外文原文
Pulse-width modulation
Pulse-width modulation (PWM)is a modulation technique that conforms the width of the pulse, formally the pulse duration, based on modulator signal information. Although this modulation technique can be used to encode information for transmission, its main use is to allow the control of the power supplied to electrical devices, especially to inertial loads such as motors. In addition, PWM is one of the two principal algorithms used in photovoltaic solar battery chargers,[1]
The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.
The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove, 120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of on time to the regular interval or period of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.
The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM also works well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.
PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
History
In the past, when only partial power was needed (such as for a sewing machine motor), a rheostat (located in the sewing machines foot pedal) connected in series with the motor adjusted the amount of current flowing through the motor, but also wasted power as heat in the resistor element. It was an inefficient scheme, but tolerable because the total power was low. This was one of several methods of controlling power. There were others—some still in use—such as variable autotransformers, including thetrademarked Autrastat for theatrical lighting; and the Variac, for general AC power adjustment. These were quite efficient, but also relatively costly.
For about a century, some variable-speed electric motors have had decent efficiency, but they were somewhat more complex than constant-speed motors, and sometimes required bulky external electrical apparatus, such as a bank of variable power resistors or rotating converter such as Ward Leonard drive.
However, in addition to motor drives for fans, pumps and robotic servos, there was a great need for compact and low cost means for applying adjustable power for many devices, such as electric stoves and lamp dimmers.
One early application of PWM was in the Sinclair X10, a 10 W audio amplifier available in kit form in the 1960s. At around the same time PWM started to be used in AC motor control.
Fig. 1: a pulse wave, showing the definitions of , and D.
Pulse-width modulation uses a rectangular pulse wave whose pulse width is modulated resulting in the variation of the average value of the waveform. If we consider a pulse waveform , with period , low value , a high value and a duty cycle D (see figure 1), the average value of the waveform is given by:
As is a pulse wave, its value is for and for . The above expression then becomes:
This latter expression can be fairly simplified in many cases where as . From this, it is obvious that the average value of the signal () is directly dependent on
the duty cycle D
Fig. 2: A simple method to generate the PWM pulse train corresponding to a given signal is the intersective PWM: the signal (here the red sinewave) is compared with a sawtooth waveform (blue). When the latter is less than the former, the PWM signal (magenta) is in high state (1). Otherwise it is in the low state (0).
The simplest way to generate a PWM signal is the intersective method, which requires only a sawtooth or atriangle waveform (easily generated using a simple oscillator) and a comparator. When the value of the
剩余内容已隐藏，支付完成后下载完整资料
中文翻译
一、脉冲宽度调制
脉冲宽度调制（PWM），是一种在一定的脉冲持续时间内，基于调制信号来追踪所希望达到的脉冲宽度的调制方式。虽然这种调制技术经常用于对传输信息进行编码，但是它主要的用途是控制电源装置供电到电气设备，特别是对惯性负载的供电，如电动机等。此外，PWM还是光伏太阳能电池充电器中使用的两种主要算法之一。
通过快速的转换对电源和负载之间的开关的开断进行控制，将电压的平均值（和电流）供给到负载。与开关较长的打开时相比，关断期间供给到负载的功率较高。
PWM的开关频率必须要高于负载的启动频率，也就是说，要使装置在有用功区工作。通常在电炉中，开关会在一分钟内多次切换，在一盏灯光衰减器中会达到120Hz，从几千赫兹（kHz）到几十千赫兹的电机驱动器和顺利进入几十或几百kHz的音频放大器和电脑电源供应器。
占空比描述的是持续开启时间与控制周期之比；低占空比时，系统功耗比较低，因为开关大部分时间是关闭的。工作周期用百分比表示，100%是完全工作状态。
PWM的主要优点是，开关器件的功率损耗非常低。当开关处于关闭状态时，可以说没有电流通过；当它打开时，整个开关都没有电压降。因为系统中的功率损耗等于电压和电流的乘积，因此，在这两种情况下系统的功率损耗都接近零。同时PWM技术是通过数字来控制，即通过开关性质的变化来控制，我们可以很方便地设置所需的占空比，使达到的效果更好。
PWM技术也被用在某些通信系统中，其中它的占空比被用来描述通过通信信道传播信息的比例。
二、历史发展
在过去，当只有部分功率是有需求的（例如，对于一个缝纫机马达）时候，一个可变电阻器串联连接的电动机（位于缝纫机的脚踏板）由此产生，它是用来调节流过电机的电流的大小，但电阻元件产生的热量也浪费了很多电源功率。这是一个低效率的方案，但是因为总功率同样很低，所以它也被人所接收。 这是控制功率的几种方法之一。还有其他的一些方案仍然在使用，如仍在使用可变自耦变压器的商标为lsquo;Autrastatrsquo;的舞台灯光; 和适用于一般的交流功率调节的自耦变压器。这些都是很有效的设计，但成本也比较昂贵。
大约一个世纪前，一些变速电动机有相当高的工作效率，但他们中有些比恒转速电机更复杂，有些需要体积较大的外部电气设备，如可变功率电阻堆栈或旋转转换器，病房伦纳德驱动等。
然而，除了用于风机，泵和机器人伺服系统的电机驱动模块，人们对于如何能更加简洁和低成本的应用于很多可调节功率的设备，还是有很大的需求，如电炉和灯调光器。
PWM的一个早期应用是在辛克莱的X10。在20世纪60年代，一个10瓦的音频放大器应用了这项技术。大约在同一时间，PWM开始在交流电机控制中被使用。
三、原理
图1：一个脉冲波 ，表现出的定义 ， 和D。
脉冲宽度调制采用了矩形脉冲波 ，其脉冲宽度被调制为可以形成变化的平均波形的值。如果我们考虑的脉冲波形 ，以周期 ，低电平 ，高电平 和一个占空比 D（见图1），该波形的平均值由下式给出：
如果 是脉冲波，则在 时间内，它的值为 ；在 时间内，它的值为。 上面的表达式就变成了：
在许多情况下，后面的式子还可以简化，比如在 时， 。 由此，显而易见的是该信号的平均值（ ）与占空比D有直接的参数关系。
图 2：交互式PWM是一种简单的产生对应于给定信号的PWM脉冲波形的方法：用所述信号（这里为红色正弦波）与锯齿波形（蓝色）进行比较。 当后者小于前者，PWM信号（品红色）处于高电平（1）。 否则，它处于低电平（0）。
产生PWM信号的最简单的方法是交互性方法，该方法仅需要一个锯齿或三角形波（使用振荡器容易生成）和一个比较器。当参考信号（图2中的红色正弦波）的值大于所述调制波形（蓝色）时，PWM信号（品红）是在高电平的状态，否则它是在低电平的状态。
四、时间比例
许多数字电路可以产生PWM信号（例如，许多微控制器具有PWM输出端口）。人们通常使用一个周期性的递增计数器（它直接或间接地连接到资料编号：[146835]，资料为PDF文档或Word文档，PDF文档可免费转换为Word
以上是毕业论文外文翻译，课题毕业论文、任务书、文献综述、开题报告、程序设计、图纸设计等资料可联系客服协助查找。
