Real-Time Systems. Design Principles for Distributed Embedded Applications. Herman Kopetz. Second Edition (Real-Time Systems. Design Principles for Distributed Embedded Applications. Herman Kopetz. Second Edition.pdf), страница 11

The embedded system market isdriven by the continuing improvements in the cost/performance ratio of the semiconductor industry that makes computer-based control systems cost-competitiverelative to their mechanical, hydraulic, and electronic counterparts. Among the keymass markets are the domains of consumer electronics and automotive electronics.The automotive electronics market is of particular interest, because of stringenttiming, dependability, and cost requirements that act as technology catalysts.Automotive manufacturers view the proper exploitation of computer technologyas a key competitive element in the never-ending quest for increased vehicleperformance and reduced manufacturing cost.

While some years ago, the computerapplications on board a car focused on non-critical body electronics or comfortfunctions, there is now a substantial growth in the computer control of core vehiclefunctions, e.g., engine control, brake control, transmission control, and suspensioncontrol. We observe an integration of many of these functions with the goal ofincreasing the vehicle stability in critical driving maneuvers. Obviously, an error inany of these core vehicle functions has severe safety implications.At present the topic of computer safety in cars is approached at two levels.

At thebasic level a mechanical system provides the proven safety level that is consideredsufficient to operate the car. The computer system provides optimized performanceon top of the basic mechanical system. In case the computer system fails cleanly,the mechanical system takes over. Consider, for example, an Electronic Stability201 The Real-Time EnvironmentProgram (ESP). If the computer fails, the conventional mechanical brake systemis still operational.

Soon, this approach to safety may reach its limits for tworeasons:1. If the computer-controlled system is further improved, the magnitude of thedifference between the performance of the computer-controlled system and theperformance of the basic mechanical system is further increased. A driver who isused to the high performance of the computer-controlled system might considerthe fallback to the inferior performance of the mechanical system a safety risk.2.

The improved price/performance of the microelectronic devices will make theimplementation of fault-tolerant computer systems cheaper than the implementation of mixed computer/mechanical systems. Thus, there will be an economicpressure to eliminate the redundant mechanical system and to replace it with acomputer system using active redundancy.The embedded system market is expected to grow significantly during the next10 years.

It is expected that many embedded systems will be connected to theInternet, forming the Internet of Things (IoT – see Chap. 13).1.6.2Plant Automation SystemsCharacteristics. Historically, industrial plant automation was the first field for theapplication of real-time digital computer control. This is understandable since thebenefits that can be gained by the computerization of a sizable plant are much largerthan the cost of even an expensive process control computer of the late 1960s. In theearly days, human operators controlled the industrial plants locally.

With therefinement of industrial plant instrumentation and the availability of remote automatic controllers, plant monitoring and command facilities were concentrated into acentral control room, thus reducing the number of operators required to run theplant. In the 1970s, the next logical step was the introduction of central processcontrol computers to monitor the plant and assist the operator in her/his routinefunctions, e.g., data logging and operator guidance. In the beginning, the computerwas considered an add-on facility that was not fully trusted.

It was the duty of theoperator to judge whether a set point calculated by a computer made sense andcould be applied to the process (open-loop control). In the next phase, SupervisoryControl and Data Acquisition (SCADA) systems calculated the set-points for theprogrammable logic controllers (PLC) in the plant.

With the improvement of theprocess models and the growth of the reliability of the computer, control functionshave been increasingly allocated to the computer and gradually the operator hasbeen taken out of the control loop (closed-loop control). Sophisticated non-linearcontrol techniques, which have response time requirements beyond human capabilities, have been implemented.Usually, every plant automation system is unique. There is an extensive amountof engineering and software effort required to adapt the computer system to the1.6 The Real-Time Systems Market21physical layout, the operating strategy, the rules and regulations, and the reportingsystem of a particular plant.

To reduce these engineering and software efforts, manyprocess control companies have developed a set of modular building blocks, whichcan be configured individually to meet the requirements of a customer. Comparedto the development cost, the production cost (hardware cost) is of minor importance. Maintenance cost can be an issue if a maintenance technician must be on-sitefor 24 h in order to minimize the downtime of a plant.Future Trends. The market of industrial plant automation systems is limited by thenumber of plants that are newly constructed or are refurbished to install a computercontrol system.

During the last 20 years, many plants have already been automated.This investment must pay off before a new generation of computers and controlequipment is installed.Furthermore, the installation of a new generation of control equipment in aproduction plant causes disruption in the operation of the plant with a costly lossof production that must be justified economically. This is difficult if the plant’sefficiency is already high, and the margin for further improvement by refinedcomputer control is limited.The size of the plant automation market is too small to support the massproduction of special application-specific components. This is the reason whymany VLSI components that are developed for other application domains, suchas automotive electronics, are taken up by this market to reduce the system cost.Examples of such components are sensors, actuators, real-time local area networks,and processing nodes.

Already several process-control companies have announceda new generation of process-control equipment that takes advantage the of lowpriced mass produced components that have been developed for the automotivemarket, such as the chips developed for the Controller Area Network (CAN – seeSect. 7.3.2).1.6.3Multimedia SystemsCharacteristics. The multimedia market is a mass market for specially designedsoft and firm real-time systems.

Although the deadlines for many multimedia tasks,such as the synchronization of audio and video streams, are firm, they are not harddeadlines. An occasional failure to meet a deadline results in a degradation of thequality of the user experience, but will not cause a catastrophe. The processingpower required to transport and render a continuous video stream is large anddifficult to estimate, because it is possible to improve a good picture even further.The resource allocation strategy in multimedia applications is thus quite differentfrom that of hard real-time applications; it is not determined by the given application requirements, but by the amount of available resources.

A fraction of the givencomputational resources (processing power, memory, bandwidth) is allocated to auser domain. Quality of experience considerations at the end user determine the221 The Real-Time Environmentdetailed resource allocation strategy. For example, if a user reduces the size of awindow and enlarges the size of another window on his multimedia terminal, thenthe system can reduce the bandwidth and the processing allocated to the firstwindow to free the resources for the other window that has been enlarged. Otherusers of the system should not be affected by this local reallocation of resources.Future Trends. The marriage of the Internet with smart phones and multimediapersonal computers leads to many new volume applications. The focus of this bookis not on multimedia systems, because these systems belong to the class of soft andfirm real-time applications.1.7Examples of Real-Time SystemsIn this section, three typical examples of real-time systems are introduced.

Theseexamples will be used throughout the book to explain the evolving concepts.We start with an example of a very simple system for flow control to demonstratethe need for end-to-end protocols in process input/output.1.7.1Controlling the Flow in a PipeIt is the objective of the simple control system depicted in Fig. 1.8 to control theflow of a liquid in a pipe. A given flow set point determined by a client should bemaintained despite changing environmental conditions.

Examples for such changing conditions are the varying level of the liquid in the vessel or the temperaturesensitive viscosity of the liquid. The computer interacts with the controlled objectby setting the position of the control valve. It then observes the reaction of thecontrolled object by reading the flow sensor F to determine whether the desiredeffect, the intended change of flow, has been achieved. This is a typical example ofthe necessary end-to-end protocol [Sal84] that must be put in place between thecomputer and the controlled object (see also Sect.