外文翻譯 關于PWM的外文翻譯.doc

大連交通大學2014屆本科生畢業(yè)論文外文翻譯外文原文Pulse-width modulationPulse-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.HistoryIn 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 otherssome still in usesuch 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 onthe duty cycle DFig. 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 reference signal (the red sine wave in figure 2) is more than the modulation waveform (blue), the PWM signal (magenta) is in the high state, otherwise it is in the low state.Time proportioningMany digital circuits can generate PWM signals (e.g., many microcontrollers have PWM outputs). They normally use a counter that increments periodically (it is connected directly or indirectly to the clock of the circuit) and is reset at the end of every period of the PWM. When the counter value is more than the reference value, the PWM output changes state from high to low (or low to high).3 This technique is referred to as time proportioning, particularly as time-proportioning control4 which proportion of a fixed cycle time is spent in the high state.The incremented and periodically reset counter is the discrete version of the intersecting methods sawtooth. The analog comparator of the intersecting method becomes a simple integer comparison between the current counter value and the digital (possibly digitized) reference value. The duty cycle can only be varied in discrete steps, as a function of the counter resolution. However, a high-resolution counter can provide quite satisfactory performance.PWM sampling theoremThe process of PWM conversion is non-linear and it is generally supposed that low pass filter signal recovery is imperfect for PWM. The PWM sampling theorem6 shows that PWM conversion can be perfect. The theorem states that Any bandlimited baseband signal within 0.637 can be represented by a pulsewidth modulation (PWM) waveform with unit amplitude. The number of pulses in the waveform is equal to the number of Nyquist samples and the peak constraint is independent of whether the waveform is two-level or three-level.Power deliveryPWM can be used to control the amount of power delivered to a load without incurring the losses that would result from linear power delivery by resistive means. Potential drawbacks to this technique are the pulsations defined by the duty cycle, switching frequency and properties of the load. With a sufficiently high switching frequency and, when necessary, using additional passive electronic filters, the pulse train can be smoothed and average analog waveform recovered.High frequency PWM power control systems are easily realisable with semiconductor switches. As explained above, almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states, both voltage and current are nonzero and thus power is dissipated in the switches. By quickly changing the state between fully on and fully off (typically less than 100 nanoseconds), the power dissipation in the switches can be quite low compared to the power being delivered to the load.Modern semiconductor switches such as MOSFETs or Insulated-gate bipolar transistors (IGBTs) are well suited components for high efficiency controllers. Frequency converters used to control AC motors may have efficiencies exceeding 98%. Switching power supplies have lower efficiency due to low output voltage levels (often even less than 2 V for microprocessors are needed) but still more than 7080% efficiency can be achieved.Variable-speed fan controllers for computers usually use PWM, as it is far more efficient when compared to a potentiometer or rheostat. (Neither of the latter is practical to operate electronically; they would require a small drive motor.)Light dimmers for home use employ a specific type of PWM control. Home-use light dimmers typically include electronic circuitry which suppresses current flow during defined portions of each cycle of the AC line voltage. Adjusting the brightness of light emitted by a light source is then merely a matter of setting at what voltage (or phase) in the AC halfcycle the dimmer begins to provide electrical current to the light source (e.g. by using an electronic switch such as a triac). In this case the PWM duty cycle is the ratio of the conduction time to the duration of the half AC cycle defined by the frequency of the AC line voltage (50 Hz or 60 Hz depending on the country).Voltage regulationMain article: Switched-mode power supplyPWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.Audio effects and amplificationPWM is sometimes used in sound (music) synthesis, in particular subtractive synthesis, as it gives a sound effect similar to chorus or slightly detuned oscillators played together. (In fact, PWM is equivalent to the difference of two sawtooth waves with one of them inverted.1) The ratio between the high and low level is typically modulated with a low frequency oscillator. In addition, varying the duty cycle of a pulse waveform in a subtractive-synthesis instrument creates useful timbral variations. Some synthesizers have a duty-cycle trimmer for their square-wave outputs, and that trimmer can be set by ear; the 50% point (true square wave) was distinctive, because even-numbered harmonics essentially disappear at 50%. Pulse waves, usually 50%, 25%, and 12.5%, make up the soundtracks of classic video games.A new class of audio amplifiers based on the PWM principle is becoming popular. Called Class-D amplifiers, they produce a PWM equivalent of the analog input signal which is fed to the loudspeaker via a suitable filter network to block the carrier and recover the original audio. These amplifiers are characterized by very good efficiency figures ( 90%) and compact size/light weight for large power outputs. For a few decades, industrial and military PWM amplifiers have been in common use, often for drivingservo motors. Field-gradient coils in MRI machines are driven by relatively high-power PWM amplifiers.Historically, a crude form of PWM has been used to play back PCM digital sound on the PC speaker, which is driven by only two voltage levels, typically 0 V and 5 V. By carefully timing the duration of the pulses, and by relying on the speakers physical filtering properties (limited frequency response, self-inductance, etc.) it was possible to obtain an approximate playback of mono PCM samples, although at a very low quality, and with greatly varying results between implementations.In more recent times, the Direct Stream Digital sound encoding method was introduced, which uses a generalized form of pulse-width modulation called pulse density modulation, at a high enough sampling rate (typically in the order of MHz) to cover the whole acoustic frequencies range with sufficient fidelity. This method is used in the SACD format, and reproduction of the encoded audio signal is essentially similar to the method used in class-D amplifiers. 中文翻譯一、脈沖寬度調制 脈沖寬度調制（PWM），是一種在一定的脈沖持續(xù)時間內，基于調制信號來追蹤所希望達到的脈沖寬度的調制方式。雖然這種調制技術經(jīng)常用于對傳輸信息進行編碼，但是它主要的用途是控制電源裝置供電到電氣設備，特別是對慣性負載的供電，如電動機等。此外，PWM還是光伏太陽能電池充電器中使用的兩種主要算法之一。通過快速的轉換對電源和負載之間的開關的開斷進行控制，將電壓的平均值（和電流）供給到負載。與開關較長的打開時相比，關斷期間供給到負載的功率較高。PWM的開關頻率必須要高于負載的啟動頻率，也就是說，要使裝置在有用功區(qū)工作。通常在電爐中，開關會在一分鐘內多次切換，在一盞燈光衰減器中會達到120Hz，從幾千赫茲（kHz）到幾十千赫茲的電機驅動器和順利進入幾十或幾百kHz的音頻放大器和電腦電源供應器。占空比描述的是持續(xù)開啟時間與控制周期之比；低占空比時，系統(tǒng)功耗比較低，因為開關大部分時間是關閉的。工作周期用百分比表示，100%是完全工作狀態(tài)。PWM的主要優(yōu)點是，開關器件的功率損耗非常低。當開關處于關閉狀態(tài)時，可以說沒有電流通過；當它打開時，整個開關都沒有電壓降。因為系統(tǒng)中的功率損耗等于電壓和電流的乘積，因此，在這兩種情況下系統(tǒng)的功率損耗都接近零。同時PWM技術是通過數(shù)字來控制，即通過開關性質的變化來控制，我們可以很方便地設置所需的占空比，使達到的效果更好。PWM技術也被用在某些通信系統(tǒng)中，其中它的占空比被用來描述通過通信信道傳播信息的比例。二、歷史發(fā)展在過去，當只有部分功率是有需求的（例如，對于一個縫紉機馬達）時候，一個可變電阻器串聯(lián)連接的電動機（位于縫紉機的腳踏板）由此產(chǎn)生，它是用來調節(jié)流過電機的電流的大小，但電阻元件產(chǎn)生的熱量也浪費了很多電源功率。這是一個低效率的方案，但是因為總功率同樣很低，所以它也被人所接收。 這是控制功率的幾種方法之一。還有其他的一些方案仍然在使用，如仍在使用可變自耦變壓器的商標為Autrastat的舞臺燈光; 和適用于一般的交流功率調節(jié)的自耦變壓器。這些都是很有效的設計，但成本也比較昂貴。大約一個世紀前，一些變速電動機有相當高的工作效率，但他們中有些比恒轉速電機更復雜，有些需要體積較大的外部電氣設備，如可變功率電阻堆棧或旋轉轉換器，病房倫納德驅動等。然而，除了用于風機，泵和機器人伺服系統(tǒng)的電機驅動模塊，人們對于如何能更加簡潔和低成本的應用于很多可調節(jié)功率的設備，還是有很大的需求，如電爐和燈調光器。PWM的一個早期應用是在辛克萊的X10。在20世紀60年代，一個10瓦的音頻放大器應用了這項技術。大約在同一時間，PWM開始在交流電機控制中被使用。 三、原理 圖1：一個脈沖波 ，表現(xiàn)出的定義 ， 和D。 脈沖寬度調制采用了矩形脈沖波 ，其脈沖寬度被調制為可以形成變化的平均波形的值。如果我們考慮的脈沖波形 ，以周期 ，低電平 ，高電平 和一個占空比 D（見圖1），該波形的平均值由下式給出：如果 是脈沖波，則在 時間內，它的值為 ；在 時間內，它的值為。 上面的表達式就變成了：在許多情況下，后面的式子還可以簡化，比如在 時， 。 由此，顯而易見的是該信號的平均值（ ）與占空比D有直接的參數(shù)關系。 圖 2：交互式PWM是一種簡單的產(chǎn)生對應于給定信號的PWM脈沖波形的方法：用所述信號（這里為紅色正弦波）與鋸齒波形（藍色）進行比較。 當后者小于前者，PWM信號（品紅色）處于高電平（1）。 否則，它處于低電平（0）。產(chǎn)生PWM信號的最簡單的方法是交互性方法，該方法僅需要一個鋸齒或三角形波（使用振蕩器容易生成）和一個比較器。當參考信號（圖2中的紅色正弦波）的值大于所述調制波形（藍色）時，PWM信號（品紅）是在高電平的狀態(tài)，否則它是在低電平的狀態(tài)。四、時間比例許多數(shù)字電路可以產(chǎn)生PWM信號（例如，許多微控制器具有PWM輸出端口）。人們通常使用一個周期性的遞增計數(shù)器（它直接或間接地連接到時鐘電路）來產(chǎn)生PWM信號，并且在每個PWM周期結束之后對計數(shù)器進行復位。 當計數(shù)器值大于參考值，則PWM輸出改變狀態(tài)從高向低（或者從低到高）。 這種技術被稱為時間比例，特別是控制一個固定周期中處于高電平狀態(tài)的時間比例時，這種技術也被稱為時間比例控制法。不斷以周期性遞增的復位計數(shù)器產(chǎn)生的離散的交叉鋸齒波。交叉方法的模擬比較器將當前計數(shù)器的值與數(shù)字參考值做了一個全面的比較。由計數(shù)器分辨率的函數(shù)可知，占空比僅在離散時間內變化。然而，高分辨率計數(shù)器同樣可以產(chǎn)生令人相當滿意的效果。五、PWM采樣定理PWM轉換的過程是非線性的，一般假設PWM中的低通濾波器的信號恢復過程是不完善的。 PWM的采樣定理表明，PWM轉換可以是完美的。該定理指出：任何有限基帶信號0.637都可通過脈沖寬度調制單元的振幅（PWM）波形來表示。脈沖波形的數(shù)量等于奈奎斯特的采樣數(shù)量，并且對峰值的約束不依賴于波形是二級還是三級。六、電力輸送PWM可被用于不產(chǎn)生由于通過電阻裝置的線性功率輸送而產(chǎn)生的損耗時，對傳輸?shù)截撦d的功率進行控制。其潛在的缺點是，該技術是用占空比來約束脈動，開關頻率和負載的性質。具有足夠高的開關頻率，并在需要使用額外的無源電子濾波器時，脈沖