第一篇：中英文翻譯標語

No trespassing!Violators will be shot;survivors will be shot again.嚴禁擅闖！違者將被射殺；幸存者將被再次射殺。

Those who throw objects at crocodiles will be asked to retrive them.向鱷魚丟東西的人將被要求把東西取回。

Approaching end of the motorway.即將駛出高速。

“報案中心”Police Report Centre of X X City

“您先請”則翻譯成“After you”

Staffonly 閑人勿進。

No littering 請勿亂投垃圾。

Keep right靠右駕駛。

Mind your head 小心碰頭！

Disabled Only 殘疾人專用。

Temporarily Closed 暫停服務(wù)。

Danger!High Voltage 止步！高壓危險！

Keep off the Grass.請勿踐踏草坪。

Please use handrail 請您扶好站好。

No Flash Photography 請勿使用閃光燈。

Fire escape, keep clear.安全出口，保持暢通。

Caution!Slippery When Wet 道路濕滑請小心！

Watch Your Step/ Mind the Step 下臺階請你小心。

All cycles will be removed 此處禁放自行車，違者拖走車輛。

Take Care of Your Belongings 請保管好您隨身攜帶的物品。

Fragile!Handle with Care 易碎品，請輕拿輕放。

Visitors Please Register At Office 來賓請到辦公室登記。

Children under 1 meter must be accompanied by an adult 1米以下的兒童須有成年人的陪同。Private parking.Vehicles left at owner's risk 私人停車處，違者后果自負。

Please close this gate securely on entry and exit 出入時請安全關(guān)閉此門。

Cycles are left at the owner's risk.National Maritime Museum accept no responsibility for damage or theft 請勿停放自行車，否則后果自負。國家海洋博物館不為損壞或丟失負責。

※注意身體：（錯誤）notice your bodybody 有‘遺體’之意

(正確)pay attention to your health

※我一點也不放松自己（絲毫不敢怠慢的意思）

（錯誤）I daren't loose myself(我不敢松開某樣?xùn)|西)

(正確)I daren't relax my efforts.就業(yè)問題（錯誤）employment problem(正確)unemployment

新學(xué)期已過了一個月了（錯誤）Our new term has passed one month.(正確)One month has passed since the new term began.錯welcome to our bus(again)

對thank you for riding with us

we are would be pleased to have you back in the future.錯welcome to visit my blog

對I welcome you to my blog

縮略形式 welcome to my blog（to 是介詞）

錯誤翻譯--Scattering(散播)--緊急出口

deformed man toilet（丑陋人洗手間）--殘疾人洗手間

green hand--生手，沒有經(jīng)驗

Jack is a green hand at doing this work.1.Let's go and eat.Let's go and eat our meals.(CE-Chinese Englis)

2.We were quite disappointed that he said only a few words.He only said a few sentences.He made us very disappointed.(CE)

3.He went out for a walk after supper.After supper he went out for a walk.(CE)

英語族人兩種說法都有，但中國人更趨向于后者。

4.It is Sunday today.Today is Sunday.(CE)

5.We found it impossible to get the result.We found that to get the result was impossible.(CE)

6.I was told.Some people told me.(CE)

7.He met with another Waterloo in his ambition.He got another defeat inhis ambition.(CE)

這涉及文化背景的差異。

第二篇：中英文翻譯的標語

只有勇于承擔責任，才能承擔更大的責任。

Only dare to take responsibility can bear greater duty

同德才能走得更近，同心才能走得更遠。

Same morality can move closer, same heart can go further.細微之處見公德，舉手之間顯文明。

Morality sees from subtleties, civilization shows between hands.員工以廠為榮，企業(yè)以人為本。

Employees to be proud of factory, enterprise to be oriented with people.愛護公物，珍惜資源；勤儉節(jié)約，共同發(fā)展。

Care for public property, cherish resources;diligent and thrifty, and mutual development.爭做一流員工，共造一流產(chǎn)品，同創(chuàng)一流企業(yè)。

To be the best staff, building the best products, creating the beat enterprise.檢查走馬觀花，事故遍地開花。

Check skim the surface, accident happens everywhere.安全是生命之本，違章是事故之源。

Safety is the soul of life, violation is the source of accident.簡化作業(yè)省一時，貪小失大苦一世。

Simplification work save a little time, covet little but lose big bitter the whole life.安全生產(chǎn)莫僥幸，違章操作要人命。

Safety production without fluke, peccancy operation kills life.按章操作莫亂改，合理建議提出來。

Operation chapter not modify , reasonable Suggestions put forward.多點溝通，少點抱怨；多點理解，少點爭執(zhí)。

Multipoint communication with less complaining;More understanding with less dispute.每天自我檢討，品質(zhì)自然更好。

Daily review, better quality.累積點滴改進，邁向完美品質(zhì)。

A little improvement lead to a perfect quality.品質(zhì)是生產(chǎn)出來的，不是靠檢驗出來的。

Good quality is produced not by inspection.零缺點的生產(chǎn)過程，一百分的優(yōu)質(zhì)產(chǎn)品。

production process with zero defect, products with top quality.人人有改善的能力，事事有改善的余地。

Everyone has the ability to improve, everything has the room to improve.時時尋求效率進步，事事講究方法技術(shù)。

Always seeking efficiency improvement, everything stress method and techniques 預(yù)防保養(yǎng)按時做，生產(chǎn)順暢不會錯。

Do preventive maintenance on time, the smooth flow of production won't wrong.質(zhì)量為先，信譽為重，管理為本，服務(wù)為誠。

Quality first, reputation is important,morality is the management, honesty is the service.產(chǎn)品就像一朵花，枝繁葉茂靠大家。

The product is like a flower, the flourishing depends on everybody.品質(zhì)是生命，服務(wù)是宗旨。

Quality is life, service is the tenet.全民講質(zhì)量，質(zhì)量利全民。

Everybody pay attention to quality, quality benefit to all people.品質(zhì)觀念把握好，成品出貨不苦惱。

Grasp the concept of high quality , finished product delivery without upset

品質(zhì)一馬當先，業(yè)績遙遙領(lǐng)先。

Quality takes the lead, achievement goes far head.

第三篇：中英文翻譯

Fundamentals This chapter describes the fundamentals of today’s wireless communications.First a detailed description of the radio channel and its modeling are presented, followed by the introduction of the principle of OFDM multi-carrier transmission.In addition, a general overview of the spread spectrum technique, especially DS-CDMA, is given and examples of potential applications for OFDM and DS-CDMA are analyzed.This introduction is essential for a better understanding of the idea behind the combination of OFDM with the spread spectrum technique, which is briefly introduced in the last part of this chapter.1.1 Radio Channel Characteristics Understanding the characteristics of the communications medium is crucial for the appropriate selection of transmission system architecture, dimensioning of its components, and optimizing system parameters, especially since mobile radio channels are considered to be the most difficult channels, since they suffer from many imperfections like multipath fading, interference, Doppler shift, and shadowing.The choice of system components is totally different if, for instance, multipath propagation with long echoes dominates the radio propagation.Therefore, an accurate channel model describing the behavior of radio wave propagation in different environments such as mobile/fixed and indoor/outdoor is needed.This may allow one, through simulations, to estimate and validate the performance of a given transmission scheme in its several design phases.1.1.1 Understanding Radio Channels In mobile radio channels(see Figure 1-1), the transmitted signal suffers from different effects, which are characterized as follows: Multipath propagation occurs as a consequence of reflections, scattering, and diffraction of the transmitted electromagnetic wave at natural and man-made objects.Thus, at the receiver antenna, a multitude of waves arrives from many different directions with different delays, attenuations, and phases.The superposition of these waves results in amplitude and phase variations of the composite received signal.Doppler spread is caused by moving objects in the mobile radio channel.Changes in the phases and amplitudes of the arriving waves occur which lead to time-variant multipath propagation.Even small movements on the order of the wavelength may result in a totally different wave superposition.The varying signal strength due to time-variant multipath propagation is referred to as fast fading.Shadowing is caused by obstruction of the transmitted waves by, e.g., hills, buildings, walls, and trees, which results in more or less strong attenuation of the signal strength.Compared to fast fading, longer distances have to be covered to significantly change the shadowing constellation.The varying signal strength due to shadowing is called slow fading and can be described by a log-normal distribution [36].Path loss indicates how the mean signal power decays with distance between transmitter and receiver.In free space, the mean signal power decreases with the square of the distance between base station(BS)and terminal station(TS).In a mobile radio channel, where often no line of sight(LOS)path exists, signal power decreases with a power higher than two and is typically in the order of three to five.Variations of the received power due to shadowing and path loss can be efficiently counteracted by power control.In the following, the mobile radio channel is described with respect to its fast fading characteristic.1.1.2 Channel Modeling The mobile radio channel can be characterized by the time-variant channel impulse response h(τ , t)or by the time-variant channel transfer function H(f, t), which is the Fourier transform of h(τ , t).The channel impulse response represents the response of the channel at time t due to an impulse applied at time t ? τ.The mobile radio channel is assumed to be a wide-sense stationary random process, i.e., the channel has a fading statistic that remains constant over short periods of time or small spatial distances.In environments with multipath propagation, the channel impulse response is composed of a large number of scattered impulses received over Np different paths，Where

and ap, fD,p, ?p, and τp are the amplitude, the Doppler frequency, the phase, and the propagation delay, respectively, associated with path p, p = 0,..., Np ? 1.The assigned channel transfer function is

The delays are measured relative to the first detectable path at the receiver.The Doppler Frequency

depends on the velocity v of the terminal station, the speed of light c, the carrier frequency fc, and the angle of incidence αp of a wave assigned to path p.A channel impulse response with corresponding channel transfer function is illustrated in Figure 1-2.The delay power density spectrum ρ(τ)that characterizes the frequency selectivity of the mobile radio channel gives the average power of the channel output as a function of the delay τ.The mean delay τ , the root mean square(RMS)delay spread τRMS and the maximum delay τmax are characteristic parameters of the delay power density spectrum.The mean delay is

Where

Figure 1-2 Time-variant channel impulse response and channel transfer function with frequency-selective fading is the power of path p.The RMS delay spread is defined as Similarly, the Doppler power density spectrum S(fD)can be defined that characterizes the time variance of the mobile radio channel and gives the average power of the channel output as a function of the Doppler frequency fD.The frequency dispersive properties of multipath channels are most commonly quantified by the maximum occurring Doppler frequency fDmax and the Doppler spread fDspread.The Doppler spread is the bandwidth of the Doppler power density spectrum and can take on values up to two times |fDmax|, i.e.，1.1.3Channel Fade Statistics The statistics of the fading process characterize the channel and are of importance for channel model parameter specifications.A simple and often used approach is obtained from the assumption that there is a large number of scatterers in the channel that contribute to the signal at the receiver side.The application of the central limit theorem leads to a complex-valued Gaussian process for the channel impulse response.In the absence of line of sight(LOS)or a dominant component, the process is zero-mean.The magnitude of the corresponding channel transfer function

is a random variable, for brevity denoted by a, with a Rayleigh distribution given by

Where

is the average power.The phase is uniformly distributed in the interval [0, 2π].In the case that the multipath channel contains a LOS or dominant component in addition to the randomly moving scatterers, the channel impulse response can no longer be modeled as zero-mean.Under the assumption of a complex-valued Gaussian process for the channel impulse response, the magnitude a of the channel transfer function has a Rice distribution given by

The Rice factor KRice is determined by the ratio of the power of the dominant path to thepower of the scattered paths.I0 is the zero-order modified Bessel function of first kind.The phase is uniformly distributed in the interval [0, 2π].1.1.4Inter-Symbol(ISI)and Inter-Channel Interference(ICI)The delay spread can cause inter-symbol interference(ISI)when adjacent data symbols overlap and interfere with each other due to different delays on different propagation paths.The number of interfering symbols in a single-carrier modulated system is given by

For high data rate applications with very short symbol duration Td < τmax, the effect of ISI and, with that, the receiver complexity can increase significantly.The effect of ISI can be counteracted by different measures such as time or frequency domain equalization.In spread spectrum systems, rake receivers with several arms are used to reduce the effect of ISI by exploiting the multipath diversity such that individual arms are adapted to different propagation paths.If the duration of the transmitted symbol is significantly larger than the maximum delay Td τmax, the channel produces a negligible amount of ISI.This effect is exploited with multi-carrier transmission where the duration per transmitted symbol increases with the number of sub-carriers Nc and, hence, the amount of ISI decreases.The number of interfering symbols in a multi-carrier modulated system is given by

Residual ISI can be eliminated by the use of a guard interval(see Section 1.2).The maximum Doppler spread in mobile radio applications using single-carrier modulation is typically much less than the distance between adjacent channels, such that the effect of interference on adjacent channels due to Doppler spread is not a problem for single-carrier modulated systems.For multi-carrier modulated systems, the sub-channel spacing Fs can become quite small, such that Doppler effects can cause significant ICI.As long as all sub-carriers are affected by a common Doppler shift fD, this Doppler shift can be compensated for in the receiver and ICI can be avoided.However, if Doppler spread in the order of several percent of the sub-carrier spacing occurs, ICI may degrade the system performance significantly.To avoid performance degradations due to ICI or more complex receivers with ICI equalization, the sub-carrier spacing Fs should be chosen as

such that the effects due to Doppler spread can be neglected(see Chapter 4).This approach corresponds with the philosophy of OFDM described in Section 1.2 and is followed in current OFDM-based wireless standards.Nevertheless, if a multi-carrier system design is chosen such that the Doppler spread is in the order of the sub-carrier spacing or higher, a rake receiver in the frequency domain can be used [22].With the frequency domain rake receiver each branch of the rake resolves a different Doppler frequency.1.1.5Examples of Discrete Multipath Channel Models Various discrete multipath channel models for indoor and outdoor cellular systems with different cell sizes have been specified.These channel models define the statistics of the 5 discrete propagation paths.An overview of widely used discrete multipath channel models is given in the following.COST 207 [8]: The COST 207 channel models specify four outdoor macro cell propagation scenarios by continuous, exponentially decreasing delay power density spectra.Implementations of these power density spectra by discrete taps are given by using up to 12 taps.Examples for settings with 6 taps are listed in Table 1-1.In this table for several propagation environments the corresponding path delay and power profiles are given.Hilly terrain causes the longest echoes.The classical Doppler spectrum with uniformly distributed angles of arrival of the paths can be used for all taps for simplicity.Optionally, different Doppler spectra are defined for the individual taps in [8].The COST 207 channel models are based on channel measurements with a bandwidth of 8–10 MHz in the 900-MHz band used for 2G systems such as GSM.COST 231 [9] and COST 259 [10]: These COST actions which are the continuation of COST 207 extend the channel characterization to DCS 1800, DECT, HIPERLAN and UMTS channels, taking into account macro, micro, and pico cell scenarios.Channel models with spatial resolution have been defined in COST 259.The spatial component is introduced by the definition of several clusters with local scatterers, which are located in a circle around the base station.Three types of channel models are defined.The macro cell type has cell sizes from 500 m up to 5000 m and a carrier frequency of 900 MHz or 1.8 GHz.The micro cell type is defined for cell sizes of about 300 m and a carrier frequency of 1.2 GHz or 5 GHz.The pico cell type represents an indoor channel model with cell sizes smaller than 100 m in industrial buildings and in the order of 10 m in an office.The carrier frequency is 2.5 GHz or 24 GHz.COST 273: The COST 273 action additionally takes multi-antenna channel models into account, which are not covered by the previous COST actions.CODIT [7]: These channel models define typical outdoor and indoor propagation scenarios for macro, micro, and pico cells.The fading characteristics of the various propagation environments are specified by the parameters of the Nakagami-m distribution.Every environment is defined in terms of a number of scatterers which can take on values up to 20.Some channel models consider also the angular distribution of the scatterers.They have been developed for the investigation of 3G system proposals.Macro cell channel type models have been developed for carrier frequencies around 900 MHz with 7 MHz bandwidth.The micro and pico cell channel type models have been developed for carrier frequencies between 1.8 GHz and 2 GHz.The bandwidths of the measurements are in the range of 10–100 MHz for macro cells and around 100 MHz for pico cells.JTC [28]: The JTC channel models define indoor and outdoor scenarios by specifying 3 to 10 discrete taps per scenario.The channel models are designed to be applicable for wideband digital mobile radio systems anticipated as candidates for the PCS(Personal Communications Systems)common air interface at carrier frequencies of about 2 GHz.UMTS/UTRA [18][44]: Test propagation scenarios have been defined for UMTS and UTRA system proposals which are developed for frequencies around 2 GHz.The modeling of the multipath propagation corresponds to that used by the COST 207 channel models.HIPERLAN/2 [33]: Five typical indoor propagation scenarios for wireless LANs in the 5 GHz frequency band have been defined.Each scenario is described by 18discrete taps of the delay power density spectrum.The time variance of the channel(Doppler spread)is modeled by a classical Jake’s spectrum with a maximum terminal speed of 3 m/h.Further channel models exist which are, for instance, given in [16].1.1.6Multi-Carrier Channel Modeling Multi-carrier systems can either be simulated in the time domain or, more computationally efficient, in the frequency domain.Preconditions for the frequency domain implementation are the absence of ISI and ICI, the frequency nonselective fading per sub-carrier, and the time-invariance during one OFDM symbol.A proper system design approximately fulfills these preconditions.The discrete channel transfer function adapted to multi-carrier signals results in

where the continuous channel transfer function H(f, t)is sampled in time at OFDM symbol rate s and in frequency at sub-carrier spacing Fs.The duration

s is the total OFDM symbol duration including the guard interval.Finally, a symbol transmitted onsub-channel n of the OFDM symbol i is multiplied by the resulting fading amplitude an,i and rotated by a random phase ?n,i.The advantage of the frequency domain channel model is that the IFFT and FFT operation for OFDM and inverse OFDM can be avoided and the fading operation results in one complex-valued multiplication per sub-carrier.The discrete multipath channel models introduced in Section 1.1.5 can directly be applied to(1.16).A further simplification of the channel modeling for multi-carrier systems is given by using the so-called uncorrelated fading channel models.1.1.6.1Uncorrelated Fading Channel Models for Multi-Carrier Systems These channel models are based on the assumption that the fading on adjacent data symbols after inverse OFDM and de-interleaving can be considered as uncorrelated [29].This assumption holds when, e.g., a frequency and time interleaver with sufficient interleaving depth is applied.The fading amplitude an,i is chosen from a distribution p(a)according to the considered cell type and the random phase ?n,I is uniformly distributed in the interval [0,2π].The resulting complex-valued channel fading coefficient is thus generated independently for each sub-carrier and OFDM symbol.For a propagation scenario in a macro cell without LOS, the fading amplitude an,i is generated by a Rayleigh distribution and the channel model is referred to as an uncorrelated Rayleigh fading channel.For smaller cells where often a dominant propagation component occurs, the fading amplitude is chosen from a Rice distribution.The advantages of the uncorrelated fading channel models for multi-carrier systems are their simple implementation in the frequency domain and the simple reproducibility of the simulation results.1.1.7Diversity The coherence bandwidth of a mobile radio channel is the bandwidth over which the signal propagation characteristics are correlated and it can be approximated by

The channel is frequency-selective if the signal bandwidth B is larger than the coherence bandwidth.On the other hand, if B is smaller than , the channel is frequency nonselective or flat.The coherence bandwidth of the channel is of importance for evaluating the performance of spreading and frequency interleaving techniques that try to exploit the inherent frequency diversity Df of the mobile radio channel.In the case of multi-carrier transmission, frequency diversity is exploited if the separation of sub-carriers transmitting the same information exceeds the coherence bandwidth.The maximum achievable frequency diversity Df is given by the ratio between the signal bandwidth B and the coherence bandwidth，The coherence time of the channel is the duration over which the channel characteristics can be considered as time-invariant and can be approximated by

If the duration of the transmitted symbol is larger than the coherence time, the channel is time-selective.On the other hand, if the symbol duration is smaller than , the channel is time nonselective during one symbol duration.The coherence time of the channel is of importance for evaluating the performance of coding and interleaving techniques that try to exploit the inherent time diversity DO of the mobile radio channel.Time diversity can be exploited if the separation between time slots carrying the same information exceeds the coherence time.A number of Ns successive time slots create a time frame of duration Tfr.The maximum time diversity Dt achievable in one time frame is given by the ratio between the duration of a time frame and the coherence time, A system exploiting frequency and time diversity can achieve the overall diversity

The system design should allow one to optimally exploit the available diversity DO.For instance, in systems with multi-carrier transmission the same information should be transmitted on different sub-carriers and in different time slots, achieving uncorrelated faded replicas of the information in both dimensions.Uncoded multi-carrier systems with flat fading per sub-channel and time-invariance during one symbol cannot exploit diversity and have a poor performance in time and frequency selective fading channels.Additional methods have to be applied to exploit diversity.One approach is the use of data spreading where each data symbol is spread by a spreading code of length L.This, in combination with interleaving, can achieve performance results which are given for

by the closed-form solution for the BER for diversity reception in Rayleigh fading channels according to [40]

Whererepresents the combinatory function，and σ2 is the variance of the noise.As soon as the interleaving is not perfect or the diversity offered by the channel is smaller than the spreading code length L, or MCCDMA with multiple access interference is applied,(1.22)is a lower bound.For L = 1, the performance of an OFDM system without forward error correction(FEC)is obtained, 9

which cannot exploit any diversity.The BER according to(1.22)of an OFDM(OFDMA, MC-TDMA)system and a multi-carrier spread spectrum(MC-SS)system with different spreading code lengths L is shown in Figure 1-3.No other diversity techniques are applied.QPSK modulation is used for symbol mapping.The mobile radio channel is modeled as uncorrelated Rayleigh fading channel(see Section 1.1.6).As these curves show, for large values of L, the performance of MC-SS systems approaches that of an AWGN channel.Another form of achieving diversity in OFDM systems is channel coding by FEC, where the information of each data bit is spread over several code bits.Additional to the diversity gain in fading channels, a coding gain can be obtained due to the selection of appropriate coding and decoding algorithms.中文翻譯 1基本原理

這章描述今日的基本面的無線通信。第一一個的詳細說明無線電頻道，它的模型被介紹，跟隨附近的的介紹的原則的參考正交頻分復(fù)用多載波傳輸。此外，一個一般概觀的擴頻技術(shù)，尤其ds-cdma，被給，潛力的例子申請參考正交頻分復(fù)用，DS對1。分配的通道傳輸功能是

有關(guān)的延誤測量相對于第一個在接收器檢測到的路徑。多普勒頻率

取決于終端站，光速c，載波頻率fc的速度和發(fā)病路徑分配給速度v波αp角度頁具有相應(yīng)通道傳輸信道沖激響應(yīng)函數(shù)圖1-2所示。

延遲功率密度譜ρ（τ）為特征的頻率選擇性移動無線電頻道給出了作為通道的輸出功能延遲τ平均功率。平均延遲τ，均方根（RMS）的時延擴展τRMS和最大延遲τmax都是延遲功率密度譜特征參數(shù)。平均時延特性參數(shù)為

有

圖1-2時變信道沖激響應(yīng)和通道傳遞函數(shù)頻率選擇性衰落是權(quán)力頁的路徑均方根時延擴展的定義為 同樣，多普勒頻譜的功率密度（FD）的特點可以定義

在移動時變無線信道，并給出了作為一種金融衍生工具功能的多普勒頻率通道輸出的平均功率。多徑信道頻率分散性能是最常見的量化發(fā)生的多普勒頻率和多普勒fDmax蔓延fDspread最大。多普勒擴散是功率密度的多普勒頻譜帶寬，可價值觀需要兩年時間| fDmax|，即

1.1.3頻道淡出統(tǒng)計

在衰落過程中的統(tǒng)計特征和重要的渠道是信道模型參數(shù)規(guī)格。一個簡單而經(jīng)常使用的方法是從假設(shè)有一個通道中的散射，有助于在大量接收端的信號。該中心極限定理的應(yīng)用導(dǎo)致了復(fù)雜的值的高斯信道沖激響應(yīng)過程。在對視線（LOS）或線的主要組成部分的情況下，這個過程是零的意思。相應(yīng)的通道傳遞函數(shù)幅度

是一個隨機變量，通過給定一個簡短表示由瑞利分布，有

是的平均功率。相均勻分布在區(qū)間[0，2π]。

在案件的多通道包含洛杉磯的或主要組件除了隨機移動散射，通道脈沖響應(yīng)可以不再被建模為均值為零。根據(jù)信道脈沖響應(yīng)的假設(shè)一個復(fù)雜的值高斯過程，其大小通道的傳遞函數(shù)A的水稻分布給出

賴斯因素KRice是由占主導(dǎo)地位的路徑權(quán)力的威力比分散的路徑。I0是零階貝塞爾函數(shù)的第一階段是一致kind.The在區(qū)間[0，2π]分發(fā)。

1.1.4符號間（ISI）和通道間干擾（ICI）

延遲的蔓延引起的符號間干擾（ISI）當相鄰的數(shù)據(jù)符號上的重疊與互相不同的傳播路徑，由于不同的延遲干涉。符號的干擾在單載波調(diào)制系統(tǒng)的號碼是給予

對于高數(shù)據(jù)符號持續(xù)時間很短運輸署<蟿MAX時，ISI的影響，這樣一來，速率應(yīng)用，接收機的復(fù)雜性大大增加。對干擾影響，可以抵消，如時間或頻域均衡不同的措施。在擴頻系統(tǒng)，與幾個臂Rake接收機用于減少通過利用多徑分集等，個別武器適應(yīng)不同的傳播路徑的干擾影響。

如果發(fā)送符號的持續(xù)時間明顯高于大的最大延遲運輸署蟿最大，渠道產(chǎn)生ISI的微不足道。這種效果是利用多載波傳輸?shù)牡胤剑堪l(fā)送符號的增加與子載波數(shù)控數(shù)目，因此，ISI的金額減少的持續(xù)時間。符號的干擾多載波調(diào)制系統(tǒng)的號碼是給予

可以消除符號間干擾由一個保護間隔（見1.2節(jié)）的使用。

最大多普勒在移動無線應(yīng)用傳播使用單載波調(diào)制通常比相鄰?fù)ǖ溃@樣，干擾對由于多普勒傳播相鄰?fù)ǖ赖淖饔貌皇且粋€單載波調(diào)制系統(tǒng)的問題距離。對于多載波調(diào)制系統(tǒng)，子通道間距FS可以變得非常小，這樣可以造成嚴重的多普勒效應(yīng)ICI的。只要所有子載波只要是一個共同的多普勒頻移金融衍生工具的影響，這可以補償多普勒頻移在接收器和ICI是可以避免的。但是，如果在對多普勒子載波間隔為幾個百分點的蔓延情況，卜內(nèi)門可能會降低系統(tǒng)的性能顯著。為了避免性能降級或因與ICI卜內(nèi)門更復(fù)雜的接收機均衡，子載波間隔財政司司長應(yīng)定為

這樣說，由于多普勒效應(yīng)可以忽略不擴散（見第4章）。這種方法對應(yīng)于OFDM的1.2節(jié)中所述，是目前基于OFDM的無線標準遵循的理念。

不過，如果多載波系統(tǒng)的設(shè)計選擇了這樣的多普勒展寬在子載波間隔或更高，秩序是在頻率RAKE接收機域名可以使用[22]。隨著頻域RAKE接收機每個支部耙解決了不同的多普勒頻率。

1.1.5多徑信道模型的離散的例子

各類離散多與不同的細胞大小的室內(nèi)和室外蜂窩系統(tǒng)的信道模型已經(jīng)被指定。這些通道模型定義的離散傳播路徑的統(tǒng)計信息。一種廣泛使用的離散多徑信道模型概述于下。造價207[8]：成本207信道模型指定連續(xù)四個室外宏蜂窩傳播方案，指數(shù)下降延遲功率密度譜。這些頻道功率密度的離散譜的實現(xiàn)都是通過使用多達12個頻道。與6頻道設(shè)置的示例列于表1-1。在這種傳播環(huán)境的幾個表中的相應(yīng)路徑延遲和電源配置給出。丘陵地形導(dǎo)致最長相呼應(yīng)。

經(jīng)典的多普勒頻譜與均勻分布的到達角路徑可以用于簡化所有的頻道。或者，不同的多普勒譜定義在[8]個人頻道。207信道的成本模型是基于一個8-10兆赫的2G，如GSM系統(tǒng)中使用的900兆赫頻段信道帶寬的測量。造價231[9]和造價259[10]：這些費用是行動的延續(xù)成本207擴展通道特性到DCS1800的DECT，HIPERLAN和UMTS的渠道，同時考慮到宏觀，微觀和微微小區(qū)的情況為例。空間分辨率與已定義的通道模型在造價259。空間部分是介紹了與當?shù)厣⑸洌@是在基站周圍設(shè)幾組圓的定義。三種類型的通道模型定義。宏細胞類型具有高達500?5000米，載波頻率為900兆赫或1.8 GHz的單元尺寸。微細胞類型被定義為細胞體積約300米，1.2 GHz或5 GHz載波頻率。細胞類型代表的Pico與細胞體積小于100工業(yè)建筑物和辦公室中的10 m階米室內(nèi)信道模型。載波頻率為2.5 GHz或24千兆赫。造價273：成本273行動另外考慮到多天線信道模型，這是不是由先前的費用的行為包括在內(nèi)。

CODIT [7]：這些通道模型定義的宏，微，微微蜂窩和室外和室內(nèi)傳播的典型案例。各種傳播環(huán)境的衰落特性是指定的在NakagamiSS）的不同擴頻碼L是長度，如圖1-3所示的系統(tǒng)。沒有其他的分集技術(shù)被應(yīng)用。QPSK調(diào)制用于符號映射。移動無線信道建模為不相關(guān)瑞利衰落信道（見1.1.6）。由于這些曲線顯示，辦法，AWGN信道的一對L時，對MC-SS系統(tǒng)性能有很大價值。

另一種實現(xiàn)形式的OFDM系統(tǒng)的多樣性是由前向糾錯信道編碼，在這里，每個數(shù)據(jù)位的信息分散在幾個代碼位。附加在衰落信道分集增益，編碼增益一個可因適當?shù)木幋a和解碼算法的選擇。

第四篇：中英文翻譯

蓄電池 battery 充電 converter 轉(zhuǎn)換器 charger

開關(guān)電器 Switch electric 按鈕開關(guān) Button to switch 電源電器 Power electric 插頭插座 Plug sockets

第五篇：中英文翻譯

特種加工工藝

介紹

傳統(tǒng)加工如車削、銑削和磨削等，是利用機械能將金屬從工件上剪切掉，以加工成孔或去除余料。特種加工是指這樣一組加工工藝，它們通過各種涉及機械能、熱能、電能、化學(xué)能或及其組合形式的技術(shù)，而不使用傳統(tǒng)加工所必需的尖銳刀具來去除工件表面的多余材料。

傳統(tǒng)加工如車削、鉆削、刨削、銑削和磨削，都難以加工特別硬的或脆性材料。采用傳統(tǒng)方法加工這類材料就意味著對時間和能量要求有所增加，從而導(dǎo)致成本增加。在某些情況下，傳統(tǒng)加工可能行不通。由于在加工過程中會產(chǎn)生殘余應(yīng)力，傳統(tǒng)加工方法還會造成刀具磨損，損壞產(chǎn)品質(zhì)量。基于以下各種特殊理由，特種加工工藝或稱為先進制造工藝，可以應(yīng)用于采用傳統(tǒng)加工方法不可行，不令人滿意或者不經(jīng)濟的場合：

1.對于傳統(tǒng)加工難以夾緊的非常硬的脆性材料； 2.當工件柔性很大或很薄時； 3.當零件的形狀過于復(fù)雜時；

4.要求加工出的零件沒有毛刺或殘余應(yīng)力。

傳統(tǒng)加工可以定義為利用機械（運動）能的加工方法，而特種加工利用其他形式的能量，主要有如下三種形式： 1.熱能； 2.化學(xué)能； 3.電能。

為了滿足額外的加工條件的要求，已經(jīng)開發(fā)出了幾類特種加工工藝。恰當?shù)厥褂眠@些加工工藝可以獲得很多優(yōu)于傳統(tǒng)加工工藝的好處。常見的特種加工工藝描述如下。

電火花加工

電火花加工是使用最為廣泛的特種加工工藝之一。相比于利用不同刀具進行金屬切削和磨削的常規(guī)加工，電火花加工更為吸引人之處在于它利用工件和電極間的一系列重復(fù)產(chǎn)生的（脈沖）離散電火花所產(chǎn)生的熱電作用，從工件表面通過電腐蝕去除掉多余的材料。

傳統(tǒng)加工工藝依靠硬質(zhì)刀具或磨料去除較軟的材料，而特種加工工藝如電火花加工，則是利用電火花或熱能來電蝕除余料，以獲得所需的零件形狀。因此，材料的硬度不再是電火花加工中的關(guān)鍵因素。

電火花加工是利用存儲在電容器組中的電能（一般為50V/10A量級）在工具電極（陰極）和工件電極（陽極）之間的微小間隙間進行放電來去除材料的。如圖6.1所示，在EDM操作初始，在工具電極和工件電極間施以高電壓。這個高電壓可以在工具電極和工件電極窄縫間的絕緣電介質(zhì)中產(chǎn)生電場。這就會使懸浮在電介質(zhì)中的導(dǎo)電粒子聚集在電場最強處。當工具電極和工件電極之間的勢能差足夠大時，電介質(zhì)被擊穿，從而在電介質(zhì)流體中會產(chǎn)生瞬時電火花，將少量材料從工件表面蝕除掉。每次電火花所蝕除掉的材料量通常在10-5～10-6mm3范圍內(nèi)。電極之間的間隙只有千分之幾英寸，通過伺服機構(gòu)驅(qū)動和控制工具電極的進給使該值保持常量。化學(xué)加工

化學(xué)加工是眾所周知的特種加工工藝之一，它將工件浸入化學(xué)溶液通過腐蝕溶解作用將多余材料從工件上去除掉。該工藝是最古老的特種加工工藝，主要用于凹腔和輪廓加工，以及從具有高的比剛度的零件表面去除余料。化學(xué)加工廣泛用于為多種工業(yè)應(yīng)用（如微機電系統(tǒng)和半導(dǎo)體行業(yè)）制造微型零件。

化學(xué)加工將工件浸入到化學(xué)試劑或蝕刻劑中，位于工件選區(qū)的材料通過發(fā)生在金屬溶蝕或化學(xué)溶解過程中的電化學(xué)微電池作用被去除掉。而被稱為保護層的特殊涂層所保護下的區(qū)域中的材料則不會被去除。不過，這種受控的化學(xué)溶解過程同時也會蝕除掉所以暴露在表面的材料，盡管去除的滲透率只有0.0025～0.1 mm/min。該工藝采用如下幾種形式：凹坑加工、輪廓加工和整體金屬去除的化學(xué)銑，在薄板上進行蝕刻的化學(xué)造型，在微電子領(lǐng)域中利用光敏抗蝕劑完成蝕刻的光化學(xué)加工(PCM)，采用弱化學(xué)試劑進行拋光或去毛刺的電化學(xué)拋光，以及利用單一化學(xué)活性噴射的化學(xué)噴射加工等。如圖6.2a所示的化學(xué)加工示意圖，由于蝕刻劑沿垂直和水平方向開始蝕除材料，鉆蝕（又稱為淘蝕）量進一步加大，如圖6.2b所示的保護體邊緣下面的區(qū)域。在化學(xué)造型中最典型的公差范圍可保持在材料厚度的±10%左右。為了提高生產(chǎn)率，在化學(xué)加工前，毛坯件材料應(yīng)采用其他工藝方法（如機械加工）進行預(yù)成形加工。濕度和溫度也會導(dǎo)致工件尺寸發(fā)生改變。通過改變蝕刻劑和控制工件加工環(huán)境，這種尺寸改變可以減小到最小。

電化學(xué)加工

電化學(xué)金屬去除方法是一種最有用的特種加工方法。盡管利用電解作用作為金屬加工手段是近代的事，但其基本原理是法拉第定律。利用陽極溶解，電化學(xué)加工可以去除具有導(dǎo)電性質(zhì)工件的材料，而無須機械能和熱能。這個加工過程一般用于在高強度材料上加工復(fù)雜形腔和形狀，特別是在航空工業(yè)中如渦輪機葉片、噴氣發(fā)動機零件和噴嘴，以及在汽車業(yè)（發(fā)動機鑄件和齒輪）和醫(yī)療衛(wèi)生業(yè)中。最近，還將電化學(xué)加工應(yīng)用于電子工業(yè)的微加工中。

圖6.3所示的是一個去除金屬的電化學(xué)加工過程，其基本原理與電鍍原理正好相反。在電化學(xué)加工過程中，從陽極（工件）上蝕除下的粒子移向陰極（加工工具）。金屬的去除由一個合適形狀的工具電極來完成，最終加工出來的零件具有給定的形狀、尺寸和表面光潔度。在電化學(xué)加工過程中，工具電極的形狀逐漸被轉(zhuǎn)移或復(fù)制到工件上。型腔的形狀正好是與工具相匹配的陰模的形狀。為了獲得電化學(xué)過程形狀復(fù)制的高精度和高的材料去除率，需要采用高的電流密度（范圍為10～100 A/cm2)和低電壓（范圍為8～30V）。通過將工具電極向去除工件表面材料的方向進給，加工間隙要維持在0.1 mm范圍內(nèi)，而進給率一般為0.1～20 mm/min左右。泵壓后的電解液以高達5～50 m/s的速度通過間隙，將溶解后的材料、氣體和熱量帶走。因此，當被蝕除的材料還沒來得及附著到工具電極上時，就被電解液帶走了。

作為一種非機械式金屬去除加工方法，ECM可以以高切削量加工任何導(dǎo)電材料，而無須考慮材料的機械性能。特別是在電化學(xué)加工中，材料去除率與被加工件的硬度、韌性及其他特性無關(guān)。對于利用機械方法難于加工的材料，電化學(xué)加工可以保證將該材料加工出復(fù)雜形狀的零件，這就不需要制造出硬度高于工件的刀具，而且也不會造成刀具磨損。由于工具和工件間沒有接觸，電化學(xué)加工是加工薄壁、易變形零件及表面容易破裂的脆性材料的首選。激光束加工

LASER是英文Light Amplification by Stimulated Emission of Radiation 各單詞頭一個字母所組成的縮寫詞。雖然激光在某些場合可用來作為放大器，但它的主要用途是光激射振蕩器，或者是作為將電能轉(zhuǎn)換為具有高度準直性光束的換能器。由激光發(fā)射出的光能具有不同于其他光源的特點：光譜純度好、方向性好及具有高的聚焦功率密度。

激光加工就是利用激光和和靶材間的相互作用去除材料。簡而言之，這些加工工藝包括激光打孔、激光切割、激光焊接、激光刻槽和激光刻劃等。

激光加工（圖6.4）可以實現(xiàn)局部的非接觸加工，而且對加工件幾乎沒有作用力。這種加工工藝去除材料的量很小，可以說是“逐個原子”地去除材料。由于這個原因，激光切削所產(chǎn)生的切口非常窄。激光打孔深度可以控制到每個激光脈沖不超過一微米，且可以根據(jù)加工要求很靈活地留下非常淺的永久性標記。采用這種方法可以節(jié)省材料，這對于貴重材料或微加工中的精密結(jié)構(gòu)而言非常重要。可以精確控制材料去除率使得激光加工成為微制造和微電子技術(shù)中非常重要的加工方法。厚度小于20 mm的板材的激光切割加工速度快、柔性好、質(zhì)量高。另外，通過套孔加工還可有效實現(xiàn)大孔及復(fù)雜輪廓的加工。

激光加工中的熱影響區(qū)相對較窄，其重鑄層只有幾微米。基于此，激光加工的變形可以不予考慮。激光加工適用于任何可以很好地吸收激光輻射的材料，而傳統(tǒng)加工工藝必須針對不同硬度和耐磨性的材料選擇合適的刀具。采用傳統(tǒng)加工方法，非常難以加工硬脆材料如陶瓷等，而激光加工是解決此類問題的最好選擇。

激光切割的邊緣光滑且潔凈，無須進一步處理。激光打孔可以加工用其他方法難以加工的高深徑比的孔。激光加工可以加工出高質(zhì)量的小盲孔、槽、表面微造型和表面印痕。激光技術(shù)正處于高速發(fā)展期，激光加工也如此。激光加工不會掛渣，沒有毛邊，可以精確控制幾何精度。隨著激光技術(shù)的快速發(fā)展，激光加工的質(zhì)量正在穩(wěn)步提高。

超聲加工

超聲加工為日益增長的對脆性材料如單晶體、玻璃、多晶陶瓷材料的加工需求及不斷提高的工件復(fù)雜形狀和輪廓加工提供了解決手段。這種加工過程不產(chǎn)生熱量、無化學(xué)反應(yīng)，加工出的零件在微結(jié)構(gòu)、化學(xué)和物理特性方面都不發(fā)生變化，可以獲得無應(yīng)力加工表面。因此，超聲加工被廣泛應(yīng)用于傳統(tǒng)加工難以切削的硬脆材料。在超聲加工中，實際切削由液體中的懸浮磨粒或者旋轉(zhuǎn)的電鍍金剛石工具來完成。超聲加工的變型有靜止（傳統(tǒng)）超聲加工和旋轉(zhuǎn)超聲加工。

傳統(tǒng)的超聲加工是利用作為小振幅振動的工具與工件之間不斷循環(huán)的含有磨粒的漿料的磨蝕作用去除材料的。成形工具本身并不磨蝕工件，是受激振動的工具通過激勵漿料液流中的磨料不斷緩和而均勻地磨損工件，從而在工件表面留下與工具相對應(yīng)的精確形狀。音極工具振動的均勻性使超聲加工只能完成小型零件的加工，特別是直徑小于100 mm 的零件。

超聲加工系統(tǒng)包括音極組件、超聲發(fā)生器、磨料供給系統(tǒng)及操作人員的控制。音極是暴露在超聲波振動中的一小塊金屬或工具，它將振動能傳給某個元件，從而激勵漿料中的磨粒。超聲加工系統(tǒng)的示意圖如圖6.5所示。音極/工具組件由換能器、變幅桿和音極組成。換能器將電脈沖轉(zhuǎn)換成垂直沖程，垂直沖程再傳給變幅桿進行放大或壓抑。調(diào)節(jié)后的沖程再傳給音極/工具組件。此時，工具表面的振動幅值為20～50μm。工具的振幅通常與所使用的磨粒直徑大致相等。

磨料供給系統(tǒng)將由水和磨粒組成的漿料送至切削區(qū)，磨粒通常為碳化硅或碳化硼。另外，除了提供磨粒進行切削外，漿料還可對音極進行冷卻，并將切削區(qū)的磨粒和切屑帶走。

Nontraditional Machining Processes Introduction

Traditional or conventional machining, such as turning, milling, and grinding etc., uses mechanical energy to shear metal against another substance to create holes or remove material.Nontraditional machining processes are defined as a group of processes that remove excess material by various techniques involving mechanical, thermal, electrical or chemical energy or combinations of these energies but do not use a sharp cutting tool as it is used in traditional manufacturing processes.Extremely hard and brittle materials are difficult to be machined by traditional machining processes.Using traditional methods to machine such materials means increased demand for time and energy and therefore increases in costs;in some cases traditional machining may not be feasible.Traditional machining also results in tool wear and loss of quality in the product owing to induced residual stresses during machining.Nontraditional machining processes, also called unconventional machining process or advanced manufacturing processes, are employed where traditional machining processes are not feasible, satisfactory or economical due to special reasons as outlined below: 1.Very hard fragile materials difficult to clamp for traditional machining;2.When the workpiece is too flexible or slender;3.When the shape of the part is too complex;4.Parts without producing burrs or inducing residual stresses.Traditional machining can be defined as a process using mechanical(motion)energy.Non-traditional machining utilizes other forms of energy;the three main forms of energy used in non-traditional machining processes are as follows: 1.Thermal energy;2.Chemical energy;3.Electrical energy.Several types of nontraditional machining processes have been developed to meet extra required machining conditions.When these processes are employed properly, they offer many advantages over traditional machining processes.The common nontraditional machining processes are described in the following section.Electrical Discharge Machining(EDM)

Electrical discharge machining(EDM)sometimes is colloquially referred to as spark machining, spark eroding, burning, die sinking or wire erosion.It is one of the most widely used non-traditional machining processes.The main attraction of EDM over traditional machining processes such as metal cutting using different tools and grinding is that this technique utilizes thermoelectric process to erode undesired materials from the workpiece by a series of rapidly recurring discrete electrical sparks between workpiece and electrode.The traditional machining processes rely on harder tool or abrasive material to remove softer material whereas nontraditional machining processes such as EDM uses electrical spark or thermal energy to erode unwanted material in order to create desired shapes.So, the hardness of the material is no longer a dominating factor for EDM process.EDM removes material by discharging an electrical current, normally stored in a capacitor bank, across a small gap between the tool(cathode)and the workpiece(anode)typically in the order of 50 volts/10amps.As shown in Fig.6.1, at the beginning of EDM operation, a high voltage is applied across the narrow gap between the electrode and the workpiece.This high voltage induces an electric field in the insulating dielectric that is present in narrow gap between electrode and workpiece.This causes conducting particles suspended in the dielectric to concentrate at the points of strongest electrical field.When the potential difference between the electrode and the workpiece is sufficiently high, the dielectric breaks down and a transient spark discharges through the dielectric fluid, removing small amount of material from the workpiece surface.The volume of the material removed per spark discharge is typically in the range of 10-5 to 10-6 mm3.The gap is only a few thousandths of an inch, which is maintained at a constant value by the servomechanism that actuates and controls the tool feed.Chemical Machining(CM)

Chemical machining(CM)is a well known non-traditional machining process in which metal is removed from a workpiece by immersing it into a chemical solution.The process is the oldest of the nontraditional processes and has been used to produce pockets and contours and to remove materials from parts having a high strength-to-weight ratio.Moreover, the chemical machining method is widely used to produce micro-components for various industrial applications such as microelectromechanical systems(MEMS)and semiconductor industries.In CM material is removed from selected areas of workpiece by immersing it in a chemical reagents or etchants, such as acids and alkaline solutions.Material is removed by microscopic electrochemical cell action which occurs in corrosion or chemical dissolution of a metal.Special coatings called maskants protect areas from which the metal is not to be removed.This controlled chemical dissolution will simultaneously etch all exposed surfaces even though the penetration rates of the material removed may be only 0.0025-0.1mm/min.The basic process takes many forms: chemical milling of pockets, contours, overall metal removal, chemical blanking for etching through thin sheets;photochemical machining(pcm)for etching by using of photosensitive resists in microelectronics;chemical or electrochemical polishing where weak chemical reagents are used(sometimes with remote electric assist)for polishing or deburring and chemical jet machining where a single chemically active jet is used.A schematic of chemical machining process is shown in Fig.6.2a.Because the etchant attacks the material in both vertical and horizontal directions, undercuts may develop(as shown by the areas under the edges of the maskant in Fig.6.2b).Typically, tolerances of ±10% of the material thickness can be maintained in chemical blanking.In order to improve the production rate, the bulk of the workpiece should be shaped by other processes(such as by machining)prior to chemical machining.Dimensional variations can occur because of size changes in workpiece due to humidity and temperature.This variation can be minimized by properly selecting etchants and controlling the environment in the part generation and the production area in the plant.Electrochemical Machining(ECM)

Electrochemical metal removal is one of the more useful nontraditional machining processes.Although the application of electrolytic machining as a metal-working tool is relatively new, the basic principles are based on Faraday laws.Thus, electrochemical machining can be used to remove electrically conductive workpiece material through anodic dissolution.No mechanical or thermal energy is involved.This process is generally used to machine complex cavities and shapes in high-strength materials, particularly in the aerospace industry for the mass production of turbine blades, jet-engine parts, and nozzles, as well as in the automotive(engines castings and gears)and medical industries.More recent applications of ECM include micromachining for the electronics industry.Electrochemical machining(ECM), shown in Fig.6.3, is a metal-removal process based on the principle of reverse electroplating.In this process, particles travel from the anodic material(workpiece)toward the cathodic material(machining tool).Metal removal is effected by a suitably shaped tool electrode, and the parts thus produced have the specified shape, dimensions, and surface finish.ECM forming is carried out so that the shape of the tool electrode is transferred onto, or duplicated in, the workpiece.The cavity produced is the female mating image of the tool shape.For high accuracy in shape duplication and high rates of metal removal, the process is operated at very high current densities of the order 10-100 A/cm2,at relative low voltage usually from 8 to 30 V, while maintaining a very narrow machining gap(of the order of 0.1 mm)by feeding the tool electrode with a feed rate from 0.1 to 20 mm/min.Dissolved material, gas, and heat are removed from the narrow machining gap by the flow of electrolyte pumped through the gap at a high velocity(5-50 m/s), so the current of electrolyte fluid carries away the deplated material before it has a chance to reach the machining tool.Being a non-mechanical metal removal process, ECM is capable of machining any electrically conductive material with high stock removal rates regardless of their mechanical properties.In particular, removal rate in ECM is independent of the hardness, toughness and other properties of the material being machined.The use of ECM is most warranted in the manufacturing of complex-shaped parts from materials that lend themselves poorly to machining by other, above all mechanical methods.There is no need to use a tool made of a harder material than the workpiece, and there is practically no tool wear.Since there is no contact between the tool and the work, ECM is the machining method of choice in the case of thin-walled, easily deformable components and also brittle materials likely to develop cracks in the surface layer.Laser Beam Machining(LBM)

LASER is an acronym for Light Amplification by Stimulated Emission of Radiation.Although the laser is used as a light amplifier in some applications, its principal use is as an optical oscillator or transducer for converting electrical energy into a highly collimated beam of optical radiation.The light energy emitted by the laser has several characteristics which distinguish it from other light sources: spectral purity, directivity and high focused power density.Laser machining is the material removal process accomplished through laser and target material interactions.Generally speaking, these processes include laser drilling, laser cutting, laser welding, and laser grooving, marking or scribing.Laser machining(Fig.6.4)is localized, non-contact machining and is almost reacting-force free.This process can remove material in very small amount and is said to remove material “atom by atom”.For this reason, the kerf in laser cutting is usually very narrow , the depth of laser drilling can be controlled to less than one micron per laser pulse and shallow permanent marks can be made with great flexibility.In this way material can be saved, which may be important for precious materials or for delicate structures in micro-fabrications.The ability of accurate control of material removal makes laser machining an important process in micro-fabrication and micro-electronics.Also laser cutting of sheet material with thickness less than 20mm can be fast, flexible and of high quality, and large holes or any complex contours can be efficiently made through trepanning.Heat Affected Zone(HAZ)in laser machining is relatively narrow and the re-solidified layer is of micron dimensions.For this reason, the distortion in laser machining is negligible.LBM can be applied to any material that can properly absorb the laser irradiation.It is difficult to machine hard materials or brittle materials such as ceramics using traditional methods, laser is a good choice for solving such difficulties.Laser cutting edges can be made smooth and clean, no further treatment is necessary.High aspect ratio holes with diameters impossible for other methods can be drilled using lasers.Small blind holes, grooves, surface texturing and marking can be achieved with high quality using LBM.Laser technology is in rapid progressing, so do laser machining processes.Dross adhesion and edge burr can be avoided, geometry precision can be accurately controlled.The machining quality is in constant progress with the rapid progress in laser technology.Ultrasonic Machining(USM)

Ultrasonic machining offers a solution to the expanding need for machining brittle materials such as single crystals, glasses and polycrystalline ceramics, and for increasing complex operations to provide intricate shapes and workpiece profiles.This machining process is non-thermal, non-chemical, creates no change in the microstructure, chemical or physical properties of the workpiece and offers virtually stress-free machined surfaces.It is therefore used extensively in machining hard and brittle materials that are difficult to cut by other traditional methods.The actual cutting is performed either by abrasive particles suspended in a fluid, or by a rotating diamond-plate tool.These variants are known respectively as stationary(conventional)ultrasonic machining and rotary ultrasonic machining(RUM).Conventional ultrasonic machining(USM)accomplishes the removal of material by the abrading action of a grit-loaded slurry, circulating between the workpiece and a tool that is vibrated with small amplitude.The form tool itself does not abrade the workpiece;the vibrating tool excites the abrasive grains in the flushing fluid, causing them to gently and uniformly wear away the material, leaving a precise reverse from of the tool shape.The uniformity of the sonotrode-tool vibration limits the process to forming small shapes typically under 100 mm in diameter.The USM system includes the Sonotrode-tool assembly, the generator, the grit system and the operator controls.The sonotrode is a piece of metal or tool that is exposed to ultrasonic vibration, and then gives this vibratory energy in an element to excite the abrasive grains in the slurry.A schematic representation of the USM set-up is shown in Fig.6.5.The sonotrode-tool assembly consists of a transducer, a booster and a sonotrode.The transducer converts the electrical pulses into vertical stroke.This vertical stroke is transferred to the booster, which may amplify or suppress the stroke amount.The modified stroke is then relayed to the sonotrode-tool assembly.The amplitude along the face of the tool typically falls in a 20 to 50 μm range.The vibration amplitude is usually equal to the diameter of the abrasive grit used.The grit system supplies a slurry of water and abrasive grit, usually silicon or boron carbide, to the cutting area.In addition to providing abrasive particles to the cut, the slurry also cools the sonotrode and removes particles and debris from the cutting area.