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 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 prominent 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. Through 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 only 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 also 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 required 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 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.
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 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, communicate 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 communications 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 tasked 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 where 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 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 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 integrate the motion control configuration into the logic controller programming software package.
Programming Languages
A facet of the PLC that reflects both the past and the future is programming 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 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 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 technology gives the PLC a faster, more powerful processor with more me
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我们每个人都知道，在科技和技术的世界里，只有一个永恒的真理，那就是变化。这在可编程逻辑控制器（PLC）及其各种应用的开发中尤其明显。自从三十多年前PLC被引进以来，PLC已成为数十万控制系统在广泛的行业的基石。
从本质上讲，PLC是高度专业化的语言编程的工业化的计算机，并继续在计算机和信息技术世界的技术进步中获益。其中最突出的是小型化和通信。
微型化的PLC
当PLC首次推出，它的大小是一个重大改进 - 这与更换了数百个硬接线继电器和计时器有关。一个镶嵌了CPU和I / O的典型的单元壳大致相当于19寸视机的大小。从20世纪80年代到20世纪90年代初，模块化的PLC逐渐往微型化发展，与此同时，它的容量和性能得到增加。近年来，更小型的PLCy已经发展为纳米和微观类，具备了以前只有在大型PLC上才有的功能。这使得为了附加额外的特征和性能而不是增加I/O计数来指定一个较大的PLC显得不必要，因为即使那些纳米级的PLC也具备以太网通信、运动控制、自动调谐的嵌入式PID、远程连接等其他更多的功能。
现在，因为PLC能执行运动控制，数据采集，RTU（远程遥测单元），甚至一些集成HMI（人机接口）等功能，所以现在PLC也已经配置齐全从而代替单一的过程控制器。以前，这些功能往往需要自己的专用控制器和软件，加上用于离散控制和联锁独立的PLC。
强大的通信功能
近些年来，发生的最显著的变化可能在通信领域。在20世纪70年代引入的Modicon Modbus通信协议所允许的PLC通过标准电缆进行通信。这相当于放置的PLC更接近真实世界的设备，并在主面板传送回给其他系统控制的能力。
在过去的30年里，我们真实地目睹了数以百计的专利化和标准化协议的开发，每个都有自己独特的优势。今天的PLC已经成为数据编译器和信息网关。它们与条码扫描仪和打印机，以及温度和模拟传感器接口相连。它们需要多协议支持，能够与进程中的其他设备连接。进而，他们需要所有这些功能，同时保持成本效益和程序的简洁化。
另一个改进主要来自于计算机领域，确切地说，它革命化了PLC的编程方式、互相通信、与将接口用于HMI、SCADA和DCS的PC的通讯。
在过去的五年中，车间级以太网通讯的应用已经多了一倍。尽管串行通信仍然很受欢迎，可靠，但以太网迅速地成为值得选择的通信媒体，它有着不能被忽视的优点，例如：网速快、设置简单、布线方便、现成网络组件的可用性高、嵌入式通信设置 便捷。
集成运动控制
在PLC一直负责的另一个责任是运动控制。从简单的开环到多轴应用，该趋势一直到这个功能集成到PLC的硬件和软件。
有很多系统在快速运行中需要精确的控制，这些应用中，独立的PLC效果很好。许多纳米和微米PLC是可用内置于控制器的高速计数功能和高频脉冲输出，使得它们是开环控制的可行解决方案。
一个需要注意的是，控制器不知道输出装置的控制序列期间的位置。在另一方面，它的主要优点是成本低。以前即使是简单的运动控制需要昂贵的选件模块，有时为了满足系统的要求，它不能用于更精密的控制平台中。
更复杂的运动控制应用需要更高精度的定位硬件和软件，而许多PLC提供高速选择模块与伺服驱动器接口。大多数驱动都可以从主机（PLC或PC）控制接受传统命令，或者提供自身的内部运动控制。将运动控制配置集成到逻辑控制器编程的软件程序包将成为一种趋势。
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标准涉及的编程语言，并定义了两个图形和两个文本PLC编程语言标准:梯形逻辑（图形）、功能框图（图形）、结构化文本（文本）、Instruction列表（文本）。
该标准还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的顺控功能图元素。根据标准，很多厂商提供至少两种这些语言作为编程的PLC的选择。讽刺的是，PLC用户约96％的最近还在用梯形图来构建自己的PLC代码。看来，梯形逻辑仍然是首选，因为它的表现如此之好了这么久。
硬件平台
现代PLC已经在它们的CPU中纳入了许多类型的商业的非定制（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。 这些先进的技术还为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，而不会牺牲控制工业的所要求严格性为代价。
新技术已经创造了另一种控制器---可编程自动化控制器（PAC）。有所不同的地方是因为PAC在软件和硬件上都使用了开放式的模块化的体系结构，而且采用了标准网络接口，语言和协议传统PLC。它们可以被看作是一个置于工业类PLC程序包里的PC。
展望未来
来自里德研究小组2005年的PLC产品聚焦研究指出了一些对PLC用户，机器制造商和采购决策人员越来越重要的因素是：
- 联网能力和实用性
以太网正在引领通信领域。不仅是新的协议逐渐平面化，许多已经使用了许多年的工业标准化协议被移植到以太网平台。这些措施包括的Modbus（MODBUSTCP），DeviceNet的（以太网/ IP）和Profibus（PROFINET）。PLC的以太网通讯模块是具有高速性能和灵活的协议群。此外，许多PLC的CPU现在可以与板载以太网端口，节省I / O插槽的空间。PLC将会开发更先进的连通性，使他可以将信息传送到其它PLC系统、控制系统、数据采集（SCADA）系统和企业资源计划（ERP）系统上。此外，无线通信将继续得到普及。
- 用PC连接PLC输入/输出（I/O）的能力
相同的趋势，以前与PLC联网受益很多，但是现在已经迁移到I / O水平。许多PLC厂商都支持最公认的现场总线网络，使PLC的I / O分配在更远的位置，或者它以前被认为是几乎不可能的位置。这使用个人电脑通过接口卡接进PLC的I/O子系统成为可能，这种接口主要由plc生产商和第三方开发商提供的。现在，这些具有挑战性的位置可以用今天的PC进行监控。在不需要工业级控制引擎的情况下，用户可以以较低的成本利用更先进的软件包和硬件的灵活性。
3．使用通用编程软件编制多对象/平台的能力
在过去，大家都认为一个智能控制器需要复杂的编程，现在已经不会再出现这种情况。 用户不再仅仅是训练有素的程序员，如设计工程师或系统集成商，而是期望用上更熟悉的设计和更人性化的软件的最终用户。基于Windows的外观和感觉，用户熟悉他们，使得他们的个人电脑已经成为最接受的图形用户界面。开始的时候用于为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（这些功能配置更直观）。PLC制造商也开始实现多样化的功能 ，让你在配置逻辑，HMI，运动控制等专业功能的编程集成的时候只需学习一个编辑软件包。最终用户的最终愿望可能是得到一个 无缝地为多个PLC和子系统编程的软件包。毕竟，无论是安装在戴尔，惠普或IBM的电脑，微软的Windows操作系统和应用程序都是同样地运行，这就方便了用户。
总体而言，PLC用户满意目前提供的产品，与此同时，他们密切关注新趋势，并且在有好处的地方用上新的plc产品。通常情况下，新的PLC需要新的技术以保证它们在工业界更容易被采纳。
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