Communications: electrical – Selective – Interrogation response
Reexamination Certificate
2000-09-11
2004-11-30
Horabik, Michael (Department: 2635)
Communications: electrical
Selective
Interrogation response
Reexamination Certificate
active
06825754
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to communication devices, and more particularly to radio frequency identification (RFID) devices providing increased tag activation distances.
BACKGROUND OF THE INVENTION
In general, an RFID system consists of one or more tags, a tag reader, and a host computer system. Tags are devices that can come in many sizes and form factors, but are usually small and lightweight. Tags are commonly used as portable data devices that wirelessly communicate with RFID readers at distances ranging from a few millimeters to several meters. The information stored in a tag can be used, for example, to identify an individual or object carrying the tag.
RFID technology is used in a variety of applications because of its convenience and flexibility. An example application for RFID technology is a building security system. As part of a building security system, RFID systems are used to grant access only to individuals carrying authorized tags (or cards). When an individual places their card in the vicinity of the reader, the reader interrogates the card and obtains identification information stored in the card. After further processing, the reader communicates the individual's identification (“ID”) code to a host computer in the security system. If the ID code received by the host computer system is authorized, the door is unlocked to permit access to the building.
RFID systems are also used to detect specific items and link those items with other information and events. RFID systems can be used, for example, to track products being built in a factory, to trigger manufacturing steps to occur, to assist in inventory control, etc. Read-only tags are ones in which the data is programmed once, and the tag only sends the stored information to the reader. Read-write tags have the ability to be reprogrammed to suit the needs of the application. Therefore, read-write tags can be used as portable databases, eliminating the need for central databases.
Most RFID tags contain functional electronics in the form of an integrated circuit, or IC, to store and process data, and to perform communication functions. RFID tags also contain an antenna, which is used as the radio frequency interface with the reader. The IC requires power to operate, which can be supplied by a battery. Most applications, however, require tags to be small and inexpensive, so batteryless, or “passive”, tags are in very wide use. Passive tags receive energy from the radio frequency (“RF”) field generated by a reader, and the IC converts the RF to direct current (“DC”) operating power for itself. Once operating, the IC communicates with the reader, which has an antenna system for transmission and reception of signals. Power and data are transferred between tag and reader through one or more antennae in each device. The reader antenna used to couple power and/or information from the reader to the tag is called the exciter antenna. The reader antenna used to receive information from the tag is called the receive antenna.
A key performance parameter of all RFID systems is read range. Read range is the distance between the card and reader at which the reader captures the data transmitted by the card. Read range is a function of two factors. Tag activation distance is the distance between the tag and the reader at which the tag receives sufficient energy to power the IC. The tag can then begin to send data to the reader. Receiver sensitivity controls the distance at which the reader can receive the transmitted data. If receiver sensitivity is low or is compromised by interfering signals the tag must be moved closer to the reader until the data signal strength exceeds the reader receiver sensitivity. Internally or externally generated noise may effectively reduce the sensitivity of the receiver. Thus the read range of an RFID system can never be greater than the tag activation distance.
Some tag-reader systems communicate via magnetic fields, while other types of systems communicate via electric fields. Electric field tags offer advantages in cost, size, weight and flexibility compared with magnetic field tags. Many applications demand small, compact and inexpensive readers, as well. Shrinking the size of electric field RFID readers, however, presents unique design challenges. Without addressing these challenges, reader performance is significantly impaired.
Two types of electric field RFID reader and tag systems exist, and are referred to as monopole and dipole systems. A monopole electric field RFID reader, or RFID device, has a single exciter antenna, or exciter electrode, driven by a voltage source that is referenced to an impedance that is common with the environment (“common impedance”) in which an electric field tag is being used. The tag has two antennae, or electrodes, the first of which is preferentially coupled to the impedance that is common with the RFID device. For example, the preferential coupling impedance may be formed by a person holding the tag, while the common impedance may be formed by earth ground. The total return impedance, defined as the sum of the preferential coupling impedance and the common impedance, may be resistive, capacitive, inductive, or any combination thereof. When the tag is close enough to the RFID device, displacement current will flow from the exciter electrode to the second tag electrode through the capacitance that exists between them. Current will then flow through the tag IC, and back to the reference terminal of the exciter voltage source through the total return impedance. If sufficient current flows, the tag IC will become activated. In general, the total return impedance is much lower than the reactance of the very small capacitance that exists between the exciter electrode and the second tag electrode. Therefore, the displacement current that activates the tag is limited primarily by the small capacitance between the reader and the tag.
Dipole electric field RFID devices contain two exciter electrodes whose voltages are opposite in polarity but balanced about a common impedance path such as earth ground or the chassis of equipment. Dipole systems do not require preferential coupling of one tag electrode to a common impedance of the system, although preferential coupling may be utilized in some cases. Because the pair of tag electrodes is coupled to the pair of RFID device exciter electrodes through a series combination of two small capacitance values, the effective tag-reader coupling impedance is much larger in dipole systems than in monopole systems. Therefore, the tag activation distance of a dipole electric field RFID system is substantially smaller than that of a similarly sized monopole electric field RFID system. Monopole electric field RFID systems are used in more applications than are dipole systems because of the improved coupling efficiency and large tag activation distance that is achievable.
For simplicity, the following discussion will describe electric field RFID devices in terms of monopole systems, even though the concepts also apply to dipole systems. For clarity, only the excitation function of electric field RFID systems will be described. Other functional elements that are required for full RFID device functionality, such as the receive electrode(s), receiver, demodulator, decoder, processor, I/O circuitry, etc., are understood by those skilled in the art, and are not relevant to this discussion.
FIG. 1
is a simplified side pictorial/schematic view of a system containing the electric field RFID device
14
and the tag
5
, illustrating the excitation portion of the system. In
FIG. 1
, the exciter antenna assembly
1
is comprised of the exciter electrode
2
(e.g., antenna plate, etc.) which is illustrated as an electrically conductive layer, or sheet, disposed upon the dielectric substrate
3
, which can be fabricated using well-known technologies such as, but not limited to, printed circuit board technology. The exciter voltage source
4
generates a high alternating current (“AC”) voltage that is connec
Bangachon William
Horabik Michael
Hughes Terri S.
Miller Thomas V.
Motorola Inc.
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