IntroductionstoPLC

1、Lesson 19 Introductions to PLC A PLC (i.e. Programmable Logic Controller) is a device that was invented to replace the necessary sequential relay circuits for machine control. PLC(即可編程邏輯控制器)是機(jī)械控制中為替代必要的繼電器時(shí)序電路而發(fā)明的一種設(shè)備。The PLC works by looking at its inputs and depending upon their state, turning on/

2、off its outputs. PLC 工作時(shí)通過(guò)查詢輸入端并根據(jù)其狀態(tài)打開(kāi)或關(guān)閉輸出。The user enters a program, usually via software or programmer, that gives the desired results.用戶通常用軟件或編程器輸入程序，從而獲得期望的結(jié)果。 PLCs are used in many “real world” applications.很多實(shí)際應(yīng)用都采用PLC。 If there is industry present, chances are good that there is a PLC prese

3、nt.工業(yè)生產(chǎn)中應(yīng)用PLC的可能性很高。If you are involved in machining, packaging, material handling, automated assembly or countless other industries, you are probably already using them. 如果你正在進(jìn)行機(jī)械制造、產(chǎn)品包裝、材料處理、自動(dòng)化裝配及無(wú)數(shù)其他工業(yè)生產(chǎn)，你可能已經(jīng)用到了PLC。If you are not, you are wasting money and time. Almost any application that need

4、s some type of electrical control has a need for a PLC.如果沒(méi)有用到，那就是在浪費(fèi)金錢(qián)和時(shí)間。幾乎所有需要電氣控制的地方都需要PLC。 For example, lets assume that when a switch turns on we want to turn a solenoid on for 5 seconds and then turn it off regardless of how long the switch is on for. 例如，假定在開(kāi)關(guān)閉合時(shí)我們需要一個(gè)線圈接通5 秒，然后不管開(kāi)關(guān)接通多長(zhǎng)時(shí)間都將線圈斷

5、開(kāi)。We can do this with a simple external timer. But what if the process included 10 switches and solenoids? We would need 10 external timers. 我們可以通過(guò)一個(gè)簡(jiǎn)單的外部定時(shí)器來(lái)實(shí)現(xiàn)。但是假如該過(guò)程有十個(gè)開(kāi)關(guān)和線圈呢？我們就需要十個(gè)外部定時(shí)器。What if the process also needed to count how many times the switch individually turned on? We need a lot of e

6、xternal counters.如果這個(gè)過(guò)程需要分別記錄每個(gè)開(kāi)關(guān)開(kāi)啟的次數(shù)呢？我們又需要很多外部計(jì)數(shù)器。 As you can see, the bigger the process the more of a need we have for a PLC. We can simply program the PLC to count its inputs and turn the solenoids on for the specified time.由此可見(jiàn)，系統(tǒng)越大，我們就越需要PLC。我們可以簡(jiǎn)單地用PLC編程來(lái)對(duì)輸入信號(hào)進(jìn)行計(jì)數(shù)，并在規(guī)定的時(shí)間接通線圈。 We will take

7、a look at what is considered to be the “top 20” PLC instructions. It can be safely estimated that with a firm understanding of these instructions one can solve more than 80% of the applications in existence. 我們考察一下哪些是PLC 中最常用的20 條指令。保守地估計(jì)一下，如果真正地掌握了這些指令，就能解決80%以上現(xiàn)存的應(yīng)用問(wèn)題。 Thats right, more than 80%!

8、Of course well learn more than just these instructions to help you solve almost ALL your potential PLC applications. 是的，80%以上！當(dāng)然，我們要學(xué)習(xí)的指令比這些更多，以幫助你解決幾乎所有潛在的PLC應(yīng)用問(wèn)題。The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data, as shown in Fig. 17.1. PLC 主要由中央處

9、理器(CPU)、存儲(chǔ)器和輸入、輸出電路構(gòu)成，如圖17.1 所示。We can actually consider the PLC to be a box full of hundreds or thousands of separate relays, counters, timers and data storage locations. 我們可以將PLC看成是一個(gè)裝滿了成百上千個(gè)獨(dú)立的繼電器、計(jì)數(shù)器、定時(shí)器以及數(shù)據(jù)存儲(chǔ)器的盒子。Do these counters, timers, etc. really exist? No, they dont “physically” exist but

10、 rather they are simulated and can be considered software counters, timers, etc. 這些計(jì)數(shù)器、定時(shí)器等是不是真的存在呢？不，它們都是模擬的，物理上并不存在，但可以將它們看成是軟計(jì)數(shù)器、軟定時(shí)器等。These internal relays are simulated through bit locations in registers.這些內(nèi)部繼電器是用寄存器中的位單元模擬出來(lái)的。 What does each part do?各個(gè)部分是如何工作的呢？ INPUT RELAYS-(contacts) These a

11、re connected to the outside world. They physically exist and receive signals from switches, sensors, etc. Typically they are not relays but rather they are transistors.輸入繼電器(觸點(diǎn))這些繼電器連接外部電路。它們是實(shí)際存在的，并接收來(lái)自開(kāi)關(guān)、傳感器等的信號(hào)，通常是晶體管而非繼電器。 INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the

12、outside world nor do they physically exist. 內(nèi)部通用繼電器(觸點(diǎn))它們不從外部設(shè)備接收信號(hào)，也非物理上存在的。They are simulated relays and are what enables a PLC to eliminate external relays.它們是模擬的繼電器，用以消除PLC的外部繼電器。 There are also some special relays that are dedicated to performing only one task.此外還有一些特殊繼電器，專門(mén)執(zhí)行一項(xiàng)任務(wù)。Some are alwa

13、ys on while some are always off. Some are on only once during power-on and are typically used for initializing data that was stored.其中一些是常開(kāi)的，一些是常閉的。有一些僅在電源上電時(shí)導(dǎo)通一次，通常用來(lái)初始化存儲(chǔ)的數(shù)據(jù)。 COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pulses. 計(jì)數(shù)器 它

14、們也非物理上存在的，而是模擬的計(jì)數(shù)器，可通過(guò)編程來(lái)對(duì)脈沖進(jìn)行計(jì)數(shù)。Typically these counters can count up, down or both up and down. 通常它們可進(jìn)行加計(jì)數(shù)、減計(jì)數(shù)或同時(shí)進(jìn)行加減計(jì)數(shù)。Since they are simulated, they are limited in their counting speed. 因?yàn)樗鼈兪怯密浖M的，計(jì)數(shù)速度就有限。Some manufacturers also include high-speed counters that are hardware based. 一些制造商提供了基于硬件的

15、高速計(jì)數(shù)器。We can think of these as physically existing. Most times these counters can count up, down or up and down.這樣的計(jì)數(shù)器可以認(rèn)為是物理上存在的。這些計(jì)數(shù)器多數(shù)情況下可以進(jìn)行加計(jì)數(shù)、減計(jì)數(shù)或同時(shí)進(jìn)行加減計(jì)數(shù)。 TIMERS-These also do not physically exist. They come in many varieties and increments.定時(shí)器 它們也非物理上存在的，分為多種類型和定時(shí)單位。The most common type is a

16、n on-delay type. Others include off-delay and both retentive and non-retentive types. Increments vary from 1ms through 1s.最常用的一種類型是延時(shí)導(dǎo)通型。其他類型還有延時(shí)斷開(kāi)型、記憶和非記憶型。定時(shí)單位的范圍是1ms 到1s。 OUTPUT RELAYS-(coils) These are connected to the outside world.輸出繼電器(線圈) 該部分連接到外圍電路。They physically exist and send on/off sign

17、als to solenoids, lights, etc. They can be transistors, relays, or triacs depending upon the model chosen.它們是物理上存在的，并給線圈、燈等發(fā)送開(kāi)關(guān)信號(hào)。輸出繼電器可以是晶體管、繼電器或可控硅，取決于選擇的型號(hào)。 DATA STORAGE-Typically there are registers assigned to simply store data. They are usually used as temporary storage for math or data manipu

18、lation. 數(shù)據(jù)存儲(chǔ)器 它們通常是用來(lái)存儲(chǔ)數(shù)據(jù)的寄存器，一般作為運(yùn)算或數(shù)據(jù)處理的暫存器。They can also typically be used to store data when power is removed from the PLC. 在PLC斷電時(shí)通常還可用來(lái)存儲(chǔ)數(shù)據(jù)。Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!再次接通電源后，其內(nèi)容與斷電前相同，非常方便且必要。 A PLC works by

19、continually scanning a program. We can think of this scan cycle as consisting of 3 important steps, as shown in Fig. 17.2.PLC 是通過(guò)連續(xù)掃描一個(gè)程序來(lái)工作的。我們可以認(rèn)為掃描周期是由3 個(gè)主要階段組成的，如圖17.2 所示。There are typically more than 3 but we can focus on the important parts and not worry about the others. 當(dāng)然有多于三個(gè)階段的情況，但我們可關(guān)注重要

20、的環(huán)節(jié)，忽略其他環(huán)節(jié)。Typically the others are checking the system and updating the current internal counter and timer values.其他階段通常正在檢查系統(tǒng)及更新內(nèi)部計(jì)數(shù)器和定時(shí)器的當(dāng)前值。 Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off.第一步檢查輸入狀態(tài)首先PLC檢查每一個(gè)輸入是否接通。In other words, is the sensor

21、connected to the first input on? How about the second input? How about the third 換句話說(shuō)就是，與第一個(gè)輸入端連接的傳感器接通了嗎？第二個(gè)輸入呢？第三個(gè)輸入呢？It records this data into its memory to be used during the next step.PLC 將這些數(shù)據(jù)記錄到存儲(chǔ)器中，以便在下一階段使用。 Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time

22、. 第二步執(zhí)行程序然后PLC一次一條指令地執(zhí)行程序。Maybe your program said that if the first input was on then it should turn on the first output. 你的程序可能要求第一個(gè)輸入接通時(shí)，就接通第一個(gè)輸出。Since it already knows which inputs are on/off from the previous step, it will be able to decide whether the first output should be turned on based on the state of the first input. 因?yàn)樵谏弦徊揭呀?jīng)知道輸入端的開(kāi)關(guān)狀態(tài)，根據(jù)上一步中第一個(gè)輸入端的狀態(tài)，就可以確定是否應(yīng)接通第一個(gè)輸出。It will store the execution results for use later during the next step.PLC將執(zhí)行結(jié)果存儲(chǔ)起來(lái)，以供下一步使用。 Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of th