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The Programmable Logic Controller
Early machines were controlled by mechanical means using cams, gears, levers and other basic mechanical devices. As the complexity grew, so did the need for a more sophisticated control system. This system contained wired relay and switch control elements. These elements were wired as required to provide the control logic necessary for the particular type of machine operation. This was acceptable for a machine that never needed to be changed or modified, but as manufacturing techniques improved and plant changeover to new products became more desirable and necessary, a more versatile means of controlling this equipment had to be developed. Hardwired relay and switch logic was cumbersome and time consuming to modify. Wiring had to be removed and replaced to provide for the new control scheme required. This modification was difficult and time consuming to design and install and any small "bug" in the design could be a major problem to correct since that also required rewiring of the system. A new means to modify control circuitry was needed. The development and testing ground for this new means was the U.S. auto industry. The time period was the late 1960's and early 1970's and the result was the programmable logic controller, or PLC. Automotive plants were confronted with a change in manufacturing techniques every time a model changed and, in some cases,  for changes on the same model if improvements had to be made during the model year.  The PLC provided an easy way to reprogram the wiring rather than actually rewiring the control system.
The PLC that was developed during this time was not very easy to program. The language was cumbersome to write and required highly trained programmers. These early devices were merely relay replacements and could do very little else. The PLC has at first gradually, and in recent years rapidly developed into a sophisticated and highly versatile control system component. Units today are capable of performing complex math functions including numerical integration and differentiation and operate at the fast microprocessor speeds now available. Older PLCs were capable of only handling discrete inputs and outputs (that is, on-off type signals), while today's systems can accept and generate analog voltages and currents as well as a wide range of voltage levels and pulsed signals. PLCs are also designed to be rugged. Unlike their personal computer cousin, they can typically withstand vibration, shock, elevated temperatures, and electrical noise to which manufacturing equipment is exposed.
As more manufacturers become involved in PLC production and development, and PLC capabilities expand, the programming language is also expanding. This is necessary to allow the programming of these advanced capabilities. Also, manufacturers tend to develop their own versions of ladder logic language (the language used to program PLCs).  This complicates learning to program PLC's in general since one language cannot be learned that is applicable to all types. However, as with other computer languages, once the basics of PLC operation and programming in ladder logic are learned, adapting to the various manufacturers’ devices is not a complicated process. Most system designers eventually settle on one particular manufacturer that produces a PLC that is personally comfortable to program and has the capabilities suited to his or her area of applications.
It should be noted that in usage, a programmable logic controller is generally referred to as a “PLC” or “programmable controller”. Although the term “programmable controller” is generally accepted, it is not abbreviated “PC” because the abbreviation “PC” is usually used in reference to a personal computer. As we will see in this chapter, a PLC is by no means a personal computer.
Programmable controllers (the shortened name used for programmable logic controllers) are much like personal computers in that the user can be overwhelmed by the vast array of options and configurations available. Also, like personal computers, the best teacher of which one to select is experience. As one gains experience with the various options and configurations available, it becomes less confusing to be able to select the unit that will best perform in a particular application.
The typical system components for a modularized PLC are:
1. Processor.
The processor (sometimes call a CPU), as in the self contained units, is generally specified according to memory required for the program to be implemented. In the modularized versions, capability can also be a factor.  This includes features such as higher math functions, PID control loops and optional programming commands. The processor consists of the microprocessor, system memory, and serial communication ports for printer, PLC LAN link and external programming device and, in some cases, the system power supply to power the processor and I/O modules.
2. Mounting rack.
This is usually a metal framework with a printed circuit board backplane which provides means for mounting the PLC input/output (I/O) modules and processor. Mounting racks are specified according to the number of modules required to implement the system. The mounting rack provides data and power connections to the processor and modules via the backplane. For CPUs that do not contain a power supply, the rack also holds the modular power supply. There are systems in which the processor is mounted separately and connected by cable to the rack. The mounting rack can be available to mount directly to a panel or can be installed in a standard 19" wide equipment cabinet. Mounting racks are ascendable so several may be interconnected to allow a system to accommodate a large number of I/O modules.
3. Input and output modules.
Input and output (I/O) modules are specified according to the input and output signals associated with the particular application. These modules fall into the categories of discrete, analog, high speed counter or register types.
Discrete I/O modules are generally capable of handling 8 or 16 and, in some cases 32, on-off type inputs or outputs per module. Modules are specified as input or output but generally not both although some manufacturers now offer modules that can be configured with both input and output points in the same unit. The module can be specified as AC only, DC only or AC/DC along with the voltage values for which it is designed.
Analog input and output modules are available and are specified according to the desired resolution and voltage or current range. As with discrete modules, these are generally input or output; however some manufacturers provide analog input and output in the same module. Analog modules are also available which can directly accept thermocouple inputs for temperature measurement and monitoring by the PLC.
Pulsed inputs to the PLC can be accepted using a high speed counter module. This module can be capable of measuring the frequency of an input signal from a tachometer or other frequency generating device. These modules can also count the incoming pulses if desired. Generally, both frequency and count are available from the same module at the same time if both are required in the application.
Register input and output modules transfer 8 or 16 bit words of information to and from the PLC. These words are generally numbers (BCD or Binary) which are generated from thumbwheel switches or encoder systems for input or data to be output to a display device by the PLC.
Other types of modules may be available depending upon the manufacturer of the PLC and it's capabilities. These include specialized communication modules to allow for the transfer of information from one controller to another. One new development is an I/O Module which allows the serial transfer of information to remote I/O units that can be as far as 12,000 feet away.
4. Power supply.
The power supply specified depends upon the manufacturer's PLC being utilized in the application. As stated above, in some cases a power supply capable of delivering all required power for the system is furnished as part of the processor module. If the power supply is a separate module, it must be capable of delivering a current greater than the sum of all the currents needed by the other modules. For systems with the power supply inside the CPU module, there may be some modules in the system which require excessive power not available from the processor either because of voltage or current requirements that can only be achieved through the addition of a second power source. This is generally true if analog or external communication modules are present since these require ± DC supplies which, in the case of analog modules, must be well regulated.
5. Programming unit.
The programming unit allows the engineer or technician to enter and edit the program to be executed. In it's simplest form it can be a hand held device with a keypad for program entry and a display device (LED or LCD)  for viewing program steps or functions, as shown. More advanced systems employ a separate personal computer which allows the programmer to write, view, edit and download the program to the PLC. This is accomplished with proprietary software available from the PLC manufacturer. This software also allows the programmer or engineer to monitor the PLC as it is running the program. With this monitoring system, such things as internal coils, registers, timers and other items not visible externally can be monitored to determine proper operation. Also, internal register data can be altered if required to fine tune program operation. This can be advantageous when debugging the program. Communication with the programmable controller with this system is via a cable connected to a special programming port on the controller. Connection to the personal computer can be through a serial port or from a dedicated card installed in the computer.
A Programmable Controller is a specialized computer. Since it is a computer, it has all the basic component parts that any other computer has; a Central Processing Unit, Memory, Input Interfacing and Output Interfacing.
The Central Processing Unit (CPU) is the control portion of the PLC. It interprets the program commands retrieved from memory and acts on those commands. In present day PLC's this unit is a microprocessor based system. The CPU is housed in the processor module of modularized systems.
Memory in the system is generally of two types; ROM and RAM. The ROM memory contains the program information that allows the CPU to interpret and act on the Ladder Logic program stored in the RAM memory. RAM memory is generally kept alive with an on-board battery so that ladder programming is not lost when the system power is removed. This battery can be a standard dry cell or rechargeable nickel-cadmium type. Newer PLC units are now available with Electrically Erasable Programmable Read Only Memory (EEPROM) which does not require a battery. Memory is also housed in the processor module in modular systems.
Input units can be any of several different types depending on input signals expected as described above. The input section can accept discrete or analog signals of various voltage and current levels. Present day controllers offer discrete signal inputs of both AC and DC voltages from TTL to 250 VDC and from 5 to 250 VAC. Analog input units can accept input levels such as ±10 VDC, ±5 VDC and 4-20 mas. Current loop values. Discrete input units present each input to the CPU as a single 1 or 0 while analog input units contain analog to digital conversion circuitry and present the input voltage to the CPU as binary number normalized to the maximum count available from the unit. The number of bits representing the input voltage or current depends upon the resolution of the unit. This number generally contains a defined number of magnitude bits and a sign bit. Register input units present the word input to the CPU as it is received (Binary or BCD).
Output units operate much the same as the input units with the exception that the unit is either sinking (supplying a ground) or sourcing (providing a voltage) discrete voltages or sourcing analog voltage or current. These output signals are presented as directed by the CPU. The output circuit of discrete units can be transistors for TTL and higher DC voltage or Triads for AC voltage outputs. For higher current applications and situations where a physical contact closure is required, mechanical relay contacts are available. These higher currents, however, are generally limited to about 2-3 amperes. The analog output units have internal circuitry which performs the digital to analog conversion and generates the variable voltage or current output.
The first thing the PLC does when it begins to function is update I/O. This means that all discrete input states are recorded from the input unit and all discrete states to be output are transferred to the output unit. Register data generally has specific addresses associated with it for both input and output data referred to as input and output registers. These registers are available to the input and output modules requiring them and are updated with the discrete data. Since this is input/output updating, it is referred to as I/O Update. The updating of discrete input and output information is accomplished with the use of input and output image registers set aside in the PLC memory. Each discrete input point has associated with it one bit of an input image register. Likewise, each discrete output point has one bit of an output image register associated with it. When I/O updating occurs, each input point that is ON at that time will cause a 1 to be set at the bit address associated with that particular input. If the input is off, a 0 will be set into the bit address. Memory in today's PLC's is generally configured in 16 bit words. This means that one word of memory can store the states of 16 discrete input points. Therefore, there may be a number of words of memory set aside as the input and output image registers. At I/O update, the status of the input image register is set according to the state of all discrete inputs and the status of the output image register is transferred to the output unit. This transfer of information typically only occurs at I/O update. It may be forced to occur at other times in PLC's which have an Immediate I/O Update command. This command will force the PLC to update the I/O at other times although this would be a special case.
Before a study of PLC programming can begin, it is important to gain a fundamental understanding of the various types of PLCs available, the advantages and disadvantages of each, and the way in which a PLC executes a program. The open frame, shoebox, and modular PLCs are each best suited to specific types of applications based on the environmental conditions, number of inputs and outputs, ease of expansion, and method of entering and monitoring the program. Additionally, programming requires a prior knowledge of the manner in which a PLC receives input information, executes a program, and sends output information. With this information, we are now prepared to begin a study of PLC programming techniques.
When writing programs for PLCs, it is beneficial to have a background in ladder diagramming for machine controls. This is basically the material that was covered in Chapter 1 of this text. The reason for this is that at a fundamental level, ladder logic programs for PLCs are very similar to electrical ladder diagrams. This is no coincidence. The engineers that developed the PLC programming language were sensitive to the fact that most engineers, technicians and electricians who work with electrical machines on a day-to-day basis will be familiar with this method of representing control logic. This would allow someone new to PLCs, but familiar with control diagrams, to be able to adapt very quickly to the programming language. It is likely that PLC programming language is one of the easiest programming languages to learn.
 
 
 
 
可编程逻辑控制器
早期的机器通过机械手段使用凸轮，齿轮，控制杆和其他基本的机械装置。随着复杂性的增长，所以没有一个更复杂的控制系统的需要。该系统包含有线继电器和开关控制元件。这些元素是有线的需要提供机器操作的特定类型的必要的控制逻辑。这是可以接受的，不需要改变或修改的机器，但作为制造技术的提高和植物转换新产品变得更可取的，必要的，更灵活的控制装置已被开发。硬继电器和开关逻辑是繁琐和耗时的修改。布线必须被删除并替换为新的控制方案的要求。这个修改是困难的，并安装任何小的“错误”在设计中可以自认为还需要重装系统的一个主要问题是正确设计耗时。修改控制电路的一种新方法是必要的。这种新的方法和试验场的发展是美国汽车业。时间是1960年代后期和1970年初的结果是可编程逻辑控制器PLC，汽车厂都面临着一个改变制造技术的每一次模型的改变，在某些情况下，在同一模型的变化，如果改革必须在年度车型。PLC提供的编程布线而不是重新控制系统的一个简单的方法。
PLC是在这个时候发展不是程序很容易。语言是繁琐和需要训练有素的程序员写的。这些早期的设备仅仅是继电器更换和能做的很少的人。PLC已逐渐在第一，近年来迅速发展成为一个复杂的和高度灵活的控制系统的组成。单位今天能够执行复杂的数学函数的数值积分和微分和操作在快速的微处理器速度现在可用。年长的PLC只能处理离散的输入和输出（即，开关型信号），而今天的系统可以接受并产生模拟电压和电流以及一系列的电压水平和脉冲信号。PLC的设计也非常坚固。不像他们的个人电脑的表弟，他们通常可以承受冲击，振动，温度升高，电气噪声，制造设备暴露。
随着更多的厂商参与PLC的产生和发展，与PLC的功能拓展，编程语言也不断扩大。这是要让这些先进的编程能力。同时，制造商倾向于发展自己的梯形逻辑语言的版本（使用PLC程序语言）。这种复杂的学习PLC程序的一般从一种语言无法得知是适用于所有类型的。然而，与其他计算机语言，一旦在梯形逻辑PLC操作和编程的基础知识学习，适应不同厂商的设备不是一个复杂的过程。大多数的系统设计师最终定居在一个特定的制造商生产的PLC程序，是个人的舒适和有能力适应他或她的地区的应用。
应该指出的是，在使用，可编程逻辑控制器一般称为“PLC”或“可编程控制器”。尽管术语“可编程控制器”是公认的，这不是因为缩写“PC”的缩写“PC”通常是指个人计算机。正如我们将看到在这一章中，PLC是不是一个个人计算机。
可编程控制器（简称用于可编程逻辑控制器）很像个人电脑，用户可以被大量的选项和配置。同时，像个人电脑一样，其中一个选择是体验最好的老师。作为一个经验的各种选项和配置，就可以选择单位，最好在一个特定的应用执行减少困惑。
一个模块化的PLC典型的系统元件：
1．处理器
处理器（有时称为CPU），在自给自足的单位，一般是根据项目实施所需内存指定。在模块化的版本，也可以是一个因子的能力。这包括如高等数学的功能特点，PID控制回路和可选的编程命令。处理器包括微处理器，内存，串行通信端口的打印机，PLC的局域网连接和外部编程的装置，在某些情况下，系统的供电电源处理器和I/O模块。
2．安装架
这通常是一个与印刷电路板的背板安装PLC输入/输出提供了装置的金属框架（I / O）模块和处理器。安装架上根据需要实现系统模块指定的数目。安装架通过背板的处理器和模块提供数据和电源连接。CPU不包含电源，在机架上还拥有模块化电源。其中有系统的处理器分别安装和连接电缆架。安装架可直接安装到面板或可以安装在标准的19“宽的设备柜。安装架上的级联这么几个可以相互连接，使系统能够容纳大量的I / O模块。
3．输入和输出模块
输入输出（I / O）模块是根据与特定应用程序相关的输入和输出信号的规定。这些模块分为离散的，模拟的类别，高速计数器或寄存器类型。
离散I / O模块通常能够处理8或16和32，在某些情况下，开关型的输入或输出模块。模块被指定为输入或输出，但一般不都虽然现在一些厂家提供的模块，可以配置在同一单元的输入和输出点。该模块可以被指定为AC，DC或AC / DC随着电压值，它的设计。
模拟输入和输出模块可与指定根据所需的分辨率和电压或电流范围。与离散模块，这些通常是输入或输出；然而，一些制造商提供模拟输入和输出在同一模块。模拟模块也可直接接受温度测量热电偶输入到PLC监控。
脉冲输入到PLC可接受使用高速脉冲。该模块能够对输入信号的频率测量从转速表或频率发生装置。这些模块也可以输入脉冲如果需要。一般来说，频率和数量都可从同一个模块在同一时间，如果是在应用程序的要求。
寄存器的输入和输出模块传输8或16位字的信息和从PLC。这些词通常数（BCD码或二进制）这是从编码开关或编码器系统的输入数据是由PLC输出生成的显示装置。
其他类型的模块可以根据PLC的制造商和它的能力。这些包括专业通信模块允许信息的传递到另一个从一个控制器。一个新的发展是一个I / O模块，允许信息的串行传输到远程I / O单元可以长达12000英尺远。
4.电源
指定的电源取决于生产厂家的PLC应用中的应用。如上所述，在某些情况下，能够提供所有必需的电源为系统提供的处理器模块供电。如果电源是一个独立的模块，它必须能够提供的电流大于所有其他模块所需的电流总和。与CPU模块内的供电系统，有可能在需要权力过大，不可从处理器或者因为电压或电流的要求，只能通过第二电源达到了系统部分模块。一般来说这是事实如果模拟或外部通信模块，因为这些要求±直流电源，在模拟模块的情况下，必须调整好。
5.程序单元
编程单元允许工程师或技术人员进入和编辑要执行的程序。在它最简单的形式，它可以是一个手持设备与程序的输入和显示设备的键盘（LED或LCD）观看节目的步骤或函数，如下所示。更先进的系统使用一个单独的个人计算机，它允许程序员编写，查看，编辑和下载程序到PLC。这是专有的软件由PLC厂家完成。该软件还允许程序员或工程师监控PLC作为它正在运行的程序。该监测系统，为内部线圈，寄存器定时器之类的东西，和其他项目不可见的外部可监测以确定正确的操作。同时，内部寄存器的数据，如果需要可以微调程序操作改变。这可以是有利的调试程序时。与本系统的可编程控制器之间的通信是通过电缆连接到控制器上的一种特殊的编程口。连接到个人计算机，可以通过串口或从专用卡安装在电脑上。
可编程控制器是一种专用的计算机。因为它是一台计算机，它具有所有其它计算机的基本组成部分有；中央处理单元，存储器，输入接口和输出接口。
中央处理单元（CPU）是PLC的控制部分。它解释来自存储器的程序指令并且对这些命令。现在PLC的单元是一个基于微处理器的系统。的CPU封装在模块化系统的处理器模块。
在系统内存通常是两种类型；ROM和RAM。只读存储器包含程序信息，允许CPU解释和执行存储在随机存储器中的梯形逻辑程序。随机存储器一般保持活着的车载电池，梯形图程序不会丢失系统掉电时。这种电池可以是一个标准的干电池或可充电的镍镉型。新的PLC单元使用电可擦除可编程只读存储器（EEPROM），不需要电池。记忆也安装在模块化系统的处理器模块。
输入单元有根据输入信号将上述几种不同类型。输入部分可以接受的离散信号或模拟信号的电压和电流的水平。目前的控制器提供交流和直流电压TTL 250 VDC输入离散信号和从5到250 VAC。模拟量输入单元可以接受如±10 VDC输入水平，±5 VDC和4-20 mA。电流值。离散输入单元分别输入到CPU作为一个单一的1或0，模拟量输入单元包含模拟数字转换电路和输入电压的CPU作为二进制数的归一化的最大数可从单元。代表输入电压或电流的比特数取决于该装置的分辨率。这个数字通常有一个特定的数值位和符号比特数。寄存器的输入单元本字输入到CPU在收到（二进制或BCD）。
输出单元操作一样，除单位是下沉的输入单元（提供地）或采购（提供一个电压）的离散电压或采购的模拟电压或电流。输出单元的输出信号由CPU。离散单元的输出电路可以为TTL和更高的直流电压或交流电压输出的三端双向可控硅开关晶体管。高电流应用的情况下，一个物理接点闭合时，继电器触点可机械。这些更高的电流，然而，一般限于2~3安培。模拟输出单元的内部电路进行模拟转换数字和产生可变的电压或电流输出。
首先PLC开始运作时更新I/O，这意味着所有的离散输入状态从输入单元记录和所有将要输出的离散状态被转移到输出单元。寄存器的数据一般都有特定的地址与它的输入和输出数据称为输入和输出寄存器。这些寄存器可以输入和输出模块相连并与离散数据更新。由于这是输入/输出的更新，它被称为I / O更新。离散输入和输出信息的更新与输入和输出映像寄存器的使用搁置在PLC的内存来完成的。每一个离散输入点与它一点输入映像寄存器相关。同样，每一个离散输出点都有一个点与它相关联的输出映像寄存器。当I/O刷新时，每个输入点，在当时造成1被设置在特定输入相关的位地址。如果输入的是，0将被设置为位地址。今天的PLC的记忆一般配置在16位的字。这意味着，一个字的存储器可存储16个离散输入点的状态。因此，可能会有一些单词的记忆保留为输入和输出映像寄存器。在I / O更新，输入映像寄存器的状态是根据所有离散输入和输出映像寄存器的状态转移到输出单元的状态。这种信息传递通常只发生在I / O更新。它可能会被迫发生在PLC具有立竿见影的I / O更新命令其他的时候。这个命令将迫使PLC更新的I / O在其他时间，虽然这将是一个特殊的情况。
在PLC编程的研究开始，它是获取PLC的各种可用的类型有基本了解的重要，以及各自的优缺点，并在PLC执行程序。开放的框架，鞋盒，和模块化的PLC都是最适合的环境条件的基础上应用程序特定的类型，数量的输入和输出，易于扩展，并进入和监控程序的方法。此外，规划要求，PLC接收输入信息的先验知识，执行一个程序，并发送输出信息。有了这些信息，我们现在准备开始编程技术的研究。
编写程序时为PLC，它有利于在梯形图背景机控制。这是基本的材料，都是本文的1章。这样做的原因是，在最基本的层面上，对PLC梯形逻辑程序梯形图非常相似的电。这不是巧合。开发了PLC编程语言的工程师们的事实，大多数工程师敏感，技术人员和电工工作的电气设备在日常的基础上熟悉这代表控制逻辑的方法。这将允许一个新的PLC，但熟悉控制图，能够很快适应的编程语言。这是可能的，PLC的编程语言是最容易学的语言之一。