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可編程邏輯控制器，或可編程控制器，簡稱PLC，是工業專用裝置，用于連接和控制模擬和數字設備。PLC也是一種特殊的計算機裝置，應用于實際過程的自動化，比如控制工廠裝配流水線上的機械等。可編程邏輯控制器一般使用微處理器。程序通常可用來進行復雜的序列過程控制，并由工程師編寫。程序存儲在有備用電池的隨機存儲器或者電可擦除可編程只讀存儲器(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的形狀和大小各異。它們可以小到裝入襯衣口袋里，而比較復雜的控制系統則需要大型的PLC機柜。較小的PLC通常設計有固定的I/O接口。All PLCs have the basic structureshown in Fig9-1（所有）PLC的基本結構如圖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由圖91可見，PLC有5個主要部件：中央處理器(CPU)、程序存儲器、數據存儲器、輸出設備和輸入設備。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會運行一個程序，監測一系列不同輸入，并對輸出進行邏輯操作以實現所預期的控制。它們要有靈活的編程方式，還具有比傳統控制系統更可靠（沒有程序崩潰或機械故障）、更緊湊、更經濟的優點。程序存儲器是用于存儲邏輯控制序列指令的。開關狀態、聯鎖狀態、以前的數據值和其他的工作數據都存儲在數據存儲器中。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與其他計算機比較，最主要的區別是PLC有專門的輸入輸出裝置。它們將PLC與傳感器和執行機構相連接。PLC能讀取限位開關、溫度指示器和復雜定位系統的位置信息。有些甚至使用機器視覺。在執行機構方面，PLC能驅動任何一種電機、氣壓或液壓缸、光圈、磁性繼電器或螺線管。在簡單的PLC中，輸入輸出裝置可以內置，或者PLC中含有與專用的計算機網絡相連的外置IO模塊并插入其中。老式自動化系統要使用幾百或幾千個繼電器和凸輪定時器，于是人們發明了PLC是要用做較便宜的替代品。通常，通過對一臺PLC編程可替代數千個繼電器。最初可編程控制器用在汽車制造業中，這樣可以通過對程序的修改取代硬連線控制面板的重新連線。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經過多年的發展，其功能已經包括典型的繼電器控制、復雜的運動控制、過程控制、分布控制系統和復雜的網絡連接。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采用簡單的梯形邏輯圖來描述形成邏輯電路的方案，梯形圖源自于電氣連線圖。使用梯形圖，電氣技師們很容易找出電路原理圖的問題。選擇梯形圖主要是為了降低現有技術人員的理解難度。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，已經證明PLC非常可靠，但是可編程計算機仍然有很長的路要走。依照國際電工委員會的611313標準，現在PLC可以使用結構化的程序設計語言和邏輯基本運算來進行編程控制設備已經具有可行性了。A graphical programming notation called sequential function charts is available on certain programmable controllers某些可編程控制器可使用一種圖形編程符號，稱為順序功能流程圖表。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