Analogue Servo Fundamentals Trainer

Transcription

1 Analogue Servo Fundamentals Trainer USB

2 Analogue Servo Fundamentals Trainer USB Feedback Feedback Instruments Ltd, Park Road, Crowborough, E. Sussex, TN6 2QR, UK. Telephone: +44 (0) , Fax: +44 (0) website: Manual: USB Ed Printed in England by Fl Ltd, Crowborough Feedback Part No USB

3 Notes

4 Preface THE HEALTH AND SAFETY AT WORK ACT 1974 We are required under the Health and Safety at Work Act 1974, to make available to users of this equipment certain information regarding its safe use.+ The equipment, when used in normal or prescribed applications within the parameters set for its mechanical and electrical performance, should not cause any danger or hazard to health or safety if normal engineering practices are observed and they are used in accordance with the instructions supplied. If, in specific cases, circumstances exist in which a potential hazard may be brought about by careless or improper use, these will be pointed out and the necessary precautions emphasised. While we provide the fullest possible user information relating to the proper use of this equipment, if there is any doubt whatsoever about any aspect, the user should contact the Product Safety Officer at Feedback Instruments Limited, Crowborough. This equipment should not be used by inexperienced users unless they are under supervision. We are required by European Directives to indicate on our equipment panels certain areas and warnings that require attention by the user. These have been indicated in the specified way by yellow labels with black printing, the meaning of any labels that may be fixed to the instrument are shown below: CAUTION - RISK OF DANGER CAUTION - RISK OF ELECTRIC SHOCK CAUTION - ELECTROSTATIC SENSITIVE DEVICE Refer to accompanying documents PRODUCT IMPROVEMENTS We maintain a policy of continuous product improvement by incorporating the latest developments and components into our equipment, even up to the time of dispatch. All major changes are incorporated into up-dated editions of our manuals and this manual was believed to be correct at the time of printing. However, some product changes which do not affect the instructional capability of the equipment, may not be included until it is necessary to incorporate other significant changes. COMPONENT REPLACEMENT Where components are of a Safety Critical nature, i.e. all components involved with the supply or carrying of voltages at supply potential or higher, these must be replaced with components of equal international safety approval in order to maintain full equipment safety. In order to maintain compliance with international directives, all replacement components should be identical to those originally supplied. Any component may be ordered direct from Feedback or its agents by quoting the following information: 1. Equipment type 2. Component value 3. Component reference 4. Equipment serial number Components can often be replaced by alternatives available locally, however we cannot therefore guarantee continued performance either to published specification or compliance with international standards USB i

5 Preface DECLARATION CONCERNING ELECTROMAGNETIC COMPATIBILITY Should this equipment be used outside the classroom, laboratory study area or similar such place for which it is designed and sold then Feedback Instruments Ltd hereby states that conformity with the protection requirements of the European Community Electromagnetic Compatibility Directive (89/336/EEC) may be invalidated and could lead to prosecution. This equipment, when operated in accordance with the supplied documentation, does not cause electromagnetic disturbance outside its immediate electromagnetic environment. COPYRIGHT NOTICE Feedback Instruments Limited All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of Feedback Instruments Limited. ACKNOWLEDGEMENTS Feedback Instruments Ltd acknowledge all trademarks. IBM, IBM - PC are registered trademarks of International Business Machines. MICROSOFT, WINDOWS 2000, WINDOWS NT, WINDOWS ME, WINDOWS 98 are registered trademarks of Microsoft Corporation. ii USB

6 Contents TABLE OF CONTENTS 1 Introduction and Description Introduction Equipment Ancillary Equipment Faults Description System Discovery II Software Hardware Installation Inspection Analogue System Installing the Discovery II Environment Installation Requirements Using the Installation Program Installing the Analogue Servo Fundamentals Package Installing the Software Using the Discovery II Environment Starting Discovery II The Discovery II Environment USB TOC-1

7 Contents 6 Closed-Loop Control Systems Introduction Assignments Familiarisation Practical Initial Mechanical and Analogue Unit check Practical To Display the Waveforms Practical To display the Speed of Response of the Motor Operational Amplifier Characteristics Practical Scaling, Summation and Virtual Earth Practical Addition of ac Signals Motor, Tachogenerator and Brake Characteristics Practical Steady-State Characteristics Practical Steady-State Characteristics- Brake Load Practical Transient Response of Motor Practical Motor Time Constant Error Channel and Feedback Polarity Practical Feedback Polarity Practical Input and Output Rotation Directions The Influence of Gain Practical Step Response TOC USB

8 Contents 6 Velocity Feedback Practical Simple Velocity Feedback System Following Error Practical Following Error Practical Velocity Feedback Unstable System Practical Additional Time Constant Practical Unstable System Speed Control System Practical Closed-loop Speed Control Introduction to 3-Term Control Practical Derivative Measurement Practical Operational Amplifier Integrator Practical Term Controller Test Application of 3-Term Control Practical Proportional plus Derivative (P + D) Control Practical X-Y Display of Error Components Practical Following Error with Derivative Control Practical Elimination of Following Error Practical Elimination of Disturbance Practical Relation between Vref and Vs USB TOC-3

9 Contents 11.7 Practical 11.7 Response to Output Loading Single Amplifier Control Circuits Practical Single Amplifier P + D Control Practical Single Amplifier P + I control Practical Importance of Resistor in Amplifier Feedback Practical Single Amplifier 3-term Control Transient Velocity Feedback and Derivative Feedforward Practical Transient Velocity Feedback Practical Feedforward Transfer Functions and Closed-Loop Frequency Response Principles Practical Frequency Response of Time Constant Practical Frequency Response of Integration Practical Closed-Loop System Experiments Practical Oscillation Application of Frequency Response Method to the Control System Practical Time Constant Practical Motor Transfer Practical Proportional plus Derivative Control Practical Integral Control Practical Velocity (Tachogenerator) Control TOC USB

10 Contents Appendix A Switched Faults A-1 Appendix B Mechanical Unit Details B-1 Appendix C Re-numbering 33 Series and SFT154 C-1 Appendix D General Discovery FAQ s D-1 Appendix E Specific Discovery FAQ s E-1 Appendix F Troubleshooting F USB TOC-5

11 Contents Notes TOC USB

12 Chapter 1 Introduction and Description 1 Introduction and Description 1.1 Introduction The Servo Fundamentals Trainer is intended to provide students with a sound introduction to the principles of analogue servomechanisms, and by extension to those of closed-loop systems more generally. The consists of two units, plus software: Analogue Unit Mechanical Unit Discovery II Software which are connected as in Figure 1-1, where dotted boxes represent essential additional items. An optional Digital unit the is available which allows the Mechanical unit, , to be used in conjunction with a computer. Figure 1-1: Principal System Interconnections USB 1-1

13 Chapter 1 Introduction and Description 1.2 Equipment Qty Designation Description Analogue Unit Mechanical Unit, supplied with: 1 34-way terminated cable 1 Lead 600 mm, 4 mm plugs, 4-way 14 Leads 200 mm, 2 mm plug 6 Leads 400 mm, 2 mm plug 3 Fuse2A. 1.3 Ancillary Equipment The following items of equipment are required in addition to the trainer: 1.4 Faults Qty Designation Description 1 Power Supply Unit ±15 V dc, 1.5 A; +5V dc, 0.5 A (eg Feedback DC Power Supply PS446 or ) 1 Oscilloscope: Storage or long persistence, preferably with X-Y facility (eg Feedback ) The contains a number of switchable faults causing system malfunction and providing experience in fault finding. The faults are controlled by DIL switches on the analogue unit and the faults occur throughout the system. The relation between switch location and fault can be changed by a removable header under the unit panel. See Appendix A. A fault is introduced by setting numbered switches to the ON (upwards) position USB

14 Chapter 1 Introduction and Description 1.5 Description Mechanical Unit Contains a power amplifier to drive the motor from an analogue or switched input. The motor drives the output shaft through a 32:1 belt reduction. The motor shaft also carries a magnetic brake disc and an analogue speed transducer (tachogenerator). A two-phase pulse train for digital speed and direction sensing is also derived from tracks on the brake disc. The output shaft carries analogue (potentiometer) and digital (64 location Gray code) angle transducers. The unit contains a simple signal generator to provide low frequency test signals, sine, square and triangular waves, and requires an external power supply providing: +15V,0, 15Vat1.5A +5 V, 0, at 0.5 A The Feedback PS446 or are suitable Analogue Unit Connects to the Mechanical Unit through a 34-way ribbon cable which carries all power supplies and signals enabling the normal circuit interconnections to be made on the Analogue Unit using the 2 mm patching leads provided. The unit enables a basic system as in Figure 1-2 to be configured and contains facilities to introduce compensation to investigate improvement in overall system performance. Figure 1-2: Analogue Control System See Appendix C for details of the new numbering system and a minor change to the specification, introduced with the new numbers USB 1-3

15 Chapter 1 Introduction and Description System Analogue Unit (33-110) Figure 1-3: The Analogue Unit Figure 1-3 shows the general arrangement of the panel, interconnections are made by 2 mm plug leads and there are a few 4 mm sockets for conversion or oscilloscope connections USB

16 Chapter 1 Introduction and Description Upper portion of panel from left to right θ i, θ o These sockets give the voltage signals from the input and output shaft potentiometers. These are represented diagrammatically in the centre of the panel, the potentiometers themselves being in the Mechanical Unit. θ o This socket provides a reversed output shaft signal required for certain applications. Fault switches Error Amplifier Potentiometers P 1 and P 2 Power amplifier Motor Brake disc and magnet Tachogenerator Lower portion of panel from left to right These enable faults to be introduced. For normal (no fault) operation all switches should be down. This is used to combine potentiometer signals to provide the error. These provide system gain control and tachogenerator signal adjustment. This drives the motor. The two inputs drive the motor in opposite directions for a given input. The zero adjustment enables the motor to be rotated with no amplifier input. This is in the Mechanical Unit and drives the brake disc and tachogenerator directly, and the output shaft through a 32:1 belt reduction. These are in the Mechanical Unit and provide an adjustable load for the motor. This is mounted on the motor shaft and provides a voltage proportional to motor speed; the voltage is available with reversed polarity. ±10 V step This enables a manually switched 10 V step input to be obtained. Test signals These sockets provide ±10 V low frequency (nominally 0.1 to 10 Hz) square and triangle waveforms. The frequency control and range switch are on the Mechanical Unit. A sine wave test input is available from the Mechanical Unit External input potentiometer P 3 Controller This can be linked to any input to provide an adjustable input to the error amplifier. This contains operational amplifiers with associated networks to enable various compensating and control circuits to be introduced to improve the performance of a basic system USB 1-5

17 Chapter 1 Introduction and Description Mechanical Unit (33-100) Figure 1-4 shows the general arrangement of the panels. The unit is common to both Analogue and Digital systems. Since all signals, including supplies, for both units are available from the 34-way socket, the unit can be operated from any source of suitable signals connected to the 34-way socket. For full details refer to Appendix B Power Supplies External supplies of +15 V and 15 V at 1.5 A and of +5 V at 0.5 A are required. The input sockets (4 mm) are protected against accidental misconnection of supplies, though misconnection may blow a fuse. Figure 1-4: Mechanical Unit USB

18 Chapter 1 Introduction and Description Motor shaft Brake disc and magnet Speed tracks and readers Motor check switch Armature current signal Input shaft Test signal frequency and range switch Output shaft Digital measurement and readers Index pulse Output speed display This carries the brake disc, together with a 2-phase speed track and tachogenerator. The brake is applied by the lever projecting at the left. The lever scale is provided to enable settings to be repeated. These provide two-phase, 0-5 V square waves at 8 cycles per revolution. These signals are available on the 34-way socket but are not used in the Analogue system. This enables the motor to be rotated as an initial check. See initial check procedure in Chapter 2. This is a voltage waveform indicating the armature current with scale of 1 V/A. This carries the input potentiometer and scale and gives a signal θi in the range ±10 V. These control the internal oscillator to provide ±10 V square, triangular and sine waveforms with nominal frequency 0.1 to 10 Hz in two ranges. The square and triangular waveforms are connected to the 34-way socket. This carries the output potentiometer and digital angular measurement tracks. The potentiometer provides θ o in the range ±10V. The digital tracks give 6 bit Gray code (64 locations) information and are read by infra-red readers. The 6-bit information is supplied as 0 or 5V to six pins on the 34-way socket. At one pulse per revolution this provides an output shaft reference point for incremental control connected to a pin on the 34-way socket. This provides a direct reading of output shaft speed in r/min in the range 00.0 to 99.9, derived from the tachogenerator. Since the reduction ratio is 32:1, a motor speed of 1000 r/min gives 31.1 r/min at the output shaft USB 1-7

19 Chapter 1 Introduction and Description Display Facilities Required Many results from the Analogue System are presented as waveform displays on an oscilloscope, and since the responses are slow, perhaps taking 0.5 second or more, it is essential to have an oscilloscope with a long persistence screen or a storage oscilloscope. Using a conventional double beam oscilloscope it is convenient to trigger the trace in synchronism with any test signal to obtain a repetitive display. In the the trigger can be obtained from the square wave which is always available on the Mechanical and the Analogue Unit. If the oscilloscope has an external trigger facility this should be used so that the trace(s) remain synchronised with the Y input. The general arrangement would be as in Figure 1-5(a). Here it is assumed that the square waveform is being applied to the system and also to the EXT sync input, and a response is being displayed on Y 1. Another response could be displayedony 2. If the test frequency is changed the time base may require adjustment. Figure 1.5: Alternative Forms of Oscilloscope Display USB

20 Chapter 1 Introduction and Description If an X-Y oscilloscope is available, a very convenient form of display can be obtained as in Figure 1-5(b). The X deflection is obtained from the triangle test signal waveform and hence the spot is deflected across the screen in time T, where T is the period of the 2 square wave, and returns with a reversed scan of the same duration. Thus the display obtained is exactly as in (b), but a half cycle can be arranged to fill the screen by adjustment of the X gain. If the test frequency is changed the X deflection remains constant. Thus the display is always synchronised with the square wave and no adjustment is required. 1.7 Discovery II Software The Discovery II philosophy is to combine hardware and software to form an integrated, experiment-based, learning environment. Each Discovery II product deals with a particular subject, and comprises a number of Assignments, each of which covers a particular topic. The main emphasis is on practical work; Discovery II software is not simply a textbook on a computer screen. Assignments are divided into practicals, the objectives of which are clearly defined. Every practical is designed to contain as much circuit investigation, measurement and observation as possible. Explanatory text, diagrams and instrumentation are fully integrated. The Discovery II software learning environment contains: General instructions on how to operate the system. The objectives of each assignment. The objectives of each practical within the assignment. Background information for each practical within the assignment. Detailed instructions for each practical. Additional mathematical theory for some topics. Structured essay-type questions. Discovery II is the newest version. It is based around an Internet browser, allowing for the provision of higher quality graphics and giving a the student a richer learning environment USB 1-9

21 Chapter 1 Introduction and Description Notes USB

22 Chapter 2 Hardware Installation 2 Hardware Installation 2.1 Inspection Check the units supplied for mechanical damage. Check that the leads listed in the Equipment Section in Chapter 1 are all present. In order to check that the equipment is operating satisfactorily, the procedures on the following pages should be carried out. 2.2 Analogue System Connect together the Analogue Unit and the Mechanical Unit using the 34- way ribbon cable supplied. Connect the Mechanical Unit to a suitable power supply (Feedback Power Supply recommended) using the 4 mm lead provided. The connections are: Red +15 V Orange +5 V Black 0 V Blue 15 V Ensure all of the fault switches on the Analogue Unit are off (down). Switch on the power supply. The motor on the Mechanical Unit may revolve and the speed/rpm display should light. Adjust the power amplifier zero control to be found on the right-hand side of the Analogue Unit. The motor should drive in both directions, controllable by the zero knob. With the display switch set to RPM the display should read the output shaft speed in r/min. Set the zero control so that the motor is stopped. Set SW1 on the Analogue Unit to its centre position and the 'ext input' potentiometer, P3, fully anti-clockwise, then connect up the Analogue Unit as shown in Figure USB 2-1

23 Chapter 2 Hardware Installation Switch SW1 to +10 and slowly increase P3. The motor should drive and increase speed. Reset P3 fully anti-clockwise. Switch SW1 to 10 and slowly increase P3. The motor should drive in the opposite direction. Figure 2-1: Connections for Testing the Analogue Unit USB

24 Chapter 3 Installing the Discovery II Environment 3 Installing the Discovery II Environment Discovery II software comes in two main parts the environment and the learning packages. Each software part is supplied on its own CD-ROM. To use the learning packages, thediscovery II environment must first be installed, followed each individual learning package. This chapter details the installation of the Discovery II environment This is followed by a description of how it can be used (Chapter 5). FAQ s are provided in Appendix A and B. A description of how to install your separate learning package follows in Chapter 4 and details of the assignments available within the learning package in Chapter 7. If you are using Discovery Manager or another Managed Learning Environment (MLE), please refer to your Discovery Manager or Virtual Campus manual before continuing. 3.1 Installation Requirements In order for the Discovery II environment software to operate correctly, the computer must have the following minimum specification. Minimum CPU Pentium III 750 MHz Minimum RAM 64 MB (128 MB for Windows 2000) Operating System Windows 98/ME or Windows NT/2000 Hard Disk EIDE 6 GB Video Adapter Capable of supporting SuperVGA (800x600) Mouse Microsoft mouse (or 100% compatible) If your computer is to be used as a stand-alone workstation, a copy of the Discovery II software (environment and learning package) must be installed on each computer that will be used as a Discovery II workstation, provided your license permits. If you wish to use your computer as a part of a network, you may wish to consider installing the Discovery II software as a served-solution. Details of how to achieve this may be provided as a separate document that can be found on the CD-ROM. Sometimes, a special readme.txt file is provided on the CD-ROM. This file contains up to date information relating to the software that you are to use. Before beginning any installation, please check the distribution CD for any special instructions before following the rest of these instructions USB 3-1

25 Chapter 3 Installing the Discovery II Environment 3.2 Using the Installation Program Discovery II environment software is supplied on CD-ROM. When you place the CD-ROM in the CD drive of your computer, it should run automatically provided that Microsoft Internet Explorer 5.5, or greater, is installed on your machine. When started, a Discovery II Installations Environment page is displayed similar to the one shown in Figure 3-1. Figure 3-1: Discovery II Installations - Environment Page When the Discovery II Installations page is displayed, click on the Discovery II environment option Install (hyperlink). Note: You should NOT proceed if Netscape Navigator is started when you insert the CD-ROM. You must uninstall Netscape using the Add/Remove control panels. Discovery II will not work with Netscape Navigator. If you are unable to run Internet Explorer or to uninstall Netscape Navigator, please contact your local network manager or systems support personnel for advice USB

26 Chapter 3 Installing the Discovery II Environment When the installation begins, you will be asked a question similar to the one that is seen in Figure 3-2. This is because the installations are run from an Internet Browser, the browser wants to be sure about security. Answer Run this program from its current location and then click on the OK button to confirm. Figure 3-2: Running an Installation from a Web Page A further second prompt will ask a further security question as shown in Figure 3-3. To this, answer Yes. There will be a short pause while the installation program is loaded from the CD-ROM drive. Figure 3-3: Allowing the chosen Installation Program to Start USB 3-3

27 Chapter 3 Installing the Discovery II Environment When the installation program is running, it shows the files that are currently being copied to the hard drive of your computer and how much free space is remaining. A progress bar shows how much of the installation is remaining. If any of these do not auto-install, manually run the DiscoII.exe program from the relevant folder on the CD Manual Installation of Microsoft Internet Explorer If you find that you do not have a correct version of an Internet browser on your machine, one must be installed. To install Microsoft Internet Explorer from the CD-ROM, open the CD-ROM drive folder by using your 'windows explorer' program and go to a folder called ie5. Launch the Explorer installation program by double-clicking with the mouse on the file ie5setup.exe. This will automatically install the browser software used by Discovery II. It is important to use the version 5.5 of the Microsoft Internet Explorer program or later. Using an earlier version may cause difficulties. Even if your computer has no connection to the internet, the Internet Explorer software is required since it is this software that is used to display the Discovery II software Manual Installation of Discovery II Environment If your CD-ROM does not automatically display the Installations page as shown in Figure 3-1 and you know that you have got a correct version of the Internet Explorer program installed on your computer, you must begin the Environment Installation manually. To begin the Discovery II environment installation process manually, open the CD-ROM drive folder by using your 'windows explorer' program (accessible from your computers Start button) and double-click on the index.htm file found in the root of the CD. When the Discovery II Installation page is displayed as shown in Figure 3-1, click on the Discovery II environment option Install (hyperlink). Installation of the program is then as described for automatic installation USB

28 Chapter 4 Installing the Analogue Servo Fundamentals Package 4 Installing the Analogue Servo Fundamentals Package 4.1 Installing the Software Before commencing with the installation, you must also have installed the Discovery II environment, as described within the Chapter 3, and also the latest version of Internet Explorer. The environment allows you to gain access to all Feedback products in a consistent easy to use way. To check to see whether the Discovery II has been installed on your computer, you should go to the Add/Remove Programs option within the control panel. Figure 4-1 illustrates the entry that you should see if the Discovery II environment has been installed. Figure 4-1: Windows 98 Add/Remove Programs display If you are using the networked Discovery Manager, please refer to the Discovery Manager user manual for further details describing how to install Discovery software in a networked laboratory USB 4-1

29 Chapter 4 Installing the Analogue Servo Fundamentals Package When installing to a stand-alone version, you should begin by inserting the CD-ROM into your computer. A web-page similar to Figure 4-2 is displayed automatically: Figure 4-2: Discovery II Installations screen You should install each of the programs contained within the products section and below in the order in which they are given. When a file has been selected, you will be asked whether you wish to open or save this program, to which you must answer open. This message appears as a part of a security mechanism within Internet Explorer. When the program has started, the installation program places the software into the correct location within the Discovery environment. When the installation program has appeared to finish copying files, a short delay will be noticed until the InstallShield program disappears completely from your computer. Do not restart your computer during the installation process, otherwise you may experience some difficulties. The delay is important, since it updates the Discovery environment, allowing it to access the newly installed learning package. Note that Windows 2000 is a more secure operating system than Windows 98. Only users with administrative privileges are authorised to install the Discovery softwareona Windows 2000 machine. If you try to install without the correct permissions, an error will result. If this occurs, you should contact your institution s technical support regarding gaining temporary administrative permissions USB

30 Chapter 5 Using the Discovery II Environment 5 Using the Discovery II Environment This chapter describes how to use the Discovery II environment. 5.1 Starting Discovery II The installation software places a short-cut on your desktop like the one shown in Figure 5-1. You can start the Discovery II environment by double-clicking on this Icon: Figure 5-1 If you prefer, you can also start the environment by clicking on your computer s Start button found at the bottom left-hand side of your screen, and moving to the programs, feedback directory. Starting the Discovery II environment starts the Internet Explorer program and opens the Discovery II environment. When starting for the first time, you will be prompted with a security warning asking you whether you wish to run the software. To this question, you should tick the box, always trust software from Feedback Instruments and then click on the yes button to tell the Internet Explorer program that you always wish to trust the software. If you answer no accidentally, you should close the Internet Explorer window and restart by double-clicking on the icon again. Further information can be found in the section entitled Security Warnings. 5.2 The Discovery II Environment The Discovery II environment comprises of three parts; a content frame that is used to display the assignment and practical information, an explorer tree that is used to navigate through the learning material, and a title bar. The Discovery II explorer, the menu tree that is displayed on the left of Figure 5-2, is the central navigation control. It is used to choose assignments and launch practicals in the practical frame which occupies two thirds of the display. Shown on the current display is the Discovery II Home page the initial page that is displayed when the Discovery environment is launched USB 5-1

31 Chapter 5 Using the Discovery II Environment Title Bar Browser Tool Bar Explorer Tree Content Frame Figure 5-2: Discovery II Explorer Tree and Home The top-level menu tree displays all the Feedback Discovery II products that have been installed into your Discovery II environment. Double-clicking on these books displays a list related subject components, and then going further, this reveals a list of assignments and subsequently practicals. Figure 5-3 shows a fragment of the Explorer Tree, where some practicals are displayed. Figure 5-3: Fragment of Discovery II Explorer Tree USB

32 Chapter 5 Using the Discovery II Environment Icons used in the Explorer Tree are as follows: Subjects Covered Found within an assignment group. Details the topics that are covered within this assignment and details any pre-requisite assignments that must have been performed. Theory This is theoretical information about the assignment topic and usually deals with the more mathematical aspects. You DO NOT need to read or understand the theory to carry-out the assignment. Background The purpose and objectives of each practical are here. You MUST read the background to a practical before attempting the practical work. From the background text, you can start an associated practical. Practical Instructions for the hands-on practical work are here. During a practical, you can view a set of related questions by clicking on the question button. Questions Questions associated with each practical are here. These can be printed out and completed by the student. Answers Some learning packages contain sets of answers so that captured data from real instruments can be compared with expected results. Information The information items provide additional details about a product group. They may detail the hardware that is used for a particular product, the way that they can be configured and the type of material that a product will contain USB 5-3

33 Chapter 5 Using the Discovery II Environment Navigation using the Content Frame The Content Frame has been designed to run in Full-Screen Mode. When you are using Discovery II it is recommended that as soon as it is loaded, the Internet Explorer display is changed to Full Screen Mode. This can be performed by clicking on the Full Screen button on the Browser Toolbar (see Figure 5-2). If the Full Screen button does not exist on the tool bar, it can be added manually by pressing the right-hand mouse button when the mouse is over the Browser Toolbar. Figure 5-4: Adding the Full Screen Toolbar Button Figure 5-5 shows the Internet Explorer displaying Discovery II environment in Full Screen Mode, where the Theory pages for one of the learning packages has been opened as an example. Many pages display a control bar which can be seen at the bottom of the Content Frame. This displays a number of icons that allow you to conveniently change between sections of the learning material. The Theory and Background texts may consist of several pages. You can move between pages by clicking on forward ( ) and back ( ) buttons at any time USB

34 Chapter 5 Using the Discovery II Environment Figure 5-5: Full-Screen Display The icons that can appear on the control bar are as follows: Subjects Covered Theory Practical Questions Print USB 5-5

35 Chapter 5 Using the Discovery II Environment One or more of these buttons will automatically be displayed, apart from the Print button which will always be displayed Hiding the Explorer Tree At any time, the explorer tree can be hidden by using two direction controls located on the right of the Title Bar, as seen in Figure 5-6. Hide Tree Show Tree Figure 5-6: Title Bar Controls In normal operation of the environment, the explorer tree will be displayed during the use of the learning packages. In some cases, the whole screen may be required to display a particular concept or be needed to facilitate the operation of some practicals. Instead of using the Hide and Show buttons, the division between the explorer tree and the content tree can be altered at any time by moving the mouse to the dividing line, clicking and holding, then moving the mouse to the required position Printing Printouts from Discovery can be obtained in a number of different ways. The browser provides its own printing function on the toolbar. The print icon: should be used in preference to the browser print toolbar button to print the contents of an entire page. In some learning packages, graphs and tables are provided for result collection. These are designed to be printed and distributed to students and returned to a tutor. All theory and background pages along with practical text can be printed for students reference. Individual practicals may provide their own printing mechanisms depending upon the types of instrument and hardware that is utilised. Two instances of these include the use of the virtual oscilloscope and the virtual spectrum analyser. A print icon, similar to the one illustrated above can be found on the surface of the instrument. To learn how to use this facility, refer to specific learning package chapters or manuals USB

36 Chapter 5 Using the Discovery II Environment Security Warnings After using Discovery for a period of time, it is very likely that you may be prompted with a Security Warning as shown in Figure 5-7. These warnings do not constitute a problem with Discovery software, but are an integral part of the Internet Explorer and Windows operating system. Figure 5-7: Example Security Warning Discovery software has the potential to be loaded and run from the internet. To run, Discovery needs to access important resources of the local PC to perform file handling and to perform input/output operations. Since many computers can potentially access software over the internet, the designers of web-browsers have created a mechanism that allows only trusted software to access certain resources of your computer. Untrusted software is prevented from started by the web-browser preventing potentially malicious programs from causing damage to your computer by deleting files or accessing other computers on your local network, for example. To ensure security, software packages are issued with certificates which expire over the period of one year as a security measure. Updates to the certified Discovery software package will be made available over time to users, which will be able to be accessed through our web-site USB 5-7

37 Chapter 5 Using the Discovery II Environment If your computer is being used for Discovery only, you can reduce the level of security checks that are being displayed to avoid the constant security messages. There are several options that can be altered, one of which is shown in Figure 5-8. Figure 5-8: Altering Security Permissions CAUTION: It is advisable to use options such as these with extreme care, and only when the computer which uses Discovery is not used to access the internet USB

38 Chapter 6 Closed-Loop Control Systems 6 Closed-Loop Control Systems 6.1 Introduction What is an Automatic Control System? This is a system in which we are controlling the state of a Process, say the width and thickness of strip being rolled in a steel mill. In setting up the system we need to know what the required width and thickness are, and to set up reference or input signals to represent these values. We are able, by means of transducers, to generate similar signals to represent the actual values at the output of the process. We can then compare the actual width and thickness of the strip produced with those required. The system must be able, if there is a difference or error, to send modifying signals to an Actuator, in this case the motor and gearing controlling the roller setting The Closed-Loop Control System Thedifferenceorerror signal may be thought of as producing effects which move forward, from the point of comparison to the resulting action. The comparison itself depends on a signal which is fed back from the output of the process to be compared with the reference or input signal. The forward flow and feedback of signals form a loop around which information flows, Figure 6-1. Such a system is therefore called a closed-loop system. Figure 6.1: The Closed Control Loop USB 6-1

39 Chapter 6 Closed-Loop Control Systems Various names are given to the signals in different industrial or other contexts, but the meanings of words in any one of the columns below are much the same: input output difference reference value actual value error set value measured value deviation setpoint controlled quantity desired value demanded value Where the system is electrical, the state will normally be represented by signals expressed in volts; in our example it might be, for the width, a signal representing ten inches per volt. In this manual, the difference in the comparison will be called the error signal and the part of the system that carries out the comparison is the error channel. There is usually a power amplifying device to drive the Actuator (which in Figure 6-2 is the geared motor). It is usual for control engineers to describe their systems in a block diagram form. The block diagram below describes the type of system we shall be using in the assignments. Here there is a comparison by the error channel of the input and output, the error is then amplified to drive a motor and gearing in the forward path so that the speed or position of the output shaft can be modified. Figure 6.2: Block Diagram of an Analogue Closed-Loop System USB

40 Chapter 6 Closed-Loop Control Systems Analogue & Digital Systems In the system of Figure 6-2, it is assumed that the input and output are measured as voltages and lead to an error voltage which is amplified to operate the motor. This system has an analogue error channel since input and output are measured as continuous voltages. However, it is common practice to use digital techniques to generate the error signal in digital form, either by digitising the input and output by an analogue-to-digital (A/D) converter or by direct digital measurement techniques. The error signal is then processed in a computer to generate a digital signal to drive the motor. The motor may then be driven from a digital-to-analogue (D/A) converter or digitally by switching techniques. Thus the system may take the general form of Figure 6-3. Figure 6-3: Block Diagram of a Digital Closed-Loop System The digitising of inputs may be within the system or in an internal computer interface. The computer-generated motor command will be digital and may be converted to analogue form in the computer interface or within the system. Alternatively the command may be used to drive the motor by a switching technique. The Feedback Servo Fundamentals Trainer (33-002) provides facilities to investigate purely analogue systems as Figure 6-2, or systems involving a range of digital techniques as Figure 6-3. For the digital techniques it is necessary to use an IBM-compatible PC in which a Feedback interface unit has been installed, plus a Digital Board The Assignments in this manual relate only to the analogue system. Assignments to investigate the digital system are provided as interactive Discovery software supplied with the system USB 6-3

41 Chapter 6 Closed-Loop Control Systems Notes USB

42 Chapter 7 Assignments 7 Assignments This chapter contains assignments which relate to the Analogue and Mechanical Units, which together form analogue control systems. The assignments are: Part I Basic Elements 1 Familiarisation 2 Operational Amplifier Characteristics 3 Motor, Tachogenerator and Brake Characteristics Part II Simple Control & Speed Systems 4 Error Channel and Feedback Polarity 5 The Influence of Gain 6 Velocity Feedback 7 System Following Error 8 Unstable System 9 Speed Control System Part III Improving System Performance 10 Introduction to 3-Term Control 11 Application of 3-Term Control 12 Single Amplifier Control Circuits 13 Transient Velocity Feedback and Derivative Feedforward 14 Transfer Functions and Closed-loop Frequency Response Principles 15 Application of Frequency Response method to the Control System USB 7-1

43 Chapter 7 Assignments Notes USB

44 Assignment 1 Familiarisation 1 Familiarisation The following Practicals are included in this assignment: 1.1 Initial Mechanical and Analogue Unit check 1.2 To Display the Waveforms 1.3 To Display the Speed of Response of the Motor USB 7-1-1

45 Assignment 1 Familiarisation CONTENT The practicals in Assignment 1 provide some introduction to the before more detailed investigations are carried out. EQUIPMENT REQUIRED Qty Designation Description Analogue Unit Mechanical Unit 1 Power Supply ±15 V dc, 1.5 A; +5 V dc, 0.5 A (eg Feedback PS446 or ) 1 - Oscilloscope, storage or long-persistence, preferably with X-Y facility. (eg, Feedback ) USB

46 Assignment 1 Familiarisation OBJECTIVES When you have completed this assignment you will: Realise that the equipment comprises sub-systems which may be combined various ways to make control systems. Be familiar with two of the sub-systems, the Mechanical Unit and the Analogue Unit. KNOWLEDGE LEVEL Before you start this assignment you should: Have some experience of using an electric motor. Have some experience of handling electronic circuits. Know how to use an oscilloscope PRELIMINARY PROCEDURE The Power Supply should be connected by 4 mm-plug leads to the +15 V, +5 V, 0 V and 15 V sockets at the back of the Mechanical Unit USB 7-1-3

47 Assignment 1 Familiarisation Figure 7-1-1: The Analogue Unit USB

48 Assignment 1 Familiarisation Figure 7-1-2: The Mechanical Unit. Initially examine the Mechanical and Analogue Units without making any connections and identify all items mentioned in Figures and The digital facilities in the Mechanical Unit, speed and output tracks, will not be used with the Analogue Unit. 1.1 Practical Initial Mechanical and Analogue Unit check With the power supply switched OFF, connect its outputs to the Mechanical Unit. Set the brake fully upwards. The Analogue Unit should not be connected. Switch the power supply ON. The motor should remain stationary there may be a slight movement when the supply is actually switched USB 7-1-5

49 The output shaft speed display should show: Assignment 1 Familiarisation 0.00 This indicates that the 5V supply is operating. Hold the motor check switch to the right and the motor should run clockwise and the output speed display should indicate 15 to 25 rpm. Hold the switch left and the motor should run anti-clockwise with approximately the same speed. This test indicates that the ±15 V supplies are operating. Hold the check switch to one side and gradually lower the brake to maximum. The motor should slow down. These tests indicate that Power Supply and Mechanical Unit are operating correctly. Switch the power OFF. Connect the Analogue Unit to the Mechanical Unit by the 34-way cable. Raise the brake fully. Switch the power ON. Rotating the power amplifier zero adjustment should enable the motor to be driven in both directions up to about the same speed as before. Zero the amplifier to stop the motor. Overall the tests indicate that the system is working correctly USB

50 Assignment 1 Familiarisation 1.2 Practical To Display the Waveforms It is assumed that a suitable oscilloscope is available with: EITHER A single Y channel or preferably two Y channels when used in conjunction with a time base, with External sync input for the time base. OR A facility for X-Y operation with X and Y both able to operate with a dc input. Test Waveforms System Waveforms Connect the oscilloscope to the test signals using either the 4mm sockets in the Mechanical Unit or the 2mm terminals in the Analogue Unit. Observe that the frequency may be varied between 0.1 and 1 Hz or 1 and 10 Hz. The system waveforms may be observed either from an externally triggered display against a timebase as shown in Figure 7-1-3(a) or from an X-Y display as shown in Figure 7-1-3(b). Signal source sockets are provided on the Mechanical Unit (4 mm) and the Analogue Unit (2 mm). Figure 7-1-3: Oscilloscope Connections and Display USB 7-1-7

51 Assignment 1 Familiarisation Figure 7-1-4: Connections for Practical USB

52 Assignment 1 Familiarisation 1.3 Practical To display the Speed of Response of the Motor Set P 3 to zero and make the connections on the panel shown in Figure 7-1-4(a). Connect the oscilloscope to the system using the selected method of display. If the oscilloscope input has a 4 mm plug use the transfer socket as shown dotted. This arrangement enables the square wave test signal to be applied to the power amplifier when P 3 is adjusted. Set the test frequency to about 0.2 Hz. Set P 3 to about 30. The motor should rotate in both directions, giving speed displays as in Figure 7-1-4(b), using an X-Y connection; or against a time base as in (c). Note that the X-Y connection may give either of the two displays shown in Figure 7-1-4(b) depending on the oscilloscope in use. Examine the effect of increasing or decreasing the test frequency between 0.1 and 1 Hz. SUMMARY PRACTICAL ASPECTS This assignment has provided a general look at the basic features of the Analogue and Mechanical Units of the equipment. The last practical shows that there is a delay in the motor response to an input, which is due to the mechanical inertia of the armature. All motors exhibit this general characteristic, which has very important consequences for control system design. Special armature design can reduce the inertia greatly for small motors USB 7-1-9

53 Assignment 1 Familiarisation Notes USB

54 Assignment 2 Operational Amplifier Characteristics 2 Operational Amplifier Characteristics The following Practicals are included in this assignment: 2.1 Scaling, Summation and Virtual Earth 2.2 Addition of AC Signals USB 7-2-1

55 Assignment 2 Operational Amplifier Characteristics CONTENT The characteristics of an operational amplifier are investigated and its application to analogue signal scaling and summation is examined. EQUIPMENT REQUIRED Qty Designation Description Analogue Unit Mechanical Unit 1 Power Supply ±15 V dc, 1.5 A; +5 V dc, 0.5 A (eg, Feedback PS446 or ) 1 Oscilloscope, storage or long-persistence, preferably with X-Y facility. (eg, Feedback ) USB

56 Assignment 2 Operational Amplifier Characteristics OBJECTIVES When you have completed this assignment you will know: That an operational amplifier is a dc amplifier providing a very high negative gain. That an operational amplifier is invariably used with feedback, the nature of which almost completely determines the amplifier s behaviour. That operational amplifiers may be used to provide scaling of analogue signals and/or summation of several such signals. KNOWLEDGE LEVEL Before you start this assignment you should: Have completed Assignment 1, Familiarisation. Understand the general function of an amplifier. PRELIMINARY PROCEDURE The Analogue Unit and Mechanical Unit should be connected together by the 34-way ribbon cable. The Power Supply should be connected by 4 mm-plug leads to the +15 V, +5 V, 0 V and 15 V sockets at the back of the Mechanical Unit USB 7-2-3