DOCSLIB.ORG

Overview of Distributed Control Systems Formalisms 253

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Overview of Distributed Control Systems Formalisms 253 View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by DSpace at VSB Technical University of Ostrava Overview of distributed control systems formalisms 253 OVERVIEW OF DISTRIBUTED CONTROL SYSTEMS FORMALISMS P. Holeko Department of Control and Information Systems, Faculty of Electrical Engineering, University of Žilina Univerzitná 8216/1, SK 010 26, Žilina, Slovak republic, tel.: +421 41 513 3343, e-mail: [email protected] Summary This paper discusses a chosen set of mainly object-oriented formal and semiformal methods, methodics, environments and tools for specification, analysis, modeling, simulation, verification, development and synthesis of distributed control systems (DCS). 1. INTRODUCTION interconnected by a network for the purpose of communication and monitoring. Increasing demands on technical parameters, In the next section the problem of formalizing reliability, effectivity, safety and other the processes of DCS’s life-cycle will be discussed. characteristics of industrial control systems initiate distribution of its control components across the 3. FORMAL METHODS plant. The complexity requires involving of formal The main motivations of using formal concepts methods in the process of specification, analysis, are [9]: modeling, simulation, verification, development, and ° In the process of formalizing informal in the optimal case in synthesis of such systems. requirements, ambiguities, omissions and contradictions will often be discovered; 2. DISTRIBUTED CONTROL SYSTEM ° The formal model may lead to hierarchical semi- A common general definition of a distributed automated (or even automated) system system describes it as a system consisting of several development methods; intelligent devices cooperating for common purpose. ° The formal model can be verified for correctness Intelligent devices (microcomputers, workstations, by mathematical methods; robots etc.) support processes, which coordinate ° A formally verified subsystem can be activities and information exchange via a incorporated into a larger system with greater communication network. In order to call a device confidence that it behaves as specified; “intelligent”, it must fulfill following requirements ° Different designs can be evaluated and compared. (Fig. 1): ° Contain some kind of processor or CPU for In general, formal methods for distributed control processes realization and decision making; systems must address the following problems: ° Dispose sufficient memory capacity for I. Modeling - Select appropriate models and information storage. formal notations for adequately describing sensors/actuators controlled and control system. These notations must deal with the dynamic and reactive nature of the I/O controlled system, and allow for the proper P P RAM expression of timing properties. CPU ROM II. Verification - The verifier is presented with a IU IU IU ASIC formal mathematical model of the system, and a Comm. controller specification S of how the controlled system should P behave. The verification problem involves demonstrating that the model of the system satisfies IU IU GW IU – Intelligent Unit the specification S. GW – Gateway III. Development - In controller development a P P – Process specification S is given that the plant must satisfy ASIC – Application GW IU IU (the controller is not given). A disciplined method is Specific Integrated sought whereby designers can be helped to construct Circuit a controller so that system model satisfies S. In development the controller should be built in a Fig. 1 Example of distributed control system and modularly structured compositional fashion intelligent device structure (controller architecture) . The term Distributed Control System (DCS) IV. Synthesis – If controller development is fully denotes a control system, usually of a manufacturing automated, then such process is called synthesis. system, process or other type of dynamic system, in A chosen set of methodics, environments and which the control elements are distributed and tools fully or partially satisfying the specified 254 Advances in Electrical and Electronic Engineering requirements related to formalisms will be approach is dedicated to a special class of DCS, introduced. which are organized as several periodic processes, with nearly the same working period, but without 3.1 Sequential Function Chart common clock, and which communicate by means of shared memory through serial links or field The primary objective of Sequential Function busses. Chart (SFC++) is the implementation of graphical modeling formalism for design, validation, simulation and automatic code generation of Specification: centralized model industrial systems, graphical language for programming its control software and supervisory level control tool for control, monitoring, SCADE editor diagnostics etc. [10]. The aim of the running project Simulation Automated tests is also creation of visual programming environment, SCADE generation which integrates the advantages of object-oriented function Formal verification modeling for design and simulation and the model performance of distributed control systems (i.e. Distribution computers with real-time operating systems SCADE editor protocol interconnected via industrial networks). To bypass the differences between object-oriented model and SCADE Robust properties implementation level, on which run several parallel Distributed analyzator tasks, a standard formalism (IEC, 1988; UTE, 1992) functions is used for describing of system dynamics and programming of control system based on Sequential Environment SCADE editor Function Chart (SFC). emulation Distributed Simulation Debugging I/O Module Module implementation Automated tests model generation Formal verification Performance validation RunTime Module Local Data SCADE code Communication System SFC++ generator library Model Editor Evolution Event Module Manager Development Subsystem RunTime Subsystem code code code Fig. 3 CRISYS methodology scheme Fig. 2 SFC++ logical architecture 3.3 The ISILEIT project SFC++ is intended for control systems using single or multiprocessor computers or PLCs with The ISILEIT (Integrative Specification of real-time operating systems ((RTOS) interconnected Distributed Control Systems for the Flexible by local networks and with control buses connecting Automated Manufacturing) project aims at the the process and other devices. development of a seamless methodology for the integrated design, analysis and validation of 3.2 The Crisys project distributed production control systems [6]. Its focus is the use of existing techniques which should be The Crisys project aims at improving, improved with respect to formal analysis, simulation unification and formalization of the actual methods, and automatic code generation. The integration of techniques and tools used in the industries concerned SDL block diagrams, UML statecharts and with process control, in order to support a global collaboration diagrams formed an executable system approach when developing DCS [2],[3]. The specification language that allows specifying main result of Crisys project is quasi-synchronous reactive behavior as well as complex application approach based on synchronous language Lustre [7] specific object structures. To ensure the correctness and associated tool SCADE (Safety Critical of the design at the earliest stage, validation in form Application Development Environment) [1]. This of simulation and formal verification is integrated Overview of distributed control systems formalisms 255 into the process. The developed simulation common object-oriented problems in general environment can be used to prove that the generated domain. executable code meets the requirements. 3.5 Architecture Description Language The Architecture Description Language (ADL) System Create system Create system specification is defined as a language which disposes of model specification (FUJABA) capabilities for modeling conceptual architecture of both hardware and software systems [8]. The System object Instance of ASM language provides models, notations and tools for model V data structure V description of components and its interactions, E particularly with regard to large-scale high-level A R designs. It supports the selection of principles, Generate Instantiate system in application of architecture paradigms, abstraction executable model L I ASM meta-model and designs implementation. I F Java D I ASM model The main properties of the language include appliaction explicit specification of: C A ° components, Validate system T A Prepare ASM for ° connectors, model by model-checking ° interfaces, execution I T ° configurations. I O Model checking O input A component represents a computation unit or N data store and forms loci of computation and state. A N connector is a construction block used for modeling Run model-checker of interactions among components and for modeling Design errors or rules, which govern those interactions. The Fig. 4 ISILEIT methodology evaluation activities interfaces ensure correct connectivity and communication of components. Architectural configuration or topology is connected graph of 3.4 DCS Modeler, DCS Simulator components and connectors which describes architectural structure. During design of object-oriented simulation model of a DCS several requirements were considered [4]: 3.6 IEC 614 99 a) Dynamic behavior of each component device The IEC 61499 standard modified the Function within DCS can be considered to be a Finite State Block
Load more

Recommended publications
  • Programmable Logic Controller
    Revised 10/07/19 SPECIFICATIONS - DETAILED PROVISIONS Section 17010 - Programmable Logic Controller C O N T E N T S PART 1 - GENERAL ....................................................................................................................... 1 1.01 DESCRIPTION .............................................................................................................. 1 1.02 RELATED SECTIONS ...................................................................................................... 1 1.03 REFERENCE STANDARDS AND CODES ............................................................................ 2 1.04 DEFINITIONS ............................................................................................................... 2 1.05 SUBMITTALS ............................................................................................................... 3 1.06 DESIGN REQUIREMENTS .............................................................................................. 8 1.07 INSTALLED-SPARE REQUIREMENTS ............................................................................. 13 1.08 SPARE PARTS............................................................................................................. 13 1.09 MANUFACTURER SERVICES AND COORDINATION ........................................................ 14 1.10 QUALITY ASSURANCE................................................................................................. 15 PART 2 - PRODUCTS AND MATERIALS.........................................................................................
    [Show full text]
  • Control Theory
    Control theory S. Simrock DESY, Hamburg, Germany Abstract In engineering and mathematics, control theory deals with the behaviour of dynamical systems. The desired output of a system is called the reference. When one or more output variables of a system need to follow a certain ref- erence over time, a controller manipulates the inputs to a system to obtain the desired effect on the output of the system. Rapid advances in digital system technology have radically altered the control design options. It has become routinely practicable to design very complicated digital controllers and to carry out the extensive calculations required for their design. These advances in im- plementation and design capability can be obtained at low cost because of the widespread availability of inexpensive and powerful digital processing plat- forms and high-speed analog IO devices. 1 Introduction The emphasis of this tutorial on control theory is on the design of digital controls to achieve good dy- namic response and small errors while using signals that are sampled in time and quantized in amplitude. Both transform (classical control) and state-space (modern control) methods are described and applied to illustrative examples. The transform methods emphasized are the root-locus method of Evans and fre- quency response. The state-space methods developed are the technique of pole assignment augmented by an estimator (observer) and optimal quadratic-loss control. The optimal control problems use the steady-state constant gain solution. Other topics covered are system identification and non-linear control. System identification is a general term to describe mathematical tools and algorithms that build dynamical models from measured data.
    [Show full text]
  • Introduction to Feedback Control
    ECE4510/5510: Feedback Control Systems. 1–1 INTRODUCTION TO FEEDBACK CONTROL 1.1: What is feedback control? I Control-system engineers often face this question (or, “What is it that you do, anyway?”) when trying to explain their professional field. I Loosely speaking, control is the process of getting “something” to do what you want it to do (or “not do,” as the case may be). The “something” can be almost anything. Some obvious • examples: aircraft, spacecraft, cars, machines, robots, radars, telescopes, etc. Some less obvious examples: energy systems, the economy, • biological systems, the human body. I Control is a very common concept. e.g.,Human-machineinteraction:Drivingacar. ® Manual control. e.g.,Independentmachine:Roomtemperaturecontrol.Furnacein winter, air conditioner in summer. Both controlled (turned “on”/“off”) by thermostat. (We’ll look at this example more in Topic 1.2.) ® Automatic control (our focus in this course). DEFINITION: Control is the process of causing a system variable to conform to some desired value, called a reference value. (e.g., variable temperature for a climate-control system) = Lecture notes prepared by and copyright c 1998–2013, Gregory L. Plett and M. Scott Trimboli ! ECE4510/ECE5510, INTRODUCTION TO FEEDBACK CONTROL 1–2 tp Mp I Usually defined in terms 1 0.9 of the system’s step re- sponse, as we’ll see in notes Chapter 3. 0.1 t tr ts DEFINITION: Feedback is the process of measuring the controlled variable (e.g.,temperature)andusingthatinformationtoinfluencethe value of the controlled variable. I Feedback is not necessary for control. But, it is necessary to cater for system uncertainty, which is the principal role of feedback.
    [Show full text]
  • 8. Feedback Control Systems
    feedback control - 8.1 8. FEEDBACK CONTROL SYSTEMS Topics: • Transfer functions, block diagrams and simplification • Feedback controllers • Control system design Objectives: • To be able to represent a control system with block diagrams. • To be able to select controller parameters to meet design objectives. 8.1 INTRODUCTION Every engineered component has some function. A function can be described as a transformation of inputs to outputs. For example it could be an amplifier that accepts a sig- nal from a sensor and amplifies it. Or, consider a mechanical gear box with an input and output shaft. A manual transmission has an input shaft from the motor and from the shifter. When analyzing systems we will often use transfer functions that describe a sys- tem as a ratio of output to input. 8.2 TRANSFER FUNCTIONS Transfer functions are used for equations with one input and one output variable. An example of a transfer function is shown below in Figure 8.1. The general form calls for output over input on the left hand side. The right hand side is comprised of constants and the ’D’ operator. In the example ’x’ is the output, while ’F’ is the input. The general form An example output x 4 + D ----------------- = fD() --- = --------------------------------- input F D2 ++4D 16 Figure 8.1 A transfer function example feedback control - 8.2 If both sides of the example were inverted then the output would become ’F’, and the input ’x’. This ability to invert a transfer function is called reversibility. In reality many systems are not reversible. There is a direct relationship between transfer functions and differential equations.
    [Show full text]
  • (5) Type of Control Components, (6) Basic Control System, (7) Automatic
    F DOCV4F NT R F S 1,4 F ED 023 279 EF 002 096 Temperature Control. Honeywell Planning Guide. Honeywell, Minneapolis, Minn. Pub Date Mar 68 Note -26p. EORS Price MF -S025 HC -$I AO Descriptors -Building Eluipment, *Climate Control, *ControlledEnvironment, Guidelines, *Mechanical Equipment , *Temperature, Themil Environment Identifiers -Honeywell Presentsplanningconsiderations inselectingproper temperaturecontrol systems. Various aspects are discussedincluding--(1) adequate environmental control, (2) adequate control area, (3) control system design, (4) operators ratetheir systems, (5) type of control components, (6) basic control system,(7) automatic control systems, and (8) variables that affect systemperformance. (RH) U.S. DEPARTMENT Of HEALTH. EDUCATION & WELFARE OFFICE OF EDUCATION THIS DOCUMENT HAS BEEN REPRODUCED EXACTLY AS RECEIVED FROM THE PERSON OR ORGANIZATION ORIGINATING IT.POINTS OF VIEW OR OPINIONS STATED DO NOT NECESSARILY REPRESENT OFFICIAL OFFICE OF EDUCATION POSITION OR POLICY. 0144 '4; A xfr/. HONEYWELL PLANNING GUIDE TEMPERATURE CONTROL Man's own environment: The indoor airwe breathe, work in, play in, sleep and eat in. We heat it, cool it, dry it, addmoisture to it. It commands more of our attention each day. And wellit should. Tests have shown that it afiectsour productivity, our attitudes, our safety, even the way we think and learn. In fact,the implications of indoor environment, the atmospherewe can control, are as practical and sophisticated as today's imaginative buildingarchitecture and its multiple uses. Today, environmental control design andapplica- tion deserves the serious consideration of buildingowners, builders, engineers, and architects as wellas control system manufacturers. Choosing the proper environmental controlsystem presents a quan- dary of choice for the commercial buildingowner or builder.
    [Show full text]
  • Control System Design Methods
    Christiansen-Sec.19.qxd 06:08:2004 6:43 PM Page 19.1 The Electronics Engineers' Handbook, 5th Edition McGraw-Hill, Section 19, pp. 19.1-19.30, 2005. SECTION 19 CONTROL SYSTEMS Control is used to modify the behavior of a system so it behaves in a specific desirable way over time. For example, we may want the speed of a car on the highway to remain as close as possible to 60 miles per hour in spite of possible hills or adverse wind; or we may want an aircraft to follow a desired altitude, heading, and velocity profile independent of wind gusts; or we may want the temperature and pressure in a reactor vessel in a chemical process plant to be maintained at desired levels. All these are being accomplished today by control methods and the above are examples of what automatic control systems are designed to do, without human intervention. Control is used whenever quantities such as speed, altitude, temperature, or voltage must be made to behave in some desirable way over time. This section provides an introduction to control system design methods. P.A., Z.G. In This Section: CHAPTER 19.1 CONTROL SYSTEM DESIGN 19.3 INTRODUCTION 19.3 Proportional-Integral-Derivative Control 19.3 The Role of Control Theory 19.4 MATHEMATICAL DESCRIPTIONS 19.4 Linear Differential Equations 19.4 State Variable Descriptions 19.5 Transfer Functions 19.7 Frequency Response 19.9 ANALYSIS OF DYNAMICAL BEHAVIOR 19.10 System Response, Modes and Stability 19.10 Response of First and Second Order Systems 19.11 Transient Response Performance Specifications for a Second Order
    [Show full text]
  • Control and the Digital Computer: the Early Years
    Copyright © 2002 IFAC 15th Triennial World Congress, Barcelona, Spain CONTROL AND THE DIGITAL COMPUTER: THE EARLY YEARS Stuart Bennett Department of Automatic Control & Systems Engineering, The University of Sheffield, Mappin Street, Sheffield, S1 3JD, UK, Email: [email protected] Abstract: During the years 1950-1970 there was extensive development of control theory and its application. This paper explores the influence of the rapid development of the digital computer and associated enabling technologies on the field of control systems. A brief outline of digital computer and control theory development is given followed by an account of the effect of the digital computer on process control applications. Copyright © 2002 IFAC Keywords: history, digital computer, process control, control theory 1. INTRODUCTION New York Times in the USA devoted considerable column inches to the debate. Governments, in both Text books published around 1950 expounded the the USA and the UK, instituted enquiries into the frequency domain techniques which had been used subject and professional engineering institutions during the war for the design (by trial and error) of began to recognise automatic control as a specific linear single variable systems. However, as many of sub-discipline worthy of the formation of sections contributors to the conferences of 1951 (at Cranfield, (Bennett, 1993). A report produced, in 1956, for the UK) and 1953 (New York, USA) explained, typical UK government saw a combination of real-world problems were non-linear, complex, mechanisation, automatic control and the newly multivariable, many involving discrete as well as emerging digital computer as providing the means to continuous data; they also expressed the need for overcome labour shortages and keep the economy optimum not just adequate controller performance.
    [Show full text]
  • A Condensed Guide to Automation Control System Specification, Design and Installation
    A Condensed Guide to Automation Control System Specification, Design & Installation: White Paper, Title Page A Condensed Guide to Automation Control System Specification, Design and Installation By Tom Elavsky AutomationDirect WHITE PAPER The #1 Value in Automation.™ A Condensed Guide to Automation Control System Specification, Design & Installation: White Paper, pg. 2 The following is Part 1 of a four-part series of articles on Control System Design that can act as a general guide to the specification, design and installation of automated control systems. The information and references are presented in a logical order that will take you from the skills required to recognize an operation or process suited for automating, to tips on setting up a program, to maintaining the control system. Whether you are an expert or a novice at electrical control devices and systems, the information presented should give you a check list to use in the steps to implementing an automated control system. Electrical control systems are used on everything from simple pump controls to car washes, to complex chemical processing plants. Automation of machine tools, material handling/conveyor systems, mixing processes, assembly machines, metal processing, textile processing and more has increased productivity and reliability in all areas of manufacturing, utilities and material processing. You may have come to realize that an operation or process used to produce your end product is very laborious, time consuming, and produces inconsistent results. You may have also visualized ways that would allow you to automate the operation. Automating the process will reduce the amount of manual labor, improve throughput and produce consistent results.
    [Show full text]
  • Five Key Features Required for a Perfect Fit Distributed Control System Identify the Right DCS Solution for Your Industrial Operation
    Five Key Features Required for a Perfect Fit Distributed Control System Identify the Right DCS Solution for Your Industrial Operation ©2015 Honeywell International Inc. All Rights Reserved. Five Key Features Required for a Perfect Fit Distributed Control System Introduction For industrial organizations, it is imperative to increase uptime Although Distributed Control System and improve reliability. Plants seek to boost the effectiveness of (DCS) technology offers significant their operations teams, and also require faster and more accurate advantages for modern industrial engineering and greater maintenance efficiency. In addition, operations, the right solution should there is a need for higher speed processing and better control be purpose-built with the latest in manufacturing applications. technology for your industry and application needs. ©2015 Honeywell International Inc. 2 All Rights Reserved. Five Key Features Required for a Perfect Fit Distributed Control System Are These Questions on Your Mind? • How do you ensure • What if you could maintain • How do you • What if you could quickly your plant has steady and efficient ensure your plant respond to changes in the an uninterrupted operation to ensure products personnel have the market that may require process execution? are manufactured according tools to optimize shifts in production type to your customer’s quality production? or volume, or the start-up and delivery standards? of new manufacturing facilities? If yes, what you’re really looking for is a reliable and flexible DCS with features tailored to your control applications... FPO ©2015 Honeywell International Inc. 3 All Rights Reserved. Five Key Features Required for a Perfect Fit Distributed Control System ©2015 Honeywell International Inc.
    [Show full text]
  • Simple Control Systems
    4 Simple Control Systems 4.1 Introduction In this chapter we will give simple examples of analysis and design of control systems. We will start in Sections 4.2 and 4.3 with two systems that can be handled using only knowledge of differential equations. Sec- tion 4.2 deals with design of a cruise controller for a car. In Section 4.3 we discuss the dynamics of a bicycle, many of its nice properties are due to a purely mechanical feedback which has emerged as a result of trial and error over a long period of time. Section 3.3 is a suitable prepara- tion for Sections 4.2 and 4.3. Differential equations are cumbersome for more complicated problems and better tools are needed. Efficient meth- ods for working with linear systems can be developed based on a basic knowledge of Laplace transforms and transfer functions. Coupled with block diagrams this gives a very efficient way to deal with linear systems. The block diagram gives the overview and the behavior of the individual blocks are described by transfer functions. The Laplace transforms make it easy to manipulate the system formally and to derive relations between different signals. This is one of the standard methods for working with control systems. It is exploited in Section 4.4, which gives a systematic way of designing PI controllers for first order systems. This section also contains material required to develop an intuitive picture of the proper- ties of second order systems. Section 4.5 deals with design of PI and PID controllers for second order systems.
    [Show full text]
  • Principles of Control Theory As Applied to a Thermostat Joseph Peter Stidham
    University of Richmond UR Scholarship Repository Master's Theses Student Research 8-1972 Principles of control theory as applied to a thermostat Joseph Peter Stidham Follow this and additional works at: http://scholarship.richmond.edu/masters-theses Part of the Chemistry Commons Recommended Citation Stidham, Joseph Peter, "Principles of control theory as applied to a thermostat" (1972). Master's Theses. Paper 1006. This Thesis is brought to you for free and open access by the Student Research at UR Scholarship Repository. It has been accepted for inclusion in Master's Theses by an authorized administrator of UR Scholarship Repository. For more information, please contact [email protected]. "PRINCIPLES OF CONTROL THEORY ~S APPLIED TO A THERMOSTAT BY JOSEPH PETER STIDHAM A THESIS SUBMITTED TO THE GRADUATE FACULTY OF THE UNIVERSITY OF RICHMOND IN CANDIDACY FOR THE DEGREE OF MASTER OF SCIENCE IN CHEMISTRY A.UGUST 1972 APPROVAL: LIBRA.RY UNIVERSITY OF RICHMOND VIRGINIA ACKNOWLEDGEMENT I am ~rateful to Dr. James _E. WoPshama Jr .. for his guidance in the research and prepar~tion of this paper. JNIVERSITY OF RICHMOND TABLE OF CONTENTS Introduction 1 Historical 2 Theory 6 Block Diagrams 11 Root Locus Method 15 Development of the Control Equations for a Controlled Temperature Bath 24 Experimental 31 Thermistor Transfer Function KH 32 Control Element Transfer Function KF 34 Heat ~oss to Room 34 Integrating Amplifier Time Constant 36 System Time Delays 37 Calculations 39 Control System 39 Temperature Recorder 42 Calculated Response
    [Show full text]
  • Feedback Control Theory
    Feedback Control Theory John Doyle, Bruce Francis, Allen Tannenbaum c Macmillan Publishing Co., 1990 Contents Preface iii 1 Introduction 1 1.1 IssuesinControlSystemDesign . ........ 1 1.2 WhatIsinThisBook ................................ ... 7 2 Norms for Signals and Systems 13 2.1 NormsforSignals................................. ..... 13 2.2 NormsforSystems ................................. 15 2.3 Input-OutputRelationships . ......... 18 2.4 PowerAnalysis(Optional). ........ 19 2.5 Proofs for Tables 2.1 and 2.2 (Optional) . ........... 21 2.6 Computing by State-Space Methods (Optional) . ............ 24 3 Basic Concepts 31 3.1 BasicFeedbackLoop............................... ..... 31 3.2 InternalStability ............................... ....... 34 3.3 AsymptoticTracking. .. .. .. .. .. .. .. .. ...... 38 3.4 Performance..................................... 40 4 Uncertainty and Robustness 45 4.1 PlantUncertainty ................................ ..... 45 4.2 RobustStability ................................. ..... 50 4.3 RobustPerformance ............................... ..... 53 4.4 RobustPerformanceMoreGenerally . ......... 58 4.5 Conclusion ...................................... 59 5 Stabilization 63 5.1 Controller Parametrization: Stable Plant . .............. 63 5.2 CoprimeFactorization . ....... 65 5.3 Coprime Factorization by State-Space Methods (Optional)............... 69 5.4 Controller Parametrization: General Plant . .............. 71 5.5 AsymptoticProperties . ....... 73 5.6 Strong and Simultaneous Stabilization . ...........
    [Show full text]