机电专业英语讲义.ppt.Convertor

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一 1.1 Bipolar junction transistors

Today, industrial electronic systems employseveral devices that are described by the term transistor. Each type of transistor has different characteristics and operational conditions that are used to distinguish it from others.

每种晶体管都有区别于其它种类的特征和工作环境。

In the first part of this discussion, we are concerned with the . Structurally, this transistor is described as bipolar because it has two different current-carrier polarities.

首先探讨双极型晶体管。从结构上看，这种晶体管被称为双极是因为它具有两种不同的电流载流子电极。Holes are positive current carriers, whereas electrons are negative current carriers.

空穴是正的载流子，电子是负的电流载流子。

Two distinct kinds of semiconductor crystals are connected together by a common element. 两种截然不同的半导体晶体通过公共元件连接在一起。

The structure of this device is similar to that of two diodes connected back to back, with one crystal being common to both junctions. The center material is usually made thinner than the two outside pieces. 该器件的结构类似于背靠背连接的两个二极管的结构，一个晶体同时属于两个结。其中间的材料通常比两个外边的作得窄些。

In the operation of a single PN diode junction, forward biasing causes conduction and reverse biasing causes nonconduction. 单个PN二结的工作情况是：正向偏置促使其导通，反向偏置导致其截止。

In a transistor, this rule does not apply directly because two junctions are involved. 因为晶体管包含两个结，该规则不能直接应用

For example, when the emitter-base junction is forward biased, it causes a large amount of IE to enter the base region. 例如，其发射-基极处于正向偏置时，它会促使大量的IE 进入基区。

Reverse biasing of the base-collector junction would ordinarily restrict this current. 而基-集电极的反向偏置会限制该电流。

But due to the thin base structure, IE will immediately enter into the collector current IC or output current. 但因基区窄，IE会很快进入集极电流IC或输出电流.

Forward biasing of the emitter-base junction therefore alters or reduces the reverse biasing effect of the base-collector junction in normal transistor operation. 因而，在一般的晶体管工作中，发射-基结的正向偏置会改变或减少基-集结反向偏置的影响。

A transistor is primarily classified as a current-operating device. This means that the output or collector IC will occur only when the emitter-base is forward biased and producing base current. 晶体管被划分为基本电流型器件。这意味着仅仅当发射-基结处于正偏且有基极电流IB时才会有其输出或集电极电流IC

When the base current ceases, collector current stops and the transistor become nonconductive. This condition is called cut off. 当基极电流消失时，集电极电流停止，晶体管处于非导通状态。这种情况就称为截止。

On the other hand, if an excessive amount of base current occurs, the transistor is driven into saturation. When this condition occurs, a further increase in IB will not cause a corresponding change in IC. 相反，如果基极电流过大，晶体管处于饱和状态。在这种情况下，再增加IB不会引起IC的相应变化。

When amplitude control is being achieved, a transistor is rarely operates in the saturation region. When a transistor is used as a switch, it usually operates in the saturation region. 进行幅值控制时晶体管几乎不会处于饱和区。而当它作为开关使用时，经常工作在饱和区。

1.2 UJT Thyristors and SCRS

Thyristors should not be confused with s or field-effect transistors. 不应将晶闸管与双结型晶体管（s，BJTs）或场效应管（field-effect transistors，FETs）混淆起来.

BJTs and FETs are both capable of performing switching operations. As a rule, these devices are not as efficient as thyristor and do not have the power-handling capability. BJTs与FETs都能够完成开关操作。按照惯例，这些器件并没有晶闸管那么有效率且不具备功率（电力）处理能力。Thyristors are used as power control devices, whereas

transistors are primarily used in amplifying applications. 晶闸管通常用作电力控制器件，而晶体管主要用于信号放大。

A variety of unidirectional thyristors is now being used in industrial electronic applications. One of the first to be developed was the silicon-controlled rectifier(SCR). The term thyristor was derived form the words thyratron-transistor. 工业电子应用系统目前使用了很多种单向晶闸管。可控硅整流器（silicon-controlled rectifier, SCR）就是最初开发出的三极管之一。术语晶闸管“thyristor”源自于单词“thyratron-transistor”。

An SCR is classified as a reverse blocking triode thyristor. This means that the device has conductibility in only one direction. When its anode and cathode are reverse biased, the device will not conduct. SCR被归类为反向阻断晶闸管。这意味着该器件仅在一个方向上导通。当其阳极、负极处于反偏时，它不导通。

The silicon-controlled switch(SCS) and the programmable unijunction transistor(PUT) are two other thyristors of this classification. 可控硅开关(silicon-controlled switch, SCS)和可编程单结晶体管(programmable unijunction transistor, PUT)是该类的另外两种晶闸管。

2.1 op-amp

A person working with an op-amp does not ordinarily need to be concerned with its internal construction. It is helpful, however, to have some general understanding of what the internal circuitry accomplishes. 使用运算放大器工作的人一般并不需要关心其内部结构。然而，了解其内部电路所具备功能的一般常识是很有帮助地。

This permits the user to see how the device performs and indicates some of its limitations as a functioning unit. 这就让用户明白该器件是怎样工作的，当然也显示出其作为一个功能单元的一些局限。

The first stage or input of an op-amp is usually a differential amplifier. This amplifier has two inputs, which are labeled V1 and V2. 运算放大器的输入或者其第一级通常是一个差分放大器。该放大器有两个分别标为V1、V2的输入端。

It provides high gain of the signal difference supplied to the two inputs and low gain for common signals applied to both inputs simultaneously. 它对同时施加在两个输入端的差分、共模信号分别提供高、低增益。

The input impedance is high to any applied signal. The output of the amplifier is generally two signals of equal amplitude and 180o out of phase. This could be described as a push-pull input and output. 对于任意信号，输入阻抗均很大。它一般输出幅值相同而相位相差180o的两路信号。这可称为推拉式输入输出。

4.1 Flip-Flops

Flip-Flops are commonly used to generate signals, shape waves, and achieve division. 触发器通常用于产生信号、整形波形、实现分频。

In addition to these operations, a flip-flop may also be used as a memory device. 除此之外，它也可用作存储器件。In this capacity, it can be made to hold an output state even when the input is completely removed. It can also be made to change its output when an appropriate input signal occurs. 至于这方面，甚至当输入信号完全去除时它都能保持输出状态。当出现合适的输入信号时，也可用它来改变其输出状态。

In many digital system applications, must be set and cleared at specific times with respect to other operation circuits. 许多数字应用系统在特定时刻参照其其它运算电路须要求对触发器进行置位、清零。

This type of operation can be achieved by manipulating in step with a clock pulse. 这类操作可通过逐步控制触发器实现。

In this case, the appropriate RS inputs and clock pulse must all be present in order to cause a state change. A device of this type is called an RS triggered flip-flop or simply an RST flip-flop. 在这种情况下，须要有合适的RS输入信号和时钟脉冲以改变触发器的状态。这种器件就称为RS触发的触发器或者简称为RST触发器。

One of the most versatile and important logic devices of digital system is the counter. This device, as a general rule, can be employed to count a wide varity of objects in a number of different digital system applications. 计数器是数字系统的最通用、重要的逻辑器件之一。作为惯例，它可对许多不同数字应用系统中的大量物体进行计数。

While this device may be called upon to count an endless number of objects, it essentially counts only one thing, electronic pulses. 尽管可以说该器件能对数不清的物体进行计数，本质上，它仅仅对一种物体进行了计数，即电脉冲。

These pulses may be produced electronically by a clock mechanism, electromechanically, photoelectrically, acoustically, or by a number of other processes. The basic operation of the counter, however, is completely independent of the pulse generator. 这些脉冲可由机电式、电子光学式、声音式或许多其它形式的脉冲机构产生。然而，计数器的基本工作完全独立于这些脉冲产生器。

A common application of the digital counter is used to count numerical information in binary form. 数字计数器的一个普通应用就是对二进制形式的数字信息进行计数。

This type of device simply employs a number of connected so that the Q output of the first device drives the trigger or clock input of the next device. Each flip-flop therefore has a divide-by-2 function. 这类器件仅仅使用了大量的触发器，这些触发器顺次连接起来以便于第一个触发器的输出驱动下一个触发器的触发引脚或者时钟输入。每个触发器因此具有2分频功能。

4.2 Digital System Decoding

5.1Filter circuit

For many applications, a smooth dc output voltage with the ac ripple removed is required. 许多应用系统需要一个去除交流脉动的光滑直流电压。

ts used removed ac vatiations of rectified direct cuurent are filter circuits. 去除整流后直流电流中交流脉动的电路就称为滤波电路。

The output of a rectifier has a dc value and ac ripple value as shown in Figure 1.30. 如图1.3所示，整流器的输出包括直流成分和交流脉动成分。

To gain a relative index of the amount of ac variation, the ripple factor of the output waveform of a rectifier can be determined by 为了获得整流器输出波形中交流变化量的相对大小，其脉动系数可表示为：

Where r = ripple factor, Vr(rms) = rms value of the ac component, Vdc = average value of the rectified dc voltage.式中，r、Vr(rms)、Vdc分别表示纹波系数、交流部分的均方根值、整流后的直流电压平均值。

A different value of filter capacitor would result in a change in the rate of discharge. 滤波器的电容大小不一样会导致其放电率的变化。

If C discharged a very small amount, the value of Vdc would be closer to the value of Vmax. 如果C放电量非常小，Vdc的值更接近Vmax。

With light loads (high resistance), the capacitor filter will supply a high dc voltage with little ripple. 负载轻（大电阻）的电容滤波器能提供很小波动的高直流电压。

However, with a heavy (low resistance) load connected, the dc voltage level will drop due to a greater ripple. 然而，若负载重（小电阻），较大脉动导致了直流电压的降低。

The increased ripple is caused by the lower resist- ance discharge path for the filter capacitor. 增加的脉动是由和滤波电容形成放电通路的较小的电阻引起的。

The effect of a heavier load on the filter capacitor is shown in Figure 1.31(D). 电容滤波器带重负载的效果如图1.31(D)所示。

5.2 Direct-current Regulation

The concept of voltage regulation can be understood by refering to the formula 参照公式,可以最好地理解概念：电压调整。

where VR = voltage regulation in percent, VNL = no-load terminal voltage, and VFL = rated full-load terminal voltage 式中，VR、VNL、VFL分别表示百分比表示的电压调整、空载下的终止电压、额定负载下的终止电压

Output voltage is compared with a reference voltage by a high-gain error amplifier. Operation of the three-terminal regulator is based on the applied input voltage and developed output voltage. 三端调整器的工作基于输入电压与输出电压。高增益误差放大器将输入电压与参考电压进行比较。

Any difference in voltage will be sensed by the error amplifier. 误差放大器感知两个电压的任何变化。

The output of the error amplifier then controls conduction of the series-pass transistor. 其输出控制串联三极管的传导。

This transistor responds as a variable resistor. 它和负载电流的变化相反，以维持其输出电压在一个恒定的值。

It changes inversely with the load current to maintain the output voltage at a constant value. 该三极管在这里作为可变电阻使用。

An increase in output voltage would cause a corres- ponding increase in the conduction of the series-pass transistor. 输出电压的增大会导致串联三极管传导的相应增加。

This in turn would cause more voltage drop across the series resistor, which would lower the output voltage. 相应地，降落在串联电阻上的电压会增大，从而降低输出电压。

A decrease in output voltage would cause less conduction of the pass transistor. 输出电压的减小会导致串联三极管传导的相应降低.

Reduced current through the series resistor will cause less voltage drop. The output voltage would therefore increase in value to compensate for the decrease in voltage. 减少的电流降低了串联电阻上的电压，因而提高了输出电压

7.1 Phase Control

Shifting the phase of gate-cathode ac voltage with respect to the anode-cathode voltage can be used to control the conduction of an SCR. 参照阳极电压，移动门极交流电压的相位可以控制SCR的导通。

In phase-control applications, the shape and amplitude of the gate voltage remain unchanged while its phase is shifted with respect to time. 在相位控制应用中，参照时间移动相位时门电压的形状、大小保持不变。

Capacitors and inductors are commonly used to achieve this type of control. 通常利用电容、电感实现这种控制。Phase shifting of an ac sine wave usually involves an LR or an LC component combination. 正弦交流波的相位移动经常包括LR或者LC组件。

These components are used to alter the current and voltage relationship of the circuit. 利用这些组件改变电路的电流与电压之间的关系。

In an LR combination, inductance causes the current to lag behind the voltage by as much as 90°. 在LR组件中，电感上的电流滞后其电压90o。

Altering the resistive component of this combination causes a total circuit impedance (Z) change, which in turn produces a phase change. 改变该组件中的电阻会导致整个电路阻抗的变化，其相位相应地发生了变化。

An RC network, by comparison, causes the current to lead the coltage by as much as 90°. 比较而言，RC网络中电容上的电流超前其电压90o。

Adjusting the value of R in this type of circuit changes the total impedance, which in turn causes a phase change. 调解R的大小从而改变整个阻抗，其相位相应地发生了变化。

A common way to show the phase relationship of ac current and voltage is to represent these values by lines called phasor. 显示交流电中电流、电压相位关系的通用方法是用称为相图的线表示。

The line length of a phasor is used to show its value, while its direction is used to show phase relationships. 相图的线长代表了大小，方向代表了其相位关系。

7.2 Unijuncity transisitor pulse triggering

Pulse triggering of an SCR can be accomplishied with a unijunction transistor or UJT. SCR的脉冲触发可借助于单结晶体管（unijunction transistor, UJT）实现。

This device is commonly described as a voltage-controlled diode. 这种设备通常称为电压控制二极管。

When used to trigger an SCR into conduction, UJTs can be designed to respond to low-level signals from other inputs. 触发SCR时，UJTs常用来响应来自于其它输入端的低级别信号。

The output of photocells, thermocouples, and other transducers can be used to energize the input of a UJT, which in turn will trigger the conduction of an SCR. 光电管、热电偶以及其它变换器的输出可激活UJT的输入，相应地触发SCR，让其处于导通状态。

Through this type of circuitry, it is possible to control a large SCR current with an extremely small input signal.通过该类电路，非常小的输入信号可控制大的SCR电流。

A unijunction transistor is a three-terminal, single-junction, solid-state device that has unidirectional conductivity. UJT 是一个单向导通的三端、单结、固态器件.

As illustrated in Fig. 1.47, a small bar of N-type silicon is mounted on a ceramic base. 如图1.47所示，陶瓷基部贴有N-型硅片.

Leads attached to the silicon bar are called base 1(B1) and base 2(B2). 附着在硅片上的导线称为基1（B1）和基2（B2）。The emitter (E) is formed by fusing an wire to the opposite side of the silicon bar. 将金属导线融化在硅片的另一侧就形成了发射极。

The emitter is oriented so that it is closer to B2 than B1. 发射极的位置更靠近B2而非B1。

A PN junction is formed by the emitter and the silicon bar. 发射极和硅片间形成了PN结。

The arrow of the UJT symbol points N, which indicates that the emitter is P and the silicon bar is N material. UJT的符号箭头指向N，这表示发射极是P型材料而硅片是N型材料。

The arrow of the symbol is slanted to distinguish the UJT from the N-channel JEET symbol. 为区分N沟道JFET的符号，其符号箭头倾斜。

二 1.1 Introduction to Control System

The design of people to control nature’s forces successfully has been the catalyst for progress throughout history. 人类成功征服自然的设计一直促进着整个历史的发展、进步。

Our goal has been to control these forces in order to help perform physical tasks which were beyond our own capabilities. 我们的目标一直是控制这些自然力，以帮助我们完成我们自身无法完成的体力工作。

During the dynamic and highly motivated 20th cen-tury, the control-system engineering has transform-ed many of our hopes and dreams into reality. 在快速发展的20世纪，控制系统业界已经把我们许多希望和梦想变为现实。Control-system engineers have made very important contributions to our advancements. 控制系统工程师在这方面做出了重大贡献，

As we look back, control-system engineers have made contributions to robotics; space-vehicle systems, including the successful accomplishment of the lunar soft landing; aircraft autopilots and controls; control-systems for ships and submarines, and automatic control systems for hydrofoil, surface-effect ships, high-speed rail system; and most recently, control system for the magnetic-levitation rail systems.

控制系统工程师在机器人方面做出了重大贡献，包括成功实现月球软着陆的航空系统，飞行器的自动驾驶仪与控制器，轮船与潜艇的控制系统，以及水翼艇、航空母舰、高速列车系统等的自动控制系统和最近的磁悬浮列车系统的控制系统。

Yet all these types of control, if they are to be successful, have certain features in common. 然而，所有成功的各种控制系统具有某些共同的特征。

A control system is the collection of components connected in such a way as to effect control over certain aspects of the domain in which the system operates. 一个控制系统是一些部件的集合，这些部件按某种方式连接在一起以有效控制被控对象的某些特定方面。

In addition, Control systems exist that require no human intercation, such as aircraft automatic plots and auomobile cruise control systems. 另外，有些控制系统不需要人的参与，如飞行器的自动驾驶与汽车的巡航控制系统。

In dealing with control systems, particularly engineering control systems, we will deal with a variety of components, indicating that the subject is an interdisciplinary one. 在控制系统，特别是工程控制系统中，需要处理多种不同的部件，这意味着控制是一门交叉学科。

The control engineer needs a working knowledge of mechanics, electronics, electrical machines, fluid mechanics, thermodynamics, structures, material properties,and so on. 控制工程师需要机械、电子、电子机械、流体机械、热动力、结构、材料属性等方面的知识。

Computers are used widely to implement control schemes, and an increasing knowledge of information technology and software engineering is therefore being demanded of control engineers. 计算机广泛地用来实施控制方案，要求控制工程师们具备日益增长的信息技术和软件工程知识。

Obviously not every control system contains elements from each of the abvoe domains, but most useful control systems contain elements from more than one discipline. 显然，并不是每个控制系统的部件涉及上述所有领域，但是绝大多数有用的控制系统的部件所涉及的学科不少于上述中的一种。

2.1 Performance Specification of Control System

The design of any engineering system or component requires a specification of the job to be done, or of certain essential properties the system must have. 任何工程系统或部件的设计均需要所做工作或者系统必须具备的某些本质属性的一个指标。

This specification should be fairly precise; it is useful to know when a given design is just good enough for the job at hand since better quality almost invariably results in more complex, difficult and expensive designs. Automatic control systems are no exception.

该指标应非常精确。对于即将进行的工作，它有助于让人们了解给定设计在何时才做得足够好，而更好的质量总是会导致更复杂、困难、昂贵的设计。自动控制系统也不例外。

Control behavior of scalar feedback systems involves both steady-state and transient responses. 反馈系统的控制特性包含稳态响应和瞬态响应两种。

The same two categories are traditionally used for the performance specfications of feedback systems. 习惯上将这两类用于反馈系统的性能指标。

Steady-state performance of a feedback system is generally described in terms of stability and accuracy. 反馈系统的稳态性能一般用术语稳定性和精度来描述。The degree of stability is an extremely important part of describing a control system’s performance criteria. 稳定度是描述一个控制系统性能标准的一个极为重要的部分。

A feedback control system must be stable even when the system is subjected to command signals, extraneous inputs anywhere within the loop, power-supply variations and changes in parameters of the feedback loop. 甚至当系统受控于指令信号时，无论环内任何处的外部输入，还是电源的变化与反馈环内参数的改变，反馈控制系统必须是稳定的。

In the time-domain, system order refers to the highest derivative of the controlled quantity in the equation describing the control system’s dynamics. System order is very significant param eter for characterizing a system. 在时域中，系统阶数指的是描述系统动态方程中受控量的最高阶导数。系统阶数是表征系统的非常重要的参数。

2.2 Second-Order Systems

From the frequency-domain viewpoint, system order refers to the highest power of s in the denominator of the closed- loop transfer function of a system. 从频域的角度，系统阶数指的是系统闭环传递函数的分母中s的最高幂指数。Second-Order Systems are very important to the control-system engineering. 对于从控制系统工程，二阶系统非常重要。

This type of system characterizes the dynamics of many control-system applications found in the fields of servomechanisms, space-vehicle control, chemical process control, bioengineering, aircraft control system, ship controls, etc. 这类系统表征了现有的伺服机构、太空交通工具控制、化学过程控制、生物工程、飞行器控制系统、轮船控制等许多应用控制系统的动态。

It is interesting to note that most control system designs are based on second order system analysis. 有趣地发现，绝大多数控制系统的设计基于二阶系统分析。

Even if the system is of higher order , as it usually is, the system may be approximated by a second-order system in order to obtain a first approximation for preliminary design purposes with reasonable accuracy. 甚至如果系统阶数更高，如同通常那样，系统可由二阶系统进行近似以获得具有相当精度的初步设计的一个最初估计。

A more exact solution can then be obtained in terms of departures from the performance of a second-order system. （然后）从二阶系统的性能着手，可以获得更精确的解决方案。

5.1 Digital Control System

It has been noted already that digital electronics and digital computers play a vital role in modern control engineering. 众所周知，数字电子和数字计算机在当今控制工程中扮演着非常重要的角色。

Sophisticated microprocessor-based control systems are incorporated into machine tools, industrial robots, and domestic audio equipment. 复杂的基于微处理器的控制系统将机床、工业机器人和市内音响设备整合在一起。This unit is concerned with a class of feedback control system in which the signal at one or more points appears as a

train of pulses rather than as a continuous function. 它与一类在一点或多点以脉冲而非连续函数方式出现轨迹信号的反馈控制系统有关。

Such a system is known as a Digital Control System, in contrast to a continuous control system, where the signal is continuous everywhere. 相比于始终为连续信号的连续控制系统，该系统就是数字控制系统。

Consider Fig. 2.25, which represents a general, single-loop and digital control scheme. 图2.25表示了一种通用的单环数控方案。

Here the measured value of the controlled variable is converted at regular intervals into a digital representation by means of an analogue-to-digital converter(ADC). 其中，受控量的测量值借助于模/数转换器（ADC）转换成数字量。

These digitized samples of the controlled variable are made available to the digital processor, which uses them, together with supplied information about the desired output, to compute the necessary control action. 这些数字量通常被转换成数字处理器可接受的形式。连同期望输出对应的输入信息，处理器计算出需要的控制指令。

The processor may be implemented either in hardware, or as a computer program. 处理器可由硬件实现，也可由计算机程序实现。

The output of the processor may be viewed as a sequence of numerical values which, after conversion to analogue form by means of a digital-to-analogue converter(DAC), provides the control action drive the plant. 处理器的输出可看作一序列的数字值，之后借助于数/模转换器将之转换成模拟信号以控制设备的驱动。

6.1 State Equation

Designing, stabilizing, and compensating such a system is obviously more complex than a SISO system, and the problem becomes more difficult as the number of inputs and outputs increase. 设计、稳定和折衷协调这样一个系统比一个单输入单输出系统明显复杂得多。由于输入输出量的增加，这些问题也变得更困难。

Clearly, analyzing this system using classical meth-ods would be very tedious and error prone. 显然，利用传统方法分析该系统将非常单调而且易出错。An alternative approach for handing MIMO syst-ems is in common use and is uaually referred to as modern control theory. 通常采用现代控制理论处理多输入多输出系统。

Its principle advantages are that it is easily implemented on a digital computer, it handles nonlinear systems and time-varying systems, and it provides additional insights into system behavior that transfer function analysis does not. 现代控制理论最基本的优点在于它易于在计算机上实现，能处理非线性系统和时变系统，能额外提供传递函数分析所不及的系统内部行为的洞察。

It should be pointed out that,except for simple systems, modern control theory relies heavily on computers, and the design of system controllers may appear somewhat abstract. 应该指出，除了简单系统，现代控制理论严重地依赖于计算机，而且系统的设计也显得有点深奥。

Even so these methdos do offer powerful tools for ana-lyzing not just MIMO systems, but SISO systems too. 这些方法的确不仅为分析多输入多输出系统提供了强大的工具，而且对于单输入单输出系统，也是如此。

The basis of modern control theory is the concept of state and of state variables:现代控制理论的基础是状态以及状态变化的概念：

State variables maybe defined as the minimum set of variables such that a knowledge of the values of these variables at time t0, together with any inputs u to the system for t≥t0, will allow the behavior of the system to be determined for t ≥t0. 状态变量可定义为变量的最小集合，以便于对这些变量在时间t0时的值的理解连同t > t0时施加到系统的任意输入确定系统t > t0时的行为。

三 1.2 The Application of computer

For the most part, human beings can do whatever computers can do, but computers can do it with much greater speed and accuracy. This is in spite of the fact that computers perform all their calculations and operations one step at a time. 在绝大多数情况下，人类能做计算机所作的任何事。尽管计算机在任何时刻只能完成其所有计算或操作的一步，但它做得比人类更快速，更精确。

For example, a human being can take a list of 10 numbers and find their sum all in one operation by listing the numbers one over the other and adding them column by column. 例如，人类通过列表、逐列（位）相加，通过一步运算能

把10个数据列在一起并得到所有数据的累加和。

A computer, on the other hand, can add numbers only two at a time, so adding this same list of numbers will take nine actual addition steps. 而计算机在任何时刻只能将两个数相加，因而同样的10个数据需要作9步加法操作。

Of course, the fact that the computer requires only less than a microsecond per step makes up for this apparent inefficiency. 当然，计算每步仅需不到1微妙的时间弥补了其显而易见的低效。

A computer is faster and more accurate than people, but unlike most people it has to be given a complete set of instructions that tell it exactly what to do at each step of its operation. 计算机比人类工作的快、准确，但并不像绝大多数人，它需要一套告诉它每步该做什么的完整指令集。

This set of instructions ,called a program, is pre-pared by one or more persons for each job the computer is to do. 这套指令集，通常也成为程序，由一人或者多人准备。

These programs are placed in the computer’s memory unit in binary-coded form, with each instruction having a unique code. 这些程序以二进制码的形式安放在计算机的存储单元中，每条指令对应于一个独特的代码。

The computer takes these instruction codes from memory one at a time and performs the operation called for by the code. 计算机从存储器中获得指令代码并完成代码所对应的操作。Much more will be said on this later. 关于这方面，之后将进行更多的探讨。

四2.1 DC Motors

A dc motor is a dc generator with the power flow reversed. 一台直流电动机就是一台具有功率可逆的直流发电机。In the dc motor electrical energy is converted to mechanical form. 在直流电动机中，电能转换成机械能。

There are three types of dc motor: the shunt motor, the cumulatively compounded motor, and the series motor. 直流电动机共有3种：并励电动机、复励电动机和串励电动机。

The compound motor is prefixed with the word cumulative in order to stress that the connections to the series-field winding are such as to assure that the series-field flux aids the shunt-field flux. 混合电动机被附以前缀单词“cumulative”的目的是为了强调连到串磁场绕组上的连接是为了确保串励绕组上的磁通能有助于并励磁场的磁通。

The series motor, unlike the series generator, finds wide application, especially for traction-type loads. 不象并励直流发电机，并励电动机得到了广泛的应用，特别对于牵引类型的负载。Hence due attention is given to this machine in the treatment that follows. 因此，论文之后将对它进行探讨。

The performance of the dc motor operating in any one of its three modes can conveniently be described in terms of an equivalent circuit, a set of performance equations. 运行在三种模式中的任何一种模式下的直流电动机的性能可以很方便地由等价电路、性能方程集描述。

The equivalent circuit is depicted in Fig.4.6. 等价电路如图4.6所示。

It is worthwhile to note that now the armature induced voltage is treated as a reaction or counter emf. 将电枢的感应电压作为反作用或者反电动势非常有价值。

By imposing constrains similar to those applied to the dc generator, we obtain the correct equivalent circuit for the desired mode of operation. 类似于直流发电机，通过强加约束条件，就可以正确获得期望工作模式下的等价电路。For example, for a series motor the appropriate equivalent circuit results upon removing Rf from the circuit of Fig. 4.6. 例如，对于串励电动机，通过去掉图中的Rf即可获得合适的等价电路。

The armature winding is replaced by a source voltage having the induced emf Ea and a resistance Ra, which represents the armature circuit resistance. 电枢绕组由具有感应电动式Ea和代表电枢电阻Ra的电压源代替。The series field is replaced by its resistance Rs, likewise for the shunt field. 串励场由其电阻Rs替代。同样地，可以得到并励直流电动的等效电路。