毕业设计英文翻译Programmable_logic

Programmable logic controller

Cynthia Cooper

From Wikipedia, the free encyclopedia

A 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 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 a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.

Features

Control panel with PLC (grey elements in the center). The unit consists of separate elements, from left to right; power supply, controller, 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, 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 arrangements may be built into a simple PLC,

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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 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 logic 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 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 communication 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 standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

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PLC compared with other control systems

PLCs are well-adapted to a range of automation tasks. These are typically industrial processes in manufacturing 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 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 simple 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 produced 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 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.

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Very complex process control, such as used in the chemical industry, may require algorithms 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, derivative\" 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 be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.

Digital and analog signals

Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Push buttons, limit 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, values 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 depending on the device and the number of bits available to store the data. As PLCs typically use 16-bit signed

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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 voltage 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.

Example

As an example, say a facility needs to store water in a tank. The water is drawn from the tank by another 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 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 load 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 adjusts 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.

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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 and prevent water hammer. Backup and maintenance methods can make a real system very complicated.

System scale

A 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 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 Interface) devices such as keypads or PC-type workstations. Some of today's 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.

Programming

Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals. they were designed to replace relay logic systems. These PLCs were programmed in \"ladder logic\Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional 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.

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Recently, the International standard IEC 61131-3 has become popular. IEC 61131-3 currently defines five programming languages for programmable 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 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 models may not be directly compatible.

User interface

PLCs 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. HMI's are also referred to as MMI's (Man Machine Interface) and 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 Acquisition) system or web browser.

Communications

PLCs usually have built in communications ports usually 9-Pin RS232, and optionally for RS485 and Ethernet. Modbus or DF1 is usually included as

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one of the communications protocols. Others' options include various fieldbuses such as DeviceNet or Profibus.

History

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 facilities 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 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 servicing 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 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

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machines. Well known PLC brands are Toshiba, Siemens, Allen-Bradley, ABB, Mitsubishi, Omron, and General Electric.

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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。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。

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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）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

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数字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合，PLC将会临时关闭进水阀门防止水的溢出。

一个模拟系统可能会使用水位的压力传感器、可调（节流）出水阀和调节阀来逐渐提高水箱水位。

在这一系统中，为了避免损坏阀的扰动调节器，很多的PLC加入“滞后”环节，基本上实现了“死区”的活动。 技术员通过调整死区，使阀只在有一个显着变化率的时刻动作。这将依次把阀门的动作减到最少，从而减少其磨损。

一个真正的系统可能把这两种做法结合起来，使用浮动开关和简单阀门来防止泄漏，使用速率传感器和速度阀来优化水位上升速率并防止水锥的发生。考虑到支持性的维护方法，一个实际的系统将会非常复杂。

系统规模

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一个小型PLC将有固定数量的I/O接口。通常情况下，如果相应的模型并没有足够的I/O接口，则需要扩展I/O口。

在较大的I/O系统中，PLC可能使用点对点（P2P）的处理器通信。这使得一个复杂过程的单独部分有各自的控制，同时又允许各子系统同级之间进行通信联系。这些通讯联系常常被用于HMI（人机界面）设备，如键盘或PC型工作站。当今某些PLC可以通过各种各样的设备进行通信联系，包括RS-485接口，同轴电缆，和具有速度高达100兆比特/秒的网络控制I/O的以太网。

编程

早期的PLC（直至20世纪80年代中期）使用专有的编程渠道或特殊用途的编程终端进行编程。它们是用来取代继电器逻辑系统的。这些PLC用梯形图编程，其非常类似于继电器逻辑示意图。现代的PLC可以以各种方式编程，从梯形图到更传统的编程语言，如BASIC和C，另一种高级程序设计语言是State Logic，它是基于状态转移图设计PLC程序的。

最近，国际标准IEC61131-3已经流行。目前的IEC61131-3为可编程控制系统定义了5编程语言：FBD（功能框图），LD（梯形图），ST（结构化文本），IL（指示表）和SFC（顺序功能图）。这些技术强调动作的逻辑组织。

所有制造商PLC编程的基本概念是共同的，分歧在于I/O的选择，存储组织的安排和指令集，所以不同生产商之间的PLC程序从来都不可以完全的互换。即使是同一制造商同一产品线，不同的模式也可能无法兼容。

用户界面

PLC为了配置，报警或日常控制的目的可能需要人机互动交流。HMI（人机界面）因此而产生。人机界面包括MMI（人机接口）和GUI（图形用户界面）。

一个简单的系统可以使用按钮和灯来与用户互动。文字显示以及图形化的触摸屏也可能用到。最现代的PLC可以通过网络连接到其他的系统，如运行SCADA（监控和数据采集系统）的计算机系统或网络浏览器。

通信

PLC的内置通信端口通常使用9针的RS232串口 ，并选择性使用RS485的和以太网。Modbus协议或DF1是常用的一种通信协议。其他通信协议包括各种现场总线，如DeviceNet现场总线或Profibus现场总线等。

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历史

PLC是应美国汽车业发展的需要而产生的。在PLC产生之前，汽车使用继电器进行顺序控制和安全联锁逻辑，制造定时器和专用闭环控制器。每年模型转换是非常耗时和昂贵的，而且中继系统需要有熟练的电工才能操作。在1968年通用汽车公司发出一份为硬连线中继系统寻找一个电子替代品的建议书。

中标投标书来自贝德福德，马萨诸塞州的贝德福德联营公司。第一个PLC，被命名为084 ，因为它是贝德福德的第八十四个项目的结果。贝德福德联营协会创建了一个新的公司，它致力于开发，制造，销售和服务这一新产品：莫迪康，它代表模块化数字控制器。该项目主持者切尼莫雷，被认为是“PLC之父”。该莫迪康品牌在1977年被古尔德电子收购，后来又被德国AEG公司收购，然后是法国施耐德电气，也是目前的所有者。

汽车行业仍然是最大的一个PLC用户，莫迪康仍然标称一些控制器的模型。PLC应用在许多不同的行业和机器上，如包装机和半导体设备。众所周知，PLC的品牌有东芝，西门子，艾伦布拉德利，ABB，三菱，欧姆龙和通用电气等。