1、 附录Programmable logic controllerCynthia 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-purpose compu
2、ters, 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 a real
3、 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, controller, rel
4、ay 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, analog
5、 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 arrangements
6、may be built into a simple PLC, 附录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 progr
7、ammed 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 logic in
8、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 a fo
9、rm 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 communica
10、tion 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 standard
11、s-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.附录PLC compared with other control systemsPLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing
12、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 PLC i
13、s 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 simpl
14、e 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 produ
15、ced 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 programming
16、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 algorithm
17、s and performance beyond the capability of even high-附录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, derivative“
18、or “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
19、be 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
20、 switches, 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, val
21、ues 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 accurac
22、y depending 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. 附录Analog signals can use v
23、oltage 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
24、system, 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
25、 so that 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 loa
26、d cell, 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 adju
27、sts this 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 refi
28、ll 附录rates 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 lar
29、ger I/O systems 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 Inte
30、rface) devices 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 program
31、ming panels 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 traditiona
32、l programming 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
33、 for programmable 附录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 operat
34、ions.While the 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 manufactu
35、rer, different 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
36、GUI (Graphical 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
37、Acquisition) system 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 Profib
38、us. History附录The 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 fac
39、ilities for the yearly 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
40、relay systems.The 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 servicin
41、g this new product: 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
42、 Schneider Electric, 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,
43、Siemens, Allen-Bradley, ABB, Mitsubishi, Omron, and General Electric.附录附录 B 中文翻译可编程逻辑控制器辛西娅.库珀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备用电池或非易失性存储器中。PLC 要求实时系统的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
44、结果。特征PLC 的控制面板（灰色元素的中心） ，它的每个单位都是由单独的元素组成的，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） ，并配备了适合于各种 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
45、置，可以构建一个简单的 PLC 系统。这个 PLC 系统可以通过外部I/O 模块连接到一个计算机网络上。PLC 的出现改变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，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。这项计划符号的选择使用可以降低培训其现有的技术人员的要求。附录而其他早期的 PLC
46、 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
47、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 PLC 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
48、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。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“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）附录将代替使用。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
49、或离散信号表现为二进制开关信号，收益率只是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。模拟型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。当水位下降使得水箱空箱开关关闭时，PLC 将会打开阀门使更多的水进入。一旦水位升高到一定高度使得满箱开关闭
