1.0 INTRODUCTION
Scanning is considered as electronics imaging. An electronic imaging system usually consists of an input scanner which converts an optical image into electrical signal. This is followed by electronic hardware and software for processing or manipulation of the signal and for storage and/or transmission to an output scanner. The latter converts the final version of the signal back into optical (visible) image, typically for transient (softcopy) or permanent (hardcopy) display to a human observer. The scanning system can be divided by using infrared technology and radio frequency technology.
1.1 Wireless Communication / RF Technology
Wireless communication systems require frequency signals for the efficient transmission of information. Since the signal frequency is inversely related to its wavelength, antennas operating at RFs and microwaves have higher radiation efficiencies. Radio Frequency (RF) refers specifically to the electromagnetic field, or radio wave, that is generated when an alternating current is input to an antenna. This field can be used for wireless broadcasting and communications over a significant portion of the electromagnetic radiation spectrum from about 9 kilohertz (kHz) to thousands of gigahertz (GHz). As the frequency is increased beyond the RF spectrum, electromagnetic energy takes the form of infrared, visible light, ultraviolet, X-rays and gamma rays.
Further, their size is relatively small and hence convenient for mobile communication. Another factor that favors RFs and microwaves is that the transmission of broadband information signals requires a high-frequency carrier signal. Wireless technology has been expending very fast. In addition to the traditional applications in communication, such as radio and television. RF and microwaves signal are being used in works and personal communication service. Keyless door entry, radio frequency identification (RFID), monitoring of patients in hospital or a nursing home, cordless keyboards for computers are and many measuring and instrumentation systems used in manufacturing some of the other areas where RF technology is being used and operate at infrared or visible light frequencies.
The RF spectrum is divided into several ranges, or bands. Each of these bands, other than the lowest frequency segment, represents an increase of frequency corresponding to an order of magnitude (power of ten). The electromagnetic spectrum is a continuum of all electromagnetic waves arranged according to frequency and wavelength. Electromagnetic radiation is classified into types according to the frequency and length of the wave. Visible light that comes from a lamp in your house or radio waves transmitted by a radio station are just two of the many types of electromagnetic radiation. An electromagnetic wave consists of the electric and magnetic components. These components repeat or oscillate at right angles to each other and to the direction of propagation, and are in phase with each other.
Figure 1 : An electromagnetic waves
These frequencies make up part of the electromagnetic radiation spectrum such as below:
· Ultra-low frequency (ULF) -- 0-3 Hz
· Extremely low frequency (ELF) -- 3 Hz - 3 kHz
· Very low frequency (VLF) -- 3kHz - 30 kHz
· Low frequency (LF) -- 30 kHz - 300 kHz
· Medium frequency (MF) -- 300 kHz - 3 MHz
· High frequency (HF) -- 3MHz - 30 MHz
· Very high frequency (VHF) -- 30 MHz - 300 MHz
· Ultra-high frequency (UHF)-- 300MHz - 3 GHz
· Super high frequency (SHF) -- 3GHz - 30 GHz
· Extremely high frequency (EHF) -- 30GHz - 300 GHz
1.2 Radio waves
Radio waves can propagate from transmitter to receiver in four ways: through ground waves, sky waves, free space waves, and open field waves. Ground waves exist only for vertical polarization, produced by vertical antennas, when the transmitting and receiving antennas are close to the surface of the earth. The transmitted radiation induces currents in the earth, and the waves travel over the earth's surface, being attenuated according to the energy absorbed by the conducting earth. The reason that horizontal antennas are not effective for ground wave propagation is that the horizontal electric field that they create is short circuited by the earth.
Figure 2 : Geometry of Tropo-Scatter Signal Propagation
Ground wave propagation is dominant only at relatively low frequencies, up to a few MHz, so it needn't concern us here. Sky wave propagation is dependent on reflection from the ionosphere, a region of rarified air high above the earth's surface that is ionized by sunlight (primarily ultraviolet radiation). The ionosphere is responsible for long-distance communication in the high-frequency bands between 3 and 30 MHz. It is very dependent on time of day, season, longitude on the earth, and the multiyear cyclic production of sunspots on the sun. It makes possible long-range communication using very low power transmitters. Most short-range communication applications that we deal with in this chapter use VHF, UHF, and microwave bands, generally above 40 MHz. There are times when ionospheric reflection occurs at the low end of this range, and then sky wave propagation can be responsible for interference from signals originating hundreds of kilometers away.
The most important propagation mechanism for short-range communication on the VHF and UHF bands is that which occurs in an open field, where the received signal is a vector sum of a direct line-of-sight signal and a signal from the same source that is reflected off the earth. Later we discuss the relationship between signal strength and range in line-of-sight and open field topographies. The range of line-of-sight signals, when there are no reflections from the earth or ionosphere, is a function of the dispersion of the waves from the transmitter antenna. In this free-space case the signal strength decreases in inverse proportion to the distance away from the transmitter antenna.
2.0 Radio Frequency Identification (RFID)
Radio Frequency Identification (RFID) is a system that facilitates the tracking of objects, primarily for inventory tracking, via a three-part technology comprised of a reader, a transceiver with decoder and a transponder (RF Tag). RFID is a wireless system that works in conjuction with an organization’s information technology infrastructure to improve business processes such as inventory management and efficiency in supply chain management.
The RFID is not a new technology. For example, the principles of RFID were employed by the British in World War II to identify their aircraft using the IFF system (Identify Friend or Foe). Later, work on access control that is more closely related to modern RFID, was carried at Los Alamos National Laboratories during the 1960s, RFID tags incorporated in employee badges enabled automatic identification of people to limit access to secure areas, and had the additional advantage that it made the badges hard to forge. For many years this technology has been relatively obcure, although it has been adopted in various niche domains, such as to identify animals, make toys interactive, improve car-key designs, label airline luggage, time marathon runners, prevent theft, enable automatic toll-way billing (smart tag), and many forms of ID badge for access control. Today, it is even being applied to validate money and passports, and as a tamper safeguard for product packing.
Figure 3: RFID chip
2.1 RFID Topology
An RFID system consists of a tag made up of a microchip with an antenna, and an interrogator or reader with an antenna. The reader sends out electromagnetic waves. The tag antenna is tuned to receive these waves. A passive RFID tag draws power from the field created by the reader and uses it to power the microchip's circuits. The chip then modulates the waves that the tag sends back to the reader, which converts the new waves into digital data. In its minimalist configuration the micro-topology requires just four sub-systems, as follows:
i. Tag
ii. Reader
iii. Air Interface
iv. Computer Communication and Control
The most demanding macro-topology involves pervasive tagging where sophisticated Readers are simultaneously interrogating multiple tags in a dynamic environment. In this scenario, anti collision algorithms are required in addition to data handling processes for large velocity data streams.
Figure 4 :Basic RFID system consists of three components
a. Tags
The basic RFID building blocks are miniature electronic devices known as Tags which talk to Readers. The RFID tags, also known as transponder, are usually small pieces of material, typically comprising three components: an antenna, a microchip unit containing memory storage an encapsulating material. Tag are embedded or attached to an item. The Tag has memory which stores information as either read only, write once or unlimited read/write. Tags typically range in size from a postage stamp to a book, depending on read distance and features. RFID tags come in a wide variety of shapes and sizes.
Implementation of tags is animal tracking tags, inserted beneath the skin, can be as small as a pencil lead in diameter and one-half inch in length. Tags can be screw-shaped to identify trees or wooden items, or credit-card shaped for use in access applications. The anti-theft hard plastic tags attached to merchandise in stores are RFID tags. In addition, heavy-duty 5- by 4- by 2-inch rectangular transponders used to track intermodal containers or heavy machinery, trucks, and railroad cars for maintenance and tracking applications are RFID tags.
Figure 5 : RFID Tags
RFID tags are categorized into active and passive. They are fundamentally distinct technologies with substantially different capabilities. Both of the technology use radio frequency energy to communicate between a tag and a reader, the method of powering the tags is different. Active RFID tags are powered by an internal battery or internal power source continuously power the tag and its RF communication circuitry and are typically read/write, i.e., tag data can be rewritten and/or modified.
While passive RFID tags operate without a separate external power source and obtain operating power generated from the reader. The passive RFID relies on RF energy transferred from the reader to be tag to power the tag. Passive tags are consequently much lighter than active tags, less expensive, and offer a virtually unlimited operational lifetime.
Item / Active RFID / Passive RFIDTag power source / Internal to tag / Energy transferred from reader via RF
Tag battery / Yes / No
Availability of Tag Power / Continuous / Only within field of reader
Required signal strength from reader to Tag / Low / High (must power the tag)
Available signal strength from tag to Reader / High / Low
Table 1 : Technical differences between Active and Passive RFID technologies
While this distinction may seem minor on the surface, its impact on the functionality of the system is significant. Passive RFID either 1) reflects energy from the reader or 2) absorbs and temporarily stores a very small amount of energy from the reader’s signal to generate its own quick response. In either case, passive RFID operation requires very strong signals from the reader, and the signal strength returned from the tag is constrained to very low levels by the limited energy.
Active RFID / Passive RFIDCommunication range / Long range (100m and above) / Short or very short range (3m or less)
Multi-tag collection / i. Collect 1000s of tags over a 7 acre region from a single reader.
ii. Collects 20 tags moving at more than 100 mph / i. Collect’s hundreds of tags within 3 meters from a single reader.
ii. Collects 20 tags moving at 3 mph or slower.
Sensor capability / Ability to continuously monitor and record sensor input; data/time stamp for sensor events / Ability to read and transfer sensor values only when tag is powered by reader; no date/time stamp
Data storage / Large read/write data storage (128kb) with the sophisticated data search and access capabilities available / Small read/write data storage (e.g. 128 bytes)
Table 2 : Functional capabilities of Active and Passive RFID technologies
Item / Characteristics / TechnologyBoxes individual luggage / Structured, orderly process for loading / Passive RFID
Unit Load Device / · Unstructured movement throughout airport facility
· Security requirements / Active RFID
Boxes Cartons Individual Items / Structured, orderly process for loading-dedicated loading stations conveyors / Passive RFID
Pallet / Structured or unstructured movement, depanding on situation / Passive RFID or Active RFID
Intermodal Container / · Security requirements
· Area monitoring within ports, terminals
· Roadside monitoring / Active RFID
Chassis, rail car, other conveyance / · Area monitoring within ports, terminals
· Roadside monitoring
· Intransit visibility / Active RFID
Table 3 : Complementary use of Active and Passive RFID
As usual every electrical applicant has their own problem of implementation. Such as the RFID tags with the tag collision. Tag collision occurs when more than one transponder reflects back a signal at the same time, confusing the reader. Different vendors have developed different systems for having the tags respond to the reader one at a time. These involve using algorithms to "singulate" the tags. Since each tag can be read in milliseconds, it appears that all the tags are being read simultaneously.
b. Reader
The Reader is able to talk to the Tag using radio waves over the air to send or receive information. The distance between the Tag and Reader for the radio waves to be strong enough for the devices to talk with each other is an important specification in building a reliable RFID system. Once you have reliable radio communications between the Tag and the Reader the system may take action based on results of their communication. RFID may send information downstream to your legacy systems or update digital information stored on the Tag. This wide range of options and the real time capability of RFID give it exciting new capabilities, distinct advantages and specific costs to build its infrastructure. Types of ActiveWave RFID Readers. There are several types of Readers available. Please refer to data sheets for more details.
Fixed Reader AC Power / 120/230 Volts ACFixed Reader DC Power / 12 Volt DC
PC-Card Reader / For use with portable devices in trucks, forklifts, etc.
Handheld Reader / Has a wireless link to standard ActiveWave RFID readers
The disadvantage of the reader is the RFID reader overlaps with another reader called reader collision. This causes two different problems: