RFID技術(shù)外文文獻

1、外文文獻Current RFID TechnologyThis section describes out of which parts RFID tags consist of, how they work in principle, and what types of tags do exist. It focuses on how tags are powered and what frequency ranges is used. The section concludes by covering a few important standards.RFID transponders

2、(tags) consist in general of: Micro chip, Antenna, Case, Battery (for active tags only)The size of the chip depends mostly on the Antenna. Its size and form is dependent on the frequency the tag is using. The size of a tag also depends on its area of use. It can range from less than a millimeter for

3、 implants to the size of a book in container logistic. In addition to the micro chip, some tags also have rewritable memory attached where the tag can store updates between reading cycles or new data like serial numbers.A RFID tag is shown in figure 1. The antenna is clearly visible. As said before

4、the antenna has the largest impact of the size of the tag. The microchip is visible in the center of the tag, and since this is a passive tag it does not have an internal power source In principle an RFID tag works as follows: the reading unit generates an electro-magnetic field which induces a curr

5、ent into the tags antenna. The current is used to power the chip. In passive tags the current also charges a condenser which assures uninterrupted power for the chip. In active tags a battery replaces the condenser. The difference between active and passive tags is explained shortly. Once activated

6、the tag receives commands from the reading unit and replies by sending its serial number or the requested information. In general the tag does not have enough energy to create its own electro-magnetic field, instead it uses back scattering to modulate (reflect/absorb) the field sent by the reading u

7、nit. Because most fluids absorb electro-magnetic fields and most metal reflect those fields the reading of tags in presence of those materials is complicated.During a reading cycle, the reader has to continuously power the tag. The created field is called continuous wave, and because the strength of

8、 the field decreases with the square of the distance the readers have to use a rather large power. That field overpowers any response a tag could give, so therefore tags reply on side-channels which are located directly below and above the frequency of the continuous wave.1. Energy SourcesWe disting

9、uish 3 types of RFID tags in relation to power or energy: Passive, Semi-passive, Active Passive tags do not have an internal power source, and they therefore rely on the power induced by the reader. This means that the reader has to keep up its field until the transaction is completed. Because of th

10、e lack of a battery, these tags are the smallest and cheapest tags available; however it also restricts its reading range to a range between 2mm and a few meters. As an added benefit those tags are also suitable to be produced by printing. Furthermore their lifespan is unlimited since they do not de

11、pend on an internal power source.The second type of tags is semi-passive tags. Those tags have an internal power source that keeps the micro chip powered at all times. There are many advantages: Because the chip is always powered it can respond faster tore quests, therefore increasing the number of

12、tags that can be queried per second which is important to some applications. Furthermore, since the antenna is not required for collecting power it can be optimized for back scattering and therefore increasing the reading range. And last but not least, since the tag does not use any energy from the

13、field the back scattered signal is stronger, increasing the range even further. Because of the last two reasons, a semi-active tag has usually a range larger than a passive tag.The third type of tags is active tags. Like semi-active tags they contain an internal power source but they use the energy

14、supplied for both, to power the micro chip and to generate a signal on the antenna. Active tags that send signals without being queried are called beacons. An active tags range can be tens of meters, making it ideal for locating objects or serving as landmark points. The lifetime is up to 5 years.2.

15、 Frequency BandsRFID tags fall into three regions in respect to frequency: Low frequency (LF, 30- 500kHz), High frequency (HF.10-15MHz), Ultra high frequency (UHF, 850- 950MHz, 2.4-2.5GHz, 5.8GHz)Low frequency tags are cheaper than any of the higher frequency tags. They are fast enough for most appl

16、ications, however for larger amounts of data the time a tag has to stay in a readers range will increase. Another advantage is that low frequency tags are least affected by the presence of fluids or metal. The disadvantage of such tags is their short reading range. The most common frequencies used f

17、or low frequency tags are 125-134.2 kHz and 140-148.5 kHz.High frequency tags have higher transmission rates and ranges but also cost more than LF tags. Smart tags are the most common member of this group and they work at 13.56MHz. UHF tags have the highest range of all tags. It ranges from 3-6 mete

18、rs for passive tags and 30+ meters for active tags. In addition the transmission rate is also very high, which allows to read a single tag in a very short time. This feature is important where tagged entities are moving with a high speed and remain only for a short time in a readers range. UHF tags

19、are also more expensive than any other tag and are severely affected by fluids and metal. Those properties make UHF mostly useful in automated toll collection systems. Typical frequencies are 868MHz (Europe), 915MHz (USA), 950MHz (Japan), and 2.45GHz.Frequencies for LF and HF tags are license exempt

20、 and can be used worldwide; however frequencies for UHF tags differ from country to country and require a permit.3. StandardsThe wide range of possible applications requires many different types of tags, often with conflicting goals (e.g. low cost vs. security). That is reflected in the number of st

21、andards. A short list of RFID standards follows: ISO11784, ISO11785, ISO14223, ISO10536, ISO14443, ISO15693, ISO18000. Note that this list is not exhaustive. Since the RFID technology is not directly Internet related it is not surprising that there are no RFCs available. There cent hype around RFID

22、technology has resulted in an explosion in patents. Currently there are over 1800 RFID related patents issued (from 1976 to 2001) and over 5700 patents describing RFID systems or applications are backlogged.4. RFID SystemsA RFID reader and a few tags are in general of little use. The retrieval of a

23、serial number does not provide much information to the user nor does it help to keep track of items in a production chain. The real power of RFID comes in combination with a backend that stores additional information such as descriptions for products and where and when a certain tag was scanned. In

24、general a RFID system has a structure as depicted in figure 2. RFID readers scan tags, and then forward the information to the backend. The backend in general consists of a database and a well defined application interface. When the backend receives new information, it adds it to the database and if

25、 needed performs some computation on related fields. The application retrieves data from the backend. In many cases, the application is collocated with the reader itself. An example is the checkout point in a supermarket (Note that the given example uses barcodes instead of RFID tags since they are

26、more common; however, the system would behave in exactly the same way if tags were used). When the reader scans the barcode, the application uses the derived identifier to look up the current price. In addition, the backend also provides discount information for qualifying products. The backend also

27、 decreases the number of available products of that kind and notifies the manager if the amount falls below a certain threshold.This section describes how RFID tags work in general, what types of tags exist and how they differ. The three frequency ranges that RFID tags typically use are LF, HF, and

28、UHF. Also the difference between passive, semi-passive, and active tags was explained and their advantages and disadvantages were compared. The section concluded by looking at different standards and showed the great interest of the industry by counting the number of issued and backlogged patents US

29、 Patent Office.翻譯：當前的RFID技術(shù)該節(jié)描述的是RFID標簽由哪些部分組成、工作原理和確實存在的標簽類型，關(guān)注標簽的供電方式和使用頻率范圍。這部分也總結(jié)了一些重要的標準。RFID應答器的一般組成:微芯片、天線、線圈和電池(僅適用于有源標簽)。芯片的大小主要取決于天線，它的規(guī)模和形式的取決于標簽的使用頻率，也取決于它的使用面積。它的大小范圍可從不到一毫米的植入體大到一本關(guān)于集裝箱物流的書。除了微型芯片,有些標簽也附有可重寫內(nèi)存，這樣標簽就可儲存更新閱讀周期之間的或新的數(shù)據(jù)，如序號。如圖1所示的RFID標簽。天線清晰可見。正如前面所說的，天線對標簽大小的影響最大。在標簽的中心可看

30、見的是芯片。因為這是一個無源標簽所以無內(nèi)部的能源。RFID標簽工作原理是如下:閱讀單元產(chǎn)生電磁場引導電流流進標簽的天線。該電流用以給芯片提供能源。在無源標簽中該電流還為冷凝器充電，以保證芯片的不間斷供電。在有源標簽中電池取代了冷凝器。有源和無源標簽的區(qū)別是短期內(nèi)的信息闡釋。一旦被激活的標簽收到閱讀的命令它就可以發(fā)送序列號或所要求的信息?？偟膩碚f,標簽沒有足夠的能量來創(chuàng)造自己的電磁場, 相反它可以采用反向散射調(diào)制（反映/吸收）來產(chǎn)生由閱讀單元發(fā)射的電磁場。由于大多數(shù)流體吸收電磁場和大多數(shù)金屬反射這些場，故可使用的標簽閱讀材料是復雜的在一次循環(huán)解讀中, 閱讀器不得不持續(xù)給標簽供電。它所建立的場將產(chǎn)

31、生連續(xù)波,因為磁場的強度隨距離的平方而減少，故閱讀器必須有一個相當大的能源。該場迅速響應標簽給的任何指示, 因此標簽位于正下方的側(cè)渠道可以響應上述連續(xù)波的頻率。1.能源我們辨別三種不同的RFID電子標簽的能量或能源：被動、半被動和主動。被動式標簽沒有內(nèi)部電源,因此它們的能量來源于閱讀器。這意味著閱讀器必須保持磁場直到轉(zhuǎn)換完成。由于沒有電池,故這些都是可用的最小和最便宜的標簽。但它的閱讀范圍可從2毫米和幾米。這些標簽的另一個好處是適用于印刷生產(chǎn)。此外，因為它不依賴于內(nèi)部電源，所以它們的壽命是無限的。第二種類型是半被動式標簽。這些標簽都有內(nèi)部電源可在任何時候都給微芯片供電。它有許多優(yōu)點:由于芯片在

32、持續(xù)帶電的情況下反應迅速,因此可以增加每秒查詢的標簽數(shù)量，這是非常重要的應用。此外,由于天線不需要收集能量，故可以優(yōu)化用以反向散射和回歸來增加閱讀范圍。最后但并非不重要，因為標簽不使用任何磁場能量所以反向散射的信號越強，閱讀范圍更廣。由于最后兩個原因，半被動標簽通常比被動標簽應用范圍更廣泛。第三種類型是主動式標簽。類似于半主動標簽，它的內(nèi)部也有能源但它的能源用于兩個方面：給微芯片供電和使天線產(chǎn)生信號。主動式標簽發(fā)送信號而不被質(zhì)疑,這被稱為信標。主動標簽可查詢的范圍是幾十公尺,從而使其適宜于定位對象或理想標志點。壽命長達5年的。2.頻帶RFID電子標簽按照頻率分為三個部分:低頻(LF,30-500千赫)、高頻 (HF、10-15兆赫)、超高頻 (UHF),（850-950兆赫,2.4-2.5兆赫,5.8兆赫)。低頻標簽比任何高頻率的標簽都便宜。對于大多數(shù)應用程序來講,它們的響應速度很快。 但是，留在閱讀器的大量標記數(shù)據(jù)的時間范圍將增加。另一個優(yōu)點是低頻標簽由于流體的存在或金屬的存在而受到的影響最小。這類標簽的缺點是它們識別范圍很短。最常見的低頻標簽頻率是125-134.2千赫和140-148.5千赫。 高