《毕业设计-外文原文4万字符.doc》由会员分享，可在线阅读，更多相关《毕业设计-外文原文4万字符.doc（21页珍藏版）》请在课桌文档上搜索。
1、-Information ControlAutomatic ControlAutomatic control has played a vital role in the advance of engineering and science. In addition to its e*treme importance in space-vehicle systems, missile-guidance systems, robotic systems, and the like, automatic control has bee an important and integral part
2、of modern manufacturing and industrial processes. For e*ample, automatic control is essential in the numerical control of machine tools in the manufacturing industries, in the design of autopilot systems in the aerospace industries, and in the design of cars and trucks in the automobile industries.
3、It is also essential in such industrial operations as controlling pressure, temperature, humidity, viscosity, and flow in the process industries.Since advances in the theory and practice of automatic control provide the means for attaining optimal performance of dynamic systems, improving productivi
4、ty, relieving the drudgery of many routine repetitive manual operations, and more, most engineers and scientists must now have a good understanding of this field.Control engineering is based on the foundations of feedback theory and linear system analysis, and it integrates the concepts of network t
5、heory and munication theory. Therefore control engineering is not limited to any engineering discipline but is equally applicable to aeronautical, chemical, mechanical, environmental, civil, and electrical engineering. For e*ample, a control system often includes electrical, mechanical, and chemical
6、 ponents. Furthermore, as the understanding of the dynamics of business, social, and political systems increases, the ability to control these systems will also increase.A control system is an interconnection of ponents forming a system configuration that will provide a desired system response. The
7、basis for analysis of a system is the foundation provided by linear system theory, which assumes a cause-effect relationship for the ponents of a system. Therefore a ponent or process to be controlled can be represented by a block, as shown in Figure 8.1. The input-output relationship represents the
8、 cause-and-effect relationship of the process, which in turn represents a processing of the input signal to provide an output signal variable, often with a power amplification.In general, control systems can be categorized as being either open-loop or closed loop. The distinguishing feature between
9、these two types of control systems is the use of feedback parison for closed-loop operation.1Open-loop Control SystemAn open-loop control system utilizes a controller or control actuator to obtain the desired response, as shown in Figure 8.2. An open-loop system is a system without feedback, simples
10、t form of controlling devices. Figure 8.3 illustrates a simple tank-level control system. We wish to hold the tank level h within reasonable acceptable limits even though the outlet flow through value V1 is varied. This can be achieved by irregular manual adjustment of the inlet flow rate by valve V
11、2. The system is not a precision system,as it does not have the capability of accurately measuring the output flow rate through valve V1 ,the input flow rate through valve V2, or the tank level. Figure 8.4 shows the simple relationship that e*ists in this system between the input (the desired tank l
12、evel) and the output (the actual tank level). This control system does not have any feedback parison,and the term open loop is used to describe this absence.Figure 8.3 Tank-level control systemOutput(actual tank level)OuputInputFigure 8.1 Process to be controlledProcessOutputActuatingdeviceProcessDi
13、sireouputresponseFigure 8.2 An open-loop systemhControl system(valueand operator)Input(desiredtank level)Figure 8.4 Tank-level control system black diagram2Closed-loop Control SystemIn contrast to an open-loop control system, a closed-loop control system utilizes an additional measure of the actual
14、output to pare the actual output with the desired output response. The measure of the output is called the feedback signal. A simple closed-loop feedback control system is shown in Figure 8.5. A feedback control system is a control system that tends to maintain a prescribed relationship of one syste
15、m variable to another by paring functions of these variables and using the difference as a means of control.OutputparisonControllerProcessDesire outputresponseMeasurementFigure 8.5Close-loop control systemClosed-loop control systems derive their valuable accurate reproduction of the input from feedb
16、ack parison. An error detector derives a signal proportional to the differences between the input and outputThe closed-loop control system drives the output until it equals the input and the error is zeroAny differences between the actual and desired output will be automatically corrected in a close
17、d-loop control systemThrough proper design the system can be made relatively independent of secondary inputs and changes in ponent characteristicsFigure 8.6 illustrates an automatic tank-level control version of the system shown in Figure 8.3. It can maintain the desired tank level h within quite ac
18、curate tolerances even through the output flow rate through value V1 is varied. If the tank level is not correct,an error voltage is developedThis is amplified and applied to a motor drive that adjusts value V2 in order to restore the desired tank level by adjusting the inlet flow rateA block diagra
19、m analogous to this system is shown in Figure 8.7. Because feedback parison is present,the term closed-loop is used to describe the systems operation. Input(desiredtank level)h+v-vPoweramplifierMotordriveFloatErrorFigure 8.6Automatic tank-level control systemPower amplifierTankFloatMotordriveFigure
20、8.7 Black diagram of automatic tank-level control systemOutput(actual tank level)Error+-Due to the increasing ple*ity of the system under control and the interest in achieving optimum performance, the importance of control system engineering has grown in the past decade. Furthermore, as the system b
21、ee more ple*, the interrelationship of many controlled variables must be considered in the control scheme. A block diagram depicting a multivariable control system is shown in Figure 8.8.The introduction of feedback enables us to control a desired output and can improve accuracy, but it requires att
22、ention to the issue of stability of response.ControllerProcessMeasurementDesiredoutputresponseOutputvariablesFigure 8.8Mulutivariable control systemMicrocontrollerA microcontroller (sometimes abbreviated C, uC or MCU) is a small puter on a single integrated circuit containing a processor core, memor
23、y, and programmable input/output peripherals. Program memory in the form of NOR flash is also often included on chip, as well as a typically small amount of RAM. Microcontrollers are designed for embedded applications, in contrast to the microprocessors used in personal puters or other general purpo
24、se applications. Microcontrollers are used in automatically controlled products and devices, such as automobile engine control systems, implantable medical devices, remote controls, office machines, appliances, power tools, toys and other embedded systems. By reducing the size and cost pared to a de
25、sign that uses a separate microprocessor, memory, and input/output devices, microcontrollers make it economical to digitally control even more devices and processes. Mi*ed signal microcontrollers are mon, integrating analog ponents needed to control non-digital electronic systems.Some microcontrolle
26、rs may use four-bit words and operate at clock rate frequencies as low as 4 kHz, for low power consumption (milliwattsor microwatts). They will generally have the ability to retain functionality while waiting for an event such as a button press or other interrupt; power consumption while sleeping (C
27、PU clock and most peripherals off) may be just nanowatts, making many of them well suited for long lasting battery applications. Other microcontrollers may serve performance-critical roles, where they may need to act more like a digital signal processor (DSP), with higher clock speeds and power cons
28、umption.Embedded designA microcontroller can be considered a self-contained system with a processor, memory and peripherals and can be used as an embedded system. The majority of microcontrollers in use today are embedded in other machinery, such as automobiles, telephones, appliances, and periphera
29、ls for puter systems. These are called embedded systems. While some embedded systems are very sophisticated, many have minimal requirements for memory and program length, with no operating system, and low software ple*ity. Typical input and output devices include switches, relays, solenoids, LEDs, s
30、mall or custom LCD displays, radio frequency devices, and sensors for data such as temperature, humidity, light level etc. Embedded systems usually have no keyboard, screen, disks, printers, or other recognizable I/O devices of a personal puter, and may lack human interaction devices of any kind.1In
31、terrupts Microcontrollers must provide real time (predictable, though not necessarily fast) response to events in the embedded system they are controlling. When certain events occur, an interrupt system can signal the processor to suspend processing the current instruction sequence and to begin an i
32、nterrupt service routine (ISR, or“interrupt handler). The ISR will perform any processing required based on the source of the interrupt before returning to the original instruction sequence. Possible interrupt sources are device dependent, and often include events such as an internal timer overflow,
33、 pleting an analog to digital conversion, a logic level change on an input such as from a button being pressed, and data received on a munication link.Where power consumption is important as in battery operated devices, interrupts may also wake a microcontroller from a low power sleep state where th
34、e processor is halted until required to do something by a peripheral event.2Programs Microcontroller programs must fit in the available on-chip program memory, since it would be costly to provide a system with e*ternal, e*pandable, memory. pilers and assemblers are used to convert high-level languag
35、e and assembler language codes into a pact machine code for storage in the microcontrollers memory. Depending on the device, the program memory may be permanent, read-only memory that can only be programmed at the factory, or program memory may be field-alterable flash or erasable read-only memory.
36、Higher integration In contrast to general-purpose CPUs, micro-controllers may not implement an e*ternal address or data bus as they integrate RAM and non-volatile memory on the same chip as the CPU. Using fewer pins, the chip can be placed in a much smaller, cheaper package.Integrating the memory an
37、d other peripherals on a single chip and testing them as a unit increases the cost of that chip, but often results in decreased net cost of the embedded system as a whole. Even if the cost of a CPU that has integrated peripherals is slightly more than the cost of a CPU and e*ternal peripherals, havi
38、ng fewer chips typically allows a smaller and cheaper circuit board, and reduces the labor required to assemble and test the circuit board.This integration drastically reduces the number of chips and the amount of wiring and circuit board space that would be needed to produce equivalent systems usin
39、g separate chips. Furthermore, on low pin count devices in particular, each pin may interface to several internal peripherals, with the pin function selected by software. This allows a part to be used in a wider variety of applications than if pins had dedicated functions. Micro-controllers have pro
40、ved to be highly popular in embedded systems since their introduction in the 1970s.Some microcontrollers use a Harvard architecture: separate memory buses for instructions and data, allowing accesses to take place concurrently. Where a Harvard architecture is used, instruction words for the processo
41、r may be a different bit size than the length of internal memory and registers; for e*ample: 12-bit instructions used with 8-bit data registers. The decision of which peripheral to integrate is often difficult. The microcontroller vendors often trade operating frequencies and system design fle*ibili
42、ty against time-to-market requirements from their customers and overall lower system cost. Manufacturers have to balance the need to minimize the chip size against additional functionality. Microcontroller architectures vary widely. Some designs include general-purpose microprocessor cores, with one
43、 or more ROM, RAM, or I/O functions integrated onto the package. Other designs are purpose built for control applications. A micro-controller instruction set usually has many instructions intended for bit-wise operations to make control programs more pact. For e*ample, a general purpose processor mi
44、ght require several instructions to test a bit in a register and branch if the bit is set, where a micro-controller could have a single instruction to provide that monly-required function. Microcontrollers typically do not have a math coprocessor, so floating point arithmetic is performed by softwar
45、e.Programming environmentsMicrocontrollers were originally programmed only in assembly language, but various high-level programming languages are now also in mon use to target microcontrollers. These languages are either designed specially for the purpose, or versions of general purpose languages su
46、ch as the C programming language. pilers for general purpose languages will typically have some restrictions as well as enhancements to better support the unique characteristics of microcontrollers. Some microcontrollers have environments to aid developing certain types of applications. Microcontrol
47、ler vendors often make tools freely available to make it easier to adopt their hardware. Many microcontrollers are so quirky that they effectively require their own non-standard dialects of C, such as SDCC for the 8051, which prevent using standard tools (such as code libraries or static analysis to
48、ols) even for code unrelated to hardware features. Interpreters are often used to hide such low level quirks. Interpreter firmware is also available for some microcontrollers. For e*ample, BASIC on the early microcontrollers Intel 8052; BASIC and FORTH on the Zilog Z8 as well as some modern devices.
49、 Typically these interpreters support interactive programming. Simulators are available for some microcontrollers, such as in Microchips MPLAB environment and the Revolution Education PICA*E range. These allow a developer to analyze what the behavior of the microcontroller and their program should be if they were using the actual part. A simulator wi
