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CALTECH CDS 101 - Implementation

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Chapter 10Implementation10.1 IntroductionSo far we have focused on concepts and ideas. Since controllers are ac-tual devices we will now briefly discuss what controllers look like physically.This is difficult for two reasons, control are implemented in many differenttechnologies and the application areas are very wide. Technology has alsochanged significantly over the years. We will therefore concentrate on theprinciples and give a few practical illustrations.Controllers have been described as differential equations or transfer func-tions. To implement a controller it is necessary to construct a device thatsolves differential equations in real time. In addition it is necessary to con-nect the computing device to the process which is done using sensors andactuators. A controller also has other functions apart from the control al-gorithm itself. Most controllers have facilities for man-machine interaction.Controllers can also operate in different modes. When discussing implemen-tation we must consider issues such as mode, switches, safety, reliability anduser interfaces.Early controllers were implemented with devices that combined sensing,computing and actuation. The centrifugal governor is a typical example.Later as technology developed the different functions were separated. Com-puting can be done in many different ways using a wide range of technologies.Analog computing was used in the early controllers but today it is typicallyperformed with digital computers. Analog control is still used in devicesthat require very high speed. It is also commonly used in MEMS devicesbecause analog controllers require less silicon surface than digital controllers.Feedback has had a central role in the development of sensing, actuation175176 CHAPTER 10. IMPLEMENTATIONPSfrag replacementsryuControllerFigure 10.1: An abstract representation of a controller with two degrees offreedom.and computing. Many advances in the technology have been associatedwith the introduction of feedback in the devices. There have thus been closesynergies between the development of ideas and devices.A significant standardization has also occured in specific application do-mains. The advantages of standardization was recognized early in processcontrol where standards for signal transmission were introduced. This madeit possible to have different suppliers of sensors, actuators and controllers. Itwas also possible to collect all controllers into central operating rooms so thatoperators could have a good overview over large manufacturing processes.Standardization of digital communication is also underway but progress hasbeen slow because of special vendor interest. An interesting feature is thatthe internet protocol is beeing used increasingly.Feedback has had an essential role both in analog computing and inimplementation of controllers. The devices used to implement controllersare amplifiers and systems with dynamics. The amplifiers can be nonlinearbecause linear behavior can be created using feedback. The development ofcontrollers is interesting, great ingenuity has often been demonstrated and awide range of technologies have been used. Today two technologies are pre-dominant. Analog controllers based on operational amplifiers and computercontrol. Since biological systems use pulse based computing extensively wealso include a section on that.10.2 Sensing Actuation Computing and Commu-nicationFor the most part of this book a controller has simply been viewed as abox with two inputs, the reference r, the measured process variable y, andone output, the control variable u as illustrated in Figure 10.1. The linearbehavior of a controller is typically described by a transfer function or adifferential equation. A more detailed representation of a controller is givenin Figure 10.2. In this figure the controller is decomposed into three blocks,10.2. SENSING ACTUATION COMPUTING AND COMMUNICATION177PSfrag replacementsryuSensingComputingActuationFigure 10.2: A more detailed representation of a controller.representing sensing, computing and actuation. The sensor converts thephysical process variable to a representation that can be handled by thecomputing device. The computing device performs the operation on thesignal expressed by the controller transfer function. The actuator convertsthe result of the computations to a physical variable that can influence theprocess. Actuation requires energy or forces.Many processes have large dimensions and the control system has tocover a large area. For large processes it is therefore common to centralizethe computing in a control room or master computer. It is then necessaryto connect sensors actuators and computing functions. If signals are repre-sented electrically by currents and voltages the connections are done withwires. If the system have many sensors and actuators there will be a lot ofwires and then is is highly advantageous to use communication networks. Awide variety of networks can be used, electrical, optical and wireless. Thedimensions of the systems can vary widely. A CD player is about 0.1 m.Systems for a car have sizes of a few meters. Climate control of buildingshave sizes of hundreds of meters. For large paper mills the distances canbe of the order of kilometers. Power systems cover several nations and theInternet covers practically the whole planet.Figure 10.3a shows some of the trends in sensing, actuation, and com-putation in automotive applications. As in many other applicationareas, the number of sensors, actuators, and microprocessors is increasingdramatically, as new features such as antilock brakes, adaptive cruise con-trol, active restraint systems, and enhanced engine controls are brought tomarket. The cost/performance curves for these technologies, as illustratedin Figure 10.3b, is also insightful. The costs of electronics technologies, suchas sensing, computation, and communications, is decreasing dramatically,enabling more information processing. Perhaps the most important is therole of communications, which is now inexpensive enough to offer many newpossibilities.Modern control engineering is also closely related to the integration ofsoftware into physical systems. Virtually all modern control systems areimplemented using digital computers. Often they are just a small part of178 CHAPTER 10. IMPLEMENTATIONActuatorsSensorsFunctionsProjected(a) Engine Control Electronics010203040501986 1989 1995 19981980 1983Year1992ComputationSensing CommunicationsActuation(b)


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