Patent Application
20110139877
The present disclosure is directed generally to radio frequency identification (RFID) tags and, more particularly, RFID tags used in a system to detect the presence of items carrying such a tag.
RFID systems are comprised of two components—the tag, sometimes called a transponder, and a reader, sometimes called an interrogator. The tag acts as a programmable data storage device. The reader establishes a wireless communication link with the tag and reads the data stored in the tag. Both the reader and the tag have an antenna for communicating with one another.
There are many different ways to categorize RFID systems available today. One way to categorize systems is based on the type of antenna that is used in the tag. One type of antenna is disc-shaped and uses a circularly wound coil while another type is cylindrically shaped and uses a wire wound around a rod shaped ferrite core. The ferrite core may also be rectangular. Another way to categorize systems it to distinguish between passive systems which use a tag that relies on power harvested from the field created by the reader and antenna or the field in the environment, versus active systems in which the tag contains a power source such as a battery. Passive systems may be further categorized as near field systems that inductively couple to the reactive energy circulating around the reader's antenna and far field systems that couple to the radiated power contained in electromagnetic waves propagating in free space from the antenna of the reader. Systems can be categorized according to the frequency at which they operate. Two common frequencies of operation are 125 kHz and 13.56 MHz. Another way to categorize RFID systems is based on the communication scheme by which the tag and reader talk to one another. Regardless of these various ways of categorizing RFID systems, there are some fundamental principles that RFID systems must follow. One of those principles, described below, is that for maximum coupling between the tag and the reader, the tag should act as a resonant circuit that resonates at the operating frequency of the reader.
For lower frequencies such as 125 kHz and 13.56 MHz, the coupling between an RFID tag and reader is similar to the inductive coupling between the primary winding of a transformer and the secondary winding of the transformer. When the antenna connected to the reader (which in most cases is much larger than the antenna of the tag) is energized with an alternating current, an alternating magnetic field is created around the antenna. When an RFID tag is brought within this magnetic field, the antenna of the tag extracts energy from the magnetic field set up by the reader. Generally, the larger the tag antenna, the more energy that can be extracted from the magnetic field. Maximum coupling occurs when the tag resonates at the frequency of operation, e.g., 125 kHz or 13.56 MHz. Designing a tag that will resonate at the frequency of operation is based upon the following known equation:
resonant frequency=1/(2Π√{square root over (LC)}) (1)
Equation (1) relates the resonate frequency (e.g , 125 kHz or 13.56 MHz) to the values of inductance (L) and capacitance (C) of the tag antenna. The values of the inductance and capacitance of the tag antenna are known based on the physical characteristics of the antenna and chip capacitance. It is common for the inductance and capacitance of the tag antenna to not satisfy equation (1), so often a capacitor is added to the antenna circuit to cause equation (1) to be satisfied. Also, the number of coil turns may be adjusted thus adjusting the value of (L). When the values of (L) and (C) are fixed so that equation (1) is satisfied, the tag is said to be tuned, and the capacitor added to the antenna circuit is sometimes referred to as a tuning capacitor. Often, this capacitance is integrated into the integrated circuit chip.
Although the tuning of a tag may seem to be a straight forward matter, uncertainties caused by environmental conditions or a changing environment can quickly cause problems. For example, it is known that metals reflect higher frequency signals e.g., 13.56 MHz, to a greater extent than lower frequency signals e.g., 125 kHz. Metals also cause eddy currents, electrical currents flowing in the metal adjacent to a magnetic field. These eddy currents dissipate power meaning there is less power available for the tag. Finally, the proximity of metal causes the stray capacitance in the system to change from the value of the stray capacitance without the metal present, causing the tag to become detuned. One example is the interwinding capacitance in the tag antenna. Metal in close proximity to the tag antenna will change the capacitance and therefore the tuning. All of these effects reduce the distance at which a reader can detect a tag and interfere with the communication between the reader and the tag. Added to those problems is the fact that metal objects are often big and heavy compared to the size and weight of an RFID tag, and metal objects are often subjected to extremely harsh environments, such as a sterilizing environment. As a result, the use of RFID tags on metal objects is very problematic.
The work by Senba et al. attempts to address some of these problems. For example, U.S. Pat. No. 6,897,827 issued May 24, 2005 and entitled Installation Structure for RFID Tag, Method for Installing RFID Tag, and Communication Using Such RFID Tag discloses an RFID tag installing structure for installing a microminiaturized RFID tag having a cylindrical antenna coil to a conductive member. An RFID tag having a cylindrical antenna coil and shaped into a rod is installed such that the axial direction of the RFID tag is parallel to the installation surface composed of the bottom surface of an installation groove made in a conductive member and is in contact with the installation surface.
Another patent to Senba et al., U.S. Pat. No. 6,927,738 issued Aug. 9, 2005 and entitled Apparatus and Method for a Communications Device, discloses a sheet-like amorphous magnetic material being arranged in a manner extending from a magnetic flux generating portion of a concentric disk-shaped antenna coil of an RFID tag serving as the communication device to an outer area of the antenna coil.
Yet another patent to Senba et al., U.S. Pat. No. 7,088,249 issued Aug. 8, 2006 and entitled Housing Structure for RFID Tag, Installation Structure for RFID Tag, and Communication Using Such RFID Tag, discloses providing a novel installation structure for an RFID tag, which effectively protects the RFID tag from external stress or impact during the storage, transportation and usage, and allows communication with an external device. The '249 patent also discloses providing a novel installation structure for an RFID tag, which enables communication with the external device even if the RFID tag is installed on a conductive member such as a metal member, and the surface thereof is covered with a protective member typically made of a metal which has an excellent strength and durability. The '249 patent also discloses providing a communication method using an RFID tag surrounded by a conductive member typically made of a metal. Even if an RFID tag is housed in a container typically made of a conductive material such as a metal having a large mechanical strength, the RFID tag can communicate with an external read/write terminal as mediated by leakage magnetic flux if only a flux leakage path composed for example of a gap is formed in such container.
U.S. Pat. No. 3,594,805 is directed to aerials comprising a ferrite rod disposed within a longitudinally split sleeve of electrically conducting material and where a substantially uniform capacitance exists or is provided across the split. According to this invention, the resonant frequency of the aerial can be adjusted by varying the inductance of the split sleeve disposed around the ferrite rod.
Although work has been done toward providing RFID tags that can be used to tag and track metal items, tag-to-tag coupling of closely spaced tags, detuning, and physical damage remain serious issues. Thus, a need exists for a rugged, economical RFID tag that can remain tuned when attached to metal objects or brought into close contact with other tags and can work in an environment containing large numbers of metal objects while maintaining maximum read distances and high signal to noise ratios for the received communications.
The present disclosure is directed to an RFID tag comprising a magnetic core, a wire wrapped around the core, and an integrated circuit electrically connected to the wire. A metal sleeve has an open first end and an open second end opposing the first end. The sleeve further has at least one impediment to the flow of eddy currents formed between the first and the second ends. The impediment may take a virtually infinite number of forms including various types and combinations of slits, non contacting but overlapping sections (edges) of the sleeve, perforations, or nonmagnetic or nonmetallic portions formed in the sleeve. An encapsulating/potting material is used to fix the wire wrapped core and the circuit within the sleeve, while preventing electrical contact of the coil circuit with the metal sleeve.
The present disclosure is also directed to a method of constructing an RFID tag, comprising: winding a wire about a core; connecting an integrated circuit to the wire; inserting the wire wound core and circuit into a metallic sleeve having open opposing ends and at least one impediment to the flow of eddy currents formed between the open opposing ends; and fixing the wire wound core and circuit within the metallic sleeve. The method can include connecting an optional tuning capacitor if needed, or means to tune coil inductance.
The RFID tag disclosed herein has many benefits. The metal sleeve creates a known electrical environment for the RF components of the tag that effectively isolates the RF components from the environment outside of the sleeve and allows the resonant frequency to remain stable. A primary factor of this environment is the inter-winding capacitance. This is what is believed will change when a prior art tag is brought near metal. Bringing metal nearby a prior art tag also adds resistance, or loss, into the overall system. The end result of such effects is a shift in tuning which then makes the prior art tag resonate at a different frequency than the frequency at which the reader is designed to communicate. The observed effect on a prior art tag is therefore a drastic reduction in read range or the complete inability to read a tag when the tag is very near metal. Another benefit of the metal sleeve isolating the RF components from the environment is that the tags are not de-tuned when brought into close proximity of other RFID tags (including other like tags). This is distinct from the benefit of not detuning around metal. Coupling and subsequent detuning of tags that are closely packed into a volume is a common problem in RFID system design. Another benefit of the RFID tag disclosed herein is that the impediment (e.g., slit) in the metal sleeve stops eddy currents which can cause power and signal losses. This encapsulated, metallic sleeve configuration with the air gap is a novel form factor. Those advantages and benefits, and others, will be apparent from the detailed description below.
For the present disclosure to be easily understood and readily practiced, preferred embodiments will now be described, for purposes of illustration and not limitation, in conjunction with the following figures.
Turning now to
As shown in
The combination 27 of the RFID chip 24, the wire 22 wrapped core 10, and optional tuning capacitor 27 is inserted into a metal sleeve 26 shown in
In one embodiment, the finished RFID tag 12, sometimes referred to as a transponder, is designed to operate at a 13.56 MHz frequency or other frequencies which use inductive coupling for communications and energy transfer, for use on metal instruments, such as instrument 14 shown in
The sleeve 26 preferably has a complete air gap or slit 34 running lengthwise from the first open end 28 to the second open end 30 although there are infinite patterns of slits that could work. Because the slit doesn't need to be straight, the slit could be wavy, an “interlocking finger” pattern, etc. There could also be overlapping edge portions with vertical separation between layers such that from the outside looking at the tag, it might appear to have no slit. (See
The dimensions of the slit are preferred to be a small percentage of the circumference of the tag. The prototypes we have tested have a slit having a width that is 15% of the circumference of the metal sleeve. We believe slits anywhere from 1-25% will work, with 15% being a presently preferred embodiment. Slits of a width that is above 25% of the circumference of the sleeve may result in diminishing effectiveness of the slit.
Other methods may be used to reduce the effects of eddy currents while still providing the benefits of a consistent electrical background. One other method contemplates using a perforated metal sleeve. Another method contemplates the use of a sleeve having non-metallic regions interspersed through out the sleeve. (See
Consideration should be given to the detail around the slit, in particular the width of the slit and the cross-section of the sleeve material. The two side walls of the slit, brought very near to each other, could create a capacitance that would allow for current to flow. At the intended frequencies of operation, the slit would need to be extremely narrow for capacitance to begin to have an effect. We believe that the slit size would be within the tolerance of most production equipment, so the mere act of designing a slit that can be manufactured would eliminate designs where capacitance is an issue. Also realize that the sleeve thickness can be adjusted without consequence, as long as a thickness greater than the electrical skin depth is maintained.
There may be some designs that result in some capacitance across the slit. By controlling the configuration of the edges 29 of the sleeve 26 at the slit 34 as shown in
It is believed that the longitudinal slit 34 running from the first open end 28 to the second open end 30 presents an impediment or barrier to the flow of eddy currents. The purpose of the metal sleeve 26 is to provide a large metal presence which controls the tuning of the finished tag 12. Once the finished tag 12 is tuned, placing the finished tag 12 in the presence of metallic objects does not detune the finished tag 12 because it is believed that the presence of the metal objects is inconsequential due to the close proximity of the metal sleeve 26 in conjunction with the combination 27. In effect, the metal sleeve 26 isolates the RFID chip 24 from the environment outside of the sleeve 26. The wire 22 forms a coil which is tuned (perhaps with the aid of a tuning capacitor or by controlling the number of windings) to interact with only the magnetic sleeve 26 such that any additional metal next to the outside of the sleeve 26 does not disturb the interwinding capacitance which will disturb the resonant frequency of the tuned tag 12.
A benefit of using some type of slit over other types of impediments is that we can leverage the concept of the slit to create a “switchable” RFID tag. By physically closing the slit, either by directly pushing or squeezing the sleeve until the slit is closed, or by closing a jumper circuit or electrical switch (see
Another embodiment of the present invention is disclosed in
A closer look at the finished tag 42 shown in
As will be apparent from the foregoing description of the embodiments of the present invention, the particular shape or configuration of the ferrite cores 10, 36 is not critical to the operation of the present invention. Furthermore, the particular shape or configuration of the metallic sleeves 26, 38, whether square, rectangular, or circular, among others, is not critical to the operation of the present invention. Providing the metal sleeve with a slit provides a means for the flux lines of the ferrite core to complete their electromagnetic circuit and maintain optimal readability. Although the slits illustrated in the embodiments discussed above extend perpendicularly from one open end to the other open end of the sleeve, the slits may extend at an angle. Other types of discontinuities or impediment to the flow of eddy currents may also be used as discussed above and as shown in
Turning now to
Readability testing was performed using the following equipment and settings:
As the instrument wand (reading antenna) is swept over the tag, there exists a best case read range for tags positioned vertically with respect to the reading antenna as shown in
The read range was tested for three tag types, commercially available Tagsys™ sponge tags, the RFID tags of the present disclosure by themselves, and the RFID tags of the present disclosure attached to metal instruments. These tests were done using the best case vertical read range (see
It can be seen that there is no difference in read range when a tag is welded to a metal instrument. There is a 1.75 inch increase in read range if reader power is increased from 2 W to 8 W. It is possible to run this wand using a low power, if maximum read range is not required. This reduction to practice of the invention can effectively allow reading of tagged surgical items, even when in a metal tray, from at worst case, four inches away when using lowest reader power settings. This reduction to practice could be optimized further by improving reader antenna and detection characteristics.
Turning now to
While the disclosure has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope of the embodiments. Thus, it is intended that the present disclosure cover the modifications and variations of this disclosure provided they come within the scope of the appended claims and their equivalents.
