GB1563507A - Electromagnetic sensor system - Google Patents

Description

(54) ELECTROMAGNETIC SENSOR SYSTEM (71) We, MINNESOTA MINING AND MANUFACTURING COMPANY, a corporation organized and existing under the laws of the State of Delaware, United States of America, of 3M Center, Saint Paul, Minnesota 55101, United States of America" do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: The present invention relates in general to electromagnetic reading of magnetically encoded cards, and more specifically to a sensor system for detecting magnetized bits in regions in such cards.

Various readers and sensing devices for detecting magnetized bits in regions in magnetically encoded cards are known in the art.

Although the prior sensors appear to provide satisfactory sensing of bits .125 inch (3 mm) or larger in diameter, there is an increasing need for sensors that will detect bits of much smaller diameters in order that considerably more information may be encoded in a credit card or the like. Furthermore, as more and more information is encoded in a card the importance of reliability of operation and simplicity of construction of a sensor for reading encoded cards is likewise increased.

The present invention provides an improved electromagnetic sensor system for detecting data magnetically encoded in a document, said system comprising a housing with at least one planar surface area, a two dimensional planar array of bit indicators disposed in said housing in a parallel relationship to said surface area, which bit indicators each include at least one ferromagnetic core that is magnetizable to one or other of two alternate stable remanent magnetized states, and a control member centrally disposed through said core and including drive means that provides a magnetic field for switching the magnetization of said ferromagnetic core from one stable remanent state to the other and also including sense means in which an electrical signal is induced during each stable state transition, a plurality of magnetized bias field sources, each field source being associated with one a respective of said cores and having a magnetic field to magnetize said associated core in a magnetized state that prevents the inducement of the stable remanent magnetized states and transitions therebetween, and a magnetically encoded document disposed in surface-to-surface contact with the planar surface of said housing and containing data represented by a plurality of magnetic data bits some of which have a magnetic field substantially opposite in magnitude and direction to that of each of said bias field sources, said data bits located in various bit regions arranged in a two dimensional pattern that includes at least one bit region aligned with a respective one of said bit indicators so that the magnetic fields of some of said data bits substantially cancel the magnetic fields of the bias field sources magnetizing the bit indicators aligned with the bit regions in which said data bits are located.

In the absence of any external magnetic field, for example caused by the said bias field sources or said magnetically encoded document. an electrical pulse applied to the drive means of the indicators causes a first stable remanent magnetized state to be induced in the indicator cores. The squareness ratio of the cores is high and such magnetized state remains virtually unchanged until a second electrical pulse opposite to the first pulse is applied to the drive means for producing a transition of the magnetization in the cores from the first state to a second remanent state. This transition between stable remanent states occurs abruptly and induces a sharp electrical signal pulse in the sense means, which signal can be readily detected by common electronic circuitry.

The magnetic field sources normally influence the respective cores with a magnetic field similar to that induced by the bit of a magnetically encoded card. The field sources, therefore, prevent the indicator cores from switching between magnetized stable states.

Thus, core stable state transitions do not occur unless the cores are near a magnet ized bit that provides a magnetic field opposite in polarity and substantially equal to the field of the bias field sources so that there is a cancellation of the two fields. Upon such cancellation, drive pulses thereafter cause the stable state of the cores to switch and result in sensing indications in the sense means. Accordingly, there are provided readily detectable pulses only when the cores of the sensor are in the magnetic influence of a specified type of encoded card bit.

A preferred embodiment is described in connection with the following drawings wherein: Fig. 1 is a schematic plan view of an elec- tromagnetic sensor with portions cut away to show the interior arrangement of the cores; Fig. 2 is an enlarged fragmentary perspective view of the sensor of Fig. 1 with portions cut away; Fig. 3A is an enlarged perspective view of a bit indicator included in the sensor of Fig. 1 and magnetized in a first stable remanent state; Fig. 3B is a graph of a hysteresis loop indicating the magnetic characteristics of the bit indicator of Fig. 3A; Fig. 3C is an enlarged perspective view of the bit indicator of Fig. 3A magnetized in a second stable remanent state; Fig. 4 is an enlarged perspective view of the bit indicator of Fig. 3A interposed between a magnetic field source and a magnetized bit of a magnetically encoded card;; Fig. 5A is an enlarged perspective view of a bit indicator that may be utilized in the sensor of Fig. 1 in substitution for the indicator of Fig. 3A; Fig. SB is an enlarged perspective view of another type of bit indicator construction that may be utilized in the sensor of Fig. 1; Fig. SC is an enlarged perspective view of yet another type of bit indicator that may be utilized in the sensor of Fig. 1; and Fig. 6 is an enlarged perspective view of a complex bit indicator that may be utilized in the sensor of Fig. 1; and Fig. 7 is a perspective view of a magnetically encoded card having a plurality of magnetized regions.

Referring now to the drawings and with reference first to Figs. 1 and 2, an electromagnetic sensor 1 is shown that represents a preferred embodiment of the present invention. The sensor 1 is particularly advantageous for employment with a card reader that detects the presence of data bits located in regions of a magnetically encoded card. Such bits may each be in the form of a single magnetized section, or of a pair of magnetized sections interfaced together at a common boundary. The sensor 1 is shown as having a self contained housing 2, but for practical considerations it may be desirable to build the sensor 1 as an integral part of a card reader with which it is employed.

The sensor housing 2 has a relatively thin rectangular configuration and is formed of a top wall 3, a bottom wall 4, sidewalls 8 and 9, and end walls 10 and 11. A backing card 12 is positioned in the housing 2 and is parallel to the top wall 3. Mounted on the card 12 in close proximity to the top wall 3 is a planar array of a plurality of minute bit indicators 13 arranged in a number of rows and columns, but such arrangement is not essential to the present invention.

Instead, the indicators 13 may be positioned in various arrangements to conform with the location of magnetized bits in the encoded cards that the sensor 1 may be utilized for reading.

As indicated in Fig. 3A, the bit indicators 13 each include a toroidal, ferrite core 14 through which a control means 16 is disposed. A pair of electrical conductors 17 and 18 form the control means 16 and serve respectively as a drive means and a sense means. Rather than using two conductors in the control means it may be desirable to use a single conductor that acts as both the drive and sense means. It would also be possible to employ more than one of the conductors 17 to form the drive means such as presently used in addressing computer memories. The core 14 is preferably formed from a ferromagnetic, ceramic material such as magnesium-manganese ferrites or other spinel ferrites and has a rectangular hysteresis loop 19, of the type shown in the graph 3B, with two remanent magnetized stable states 20 and 21.The term "ferromagnetic" is used herein to include both ferromagnetic and ferrimagnetic.

Referring now to Fig. 3B in conjunction with Fig. 3A, in the absence of any external magnetic field influencing the cores 14, such as for example the card 30, a d-c pulse, represented by the arrow 22, directed through the drive conductor 17 results in a magnetic field Hm of the form of closed loop concentric lines that induce a similarly configured stable remanent magnetized state that circles the conductor 17. The magnetic flux 23 resulting from this magnetization state is substantially confined in the core 14.

Upon termination of the pulse 22 through the drive conductor 17, the magnetic field Hm relaxes and the magnetic flux density decreases a small amount to the stable state 20.

The squareness ratio of the core 14 is high in order that the magnetization of the core 14 remains virtually unchanged from the stable state 20 even in the presence of a reverse magnetizing force approaching the coercive force. As indicated by Figs. 3B and 3C, however, under the same external conditions the application of a second d-c pulse 24 in the drive conductor 17 (such pulse being equal in magnitude but opposite in direction to the pulse 22) produces a reverse magnetizing field - H1 in the direction 29 that abruptly reverses the magnetization state of the core 14 in the same direction 29 to result in a magnetic flux density --B,, also in the direction 29.

The reverse magnetic field --H,, falls to zero at the termination of the current pulse 24 and the negative magnetic flux density -B, decreases slightly to the stable remanent state 21. A second d-c pulse 22 would thereafter cause another abrupt reversal in the magnetization of the core 14 and return it back to the magnetic flux density Bnl.

Thus, by directing d-c pulses through the drive conductor 17-first in one direction and then in the opposite direction-the magnetized stable states of the core 14 are abruptly reversed. Each abrupt reversal of the magnetic flux in the core 14 induces an easily detectable short duration discrete output pulse represented in Fig. 3C by the arrow 28 in the sense conductor 18, and standard electronic circuitry (not shown) may be employed to receive the pulses 28 for detecting the flux reversals that occur.

In contrast to using d-c pulses in the drive conductor 17, a-c pulses may be applied to the drive conductor 17 to produce a series of abrupt reversals between the two stable magnetized states 20 and 21.

The sensor 1 includes in addition a plurality of bias magnetic field sources 34 (shown only in Fig. 4) that are disposed in the sensor 1 so that one field source 34 is positioned immediately beneath each indicator 13. The field sources 34 can be provided by using permanent magnets or current carrying conductors that may be adapted to change bias field direction or magnitude when necessary to sense bits of different polarity or size. The purpose of the sources 34 is to bias the cores 14 with a sufficiently strong unidirectional field, indicated by the arrows 38 in Fig. 4, so that the cores 14 are normally in a nonremanent or unstable magnetized third state that inhibits magnetization in the cores 14 from existing in the stable state 20 or 21, or switching therebetween.

The strength of the magnetic fields of the field sources 34 should be carefully selected in order that they approximately equal the magnitude of the magnetic fields of the bits 31 in the magnetically encoded cards 30 with which the sensor 1 is to be employed.

As indicated by Fig. 4, the magnetic fields of the bits 31 and the field sources 34 are opposite in direction and serve to cancel one another. Upon such field cancellation, appropriate electrical pulses in the drive conductor 17 of the indicator 13 thereafter induce one of the magnetized stable states 20 or 21 and subsequent transitions therebetween in the core 14. Thus, with the addition of the field sources 34 the bit indicators 13 signal the presence of an encoded bit by induced electrical pulses in the sense conductor 18.

Although the use of the field sources 34 allows the sensor 1 to provide electrical sense pulses in the conductor 18 only when detecting encoded bits 31 of one type of magnetic polarity, the bias field provided by the field sources 34 may be reversed to detect bits of opposite polarities and, thus, the use of the field sources 34 is highly advantageous in the present invention. Furthermore, the sources 34 permit the indicators 13 to operate within a range of magnetic fields that may be designed to allow normal variations in bit size, to permit the sensing of encoded cards with unmagnetized or oppositely magnetized background areas, and also to provide a means for ignoring bits 31 of the wrong size and polarity in an encoded card where it is desirable to conceal the encoded data.

The sensor 1 may be modified by employing other types of bit indicators as an alternative to the above construction of the indicators 13. As shown in Fig. 5A bit indicators may be formed with two relatively thin layers 39 and 40 of the ferromagnetic film employed as a core 41 to encircle a control means 42, or the bit indicators may be formed with a magnetic foil core 43, as shown in Fig. 5B. A third form of bit indicators (shown in Fig. SC) may include a control means 44 that is plated with a ferromagnetic core material 45.

In some instances, it may be desirable to have larger output sensing pulses in the sense conductors 18 or a greater sensitivity to small field bits than provided by the use of a bit indicator 13 with a single core 14 for each magnetized bit of an encoded card.

Such increased performance may be achieved through the use of a complex bit indicator 46 shown in Fig. 6 together with the magnetically encoded card 30. The indicator 46 has three toroidal ceramic cores 7, but any number of cores 47 may be employed to obtain the desired output voltages or sensitivity. Similar to the bit indicators 13, the indicator 46 has a control means 48 formed of a drive conductor 49 and a sense conductor 50 that are directed through the cores 47 so that the pulses induced in the sense conductors 50 are additive. Greater sensitivity results from lengthening one direction of the core member because the permeability of the core member increases in that direction.

Thus, as shown and described, the present invention provides a sensor 1 that is simplistic in construction but yet provides an im proved capability for detecting magnetic bits in magnetically encoded cards. In the above described embodiment, the sensor 1 is adapted to provide an indication of the presence of a magnetic bit without regard to holding such data in memory.

Referring again to Figs. 2 and 3A, each of the drive conductors 17 of the bit indicators 13 is pulsed with a sufficient d-c pulse to induce magnetization of their respective cores 14 in the first magnetized stable state 20. A magnetically encoded card 51, shown in Fig. 7, is then positioned on the top wall 3 of the sensor housing 2 to be in close proximity to the planar array of the indicators 13. The card 51 is composed of a plurality of bit regions 52 equal to the number of bit indicators 13 in the sensor 1. Each bit region 52 is aligned directly above one of the indicators 13 and may or may not include a magnetized data bit.

Certain of the bit locations 52 are magnetized to represent one type of digital information such as a "1", and the remaining bit locations 52 are left unmagnetized to represent a "0". With the card 51 positioned on the housing top wall 3 of the sensor 1, the field of each magnetized bit location magnetically induces the core 14 of its corresponding bit indicator 13 in a nonremanent third magnetized state. During the time the card is retained in close proximity to the indicator 13, subthreshold electrical pulses that tend to induce the magnetized stable state 21 in the cores 14 are supplied to all the drive conductors 17 and persist until after removal of the card 51 from the sensor 1. Such pulses are subthreshold pulses in that they are not sufficient by themselves to produce a reversal of the magnetized stable states of the cores 14 from the remanent state 20 to the state 21.Nevertheless when the card 51 is removed from the sensor 1, the persistent subthreshold pulses provide a sufficient magnetic field to induce the cores 14 that are in the third nonremanent magnetized state to the stable state 21 because less energy is required to induce the stable state 21 in the cores 14 as the nonremanent state is relaxed than is required to switch the cores 14 from the stable state 20 to the stable state 21. Accordingly, the persistent subthreshold pulses do not effect switching in the cores 14 that were not induced with the non-remanent field.

WHAT WE CLAIM IS:- 1. An electromagnetic sensor system for detecting data magnetically encoded in a document, said system comprising a housing with at least one planar surface area, a two dimensional planar array of bit indicators disposed in said housing in a parallel relationship to said surface area, which bit indicators each include at least one ferromagnetic core that is magnetizable to one or other of two alternate stable remanent magnetized states, and a control member centrally disposed through said core and including drive means that provides a magnetic field for switching the magnetization of said ferromagnetic core from one stable remanent state to the other and also including sense means in which an electrical signal is induced during each stable state transition, a plurality of magnetized bias field sources, each field source being associated with a respective one of said cores and having a magnetic field to magnetize said associated core in a magnetized state that prevents the inducement of the stable remanent magnetized states and transitions therebetween, and a magnetically encoded document disposed in surface-to-surface contact with the planar surface of said housing and containing data represented by a plurality of magnetic data bits some of which have a magnetic field substantially opposite in magnitude and direction to that of each of said bias field sources, said data bits located in various bit regions arranged in a two dimensional pattern that includes at least one bit region aligned with a respective one of said bit indicators so that the magnetic fields of some of data bits substantially cancel the magnetic fields of the bias field sources magnetizing the bit indicators aligned with the bit regions in which said data bits are located.

2. A sensor system as recited in claim 1 wherein said cores each comprise a substantially toroidal member having an open center through which said control means is disposed.

3. A sensor system as recited in claim 1 wherein said cores each comprise two layers of ferromagnetic film between which said control means is disposed.

4. A sensor system as recited in claim 1 wherein each of said cores comprise a ring of ferromagnetic foil.

5. A sensor system substantially as herein described with reference to the accompanying drawings.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (5)

**WARNING** start of CLMS field may overlap end of DESC **. proved capability for detecting magnetic bits in magnetically encoded cards. In the above described embodiment, the sensor 1 is adapted to provide an indication of the presence of a magnetic bit without regard to holding such data in memory. Referring again to Figs. 2 and 3A, each of the drive conductors 17 of the bit indicators 13 is pulsed with a sufficient d-c pulse to induce magnetization of their respective cores 14 in the first magnetized stable state 20. A magnetically encoded card 51, shown in Fig. 7, is then positioned on the top wall 3 of the sensor housing 2 to be in close proximity to the planar array of the indicators 13. The card 51 is composed of a plurality of bit regions 52 equal to the number of bit indicators 13 in the sensor 1. Each bit region 52 is aligned directly above one of the indicators 13 and may or may not include a magnetized data bit. Certain of the bit locations 52 are magnetized to represent one type of digital information such as a "1", and the remaining bit locations 52 are left unmagnetized to represent a "0". With the card 51 positioned on the housing top wall 3 of the sensor 1, the field of each magnetized bit location magnetically induces the core 14 of its corresponding bit indicator 13 in a nonremanent third magnetized state. During the time the card is retained in close proximity to the indicator 13, subthreshold electrical pulses that tend to induce the magnetized stable state 21 in the cores 14 are supplied to all the drive conductors 17 and persist until after removal of the card 51 from the sensor 1. Such pulses are subthreshold pulses in that they are not sufficient by themselves to produce a reversal of the magnetized stable states of the cores 14 from the remanent state 20 to the state 21.Nevertheless when the card 51 is removed from the sensor 1, the persistent subthreshold pulses provide a sufficient magnetic field to induce the cores 14 that are in the third nonremanent magnetized state to the stable state 21 because less energy is required to induce the stable state 21 in the cores 14 as the nonremanent state is relaxed than is required to switch the cores 14 from the stable state 20 to the stable state 21. Accordingly, the persistent subthreshold pulses do not effect switching in the cores 14 that were not induced with the non-remanent field. WHAT WE CLAIM IS:-

1. An electromagnetic sensor system for detecting data magnetically encoded in a document, said system comprising a housing with at least one planar surface area, a two dimensional planar array of bit indicators disposed in said housing in a parallel relationship to said surface area, which bit indicators each include at least one ferromagnetic core that is magnetizable to one or other of two alternate stable remanent magnetized states, and a control member centrally disposed through said core and including drive means that provides a magnetic field for switching the magnetization of said ferromagnetic core from one stable remanent state to the other and also including sense means in which an electrical signal is induced during each stable state transition, a plurality of magnetized bias field sources, each field source being associated with a respective one of said cores and having a magnetic field to magnetize said associated core in a magnetized state that prevents the inducement of the stable remanent magnetized states and transitions therebetween, and a magnetically encoded document disposed in surface-to-surface contact with the planar surface of said housing and containing data represented by a plurality of magnetic data bits some of which have a magnetic field substantially opposite in magnitude and direction to that of each of said bias field sources, said data bits located in various bit regions arranged in a two dimensional pattern that includes at least one bit region aligned with a respective one of said bit indicators so that the magnetic fields of some of data bits substantially cancel the magnetic fields of the bias field sources magnetizing the bit indicators aligned with the bit regions in which said data bits are located.

2. A sensor system as recited in claim 1 wherein said cores each comprise a substantially toroidal member having an open center through which said control means is disposed.

3. A sensor system as recited in claim 1 wherein said cores each comprise two layers of ferromagnetic film between which said control means is disposed.

4. A sensor system as recited in claim 1 wherein each of said cores comprise a ring of ferromagnetic foil.

5. A sensor system substantially as herein described with reference to the accompanying drawings.