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PLC or DCS: What is the difference?
Turn the clock back 10-15 years: The programmable logic controller (PLC) is king of machine control while the distributed control system (DCS) dominates process control. If you manufacture plastic widgets, you speak PLC. If you produce chemicals, you speak DCS.
Today, the two technologies share kingdoms as the functional lines between them continue to blur. We now use each where the other used to rule. However, PLC still dominate high-speed machine control, and DCS prevail in complex continuous processes.
The early DCS looked dramatically different from the early PLC. Initially, the DCS performed the control functions of the analog panel instruments it replaced, and its interface mimicked their panel displays. DCS then gained sequence logic capabilities to control batch processes as well as continuous ones. DCS performed hundreds of analog measurements and controlled dozens of analog outputs, using multi-variable Proportional Integral Derivative (PID) control. With the same 8-bit microprocessor technology that gave rise to the DCS, PLC began replacing conventional relay/solid-state logic in machine control. PLC dealt with contact input/output (I/O) and started/stopped motors by performing Boolean logic calculations.
The big change in DCS over the past 20 years is its move from proprietary hardware to the personal computer (PC) and standard LAN technologies. With each advance in PC power, DCS have moved up in power. PCs gave us speedy, responsive, multi-media, windowed, operator-process interfaces (OPI). Relational databases and spreadsheet software enhance the ability of DCS to store and manipulate data. Artificial intelligence (AI) technology gives us "smart" alarming. Today's DCS architecturally looks much like the DCS of 20 years ago, but tomorrow's DCS may control through networked "smart" devices-with no I/O hardware of its own.
Most DCS offer redundant controllers, networks, and I/Os. Most give you "built-in" redundancy and diagnostic features, with no need for user-written logic.
DCS allow centralized configuration from the operator or engineering console in the control room. You can change programming offline, and download without restarting the system for the change to be effective.
DCS allow inter-controller communications. You can do data exchange in most DCS systems ad hoc (no need for predefined data point lists). You access data by tag name, regardless of hardware or location.
DCS use multi-tasking operating systems, so you can download and run applications aside from the real-time control functions and still do fractional-second control. DCS now come in "micro" systems, to price-compete with PLC but with full DCS features and capabilities.
The typical DCS has integrated diagnostics and standard display templates that automatically extend/update when your database changes. This database is central to the system-you don't have different databases sitting in the controllers.
DCS have user-friendly configuration tools, including structured English, control block libraries, SFC (sequential function chart), and even RLL (relay ladder logic).
Most DCS allow graphical configuration, provide online diagnostics, and are self-documenting. Most provide for user-defined control blocks or customized strategies. The controllers execute control strategies as independent tasks; thus, making changes to part of the control logic has no impact on the rest.
An important difference between DCS and PLC is how vendors market them. DCS vendors typically sell a complete, working, integrated, and tested system; offering full application implementation. They offer many services: training, installation, field service, and integration with your Information Technology (IT) systems. A DCS vendor provides a server with a relational database, a LAN with PCs for office automation, networking support and integration of third-party applications and systems. The DCS vendor tries to be your "one-stop shop." The PLC is more of a "do-it-yourself" device, which is sometimes simpler to execute.
When PLC was solely replacements for hard-wired relays, they had only digital I/O, with no operator interface or communications. Simple operator interfaces appeared, then evolved into increasingly complex interfaces as PLC worked with increasingly complex automation problems. We went from a panel of buttons and I/O-driven lamps to PLC full-color customized graphic displays that run on SCADA software over a network.
PLC now have many DCS-like control functions (e.g., PID algorithms) and analog I/O. They've moved past their birthplace: the digital world (switch and binary sensor inputs and output contacts to run motors and trigger solenoids).
PLC is fast: They run an input-compute-output cycle in milliseconds. On the other hand, DCS offer fractional second (1/2 to 1/10) control cycles. However, some DCS provide interrupt/event-triggered logic for high-speed applications.
PLC is simple, rugged computers with minimal peripherals and simple OS. While increasing reliability, PLC simplicity is not conducive to redundancy. Thus, fully redundant variations of PLC, with their added hardware and software, sometimes suffer from a reduction in their reliability-a characteristic PLC are famous for.
Data exchange typically requires you to pre-assign data registers and hard code their addresses into the logic. If you add registers or need to reassign data, you typically have to deal manually with the Domino Effect.
Typical PLC Relay Ladder Logic (RLL) languages include function blocks that can perform complex control and math functions (e.g., PID algorithms). Complex multi-loop control functions (e.g., cascade management and loop initialization) are not typical. For functions too messy to implement in RLL, most PLC provide a function block that calls a user-written program (usually in BASIC or C).
PLC typically operates as "state" machines: They read all inputs, execute through the logic, and then drive the outputs. The user-written logic is typically one big RLL program, which means you may have to take the whole PLC off-line to make a change of any size. You also run into database synchronization problems because of the separation of PLC and the Man Machine Interface (MMI) software packages, as opposed to the central databases of DCS.
A PLC will run in a stand-alone configuration. A DCS controller normally expects an operator interface and communications, so it can send alarms, messages, trend updates, and display updates.
Many PLC installations use interface software from third-party vendors for improved graphics and various levels of alarming, trending, and reporting. The PLC and MMI software normally interact by sitting on the network and using the register exchange mechanism to get data from and to the various PLC. This type of communication presumes you have preassigned data registers and can fetch data on an absolute address basis. This can lead to data processing errors (e.g., from the wrong input) you won't encounter with the central database of a DCS.
Some PLC use proprietary network, and others can use LANs. Either way, the communication functions are the same-fetch and put registers. This can result in bottlenecking and timing problems if too many PCs try communicating with too many PLC over a network.
A PLC may have a third-party package for operator interfaces, LAN interface to PCs and peripherals, PLC data highway or bus, redundant controllers with local and distributed I/O, local MMI and local programming capability. The PLC would have redundant media support, but not the redundant communication hardware or I/O bus hardware you'd find in a DCS. A PLC would have preprogrammed I/O cards for specific signal types and ranges.
In the beginning, there was the microprocessor. This small chip, with its integrated functionality, laid the technological foundation for the birth of PLC and DCS. The first PLC application was in the automobile industry in 19'70. Necessity to satisfy application requirements, as requested by General Motors, became the mother of invention for the PLC in discrete manufacturing. The PLC was first used commercially in machine control applications for metal cutting, hole drilling, material handling, assembly and testing for GM's Hydramatics Model 400 automatic transmission.
The PLC replaced hard-wired electromechanical relays and provided greater flexibility by eliminating the necessity of reworking hard-wired panels to accommodate process and application changes. The PLC allowed engineers to make faster and easier production changes that translated into dramatic reductions in both cost and cycle time. PLC have dramatically evolved since their inception and have greater functionality and application. Advances in operator interface software have provided engineers with a user friendly environment and a configurable window into all process control applications.
The DCS was introduced in the mid 1970s and revolutionized the process control industry. The DCS was the first practical and comprehensive replacement by large and bulky hard-wired custom control panels. At that time, DCS control philosophy was centralized and liability was minimized through distribution over the entire control system. It offered for the first time, the ability for all engineering disciplines to program a process control computer through configurable software without necessarily requiring high level programming skills. DCS provided the user with larger boundaries on flexibility, speed of control, security, manufacturing consistencies and reliability than ever before.
Generally in-process control, the procurement of a control system is the largest single expenditure automation engineers are faced with. The control system, in most cases, is the brain and nervous system of a production facility. The purpose of the control system is to supervise, control, monitor, schedule, document and record process parameters that are vital to production. Since automation is critical to manufacturing, selection of a control system, by definition, is equally important. Trying to decide between control systems with similar functionality can be very subjective, confusing and time consuming.
Once the Want objectives have been defined, a weight is assigned to each with respect to its relative importance. The most important objective is given a weight of 10, all other objectives are weighted in comparison with the first, from 10 down to 1. All Want objectives are evaluated and scored for degree of compliance, on a scale from 1 to 10. The weighted Want objectives are then multiplied by the score to result in a weighted score. Selection is awarded to the control system with the highest total weighted score. This structured analytical process for control system selection, is efficient, less subjective and quantifiable. Engineers can use these tools easily to quantify and justify both technical and commercial selection of PLC/DCS control systems.
Comparing the differences between the functions of PLC and DCS for the purposes of selection has become more difficult for engineers today. There no longer exists a clear black and white difference in application and functionality that was so evident in the 1970s. PLC over the past two decades have entered applications that were traditionally in the DCS domain. This trend continued into the 1980s and 1990s and has, to some degree, polarized control engineers to voice strong preferences in favor of application solutions using PLC, DCS or a hybrid of both in certain applications. The continued encroachment of PLC into the DCS domain has consistently increased from sharing limited common functionality in the 1980s to major overlapping of functions in the 1990s. The DCS has also taken advantage of new technology to improve its performance to handle discrete signals. Some DCS have incorporated functionality such as relay ladder logic, function block and structured text programming that were traditionally found in PLC. By the year 2000, expect to see a functional convergence of the two systems into one unified PLC/DCS control system for large applications, costing the same as today's PLC-based control systems.
Comparing and choosing between PLC and DCS based control systems can sometimes be quite difficult, tedious and somewhat subjective, depending on the application. The user's technical background as well as work experience. The debate between which system is a better choice when it comes to application, cost and performance, will be decided only by the user in his final selection. The intention of the following common system issues is to highlight differences most often encountered in the selection process.
Cost Initial hardware and software costs, in almost all cases, are less for PLC than DCS However, in a large system that requires heavy integration and custom coding PLC software cost may be higher and will cancel any cost savings that may have been realized initially. It is essential for this reason to define accurately and completely all system functions and needs in the form of a user's requirement document and carefully evaluate compliance of control system proposals accordingly.
Batch sequencing in large processes that require production of multiple products and/or batches, including recipe management, the DCS outperforms the PLC. However, in small, dedicated batch production with limited recipes, where batch management is not critical, PLC is a cost-effective and practical solution.
Security DCS networks are designed to offer high availability and full redundancy in all system components, with no single point of failure. Tight coupling between operator interface, controllers and system software allows greater security and assurance that all components will work well together. Not all levels of PLC can offer the same system-wide redundancy, and similar DCS features must be designed in to the PLC by the user.
Reliability Historically,reliability for both PLC and DCS has a solid track record and is rated equal.
Diagnostics,Certain PLC and DCS can be considered almost equal in diagnostics. However, DCS can handle system diagnostics more easily. Both can go down to reading diagnostics at a point level as well as health of hi way, 1/O cards and CPU loading. Since system reliability is very high for both PLC and DCS, this may not be a major consideration.
OEM integration The PLC is predominantly used as embedded automation by OEMs because of its ideal application and low hardware cost. Most small machine operations can not support an expensive DCS solution. The choice of using a PLC-based control system is clear when integrating multiple OEM pieces of automated equipment. Integrating multiple OEM PLC using third party PC-based SCADA software is a natural choice. A DCS can also be used as a front-end to interface with OEM PLC. Using this type of hybrid DCS/PLC configuration will require additional software programming. OEM proprietary and warranty issues will surface if the user chooses to tie OEM PLC and/or field devices directly to the DCS.
Now, emerging field protocol standards for digital communications, such as FieldBus and Profibus will lay the foundation for additional reduction in project costs and provide faster, more accurate communications. This will complement the future trend to shift more system intelligence from control system controllers down to the field devices, such as smart transmitters and intelligent valves. This trend will off-load some of the PLC/DCS processing capacity for more critic applications. In summary, the innovative use and immediate utilization of current PLC/DCS technology is key in maintaining a competitive edge in a global economy.
PLC and DCS overlap in their features, but also have distinct strengths and weaknesses. When deciding between the two, know who will deliver and support your system, and how they will do it.
PLC或DCS系统：有什么区别？
时光倒流10至15年：可编程逻辑控制器（PLC）是计算机控制的国王，而分布式控制系统（DCS）则主导着过程控制。如果您制造塑料部件，你支持的PLC；但是如果你生产的化学品，你则选用集散控制系统。
今天，这两种技术之间的职能划分份额继续模糊不清。两者已经相互应用在对方曾经控制的领域。然而，PLC在高速机的控制方面仍然占主导地位的，DCS系统盛行在在复杂的连续过程方面。
早期的DCS系统看起来与早期的PLC有着显着的不同。最初，集散控制系统取代了模拟仪表板的控制功能，它的界面模仿他们的显示器。 DCS系统引入逻辑顺序控制，以此获得了间歇过程和持续的控制功能。DCS系统采用多变量比例积分微分（PID）控制，进行模拟测量和模拟控制几十个几百个输出。与同级别的8位微处理器技术兴起了集散控制系统相似，PLC开始取代传统的机械继电器/固态逻辑控制。PLC通过执行布尔逻辑运算控制电动机的输入处理/输出（I/O）和开始/停止功能。
集散控制系统在过去20年的巨大变化是其从专有硬件转移到个人计算机（PC）以及局域网标准技术上。随着电脑电源的发展，DCS系统在电力上也获得了提升。电脑可以给了我们快捷，反应灵敏，多媒体，窗口，操作流程接口（OPI的）。关系型数据库和电子表格软件增强了DCS系统存储和处理数据的能力。人工智能（AI）技术为我们提供了令人震惊的“智能”。今天的DCS系统体系结构上看起来很和20年前的DCS很像，但是明天的集散控制系统或许可以通过“智能”网络控制设备而不需要自身的I/O接口。
大多数DCS系统提供冗余控制器，网络和I/O接口。大部分允许用户“内置”冗余和诊断功能，而不需要用户编写控制程序。 DCS系统允许操作者或工程师在控制室集中配置控制系统。您可以离线编程，无需重新启动便可下载程序并使系统重新配置生效。
DCS系统允许内部通信。你可以在大多数DCS系统方案（无需提前定义数据交换点列表）中交换数据。您可通过标记名称访问数据而与硬件或地点无关。
DCS系统采用多任务操作系统，所以你可以在运行实时控制功能的同时下载并运行其他程序，并且可以运行少数的第二控制。 在价格方面，与PLC相比，实现相同的系统特性和功能，DCS系统只是个“微型”系统。
典型的集散控制系统具有集成诊断和标准模板功能，可以自动显示更新您的数据库。数据库是系统的核心，在控制系统中你只有一个数据库。
DCS系统具有用户友好的配置工具，包括结构化英语，控制块库，SFC（顺序功能图表），和RLL（继电器梯形图）。大多数DCS系统允许图形化配置，提供在线诊断，并自我记录。大多数为用户提供定义的控制块或定制策略。该控制器的控制策略是独立执行的任务，因此，更改部分系统的控制对其它部分没有影响。
DCS系统和PLC之间的一个重要区别在于厂商的市场推广方式。集散控制系统供应商通常销售完整的、工作的、集成和测试系统;提供全面的执行系统。他们提供多种服务：培训，安装，现场服务，并与您的信息技术（IT）系统的集成。一个DCS系统供应商提供了一个关系数据库，与办公自动化网络的连接，网络支持以及第三方应用软件和系统的集成。DCS厂商试图成为你的“一站式商店”。 PLC的只是一个“做自己动手”装置，更易于执行。
当PLC的作为硬接线继电器的替代品出现时，他们只有数字I/O，没有操作界面或通信接口。简单的操作界面出现后，此后随着PLC工作在日益复杂的自动化问题上，操作界面的演化也越来越复杂。从面板的按钮和I/O驱动灯的PLC到全彩色定制，可以运行在SCADA软件上的网络话图形显示。
现在有很多PLC的集散式控制功能（例如，PID控制算法）和模拟的I/O。他们超越了过去的产品类型：数码世界（开关和传感器的输入和输出联合控制二进制运行电机工作）。
PLC的运算速度快：他们以毫秒计算单位输入输出周期。另一方面，DCS系统提供微秒（1/2到1/10）控制周期。然而，一些集散控制系统提供高速中断触发器的应用方式。
PLC是简单的，以最少的外围设备和简单的操作系统来强化计算机。虽然提高了可靠性，但是PLC的不能解决冗余的问题。因此，完全冗余（“热”自动，无扰）的变化与PLC添加的硬件和软件有关，有时是受到其可靠性的影响，这是PLC一个有名的问题。
数据交换通常需要你预先分配数据寄存器和硬编码的地址。如果您添加或需要重新分配寄存器数据，您需要人工处理由此引发的骨牌效应。
典型的PLC继电器梯形逻辑（RLL的）语言，包括功能模块，可以执行复杂的控制块和数学函数（例如，PID控制算法）。复杂的多回路控制功能（例如，梯级管理和初始化循环）并不典型。对于功能太乱RLL的实施，大部分PLC提供了一个功能块来调用用户编写的程序（通常在BASIC或C）。
PLC的通常运作为“规定”的机器：他们阅读的所有输入，执行逻辑程序，然后驱动输出。用户编写的逻辑程序是一个典型的RLL的方案，这意味着你可能要使PLC的离线无论用户做出任何大小的改变。您还遇到由于PLC和人机界面（MMI）软件包分离而带来的数据库同步问题，而不是DCS系统的中央数据库。
可编程控制器将运行在一个独立的配置。一般的DCS控制器需要操作界面和通信接口，用来发送警报，信息，趋势更新，并显示更新。
许多PLC用户使用第三方供应商的人机界面软件来改进的图形和其它各级报警，趋势，报告功能。 PLC和人机界面软件通常会通过在交互网络上注册和使用的交流机制来实现数据在各PLC之间的交换。这种类型的通信方式假设你预先指定了数据寄存器，可以读取绝对地址上的数据。这可能导致数据处理错误（例如：从错误的输入），你不会在DCS系统的中央数据库遇到此类问题。
一些PLC的使用专有网络，其它的可以使用局域网。无论哪种方式，都是一样的通信功能和寄存器读取方式。这可能会导致瓶颈和时间延迟，如果太多的个人电脑通过网络尝试与很多PLC的通信的话。
编程控制器可能有一个第三方的操作界面，局域网PC和外设接口，PLC的数据总线，冗余本地控制器和分布式I/O，本地人机界面和本地化的编程空间。PLC会有冗余的媒体支持，但是DCS系统不会有多余的通信硬件或I/O总线。PLC有预先编程的针对特定的信号类型和范围的I/O卡。
起初，在微处理器时代。这个具有集成功能的小芯片奠定了PLC和DCS产生的技术基础。PLC的首次应用是在1970的汽车行业。为了满足通用汽车公司的要求，PLC的产生式在不相关的制造行业。PLC首次使用在控制金属切削，钻孔，材料处理，组装和通用公司的Hydramatic模型试验机400的自动变速器控制上。
PLC取代硬连线并通过消除为适应流程和应用改变而做出硬件改动提供了更大的灵活性。PLC允许工程师更快，更容易生产并兼顾到成本和周期工作时间。自成立以来PLC取得了很大的发展，有了更强的功能和应用。操作界面软件的进展为工程师提供了一个友好的环境以及全过程控制的可配置窗口。
DCS系统产生于20世纪70年代中期并革新了过程控制行业。该集散控制系统是第一个实用的，全面的大型替代了庞大的硬连接控制线路。当时，DCS控制是集中分布式的通过分布式控制减少系统的负载。它首次提出，使所有的工程师可以通过可配置的软件程序来编程控制生产过程，而不一定需要高层次的编程技能。集散控制系统为用户提供了更大的灵活性，快速控制，安全，生产的连续性和可靠性有了较大的提高。
一般来说，在过程控制中，控制系统采购是自动化工程师面临着的最大的一笔开支。该控制系统在大多数情况下，是生产设施的大脑和神经系统。该控制系统的目的，是为了监督，控制，监控，调度，记载和记录工艺参数，这些对生产是至关重要的。由于自动化是生产的关键所在，选择一个控制系统的，也是同样重要的。试图在具有类似功能的控制系统之间选择是很主观，混乱和费时的。
一旦目标确定，各方面因素会被根据重要性分配比重。最重要的目的获得10的比重，其他因素与第一个相比较后给出比重值，从10到1。所有预期目的都按照评估获得的相应的比重，从1到10的范围。预期目的的加权值乘以得分后产生最后的加权值。最后选择获得最高分的控制系统。这种结构化的控制系统选择分析的过程，是有效的，较少主观和量化的。工程师们可以使用这些工具来简单化的评估PLC/DCS控制系统在技术和商业的因素。
以选择系统为目的的比较PLC和DCS系统之间的功能差异，对工程师来说更加困难了。在应用和功能上已不存在一个像20世纪70年代那么明显的差异了。在过去20年里，PLC已进入是在传统的DCS应用领域。这一趋势到持续到20世纪80年代和90年代，并在一定程度上延续，分化了控制工程师的选择，选择PLC，DCS系统或混合在某些应用中。PLC对DCS系统领域的侵入，从20世纪80年代分享有限的相似功能，到20世纪90年代主要功能的重叠。DCS系统也采取了新的技术手段来提高其控制性能，以处理离散信号。DCS系统也扩展新的应用，如梯形逻辑功能，功能块和结构化文本编程这些PLC的传统应用。到2000年，有可能看到一个统一的PLC/DCS大型应用控制系统。耗资与今天基于PLC的控制系统一样。
在PLC与DCS控制系统之间比较和选择，有时是相当困难的，单调乏味，有些主观的，基于应用的。用户的技术背景以及工作经验。两个系统之间的孰好孰坏的争辩，有关应用，成本和性能这些因素，将决定用户最后的选择。以下普通系统问题的目的是要突出最常在选择过程中遇到的问题。
成本：最初的硬件和软件成本，几乎所有的情况下，DCS系统价格要比PLC的高。然而，在一个大系统中，需要大量集成和定制编程，PLC的软件成本可能会更高，将消耗用户之前的节省。基于这个至关重要的原因，要确定在用户的要求，文件的准确形式，完整的系统功能和需要仔细评估遵守相应的控制要求。
批次顺序：在需要的多种产品和/或批次的控制过程中，包括配方管理，DCS系统优于PLC控制。然而，对于较小的固定产品的生产过程，批次管理不是最重要的，PLC更符合成本效益，是切实可行的解决办法。
安全：DCS网络设计为所有系统提供高可用性和全面冗余，没有单点故障。操作界面之间的紧密结合，控制器和系统软件耦合允许更大的安全和保证，保证所有组件一起工作。并不是所有的PLC级别都可以提供相同的系统冗余，而类似的DCS系统特性必须由用户在PLC中设计。
可靠性：从历史上看，无论PLC和DCS系统均具有良好的跟踪记录能力和高可靠性。
诊断：PLC和DCS系统在诊断功能上几乎相同。然而，DCS系统可以更容易地处理系统诊断。两种系统均可深入诊断系统，1/O接口和CPU负荷诊断。由于PLC和DCS系统的可靠性都非常高，这可能不是一个主要的考虑因素。
代工生产整合：PLC是作为OEM嵌入式自动化平台的的理想选择，因为其良好的应用和较低的硬件成本。大多数小生产商不能支持的解决方案昂贵的DCS系统。PLC整合了多种自动化设备的OEM部件，所以利用一个基于PLC的控制系统是简单的。整合多种PLC的系统，自然选择第三方的基于PC的监控软件。DCS系统还可以用来作为一个前端界面与原始设备制造商的PLC系统相连接。使用这种混合型DCS/PLC的系统将需要额外的软件支持。如果用户选择将PLC系统或乡长设备直接与DCS系统相连接，OEM系统专有性和保修问题将出现。
现在，新的现场通信协议标准的出现，如FieldBus和ProfiBus，将使项目成本进一步降低并提供更快，更准确的通信。这将完善未来是将有更多系统的智能控制设备作为现场设备使用的趋势，如智能变送器，智能阀门。这一趋势将使PLC/ DCS系统有更强的处理能力。总之，创新技术的使用和当前的PLC/DCS技术利用是保持全球经济中竞争力的关键。
PLC和DCS系统的功能重叠，但也有不同的强项和弱点。在决定两者之间，没有人知道那个会提供并支持您的系统，以及它们将如何做到这一点。
