Real Time System By Liu Pdf
This valuable reference provides a comprehensive treatment of the technology known as RMA (rate-monotonic analysis) method. It also covers the tremendous recent advances in real-time operating systems and communications networks—emphasizing research results that have been adopted in state-of-the-art systems. Describing how and discussing why, this book uses insightful illustrative examples to convey technology transition in the last ten years. Coverage includes commonly used approaches to hard real-time scheduling, clock-driven scheduling, scheduling aperiodic and sporadic jobs in priority-driven systems, resources and resource access control, real-time communications, and operating systems. For systems architects, designers, chief scientists and technologists, and systems analysts. File:PDF, 19.86 MBIntegre Technical Publishing Co., Inc.C H A P T E RLiuJanuary 13, 2000 8:45 a.m.chap1page 11Typical Real-Time ApplicationsFrom its title, you can see that this book is about real-time (computing, communication, andinformation) systems.
Rather than pausing here to define the term precisely, which we willdo in Chapter 2, let us just say for now that a real-time system is required to complete itswork and deliver its services on a timely basis. Examples of real-time systems include digitalcontrol, command and control, signal processing, and telecommunication systems. Every daythese systems provide us with important services. When we drive, they control the engineand brakes of our car and regulate traffic lights.
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When we fly, they schedule and monitor thetakeoff and landing of our plane, make it fly, maintain its flight path, and keep it out of harm’sway. When we are sick, they may monitor and regulate our blood pressure and heart beats.When we are well, they can entertain us with electronic games and joy rides. Unlike PCs andworkstations that run nonreal-time applications such as our editor and network browser, thecomputers and networks that run real-time applications are often hidden from our view. Whenreal-time systems work correctly and well, they make us forget their existence.For the most part, this book is devoted to real-time operating systems and communication protocols, in particular, how they should work so that applications running on them canreliably deliver valuable services on time.
From the examples above, you can see that malfunctions of some real-time systems can have serious consequences. We not only want suchsystems to work correctly and responsively but also want to be able to show that they indeeddo. For this reason, a major emphasis of the book is on techniques for validating real-timesystems. By validation, we mean a rigorous demonstration that the system has the intendedtiming behavior.As an introduction, this chapter describes several representative classes of real-time applications: digital control, optimal control, command and control, signal processing, tracking,real-time databases, and multimedia. Their principles are out of the scope of this book. Weprovide only a brief overview in order to explain the characteristics of the workloads generated by the applications and the relation between their timing and functional requirements.
Inlater chapters, we will work with abstract workload models that supposely capture the relevant characteristics of these applications. This overview aims at making us better judges ofthe accuracy of the models.In this chapter, we start by describing simple digital controllers in Section 1.1. They arethe simplest and the most deterministic real-time applications. They also have the most stringent timing requirements. Section 1.2 describes optimal control and command and controlapplications. These high-level controllers either directly or indirectly guide and coordinate1Integre Technical Publishing Co., Inc.2Chapter 1LiuJanuary 13, 2000 8:45 a.m.chap1page 2Typical Real-Time Applicationsdigital controllers and interact with human operators. High-level controllers may have significantly higher and widely fluctuating resource demands as well as larger and more relaxedresponse time requirements.
Section 1.3 describes signal processing applications in generaland radar signal processing and tracking in particular. Section 1.4 describes database andmultimedia applications.
Section 1.5 summarizes the chapter.1.1DIGITAL CONTROLMany real-time systems are embedded in sensors and actuators and function as digital controllers. Figure 1–1 shows such a system. The term plant in the block diagram refers to acontrolled system, for example, an engine, a brake, an aircraft, a patient. The state of the plantis monitored by sensors and can be changed by actuators. The real-time (computing) systemestimates from the sensor readings the current state of the plant and computes a control outputbased on the difference between the current state and the desired state (called reference inputin the figure). We call this computation the control-law computation of the controller.
Theoutput thus generated activates the actuators, which bring the plant closer to the desired state.1.1.1Sampled Data SystemsLong before digital computers became cost-effective and widely used, analog (i.e., continuoustime and continuous-state) controllers were in use, and their principles were well established.Consequently, a common approach to designing a digital controller is to start with an analogcontroller that has the desired behavior. The analog version is then transformed into a digital (i.e., discrete-time and discrete-state) version.
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The resultant controller is a sampled datasystem. It typically samples (i.e., reads) and digitizes the analog sensor readings periodicallyand carries out its control-law computation every period.
The sequence of digital outputs thusproduced is then converted back to an analog form needed to activate the actuators.A Simple Example. As an example, we consider an analog single-input/single-outputPID (Proportional, Integral, and Derivative) controller. This simple kind of controller is commonly used in practice.
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The analog sensor reading y(t) gives the measured state of the plantat time t. Let e(t) = r (t) − y(t) denote the difference between the desired state r (t) and themeasured state y(t) at time t.