【正文】 and micro PLCs are available with highspeed counting capabilities and highfrequency pulse outputs built into the controller, making them a viable solution for openloop control. The one caveat is that the controller does not know the position of the output device during the control sequence. On the other hand, its main advantage is cost. Even simple motion control had previously required an expensive option module, and at times was restricted to more sophisticated control platforms in order to meet system requirements. More sophisticated motion applications require higherprecision positioning hardware and software, and many PLCs offer highspeed option modules that interface with servo drives. Most drives today can accept traditional mands from host (PLC or PC) controls, or provide their own internal motion control. The trend here is to integrate the motion control configuration into the logic controller programming software package.Programming LanguagesA facet of the PLC that reflects both the past and the future is programming language. The IEC 611313 standard deals with programming languages and defines two 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 chart 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。專業(yè)英文文獻(xiàn)翻譯； PLCs Past, Present and FutureEveryone knows there39。電信學(xué)院畢業(yè)設(shè)計(jì)任務(wù)書(shū)題目 基于PLC的污水池液位控制系統(tǒng)設(shè)計(jì) 學(xué)生姓名 班級(jí) 學(xué)號(hào) 題目類型 工程設(shè)計(jì) 指導(dǎo)教師 魏祥林 系主任 一、畢業(yè)設(shè)計(jì)（論文）的技術(shù)背景和設(shè)計(jì)依據(jù)： 與傳統(tǒng)的人工調(diào)節(jié)液位控制系統(tǒng)相比，變頻液位自動(dòng)控制系統(tǒng)具有節(jié)能效果顯著、人工勞動(dòng)強(qiáng)度低，系統(tǒng)工作可靠、自動(dòng)化程度高等優(yōu)點(diǎn)。s only one constant in the technology world, and that39。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 soft