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(即可编程逻辑控制器)是机械控制中为替代必要的继电器时序电路而发明的一种设备。The PLC works by looking at its inputs and depending upon their state, turning on/

2、off its outputs. PLC 工作时通过查询输入端并根据其状态打开或关闭输出。The user enters a program, usually via software or programmer, that gives the desired results.用户通常用软件或编程器输入程序，从而获得期望的结果。 PLCs are used in many “real world” applications.很多实际应用都采用PLC。 If there is industry present, chances are good that there is a PLC prese

3、nt.工业生产中应用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. 如果你正在进行机械制造、产品包装、材料处理、自动化装配及无数其他工业生产，你可能已经用到了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.如果没有用到，那就是在浪费金钱和时间。几乎所有需要电气控制的地方都需要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，假定在开关闭合时我们需要一个线圈接通5 秒，然后不管开关接通多长时间都将线圈断

5、开。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. 我们可以通过一个简单的外部定时器来实现。但是假如该过程有十个开关和线圈呢？我们就需要十个外部定时器。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.如果这个过程需要分别记录每个开关开启的次数呢？我们又需要很多外部计数器。 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.由此可见，系统越大，我们就越需要PLC。我们可以简单地用PLC编程来对输入信号进行计数，并在规定的时间接通线圈。 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. 我们考察一下哪些是PLC 中最常用的20 条指令。保守地估计一下，如果真正地掌握了这些指令，就能解决80%以上现存的应用问题。 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！当然，我们要学习的指令比这些更多，以帮助你解决几乎所有潜在的PLC应用问题。The PLC mainly consists of a CPU, memory areas, and appropriate circuits to receive input/output data, as shown in Fig. 17.1. PLC 主要由中央处

9、理器(CPU)、存储器和输入、输出电路构成，如图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. 我们可以将PLC看成是一个装满了成百上千个独立的继电器、计数器、定时器以及数据存储器的盒子。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. 这些计数器、定时器等是不是真的存在呢？不，它们都是模拟的，物理上并不存在，但可以将它们看成是软计数器、软定时器等。These internal relays are simulated through bit locations in registers.这些内部继电器是用寄存器中的位单元模拟出来的。 What does each part do?各个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.输入继电器(触点)这些继电器连接外部电路。它们是实际存在的，并接收来自开关、传感器等的信号，通常是晶体管而非继电器。 INTERNAL UTILITY RELAYS-(contacts) These do not receive signals from the

12、outside world nor do they physically exist. 内部通用继电器(触点)它们不从外部设备接收信号，也非物理上存在的。They are simulated relays and are what enables a PLC to eliminate external relays.它们是模拟的继电器，用以消除PLC的外部继电器。 There are also some special relays that are dedicated to performing only one task.此外还有一些特殊继电器，专门执行一项任务。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.其中一些是常开的，一些是常闭的。有一些仅在电源上电时导通一次，通常用来初始化存储的数据。 COUNTERS-These again do not physically exist. They are simulated counters and they can be programmed to count pulses. 计数器 它

14、们也非物理上存在的，而是模拟的计数器，可通过编程来对脉冲进行计数。Typically these counters can count up, down or both up and down. 通常它们可进行加计数、减计数或同时进行加减计数。Since they are simulated, they are limited in their counting speed. 因为它们是用软件模拟的，计数速度就有限。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、高速计数器。We can think of these as physically existing. Most times these counters can count up, down or up and down.这样的计数器可以认为是物理上存在的。这些计数器多数情况下可以进行加计数、减计数或同时进行加减计数。 TIMERS-These also do not physically exist. They come in many varieties and increments.定时器 它们也非物理上存在的，分为多种类型和定时单位。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.最常用的一种类型是延时导通型。其他类型还有延时断开型、记忆和非记忆型。定时单位的范围是1ms 到1s。 OUTPUT RELAYS-(coils) These are connected to the outside world.输出继电器(线圈) 该部分连接到外围电路。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.它们是物理上存在的，并给线圈、灯等发送开关信号。输出继电器可以是晶体管、继电器或可控硅，取决于选择的型号。 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. 数据存储器 它们通常是用来存储数据的寄存器，一般作为运算或数据处理的暂存器。They can also typically be used to store data when power is removed from the PLC. 在PLC断电时通常还可用来存储数据。Upon power-up they will still have the same contents as before power was removed. Very convenient and necessary!再次接通电源后，其内容与断电前相同，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 是通过连续扫描一个程序来工作的。我们可以认为扫描周期是由3 个主要阶段组成的，如图17.2 所示。There are typically more than 3 but we can focus on the important parts and not worry about the others. 当然有多于三个阶段的情况，但我们可关注重要

20、的环节，忽略其他环节。Typically the others are checking the system and updating the current internal counter and timer values.其他阶段通常正在检查系统及更新内部计数器和定时器的当前值。 Step 1-CHECK INPUT STATUS-First the PLC takes a look at each input to determine if it is on or off.第一步检查输入状态首先PLC检查每一个输入是否接通。In other words, is the sensor

21、connected to the first input on? How about the second input? How about the third 换句话说就是，与第一个输入端连接的传感器接通了吗？第二个输入呢？第三个输入呢？It records this data into its memory to be used during the next step.PLC 将这些数据记录到存储器中，以便在下一阶段使用。 Step 2-EXECUTE PROGRAM-Next the PLC executes your program one instruction at a time

22、. 第二步执行程序然后PLC一次一条指令地执行程序。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个输入接通时，就接通第一个输出。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. 因为在上一步已经知道输入端的开关状态，根据上一步中第一个输入端的状态，就可以确定是否应接通第一个输出。It will store the execution results for use later during the next step.PLC将执行结果存储起来，以供下一步使用。 Step 3-UPDATE OUTPUT STATUS-Finally the PLC updates the status of th