test2-4_PLC概述-back-all.doc

PLC OverviewPLC概述Programmable logic controllers，or programmable controllers，or PLCs for short，are specialized industrial devices for interfacing to and controlling analog and digital devicesThey are also special computer devices used for automation of real-world processes，such as control of machinery on factory assembly linesThe PLC usually uses a microprocessorThe program can often be used to control complex sequencing and is often written by engineersThe program is stored in battery-backed memory or EEPROMs可编程逻辑控制器，或可编程控制器，简称PLC，是工业专用装置，用于连接和控制模拟和数字设备。PLC也是一种特殊的计算机装置，应用于实际过程的自动化，比如控制工厂装配流水线上的机械等。可编程逻辑控制器一般使用微处理器。程序通常可用来进行复杂的序列过程控制，并由工程师编写。程序存储在有备用电池的随机存储器或者电可擦除可编程只读存储器(EEPROM)中。PLCs come in many shapes and sizesThey can be so small as to fit in your shirt pocket，while more involved control systems require large PLC racksSmaller PLCs are typically designed with fixed I/O points.PLC的形状和大小各异。它们可以小到装入衬衣口袋里，而比较复杂的控制系统则需要大型的PLC机柜。较小的PLC通常设计有固定的I/O接口。All PLCs have the basic structureshown in Fig9-1（所有）PLC的基本结构如图9.1所示。From Fig9-1，the PLC has five main units：the central processing unit (CPU)，the programme memory，the data memory，the output devices and the input devices由图91可见，PLC有5个主要部件：中央处理器(CPU)、程序存储器、数据存储器、输出设备和输入设备。The CPU is dedicated to run one program that monitors a series of different inputs and logically manipulates the outputs for the desired controlThey are meant to be very flexible in how they can be programmed while also providing the advantages of high reliability (no program crashes or mechanical failures)，compact and economical over traditional control systemsThe programme memory is used for storing the instructions for the logical control sequenceThe status of switches，interlocks，past values of data and other working data is stored in the data memoryCPU会运行一个程序，监测一系列不同输入，并对输出进行逻辑操作以实现所预期的控制。它们要有灵活的编程方式，还具有比传统控制系统更可靠（没有程序崩溃或机械故障）、更紧凑、更经济的优点。程序存储器是用于存储逻辑控制序列指令的。开关状态、联锁状态、以前的数据值和其他的工作数据都存储在数据存储器中。The main differences from other computers are the special input/output arrangementsThese connect the PLC to sensors and actuatorsPLCs can read limit switches，temperature indicators and the positions of complex positioning systemsSome even use machine visionOn the actuator side，PLCs drive any kind of electronic motors，pneumatic or hydraulic cylinders，diaphragms，magnetic relays or solenoidsThe input/output arrangements may be built into a simple PLC，or the PLC may have external I/O modules attached to a proprietary computer network that plugs into the PLCPLCs were invented as less expensive replacements for older automated systems that would use hundreds or thousands of relays and cam timersOften，a single PLC can be programmed to replace thousands of relaysProgrammable controllers were initially adopted by the automotive manufacturing industry，where software revision replaced the rewiring of hard-wired control panels与其他计算机比较，最主要的区别是PLC有专门的输入输出装置。它们将PLC与传感器和执行机构相连接。PLC能读取限位开关、温度指示器和复杂定位系统的位置信息。有些甚至使用机器视觉。在执行机构方面，PLC能驱动任何一种电机、气压或液压缸、光圈、磁性继电器或螺线管。在简单的PLC中，输入输出装置可以内置，或者PLC中含有与专用的计算机网络相连的外置IO模块并插入其中。老式自动化系统要使用几百或几千个继电器和凸轮定时器，于是人们发明了PLC是要用做较便宜的替代品。通常，通过对一台PLC编程可替代数千个继电器。最初可编程控制器用在汽车制造业中，这样可以通过对程序的修改取代硬连线控制面板的重新连线。The functionality of the PLC has evolved over the years to include typical relay control，sophisticated motion control，process control，distributed control systems and complex networkingPLC经过多年的发展，其功能已经包括典型的继电器控制、复杂的运动控制、过程控制、分布控制系统和复杂的网络连接。The earliest PLCs expressed all decisions making logic in simple ladder logic inspired from the electrical connection diagramsThe electricians were quite able to trace out circuit problems with schematic diagrams using ladder logicThis was chosen mainly to reduce the apprehension of the existing technicians最早的PLC采用简单的梯形逻辑图来描述形成逻辑电路的方案，梯形图源自于电气连线图。使用梯形图，电气技师们很容易找出电路原理图的问题。选择梯形图主要是为了降低现有技术人员的理解难度。Today，the PLCs have been proven very reliable，but the programmable computer still has a long way to goWith the IEC6 1131-3 standard，it is now possible to program these devices using structured programming languages，and logic elementary operations如今，已经证明PLC非常可靠，但是可编程计算机仍然有很长的路要走。依照国际电工委员会的611313标准，现在PLC可以使用结构化的程序设计语言和逻辑基本运算来进行编程控制设备已经具有可行性了。A graphical programming notation called sequential function charts is available on certain programmable controllers某些可编程控制器可使用一种图形编程符号，称为顺序功能流程图表。However，it should be noted that PLCs no longer have a very high cost(often thousands of dollars)，typical of a“generic”solutionModem PLCs with full capabilities are available for a few hundred USDThere are other ways for automating machines，such as a custom microcontroller-based design，but there are differences among both：PLCs contain everything needed to handle high power loads right out of the box，while a microcontroller would need an electronics engineer to design power supplies，I/O modules，etcAlso a microcontroller based design would not have the flexibility of in-field programmability of