专业英语第10单元作业.docx

1、专业英语第10单元作业10 Introduction of Programmable ControllersFrom a simple heritage, these remarkable systems have evolved to not only replace electromechanical devices, but to solve an ever-increasing array of control problems in both process and nonprocess. By all indications, these microprocessor powere

2、d giants will continue to break new ground开辟新天地 in the automated factory into the 1990s.10. 1 HISTORYIn the 1960s, electromechancal devices were the order of the day风尚 as far as control was concerned. These devices, commonly known as relays, were being used by the thousands to control many sequentia

3、l-type manufacturing process and stand-alone machines. Many of these relays were in use in the transportation industry, more specifically, the automotive industry. These relays used hundreds of wires and their interconnections to effect a control solution. The performance of a relay was basically re

4、liableat least as a single device. But the common applications for relay panels（继电器盘，配电盘） called for要求 300 to 500 or more relays, and the reliability and maintenance issues associated with supporting these panels became a very great challenge. Cost became another issue, for in spite of the low cost

5、of the relay itself, the installed cost of the panel could be quite high. The total cost including purchased parts, wiring, and installation labor, could range from $30$50 per relay. To make mattes worse, the constantly changing needs of a process called for recurring modifications of a control pane

6、l. With relays, this was a costly prospect, as it was accomplished by a major rewiring effort on the panel. In addition, these changes were sometimes poorly documented（验证）, causing a second-shift maintenance nightmare months later. In light of（按照） this, it was not uncommon to discard（放弃） an entire c

7、ontrol panel in favor of a new one with the appropriate components wired in a manner suited for the new process. Add to this the unpredictable, and potentially high, cost of maintaining these systems as on high-volume motor vehicle production lines, and it became clear that something was needed to i

8、mprove the control process-to make it more reliable, easier to troubleshoot故障检修, and more adaptable to changing control needs.以上女生完成That something, in the late 1960s, was the first programmable controller. This first revolutionary system was developed as a specific response to the needs of the major

9、 automotive manufacturers in the United States. These early controllers, or programmable logic controllers(PLC), represented the first systems that could be used on the factory floor场地, could have there logic changed without extensive rewiring or component changes, and were easy to diagnose and repa

10、ir when problems occurred.It is interesting to observe the progress that has been made in the past 15 years in the programmable controller area. The pioneer products of the late 1960s must have been confusing and frightening to a great number of people. For example, what happened to the hardwired硬接线

11、 and electromechanical devices that maintenance personnel were used to repairing with hand tools? They were replaced with computers disguised（伪装） as electronics designed to replace relays. Even the programming tools were designed to appear as relay equivalent presentations. We have the opportunity n

12、ow to examine the promise, in retrospect回顾, that the programmable controller brought manufacturing.以上719完成10.2 BASIC CONCEPTSAll programmable controllers consist of the basic functional blocks shown in Fig. 10.1. Well examine each block to understand the relationship to the control system. First we

13、look at the center, as it is the heart (or at least the brain) of the system. It consists of a microprocessor, logic memory for the storage存储 of the actual control logic, storage or variable memory（变量存储器） for use with date that will ordinarily change as a function of the control program execution执行,

14、 and a power supply to provide electrical power for the processor and memory. Next comes the I/O block. This function takes the control level signals for the CPU and converts them to voltage and current levels suitable for connection with factory grade sensors and actuators. The I/O type can range f

15、rom digital (discrete or on/off), analog (continuously variable), or a variety of special purpose smart I/O which are dedicated to a certain application task. The programmer编程器 is shown here, but it is normally used only to initially configure and program a system and is not required for the system

16、to operate. It is also used in troubleshooting a system, and can prove to be a valuable tool in pinpointing the exact cause of a problem. The field devices shown here represent the various sensors and actuators connected to the I/O. These are the arms, legs, eyes, and ears of the system, including p

17、ushbuttons, limit switches, proximity switches, photosensors, thermocouples热电偶, RTDs电阻式温度传感器, position sensing devices, and bar code reader条形码阅读器 as input; and pilot lights指示灯, display devices, motor starters, DC and AC drives, solenoids螺线管, and printers as outputs. No single attempt could cover its

18、 rapidly changing scope, but three basic characteristics can be examined to give classify an industrial control device as a programmable controller.(1) Its basic internal operation is to solve logic from the beginning of memory to some specified point, such as end of memory or end of program. Once t

19、he end is reached, the operation begins again at the beginning of memory. This scanning扫描 process continues from the time power is supplied to the time it is removed.(2) The programming logic is a form of a relay ladder diagram梯形图. Normally open, normally closed contacts, and relay coils are used wi

20、thin a format utilizing a left and a right vertical rail（左母线和右母线）. Power flow能流 (symbolic positive electron flow) is used to determine which coils or outputs are energized or deenergized.(3) The machine is designed for the industrial environment from its basic concept; this protection is not added a

21、t a later date. The industrial environment includes unreliable AC power, high temperatures (0 to 60 degree Celsius), extremes of humidity, vibrations, RF（Radio Frequency射频） noise, and other similar parameters.10.3 GENERAL APPLICATION AREAS The programmable controller is used in a wide variety of con

22、trol applications today, many of which were not economically possible just a few years ago. This is true for two general reasons:their cost effectiveness费效比 (that is, the cost per I/O point )has improved dramatically with the falling prices of microprocessors and related components, and the ability

23、of the controller to solve complex computation and communication tasks has made it possible to use it where a dedicated computer专用计算机 was previously used.以上718完成Applications for programmable controllers can be categorized in a number of different ways, including general and industrial application ca

24、tegories. But it is important to understand the framework in which controllers are presently understood and used so that the full scope of present and future evolution can be examined. It is through the power of applications that controllers can be seen in their full light角度. Industrial applications

25、 include many in both discrete manufacturing独立制造 and process industries过程工业. Automotive industry applications, the genesis of the programmable controllers, continue to provide the largest base of opportunity. Other industries, such as food processing食品加工 and utilities, provide current development op

26、portunities.There are five general application areas in which programmable controllers are used. A typical installation will use one or more of these integrated to form a complete solution to the control system problem. The five general areas are explained briefly below.10.3.1 Sequence control This

27、is the largest and most common application for programmable controllers today, and is the closest to traditional relay control in its sequential nature. Because of the very general nature of this category, it is sometimes difficult to understand the breadth宽度 of power that it brings to so many appli

28、cations. From an applications standpoint, sequence control is found on individual machines or machine lines, on conveyor and packaging machinery, and even on modern elevator control systems.10.3.2 Motion Control This is the integration of linear or rotary motion control in the programmable controlle

29、r. This could be a single or multiple axis drive多轴驱动 system control, and can be used with servo伺服, stepper步进, or hydraulic液压 drives. In early systems, a stand-alone servo drive would be connected to the programmable controller with a series of individual conductors to discrete inputs and outputs. Ne

30、wer systems integrate this functionality directly into the I/O racks机架 through the use of special I/O boards dedicated to motion control. This eliminates消除 the need to interface接口 the two devices together with discrete I/O. Programmable controller motion control applications include an unending vari

31、ety of machinery; metal cutting 金属切割(grinders磨床), metal forming金属成形 (press brake碾压成型), assembly machines机器组装, and multiple axes of motion can be coordinated for both discrete part and process industry applications. Examples of these would include Cartesian robots笛卡尔坐标机器人, and many web related proces

32、ses that is, film涂膜, rubber橡胶, and nonwoven非纺织的 textile纺织品（即无纺布） systems.10.3.3 Process control This is the ability of the programmable controller to control a number of physical parameters such as temperature, pressure, velocity, and flow. This involves the use of analog (continuously variable) I/O

33、 to achieve a (PID), software always allow the programmable controller, retaining the best features of each. Typical examples of applications include plastic injection molding注射成型 machines, extrusion process挤出成型 machines, heat treat furnaces热处理炉, and many other batch-type分批式 control applications.10.3.4 Data Management 数据管理