The main factor that characterizes the RFID system is the Tags. Before we dive into the details of the tags it’s important to know about the readers, it is to be noted that generally a Passive Tag reader can’t read an active tag and vice versa, however these days there are these multi-protocol readers which can read more then one type of tags. In the next sections we will focus on the different types of RFID systems and we will cover in this section the following types of tags:
Passive RFID systems
SAW RFID Tags
Semi Passive RFID systems
Active RFID systems
The RFID reader is a device that creates an electromagnetic signal, which is transmitted to the RFID tags through one or more antennas. Under normal operation, the reader is continuously transmitting the electromagnetic signal in search of one or more RFID tags. The RFID reader also performs a second function of monitoring for electromagnetic signals from the RFID tags via the same antenna.
Passive RFID Tags
The Passive tags do not contain a power source. To power the tag circuitry, the tag relies on electromagnetic power obtained from the RFID systems antenna. Since passive tags do not contain their own power sources, the designs can be simpler and less expensive.
They can also have an unlimited shelf life in comparison to active tags. This has made the passive tag the focus of most government and commercial RFID mandates. The downside of all passive tags is their extremely limited range. Since passive tags depend on power from the reader and antenna, with the current technology, passive tags must be in close proximity to the reader and antenna in order to obtain sufficient power to transmit a signal.
Many RFID experts believe that passive tags are the future of RFID. In the last few years, the unit price of passive RFID tags has steadily gone down in cost. This is a result of, at least, increased scales of production. Some industry analysts believe that when the cost of individual tags reaches five cents, significant acceptance will be achieved. At that point, RFID tags may be placed on many consumables. Should this happen, the vision of consumers bypassing the checkout counter may soon follow.
SAW RFID Tags
SAW (surface acoustic wave) tags are passive RFID tags that operate in a fundamentally different way from typical RFID tags. Typical RFID tags are based on semiconductor physics to provide power for transmission of their ID number. SAW tags convert an incoming wave from the interrogator into nano-scale surface acoustic waves on the surface of the chip. The wave travels past a set of acoustic wave reflectors that encode the wave into a unique pulse train. This pulse train is converted to a radio wave to be sent back to the reader.
Semi Passive RFID Tags
Tags can also be designed with features found in both passive and active tags. These are attempts to retain the advantages while eliminating the disadvantages of each type. Semi-active tags typically use an internal battery to power circuitry that is internal to the tag itself. Typically, circuitry on semi-active tags includes sensors for monitoring environmental conditions such as temperature and humidity. Sensors can also be powered to detect vibration or movement. These are typically used to monitor the possibility of damage or unauthorized movement during transport or storage.
However, in contrast to active tags, the semi-active tag does not use its internal power source to communicate with the antenna. For communications functions, the semi-passive tag relies on electromagnetic field power received from the system’s antenna. By conserving its internal power in this manner, the internal battery life can be greatly extended.
Active RFID Tags
In contrast to passive tags, active tags contain an onboard power source. This is usually in the form of a small battery. The battery powers both the tag’s internal circuitry and the onboard antenna. The additional circuitry required by the battery as well as the battery itself requires that active tags be larger and more expensive than passive tags. Many active tags, for example, have plastic housings. These cannot simply be adhered to high-volume inventory in the same manner as a film or Mylar-based passive tag. Because of this, specific consideration must be made to affixing the active tag to the inventory or pallet being tracked.
As a result of the additional power offered by the battery, the range of active tags is generally far superior to that of passive tags. Active tags can have transmission ranges measured in hundreds or even thousands of feet instead of just a few feet, as is normal in the case of passive tags. Active tags conserve battery power by normally existing in a sleep mode. The tag is woken up or activated by entrance into an RFID system interrogation zone.
The powered tag then provides data to the RFID system as requested. The ability to normally exist in a sleep mode greatly lengthens the operational life of an active tag. The minimal power consumption in the normal sleep mode enables many tags to remain operational for several years. The actual length of the battery life will be dependent on the number of times that the tag is activated. Thus, the RFID engineer will have to design or set up an active RFID system so that in the event that tagged material is stored within an interrogation zone the tags will not be continuously activated until their batteries are exhausted.
RFID active tag batteries come in many shapes and sizes. Many RFID-specific batteries superficially resemble normal commercial equivalents. However, the RFID batteries are likely to function at a higher voltage of 3.6 versus 1.5 for smaller cells, as in the case with many defense-related RFID tags. To avoid potential damage to both RFID and other conventional battery equipment, it is imperative that control be maintained over both the storage and replacement of RFID-specific batteries.
The high power demands of RFID active tag batteries may also require different battery chemistry than conventional equipment. Whereas most electronics utilize alkaline, nickel-cadmium, or nickel metal hydride batteries, RFID batteries are more likely to be based on substantially more costly advanced battery technology such as lithium chemistry. The higher cost associated with lithium batteries may also lead the RFID tag manufacturer to produce rechargeable systems.
Active tags can also be more sophisticated than passive tags. In some cases, active tags can be interfaced with other technologies such as the global positioning system (GPS) and/or satellite communication systems. The global positioning system is a set ofU.S.government satellites orbiting around the earth. A GPS receiver communicates with the GPS satellites. Nowadays, receivers can be found in cell phones, radios, and wristwatches. If the minimum required number of satellite signals can be acquired by the GPS receiver, the system can determine its location within as little as ten meters. This means that GPS interfaced tags can be placed on large shipping containers or tractor trailer rigs to determine both the identification and location of product.
Tags primarily operate at either high frequency (HF) or ultra-high frequency (UHF). HF is most often 13.56 MHz, while UHF can range from 902 to 928 in theUnited States. The 2400–2500 MHz range may also use. Some active tags for specialized applications may utilize microwave frequencies. The use of either HF or UHF tags for more normal applications is dependent on the range required and the materials present in the system.
HF tags are generally limited in ranges measured in inches. This lends HF tags to inventory applications where the items are in close proximity to each other and in close proximity to a reader. UHF tags operating between 902 and 928 MHz, on the other hand, can be utilized out to several feet or even yards. The greater range of UHF tags makes them more applicable to shipping dock type applications; 2400- to 2500-MHz tags may have a range between one and four feet. Both the packaging material and the material itself is a significant RFID system issue as some materials are known as radio frequency absorbing while others are radio frequency reflecting. Examples of radio frequency reflecting materials are metallic items or containers. The RF reflecting characteristic of metals can prevent the tag antenna from absorbing sufficient RF energy to be powered by defecting the RF wave. Examples of RF absorbing materials are liquids, moisture. Liquids reduce the effectiveness of the RF wave by absorbing the energy. The reduced strength RF signal then does not have sufficient power to activate the tag.
When the tag enters the interrogation zone, the data stored in the tag is transmitted to the RFID reader antenna. The data can be ASCII, hex characters, or decimal characters. The data that is stored in the tag is dependent on the tag’s writing capability. The three general types of writing capabilities are:
Write once, read many
Read-only tags are tags where the identification data is entered by the tag’s manufacturer. Thus, these types of tags must be either specified by the manufacturer and accepted by the purchaser or specified by the purchaser. In many cases, the identification data is used by a number of different organizations. Therefore, it is actually easier to control if the identification data is assigned by the manufacturer. A typical example is the E-ZPass toll way system. A vehicle is assigned a tag with a specific number regardless of which toll way system the tag is purchased through. Since the tag number is controlled, it may be used on other toll way systems in the tag consortium.
Write once; read many (WORM) tags are not programmed by the manufacturer. The purchaser is given the opportunity to write the identification data to the tag. However, with the WORM type of tag, this identification data cannot be erased. This means that once the data is written it cannot be changed. However, in some cases, if additional memory space is available, additional identification data can be added. Generally, in the event that incorrect data is written to the tag, the tag must be discarded.
As with WORM tags, read/write tags are not programmed by the manufacturer. It is the purchaser who programs the tags. The advantage of the read/write tag is that the purchaser can reprogram the identification data held by the tag. Thus, any identification data writing errors can be corrected. Read/write tags are generally the most sophisticated type of the three types of tags. Often additional information may be stored. It is also possible to lock certain areas of the tag’s memory so that it cannot be erased.
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