WO1995033344A1

WO1995033344A1 - Reader having ferromagnetic core field coil

Info

Publication number: WO1995033344A1
Application number: PCT/US1995/006777
Authority: WO
Inventors: Michael L. Beigel; Steven R. Frank
Original assignee: Avid Marketing, Inc.
Priority date: 1994-05-27
Filing date: 1995-05-26
Publication date: 1995-12-07
Also published as: AU2692995A

Abstract

A reader (10) for an identification system utilizing a field coil (60) having a ferromagnetic core allowing the reader to have a substantially reduced size. The reader optimizes the shape of the ferromagnetic core of the field coil to maximize the ability of the reader to extract information from an embedded or implanted identification tag, despite variations in the location and orientation of the tag. The ferromagnetic core (110) is preferably a block shape having the field coil (112) wrapped around the long axis of the block.

Description

READER HAVING FERROMAGNETIC CORE FIELD COIL

Field of the Invention

The present invention relates generally to electronic inductive identification systems. The invention specifically relates to a reader including a field coil having a ferromagnetic core which enhances the reader's ability to read a tag in a radio frequency coupled identification systems.

Background of the Invention

Identification systems relying on radio frequency based communication between a reader and a transponder or tag of various types for identifying animals and objects are currently in use in a number of applications. Generally, identification systems which have a tag which transmits an ID signal simultaneous to its being energized by an electro¬ magnetic field from a reader are termed full-duplex systems. Identification systems including a tag capable of receiving and converting a transmitted "charging" signal to charge a capacitor or power storage element, which is then discharged to allow the broadcast of a signal received by a reader which is in a "silent" or non-broadcasting mode, are termed half- duplex systems. For either of the foregoing identification systems, the tags are very small, although as a general rule a full duplex tag will be smaller than a half-duplex tag since a power storage element is required in the latter.

For either half-duplex or full-duplex identification systems, a particular advantage is that the tag which is to be used in the identification of an animal, element or object is implanted or embedded. As a result, the precise location or orientation of the tag - is not readily identifiable. Further, the location and orientation of the tag once implanted or embedded may be variable, both initially and over time if migration of the tag occurs. The tag for an inductive identification systems generally includes a memory element coupled to an antenna such as an inductive coil which facilitates coupling with an inductive power supply. The reader usually includes a battery power supply and a field coil for transmitting an electromagnetic field to the tag. The field is received by the tag and converted through induction to a direct current power supply signal to run the tag circuitry in a full duplex tag, or stored in the capacitor of a half-duplex tag. The tag transmits the identification data to the reader from the tag memory, and the reader can display the data. The inductive identification systems thus permit powering an identification tag transponder by an electro agnetically coupled energizer reader, and the transmission of an ID signal through the coil in the tag.

An example of a reader and tag system, which reads tag data by providing a variable loading means in the tag, is disclosed in U.S. Patent Nos. 4,517,563 and 4,333,072 herein incorporated by reference. The readers for identification systems which have gained the widest commercial success have generally relied upon a transmission coil which is mounted, along with the associated reader circuitry, within a plastic housing. The transmission coil is "air wound" in that the coil is simply a fine wire coiled to define a three to six inch diameter circle, or rounded off square, having no ferrite core or components. Accordingly, readers having these cores must be sized to accommodate the transmission coil, and therefore tend to be somewhat bulky. A limited number of readers having a cylindrical ferrite core field coil have also been produced with limited success.

As one example of an air wound field coil, the field coil may have a single coil wound around an oval plastic core approximately 4 5/8 inches long and 3 3/4 inches wide. The field coil is wound with 90 to 100 turns of 28-gauge wire, yielding a coil with approximate inductance of 2.3 mH and approximate impedance of 7.6 ohms. Such a coil has produced an effective reading distance of about 4 1/2 inches.

Users of inductively coupled identification systems desire to have an efficient high-power reader system which increase the practical distance by which the reader and tag can be operationally separated. Moreover, users desire a system in which the reader is compact, yet still has a satisfactory reading characteristic which is capable of coupling with a tag despite the variable orientation of the tag.

Summary of the Invention

The present invention details a reader for either an identification system utilizing a field coil having a ferromagnetic core allowing the reader to have a substantially reduced size. The reader optimizes the shape of the ferromagnetic core of the field coil to maximize the ability of the reader to extract information from an embedded or implanted identification tag, despite variations in the location and orientation of the tag. The ferromagnetic core in the preferred embodiment has a block shape with the field coil wrapped around the long axis of the block. A number of alternative embodiments of the shape of the ferromagnetic core of the field coil are also disclosed. The new reader can be can be used in full-duplex and half-duplex identification systems.

Brief Description of Drawings

FIG. 1 is a perspective view of an inductive identification system including a reader and a transponder; FIG. 2 is a block diagram of an exemplary circuit of the reader of Fig 1;

FIG. 3 depicts one side of the internal circuitry of the reader of Fig. 1;

FIG. 4 depicts the opposite side of the internal circuitry of the reader of FIG. 1; FIG. 5 is a perspective view of the field coil of the reader of Fig. 1;

FIG. 6 is a graphic illustration of the magnetic field in the X-Y plane surrounding the field coil of Fig. 4;

FIG. 7 is a graphic illustration of the magnetic field in the Y-Z plane surrounding the field coil of Fig. 4 ;

FIG. 8 is a graphic illustration of the orientation of the magnetic vectors for the field coil of Fig 4;

FIG. 9 depicts another alternative design for the field coil; FIG. 10 depicts another alternative design for the field coil;

FIG. 11 depicts another alternative design for the field coil;

FIG. 12 depicts another alternative design for the field coil;

FIG. 13 depicts another alternative design for the field coil;

FIG. 14 depicts another alternative design for the field coil; FIG. 15 is a graphic illustration of the magnetic field in the X-Y plane surrounding a field coil having a cylindrical core; and

FIG. 16 is a graphic illustration of the orientation of the magnetic vectors for the field coil of Fig. 13. Detailed Description of the Preferred Embodiments

Fig. 1 depicts a perspective view of an inductive reader 10 according to the present invention for use in reading an identification signal from a transponder or tag 12. The reader 10 includes a case 14, preferably formed from plastic, and a display 16, such as an alpha-numeric display. The reader 10 also includes a power switch 18 and a signal transmit key 20.

The tag 12 of Fig. 1 may be either a full duplex or a half-duplex tag as generally known and currently in use in a variety of applications. For purposes of the following description of the reader 10, the particular configuration of the tag 12 is significant only to the extent that the tag 12 includes a coil or antenna operative with the reader 10, and the signal generated thereby.

Fig. 2 depicts a representative signal transmission and tag reader circuit for the reader 10, although it is to be understood that other equivalent systems and circuitry could be substituted. In Fig. 2, the reader 10 and the tag 12 are shown in block diagrams. Figs. 3 and 4 depict the circuitry of the reader 10 removed from the case 14 of Fig. 1. Fig. 3 depicts one side of the internal circuitry of the reader of Fig. 1, and Fig. 4 depicts the opposite side of the internal circuitry of the reader of FIG. 1. As shown in the block diagram of Fig. 2 and the circuit configurations of Figs. 3 and 4, the reader 10 includes a coil circuit 30 coupled to a coil driver circuit 32. The coil driver circuit 32 includes an oscillator 34 producing regular reoccurring pulses in a drive signal 36 on an oscillator output line 38 at a transmission frequency F(t) . The drive signal 36 may be a sine wave, triangle wave, square wave, or other waveform with a pulse time or period corresponding to the desired transmission frequency.

The drive signal 36 is provided to a signal driver element 40, including a positive driver 42 which outputs the F(t) waveform at zero degrees (0°) and a negative driver 44 which outputs the same signal inverted or shifted one hundred eighty degrees (180°) . Thus, the signal driver element 40 transforms the drive signal 36 into first and second complementary pulse trains 46 and 48. The pulse trains 46 and 48 are amplified by amplifiers 50 and 52, respectively, then coupled to the coil circuit 30.

The coil circuit 30 may include dual capacitors 56, 58 coupled to receive the pulse trains 46 and 48 from the amplifiers 50 and 52. The capacitors 56 and 58 are coupled in turn to opposite ends of a field coil 60, so that each input of the field coil 60 is coupled to one of the pulse trains 46 and 48 through a separate capacitor.

The field coil 60 produces a time varying electromagnetic field when excited by the pulse trains 46 and 48. The electromagnetic field propagates in three-dimensional space around the field coil 60. The tag 12 includes a receiving coil 70 receiving and inducing a tag power supply voltage from the electromagnetic field generated by field coil 60. The tag 12 also includes a memory 72 and other circuitry 74 for variably loading or energizing the receiving coil 70 of the tag 12 reflecting data output from the memory 72.

For a full-duplex system, the tag 12 can have the structure of the tag disclosed in U.S. Patent 4,333,072 (Beigel) . For a half-duplex system, the circuitry 74 would include a power storage element such as a capacitor (not shown) . When the tag 12 is in proximity with the reader 10, the tag 12 is energized and produces an identification signal which is received by the field coil 60 of the reader 10. Following receipt of the identification signal by the field coil 60 of the reader 10, the identification signal is processed or decoded in a signal processing circuit 80. The signal processing circuit 80 may include a rectifier 82 and filter 84 as shown in Fig. 2. Alternatively, the signal processing circuit 80 can include a low pass filter coupled to a comparator such as that disclosed in Beigel U.S. Patent 4,333,072. Other types of signal processing may be required, and are contemplated herein, dependent upon the particular encoding features of the tag 12. The reader may also include a microprocessor or central processing unit (CPU) 90 coupled to receive the output from the signal processing circuit 80. The CPU 90 may including decoding and display circuitry and/or software to translate the identification signal into usable data according to a predetermined format for information retrieval or transmission purposes. The CPU 90 can be an Intel 80C51 processor and can operate in cycles synchronized by a clock 92. The clock 92 can also be coupled to the oscillator 34, creating a synchronous system, using a clock signal provided on line 94 from the CPU 90 to the oscillator 34. The clock 92 thereby determines the operating frequency of the entire system.

The CPU 90 drives the display 16 which is preferably a commercially available alphanumeric dot matrix liquid crystal display (LCD) or similar device. Preferably the display comprises a commercially available one line by sixteen character alpha-numeric display, such as Amperex model AMX116 or equivalent. As an optional accessory, the reader 10 can include an input/output (I/O) interface 96 to an external device 98, such as a conventional RS-232 serial interface. The reader 10 can be powered by a conventional regulated direct current (DC) power supply 100, preferably using a battery 102 as an input current source or an external D.C. supply.

As shown in Fig. 3, the field coil 60 is positioned proximate one end of the reader 10, and attached to a main circuit board 88. The circuit board 88 also has the display 16 mounted ofn the same side as the field coil 60, as well as a piezoelectric tone generator 86 which emits a tone upon activation, and to confirm that a tag has been read.

The field coil 60 includes a block shaped ferromagnetic core 110, having a conductive wire 112 wrapped about the core 110. The wire 112 is preferably a fine gage insulated copper wire. The wire 112 is wrapped to encircle the long axis of the core 110, for example using 220 turns of #32 gage wire. Prior to wrapping with the wire 112, the core 110 is preferably coated with a rubbery silicon 114 and/or tape 116 to both protect the core 110 from being accidentally damaged and space the core 110 from the wire 112 to decrease core saturation.

The field coil 60 is illustrated in the perspective view of Fig. 5 removed from the circuit board 88. The core 110 of the field coil 60 preferably has a rectangular block shape. The ferromagnetic core 110 preferably has a length along the axis of the coil, of between about one and four inches, however larger coils to six or eight inches long may be used in some applications. Further, the field coil preferably has a length to width ratio of between about 1 to 1 and 5 to 1, and a width to thickness ratio of between about 1 to 1 and 10 to 1. The dimensions for the core 110 as illustrate are about two and one half inches by one inch by one quarter inch. Thus, the cross sectional area perpendicular to the long axis of the core is about .25 square inches, while the profile required to contain the field coil 60 is minimized.

In operation, the reader 10 radiates the electromagnetic field into space at a specific frequency. The receiving coil 70 of tag 12 is resonant at F(t) , so that the receiving coil 70 obtains energy from the radiated field and inductively converts the field to an adequate internally generated supply voltage and current for the circuitry 74 in the tag 12. The tag circuitry 74 may include a power supply, variable loading element, clocking section and sequencing section (not shown) , coupled to the programmable memory 72. The power supply converts AC energy at the resonant frequency into a DC supply voltage to power the electronic circuits in the tag 12 reliably over as wide a range of input energy values as possible. The reading distance of a full duplex system is generally proportional to the amount of mutual inductance or coupling generated between the receiving coil 70 of the tag 12 and the field coil 60 of the reader 10. At the frequency range of generation (100 to 400 KHZ) these systems obey the physics of "steady state (DC)" near-field magnetic coupling. The coupling between the receiving coil 70 and the field coil 60 is proportional to the crossing of field lines of the attached magnetic field of the field coil 60 which supplies power to the tag 12 and the "virtual" magnetic field of the receiving coil 70 which variably loads the field coil 60 or otherwise produces an inductive signal in the field coil 60 of reader 10.

For full duplex systems, therefore, the reading distance is a complex function of the orientation between the receiving coil 70 and the field coil 60. In designing a hand-held portable reader, a design objective is to enable the user to read a tag with a minimum of effort. Since the exact location and orientation of an implanted or embedded tag 12 is unknown, the field coil 60 is optimized to provide the maximum opportunity for reading the tag 12. In practice, the reader 10 is usually brought into the vicinity of the tag 12 and then moved with a spiraling motion until the tag is detected.

The field coil 60 preferably has a size and shape and winding configuration which results in the highest probability of reading a specified tag 12 for a given "reader" size. The tag 12 would theoretically be at any angular orientation with respect to the field coil 60. The reading range distance may be specified within some limits, but the probability of obtaining a reading within that distance should be high, independent of the orientation of the tag, as long as the field coil 60 of the reader 10 is moved around in a specific area with respect to the receiving coil 60 of the tag 12.

Figs. 6 and 7 graphically illustrate the magnetic field produced by the field coil 60, with the magnitude of a field in the X-Y plane being illustrated in Fig. 6, in the Y-Z plane in Fig. 7, and the orientation or magnetic vectors of the magnetic field emanating from the axial ends of the field coil 60 represented by the arrows 100 in Fig. 8. The lines 102, 104, 106 and 108 in Figs. 6 and 7 represent the equal potential magnetic field strength at 40 A/m (Amp/meter) , 30 A/m, 20 A/m and 10 A/m, respectively, surrounding the field coil 60, when it is energized by a particular input signal having a defined current level. As will be apparent to those skilled in the art, the magnitude of the field will vary with variations in the input signal supplied to the field coil 60.

In Figs. 6 and 7, the lines 102 - 108 illustrate the magnetic flux strength at varying distances from the field coil 60. For a given tag 12 having a specific coil configuration, it may be assumed that the tag coil will receive enough energy to allow coupling of information with the reader 10 at a determinable threshold level, and particular orientation with respect to the magnetic vector. Generally, large tag coils achieve coupling at greater distances, hence lower field strengths, than small tag coils.

Fig. 8 is a graphic depiction of the vector field surrounding the field coil of Fig. 6. As may be appreciated from the respective Figs. 6 and 8, the magnitude and to a lesser degree the shape of the equal potential magnetic field surrounding the respective field coils is impacted by the shape of the core of the field coil, when the energizing field and winding configurations are made equivalent.

A first alternative embodiment of the field coil is depicted in Fig. 9, wherein a ferromagnetic core 140 of the field coil 60 has a generally oblong shape wherein the respective axial ends 142, 144 are rounded. However, the ferromagnetic core 120 retains the generally rectangular cross section. The design of the ferromagnetic core 120 modifies the magnetic field produced by the field coil 60 and enhances the ability of the reader 10 to couple with a tag 12 for particular orientations of the tag 12. In addition, the case 146 for the reader may be modified to closely proximate the shape of the field coil 60. As in the embodiments of Figs. 1 and 3, the field core of Fig. 9 includes plurality of windings 148.

FIG. 10 depicts another alternative design for the field coil, wherein the ferromagnetic core 160 has a block shaped center section 162 and one-quarter hemispherical end sections 164, 166. The case 170 for enclosing the field coil is modified similar to that of Fig. 9 to have rounded corners closely proximating the rounded portions of the respective ends 164, 166 of the ferromagnetic core 160. As in the embodiments of Figs. 1 and 3, the field core of Fig. 10 includes plurality of windings 168. FIG. 11 depicts another alternative design for the field coil, wherein the field coil has a generally square shaped ferromagnetic element 180 which is almost completely encircled by the windings 182. As illustrated, the case 184 for this embodiment is generally squared off and closely proximates the outer dimensions of the square shaped field core.

FIG. 12 depicts another alternative design for the field coil, wherein the field coil includes a ferromagnetic element 190 including a generally blocked shaped center section 192 and triangular end sections 194 and 196. As above, the windings 198 encircle the generally block shaped center section 192. In addition, the housing is modified to closely proximate the surfaces of the field coil.

FIG. 13 depicts yet another alternative embodiment of the field coil, wherein the field coil includes a disk shaped ferromagnetic element 200 having a field coil 202 wrapped around the center section leaving portions 204 and 206 partially exposed. This design is a variation on the designs according to Figs. 9 and 10, and the case enclosing the field coil has a generally rounded end portion. FIG. 14 depicts still another alternative embodiment of the field coil, wherein the field coil includes a part- circular ferromagnetic element 210 having a winding 212 wrapped thereabout and leaving exposed ends 214, 216. As may be appreciated, the case enclosing the field coil having the part-circular ferromagnetic element 210 has a rounded profile at the end thereof, accommodating the field coil.

For any of the ferromagnetic coils of the field coils depicted in the foregoing figures, it should be recognized that the magnetic flux lines emanating from the field coil exit perpendicular to the respective end portions of the ferromagnetic element. Accordingly, the designs according to Fig. 9 modifies the shape of the magnetic field surrounding the respective field coils thereby impacting the shape of the envelope in which a tag can be read. The plurality of designs is depicted to emphasize the variations which may be utilized while still maintaining a relative low thickness and profile for the reader, while minimizing the size of the field coil by taking advantage of the increase magnetic field generated by the use of a ferromagnetic core within the field coil. For any of the foregoing embodiments, the ferromagnetic core may be formed from a ferrite material. Alternatively, the ferromagnetic core can be formed from a magnetic amorphous metal having the required ferromagnetic property, or sheets of magnetic amorphous metal having the required ferromagnetic properties. These types of amorphous materials are currently being used in transformers to provide enhanced magnetic properties and reduce the losses associated with transmission of an electromagnetic field.

Preferred magnetic amorphous metals are materials having the general formulas M_aY_bZ_c and M_dY_e, which are disclosed by U.S. Patent No. 3,856,513 issued to Chen et al. A variety of techniques are known in the art for producing these materials in the form of wires, ribbons and thin films. In general, the materials are formed by rapidly quenching the alloy from the

ΛO melt at rates of at least 10 C/s, and more preferably, of at least 1060C,/s

Figs. 15 and 16 is a model graphically illustrating the magnetic field produced by a field coil having cylindrical ferrite core, for purposes of comparison to the field produced by the field coil having a block shaped core of the present invention. As in Fig. 6, the magnitude of the field in the X-Y plane is illustrated in Fig. 15, and the orientation or magnetic vectors of the magnetic field emanating from the axial ends of the field coil is shown in Fig. 16. In the representations according to Figs. 15 and 16, the diameter of the cylindrical ferrite core is equal to the thickness of the block shaped ferrite core modeled in Figs 6 and 8. As may be appreciated, the magnetic field illustrated in Fig. 6 encloses a significantly greater envelope, for any particular magnitude, than does the field shown in Fig. 15.

In view of the foregoing, it may be appreciated that the block shaped core 110 for the field coil 60 of Fig. 5 optimizes the area within which a reader can successfully couple with a tag 12, while minimizing the profile of the reader 10. The alternative designs above also take advantage of this feature to a lesser degree, and may provide enhanced reading capability by virtue of the differences in the shape of the magnetic fields which will be generated by each respective field coil. These features are highly desirable in the design of the reader 10, and its operation by the user. Moreover, the field coil 60 disclosed herein provides a substantial increase in the transmitted power for a given power supply input voltage, and an increase in signal sensitivity for a given variation in tag output signals. The present invention contemplates many variations and alternative embodiments. Numerous alternate circuits are possible to drive the field coil, and to create different signal transmission protocols, each of which might optimally use different filtering, recognition, and detection circuits. Thus, the scope of the present invention should not be limited to the foregoing detailed description, but rather should be only by the proper literal and equivalent scope of the appended claims.

Claims

WHAT IS CLAIMED IS:

1. A reader for an inductive coupled tag identification system, comprising: a field coil having a non-cylindrical ferromagnetic core, said ferromagnetic core having a length extending from respective ends of said ferromagnetic core and a long axis,, said field coil and said ferromagnetic core being operative to produce a magnetic field emitted from said ferromagnetic core at an angle of not more than ninety degrees to the long axis; a driver circuit electrically connected to provide an alternating voltage to said field coil; and a signal analyzing circuit electrically connected to said field coil to obtain and analyze a tag identification signal from said field coil.

2. The reader of claim 1, wherein the field coil further comprises a wire wrapped around said core a plurality of times to encircle the core about its long axis.

3. The reader of claim 1, wherein the wire of the field coil is insulated copper wire

4. The reader of claim 1, wherein the field coil further comprises a generally block shaped core.

5. The reader of claim 1, wherein the field coil further comprises a generally block shaped core having a length of between about one and four inches and a length to width ratio of between about 1 to 1 and 5 to 1.

6. The reader of claim 1, wherein the field coil further comprises a generally block shaped core having a length to width ratio of between about 1 to 1 and 5 to 1.

7. The reader of claim 6, wherein the field coil has a width to thickness ratio in the range of between about 1 to 1 and 10 to 1.

8. The reader of claim 1, wherein the field coil is rectangularly-shaped and has a width to thickness ratio in the range of between about 1 to 1 and 10 to 1.

9. The reader of claim 1, wherein the core of the field coil is a generally oblong shaped ferromagnetic element having a generally rectangular cross section.

10. The reader of claim 1, wherein the core has a part circular profile.

11. The reader of claim 1, wherein the core of the field coil includes a generally block shaped center section and one- quarter hemispherical end sections.

12. The reader of claim 1, wherein the core of the field coil includes a generally block shaped center section and triangular end sections.

13. The reader of claim 1, wherein the core has a disk shape.

14. The reader of claim 1, wherein the core is formed from a material selected from the group consisting of ferrite and ferromagnetic amorphous metals.

15. An inductively coupled tag reader system, comprising: a tag having an identification code memory element coupled to a receiving coil; and a reader including a field coil having anon-cylindrical ferromagnetic core, a driver circuit electrically connected to provide an alternating voltage to said field coil, and a signal analyzing circuit electrically connected to said field coil to obtain and analyze a tag identification code signal from said field coil; wherein said ferromagnetic core has a length extending from respective ends of said ferromagnetic core and a long axis, said field coil and said ferromagnetic core being operative to produce a magnetic field emitted from said ferromagnetic core at an angle of not more than ninety degrees to the long axis

16. The inductively coupled tag reader system of claim 15, wherein the field coil further comprises a core having a length along a magnetic axis of between about one and eight inches and a length to width ratio of between about 1 to 1 and 3 to 1.

17. The inductively coupled tag reader system of claim 15, wherein the field coil further comprises a generally block shaped core having a length to width ratio of between about 1 to 1 and 3 to 1.