自动化外文翻译5000字

自动化制造系统与PLC关系

控制工程随着时间的演变。过去的人们主要致力于控制方面研究。最近电力已被应用于控制，早期电气控制是基于继电器的。这些继电器使其可以在没有机械开关的情况下被开动和关闭。这是通常使用继电器进行简单的逻辑控制的方法。低成本计算机的发展带来了新的革命，可编程逻辑控制器（PLC）出现于70年代，它已成为制造控制的最常见选择。PLC的功能受到越来越多的工厂欢迎并可能作为主要控制手段再今后的一段时间内。而这其中绝大部分原因是因为PLC它的优点很多。

1.1梯形逻辑

梯形逻辑编程法是主要的PLC编程方法。正如之前所说，梯形逻辑已发展到模仿继电器逻辑。通过选择简单的梯形逻辑编程法，培训工程师和商人所需要的金钱极大的减少。现代控制系统仍然包括继电器，但这些都是很少的逻辑使用。字母a继电器是一个简单的装置，它使用一个磁场来控制开关，如图图 1.1。当电压作用于输入线圈产生的磁场，产生电流领域。拉起磁场的金属开关，再实现它的接触和联系，关闭开关。

图1.1 简单的布局和继电器电路图

继电器的工作方式，让一个电源开关关闭另一（通常是高电流）电源，同时保持他们孤立。一个简单的例子，控制继电器应用，见图 1.2。在这方面，左边第一个接力是通常使得

系统关闭，并允许电流流动，直到电压加到输入端甲，第二个中继器通常是开放的，不会允许目前的速度，目前的输入二是流经前两个继电器然后电流流通过在第三继电器线圈，并关闭输出C.此电路的开关会通常应用在制定阶梯逻辑形式。这可以被解释为将C逻辑作用，如果A关闭B合上的话。

图1.2一个简单的继电器控制器

图1.2中的例子没有显示整个控制系统，只有逻辑。当我们考虑一个PLC有输入，输出，和逻辑。图1.3显示的更全面。这里有两个按钮的输入。我们可以想像激活24V直流在PLC 继电器线圈的输入。反过来驱动器是一个输出继电器，开关115伏交流电，结果打开了一盏灯。请注意，在实际情况下PLC的输入继电器，常常又是输出继电器。PLC梯形图逻辑其实一种计算机程序，用户可以输入和更改它。注意，两个输入的推按钮常开，但里面的PLC梯形图逻辑有一个常开触点，和一个常闭触点。在PLC梯形逻辑图不需要匹配输入或输出。许多初学者会被抓住这点试图使阶梯逻辑匹配它的输入类型。

图1.3继电器PLC的简图

许多继电器也有多个输出（抛出），这允许输出继电器可以同时输入。图1.4所示的电路是一个例子，它是在电路里称为印章。此电路的电流流过两个电路的分支，通过接触标签A或B的输入端，B只相对乙输出。如果B是关闭的，而A是通电，那么B将打开。如果B打

开，然后输入，B将打开。打开后，乙在输出，乙将不会关闭。

图1.4电路

1.2编程

第一个是PLC的编程，一个基础技术的继电器逻辑接线示意图。虽然这就不需要教电工，技术员和工程师电脑编程 - 但是，这种方法一直是被认可的，这是现在最常见的PLC的编程技术。梯形逻辑的一个例子，图 1.5。为了解释这个图，想像左手垂直线方向，我们称之为热铁路。在右边是中立轨道。图中有两个人物，每个梯级有输入（2垂直线）和组合输出（圆圈）。如果输入是打开，或正确的组合可以关闭热流量通过铁路的输入，使得电力输出，最后中立铁路。输入来自于一个传感器，开关，或任何其他类型的传感器。输出会有是一些外围的PLC设备，开启或关闭则是如灯光或马达之类的。在发出指令后，有常开和常闭2种出点。这意味着，如果输入A和B是关闭，然后将输出并激活它。任何其他组合输入值将导致输入被关闭。

图1.5一个简单的梯形逻辑图

第二个梯级图1.5更复杂，其中有多种组合的输入，输出Y将开机。在最左边的部分发出声响，流过顶端，如果C和D是关闭的。电流也可以（和同时）流经底部，如果E和F都为真。这将使得大部分响起，然后，如果是G或H输出y的话，我们将在后面的章节解释这些。

还有其他的PLC编程方法。最早的一个技术涉及的记忆指令。这些指令由阶梯逻辑图编写，并输入到PLC的编程，通过简单的终端。图1.6是一个记忆法的例子。在这个例子中，读取指令一次一行从上到下的时间。第一行00000的指令LDN（输入负载而不是输入答）这将检查输入到PLC，如果将它关闭记得1 1（或真），如果它会记住一个0（或假）。下一行使用一个（输入负载）语句看看输入。如果输入的是一个0，如果输入记得它是1（注意：这

是相反的）。该声明回顾与最后两个数字记住，如果都真正的结果是1，否则结果是0。这一结果现在取代了两个数字，只有一个数字记忆中。这个过程重复行00003和00004，但是，当这些完成现在有三个数字的记忆中。

最古老的数字是从与，较新的数字是从两个工作点处显示的，并且符合00005，结合从最后的结果和指示工作点处，现在有两个数字的记忆中。指令采用现在剩下的两个数字，如果一方是1的结果是1，否则结果是0。这一结果可替代两个数字，现在有一个数字在这。最后一个指令是存储量，则看最后一个值储存，如果是1，输出将被打开，如果是0输出将被关闭。

图1.6的一个助记符和等效梯形逻辑实例

图1.6梯形逻辑程序，相当于记忆程序。即使你有梯形逻辑编程，PLC的，将被转换为记忆形式使用前由PLC。在过去的记忆节目是最共同的，但现在是常见的用户甚至看到记忆程序。顺序功能图（SFCs）已经制定，以适应规划更先进的系统。这是类似于流程图，但更强大。在图1.7中看到的例子是做两件不同的事情。要阅读图表，顶部是说，地方开始启动。下面这存在着双重的水平线，上面写着遵循两个路径。因此，临立会开始跟随在左，右支另一方面，同时双方分开。在左边有两个功能，第一个是拉功能的权力。此函数将运行至决定这样做，和电力下来后功能会。在右边是闪光功能，这将运行直到它完成。看看这些职能不明，但每个例如启动功能，将一个小梯形逻辑程序。这种方法有很大不同的流程图因为它没有按照流程图通过一个单一的路径。

图1.7的一个顺序功能图例子

结构化文本编程已经发展成为一个更现代的编程语言。这是很相似，如BASIC语言。一个简单的例子所示图1.8。此示例使用一个PLC的内存位置岛该内存位置为整数，也将在后面解释这本书。该计划的第一行设置值为0。下一行开始一个循环，并将在循环返回。下一行回顾我珍惜的位置，给它加1，并返回到相同的位置。下一行检查是否应该退出循环。如果我是大于或等于10，那么循环将退出，否则计算机将返回到重复的声明继续从那里。每次程序通过这个循环时，i去将增加1至值达到10。

图1.8一个结构化文本程序范例

2.1 PLC的连接

当一个进程被控制的PLC，它使用传感器的输入作出决定和更新输出，可驱动器，如图2.1所示。这个过程是一个真正的进程将随时间而改变。执行器将驱动系统，以新的国家（或模式操作）。这意味着，该控制器是由传感器提供，如果输入有限不可用时，控制器将无法检测的条件。

图2.1控制器和分离过程

控制回路是临立会读的投入不断循环，解决了阶梯逻辑，然后更改输出。如同任何电脑不会发生即时。图2.2显示了PLC的基本操作周期。当电源开启最初的PLC做了快速完整性检查，以确保硬件正常工作。如果有问题，临立会停止，并说明有错误。例如，如果PLC的功率下降，即将引爆这将导致故障类型之一。如果临立会通过的完整性检查，然后将扫描（读取）所有的投入。输入值后，存储在内存中的阶梯逻辑将扫描（解决）使用存储的值 - 不是当前值。这样做是为了防止当输入逻辑问题期间更改梯子逻辑扫描。当梯子逻辑扫描完成的产出将扫描（输出值将被更改）。之后系统将可以追溯到做完整性检查，和循环继续下去。不同于一般的计算机，整个程序将被每次扫描运行。对每个阶段的典型是时代的毫秒秩序。

图2.2 PLC的扫描周期

2.2梯形逻辑输入

PLC的输入很容易代表梯形逻辑。在图2.3有三个类型的显示的投入。前两个是常开和常闭投入，讨论以前。 IIT的（立即输入）函数允许后才能投入读输入扫描，而梯形逻辑被扫描。这使得梯形逻辑研究输入值往往超过一个周期。（注：本指令是不可用在ControlLogix 处理器，但仍然可以用旧型号的。）

图2.3梯形逻辑图输入

2.3梯形逻辑输出

在梯形逻辑有多种类型的产出，但这些都不是一贯可在所有的PLC。产出部分将外部连接的设备以外PLC的，但它也可以用在PLC内部存储器位置。 6种输出显示在图2.4。第一个是正常的输出，输出时活力会打开，和激励输出。用斜线通过圆是正常在输出。当通电输出将关闭。这种类型的输出上没有所有的PLC类型。当最初活力的OSR（一炮接力）指令将打开一个扫描，但后来被扫描后，就所有关闭，直到它关闭。的L（锁）和U（解锁）指令可以用来锁定输出。当一个L输出带旺输出会变成无限期，即使输出线圈deenergized。输出可只有关闭使用的U输出。最后一个指令是互操作性测试（立即输出）这将允许产出，而不必为梯形逻辑等待扫描更新为完成。

3.1输入和输出

在投入和产出，到PLC是必要的监测和控制的过程。输入和输出都可以分为两种：基本类型的逻辑或连续。考虑一个灯泡的例子。如果它只能打开或关闭，这是合乎逻辑的控制。如果光线可以使变暗淡不同层次，它是连续的。连续价值观似乎更直观的，但逻辑值是首选，因为它们让更多的确定性和简化控制。

因此，大多数控件的应用程序（和PLC）和逻辑投入使用输出对于大多数应用。因此，我们将讨论逻辑I / O和休假连续的I / O后。对执行器输出使PLC在导致一些事情发生的过程。字母a执行器的流行短名单如下，以相对受欢迎。电磁阀 - 逻辑输出，可以切换液压或气动流。灯 - 这通常可以采用直接从PLC输出逻辑输出板。马达起动器 - 电机常常引起

人们的电流时，开始大量的，因此他们需要电动机起动器，基本上大的继电器。伺服电机 - 从PLC的连续输出可以命令变速或立场。从PLC的产出常常继电器，但它们也可以固体电子学例如DC输出或输出的双向交流晶体管。连续输出要求特别输出卡与数字到模拟转换器。输入来自传感器转化为电信号的物理现象。

传感器典型的例子是下面列出的普及相对顺序。接近开关 - 使用电感，电容或光线来检测对象的逻辑。开关 - 机械机制，将打开或关闭电接触的逻辑信号。电位器 - 不断措施角位置，使用性。LVDT（线性可变差动变压器） - 线性位移的措施不断用磁耦合。从PLC的产出常常继电器，但它们也可以固体电子学例如DC输出或输出的双向交流晶体管。连续输出要求特别输出卡与数字到模拟转换器。输入来自传感器转化为电信号的物理现象。传感器典型的例子是下面列出的普及相对顺序。接近开关 - 使用电感，电容或光线来检测对象的逻辑。开关 - 机械机制，将打开或关闭电接触的逻辑信号。电位器 - 不断措施角位置，使用性。LVDT（线性可变差动变压器） - 线性位移的措施不断用磁耦合。

3.1.1 PLC的输入

在较小的投入通常是内置在购买时指定的PLCPLC的。对于较大的PLC的投入是作为模块或信用卡购买，8或16投入的每张卡上同一类型。为了便于讨论，我们将讨论所有的投入如果他们已经购买的卡。下面的列表显示了典型的输入电压范围，大约是为了普及。PLC的输入卡很少供电，这意味着一个外部电源需要提供的投入和传感器等。图3.1中的例子显示了如何连接到一个AC输入卡。

图3.1 AC输入卡和梯形逻辑

在这个例子中有两个输入，一个是常开按钮，和第二个是一个温度开关，或热继电器。（注意：这些符号是标准稍后将讨论在这一章。）的开关都采用了积极/炎热的输出器24VAC电源- 这就像在一个直流电源正端的。权力是提供给对两个开关的左侧。当开关打开有传递到输入卡没有电压。如果任一电源开关，将关闭提供给输入卡。在这种情况下投入1和3是使用 - 通知，开始投入在0。输入卡比较这些电压的共同。如果输入电压范围内一个给定的容差范围的投入，将开关。梯形逻辑图中显示为的投入。在这里它使用的ControlLogix艾伦布拉德利符号。在顶部的标签（变量名）为在机架上。输入卡（'我'）是在插槽3，因此该卡的地址是鲍勃：3.I.Data.x，其中，'x'是输入位的数字。这些地址也可以给定别名标签，使较少的阶梯逻辑混乱。

许多初学者成为混淆在连接电路中需要上面的。关键是要记住单词的电路，这意味着有一个完整的循环电压必须能够遵循。在图3.1之后，我们可以启动电路（循环）在电源。路径穿过交换机，通过输入卡，再回到电源回流的地方通过向启动。在实施全面的PLC那里将是每一个都必须完成许多电路。第二个重要的概念是共同的。在这里，中立的电力供应是共同的，或参考电压。实际上，我们选择了这是我们为0V参考，和所有其他电压测量相对于它。如果我们有一个第二个电源，我们还需要连接的中性，使这两个中立国将连接到同样普遍。通常的共同和地面也会无所适从。常见的是一个参考，或基准电压，用于为0V使用，但地面是用于防止冲击和损害到设备。地面连接到下一个金属管道或在网格建设地面。这是连接到建筑物的电气系统，对电源插座，在电气设备的金属案件有关。当电源流经地上是坏的。不幸的是许多工程师，制造混淆的地面和共同的。这是很常见找到一个与地面和共同错误标签供电。

最后一个概念，初学者往往陷阱是每个输入卡是孤立的。这意味着，如果你有一个共同的连接只有一个卡，那么其他卡未连接。当发生这种情况的其他卡将无法正常工作。您必须连接共同为每个输出卡。

3.1.2 输出模块

正如输入模块，输出模块很少提供任何权力，而是作为开关。外部电源连接到输出卡和卡开关电源或关闭每个输出。典型的输出电压下面列出，并大致排序受欢迎。

这些卡通常有8至16个相同类型的输出，就可以买到不同的额定电流。一种常见的选择，采购卡的继电器输出，晶体管或可控硅。继电器是最灵活的输出设备。他们有能力开关交流

和直流输出。但是，他们会更慢（约10ms的切换是典型），他们是笨重的，他们花费更多，而且会磨损周期后，数百万人。继电器输出通常被称为干触点。晶体管是有限的直流输出，和双向的仅限于交流输出。晶体管和可控硅输出称为切换输出。

一个独立的继电器，致力于每个输出。这使得混合电压（AC或DC和电压水平，直至最高），以及绝缘输出以保护其他产出和PLC。响应时间往往大于10毫秒。此方法是最不敏感的电压变化和尖峰。交换产出 - 电压提供给PLC的卡，卡切换到使用固态电路（晶体管，晶闸管等）双向不同的输出适合交流设备需要小于1A。晶体管输出NPN或使用PNP晶体管高达1A典型。它们的反应时间大大低于1毫秒。

Automating Manufacturing Systems with PLCs Control engineering has evolved over time. In the past humans were the mainmethod for controlling a system. More recently electricity has been used for control andearly electrical control was based on relays. These relays allow power to be switched on

and off without a mechanical switch. It is common to use relays to make simple logicalcontrol decisions. The development of low cost computer has brought the most recent revolution,the Programmable Logic Controller (PLC). The advent of the PLC began in the1970s, and has become the most common choice for manufacturing controls.PLCs have been gaining popularity on the factory floor and will probably remainpredominant for some time to come. Most of this is because of the advantages they offer.

1.1 Ladder logic

Ladder logic is the main programming method used for PLCs. As mentioned before, ladder logic has been developed to mimic relay logic. logic diagrams was a strategic one. By selecting ladder logic as the main programming method, the amount of retraining needed for engineers and tradespeople was greatly reduced.

Modern control systems still include relays, but these are rarely used for logic. A relay is a simple device that uses a magnetic field to control a switch, as pictured in Figure 1.1. When a voltage is applied to the input coil, the resulting current creates a magnetic field. The magnetic field pulls a metal switch (or reed) towards it and the contacts touch, closing the switch.

Figure 1.1 Simple Relay Layouts and Schematics Relays are used to let one power source close a switch for another (often high current) power source, while keeping them isolated. An example of a relay in a simple control application is shown in Figure 1.2. In this system the first relay on the left is used as normally closed, and will allow current to flow until a voltage is applied to the input A. The second relay is normally open and will not allow current to flow until a voltage is applied to the input B. If current is flowing through the first two relays then current will flow through the coil in the third relay, and close the switch for output C. This circuit would normally be drawn in the ladder logic form. This can be read

logically as C will be on if A is off and B is on.

Figure 1.2 A Simple Relay Controller

The example in Figure 1.2 does not show the entire control system, but only the logic. When we consider a PLC there are inputs, outputs, and the logic. Figure 1.3 shows a more complete representation of the PLC. Here there are two inputs from push buttons.We can imagine the inputs as activating 24V DC relay coils in the PLC. This in turn drives an output relay that switches 115V AC, that will turn on a light. Note, in actual PLCs inputs are never relays, but outputs are often relays. The ladder logic in the PLC is actually a computer program that the user can enter and change. Notice that both of the input push buttons are normally open, but the ladder logic inside the PLC has one normally open contact, and one normally closed contact. Do not think that the ladder logic in the PLC need so match the inputs or outputs. Many beginners will get caught trying to make the ladder logic match the input types

.

Figure 1.3 A PLC Illustrated With Relays

Many relays also have multiple outputs (throws) and this allows an output relay to also be an input simultaneously. The circuit shown in Figure 1.4 is an example of this, it is called a seal in circuit. In this circuit the current can flow through either branch of the circuit, through the contacts labelled A or B. The input B will only be on when the output B is on. If B is off, and A is energized, then B will turn on. If B turns on then the input B will turn on, and keep output B on even if input A goes off. After B is turned on the output B will not turn off.

Figure 1.4 A Seal-in Circuit

1.2 Programming

The first PLCs were programmed with a technique that was based on relay logic wiring schematics. This eliminated the need to teach the electricians, technicians and engineers how to program a computer - but, this method has stuck and it is the most common technique for programming PLCs today. An example of ladder logic can be seen in Figure 1.5. To interpret this diagram imagine that the power is on the vertical line on the left hand side, we call this the hot rail. On the right hand side is the neutral rail. In the figure there are two rungs, and on each rung there are combinations of inputs (two vertical lines) and outputs (circles). If the inputs are opened or closed in the right combination the power can flow from the hot rail, through the inputs, to power the outputs, and finally to the neutral rail. An input can come from a sensor, switch, or any other type of sensor. An output will be some device outside the PLC that is switched on or off, such as lights or motors. In the top rung the contacts are normally open and normally closed. Which means if input A is on and input B is off, then power will flow through the output and activate it. Any other combination of input values will result in the output X being off.

Figure 1.5 A Simple Ladder Logic Diagram

The second rung of Figure 1.5 is more complex, there are actually multiple combinations of inputs that will result in the output Y turning on. On the left most part of the rung, power could flow through the top if C is off and D is on. Power could also (and simultaneously) flow through the bottom if both E and F are true. This would get power half way across the rung, and then if G or H is true the power will be delivered to output Y. In later chapters we will examine how to interpret and construct these diagrams.

There are other methods for programming PLCs. One of the earliest techniques involved mnemonic instructions. These instructions can be derived directly from the ladder logic diagrams and entered into the PLC through a simple programming terminal. An example of mnemonics is shown in Figure 1.6. In this example the instructions are read one line at a time from top to bottom. The first line 00000 has the instruction LDN (input load and not) for input A. . This will examine the input to the PLC and if it is off it will remember a 1 (or true), if it is on it will remember a 0 (or false). The next line uses an LD (input load) statement to look at the input. If the input is off it remembers a 0, if the input is on it remembers a 1 (note: this is the reverse of the LD). The AND statement recalls the last two numbers remembered and if the are both true the result is a 1, otherwise the result is a 0. This result now replaces the two numbers that were recalled, and there is only one number remembered. The process is repeated for lines 00003 and 00004, but when these are done there are now three numbers remembered.

The oldest number is from the AND, the newer numbers are from the two LD instructions. The AND in line 00005 combines the results from the last LD instructions and now there are two numbers remembered. The OR instruction takes the two numbers now remaining and if either one is a 1 the result is a 1, otherwise the result is a 0. This result replaces the two numbers, and there is now a single number there. The last instruction is the ST (store output) that will look at the last value stored and if it is 1, the output will be turned on, if it is 0 the output will be turned off.

Figure 1.6 An Example of a Mnemonic Program and Equivalent Ladder Logic

The ladder logic program in Figure 1.6, is equivalent to the mnemonic program. Even if you have

programmed a PLC with ladder logic, it will be converted to mnemonic form before being used by the PLC. In the past mnemonic programming was the most common, but now it is uncommon for users to even see mnemonic programs.

Sequential Function Charts (SFCs) have been developed to accommodate the programming of more advanced systems. These are similar to flowcharts, but much more powerful. The example seen in Figure 1.7 is doing two different things. To read the chart, start at the top where is says start. Below this there is the double horizontal line that says follow both paths. As a result the PLC will start to follow the branch on the left and right hand sides separately and simultaneously. On the left there are two functions the first one is the power up function. This function will run until it decides it is done, and the power down function will come after. On the right hand side is the flash function, this will run until it is done. These functions look unexplained, but each function, such as power up will be a small ladder logic program. This method is much different from flowcharts because it does not have to follow a single path through the flowchart..

Figure 1.7 An Example of a Sequential Function Chart

Structured Text programming has been developed as a more modern programming language. It is quite similar to languages such as BASIC. A simple example is shown in Figure 1.8. This example uses a PLC memory location i. This memory location is for an integer, as will be explained later in the book. The first line of the program sets the value to 0. The next line begins a loop, and will be where the loop returns to. The next line recalls the value in location i, adds 1 to it and returns it to the same location. The next line checks to see if the loop should quit. If i is greater than or equal to 10, then the loop will quit, otherwise the computer will go back up to the REPEAT statement continue from there. Each time the program goes through this loop i will increase by 1 until the value reaches 10.

Figure 1.8 An Example of a Structured Text Program

2.1 PLC Connections

When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators, as shown in Figure 2.1. The process is a real process that will change over time. Actuators will drive the system to new states (or modes of operation). This means that the controller is limited by the sensors available, if an input

is not available, the controller will have no way to detect a condition.

Figure 2.1 The Separation of Controller and Process

The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any computer this does not happen instantly. Figure 2.2 shows the basic operation cycle of a PLC. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly.If there is a problem the PLC will halt and indicate there is an error. For example, if the PLC power is dropping and about to go off this will result in one type of fault. If the PLC passes the sanity check it will then scan (read) all the inputs. After the inputs values are stored in memory the ladder logic will be scanned (solved) using the stored values not the current values. This is done to prevent logic problems when inputs change during the ladder logic scan. When the ladder logic scan is complete the outputs will be scanned (the output values will be changed). After this the system goes back to do a sanity check, and the loop continues indefinitely. Unlike normal computers, the entire program will be run every scan. Typical times for each of the stages is in the order of milliseconds.

Figure 2.2 The Scan Cycle of a PLC

2.2 Ladder Logic Inputs

PLC inputs are easily represented in ladder logic. In Figure 2.3 there are three types of inputs shown. The first two are normally open and normally closed inputs, discussed previously. The IIT (Immediate InpuT) function allows inputs to be read after the input scan, while the ladder logic is being scanned. This allows ladder logic to examine input values more often than once every cycle. (Note: This instruction is not available on the ControlLogix processors, but is still available on older models.)

Figure 2.3 Ladder Logic Inputs

2.3 Ladder Logic Outputs

In ladder logic there are multiple types of outputs, but these are not consistently available on all PLCs. Some of the outputs will be externally connected to devices outside the PLC, but it is also possible to use internal memory locations in the PLC. Six types of outputs are shown in Figure 2.4. The first is a normal output, when energized the output will turn on, and energize an output. The circle with a diagonal line through is a normally on output. When energized the output will turn off. This type of output is not available on all PLC types. When initially energized the OSR (One Shot Relay) instruction will turn on for one scan, but then be off for all scans after, until it is turned off. The L (latch) and U (unlatch) instructions can be used to lock outputs on. When an L output is energized the output will turn on indefinitely, even when the output coil is deenergized. The output can only be turned off using a U output. The last instruction is the IOT (Immediate OutpuT) The last instruction is the

IOT (Immediate OutpuT)that will allow outputs to be updated without having to wait for the ladder logic scan to be completed.

3.1 INPUTS AND OUTPUTS

Inputs to, and outputs from, a PLC are necessary to monitor and control a process. Both inputs and outputs can be categorized into two basic types: logical or continuous. Consider the example of a light bulb. If it can only be turned on or off, it is logical control. If the light can be dimmed to different levels, it is continuous. Continuous values seem more intuitive, but logical values are preferred because they allow more certainty, and simplify control.

As a result most controls applications (and PLCs) use logical inputs and outputs for most applications. Hence, we will discuss logical I/O and leave continuous I/O for later. Outputs to actuators allow a PLC to cause something to happen in a process. A short list of popular actuators is given below in order of relative popularity. Solenoid Valves - logical outputs that can switch a hydraulic or pneumatic flow. Lights - logical outputs that can often be powered directly from PLC output boards.Motor Starters - motors often draw a large amount of current when started, so they require motor starters, which are basically large relays. Servo Motors - a continuous output from the PLC can command a variable speed or position. Outputs from PLCs are often relays, but they can also be solid state electronics such as transistors for DC outputs or Triacs for AC outputs. Continuous outputs require special output cards with digital to analog converters. Inputs come from sensors that translate physical phenomena into electrical signals.

Typical examples of sensors are listed below in relative order of popularity.Proximity Switches - use inductance, capacitance or light to detect an object logically. Switches - mechanical mechanisms will open or close electrical contacts for a logical signal. Potentiometer - measures angular positions continuously, using resistance. LVDT (linear variable differential transformer) - measures linear displacement continuously using magnetic coupling. Inputs for a PLC come in a few basic varieties, the simplest are AC and DC inputs. Sourcing and sinking inputs are also popular. This output method dictates that a device does not supply any power. Instead, the device only switches current on or off, like a simple switch. Sinking - When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied. Sourcing - When active, current flows from a supply, through the output device and to ground. This method is best used when all devices use a single supply voltage. This is also referred to as NPN (sinking) and PNP (sourcing). PNP is more popular. This will be covered in detail in the chapter on sensors.

3.1.1 Inputs

In smaller PLCs the inputs are normally built in and are specified when purchasing the PLC. For larger PLCs the inputs are purchased as modules, or cards, with 8 or 16 inputs of the same type on each card. For discussion purposes we will discuss all inputs as if they have been purchased as cards. The list below shows typical ranges for input voltages, and is roughly in order of popularity. PLC input cards rarely supply power, this means that an external power supply is needed to supply power for the inputs and sensors. The example in Figure 3.1 shows how to connect an AC input card.

Figure 3.1 An AC Input Card and Ladder Logic

In the example there are two inputs, one is a normally open push button, and the second is a temperature switch, or thermal relay. (NOTE: These symbols are standard and will be discussed later in this chapter.) Both of the switches are powered by the positive/ hot output of the 24Vac power supply - this is like the positive terminal on a DC supply. Power is supplied to the left side of both of the switches. When the switches are open there is no voltage passed to the input card. If either of the switches are closed power will be supplied to the input card. In this case inputs 1 and 3 are used - notice that the inputs start at 0. The input card compares these voltages to the common. If the input voltage is within a given tolerance range the inputs will switch on. Ladder logic is shown in the figure for the inputs. Here it uses Allen Bradley notation for ControlLogix. At the top is the tag (variable name) for the rack. The input card (’I’) is in slot 3, so the address for the card is bob:3.I.Data.x, where ’x’ is the input bit number. These addresses can also be given alias tags to make the ladder logic less confusing.

Many beginners become confused about where connections are needed in the circuit above. The key word to remember is circuit, which means that there is a full loop that the voltage must be able to follow. In Figure 3.1 we can start following the circuit (loop) at the power supply. The path goes through the switches, through the input card, and back to the power supply where it flows back through to the start. In a full PLC implementation there will be many circuits that must each be complete. A second important concept is the common. Here the neutral on the power supply is the common, or reference voltage. In effect we have chosen this to be our 0V reference, and all other voltages are measured relative to it. If we had a second power supply, we would also need to connect the neutral so that both neutrals would be connected to the same common. Often common and ground will be confused. The common is a reference, or datum voltage that is used for 0V, but the ground is used to prevent shocks and damage to equipment. The ground is connected under a building to a metal pipe or grid in the ground.

This is connected to the electrical system of a building, to the power outlets, where the metal cases of electrical