【正文】 elements to organize programs for sequential and parallel control processing. Based on the standard, many manufacturers offer at least two of these languages as options for programming their PLCs. Ironically, approximately 96 percent of PLC users recently still use ladder diagrams to construct their PLC code. It seems that ladder logic continues to be a top choice given it39。s performed so well for so long.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, more powerful processor with more memory at less cost. These advances have also allowed the PLC to expand its portfolio and take on new tasks like munications, data manipulation and highspeed motion without giving up the rugged and reliable performance expected from industrial control equipment. New technology has also created a category of controllers called Programmable Automation Controllers, or PACs. PACs differ from traditional PLCs in that they typically utilize open, modular architectures for both hardware and software, using de facto standards for network interfaces, languages and protocols. They could be viewed as a PC in an industrial PLClike package.The FutureA 2005 PLC Product Focus Study from Reed Research Group pointed out factors increasingly important to users, machine builders and those making the purchasing decisions. The top picks for features of importance were. * The ability to network, and do so easily. Ethernet munications is leading the charge in this realm. Not only are new protocols surfacing, but many of the industry de facto standard serial protocols that have been used for many years are being ported to Ethernet platforms. These include Modbus (ModbusTCP), DeviceNet (Ethernet/IP) and Profibus (Profinet). Ethernet munication modules for PLCs are readily available with highspeed performance and flexible protocols. Also, many PLC CPUs are now available with Ethernet ports on board, saving I/O slot space. PLCs will continue to develop more sophisticated connectivity to report information to other PLCs, system control systems, data acquisition (SCADA) systems and enterprise resource planning (ERP) systems. Additionally, wireless munications will continue to gain popularity. * The ability to network PLC I/O connections with a PC. The same trends that have benefited PLC networking have migrated to the I/O level. Many PLC manufacturers are supporting the most accepted fieldbus networks, allowing PLC I/O to be distributed over large physical distances, or located where it was previously considered nearly impossible. This has opened the door for personal puters to interface with standard PLC I/O subsystems by using interface cards, typically supplied by the PLC manufacturer or a third party developer. Now these challenging locations can be monitored with today a PC. Where industrialgrade control engines are not required, the user can take advantage of more advanced software packages and hardware flexibility at a lower cost. * The ability to use universal programming software for multiple targets/platforms. In the past it was expected that an intelligent controller would be plex to program. That is no longer the case. Users are no longer just trained programmers, such as design engineers or systems integrators, but endusers who expect easiertouse software in more familiar formats. The Windowsbased look and feel that users are familiar with on their personal puters have bee the most accepted graphical user interface. What began as simple relay logic emulation for programming PLCs has evolved into languages that use higher level function blocks that are much more intuitive to configure. PLC manufacturers are also beginning to integrate the programming of diverse functions that allow you to learn only one package in configuring logic, HMI, motion control and other specialized capabilities. Possibly the ultimate wish of the enduser would be for a software package that could seamlessly program many manufacturers PLCs and subsystems. After all, Microsoft Windows operating system and applications work similarly whether installed on a Dell, HP or IBM puter, which makes it easier for the user. Overall, PLC users are satisfied with the products currently available, while keeping their eye on new trends and implementing them where the benefits are obvious. Typically, new installations take advantage of advancing technologies, helping them bee more accepted in the industrial world. PLC的過去、現(xiàn)在與未來眾所周知，科技世界里只有一個永恒真理，那就是變化。這在可編程邏輯控制器（PLC）及其各種應(yīng)用的發(fā)展過程中尤為明顯。自從三十多年前將PLC引進以來，PLC已經(jīng)在廣泛的工業(yè)領(lǐng)域中成為幾十萬控制系統(tǒng)的基礎(chǔ)。從本質(zhì)上講，PLC是一種用高度專業(yè)化語言編程的工業(yè)計算機，并繼續(xù)受益于計算機和信息技術(shù)領(lǐng)域的技術(shù)進步。它的最突出之處是小型化和通信功能。 微型化的PLC在最初引進PLC的時候，主要改進它的體積，這與替換了數(shù)百個硬接線繼電器和計時器有關(guān)。一個嵌有CPU和I/O的典型單元有大約19寸電視機那么大。從20世紀(jì)80年代到20世紀(jì)90年代初，模塊化的PLC逐漸微型化，同時它的容量和性能也得到了提高。近年來，更小型PLC已經(jīng)發(fā)展到納米級和微型級，它們已具有以前只在大型PLC上才有的特點。因此僅為了額外特性或性能而不是增加I/O容量而具體指定一個大型的PLC變得不必要，因為即使納米級PLC也具備以太網(wǎng)通信、運動控制、自動調(diào)諧的嵌入式PID、遠(yuǎn)程連通性等更多的功能?，F(xiàn)在，由于PLC能執(zhí)行運動控制、數(shù)據(jù)采集，遠(yuǎn)程終端單元（RTU）甚至一些集成人機介面（HMI）等功能，因此PLC在很多應(yīng)用中也已配置齊全從而替代單一的過程控制器。以前，這些功能通常要求他們自身內(nèi)置實現(xiàn)這些功能的控制器和軟件，此外，還需要一個用于離散控制和互鎖的獨立的PLC。強大的通信功能近年來，最有意義的變化也許發(fā)生在通信領(lǐng)域。在20世紀(jì)90年代，Modicon推行的Modbus通信協(xié)議，允許PLC通過標(biāo)準(zhǔn)電纜進行通信。這為PLC更好地適用于現(xiàn)存的設(shè)備提供了可能性，并且向主板上的其它控制系統(tǒng)通信成為可能。在過去的30年里，我們真實地目睹了數(shù)百個專利化協(xié)議和標(biāo)準(zhǔn)化協(xié)議的發(fā)展，每一個協(xié)議都有自己獨特的優(yōu)勢?，F(xiàn)在，PLC已成為數(shù)據(jù)編譯器和信息網(wǎng)關(guān)，它們必須接入條形碼掃描器和打印機，還有溫度和模擬傳感器。在過程控制中，它們需要支持多種協(xié)議，以便它們能和其它設(shè)備通信。此外，在它們?nèi)烤邆溥@些功能的同時，它們?nèi)匀灰懈叩男詢r比而且編程簡單。另一個主要改進來自于計算處理領(lǐng)域。確切地說，它革命化了PLC的編程方式、互相通信、與用于HMI、SCADA和DCS的PC有接口。在過去的五年中，車間級以太網(wǎng)通信的應(yīng)用已經(jīng)翻了一倍。盡管串行通信仍然很受歡迎并且很可靠，但以太網(wǎng)快速地成為值得選擇的通信媒體，它有著不能被忽視的優(yōu)勢，例如：網(wǎng)速、設(shè)置簡單、布線方便、現(xiàn)成網(wǎng)絡(luò)組件的可用性、嵌入式通信設(shè)置集成運動控制另一個分配給PLC的任務(wù)是運動控制。從簡單的開環(huán)控制到多軸應(yīng)用來看，在PLC的軟件和硬件中集成運動控制已經(jīng)成為一個趨勢。很多系統(tǒng)在快速運行時要求精確的控制，但并不是在超高速運行時的絕對精準(zhǔn)。單機PLC在一些系統(tǒng)上也能很好地運行。許多納米級和微型級PLC都有高速運算能力和控制器內(nèi)置的高頻脈沖輸出能力，使它們成為開環(huán)控制的可行解決方案。一方面要提醒的是控制器在控制順序上不能確定輸出設(shè)備的位置。另一方面要提醒的是它的主要優(yōu)勢在于它的成本。以前，即使簡單的運動控制也要求有一個昂貴的選擇模塊。有時為了滿足系統(tǒng)需求，它不能用于更精密的控制平臺中。越精密的運動控制系統(tǒng)要求越高精度的定位硬件和軟件，而許多PLC都提供高速選擇模塊接入伺服驅(qū)動?，F(xiàn)在，許多驅(qū)動都兼容來自控制主機（PLC或PC）的傳統(tǒng)命令，或者提供自身的內(nèi)部運動控制。將運動控制組態(tài)軟件集成在P