1、外文文献原文：Programmable logic controllerBy Cynthia CooperFrom Wikipedia, the free encyclopediaA programmable logic controller or simply programmable controller is a digital computer used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpos
2、e computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of
3、 a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.FeaturesControl panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controll
4、er, relay units for input and output.The main difference from other computers is that PLCs are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches,
5、 analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrang
6、ements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be
7、programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed.Many of the earliest PLCs expressed all decision making logi
8、c in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used
9、 a form of instruction list programming, based on a stack-based logic solver.The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and comm
10、unication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.Under the IEC 61131-3 standard, PLCs can be programmed using sta
11、ndards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.PLC compared with other control systemsPLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturi
12、ng where the cost of developing and maintaining the automation system is high relative to the total cost of the automation.PLCs contain input and output devices compatible with industrial pilot devices and controls.PLC applications are typically highly customized systems so the cost of a packaged PL
13、C is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a generic solution。 For high volume or very sim
14、ple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.A microcontroller-based design would be appropriate where hundreds or thousands of units will be pro
15、duced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programmin
16、g of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.Very complex process control, such as used in the chemical industry, may require algorit
17、hms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customized solutions; for example, aircraft flight controls.PLCs may include logic for single-variable feedback analog control loop, a proportional, integral,differential o
18、r PID controller. A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be
19、used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.Digital and analog signalsDigital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit sw
20、itches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. For example, a PLC might use 24 V DC I/O, with values above 22 V DC representing On, values
21、 below 2VDC representing Off, and intermediate values undefined. Initially, PLCs had only discrete I/O.Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy d
22、epending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed binary processors, the integer values are limited between -32,768 and +32,767. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltag
23、e or current with a magnitude proportional to the value of the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.ExampleAs an example, say a facility needs to store water in a tank. The water is drawn from the tank by another syste
24、m, as needed, and our example system must manage the water level in the tank.Using only digital signals, the PLC has two digital inputs from float switches (tank empty and tank full). The PLC uses a digital output to open and close the inlet valve into the tank.When the water level drops enough so t
25、hat the tank empty float switch is off (down), the PLC will open the valve to let more water in. Once the water level raises enough so that the tank full switch is on (up), the PLC will shut the inlet to stop the water from overflowing.An analog system might use a water pressure sensor or a load cel
26、l, and an adjustable (throttling)dripping out of the tank, the valve adjusts to slowly drip water back into the tank.In this system, to avoid flutter adjustments that can wear out the valve, many PLCs incorporate hysteresis which essentially creates a deadband of activity. A technician adjusts this
27、deadband so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.A real system might combine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates
28、and prevent water hammer. Backup and maintenance methods can make a real system very complicated.System scaleA small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model does not have enough I/O.PLCs used in larger I/O sys
29、tems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI (Human-Machine Interface) devi
30、ces such as keypads or PC-type workstations. Some of todays PLCs can communicate over a wide range of media including RS-485, Coaxial, and even Ethernet for I/O control at network speeds up to 100 Mbit/s.ProgrammingEarly PLCs, up to the mid-1980s, were programmed using proprietary programming panels
31、 or special-purpose programming terminals. they were designed to replace relay logic systems. These PLCs were programmed in ladder logic, which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming
32、 languages such as BASIC and C. Another method is State Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams.Recently, the International standard IEC 61131-3 has become popular. IEC 61131-3 currently defines five programming languages for programm
33、able control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.While the
34、fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different
35、models may not be directly compatible.User interfacePLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A Human-Machine Interface (HMI) is employed for this purpose. HMIs are also referred to as MMIs (Man Machine Interface) and GUI (Graphical
36、User Interface).A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) sy
37、stem or web browser.CommunicationsPLCs usually have built in communications ports usually 9-Pin RS232, and optionally for RS485 and Ethernet. Modbus or DF1 is usually included as one of the communications protocols. Others options include various fieldbuses such as DeviceNet or Profibus. HistoryThe
38、PLC was invented in response to the needs of the American automotive industry. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using relays, timers and dedicated closed-loop controllers. The process for updating such facilities for the y
39、early model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an electronic replacement for hard-wired relay systems.The
40、 winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates eighty-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new produc
41、t: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the father of the PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by French Schneider Electric
42、, the current owner.The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models. PLCs are used in many different industries and machines such as packaging and semiconductor machines. Well known PLC brands are Toshiba, Siemens, Allen-Brad
43、ley, ABB, Mitsubishi, Omron, and General Electric.5译文：可编程控制器作者：辛西娅.库珀摘自：维基自由百科全书可编程逻辑控制器或者简易可编程控制器是一种数字化的计算机，它应用于工业自动化的生产过程中，比如工厂装配生产线中机械的控制。不同于普通用途的计算机，可编程逻辑控制器是专为安排多输入和多输出而设计的，它拓展了工作温度范围，可抑制电气噪声，抗振动和干扰。.程序控制机器操作指令通常存储在有备用电池或非易失性存储器中。PLC是一个典型的实时控制系统，输出结果在一个时间范围内必须对输入条件做出响应，否则会导致意想不到的结果。特征可编程控制器的控制面
44、板（灰色元素的中心），它的每个单元都是由单个的元素组成的，由左向右分别是：电源供应器，控制器，继电器单元的输入输出。PLC和其他计算机的主要区别是它适用于各种恶劣环境条件下（如灰尘，潮湿，高温，低温等），并配备了易于扩展和连接的输入/输出端口的设备。这些设备将PLC连接到相应的传感器和信号发生器上。PLC可以定义各种开关量，模拟量（如温度和压力等）用来配置各种复杂系统的各种变量，一些PLC甚至还需要使用机器视觉。在信号发生器方面，PLC可以控制的设备有电动机，气压缸或液压缸，电磁继电器或螺线管继电器，以及一些模拟输出设备。通过输入/输出模块的配置，可以构建一个简单的PLC系统。这个PLC系统可
45、以通过外部I/O模块连接到一个计算机网络上。PLC的发明取代了过去使用成百上千的继电器，凸轮定时器，鼓音序器来构建一个自动化系统的时代。.通常，一个简单可编程控制器通过编程，以取代成千上万的继电器。可编程控制器最初应用于汽车制造业中，当软件修改取代了硬连线控制面板的重新布线，这标志着生产模式发生了彻底的改变。许多早期的PLC设计表明，在简单的梯形逻辑的决策中，已经出现了类似梯形图的电气原理图。电工们通过使用梯形逻辑能够很方便的查找出电路示意图的问题。这项可编程符号的选择为了降低培训其现有的技术人员的要求。而其他早期的PLC则使用一种基于堆栈的逻辑解决方法指令表编程的方式。 PLC的功能经过多年
46、的发展，已经包括顺序控制，运动控制，过程控制，分布式控制系统和网络控制系统等多个方面。一些现代的PLC的数据处理，储存，整理能力和通信能力，已大约相当于台式电脑了。可编程控制器的编程结合远程I/O硬件，允许通用台式电脑进行一些PLC方面的特定应用。根据国际电工委员会61131-3标准，PLC的编程可以使用基于标准的编程语言。所谓顺序功能图的图形符号编程适用于某些特定的可编程控制器。可编程控制器与其他控制系统PLC可以很好的适应各种自动化任务。这些都是典型的工业机械生产，在这些部门自动控制系统的高额费用是与其昂贵的的制造维护费用分不开的。PLC能控制输入输出设备并兼容工业试验装置和控制。通常PL
47、C应用于高度定制的系统,以便使组装PLC的成本低于设计控制器的成本。另外，在大规模生产产品方面，自定义的控制系统是经济的，由于他的组成部分成本较低。这是代替通用解决方案的最佳选择。为大批量或者简单的固定自动化任务，不同的技术被采用。例如：消费者洗碗机被成本只有几美元的机电凸轮计时器控制。微处理器的设计要考虑成百上千的组成单元，以便开发成本（电源供应器和输入输出硬件的设计）能遍布到很多销售领域，并且最终用户不需要改变控制。汽车应用是一个例子，每年有数以百万辆汽车被制造，只有极少数最终用户改变控制程序。然而，一些特殊车辆，如使用PLC系统代替定制设计控制的过境巴士因为使用量少所以开发费用不够经济。
48、复杂的过程控制，如用于化学工业的过程控制要求的算法和性能甚至超过了高性能的PLC控制。高速度和精度的控制也可能需要量身定制的解决方案，如飞机的飞行控制。PLC系统包括逻辑单变量反馈模拟控制回路，一个“比例、积分、导数”或“PID控制器”。PID回路可用于控制制造过程的温度，例如历史上的PLC通常只用于少数模拟控制回路这个过程需要成百上千的循环, 分布式控制系统（DCS）将代替使用。然而，由于PLC的功能越来越强大，DCS与PLC的界定越发的不明确。数字和模拟信号数字或离散信号表现为二进制开关信号，表现为一个闭合或关断信号。按钮、限位开关、光电传感器能提供了一个离散信号。离散信号以电压或电流的形
49、式传送，它在一个特定的范围内被设置成开和关。例如：PLC必须使用24伏直流电压的I/O口，高于22伏直流电压代表开，低于2伏直流电压代表关，中间值不确定。最初，PLC只有离散的I/O接口。模拟信号和音量控制一样，在零和满量程之间有一系列的值。这些常常被定义为PLC中的整数，它的范围取决于设备中可用来存储比特数据的存储单元的数量。由于PLC通常使用16位有符号二进制处理器，整数值被限定在-32768+32767之间。压力、温度、流量、重量往往代表的模拟信号。模拟信号可以使用与原信号成正比的电压或电流信号。例如：一个4-20mA的电流信号或者0-10V的电压信号将被转化成一个0-32767之间的整型值。事例水箱可以作为一个例子。一个设备用于向水箱中注水，另一个设备用于从水箱中取水，根据需要，我们必须建立一个系统去控制水箱的水位。若只用数字信号，PLC有两个数字输入开关量（水箱空和水箱满）。PLC有一个数字输出量去控制水箱阀门的开关。当水位下降使得水
陕公网安备 61072602000132号 违法和不良信息举报：0916-4228922