喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請放心下載，，有疑問咨詢QQ:414951605或者1304139763 ======================== 喜歡就充值下載吧。。資源目錄里展示的文件全都有，，請放心下載，，有疑問咨詢QQ:414951605或者1304139763 ======================== 英文原文：英文原文：PLCs -Past, Present and FutureJiahao WangEveryone knows theres only one constant in the technology world, and thatschange. This is especially evident in the evolution of Programmable Logic Controllers(PLC) and their varied applications. From their introduction more than 30 years ago,PLCs have become the cornerstone of hundreds of thousands of control systems in awide range of industries.At heart, the PLC is an industrialized computer programmed with highlyspecialized languages, and it continues to benefit from technological advances in thecomputer and information technology worlds. The most prominent of which isminiaturization and communications.1 The Shrinking PLCWhen the PLC was first introduced, its size was a major improvement - relativeto the hundreds of hard-wired relays and timers it replaced. A typical unit housing aCPU and I/O was roughly the size of a 19 television set. Through the 1980s and early1990s, modular PLCs continued to shrink in footprint while increasing in capabilitiesand performance (see Diagram 1 for typical modular PLC configuration).In recent years, smaller PLCs have been introduced in the nano and micro classesthat offer features previously found only in larger PLCs. This has made specifying alarger PLC just for additional features or performance, and not increased I/O count,unnecessary, as even those in the nano class are capable of Ethernet communication,motion control, on-board PID with autotune, remote connectivity and more.PLCs are also now well-equipped to replace stand-alone process controllers inmany applications, due to their ability to perform functions of motion control, dataacquisition, RTU (remote telemetry unit) and even some integrated HMI (humanmachine interface) functions. Previously, these functions often required their ownpurpose-built controllers and software, plus a separate PLC for the discrete controland interlocking.2 The Great CommunicatorPossibly the most significant change in recent years lies in the communicationsarena. In the 1970s Modiconintroduction of Modbus communications protocolallowed PLCs to communicate over standard cabling. This translates to an ability toplace PLCs in closer proximity to real world devices and communicate back to othersystem controls in a main panel.In the past 30 years we have seen literally hundreds of proprietary and standardprotocols developed, each with their own unique advantages.Todays PLCs have to bedata compilers and information gateways. They have to interface with bar codescanners and printers, as well as temperature and analog sensors. They need multipleprotocol support to be able to connect with other devices in the process. Andfurthermore, they need all these capabilities while remaining cost-effective and simpleto program.Another primary development that has literally revolutionized the way PLCs areprogrammed, communicate with each other and interface with PCs for HMI, SCADAor DCS applications, came from the computing world.Use of Ethernet communications on the plant floor has doubled in the past fiveyears. While serial communications remain popular and reliable, Ethernet is fastbecoming the communications media of choice with advantages that simply cant beignored, such as: * Network speed. * Ease of use when it comes to the setup andwiring. * Availability of off-the-shelf networking components. * Built-incommunications setups.3 Integrated Motion ControlAnother responsibility the PLC has been tasked with is motion control. Fromsimple open-loop to multi-axis applications, the trend has been to integrate thisfeature into PLC hardware and software.There are many applications that require accurate control at a fast pace, but notexact precision at blazing speeds. These are applications where the stand-alone PLCworks well. Many nano and micro PLCs are available with high-speed countingcapabilities and high-frequency pulse outputs built into the controller, making them aviable solution for open-loop control.The one caveat is that the controller does not know the position of the outputdevice during the control sequence. On the other hand, its main advantage is cost.Even simple motion control had previously required an expensive option module, andat times was restricted to more sophisticated control platforms in order to meet systemrequirements.More sophisticated motion applications require higher-precision positioninghardware and software, and many PLCs offer high-speed option modules thatinterface with servo drives. Most drives today can accept traditional commands fromhost (PLC or PC) controls, or provide their own internal motion control. The trendhere is to integrate the motion control configuration into the logic controllerprogramming software package.4 Programming LanguagesA facet of the PLC that reflects both the past and the future is programminglanguage. The IEC 61131-3 standard deals with programming languages and definestwo graphical and two textual PLC programming language standards: * Ladder logic(graphical). * Function block diagram (graphical). * Structured text(textual).Instruction list (textual).This standard also defines graphical and textual sequential function chartelements to organize programs for sequential and parallel control processing. Basedon the standard, many manufacturers offer at least two of these languages as optionsfor programming their PLCs. Ironically, approximately 96 percent of PLC usersrecently still use ladder diagrams to construct their PLC code. It seems that ladderlogic continues to be a top choice given its performed so well for so long.5 Hardware PlatformsThe modern PLC has incorporated many types of Commercial off the Shelf(COTS) technology in its CPU. This latest technology gives the PLC a faster, morepowerful processor with more memory at less cost. These advances have also allowedthe PLC to expand its portfolio and take on new tasks like communications, datamanipulation and high-speed motion without giving up the rugged and reliableperformance expected from industrial control equipment.New technology has also created a category of controllers called ProgrammableAutomation Controllers, or PACs. PACs differ from traditional PLCs in that theytypically utilize open, modular architectures for both hardware and software, using defacto standards for network interfaces, languages and protocols. They could beviewed as a PC in an industrial PLC-like package.6 The FutureA 2005 PLC Product Focus Study from Reed Research Group pointed out factorsincreasingly important to users, machine builders and those making the purchasingdecisions. The top picks for features of importance were.6.1 The ability to network, and do so easily.Ethernet communications is leading the charge in this realm. Not only are newprotocols surfacing, but many of the industry de facto standard serial protocols thathave been used for many years are being ported to Ethernet platforms. These includeModbus (ModbusTCP), DeviceNet (Ethernet/IP) and Profibus (Profinet). Ethernetcommunication modules for PLCs are readily available with high-speed performanceand flexible protocols. Also, many PLC CPUs are now available with Ethernet portson board, saving I/O slot space. PLCs will continue to develop more sophisticatedconnectivity to report information to other PLCs, system control systems, dataacquisition (SCADA) systems and enterprise resource planning (ERP) systems.Additionally, wireless communications will continue to gain popularity.6.2 The ability to network PLC I/O connections with a PC.The same trends that have benefited PLC networking have migrated to the I/Olevel. Many PLC manufacturers are supporting the most accepted fieldbus networks,allowing PLC I/O to be distributed over large physical distances, or located where itwas previously considered nearly impossible. This has opened the door for personalcomputers to interface with standard PLC I/O subsystems by using interface cards,typically supplied by the PLC manufacturer or a third party developer. Now thesechallenging locations can be monitored with today a PC. Where industrial-gradecontrol engines are not required, the user can take advantage of more advancedsoftware packages and hardware flexibility at a lower cost.6.3 The ability to use universal programming software for multipletargets/platforms.In the past it was expected that an intelligent controller would be complex toprogram. That is no longer the case. Users are no longer just trained programmers,such as design engineers or systems integrators, but end-users who expecteasier-to-use software in more familiar formats. The Windows-based look and feelthat users are familiar with on their personal computers have become the mostaccepted graphical user interface. What began as simple relay logic emulation forprogramming PLCs has evolved into languages that use higher level function blocksthat are much more intuitive to configure. PLC manufacturers are also beginning tointegrate the programming of diverse functions that allow you to learn only onepackage in configuring logic, HMI, motion control and other specialized capabilities.Possibly the ultimate wish of the end-user would be for a software package that couldseamlessly program many manufacturers PLCs and sub-systems. After all, MicrosoftWindows operating system and applications work similarly whether installed on aDell, HP or IBM computer, which makes it easier for the user.Overall, PLC users are satisfied with the products currently available, whilekeeping their eye on new trends and implementing them where the benefits areobvious. Typically, new installations take advantage of advancing technologies,helping them become more accepted in the industrial world.