1. Electrical Measuring Instruments电子测量仪表
Electrical personnel use many different types of measuring instruments. 电子技术人员使用许多不同类型的测量仪器。
Some jobs require very accurate measurements while other jobs need only rough estimates. 一些工作需要精确测量面另一些工作只需粗略估计。
Some instruments are used solely to determine whether or not a circuit is complete. 有些仪器被使用仅仅是确定线路是否完整。
The most common measuring and testing instruments are voltage testers, voltmeters, ammeters, ohmmeters, continuity testers, megohmmeters, wattmeters（功率计）, and watt-hour meters（电度表，电表）。
最常用的测量测试仪表有：电压测试仪，电压表，欧姆表，连续性测试仪，兆欧表，瓦特表还有瓦特小时表。
All meters used for measuring electrical values are basically current meters. 所有测量电值的表基本上都是电流表。
They measure or compare the values of current flowing through them. 他们测量或是比较通过他们的电流值。
The meters are calibrated and the scale is designed to read the value of the desired unit. 这些仪表可以被校准并且设计了不同的量程，以便读出期望的数值。
1.1 SAFETY PRECAUTION安全预防
Correct meter connections are very important for the safety precaution of the user and for proper maintenance of the meters. 仪表的正确连接对于使用者的安全预防和仪表的正确维护是非常重要的。
A basic knowledge of the construction and operation of meters will aid the user in making proper connections and maintaining them in safe working order. 仪表的结构和操作的基本知识能帮助使用者按安全工作程序来对他们正确连接和维护。
Many instruments are designed to be used on DC or AC only, while others can be used interchangeably. 许多仪表被设计的只能用于直流或只能用于交流，而其它的则可交替使用。
Note: It is very important to use each meter only with the type of current for which the meter is designed. 注意：每种仪表只能用来测量符合设计要求的电流类型。
Using a meter with an incorrect type of current can result in damage to the meter and may cause injury to the user. 如果用在不正确的电流类型中可能对仪表有危险并且可能对使用者引起伤害。
Some meters are constructed to measure very low values. 许多仪表被设计成只能测量很低的数值Other meters can measure extremely high values.还有些能测量非常大的数值。
CAUTION: Never allow a meter to exceed its rated maximum limit. 警告：仪表不允许超过它的额定最大值。
The importance of never allowing the actual value to exceed the maximum value indicated on the meter can not be overemphasized. 不允许被测的实际数值超过仪表最大允许值的要求再强调也不过分。
Exceeding maximum values can damage the indicating needle, interfere with proper calibration, and in some instances may cause the meter to explode, resulting in injury to the user. 超过最大值对指针有伤害，有害于正确校准，并且在某种情况下能引起仪表爆炸造成对作用者的伤害。
Some meters are equipped with over correct protection. However, a current many times greater than the instrument’s design limit may still be hazardous. 许多仪表装备了过载保护。
然而，通常情况下电流大于仪表设计的限定仍然是危险的。
2. Fundamentals of Solid-state Power Device固体功率器件的基本原理
In this chapter we focus on solid-state power devices, or power semiconductors,
only as they are being used in the power leads or power circuits to three-phase 460 V AC squirrel cage induction motors for either phase (voltage) control or frequency (speed) control. 本章将集中讨论固态功率器件或功率半导体器件，并且只研究它们在采用相控（电压控制）或频率控制（速度控制）的三相交流鼠笼式感应电机的功率电路中的应用。
2.2 SOLID-STATE POWER DEVICES固态功率器件
The five major types of power semiconductors used in solid-state AC motor control are: 有五种用于固体交流电机控制中的功率元器件：
Diodes二极管
Thyristors[e.g., silicon-controlled rectifiers (SCRs)] 晶闸管（例如：可控硅整流器SCR）
Transistors电子晶体管
Gate-turn-off thyristors门极可关断晶闸管（GTO）
Triacs 双向可控硅
SCRs and triacs are commonly used for phase controls. 晶闸管SCR和双向可控硅一般用于相位控制（相控）。
Various combinations of diodes, SCRs, transistors, and GTOs are used for speed controls. 各种二极管，晶闸管SCR，电子晶体管，门极可关断晶闸管的联合体用于频控。
The commonality of these devices is the use of crystals of silicon in the form of wafers that are layered so as to form various combinations of PN junction. 这些器件的共性是：利用硅晶体形成的薄片构成P-N结的各种组合。
The P junction is usually called the anode and N junction is usually called the cathode for diodes, SCRs, and GTOs; the corresponding terms for transistors are collector and emitter. 对二极管，SCR， GTO一般P结叫正极N结叫负极；相应的电子晶体管叫集电极和发射极。
The differences among these devices relate to how they go into and out of conduction and in their available ampere and voltage capabilities. 这些器件的区别在于导通和关断的方法及电流和电压的容量。
Let’s take a brief look at each of these devices in terms of these parameters. 让我们根据他们的参数简单看一下这些元器件。
3 Analog Electronics模拟电子
3.1 INTRODUCTION介绍
3.1.1 The Contrast between Analog and Digital Electronics模拟和数字电子学对照
We have already explored how transistors and diodes are used as switching devices to process information which is represented in digital form. 我们已经研究过三极管和二极管怎样作为开关器件用于处理以数字形式出现的信息。
Digital electronics uses transistors as electrically controlled switches: transistors are either saturated or cut off. 数字电子学中，三极管用作电子控制开关：它不是饱和就是截止。
The active region is used only in transition from one state to he other.（三极管的）动态区域仅用于从一种状态过渡到另一种状态。
By contrast, analog electronics depends on the active region of transistors and other types of amplifiers. 与之相对的是，模拟电子学依赖于三极管的动态区域和其它放大器的形式。
The Greek roots of “analog” mean “in due ratio”, signifying in this usage that information is encoded into an electrical signal which is proportional to the quantity being represented. 希腊语词根“analog”的意思是“以一定的比例”，在这里表示信息被编码成为与表达量成正比的电信号。
In Fig.3.1 our information is some sort of music, originating physically in the
excitation and resonance’s of a musical instrument.在图3.1中，我们的信息是某种音乐，物理上由乐器的激励和共鸣产生。
The radiated sound consists in the ordered movement of air molecules and is best understood ad acoustic waves. 辐射的声波由空气分子的有序运动组成，准确的理解为声波， These produce motion in the diaphragm of a microphone, which in turn produces an electrical signal. 它使话筒的振动膜移动，依次产生电信号。
The variation in the electrical signal are a proportional representation of the sound waves. 电信号的变化与声波成比例。
The electrical signal is amplified electronically, with an increase in signal power occurring at the expense of the input AC power to the amplifier. 电信号被用电子的方法放大，即利用输入放大器的交流电能将信号的功率放大。
The amplifier output drives a recording head and produces a wavy groove on a disk. 放大器的输出驱动一个记录头，并且在一个圆盘上产生起伏的沟槽。
If the entire system is good, every acoustic variation of the air will be recorded on the disk and, when the record is played back through a similar system and the signal reradiated ad sound energy be a loudspeaker, the resulting sound should faithfully reproduce the original music. 如果整个系统正常，每一次声波振动都会记录在圆盘上，并且当该记录通过类似的系统重放，信号以声能的形式从扬声器辐射出来，所产生的声音能忠实地重现原始音乐。
Electronic systems based on analog principles form an important class of electronic devices. 基于模拟原理的电子系统形成了一类重要的电子仪器。
Radio and TV broadcasting are common examples of analog systems, as are many electrical instruments used in monitoring deflection (strain gages, for example), motion (tachometers), and temperature (thermocouples). 收音机和电视机的播放是模拟系统的典型例子，许多仪器也是模拟系统，它们的应用领域包括偏差检测（应变计量器）运动控制（测速仪）和温度测量（热电偶）。
Many electrical instruments-voltmeters, ohmmeters, ammeters, and oscilloscopes-utilize analog techniques, at least in part. 许多电子仪器——电压表、电流计欧姆表、和示波器应用或至少是部分应用了模拟技术。
Analog computers existed before digital computers were developed. 模拟计算机在数字计算机成熟之前就已存在。
In an analog computer, the unknowns in a differential equation are modeled with electrical signals. 在模拟计算中，用电信号模拟微分方程的未知量。
Such signals are integrated, scaled, and summed electrically to yield solutions with modes effort compared with analytical or numerical techniques. 用电子的方法对这些信号积分、比例运算和求和以获得方程的解，比起解析或数值求解方法要容易一些。
4.1.1 What is a Digital Signal什么是数字信号
A Historical Example. 一个历史例子“Listen my children and you shall hear of the midnight ride of Paul Revere…”"听，我的孩子们，你们将会听到保罗·瑞维尔午夜策马飞奔的传奇，"According to Longfellow’s poem, Paul Revere was sent riding through the New England countryside by a signal from the bell tower of the Old North Church in Boston. 根据朗费罗的诗，保罗·瑞维尔通过波士顿老北教堂的钟楼那里给新英格兰农民发信号。
（不写：如果英军今晚无论是从陆地还是从海上来犯，在老北教堂钟楼的拱门上高挂一盏灯笼作为信号）“One if by land and two if by sea.”。
"一盏灯笼表示从陆地来，两盏灯笼表示从海上来"。
That is, one light was to be displayed if the British forces were advancing toward Concord by the road from Boston, and two lights were to be displayed if they were crossing the Mystic River to take an indirect
route)那就是说，如果英国军队从波士顿陆地向康科德前进，展示一盏灯，如果他们穿过Mystic河走间接航线，两盏灯将被展示。
The message received by the Patriot was coded in digital form. 爱国者们收到的消息就是数字形式的编码。
We would say today that the two“bits”of information were conveyed by the code(strictly speaking, two bits could indicate four possible message and would require distinguishable lights, say one red and one white). 我们今天将说这两个"位"的信息通过代码运传送(严格地说，二位能表明4个可能消息，并且要求可分辨的灯，一个红灯和一个白色的灯) .The first light signaled that the British were advancing. 第一个灯光通知英国军队正前进，The second light indicated by what route they were coming. 第2盏灯指示他们通过什么路线来。
Because only two routes of advance were envisioned, this second bit of information could be interpreted as indicating one of the two routes. 因为想象只有两条路，信息的第二位能被解释为表明两条路线之一。
Information can be communicate in digital form if the massage is capable of being defined by a series of yes/no statements. 如果信息能被一系列的YES/NO所定义，信息就能以数字形式确定。
There can be only two states of each variable used in conveying the information. 每个变量只可能有两种状态在传送信息方面使用。
Reducing information to a series of yes/no statements might appear to be a severe limitation on this method, but the method is in fact quite powerful. 这种方法把信息简化为一系列是/不看起来有局限性，但是这种方法实际上十分有用。
Numbers can be represented in base 2 and the alphabet by a digital code. 数目可以以基数2表示，字母表可以表示成数字代码。
Indeed, any situation with a finite number of outcomes can be reduced to a digital code. 的确，有限输出的任何情形都可以被简化成数字代码。
Specifically, n digital bits can represent 2∧n states of possible outcomes. 具体的，n位数字能描述成2∧n个可能状态。
Digital communication takes a well-defined code known to the parties at both ends, as in our historical example. 如同在我们的历史例子中，数字通讯能用明确定义的代码在两种情况下都通知到大家。
Analysis of the Revere Communication Code. 瑞维尔通讯码分析。
In order to fix further the idea of digital information, we shall define two digital variables which describe the Paul Revere communication system, Let B describe whether the British are coming, and L describe the route by which they are coming, provide that they are coming. 为了更进一步确定数字的信息的概念，我们将定义两个来描述保罗瑞维尔通信系统的数字变量，让B描述是否英国人正来，并且L描述他们正来的路线。
The mathematical variables, B and L, are unusual mathematical variables because each can have only two values. 数学变量，B和L，是不平常的数学变量，因为每一个只能有两个值。
We may call those two values by any names we wish :yes /no, true/false, one/zero, high /low, even black/white.我们可以用我们希望的任何名字给两值命名：是/不，真/假，一/ 0，高/低，甚至黑色/白色。
When this type of mathematics was used primarily for analysis of philosophical arguments through symbolic logic, the values of the variables were called true or false, according to the validity of the logical propositions being represented. 当这类数学被通过符号逻辑用于哲学论据分析时，变量的值被叫对还是错，根据逻辑命题被阐述的正确性。
Recently, the names one /zero have come to be preferred(首选的；优先的) by engineers and programmers dealing with digital codes. 近来，命名1/0已经开始被工程师和程序员(处理数字代码)更喜欢These names have the
obvious advantage of fitting with the binary (base2) number system for representation of numerical information, but these names occasionally lend confusion to the discussion of digital systems. 这种命名有明显的优势，适用于二进制数字系统(base2)处理数字信息，但是这些命名偶尔对数字系统的讨论造成混乱。
Nevertheless, we shall use 1/0 of one/zero as our two possible states of the digital variables B and L. 虽然如此,我们将使用ONE/ZERO或1/0作为我们数字变量B和L的两种可能的状态Thus the definitions of B and L are B=1 B=0 L=1 L=0 if the British are coming if the British are not coming if coming by sea if coming by land因此B和L的定义是如果英国人将来，B = 1 如果英国人不来，B = 0 如果通过海上来，L = 1 如果由陆路来，L = 0
The first light (B)uniquely determines if he should ride. 第一个光(B) 唯一确定他是否骑马。
When the first light appears, he mounts his horse. 当第一光出现时，他骑上他的马。
But he cannot leave until the second light (L) appears (or fails to appear), for the second light reveals the route of the British and hence defines in part the message to be announced. 但是他不能离开直到第2盏灯(L)出现(或者不出现) ,对于第2盏灯来说显示英国人的路线并且因此确定要被宣布的部分消息。
Representing Digital Information Electrically. 用电力描述数字信息。
In digital electronics, digital variables are represented by logic levels. 在数字电子方面，数字变量通过逻辑级描述。
At any given time, a voltage is expected to have one value or another, or more precisely to lie within one region or another. 在任何规定时间，电压预计有一值或者另一个，或者更确切在一个范围内。
In a typical system, a voltage between 0 and 0.8 V would be considered a digital zero, a voltage between 0.8-2 V would be forbidden; that is if the voltage fell within this range, you would known that digital equipment needs repair. 在一个典型系统里， 0和0.8 V之间的电压将被认为是数字的零，在0.8和2 V之间的电压将被禁止；如果电压属于这范围，你知道数字设备需要修理了。
这些定义显示在图4.1中These definitions are shown in Fig.4.1. As an illustration of a digital circuit, we shall consider the amplifier-switch as a NOT circuit. 作为一个数字电路的插图，我们将考虑放大器开关作为一条非门电路。
The output of a NOT circuit is the digital complement, or the opposite, of the input. 非电路的输出是输入数字的“反”或者“补码”。
First we shall represent the definition of the NOT circuit with a truth table. 首先我们将用一个真值表描述非电路的定义。
This is shown in Fig.4.2: A represents the input, which may be either 1 or 0 ；B represent the output，which may also be 1 or 0，but depends upon the input. 这在图4.2显示：A描述输入，可能1或者0；B描述输出，也可能是1或者0，但是由输入决定。
The NOT, or logical complement, operation is indicated algebraically by the equation under the truth table. 非或者逻辑补的操作，以真值表以下的那些方程式用代数方式被表达。
We shall now define logic levels for the amplifier-switch such that it performs the NOT function. 我们现在将确定放大器开关的逻辑级这样它能执行非的功能。
The input-output characteristic of the circuit is repeated in Fig.403. 电路的输入输出特性被在图4.3中重复。
Clearly, we wish 10 V to be in the region for a 1 and 0.7 V to be in the region for a 0. 显然，我们希望10 V为1区域，0.7 V在 0区域。
That is if the input were 10 V (digital 1), the output should be less than 0.7V (digital 0), and vice versa（反之亦然）. 那就是说，输入是10 V(数字1) ,输出应该少于0.7 V(数
字0) ,反之亦然。
Hence we might consider making the region for a digital 0 to be from 0～1 V, and the region for a digital 1 to be, say, from 8～10 V. 因此，我们可能考虑数字0的范围从0-1V，数字1的范围大约从8 -10 V。
This will work but leaves insufficient range for a working digital system. 这对一个工作的数字系统能很好工作，但是离开了能胜任的范围。
That is, it is desirable to broaden the range of values in the regions for 1 and 0 to allow for variations in transistors, power supply voltage, noise that might get mixed in with the signal, and the like. 那就是说，希望扩展区域1和0的有效范围，以允许晶体管和电源电压的变化以及混杂在信号里的噪音等因素的影响。
In the present case, we can by trail and error （试凑法） determine that the region 0～1.5 V as a digital 0 works well with 5～10 V as a digital 1; with these definitions the circuit operates as a NOT circuit, that is, it performs the logical complement。
These logic levels are shown in Fig.4.4. 在目前例子中，我们能以错误操作的范围0-1.5 V作为数字0，好的操作范围5 -10 V作为数字1；即由于这些定义电路作为一条并非电路，即，它执行合乎逻辑的补足。
这些逻辑级被用图4.4显示。
5 Introduction to Electrical Machines电机的介绍
5.1 Brief History of Electrical Machines电机的简要历史
Electrical machinery has been in existence for many years. 电机存在了多年。
The applications of electrical machines have expanded rapidly since their first use many years ago. 从他们多年前的第一个使用，电机的应用已经迅速扩大了。
At the present time, applications continue to increase at rapid rate. 目前，应用继续快速增加。
Thomas Edison is given credit for developing the concept of widespread generation and distribution of electrical power. 托马斯·爱迪生因为电力的普遍阶段和电力传输（配电）的概念的发展获得荣誉。
He performed developmental work on direct-current (DC) generators which were driven by steam engines. 他对被蒸气机驱动的直流(DC)发电机进行了发展的工作。
Edison’s work with electrical lights and power production led the way to the development of DC motors and associated control equipment. 爱迪生对电灯和电力产品的工作带来了直流电动机和相关控制设备的发展。
Most early discoveries related to electrical machinery operation dealt with direct-current systems. 大多数早期的发明都与处理直流电系统的电机操作有关。
Alternating-current (AC) power generation and distribution became widespread a short time later. 交流电(AC)发电机和传输后来才变得广泛。
The primary reason for converting to AC power production and distribution was that transformers could be used to increase AC voltage levels for long-distance distribution of electrical power. 转变为交流电能产生与传输的主要原因是变压器能升高交流电压电平以适适合电能的长距离传输。
Thus the discovery of transformers allowed the conversion of power production and distribution systems form DC to AC systems. 因此，变压器的发明允许电能的产生与传输系统从DC到AC系统的变换。
Presently, almost all electrical power systems produce and distribute three –phase alternating current. 目前，几乎所有电力系统产生与传输使用三相交流电。
Transformers allow the voltage produced by an AC generator to be increased while decreasing the current level by a corresponding amount. 变压器允许交流发电机产生的电压在以相应数量级减少电流时被增加。
This allows long-distance distribution at a reduced current level, reduces power losses, and increases system efficiency. 这样，允许在很低的电流下远距离传输，降低了电力损耗，增加系统效率。
The use of electrical motors has increased for home appliances and industrial and commercial applications for driving machines and sophisticated equipment. 电机的使用增加了家庭用具的使用，驱动电机和复杂设备的工业和商业上的应用也增加了。
Many machines and automated industrial equipment now require precise control. 很多机器和自动化的工业设备现在需要准确的控制。
Thus motor design and complexity has changed since early DC motors which were used primarily with railroad trains. 由于早期直流电动机主要用于铁路，因此，电动机设计和复杂性都改变了。
Motor control methods have now become more critical to the efficient and effective operation of machines and equipment. 对机器和设备的经济性和有效操作来说，电动机控制方法现在已经变得更关键。
Such innovations as servo control systems and industrial robots have led to new developments in motor design. 象伺服控制系统和工业机器人这样的革新，已经导致了在电机设计方面的新发展。
Our complex system of transportation has also had an impact on the use of electrical machines. 传输系统的复杂也已经对电机使用有影响。
Automobiles and other means of ground transportation use electrical motors for starting and generators for their battery-charging systems. 汽车及其他地面交通工具采用电机启动，并用发电机对蓄电池充电。
There has recently been emphasis in the development of electric motor-driven automobiles. 在电机驱动的汽车的发展过程中最近他们被强调了。
Aircraft use electrical machines in ways similar to automobiles. 飞机用类似于汽车的方式使用电机。
However, they also use sophisticated synchro and servo-controlled machines while in operation. 不过，他们也在运行中使用复杂的同步和伺服控制的机器。
7 Electrical Machine Control Systems电子设备控制系统
The previous chapters have dealt primarily with the types and characteristics of electrical machines. 先前的章节我们主要处理电子设备的类型和特性。
This chapter provides an overview of several power control systems which are used with electrical machines.这个章节提供了几种用于电源控制的电子设备系统的概观。
7.1 CONTROL SYMBOLS控制符号It is necessary to become familiar with the electrical symbols which are commonly used with machine control systems. 熟悉一些常用在设备控制系统的电子符号是很必须的事。
Some common machine control symbols are shown in Fig.7.1. 一些常用的设备控制符号如图7.1所示。
7.2 MACHINE CONTROL WITH SWITCHES开关控制设备
An important type of electrical machine control is the switch. 开关是一种重要
类型的电子控制设备。
Many types of switches are used to control electrical machines. 有多种类型的开关用在控制电子设备。
The function of a switch is to turn a circuit on or off; however, many more complex switch functions can be performed. 开关的功能是开断电路，但是还有很多的复杂的开关有更多的功能。
7.2.1 Toggle switches切换开关
Among the simplest type of switches are toggle switches. The symbols for several kinds of toggle switches are shown Fig.7.2.Notice the symbols that are used for various types of toggle switches. 在所有开关类型中切换开关是最简单的。
几种不同的切换开关符号如图7.2所示。
注意那些符号用于不同类型的切换开关
7.2.2 Pushbutton switches按压开关
Pushbutton switches are commonly used for machine control. 按压开关也很经常用于设备控制。
Many machine control applications use pushbuttons as a means of starting,
stopping, or reversing a motor. 很多设备控制应用中使用按压开关作为一种电动机起动，停车，或是反转。
Pushbuttons are manually operated to close or open a machine control circuit. 按压开关能用手工地操作来闭合和断开设备控制电路。
There are several types of pushbuttons used for of machines. 有几种类型的按压开关用于设备的控制。
Pushbuttons are usually mounted in enclosures called motor control stations按压开关常常安放在被称为电机控制柜的柜子里。
Ordinarily, pushbuttons are either the normally closed (NC) or normally open (NO) types. 普遍地，按压开关有常开和常闭两种类型，However, there are a few modifications. 但是有一些调整。
A normally closed pushbutton is closed until it is depressed manually and will open a circuit when it is depressed. 一个常闭按压开关是闭合的直到手工的按下，按下时断开电路。
The normally open pushbutton is open until it is manually depressed and, then, once it is depressed, it will close a circuit. The “start” pushbutton of a motor control station is of NO type, while the “stop” switches of NC type. 常开按压开关是断开的直到手工的按下，按下是闭合电路。
电机控制柜采用常开的按压类型来起动，而用常闭来停车
7.2.3 Rotary switches转动开关
Another common type of switch is the rotary switch. 另一种常用的开关类型是转动开关。
Many different switching combinations can be wired by using a rotary switch. 许多不同的开关组合连线都是通过使用转动开关实现。
The shaft of a rotary switch is attached to sets of stationary contacts.转动开关的轴附上一连串的动触点。
These moving contacts touch different sets of stationary contacts which are mounted on ceramic segments when the rotary shaft is turned to different positions. 那些动触点不同于当转轴转到不同的位置时那些安放在绝缘瓷上的一连串固定的触点。
The shaft can lock into place in an of several positions. 转轴可以锁在任何位置中的几个。
Rotary switches are usually controlled by manually turning the rotary shaft clockwise or counterclockwise. 转动开关常常通过顺时针或是逆时针转动转轴来控制。
A knob （旋钮）is normally fastened（固定） to the end of the rotary shaft to permit easier turning of the rotary shaft. 在转轴的末端常用一个旋钮来固定使得转轴的转向更容易。
7.2.4 Limit switches限位开关
Limit switches are merely on/off switches that use mechanical movement to cause a change in the operation of the electrical control circuit of a machine. 限位开关仅仅用于机械运动引起的电子设备控制电路的动作的变化的开断开关。
The electrical current developed as a result of the mechanical movement is used to limit movement of the machine or to make some change in its operational sequence. 电流的变化导致机械运动被用于限制设备的动作或者是工作顺序的改变。
Limit switches are often used in sequencing, routing（行程）, sorting（分类）, or counting（） operations in industry. 限位开关经常被用于工业上的排序、行程安排、分类、或者计数操作。
Often, they are used in conjunction with hydraulic （流体的——液压的） or pneumatic（气动的） controls, electrical relays, or other motor-operated machinery such as drill presses（钻床）, lathes（车床）, or conveyor systems. 也常与液压、气动控制，继电器，或是和钻床压紧装置、车床、传输系统的电机操作机器配合使用。
In its most basic form, a limit switch converts mechanical motion into an electrical control current. 限位开关最基本的组成，一个限位开关能把机械动作转换成电子控制电流。
Notice the cam, which is an external part that is usually mounted on a machine.
注意那个常安放在设备外部的凸轮。
The cam applies force to an actuator of the limit switch. 凸轮提供了力给限位开关的执行部件。
The actuator is the part of the limit switch that causes the internal NO or NC contacts to change state. 作为限位开关一部分的执行部件引起内部常开或常闭触点的状态改变。
The actuator operates due to either linear or rotary motion of the cam, which applies force to the limit switch. 执行部件的动作是由给限位开关提供力的凸轮的平动或转动引起的。
The other terms associated with limit switches are pretravel and overtravel. 其他与限位开关有关的术语是预冲程和过冲程。
Pretravel is the distance that the actuator must move to change the normally open or normally closed state of the limit-switch contacts. 预冲程就是执行部件移动到限位开关常开或常闭触点状态改变的距离。
Overtravel is the distance the actuator moves beyond the point where the contacts change state. 过冲程就是执行部件移动到超过触点状态改变的点的距离。
Both pretravel and overtravel settings are important in machine setups where limit switches are used. 在限位开关使用的设备设置中，预冲程和过冲程的设定都是重要的。
7.2.5 Temperature switches温控开关
Temperature switches are common types of control devices. 温控开关是一种常用的控制设备类型。
The control element of a temperature switch contains a specific amount of liquid. 温控开关的控制元素包含特殊定量的液体。
The liquid increase in volume when temperature increases. 当温度增加时液体容积增加。
Thus changes in temperature can be used to change the position of a set of contacts within the temperature-switch enclosure. 这样温度的改变可以用来改变在温控开关柜里的一连串触点的位置。
Temperature switches may be adjusted throughout a range of temperature settings. 温控开关能适应整个温度设定范围内的温度。
8.1 Control Sensor 简介
In feedback control systems- plant response is measured and compared with a reference input and the error is automatically employed in controlling the plant. 在反馈控制系统中设备的响应被测量来跟设定输入比较，然后误差叠加到控制设备上。
It follows that a measurement system is an essential component in any feedback control system and forms a vital link between the plant and the controller. 因此测量系统是一个基本的部分在任何反馈控制系统，在设备与控制器之间组成至关重要的联系。
Measurements are needed in many engineering applications. 在很多电气应用需要测量变送。
The measurement process has to he automated, however, in control systems applications. 无论如何，在控制应用系统中测量变送需要自动化。
A typical measurement system consists of one or more sensor-trans-ducer units and associated signal-conditioning and modification device. 一个典型的测试系统包含一个或多个传感器单元和与之相关的信号调理（和调整）设备。
Filtering to remove unwanted noise and amplification to strengthen a needed signal are considered signal conditioning. 过滤掉不想要的噪声和放大增强需要的信号被称为信号调理。
Analog-to-digital conversion(ADC),digital-to-analog conversion(DAC),modulation and demodulation are signal modification methods. 模数转换，数模转换，调制和解调都是信号调整的方法。
Note that signal conditioning can be considered under the general heading of signal modification. 注意信号调理被认为处于信号调整之前。
Even though data recording is an integral function in a typical data acquisition system it is not a crucial function in a feedback control system. 即使数据采样是典型的。
