adjusting the rheostat from a low value to a high value. The circuits in both cases are not opened, because they are in discontinuous systems to control the power to the actuator. 14 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

15 LEARNING TASK 1 I-5 Figure 7 shows a continuous, closed-loop system that has feedback control. The output to the actuator (a valve) is driven by the difference between a desired set-point position and a measuring element (the feedback signal). Figure 7 Continuous, closed-loop control system Now do Self-Test 1 and check your answers. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 15

16 LEARNING TASK 1 I-5 Self-Test 1 1. List three examples of process variables. 2. What is the function of a sensor? 3. What is a set point? 4. Briefly explain the difference between an open-loop and a closed-loop control system. 5. List the five system elements of a closed-loop control system. 6. Briefly explain the difference between a continuous and a discontinuous control system. Go to the Answer Key at the end of the Learning Guide to check your answers. 16 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

17 Learning Task 2: Describe common types of sensors and transducers Sensors Sensors are important parts of automatic control systems. The system can perform only as well as the accuracy of information provided to it by the sensor. Consider the process of filling a bathtub and adjusting the temperature of the water. One hand senses the water temperature while the other hand opens and closes the hot and cold water taps to achieve the desired temperature. If you wanted the bathwater to be a specific temperature, you would have to use a thermometer, instead of your hand, to achieve greater accuracy. Improved measurement always results in improved control. A system with a poor sensor and sophisticated control may not perform as well as one with a more accurate sensor and relatively basic control. Transducers Controllers usually work with electrical and pneumatic forms of energy. However, most process variables (e.g., position, speed, temperature) are not in a suitable form of energy to be controlled. This means that a sensor of a process variable has to convert the energy into a form suitable for the controller. Since many sensors convert energy from one form to another (e.g., mechanical to electrical), they are sometimes referred to as transducers. In most cases, sensors used for automatic control are transducers. A transducer used to sense the process variable and provide feedback information to the system (controller) is called a sensor. Whether to call a transducer a sensor or not depends on the transducer s function in the system. Transducers can serve additional purposes in a control system. For example, a solenoid valve accepts electrical energy and converts it into mechanical energy. The valve is a transducer used as an actuator. Analogue and digital sensors Sensors are devices used to measure the current value of a process variable and produce output signals that reflect that value. Output signals can be analogue or digital. An analogue signal (Figure 1) supplies continuous variable output that varies smoothly from 0 to 10 V, representing 0 to 10 inches of linear motion travel. The output of a digital signal (Figure 2) is in fixed, discrete steps from 0 to pulse counts, representing 0 to 10 inches of linear motion travel. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 17

18 LEARNING TASK 2 I-5 Figure 1 Sensor with analogue output signal Figure 2 Sensor with digital output signal 18 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

19 LEARNING TASK 2 I-5 Common types of sensors are: Sensors of motion Sensors of force Sensors of fluids Sensors of temperature Sensors of light Sensors of motion There are three types of motion: linear, angular and rotary. Linear motion is motion in a straight line. The amount of movement and its direction must be measured. Angular motion refers to the rotary movement of a shaft in degrees clockwise or counterclockwise from a reference position. Rotary motion is typically the speed of rotation of a shaft and its direction. Linear motion sensors Figure 3 shows a linear motion potentiometer, where the resistance of the potentiometer changes according to the mechanical movement of the shaft (a cylinder) connected to its slider. This change in resistance is an indication of the amount of linear movement. The direction of its movement is indicated by the increase or decrease in resistance. This resistance change could be converted (using an external power supply) into a corresponding electrical signal, representing a voltage or current change. Figure 3 Linear motion potentiometer attached to a cylinder This common device is very basic. It is used as a sensor for various position control systems. Since the slider of the potentiometer wipes mechanically on the resistance element, it will wear out over time. Preventive maintenance calls for checking these devices and replacing them periodically. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 19

20 LEARNING TASK 2 I-5 In a linear motion variable inductor (Figure 4), the core of the variable inductor is connected to the shaft to sense the shaft s linear movement. As the core is moved into the coil, its inductance increases. This change in the inductance indicates linear movement. The direction of movement is indicated by an increasing or decreasing value of its inductance. This type of sensor lasts longer than a linear potentiometer because it does not have contact wear. Figure 4 Linear motion variable inductor In a linear motion variable capacitor (Figure 5), the distance between the fixed plate and the movable plate varies according to the linear position. As the plate is moved closer to the fixed plate, its capacitance increases; when it is moved farther away, its capacitance decreases. Figure 5 Linear motion variable capacitor The linear variable differential transformer (LVDT) uses the principle of a differential transformer, making it very sensitive to motion. They can be made very small and can be used, with other circuit components, to provide polarity-sensitive DC voltage. The device has two identical secondary windings and a single primary winding. These windings are coupled to each other with a soft iron movable core, as shown in Figure CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

21 LEARNING TASK 2 I-5 Figure 6 Linear variable differential transformer Moving the core left or right from its null (neutral) position changes the coupling between the primary and secondary windings, inducing higher voltage in one of the secondary windings and correspondingly lower voltage in the other. The amount of voltage change in the secondary windings is proportional to the linear motion of the core. The direction of motion is indicated by which voltage output is increasing or decreasing. A single voltage signal is usually preferred to two secondary output signals. This can be obtained by connecting the two secondary windings in series opposition (called differential) connection (Figure 7). CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 21

22 LEARNING TASK 2 I-5 Figure 7 LVDT connected in a differential mode Angular motion sensors Angular motion sensors are constructed differently from linear motion sensors, but they operate on the same principles. An angular motion potentiometer resembles a linear motion potentiometer, except that the resistance element is circular, as shown in Figure 8. The slider is mounted on the shaft, and as the shaft is rotated, the resistance change is proportional to the angular motion. Change in the resistance indicates the amount of angular movement. The direction of movement is indicated by whether the resistance is increasing or decreasing. Single-turn potentiometers are suitable for angular motion not exceeding 300. Figure 8 Angular motion potentiometer 22 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

23 LEARNING TASK 2 I-5 Figure 9 shows an angular variable inductor. The change in inductance indicates angular motion; increasing and decreasing inductance indicates the direction of angular movement. This type of unit allows angular movement greater than 360. An angular variable capacitor is shown in Figure 10. The capacitor consists of a fixed set of plates, as well as a set of movable plates that are attached to the shaft. With the plates fully meshed, its capacitance is at a maximum. When the plates are fully unmeshed or open, its capacitance is at a minimum. This type of capacitor has a maximum travel of 180. Figure 9 Angular variable inductor Figure 10 Angular variable capacitor A rotary variable differential transformer (RVDT) operates on the same principle as the LVDT. Figure 11 shows an RVDT with its maximum angular travel limited to ±45 from the null position. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 23

24 LEARNING TASK 2 I-5 Figure 11 Rotary variable differential transformer (RVDT) Rotary motion sensors Sensing rotary motion requires obtaining information on the speed of rotation of a shaft, as well as its direction of rotation (clockwise or counter-clockwise). Sensors like DC/AC tachogenerators operate on a generator principle, where the induced voltage is proportional to the speed of rotation of the shaft. In an AC tachogenerator, as the speed of rotation of the shaft increases both the magnitude and the frequency of the tachogenerator s output voltage signal will increase in proportion to shaft speed. Sensors of force A bonded wire strain gauge consists of a length of fine wire bonded to a plastic sheet in the form of a grid (Figure 12). Figure 12 Bonded wire strain gauge used in a load cell In the load cell in Figure 12, the bonded wire strain gauge is cemented with special glue to a load column. When heavy force is applied to the column, the column is subjected to compression, and its overall length reduces. This change in length is usually a few thousandths of a millimetre. This change is also transferred to the fine wire of the strain gauge. Just like a stretched elastic band, when fine wire is compressed, its effective length is reduced and its cross-sectional area is increased. This translates into an overall reduction in the strain gauge resistance, in accordance with the equation: 24 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

25 LEARNING TASK 2 I-5 R where: = kl CMA R = resistance (Ω) k = constant (Ω cmil/ft.) L = length (ft.) CMA = area (cmil) As the force is removed, the fine wire of the strain gauge resumes its original resistance value. A bridge type circuit is usually used to detect the small changes in resistance. The advantage of the strain gauge is that it is capable of providing information under severe shock and vibration. These strain gauges can be used wherever there are mechanical deformations. Therefore, it can be used to sense: Tension or extension in a column when lifting a heavy weight with a crane (Figure 13) Mechanical pressure, since pressure = force/area Figure 13 Strain gauge used for sensing tension CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 25

26 LEARNING TASK 2 I-5 Figure 14 shows how strain gauges are used in weigh scale applications. Figure 14 Strain gauges used for weigh scales Sensors of fluids Fluid sensors are sensors of liquids or gases. They supply information on: Fluid pressure Fluid flow Liquid level Fluid pressure sensors Most fluid pressure sensors transform fluid pressure into a force that causes mechanical motion. Appropriate sensors of motion can be used to convert these mechanical motions into corresponding electrical signals. 26 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

27 LEARNING TASK 2 I-5 A bourdon tube (Figure 15) consists of a slightly flattened tube of flexible steel arranged in a semicircle. One end of the tube is closed and the other end is open and fixed. As the pressure in the tube increases at the open end, the closed end tends to straighten. As the pressure is reduced, the closed end of the tube moves back to its original position. Bourdon tubes are inexpensive, quite accurate, extremely rugged and reliable. Figure 15 Bourdon tube Bellows (Figure 16) are more sensitive than bourdon tubes and are used for low-pressure applications. A bellows consists of a metal box closed at one end with corrugated sides of flexible steel material. Fluid pressure can be applied from the other open end of the box with pressure fittings, causing the bellows to expand. Figure 16 Bellows CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 27

28 LEARNING TASK 2 I-5 A diaphragm (Figure 17) is a thin, circular disk made of flexible material. The edges are sealed to a box. The centre of the disk pushes away from the pressure applied at the other side of the box. Diaphragms are used for low-pressure sensing. Figure 17 Diaphragm A piezoelectric pressure transducer (Figure 18) typically uses a quartz crystal that has the property of separating charges across its opposite faces, according to the mechanical pressure applied to them. These charges leak off in time, so this type of transducer is used to sense changes of pressures where the charges do not have appreciable time to leak off. These are used typically for vibration sensing in phonograph crystal cartridges, microphones, etc. Figure 18 Piezoelectric pressure transducer Fluid flow sensors Orifice plates and manometers are used to sense rates of fluid flow (Figure 19). When fluid flows through a small orifice of fixed size, a pressure difference is created on the two sides of the plate. The size of this pressure difference depends on the rate of fluid flow. The higher the rate of flow, the higher the pressure difference. This can be measured simply by using a manometer. The manometer usually consists of a U-tube containing liquid such as mercury. Whenever there is a pressure difference between the two arms of the tube, the higher pressure pushes the liquid to the other side of the tube. The height differential is proportional to the rate of flow. An LVDT can be used to sense the level of the liquid column. 28 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

29 LEARNING TASK 2 I-5 Figure 19 Fluid flow using an orifice and a manometer A magnetic flow meter works on the generator principle. It consists of a straight section of nonmagnetic insulated pipe placed in a magnetic field. Two probes that make contact with the liquid are placed at right angles to the direction of the magnetic field. The conductive liquid flowing through the pipe will cut the magnetic field and induce voltage between the two probes. The higher the rate of liquid flow, the higher the induced voltage. The only condition of its use is that the liquid should be at least slightly conductive. Liquid level sensors A float liquid level sensor is a transducer that converts basic liquid level to a mechanical position. Figure 20 shows the unit connected to the potentiometer to provide the change in resistance for the corresponding change in liquid level. In a capacitive liquid level sensor, two probes are inserted into the liquid contained in a tank. These probes act as two plates of a capacitor; the liquid acts as a dielectric. As the liquid level changes, the capacitance between the probes also changes. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 29

30 LEARNING TASK 2 I-5 Figure 20 Float liquid level sensing An ultrasonic wave actually travels slower in a denser fluid (such as a liquid) than it travels through a fluid such as air, which is a gas. As the ultrasonic wave travels from one material to another, the change in velocity results in the ultrasonic wave changing its direction or angle of travel. Ultrasonic liquid level sensors work on the principle that ultrasonic waves are reflected by a change in fluid density (the change from gas to liquid). An ultrasonic transmitter transmits an ultrasonic signal through air towards the liquid being measured. As a result of the change in fluid density at the boundary between a gas and a liquid, the ultrasonic waves change velocity and are reflected and picked up by an ultrasonic receiver. The time delay between the transmitted signal and the received signal gives the indication of the liquid level. Temperature sensors Resistive temperature sensors depend on the change in the resistance of a material due to the change in its temperature. Metals typically have a positive temperature coefficient of resistance (i.e., as the temperature rises, its resistance also rises). Semiconductor material exhibits a negative temperature coefficient of resistance (i.e., as the temperature rises, its resistance falls). Resistance temperature detectors (RTD) are made from either fine wire or metal film (platinum, nickel or an alloy of nickel) and have positive temperature coefficients of resistance. Thermistors are typically made from semiconductor material with a negative temperature coefficient of resistance. The thermistor is more sensitive to temperature change than the RTD. It can detect minute changes in temperature, but it has poor linearity. Thermistors start with high resistance at room temperature and very low resistance at the sensing temperature. They are used for such purposes as protecting the windings of electrical machines. 30 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

31 LEARNING TASK 2 I-5 A thermocouple consists of two dissimilar metals that are joined together at one end to form a junction. When this junction is heated, a small emf (mv) develops across the two cool ends. The magnitude of this emf is directly proportional to the temperature difference between the heated junction and the cool ends (within a specific temperature range). To obtain desired temperature ranges, wires of different metal combinations are used. Figure 21 shows the thermocouple junction principle. To avoid errors in sensing, take care when installating of thermocouples to minimize the total number of junctions (especially between those of different metals) and to use special types of connectors where necessary. Figure 21 Thermocouple In a bimetallic sensor, two dissimilar metals that have different rates of expansion and contraction are welded together. As the temperature rises, the combination tends to bend toward the metal with least expansion. As the temperature falls, they return to their original shape. The bimetallic sensor converts the temperature change into corresponding mechanical movement. These are used in thermostats, overload sensors, etc. Fluid temperature sensors Fluids (liquids and gases) expand when heated, and this expansion is in direct proportion to the absolute temperature. This property means that fluid can be used as a temperature sensor. Figure 22 shows how a bulb filled with liquid, vapour or gas is used to sense temperature. As the bulb is heated, the fluid inside expands. This causes the internal pressure to increase. This increase in the internal pressure moves the bourdon tube, resulting in mechanical motion. The bourdon tube can be connected to other motion sensors to obtain information for control applications. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 31

32 LEARNING TASK 2 I-5 Figure 22 Fluid-filled bulb temperature sensor Radiation pyrometers A radiation pyrometer is often used to sense very high temperatures of metals being melted in a furnace. Under these situations, direct-contact temperature sensors would be destroyed by high temperature. The pyrometers also allow for remote measuring of temperature. The pyrometers work on the principle that as the body is heated, more and more radiant energy is given out in the form of visible and infrared rays. If the radiation is focused onto a thermopile (made up of thermocouples connected in series aiding), a voltage directly proportional to the radiant energy is generated. This, in turn, is the indication of the temperature of the remote heated body. Note that the thermopile can be replaced by other types of sensors that detect radiation (e.g., photo tube and LDR). Light sensors Light energy is a small portion of the spectrum of electromagnetic waves. In addition to visible light, light includes infrared and ultraviolet. Light sensors generally work on one of the following principles: Photoemission, where electrons are liberated from the material when light energy falls on it Photoconduction, where the resistance of the material is reduced with light Photovoltaic, where a voltage is generated in a cell due to light falling on it 32 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

33 LEARNING TASK 2 I-5 A photo tube works on the principle of photoemission, where a cathode is coated with a lightsensitive substance that releases electrons when light shines on it. The amount of current flowing from the cathode to a nearby anode depends on the amount of light falling on the cathode. A light-dependent resistor (LDR) is made from cadmium sulphide. It works on the principle of photoconduction. A photovoltaic cell (solar cell) consists of a P-N junction with a large light-collecting surface area that generates emf according to the amount of light. When light falls on the surface area of a P-type semiconductor material, the energy reaches the junction and frees holes and electrons. Holes move to P-type material, while the electrons move to N-type material. The P terminal becomes positive and the N terminal becomes negative. With photodiodes, when a P-N junction is reverse-biased, a small leakage current flows, caused by hole and electron carriers. If light energy is focused on this junction, the leakage current increases due to further separation of holes and electrons. As the light level increases, the leakage current increases correspondingly. Phototransistors provide higher current output than photodiodes for the same amount of light signal. This is because of the current gain effect of the transistor. A photo darlington transistor uses two transistors cascaded together to provide yet higher gain. A light-activated SCR (LASCR) operates similarly to a conventional SCR, except that a window is provided to shine light where the gate is connected. The action of light has the effect of causing gate current to flow and triggering the LASCR. Take care while substituting or replacing a sensor. Check for its accuracy and its suitability for use in the system. Electrical sensor signals are low level, so, while wiring the sensor to the controller, twisted pair or shielded cables should be used with proper grounding techniques. Also, while planning layout, wherever possible avoid running these and other signal control cables close to power cables. If they must be run close to power cables, they should be run at right angles to minimize electrical noise. Sometimes it is necessary to locate a controller in a control room, far from the process variable it controls. In this case, the sensor information has to be transmitted over a long distance to the controller. Since the sensor signals are low level, they are subject to signal loss and electrical noise when connected directly to the controller over long distances. Transmitters can be used to accept the sensor information locally, and provide standard output signals, where possible, for longdistance transmission. There are typically two types of output signals: electrical and pneumatic. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 33

34 LEARNING TASK 2 I-5 An electrical output signal standard is usually: 4 ma to 20 ma direct current signal range, or 10 ma to 50 ma direct current signal range A transmitter should be selected so that the input section is compatible with the sensor and the output signal is compatible with the controller. Now do Self-Test 2 and check your answers. 34 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

35 LEARNING TASK 2 I-5 Self-Test 2 1. What is a transducer? 2. Briefly explain the difference between an analogue sensor and a digital sensor. 3. List three common categories of sensors. 4. List four basic principles used to sense linear motion. 5. What device is commonly used to express rotary motion as an electrical signal? 6. What resistance parameters are changed when a strain gauge is subjected to a compression force? CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 35

36 LEARNING TASK 2 I-5 7. List three types of fluid pressure sensors. 8. Briefly explain how a piezoelectric crystal works as a sensor. 9. On what principle does a magnetic flow meter work? 10. Explain the basic difference between an RTD and a thermistor. 11. List three basic principles used in light sensors. 12. Briefly explain the function of a transmitter. Go to the Answer Key at the end of the Learning Guide to check your answers. 36 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

37 Learning Task 3: Describe the action of the controller in automatic control systems Set point As you learned in Learning Task 1, a set point provides the information to the controller about the required or desired value of the process variable. The controller then uses this information to bring the process variable to the desired value. One of the common ways of producing a set-point signal in electrical systems is by using a fixed power supply connected to the potentiometer, as shown in Figure 1. An operator can increase or decrease the desired value of the process variable simply by moving the slider clockwise or counter-clockwise, respectively. An adjustable set point is necessary in order to suit different process control system requirements at different times. Figure 1 Adjustable set point or reference signal for an electrical system Controller A controller typically consists of a comparator and an output section. The controller element block compares the set-point information with the feedback received from the sensor to determine an error. The power level of this error signal is raised to provide a sufficient output signal to operate the actuator. Comparator The comparator generates the error signal, which is the difference between the set-point signal and the feedback signal. The comparator measures how far away (deviation) the feedback signal is from the set-point reference signal, and whether it is higher or lower (the polarity). The actuator then uses this information to control the process variable. Electrical error signals can be obtained by bridge circuits or by using differential amplifiers that incorporate transistors. A typical symbol used for the comparator is shown in Figure 2. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 37

38 LEARNING TASK 3 I-5 Figure 2 Comparator symbol Output section As the process variable approaches the desired value, the error signal becomes smaller and smaller. In order for the actuator to respond to these small error signals, amplifiers are used to provide sufficient signal level at the output of the controller. There may also be a need to transform this signal into a suitable energy form that can be read by the actuator. Feedback The information about the current status of a process variable that is fed back to the controller is called feedback. There are two types of feedback: negative feedback and positive feedback. Negative feedback is the desired condition, where the error signal is the difference between the set point and the feedback signals. Since this error signal is used to bring the process variable closer to the set point, the error signal is constantly becoming smaller, approaching zero. Positive feedback occurs because of a connection error or wrong selection of system components. In positive feedback, the error signal is the sum of the set point and feedback signals, causing the process variable to move farther away from the desired value and increasing the error signal every time a comparison is made. Eventually, the system goes out of control. As a part of the system start-up procedure, check that the system is operating on a negative feedback cycle. To do this, refer to the specific system manual for details. Modes of control There are four basic modes of control: ON-OFF Proportional Proportional plus integral Proportional plus integral plus derivative (PID) 38 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

39 LEARNING TASK 3 I-5 Each mode of control progressively improves the overall system performance. ON-OFF (2-position control) In this mode of control, whenever the process variable deviates a small, predetermined amount from the set point, the output of the controller moves the actuator to either fully-on or fully-off (i.e., full power or zero power). The amount of time the system stays ON or the system stays OFF depends on the load demand on the system. In a heating system, the load demand is a result of the outside temperature falling, or how often the doors or the windows of the room are opened. If the load demand is high, the heater stays on for a longer time and off for a shorter time. This type of control is very popular because of its simplicity. It is used with systems that have slow reaction times, such as a gas water heater. Since this type of control provides full power or no power, it can lead to large overshoots. Used with microprocessors, controllers sometimes use a variation of the ON-OFF control by adjusting the duty cycle of the ON-to-OFF period. In this way, they can control the average power over a wide range from very low power to full power. Proportional For systems that require a fast reaction time, it is necessary to continuously calculate the error and modify the process variable. In these systems, there is a linear relationship between the error and the actuator manipulation. Because there is always a need for some signal for the system to operate (and depending on the deadband of the actuator), this type of control mode consistently results in a small error. This error could be made smaller by increasing the system gain the higher the gain, the lower the error. For a given system, however, there is a maximum value of this gain setting, above which the system begins to overcorrect and sometimes go into oscillations. Typically, the gain of the system is adjusted just below the point of overshoot. This, then, is the smallest error to which the system can be corrected and controlled. To improve this situation and to improve response times, other modes of control are brought into action. Proportional plus integral control (PI) The steady state error necessary for the operation of a purely proportional control system may be eliminated by using a controller with an added integral control function. The integral term is proportional to both the magnitude of the error signal and the duration of the error. The integral term in a PID controller measures the sum of the error over a period of time and gives the total steady state offset that should have been corrected for that period of time. This total accumulated error is then multiplied by the integral gain setting and is added to the controller output. The integral control function increases the speed of response, which accelerates the movement of the process towards the set point and eliminates the residual steady-state error that occurs with a pure proportional controller. However, since the integral term responds to the sum of the accumulated errors from the past and increases the controlled output accordingly, it can cause the measured process value to overshoot the set point value and oscillate. Proportional plus integral plus derivative control (PID) Derivative term measures the rate of change of the error signal and is used to (1) reduce the magnitude of the overshoot produced by the integral component; and (2) improve the combined controller-process stability. The derivative term slows the rate of change of the CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 39

40 LEARNING TASK 3 I-5 controller output. However, the derivative term also slows the response time of the controller. Controllers with a derivative function are susceptible to noise which may cause a process to become unstable if the noise and the derivative gain are sufficiently large. Proper care in the installation and termination of the control and signal cables is essential. PID Tuning PID controllers must be set up or tuned to provide the desired control over the process variable. Tuning a control loop is the adjustment of the proportional band/gain, integral gain/reset and derivative gain/rate to the best settings for the desired controller output response. Two basic requirements are (1) stability; refering to the controller s ability to remain at a given setpoint, and (2) command tracking criteria such as rise time (the amount of time to reach setpoint) and settling time (the amount of time before steady state is achieved after a process command change). Some processes must not allow an overshoot of the process variable beyond the setpoint if, for example, this would be unsafe. Other processes must minimize the energy expended in reaching a new setpoint after a command change and settling time. Even though there are only three parameters and in principle it is simple to describe, PID tuning the process controller may be a challenging and time-consuming task, since it must satisfy complex criteria within the limitations of the process control system. The table below indicates each parameter s response to an individual increase in any controller function. PID controller tuning Controller function Response time Setpoint overshoot Settling time Steady-state error Stability Proportional Decrease Increase Small change Decrease Degrade Integral Decrease Increase Increase Eliminate Degrade Derivative Minor change Decrease Decrease No effect in theory Improve Programmable logic controllers are gaining popularity because of their ability to program and to easily change operational modes. Because of this, and the PLC s capability of programmingin relay-type instructions, the PLC can handle many types of process control (assuming that the PLC scan time is acceptable). Note that in order to provide sensor information to the PLC, appropriate specialized input and output modules that are suitable for the sensor and the actuator may be required. Now do Self-Test 3 and check your answers. 40 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

41 LEARNING TASK 3 I-5 Self-Test 3 1. Why is it necessary to have an adjustable set-point signal? 2. Briefly explain the function of a controller. 3. What two signals does the comparator use to determine error signal? 4. What is the difference between positive and negative feedback? 5. Which of the two types of feedback is desirable for an automatic control system? 6. Identify two modes of control used in automatic control systems. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 41

42 LEARNING TASK 3 I-5 7. What is the typical characteristic of a proportional control mode with regard to its error? 8. What typical adjustment could be used to minimize this error? Go to the Answer Key at the end of the Learning Guide to check your answers. 42 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

43 Learning Task 4: Describe common types of electrical actuators Actuators (servomechanisms) By definition, an actuator is a device that converts a signal input to a mechanical motion. In process controls, the actuator receives signal commands from the output section of the controller and modifies the process variable accordingly. Since the output of the actuator is generally considered mechanical, they are often called servomechanisms. Generally, actuators or servomechanisms can be classified as: Electrical (or electromechanical) Hydraulic (such as oil cylinders, pistons and valves) Pneumatic (such as air bellows, diaphragms and valves) Electrical actuators Electrical actuators provide either linear motion or rotary motion. The most common types of electrical actuators are: Solenoids Motors Clutches and brakes Solenoid The solenoid (Figure 1) is an electromagnet consisting of a core-and-coil assembly and a movable plunger (armature). The solenoid is commonly designed to provide linear motion in the form of either a push or a pull, and generally a spring is used to return the plunger to its deenergized position. Solenoids are available for either AC or DC power sources. AC solenoids have cores made from laminations instead of solid iron. Note that some solenoids are designed only for intermittent duty, rather than for continuous operation. Coil conductors Mounting backplane Plunger return spring Coil winding Case Magnetic circuit path Direction of travel Energized De-energized Figure 1 Basic solenoid construction CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 43

44 LEARNING TASK 4 I-5 Motors A variety of motors are used in automated control systems. Conventional AC motors (both single-phase and three-phase types) are most popular for turning large mechanical loads. Larger DC motors are used in more specialized industrial environments. Figure 2 shows how the rotary motion of motors can be converted to linear motion using rack-and-pinion gearing. Figure 2 Rotary-to-linear motion conversion Special high-torque types of motors servomotors are used for more precise operations, such as the opening and closing of valves, positioning of antennae and so on. Accurate positioning control is commonly achieved in applications like robotics and computer disk drives by using unique motors called stepper motors. Clutches and brakes Clutches are sometimes used to couple the motor to the load device. The clutch may be used to quickly engage, disengage or even control the speed of the device it is actuating. Clutches can be either mechanical or magnetic types. Mechanical clutches are usually friction type (Figure 3), using either disks or drums. In this case, the coupling of the disks is achieved by disk-to-disk pressure exerted by the force of springs controlled by solenoids. Figure 3 Friction disk clutch Magnetic clutches can transmit torque between input and output shafts without a direct mechanical link. The eddy current clutch relies on DC field excitation to control the torque and speed of the rotating load. Figure 4 shows the basic components of a magnetic clutch assembly. 44 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

45 LEARNING TASK 4 I-5 Figure 4 Magnetic clutch assembly The same principles used by clutches can be employed to brake the motor. Stepper motors A stepper, or stepping motor, is an incremental, brushless DC motor that moves in discrete, angular steps in accordance with set sequences of current pulses that are supplied to the motor. Each pulse causes the motor to rotate a certain degree in a clockwise or counter-clockwise direction. Continuous rotation of a stepper motor results from a continuous repetition of pulse sequences supplied by digital controllers. A stepper motor, therefore, converts digital data into an angular position, or pulses into speed of rotation. The motor typically consists of a permanent magnet or a soft iron rotor with a large number of teeth. The stator of the motor is wound with four or eight poles, which also have large numbers of teeth, depending on the angular resolution (i.e., the number of degrees per step). Figure 5 is an exploded view of one type of stepper motor. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 45

46 LEARNING TASK 4 I-5 Figure 5 Exploded view of a stepper motor There are many types of motor configurations. Three basic types in general use are: Variable reluctance stepper motor Permanent magnet stepper motor Hybrid stepper motor Hybrid stepper motors are the most widely used of the three types named. They use a combination of the stator configuration of a VR stepper and the rotor design of a PM stepper motor to achieve maximum power in a small package size. They are often considered to be a permanent magnet stepper motor but use higher quality bearings, provide a smaller step angle or increased resolution; and provide greater torque. They use the same control schemes as a permanent magnet stepper motor controller. As a result only the first two types will be discussed in this Learning Task. Variable reluctance stepper motor A variable reluctance stepper motor is shown in Figure 6. It works on the principle that magnetic fields align and travel through the path of least reluctance (magnetic resistance ). 46 CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4

47 LEARNING TASK 4 I-5 CYCLE SWITCHES ANGULAR #1 #2 #3 ROTATION ON OFF OFF 00 1 OFF ON OFF 300 OFF OFF ON 600 ON OFF OFF OFF ON OFF 1200 OFF OFF ON 1500 ON OFF OFF OFF ON OFF 2100 OFF OFF ON 2400 ON OFF OFF OFF ON OFF 3000 OFF OFF ON ON OFF OFF 3600 Figure 6 Simplified diagram of a variable reluctance stepper motor In Figure 6 the reluctance stepper motor shown consists of a stator with six salient poles and a fourpole soft iron rotor. The stator windings are shown connected to a DC supply with three switches. When none of the stator coils are energized (i.e., all switches are open), the rotor has no opposition to motion and is free to move to any position. However, when the coil of pole 1 is energized via switch 1, the resulting magnetic field attracts the rotor so that it lines up as in Figure 6. If switch 1 is opened and switch 2 is closed, energizing the coil of pole 2 moves the rotor 30 clockwise to line up with pole 2. Again, if switch 2 is opened and switch 3 is closed, the rotor will move another 30 clockwise and line up with pole 3. By closing and opening the switches in the sequence 1, 2, 3, 1, the rotor can be moved clockwise. The rotor can be moved counter-clockwise by simply changing the closing and opening sequence of the switches (1, 3, 2, 1, ). In order to stop motion and hold the rotor in position, the coil of one of the poles must stay energized. In this position, the rotor stays magnetically locked, provided that the external torque does not exceed the holding torque of the motor. If the power is lost, however, this type of stepper motor does not have any residual or detent torque to hold the rotor in position. The model in Figure 6 is simplified for explaining the principles of operation. Actual designs have four or more stator and rotor poles, both with large numbers of teeth. Each tooth acts as a separate salient pole. The number of teeth on the rotor and stator determines angular motion per step. CONSTRUCTION ELECTRICIAN APPRENTICESHIP PROGRAM: LEVEL 4 47