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1、1 PLCs -Past, Present and Future Everyone knows theres only one constant in the technology world, and thats change. 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
2、cornerstone of hundreds of thousands of control systems in a wide range of industries. At heart, the PLC is an industrialized computer programmed with highly specialized languages, and it continues to benefit from technological advances in the computer and information technology worlds. The most pro
3、minent of which is miniaturization and communications. The Shrinking PLC When the PLC was first introduced, its size was a major improvement - relative to the hundreds of hard-wired relays and timers it replaced. A typical unit housing a CPU and I/O was roughly the size of a 19 television set. Throu
4、gh the 1980s and early 1990s, modular PLCs continued to shrink in footprint while increasing in capabilities and performance (see Diagram 1 for typical modular PLC configuration). In recent years, smaller PLCs have been introduced in the nano and micro classes that offer features previously found on
5、ly in larger PLCs. This has made specifying a larger 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 al
6、so now well-equipped to replace stand-alone process controllers in many applications, due to their ability to perform functions of motion control, data acquisition, RTU (remote telemetry unit) and even some integrated HMI (human machine interface) functions. Previously, these functions often require
7、d their own purpose-built controllers and software, plus a separate PLC for the discrete control and interlocking. The Great Communicator Possibly the most significant change in recent years lies in the communications arena. In the 1970s Modicon introduction of Modbus communications protocol allowed
8、 PLCs to communicate over standard cabling. This translates to an ability to place PLCs in closer proximity to real world devices and communicate back to other system controls in a main panel. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 1 页，共 7 页 - - - - - - - - -
9、 2 In the past 30 years we have seen literally hundreds of proprietary and standard protocols developed, each with their own unique advantages.Todays PLCs have to be data compilers and information gateways. They have to interface with bar code scanners and printers, as well as temperature and analog
10、 sensors. They need multiple protocol support to be able to connect with other devices in the process. And furthermore, they need all these capabilities while remaining cost-effective and simple to program. Another primary development that has literally revolutionized the way PLCs are programmed, co
11、mmunicate with each other and interface with PCs for HMI, SCADA or DCS applications, came from the computing world. Use of Ethernet communications on the plant floor has doubled in the past five years. While serial communications remain popular and reliable, Ethernet is fast becoming the communicati
12、ons media of choice with advantages that simply cant be ignored, such as: * Network speed. * Ease of use when it comes to the setup and wiring. * Availability of off-the-shelf networking components. * Built-in communications setups. Integrated Motion Control Another responsibility the PLC has been t
13、asked with is motion control. From simple open-loop to multi-axis applications, the trend has been to integrate this feature into PLC hardware and software. There are many applications that require accurate control at a fast pace, but not exact precision at blazing speeds. These are applications whe
14、re the stand-alone PLC works well. Many nano and micro PLCs are available with high-speed counting capabilities and high-frequency pulse outputs built into the controller, making them a viable solution for open-loop control. The one caveat is that the controller does not know the position of the out
15、put 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 ap
16、plications require higher-precision positioning hardware and software, and many PLCs offer high-speed option modules that interface with servo drives. Most drives today can accept traditional commands from host (PLC or PC) controls, or provide their own internal motion control. The trend here is to
17、integrate the motion control configuration into the logic controller programming software package. Programming Languages 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 2 页，共 7 页 - - - - - - - - - 3 A facet of the PLC that reflects both the past and the future is prog
18、ramming language. The IEC 61131-3 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 defin
19、es 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 re
20、cently still use ladder diagrams to construct their PLC code. It seems that ladder logic continues to be a top choice given its performed so well for so long. Hardware Platforms The modern PLC has incorporated many types of Commercial off the Shelf (COTS) technology in its CPU. This latest technolog
21、y 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 communications, data manipulation and high-speed motion without giving up the rugged and reliable performance expected from ind
22、ustrial 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
23、interfaces, languages and protocols. They could be viewed as a PC in an industrial PLC-like package. The Future A 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 f
24、eatures of importance were. * The ability to network, and do so easily. Ethernet communications 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 platf
25、orms. These include Modbus (ModbusTCP), DeviceNet (Ethernet/IP) and Profibus (Profinet). Ethernet communication modules for PLCs are readily available with high-speed performance and flexible protocols. Also, many PLC CPUs are now available with Ethernet ports on board, saving I/O slot space. PLCs w
26、ill continue to develop more sophisticated connectivity to report information to other PLCs, system 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 3 页，共 7 页 - - - - - - - - - 4 control systems, data acquisition (SCADA) systems and enterprise resource planning (ERP) s
27、ystems. Additionally, wireless communications 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, allowi
28、ng 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 computers to interface with standard PLC I/O subsystems by using interface cards, typically supplied by the PLC manufacturer or a third p
29、arty developer. Now these challenging locations can be monitored with today a PC. Where industrial-grade 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
30、multiple targets/platforms. In the past it was expected that an intelligent controller would be complex to program. That is no longer the case. Users are no longer just trained programmers, such as design engineers or systems integrators, but end-users who expect easier-to-use software in more famil
31、iar formats. The Windows-based look and feel that users are familiar with on their personal computers have become 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
32、 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 end-user would be for a softwa
33、re package that could seamlessly program many manufacturers PLCs and sub-systems. After all, Microsoft Windows operating system and applications work similarly whether installed on a Dell, HP or IBM computer, which makes it easier for the user. Overall, PLC users are satisfied with the products curr
34、ently 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 become more accepted in the industrial world. 名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心
35、整理 - - - - - - - 第 4 页，共 7 页 - - - - - - - - - 5 PLC 的过去、现在与未来众所周知，科技世界里只有一个永恒真理，那就是变化。 这在可编程逻辑控制器（PLC）及其各种应用的发展过程中尤为明显。自从三十多年前将PLC引进以来， PLC已经在广泛的工业领域中成为几十万控制系统的基础。从本质上讲， PLC是一种用高度专业化语言编程的工业计算机，并继续受益于计算机和信息技术领域的技术进步。它的最突出之处是小型化和通信功能。微型化的 PLC 在最初引进 PLC的时候，主要改进它的体积，这与替换了数百个硬接线继电器和计时器有关。一个嵌有CPU和 I/O 的典
36、型单元有大约19 寸电视机那么大。从 20 世纪 80 年代到 20 世纪 90 年代初，模块化的PLC逐渐微型化，同时它的容量和性能也得到了提高。近年来，更小型PLC 已经发展到纳米级和微型级，它们已具有以前只在大型 PLC 上才有的特点。因此仅为了额外特性或性能而不是增加I/O 容量而具体指定一个大型的PLC变得不必要，因为即使纳米级PLC也具备以太网通信、运动控制、自动调谐的嵌入式PID、远程连通性等更多的功能。现在，由于 PLC能执行运动控制、数据采集，远程终端单元（RTU）甚至一些集成人机介面 （HMI）等功能，因此 PLC在很多应用中也已配置齐全从而替代单一的过程控制器。 以前，这
37、些功能通常要求他们自身内置实现这些功能的控制器和软件，此外，还需要一个用于离散控制和互锁的独立的PLC。强大的通信功能近年来，最有意义的变化也许发生在通信领域。 在 20 世纪 90 年代， Modicon推行的 Modbus 通信协议，允许PLC 通过标准电缆进行通信。这为PLC 更好地适用于现存的设备提供了可能性，并且向主板上的其它控制系统通信成为可能。在过去的 30 年里，我们真实地目睹了数百个专利化协议和标准化协议的发展，每一个协议都有自己独特的优势。 现在，PLC已成为数据编译器和信息网关，它们必须接入条形码扫描器和打印机，还有温度和模拟传感器。在过程控制中，它们需要支持多种协议，
38、以便它们能和其它设备通信。 此外，在它们全部具备这些功能的同时，它们仍然要有高的性价比而且编程简单。另一个主要改进来自于计算处理领域。确切地说，它革命化了PLC 的编程方式、互相通信、与用于HMI、SCADA和 DCS的 PC有接口。在过去的五年中， 车间级以太网通信的应用已经翻了一倍。尽管串行通信仍然很受欢迎并且很可靠， 但以太网快速地成为值得选择的通信媒体，它有着不能被忽视的优势，例如：网速、设置简单、布线方便、现成网络组件的可用性、嵌入式通信设置集成运动控制另一个分配给 PLC的任务是运动控制。从简单的开环控制到多轴应用来看，在 PLC的软件和硬件中集成运动控制已经成为一个趋势。名师资料
39、总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 5 页，共 7 页 - - - - - - - - - 6 很多系统在快速运行时要求精确的控制，但并不是在超高速运行时的绝对精准。单机 PLC 在一些系统上也能很好地运行。许多纳米级和微型级PLC都有高速运算能力和控制器内置的高频脉冲输出能力，使它们成为开环控制的可行解决方案。一方面要提醒的是控制器在控制顺序上不能确定输出设备的位置。另一方面要提醒的是它的主要优势在于它的成本。以前，即使简单的运动控制也要求有一个昂贵的选择模块。 有时为了满足系
40、统需求， 它不能用于更精密的控制平台中。越精密的运动控制系统要求越高精度的定位硬件和软件，而许多PLC 都提供高速选择模块接入伺服驱动。现在，许多驱动都兼容来自控制主机（PLC 或PC）的传统命令，或者提供自身的内部运动控制。将运动控制组态软件集成在PLC编程软件包中将成为一种趋势。编程语言编程语言是反映 PLC历史的一个方面。国际电工委员会61131-3 标准（IEC 61131-3 ）处理了编程语言并且定义了两个图形化的和两个文本化的PLC编程语言标准：梯形逻辑（图形化）功能块图（图形化）结构化文本（文本化）指令表（文本化）这项标准也定义了为顺序控制和并行控制处理组织程序的图形化的和文本化
41、的顺控功能图元素。基于这项标准，许多PLC 生产商提供最少三种语言中的两种作为他们PLC 编程时的语言选择。讽刺的是，在近来接触的PLC使用者中，有大约96%仍然使用梯形图编写PLC 代码。由于梯形图长期的良好表现，因此梯形图编程似乎会继续成为最好的选择。硬件平台现代的 PLC已经在它们的 CPU里集成了多种的商业非定制（COTS ）技术。最新的技术为 PLC提供了一个更快，更强，存储量更大且成本更低的处理器。这些先进技术也为PLC 扩展集成和接受新任务（如通信、数据处理和高速运动）留出空间，而不会以牺牲工业控制设备所要求的严格可靠性为代价。新技术已经创造了另一种控制器可编程自动化控制器（PA
42、C ） 。PAC与传统的 PLC 有所不同，这是因为PAC在软件和硬件上都采用了开放的模块化体系结构，并且采用了网络接口、 语言和协议的已知标准。 它们可以被看作为一个置于工业的类 PLC程序包里的 PC 。展望未来来自 Reed研究团的一份 2005 年 PLC产品聚焦研究指出了一些对于PLC使用者、机器制造商和采购决策人员愈加重要的因素。排在最前的、 有重要特征的因素是：1联网能力和易用性以太网通信在通信领域中独占熬头。不只是新协议在逐渐平面化， 许多已经使用了许多年的工业标准化协议也在逐渐地接入到以太网平台上，这些协议包括Modbus (ModbusTCP) 、DeviceNet (Et
43、hernet/IP) 和 Profibus (Profinet) 。PLC的以太网通信模块已经具有高速性能和灵活性的协议群。同样，许多 PLC上的 CPU在工作时已经可以接入以太网，这就节省了 I/O 槽空间。PLC将会继续开发更成熟的连通性，使它可以将信息传输到其它PLC 系统、系统控制系统、数据采集（SCADA ）系统和企业资源计划（ERP ）系统上。另外，无线通信方式将会进一步普及。2用 PC连接 PLC输入/ 输出（ I/O ）的能力名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - -
44、 第 6 页，共 7 页 - - - - - - - - - 7 同样的一个趋势，过去与PLC 联网获益良多，但现在已经转移到了I/O 水平上。许多 PLC 生产商已经支持最受大众接受的现场总线网络，使PLC 的 I/O可以分配在广阔的区域上或者定位在以前被认为是几乎不可能的位置上。这使个人电脑通过使用接口卡接入到标准PLC 的 I/O 子系统成为可能，而这种接口卡主要是由 PLC 生产商或第三方开发商提供的。现在，这些具挑战性的广阔区域都可以由个人电脑监控。 至于在不需要技术等级控制引擎支持的地方，使用者可以利用更为先进的软件包和硬件机动性，这成本也比较低廉。3使用通用编程软件编制多对象/
45、平台的能力过去， 人们都认为一个智能型控制器需要复杂的编程，但现在不再是这样了。使用者不再只是受过培训的编程人员（如设计工程师和系统集成人员），而是在更熟悉的设计上期待能用上人性化软件的最终用户。在他们个人电脑上， 基于视窗(Windows) 的外观和触感是最为人所熟悉的，它已成为最受大众接纳的图形用户界面。开始时，用于为PLC 编程的简单继电逻辑仿真已经发展成为使用更高级功能块的编程语言（这些功能块可以更直观在配置出来）。PLC 生产商也已经开始一体化不同功能块的编程，使你在构造逻辑、HMI、动态控制和其它特定功能时只需学习一个编程软件包。 也许，最终用户的最终愿望是获得一个可以无缝地为多个 PLC和子系统编程的软件包。毕竟，无论是安装在Dell、HP 电脑上还是安装在 IBM 电脑上，微软的视窗（ Windows）操作系统和应用程序都是同样地运行，这就方便了系统用户。总的来说， PLC用户对现行可用的PLC产品感到满意。 同时，他们密切关注新的趋势，并在他们认为有利可图的地方用上新的PLC 产品。新 PLC 利用先进的技术，使它们在工业界更容易地被采纳。名师资料总结 - - -精品资料欢迎下载 - - - - - - - - - - - - - - - - - - 名师精心整理 - - - - - - - 第 7 页，共 7 页 - - - - - - - - -