HK1149374A - Enhancing the efficiency of energy transfer to/from passive id circuits using ferrite cores - Google Patents

Abstract

In one embodiment the present invention includes an RFID gaming token with a ferrite core. When the RFID gaming tokens are stacked, the ferrite cores steer the flux field from the excitation antenna through the center of the annular antennas in each token. The resulting flux field increases the efficiency of the energy transfer from the excitation antenna to the passive tags. This increased efficiency also improves the data transfer to and from the passive tags. This increased efficiency allows for reading RFID gaming tokens at a higher stack height (or at a better error rate for a given stack height) as compared to existing air core gaming tokens.

Description

Enhancing energy transfer efficiency to/from passive ID circuits using ferrite cores

Technical Field

The present invention relates to Radio Frequency Identification (RFID) tags, and in particular to RFID tags having a ferrite core.

The application claims priority from U.S. provisional application No. 61/020,543 entitled "Enhancing the Efficiency of Energy Transfer to/from Passive ID circuits Using Ferrite Cores" filed on 11.1.2008 and incorporated herein by reference.

Background

Unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

The quality factor (also referred to as "Q factor" or "Q") is a dimensionless parameter that compares the time constant of amplitude decay of an oscillating physical system with its oscillation period. Equivalently, the quality factor compares the frequency of the system oscillation with the rate at which it dissipates energy. A higher Q indicates a lower rate of energy dissipation relative to the oscillation frequency, so the oscillation disappears more slowly. With respect to RFID circuits, Q is a measure of the "quality" desired in a well-tuned circuit or other resonator.

More specifically, when driving an RFID tag with a sinusoidal drive, its resonant behavior strongly depends on Q. A resonant system responds more strongly to frequencies close to its natural frequency than to other frequencies. RFID tags with high Q resonate with a larger amplitude (at the resonant frequency) than RFID tags with a low Q factor. The amplitude of the resonant response affects the read range. To increase sensitivity and read range, many existing RFID tags have a relatively high Q. As a specific example, many existing RFID tags have a Q between 10 and 20.

However, when multiple tags are in close proximity, they tend to interact. This interaction changes the resonant frequency of its operation. Rather than a single resonance at the desired frequency, this interaction results in multiple resonances at undesirable frequencies. Thus, when the tags are in close proximity, the tags are not excited and/or data is not successfully exchanged. Various approaches may be used to mitigate this interaction, but these strategies may reduce read range. It is possible, but not always available, to compensate for this reduction in read range by increasing the power output of the reader.

Therefore, it is desirable to read RFID tags in close proximity, but to do so with an acceptable read range.

Disclosure of Invention

Embodiments of the present invention improve the read range of RFID tags. In one embodiment, the present invention includes a Radio Frequency Identification (RFID) tag. An RFID tag includes a magnetically permeable core, an antenna that receives electromagnetic energy, and tag electronics that identify the RFID tag in response to receiving the electromagnetic energy.

According to various embodiments, the magnetically permeable core may have various properties. The magnetically permeable core may be constructed of a ferrite material. The magnetically permeable core may guide a magnetic flux field of the RFID tag. The magnetically permeable core may deform a magnetic flux component of the electromagnetic energy. The deformed magnetic flux component may concentrate flux lines through the magnetically permeable core. The concentrated flux lines may couple electromagnetic energy to the antenna with increased efficiency compared to the absence of the magnetically permeable core. The RFID tag may be one of a plurality of RFID tags that, when stacked, form a stack having a solid bar of magnetically permeable material that further directs a magnetic flux field upwardly through the stack. The magnetically permeable core may guide a magnetic flux field of the RFID tag, and guiding the magnetic flux field may increase a read height of the RFID tag. The magnetically permeable core may have a magnetic permeability between 100 and 150. The magnetically permeable core may correspond to a nickel zinc ferrite material having a magnetic permeability between 100 and 150. The magnetically permeable core may improve the efficiency of energy transfer from a reader device that generates electromagnetic energy to an RFID tag. The magnetically permeable core may improve the efficiency of data transfer from a reader device that generates electromagnetic energy to an RFID tag. The magnetically permeable core may improve the efficiency of data transfer from the RFID tag to the reader device that generates the electromagnetic energy.

According to various embodiments, the tag electronics may have various attributes. The tag electronics can identify the RFID tag by modulating the carrier frequency of the electromagnetic energy. The tag electronics can identify the RFID tag by modulating the carrier frequency of the electromagnetic energy according to a modified Aloha protocol.

According to various embodiments, the antenna may have various properties. The antenna may be a loop antenna. The antenna may be a loop antenna surrounding a magnetically permeable core. The antenna may have a Q of less than 5. The antenna may have a Q of less than 1. The antenna may be untuned.

The magnetically permeable core may be one of many magnetically permeable cores within the device. The tag electronics may be part of a set of tag electronics that provides multiple identifications of the device. The antenna may be one of a plurality of antennas within a device.

The RFID tag may be one of many similar RFID tags. When the tags are stacked and a small gap is created between the cores of the tags in the stack, an increased signal strength of between 24dB and 10dB may be created when the small gap is between 0% and 9%. When the tags are stacked, the tags may be misaligned, yet the overlapping cores still guide the magnetic flux.

The RFID tag may be part of an RFID system. The RFID system may include a reader system that generates electromagnetic energy. The reader system may include a number of reader antennas that generate electromagnetic energy to read RFID tags proximate to one of the reader antennas.

The following detailed description and the accompanying drawings provide a better understanding of the nature and advantages of the present invention.

Drawings

1A-1E show an RFID gaming system 100 according to an embodiment of the invention.

Fig. 2A, 2B, and 2C show various aspects of the performance of embodiments of the present invention.

FIG. 3 is a graph showing a comparison between a prior air core RFID tag and a ferrite core RFID tag, according to embodiments of the present invention.

Fig. 4 is a waveform plot showing a basic modulation scheme for use with embodiments of the present invention.

FIG. 5 is a waveform plot showing a typical command response sequence according to an embodiment of the present invention.

Fig. 6 is a block diagram of a gaming table according to an embodiment of the present invention.

FIG. 7 is a block diagram of a motherboard according to an embodiment of the invention.

Fig. 8 is a block diagram of a daughter board according to an embodiment of the present invention.

Fig. 9 is a block diagram of an antenna according to an embodiment of the present invention.

Figure 10A is a top view (cross-sectional view) and figure 10B is a bottom view (cross-sectional view) of a token according to an embodiment of the invention.

Figure 11 is a block diagram of a token according to an embodiment of the invention.

Detailed Description

Described herein are techniques for improved Radio Frequency Identification (RFID) reading. In the following description, for purposes of explanation, numerous examples and specific details are set forth in order to provide a thorough understanding of the present invention. It will be apparent, however, to one skilled in the art that the present invention as defined by the claims may include some or all of the features in these examples alone or in combination with other features described below, and may further include modifications and equivalents of the features and concepts described herein.

Embodiments of the present invention are directed to RFID tags in a gaming environment. For ease of description, the gaming environment provides a context for describing an embodiment. It should be understood that the form factor of the RFID tag may be adjusted and that RFID tags embodying the principles of the present invention may be used in environments other than gaming environments.

Although the description uses the term "ferrite," this term should be construed broadly to refer to any type of magnetically permeable material. In general, a material is a magnetically permeable material if its magnetic permeability is greater than that of air.

One feature of an embodiment is: the energy that can be extracted by each RFID tag from the excitation (reader) antenna is increased compared to many existing RFID systems. Another feature of an embodiment is: the efficiency of energy transfer from the excitation (reader) antenna to the RFID tag is increased compared to many existing RFID systems. Another feature of an embodiment is: the efficiency of data transfer from the excitation (reader) antenna to the RFID tag is increased compared to many existing RFID systems. Another feature of an embodiment is: the efficiency of data transfer from the RFID tag back to the reader antenna is increased compared to many existing RFID systems. According to an embodiment, these increases in energy and data transfer may include one or more of the following features: an increased stack height of readable gaming tokens (resulting from increased read range); reduced read errors within a stack of gaming tokens; improved differentiation between tokens inside the "bet point" and tokens outside the bet point; and improved read times.

As described in further detail below, gaming token embodiments include RFID tags having ferrite cores that guide magnetic flux emanating from an excitation source. In a gaming environment where gaming tokens may be stacked in columns, the ferrite material in the center of the token effectively directs magnetic flux towards the top of the stack to excite all chips in the stack. The excitation source (reader antenna) may be embedded in the table top of the game table.

In contrast, typical RFID readers (using tags without ferrite cores; sometimes referred to as "air-core" tags) radiate their energy extensively. This inefficient coupling between the reader and the tag is a significant factor in the read range.

According to an embodiment, the presence of a ferrite core may enable one or more of the following features compared to air core tokens:

1. the increased coupling efficiency between the excitation (reader) source and the passive gaming tokens extends the read range of the "tag-reader" system.

2. The increased coupling efficiency between the stimulus and the passive gaming tokens allows system designers to sacrifice read range in exchange for gaming tokens having a lower "Q" with little net negative impact on read range. This lower Q can minimize sensitivity to interactions that occur when multiple tags are in close proximity, provided sufficient energy is available to energize the RFID tags.

3. Sufficient energy may be delivered to passive circuitry in the token so that a more powerful processor may be used to achieve improved data rates, security, and error detection/correction.

4. Increased coupling efficiency between gaming tokens and stimulus sources may be used to achieve improved data rates, security, and error detection/correction.

5. Controlling the shape of the h-field makes it easier to distinguish between tokens that should be read (i.e., tokens in the selected wager area) and tokens that should not be read (i.e., tokens that are not in the selected wager area). This will minimize read errors due to crosstalk between adjacent bet regions.

SUMMARY

1A, 1B, 1C, 1D, and 1E show an RFID gaming system 100 according to an embodiment of the invention.

Figure 1A shows a number of gaming tokens 102 stacked within a shield ring 104, the shield ring 104 surrounding a wager point 106. The wager point 106 may be located on a game table (not shown) on which the gaming tokens 102 may be placed for wagering. The gaming table may include a reader 107 for reading the RFID tag located within each gaming token 102, the reader 107 having an RFID antenna 105 and other RFID reader electronics (not shown).

The shield ring 104 constrains flux lines 108 generated by the gaming token 102 in response to electromagnetic energy emitted by the RFID reader. (details of the shield Loop 104 may be as described in U.S. provisional application No. 61/046,658, filed on 21/4/2008, "H-Field Shaping Using a short Loop"), the flux line 108 (also referred to as an H-Field) is generated by an RFID reader. The gaming token 102 is activated by this field. This field may be modulated by the reader (for data uplink to the token) or the gaming token 102 (for data downlink to the reader). According to an alternative embodiment shown in fig. 1E, the shield ring 104 may be omitted. (Note the difference in flux lines 108 between FIG. 1E and FIG. 1A.)

Figure 1B is a cross-sectional side view showing more detail of the gaming token 102. Each gaming token 102 includes a ferrite core 120 and a ring-shaped RFID tag 122.

Figure 1C shows a cut-away top view of the gaming token 102. The ferrite core 120 is visible, as well as a loop antenna 124 that is a component of the RFID tag 122.

Figure 1D shows a cross-sectional view of the gaming token 102. The ferrite core 120 is visible, as well as the loop antenna 124 and other electronics 126 of the RFID tag 122.

RFID tag 122 may be magnetically coupled. That is, while the electromagnetic radiation involved in RFID applications includes electric fields (e-fields) and magnetic fields (h-fields), embodiments of the present invention use magnetic fields. Essentially, a transformer is formed, with one winding in the reader and the other in the antenna 124, exciting the antenna. In many existing RFID systems, the transformer has an "air core". This air core may be inefficient, but this inefficiency may be a necessary tradeoff when the location of the RFID tag is not known. In contrast, embodiments of the present invention use a "ferrite core" to improve the performance of the transformer created by the excitation antenna and antenna 124. The use of a ferrite core is particularly effective when gaming tokens 102 are stacked and the ferrite core 120 distorts the magnetic flux component of the electromagnetic energy received from the reader. This deformation of the magnetic flux concentrates the flux lines through the ferrite core 120. The flux lines concentrated through the core 120 couple the electromagnetic energy in a more efficient manner than in a token 102 lacking the ferrite core 120. In effect, the ferrite core 120 channels the flux field to improve performance as described more fully below.

The antenna 124 extracts electromagnetic energy from the reader (e.g., from the h-field). Other electronics 126 condition the energy and use the energy to drive the processor. Modulation may be used to superimpose data delivery on a carrier wave. The processor modulates a carrier wave to perform data transfer to identify the RFID tag 122. According to an embodiment, a 13.56MHz carrier is used. The 13.56MHz carrier frequency has been found to couple well with ferrites.

Fig. 2A, 2B, and 2C show various aspects of the performance of embodiments of the present invention.

Figure 2A is a cross-sectional side view showing two stacks of gaming tokens 102. Each gaming token 102 has a ferrite core 120. The gaming token 102 has a thickness T and the ferrite core has a thickness T. The ratio T/T is referred to as "% ferrite".

Figure 2B is a perspective view showing a number of gaming tokens 102 stacked to a stack height of 3 inches. This stack height is used to compare embodiments of the present invention with other techniques.

Fig. 2C is a graph showing the performance of various materials. The x-axis shows "% ferrite" (see fig. 2A and related discussion) and the y-axis shows signal strength. (100% "% ferrite" indicates that the thickness of the ferrite core 120 is the same as the thickness of the gaming token 102.) for the material "material 2", line 202 shows 100% received signal strength in "dB". Other materials may be used to give different performance characteristics, given that line 202 is the baseline for the desired signal strength. For example, the material "material 1" shown by line 204 indicates that a thinner ferrite core 120 may be used to give the same performance as "material 2". The material "material 3" shown by line 206 indicates that a thicker ferrite core 120 is needed to give the same performance as "material 2". As can be seen, ferrite, which is a greater percentage of the thickness of the gaming token 102, gives a stronger signal. In addition, different ferrite materials have different performance characteristics.

FIG. 3 is a graph showing a comparison between a prior air core RFID tag and a ferrite core RFID tag, according to embodiments of the present invention. The x-axis shows the stack height of a stack of gaming tokens 102 (see figure 1A) and the y-axis shows the signal strength of the signal received by the topmost gaming token in the stack. For an air-core RFID tag, the wire 300 has a signal strength dB0 at the stack height H0. This stack height H0 may correspond to approximately 2.5 inches in prior systems. For ferrite core RFID tags, line 302 has a signal strength dB0 at the stack height H. Stack height H is greater than stack height H0. Thus, embodiments of the present invention allow reading at greater stack heights compared to prior systems. Accordingly, for a given stack height H0, the received signal strength of line 302 is greater than the received signal strength of line 300. Thus, embodiments of the present invention provide an improvement in read height compared to prior systems. In effect, the plurality of stacked tags creates a rod of ferrite material that directs the magnetic flux field upward through the stack.

As discussed above, many existing RFID tags have a relatively high Q. The Q is relatively high to increase sensitivity and read range. However, when multiple tags are in close proximity, they tend to interact. This interaction changes its operating resonant frequency. Thus, the tags are not excited and/or data is not successfully exchanged.

According to an embodiment of the invention, the antenna in the gaming token 102 has a relatively lower Q than in existing RFID tags. Having a relatively low Q may also be referred to as detuning. Detuning helps to solve the problem of unwanted interactions between tags, and also severely limits sensitivity and therefore read range. In contrast to many existing air core RFID tags, the proposed embodiment of the present invention uses a ferrite core 120 to compensate for this loss of read range. When these gaming tokens 102 are stacked, the ferrite core 120 concentrates the h-field and guides the flux. This allows the use of detuned tags that are not compromised by the presence of neighboring tags. The reduction in sensitivity due to detuning is compensated by the focused h-field. The results were: having the ability to read multiple tags in close proximity while maintaining excellent read range. In contrast, many existing air-core RFID tags either fail to read very closely or suffer from loss of read range.

As a particular example, the Q of many existing RFID tags is between 10 and 20, while the Q of the antenna in the gaming token 102 is close to 0 (in fact, the designer may wish to have Q as close to 0 as possible). Ideally, the antenna itself in the gaming token 102 may exist as a purely resistive load. However, in practice, there is a parasitic capacitance that helps to produce a moderate Q (less than 1). One notable factor for system designers to determine which Q is possible (to maximize read range) is the degree to which the tags are tightly coupled-which depends on the degree to which the tags are closely spaced. According to one embodiment, tags having a Q of less than 5.0 provide an acceptable read range. According to an embodiment, tags having a Q less than 1.0 provide improved read range.

According to an embodiment, the antenna in the gaming token 102 is untuned and has a resonant frequency above 13.56 MHz. The lack of a tuned antenna limits sensitivity and thus read range. As with the low Q antenna embodiments discussed above, the ferrite core 120 concentrates the h-field and guides the flux, allowing the use of untuned tags that are not damaged by the presence of neighboring tags.

Details of modulation

Fig. 4 is a graph showing a waveform 500 of a basic modulation scheme for use with an embodiment of the present invention. The x-axis corresponds to time and the y-axis corresponds to power level. The power output is modulated at about 4KHz with the amplitude shifted between 100% (equivalent to a digital 1) and 80% (equivalent to a digital 0). This selection of amplitude is a balance between maximizing the energy available to the tag and ensuring that the signal strength of the digital data is sufficiently detectable. The 256 microsecond frame is the basic frame of communication from the tag to the reader. This frame has been chosen to be long enough for the desired command and data string (see below for a detailed description of the command structure). The edges of the 256 microsecond frame (e.g., 504 and 506) are used to synchronize the tag with the reader.

Other modulation schemes may be used according to other embodiments, as desired according to design requirements. For example, the modulation frequency may be adjusted. As another example, the power levels for "1" and "0" may be shifted. As another example, the size of the data frame may be adjusted.

Label position

Typical RFID uses up to 96 bits for a unique serial number. Unlike a general RFID, the serial number of the enclosed environment does not need to be so long. Shorter sequence numbers can improve read cycle time and reduce error rates. For an embodiment, a population of 1 million (about 2E30) unique serial numbers is considered sufficient with an estimate of 1 million tokens for each of 1,000 enclosed environments. In the event that an undesirable number with poor DC balance or too many consecutive 1's or 0's is allowed to be discarded, a 31-bit base sequence number is selected in one embodiment.

A CRC check may be used to ensure that data is not corrupted during transmission. In general, the more CRC bits, the more robust error detection-but with reduced returns. According to an embodiment, a 13-bit CRC is chosen as a reasonable trade-off between robustness and overhead. According to an embodiment, the CRC may be calculated using a polynomial (e.g., 0X 1909). In other embodiments, other error detection or error correction schemes may be used.

Downlink details

FIG. 5 is a graph showing a waveform 600 of a typical command response sequence according to an embodiment of the invention. The basic command structure is a "master-slave structure" and all commands are initiated by the reader. A full read cycle begins when the 13.56MHz carrier signal is powered on and ends when all tokens have been read and the carrier is powered off. Waveform 600 shows a typical command response sequence with a "downlink" (reader-to-tag) command 602 and an "uplink" (tag-to-reader) response 604. The reader may use the read command to read the tag. Due to the statistical likelihood of collisions, a full read cycle may require multiple read commands to read all tokens.

According to an embodiment, a "sleep" command may also be implemented. The "sleep" command may be as disclosed in U.S. provisional application No. 61/031,270, filed on 25/2/2008, "Dynamic Power Absorption of a Loop Antenna for Passive RFID Tags.

Read command details

According to an embodiment, a "read" command includes two components. One component is the number of iterations. Another component is the number of windows.

The purpose of the number of iterations is to eliminate repeated collisions from the same token during subsequent "read" commands. Processing in the token firmware determines which bits of the unique token ID are used to "randomly" select the response window. Because the number of iterations for each read command in a given cycle is different, the likelihood of two token duplicate collisions is minimized. The number of iterations increases from 0 to 9. A special number of iterations 11 may be used for diagnostic purposes.

As noted above, the functionality of the modified Aloha protocol improves when the number of tags in a domain can be estimated, constrained (e.g., by limiting the physical area that can be occupied by the tags), or otherwise known. (according to one embodiment, the Number of Tags may be estimated as disclosed in U.S. provisional application No. 61/046,690, filed on 21/4/2008, "Estimating the Number of RFID Tags in the Field. one feature of the ferrite core is that the close coupling of the token to the antenna produces an antenna impedance proportional to the Number of tokens. Too many data frames result in a longer read cycle time. Too few data frames result in excessive collisions from multiple tokens responding within the same data frame. It has been determined that good results occur when the number of data frames is about 10 times the number of tokens. Alternatively, the number of tags can be estimated by using a modest number of windows and looking at how many collisions exist and then adjusting the number of windows up or down accordingly.

Table 1A shows the structure of a 16-bit "read" command, starting with the Most Significant Bit (MSB).

Bit	15	14	13-10	9-7	6-0
						Function(s)	Start bit	ReadingCommand	Number of iterations	Number of windows	Stop position
Value of	1	0	0-9	0-7	0

TABLE 1A

Table 1B shows the number of frames corresponding to the number of each window.

Number of windows	0	1	2	3	4	5	6	7
									Number of frames	32	64	128	256	512	1024	2048	Retention
Reading time	8ms	16ms	32ms	64ms	128ms	256ms	512ms	n/a

TABLE 1B

Uplink details

According to an embodiment, the uplink data rate is 4 microseconds/bit, thereby allowing the entire code ID of 46 bits to fit into a single 256 microsecond frame. The 4 microsecond rate corresponds to the clock of the microprocessor in the tag. According to one embodiment, the microprocessor in the tag is a PIC microcontroller (PIC10F206T-I/OT, available from Microchip Technology Inc., of Chandler, Arizona.) the reader (digital signal processor) samples this data multiple times, leveraging its much higher processing power as shown in Table 2, the 46-bit structure consists of a start bit, a 31-bit token ID, a 13-bit CRC, and a stop bit.

Bit	45	44-14	13-1	0
					Function(s)	Start bit	Token ID	CRC	Stop position
Value of	1	0 or 1	0 or 1	1

TABLE 2

Read cycle example

Assume that there is a stack of 30 tokens in the betting area. The reader will detect this load and specify a window number of 3 or 4. Assuming worst case (4), the corresponding frame number is 512 (or 128 milliseconds) to process this initial "read" command. Assume a collision rate of 10%, resulting in 3 tokens not being read. It also takes 35 milliseconds to put 27 tokens to sleep. The second "read" command using window number 0 corresponds to 32 frames or 8 milliseconds. If no collision is detected on this second "read" command, then the total time elapsed for the read cycle is 171 milliseconds.

According to an alternative embodiment, 44 milliseconds are used to put 27 tokens to sleep, resulting in a total time elapsed for the read cycle of 180 milliseconds.

The amplitude of the uplink signal is essentially modulated, however, the phase of the returned signal is unknown due to the phase shift from reader to tag and from tag to reader. To account for this unknown phase shift, the receiver mixes the signal to baseband, producing I and Q channels. These I and Q channels are processed to recover the data from the tag.

Fig. 6 is a block diagram of a gaming table 700 according to an embodiment of the present invention. Generally, the embodiment of FIG. 6 may be referred to as a reader system. The gaming table 700 includes eight wager points 702a through 702h (generally 702), eight antennas 704a through 704h (generally 704), eight shield rings (optional) 706a through 706h (generally 706), eight daughter boards 708a through 708h (generally 708), a motherboard 710, and a control system 712. (some descriptions of these components (e.g., wager points, antennas, and shield rings) may be repeated with descriptions of similarly named components discussed in previous sections.) the gaming table 700 may be used in conjunction with a ferrite core gaming token 102.

According to an embodiment, the gaming table 700 may include one or more of the following attributes:

1.13.56MHz carrier.

A "modified Aloha" protocol.

3. A 256 microsecond "data frame" of the modulated carrier.

4. A 16-bit data word (140KHz data rate or 7 microseconds/bit) for a command to a token. According to another embodiment, the data rate is 125KHz or 8 microseconds/bit.

5.46 bits of token ID data (500KHz data rate or 2 microseconds/bit; 31 bits for ID plus 13 bits for error detection). According to another embodiment, the data rate is 250KHz or 4 microseconds/bit. According to yet another embodiment, one start bit and one stop bit may also be transmitted.

The RF communications use a 13.56MHz carrier generated by the motherboard 710 and sent to each daughter board 708. 13.56MHz was chosen for three reasons: (1) ferrite cores are magnetically permeable at this frequency, (2) data rates are reasonable, (3) FCC frequency allocation is used for this type of application.

Even though the "ferrite core" concept may not strictly be RFID technology, the energy and data exchange utilizes a "modified Aloha" protocol that is common to many RFID systems. This protocol is chosen because it provides a fast read cycle when there is a good signal-to-noise ratio and a reasonable estimate of the number of tags within a field is known. The features of this modified Aloha protocol include:

a "master-slave" command structure, all commands initiated by the reader.

2. A number of response "windows" are defined. Ideally, the number of windows balances the need to manage conflicts (the more windows the better) with the need for fast read cycle times (the fewer windows the better).

A complete read cycle begins with the 13.56MHz carrier signal turned on to energize the tokens in the excitation field, and ends when all of the tokens in the excitation field have been read. Once powered on, the token waits for a command. The DSP on each daughter board 708 modulates the carrier to send commands and data to the tokens at a data rate that can capture the tokens (with only modest processing power). The number generator process in each token assigns a response window "randomly". According to one embodiment, the token processor uses iterations and window sizes to determine which bit in the ID number to use to define the response window. The daughter board 708 attempts to read the IDs of all chips within its antenna range. The error detection scheme identifies any conflicts. Tokens that are successfully read are put to sleep and the process is repeated. Once all token IDs have been read, the data is sent to the PC.

Token data rate is the fastest data rate (250 KHz according to one embodiment) that its internal oscillator can drive; the DSP has processing power to manage the higher data rate of the downlink from the token. This asymmetry in data rate (uplink versus downlink) is consistent with the amount of data to obtain good read cycle times (commanding much less bandwidth than token IDs). Both the 16-bit command and the 46-bit ID fit into a 256 microsecond data frame.

The number and arrangement of the wager points 702 may vary depending on the nature of the game to be played. In general, the wager point 702 may be placed at any location on the gaming table 700 near which it is desired to take measurements of the RFID tag. More specifically, for the particular example configuration described herein, "close" to yield acceptable reading performance at a distance of up to about 6 inches above the antenna 704. more specifically, the arrangement of bet points 702 on the gaming table 700 corresponds to a 21-point game where the only required information is the ID of the currently bet gaming token.

The number and arrangement of antennas 704 generally corresponds to the number and arrangement of betting points 702. For example, for a given play area, the size of the antennas 704 and the spacing therebetween may be adjusted according to a desired performance threshold, and the wager point 702 coincides with the location of the antennas 704. According to an embodiment, the antenna 704 may be rectangular in shape and may be 2 inches by 4 inches in size. According to an embodiment, the antenna 704 may be circular in shape and may be 4 inches in diameter. According to an embodiment, antenna 704 may be spaced apart from (optional) shield ring 706 by a quarter inch gap. According to one embodiment, the antenna 704 may be constructed on an FR-4 (flame retardant) printed circuit board. According to an embodiment, the antenna 704 may have a thickness of 0.031 inches.

Motherboard 710 may interface to control system 712, which control system 712 may be configured to operate in a local manner or a remote manner. When the control system 712 is local, the motherboard 710 may be connected by a connection such as USB (Universal Serial bus). When the control system 712 is remote, the motherboard 710 may be connected through a LAN (local area network) such as ethernet. The ethernet connection allows the control system 712 to be remotely located to control one or more tables 700 that are part of a larger system to monitor for fraud or reward legitimacy. Alternatively, the USB connection allows motherboard 710 to interface to a local control system 712 to help run the trial version, exclude errors in the prototype, and/or integrate different systems into a common data format. The local control system 712 may also maintain a copy of the database of "valid" IDs to ensure continuous play even in the event of a LAN crash.

Fig. 7 is a block diagram of a motherboard 710 according to an embodiment of the invention. The motherboard 710 includes a power supply 802, two control system interfaces 804a and 804b, a controller 806, and eight daughter board interfaces 808a through 808h (collectively 808).

The power supply 802 provides power to the controller 806 and the control system interfaces 804a and 804 b.

The controller 806 implements the functionality of the motherboard 710. These functions may include powering the daughter board 708, generating a carrier frequency (e.g., 13.56MHz) to drive the reader antenna, generating a clock (e.g., 4KHz) for the daughter board 708 to define command and data windows, formatting decoded serial data received from tokens, and so forth. The controller 806 may be implemented as a Complex Programmable Logic Device (CPLD) or other type of circuit structure. According to one embodiment, the controller 806 may be implemented as an XC95144XL-10TQG100C CPLD available from Xilinx corporation of san Jose, Calif.

Control system interface 804a interfaces motherboard 710 with control system 712. According to one embodiment, control system 712 is connected to motherboard 710 in a USB format and controller 806 communicates in a serial format, so control system interface 804a implements a USB to serial interface. The control system interface 804a may be constructed from the FT232RL device available from Future Technology devices International, Inc. of Glassy, UK. According to one embodiment, control system 712 is connected to motherboard 710 in an Ethernet format, so control system interface 804b implements an Ethernet connection. The control system interface 804b may be implemented by an LPC2368FBD100-S microcontroller available from NXP Semiconductors of Emnedhefin, the Netherlands.

The daughter board interface 808 provides a connection between the motherboard 710 and the daughter board 708. The system designer may choose to increase or decrease the number of sub-boards 708 depending on the number of wagers and their necessary read speed. According to one embodiment, one daughter board 708 drives one antenna 704. According to one embodiment, one daughter board 708 drives multiple antennas 704 and multiplexes the read signals. The number of simultaneous reads will affect the power required.

Figure 8 is a block diagram of a daughter board 708a according to an embodiment of the present invention. The daughter board 708a includes a transmitter 902, a receiver 904, a directional coupler 906, and an antenna switch 908.

The transmitter 902 includes an amplifier power supply 920, an amplitude modulator 922, an amplifier 924, and a band pass filter 926. Transmitter 902 receives three signals from the motherboard: a power signal PWR _ PWM, an amplitude modulated DATA signal AM _ DATA, and a 13.56MHz carrier signal. Transmitter 902 modulates a data signal onto a carrier signal and provides the modulated carrier signal to directional coupler 906.

Directional coupler 906 provides the modulated carrier signal to antenna switch 908, which antenna switch 908 provides the modulated carrier signal to antenna 704 for transmission. (according to one embodiment, daughter board 708a includes two antenna connectors and may drive both antennas antenna switch 908 determines which antenna to use.) the tag extracts the transmitted energy and in response modulates the signal further with its ID information provided by directional coupler 906 to receiver 904.

The receiver 904 includes mixers 930a and 930b, low pass filters 932a and 932b, amplifiers 936a and 936b, differential amplifiers 938a and 938b, analog-to-digital converters 940a and 940b, a programmable logic device 942, and a DSP (digital signal processor) 944. The programmable logic device 942 may be an FPGA such as the XC3S250E-4TQG144C device available from Xilinx corporation of san Jose, Calif. DSP 944 may be a TMS320F2812PGFA device available from Texas Instruments (Texas Instruments) of Dallas, Tex.

Receiver 904 receives the signal as further modulated by the tag (provided by directional coupler 906) and two versions (one shifted by 90 degrees) of the 13.56MHz carrier frequency. Receiver 904 then demodulates the tag ID information and provides the serial data to motherboard 710.

Reader antenna details

Fig. 9 is a block diagram of an antenna 704a according to an embodiment of the present invention. Antenna 704a includes connector 1002, switch 1004, matching circuit 1006, antenna loop 1008, and shield loop (optional) 1010. These components may be built on a two-sided FR-4 printed circuit board assembly.

A connector 1002 connects the antenna 704a to the daughter board 708. According to one embodiment, the connection to the daughter board 708 is a shielded coaxial cable and the connector 1002 is an SMA (micro version a) connector.

The switch 1004 is used to short the antenna loop 1008 when the antenna loop 1008 is not in use. The switch 1004 is controlled by a DC bias on the antenna cable that may be generated by the daughter card. Switch 1004 acts as an additional shield ring to reduce the field strength in the bet point of this antenna when the adjacent bet point is being actively driven. This may improve the differentiation between tokens in adjacent bet points.

The matching circuit 1006 may be a 50Ohm impedance matching circuit.

The antenna loop 1008 may be a loop antenna. Antenna loop 1008 may be in various form factors depending on the characteristics of the area to be bet and the desired performance. According to an embodiment, the antenna loop 1008 may be rectangular in shape and sized 2 inches by 4 inches. According to an embodiment, the antenna loop 1008 may be circular in shape with a diameter of 4 inches.

Details of gaming tokens

Figure 10A is a top view (cross-sectional view), figure 10B is a bottom view (cross-sectional view), and figure 11 is a block diagram of a token 1100 according to an embodiment of the invention. Token 1100 generally includes a ferrite core, an antenna, and tag electronics. More specifically, the token 1100 includes a printed circuit board 1102, an antenna 1104, a ferrite core 120, a plastic housing 1110, a bridge rectifier 1112, a current source 1114, a shunt voltage regulator 1116, a receive filter 1118, a microprocessor 1120, and a transmit transistor 1122.

The token 1100 may be, for example, a gaming token suitable for use in a casino card game (casino). Token 1100 may be circular, 1.55 inches (39.4mm) in diameter and 0.125 inches (3.18mm) thick. These parameters may be varied as desired.

The printed circuit board 1102 may be generally circular in shape so as to conform to the form factor of a circular gaming token. Printed circuit board 1102 may be an FR-4 material and have a thickness of 0.020 inches.

The antenna 1104 may be an 8-turn antenna etched onto one side (e.g., the bottom side) of the circuit board 1102. These antennas may be configured with 8 mil traces and 7 mil spacing. The inductance of the 8-turn antenna may be 3 muh. Antennas with different numbers of turns can be constructed with different balances between inductance and resistance depending on design requirements.

Many existing RFID tags use a diode rectifier followed by a voltage clamp to limit the desired operating voltage range of the tag and thereby protect the tag from over-voltage damage. In this embodiment, the power supply may be a linear power supply, with a bridge rectifier 1112, followed by a current source 1114 and then a voltage clamp (shunt voltage regulator) 1116. This structure does not clamp the voltage across the coil as is typically done in RFID tags. This linear power supply allows the tag to operate over a wide range of magnetic field strengths. This allows the tag to be read at the top of the stack (field weakest) and near the bottom of the stack (field strongest). The net effect is increased read range. The linear Power source may be as described in U.S. provisional application No. 61/031,270, filed on 25/2/2008, "Dynamic Power Absorption of a Loop Antenna for Passive RFID Tags.

Microprocessor 1120 may be a PIC microcontroller available from Microchip technology Inc. (Microchip technology Inc.), Inc., of Chandler, Arizona. The microprocessor 1120 stores the ID of the token 1100, decodes the command from the reader, and encodes the ID of the token 1100 onto a 13.56MHz carrier wave. Microprocessor 1120 includes an internal comparator coupled to receive filter 1118.

The ferrite core 120 may be as described above (see, e.g., fig. 1B). The ferrite core 120 takes advantage of the H-field and guides it through the antenna 1104. The main physical property of ferrite is its permeability. Any equivalent material that is both magnetically permeable and effectively guides the H-field through the antenna loop will improve performance. Ferrite devices are typically used at frequencies up to 1 GHz. According to one embodiment, ferrite core 120 has a magnetic permeability: 125 ± 20% (e.g., between 100 and 150).

According to one embodiment, ferrite core 120 is made of high frequency permendur NiZn ferrite, with low losses, and inductance applications up to 25 MHz. Suitable materials are "M" materials available from national magnetics Group, Burley, Pa. According to an embodiment, the ferrite core 120 is a molded plastic material with a ferrous filler or a ferrous additive. Other materials that may be used are iron powder, milled ferrite, soft iron, or nickel iron molybdenum alloys (e.g., molypermaloy), but may generally be more suitable for frequencies below 13.56 MHz.

According to one embodiment, a frequency of 13.56MHz may be used. In other embodiments different frequencies may be used.

The ferrite core 120 has a thickness that substantially conforms to the form factor of the token 1100. For example, the ferrite core 120 may have a thickness of 3.175 mm.

The ferrite core 120 has a diameter that substantially conforms to the form factor of the token 1100 and other internal components. For example, ferrite core 120 may have a diameter of 12.7 mm. This diameter allows tokens to be misaligned in the stack and provides sufficient overlap between adjacent tokens so that flux passing up through the stack is not degraded (i.e., performance is not sensitive to the degree to which the stacks are coincident). Practical ranges of diameters for ferrites in standard (round) gaming tokens may be as small as 2mm and as large as 35mm, depending on other physical requirements. Other form factors (e.g., using a plaque) may allow greater freedom in the selection and location of the ferrite elements. For example, a rectangular tile may include two circular ferrite cores 120.

The size of the ferrite core may be increased or decreased depending on the form factor and desired performance characteristics of the token 1100. There is no reason why the diameter of the ferrite core 120 may be made larger as long as it is suitable. The diameter of the ferrite can be made smaller as long as there is enough overlap so that the flux going up through the stack does not degrade. It does not seem to be very sensitive to the spacing between the antenna winding and the outer diameter of the ferrite.

The ferrite core 120 may be circular in shape, but any shape suitable for an antenna loop is acceptable. For symmetry and manufacturing convenience, a circular shape is chosen. If desired, the ferrite core 120 may be positioned at a location other than the center of the token 1100.

According to an embodiment, the diameter of the ferrite core 120 may increase up to the diameter of the token. The antenna may be wound around the outer edge of the token.

The top and bottom of the token 1100 may be covered with a logo or adhesive (not shown) to indicate the denomination or other desired information. The indicia may have a thickness of about 0.003 inches. This thickness minimizes any air gaps when stacking tokens (i.e., it helps the ferrite core in a token stack to function as if it were a single ferrite rod). If necessary, the thickness of the mark can be varied with a corresponding effect on the reading performance. Alternatively, the ferrite core may be exposed by using a ring adhesive to further minimize any air gaps in the stack. The gap performance between two tokens 1100 is given in table 3, according to an embodiment.

Voids	0％	1.75％	2.5％	4.5％	9％	17.5％	23.5％
								Gain (dB)	24	19	15.5	13	10	7	5.5

TABLE 3

"void" refers to the space between ferrite cores 120 as a percentage of the thickness of each ferrite core 120. (for example, a 20% void for a 3mm thickness corresponds to a 0.6mm void.) the void may result from identification as described above, from more robust coverage of the token 1100, from recessed features on the token 1100, and so forth. "gain" refers to the increased signal strength of a token 1100 having a ferrite core 120 as compared to a token lacking a ferrite core 120. Thus, table 3 shows: for gaps between 0% and 1.75%, the gain is between 24dB and 19 dB. The information in table 3 is dimensionless information; that is, it does not depend on the number of tokens 1100 in the stack.

The ferrite core 120 may be manufactured by cutting the ferrite core from a solid bar or by sintering the ferrite core in a special tool. Sintering allows the addition of aesthetic elements recessed on one or both facets of the ferrite that hardly degrade the performance of the ferrite. Furthermore, sintering allows designers to add features to allow molding of the ferrite to be inserted during molding of the token.

According to one embodiment, ferrous material or small ferrite beads may be added to a plastic matrix, which may then be used to mold part or all of the token 1100. In this embodiment, the ferrite core 120 and the token 1100 refer to the same structure and no separate elements for the ferrite core 120 are required.

According to one embodiment, more than one ferrite core may be in a token. According to an embodiment, a token may include more than one RFID tag (e.g., tag electronics may provide multiple tag IDs for a token). According to an embodiment, more than one antenna may be in a token.

The above description illustrates various embodiments of the invention, and examples of ways in which aspects of the invention may be practiced. The above examples and embodiments are not to be considered the only embodiments, and are presented to illustrate the flexibility and advantages of the present invention as defined by the following claims. Based on the foregoing disclosure and the following claims, other configurations, embodiments, implementations and equivalents will be apparent to those skilled in the art and may be used without departing from the spirit and scope of the invention as defined by the claims.

Claims (29)

1. A Radio Frequency Identification (RFID) tag, comprising:

a magnetically conductive core;

an antenna that receives electromagnetic energy; and

tag electronics coupled to the antenna that identifies the RFID tag in response to receiving the electromagnetic energy.

2. The RFID tag of claim 1, wherein the magnetically permeable core is comprised of a ferrite material.

3. The RFID tag of claim 1, wherein the magnetically permeable core guides a magnetic flux field of the RFID tag, wherein the magnetic flux field is caused by the electromagnetic energy.

4. The RFID tag of claim 1, wherein the magnetically permeable core deforms a magnetic flux component of the electromagnetic energy.

5. The RFID tag of claim 4, wherein the deformed magnetic flux component concentrates flux lines through the magnetically permeable core.

6. The RFID tag of claim 5, wherein the concentrated flux lines couple the electromagnetic energy to the antenna with increased efficiency compared to a case lacking the magnetically permeable core.

7. The RFID tag of claim 1, wherein the RFID tag is one of a plurality of RFID tags that, when stacked, form a stack having a solid bar of magnetically permeable material that further directs a magnetic flux field upward through the stack.

8. The RFID tag of claim 1, wherein the magnetically permeable core guides a magnetic flux field of the RFID tag, wherein the magnetic flux field is caused by the electromagnetic energy, and wherein guiding the magnetic flux field increases a read height of the RFID tag.

9. The RFID tag of claim 1, wherein the magnetically permeable core has a magnetic permeability between 100 and 150.

10. The RFID tag of claim 1, wherein the magnetically permeable core corresponds to a nickel zinc ferrite material having a magnetic permeability between 100 and 150.

11. The RFID tag of claim 1, wherein the magnetically permeable core corresponds to one of sintered ferrite, powdered ferrite, ground ferrite, soft iron, and nickel-iron-molybdenum alloy.

12. The RFID tag of claim 1, wherein the magnetically permeable core improves an efficiency of energy transfer from a reader device that generates the electromagnetic energy to the RFID tag.

13. The RFID tag of claim 1, wherein the magnetically permeable core improves data transfer efficiency from a reader device that generates the electromagnetic energy to the RFID tag.

14. The RFID tag of claim 1, wherein the magnetically permeable core improves data transfer efficiency from the RFID tag to a reader device that generates the electromagnetic energy.

15. The RFID tag of claim 1, wherein the tag electronics identifies the RFID tag by modulating a carrier frequency of the electromagnetic energy.

16. The RFID tag of claim 1, wherein the tag electronics identifies the RFID tag by modulating a carrier frequency of the electromagnetic energy according to a modified Aloha protocol.

17. The RFID tag of claim 1, wherein the antenna is a loop antenna.

18. The RFID tag of claim 1, wherein the antenna is a loop antenna that encircles the magnetically permeable core.

19. The RFID tag of claim 1, wherein the antenna has a Q of less than 5.

20. The RFID tag of claim 1, wherein the antenna has a Q of less than 1.

21. The RFID tag of claim 1, wherein the antenna is untuned.

22. The RFID tag of claim 1, wherein the RFID tag further comprises:

a plurality of magnetically permeable cores comprising the magnetically permeable core.

23. The RFID tag of claim 1, wherein the RFID tag further comprises:

a plurality of tag electronics including the tag electronics and providing a plurality of identifications of the RFID tags.

24. The RFID tag of claim 1, wherein the RFID tag further comprises:

a plurality of antennas comprising the antenna.

25. A plurality of RFID tags, wherein each RFID tag of the plurality of RFID tags comprises:

a magnetically conductive core;

an antenna that receives electromagnetic energy; and

tag electronics coupled to the antenna that identify each RFID tag in response to receiving the electromagnetic energy.

26. The plurality of RFID tags of claim 25, wherein an increased signal strength of between 24dB and 10dB results in a gap of between 0% and 9%, wherein the gap is between a first magnetically permeable core of a first RFID tag and a second magnetically permeable core of a second RFID tag.

27. The plurality of RFID tags of claim 25, wherein the plurality of RFID tags includes a first RFID tag having a first magnetically permeable core and a second RFID tag having a second magnetically permeable core, wherein the first magnetically permeable core and the second magnetically permeable core direct magnetic flux when the first magnetically permeable core and the second magnetically permeable core overlap and do not overlap.

28. A Radio Frequency Identification (RFID) system, comprising:

a reader system that generates electromagnetic energy; and

an RFID tag, wherein the RFID tag comprises:

a magnetically conductive core;

an antenna that receives the electromagnetic energy; and

tag electronics coupled to the antenna that identifies the RFID tag in response to receiving the electromagnetic energy.

29. The RFID system of claim 28, wherein the reader system comprises:

a plurality of reader antennas that generate the electromagnetic energy to read the RFID tag proximate to one of the reader antennas.