Capacitive method and apparatus for accessing information encoded by a differentially conductive pattern

[0002] The invention relates to the acquisition of encoded information, particularly barcode, that is written in a differentially conductive pattern and can be concealed from view within an envelope or other layered structure.

[0003] Much of bulk return mail is processed with at least some manual handling, especially when it contains orders. Once cut open, the envelopes are generally emptied by hand; and information from their contents is keyboarded, optically scanned, or otherwise entered into a computer. The required steps of opening the envelopes, separating their contents, and entering relevant data are expensive and time consuming. Also, data entry is subject to error, especially when information printed on the envelopes must be linked to information from their contents.

[0004] Outgoing bulk mail is also subject to sorting and other processing errors that are difficult to detect; because once sealed, their contents are concealed from view. For example, inserts containing confidential information can be placed in the wrong envelopes addressed to persons who become privy to private information of others. Many different approaches have been used to see through the envelopes and read their contents without opening them but problems plague each.

[0005] U.S. Pat. No. 5,522,921 to Custer proposes use of x-rays for reading envelope contents that are printed with special x-ray opaque materials. The x-rays penetrate the envelopes and their contents except where blocked by the special materials. A resulting shadow pattern is detected by an x-ray reading device. However, the special materials add expense and limit printing options, and the x-rays pose health risks that are difficult to justify for these purposes.

[0006] U.S. Pat. No. 5,288,994 to Berson proposes using infrared light in a similar manner to read the contents of sealed envelopes. A light source directs a beam of the infrared light through the envelopes to an optical detector that records a shadow pattern caused by different absorption characteristics between conventional inks and the paper on which they are printed. However, such filled envelopes make poor optical elements for transmitting images, even for transmissions in the infrared spectrum. Paper does not transmit the infrared images very efficiently. Irregularities in the surfaces, spacing, layering, and materials of the envelopes and their contents cause significant aberrations that can greatly diminish resolution of the images. Also, overlays of printed material on the envelopes and their contents are difficult to separate, and printed backgrounds can reduce contrast.

[0007] Except for differences in wavelength, these prior approaches are analogous to shining a flashlight through one side of an envelope in the hope of reading darker printed matter through the envelope's opposite side. X-rays penetrate paper very easily but are dangerous and require special materials to stop them. Near infrared wavelengths transmit poorly through paper, and their images are subject to aberration from optical inconsistencies and to obscuration from printed overlays or backgrounds.

[0008] U.S. Pat. No. 5,811,792 by two of the present co-inventors, Verschuur and Mitchell, Jr., proposes a combination of microwave heating and infrared viewing to access the contents of sealed envelopes. Microwave energy differentially heats conductive or dielectric patterns in the contents, and infrared detectors record thermal images of the patterns conducted to the envelopes' surfaces.

[0009] U.S. Pat. No. 5,621,200 to Irwin, Jr. et al. discloses an electronic validation system for scratch-off lottery tickets. A conductive ink containing a pattern of resistors is printed as a portion of the scratch-off material or underlying play indicia. Capacitors couple the printed resistor circuits to an electronic verification machine to verify electronic signature patterns of the resistor circuits. The electronic signatures are comparable to predetermined standards, but they do not contain information encoded in conventional formats that can be read as alphanumeric characters. Also, each ticket must be tested one at a time at a predetermined position within the verification machine.

[0010] U.S. Pat. No. 3,519,802 to Cinque et al. discloses an early attempt at authenticating credit cards with internally encoded data. Conductive plates are arranged in a pattern; and their presence, absence, or proximate orientation is detected by a capacitance sensor. However, the detection system requires the conductive plates to be bent into two offset planes that complicate manufacture and are not readily applicable to thinner substrates such as sheet materials normally enclosed by envelopes.

[0011] U.S. Pat. No. 4,591,189 to Holmen et al. discloses a more recent example of a credit card verification system in which a light-transmitting authenticating layer is sandwiched between two anti-reflective film layers. The authenticating layer is preferably vacuum deposited, such as by sputtering, but can also be formed by a printed layer of conductive ink. The impedance, conductance, or capacitance of the authenticating layer can be detected, though capacitance is not recommended for detecting discrete areas of the authenticating layer. Beyond authentication, the conductive layer does not contain any useful information.

[0012] U.S. patent application Ser. No. 09/059,985 by Verschuur, Mitchell, Jr., and Leordeanu describes a capacitive method and apparatus for reading barcode printed on inserts and hidden inside sealed envelopes. Changes in a measurement of capacitive coupling between two electrodes in the presence of the barcode is interpreted to distinguish different barcode patterns. This application is hereby incorporated by reference.

[0013] A number of factors can affect such capacitive coupling measurements including wobble of the envelopes past the electrodes, vibration of the envelope transport, different numbers or thicknesses of dielectric layers separating the barcode from the electrodes, variations in the position or angular orientation of the barcode, and overcoupling effects between adjacent bars of the barcode.

[0014] Our invention includes among its embodiments a new reader that makes capacitive coupling measurements to acquire information encoded by differentially conductive patterns. Inserts within envelopes and other hidden substrates can be printed with a conductive or dielectric ink in the form of barcode or other symbols and advanced through our reader to convey information otherwise hidden to conventional optical readers. The coupling measurements can be made simultaneously and compared with each other independently of structural, environmental, or other factors that affect the measurements collectively to distinguish features within the patterns from one another.

[0015] For example, our reader for acquiring information encoded by a differentially conductive pattern can include a plurality of electrodes positioned within one or more electrical fields generated by at least one of the electrodes. A signal processor obtains capacitive coupling measurements of the differentially conductive pattern between at least three different pairings of the electrodes as the pattern is relatively moved through the one or more electrical fields. A logic processor compares the simultaneous measurements with each other independently of variations having similar effects on the compared measurements to distinguish features of the differentially conductive pattern.

[0016] Preferably, the logic processor compares simultaneous measurements from a first grouping of the paired electrodes and uses the outcome of this comparison to trigger another comparison between simultaneous measurements from a second grouping of the paired electrodes to distinguish the features of the pattern. The former comparison can also be used to locate reference points in the pattern as the pattern is advanced past the electrodes.

[0017] One embodiment, which is particularly useful for reading barcode, arranges the electrodes in an array and positions the electrodes along a common axis that is coincident with a direction of the relative motion between the barcode and the electrodes. A first and a third electrode straddle a second electrode along the common axis. The processor compares simultaneous coupling measurements between a fourth electrode and each of the first and third electrodes to locate the reference points in the barcode and to trigger a further comparison between the coupling measurements. The further comparison, which distinguishes a bar dimension of the barcode, simultaneously compares a coupling measurement between the second and fourth electrodes to a combination of coupling measurements between the fourth electrode and each of the first and third electrodes.

[0018] The barcode dimension of interest can be bar width, which is measured along the common axis of the electrode array. When either a wide or a narrow bar is centered along the array, the capacitive couplings that involve pairings with the first and third electrodes are substantially equal. A narrow bar overlaps just the second electrode, but a wide bar overlaps portions of both the first and third electrodes as well. The electrodes are sized relative to the bar widths so that the sum of coupling measurements involving the first and third electrodes is larger than a simultaneous coupling measurement involving the second electrode when a wide bar is centered in the electrode array and smaller when a narrow bar is similarly centered.

[0019] The fourth electrode, which is preferably aligned end to end with the second electrode, can function as a transmitter; and the first, second, and third electrodes can function as separate receivers. The signal processor gathers the signals from the separate receivers and converts the signals into a more useful form for making comparisons. Alternatively, each of the first, second, and third electrodes can function as transmitters, and the fourth electrode can function as a receiver. The individual pairings of the three transmitters with the single receiver can be distinguished by transmitting a different frequency from each of the three transmitters. A demultiplexer within the signal processor isolates the different frequency signals from the different electrode pairings. More transmitters or more receivers can be paired together and distinguished by different receivers or different frequencies to gather more information about the comparative features of barcode or other information patterns.

[0020] The invention is particularly useful for processing a succession of envelopes having information encoded in their contents. The information is recorded in patterns of contrasting permittivity. Preferably, the patterns are printed with an electrically conductive ink on a dielectric medium that is inserted within the envelopes. The envelopes are transported together with their encoded contents past our new reader. Simultaneously measured variations in a plurality of capacitive couplings are compared with each other to isolate and distinguish features within the patterns. The information obtained by the relative measurements can be meaningfully interpreted and used to affect further processing of the envelopes.

[0021] For example, the information obtained from the envelopes' contents can identify intended addressees of the envelopes. The actual addressees can be read by standard optical means from the exterior of the envelopes and compared to the address information obtained from their contents to verify if they match. The further processing of the envelopes is discontinued upon detection of a mismatch. Alternatively, the address information obtained from the envelopes' contents can be used to print corresponding address information on the exterior of new envelopes. Orders for further processing can also be read from the envelopes' contents.

[0022] A system for carrying out the invention can include a transporter that conveys a succession of covered substrates imprinted with differentially conductive patterns in a first direction. An array of electrodes is positioned along the first direction and together with at least one more electrode provides at least three different capacitive couplings that are affected by the differential conductivity characteristics of the patterns. A processor performs a first comparison among a first grouping of the capacitive couplings and initiates a second comparison among a second grouping of the couplings coinciding with a predetermined outcome of the first comparison.

[0023] The two comparisons contribute to identifying features within the patterns and to distinguish them by their differing effect on the compared capacitive couplings. The processor can also match the distinguished features of the patterns to stored information about similar patterns for reading the encoded information. For example, numerical representations of the acquired patterns can be compared with a table of data describing the expected pattern or other known patterns. A sorter can be used to distinguish subsequent processing of the covered substrates based on the encoded information that would otherwise be hidden from view.

[0024] Together, the electrodes form a sensor head, which can be fixed in place relative to an advancing differentially conductive pattern or advanced with respect to a stationary differentially conductive pattern. For large-scale processing, a transporter is preferably used to move the differentially conductive patterns past the sensor head. Where such in-line processing is not appropriate, a hand-held sensor can be passed over the differentially conductive pattern to produce the desired signals. The hand-held version would also be practical for reading barcode stamped onto metal surfaces.

[0025] While our reader is especially useful for processing hidden information patterns, particularly patterns contained within envelopes, our reader can function similarly whether the information is hidden or not.

[0026] FIG. 1A is a schematic perspective view of a pair of transmitting and receiving electrodes overlying a passing conductive bar pattern.

[0027] FIG. 1B is a similar view showing dimensions of the two electrodes and the conductive bar pattern.

[0028] FIG. 2 is a diagram showing the additional components of a capacitive barcode reader incorporating the two electrodes.

[0029] FIG. 3 is a cross-sectional view of an envelope showing the relative location of the conductive bar pattern printed on one of a number of inserts inside the envelope.

[0030] FIG. 4 is a graph showing a signal expected from a single conductive bar passing by the two electrodes.

[0031] FIG. 5 is a graph contrasting signals generated by bars of differing widths passing by the electrodes at two different distances from the electrodes.

[0032] FIG. 6 is a schematic perspective view showing the relative locations of three receiving electrodes and one transmitting electrode with respect to a passing envelope containing an insert on which a barcode pattern has been printed.

[0033] FIG. 7 is a graph plotting combinations of signals from the three receiving electrodes, which can be compared to derive information about the location and width of conductive bars in the barcode pattern.

[0034] FIG. 8 is a schematic perspective view showing a similar arrangement of electrodes but whose functions of transmitting or receiving have been reversed.

[0035] FIG. 9 is a schematic plan view of the same electrode pattern next to an advancing envelope containing a conductive barcode pattern.

[0036] FIG. 10 is a diagram showing the additional components of a capacitive barcode reader incorporating the three transmitting electrodes and one receiving electrode.

[0037] FIG. 11 is a schematic end view of a narrow conductive bar centered beneath the electrodes.

[0038] FIG. 12 is a schematic end view of a wide conductive bar centered beneath the electrodes.

[0039] FIG. 13 is similar schematic end view showing capacitive coupling relationships arising from a pattern of wide and narrow bars.

[0040] FIG. 14 is a diagram of another reader in which a frequency transmitted by one of the transmitting electrodes is also transmitted by the two other transmitting electrodes in an out-of-phase relationship.

[0041] FIG. 15 is a graph plotting the different frequency signals in the presence of the same passing barcode pattern in which the transmitting electrodes are driven according to the circuit in FIG. 14.

[0042] FIG. 16 is a diagram of another reader in which the transmitting electrodes are driven both at their own frequency and at the frequency of adjacent electrodes in an out-of-phase relationship.

[0043] FIG. 17 is a graph plotting the different frequency signals in the presence of the same passing barcode pattern in which the transmitting electrodes are driven according to the circuit in FIG. 16.

[0044] FIG. 18 is a schematic perspective view showing an array of three transmitting electrodes and three receiving electrodes.

[0045] FIG. 19 is a diagram showing components of a reader incorporating the three transmitting electrodes and the three receiving electrodes.

[0046] FIG. 20 is a schematic perspective view showing an array of five transmitting electrodes and three receiving electrodes.

[0047] FIG. 21 is a diagram showing components of a reader incorporating the five transmitting electrodes and the three receiving electrodes.

[0048] FIG. 22 is a schematic perspective view of an alternative electrode array for monitoring more width of a passing envelope.

[0049] FIG. 23 is a schematic perspective view of a circuit board incorporating an electrode array and other components of our reader.

[0050] FIG. 24 is a diagram of an in-line system including a transporter for moving a series of envelopes containing hidden information encoded in differentially conductive patterns past our reader.

[0051] In FIGS. 1A and 1B, an electrically conductive bar 10 is positioned in alignment with a transmitting electrode 12 and a receiving electrode 14 of a barcode reader 20, which is shown more fully in FIG. 2. An oscillator 22 of the barcode reader 20 supplies the transmitting electrode 12 with a voltage “U_S” at a frequency “f”, preferably in a range of 20-50 kilohertz. Capacitive couplings “C₁” and “C₂” between the transmitting electrode 12 and the conductive bar 10 and between the conductive bar 10 and the receiving electrode 14 induce a voltage “U_e” in the receiving electrode 14. The voltage signal U_eis amplified by an input buffer stage 24 and further processed by a demodulator (e.g., rectifier) 26 to obtain a useful DC voltage that is proportional to the voltage received by the electrode 14.

[0052] The capacitive couplings C₁and C₂depend on three main factors: (a) areas of overlap between each of the two electrodes 12 and 14 and the conductive bar 10, (b) distances between each of the two electrodes 12 and 14 and the conductive bar 10, and (c) permittivities of the dielectric mediums interposed between each of the two electrodes 12 and 14 and the conductive bar 10. Decreasing these distances and increasing the areas of overlap and the permittivities enhance the capacitive couplings C₁and C₂.

[0053] The area dimensions of the two electrodes 12 and 14 and the conductive bar 10 are depicted in FIG. 1B. A product of a height dimension “h” and a width dimension “w” gives the area of the conductor 10. The electrodes 12 and 14 have areas corresponding to the products of height dimensions “h1” and “h2” and width dimensions “w1” and “w2”. The total overlap between the two electrodes 12 and 14 and the conductive bar 10 is generally smaller than the area (h×w) of the conductive bar 10 because the sum of the electrode height dimensions (h1+h2) is less than the height dimension “h” of the conductive bar 10. Parts of the conductive bar 10 protruding beyond the ends of the electrodes 12 and 14 contribute only marginally to the capacitive couplings C₁and C₂.

[0054] A common distance “d” between the two electrodes 12 and 14 and the conductive bar 10 is shown as an exemplary dimension in FIG. 3. An envelope 30 shown in cross section contains three paper inserts 32 separated by layers of air 34. The conductive bar 10 is printed on one of the inserts 32. The electrodes 12 and 14 seen end-on in this figure are positioned on top of the envelope 30 at the distance d from the conductive bar 10. The alternating layers of paper inserts 32 and air 34 form a dielectric medium separating the two electrodes 12 and 14 from the conductive bar 10.

[0055] If there is no electrical conductor in the vicinity of the electrodes 12 and 14, then ideally there is no voltage U_eat the receiving electrode 14. In practice, however, there is a small coupling capacity “C₀” between the electrodes 12 and 14, which produces a small residual receiving voltage U_ein the absence of an electrical conductor. Appropriate shielding can be used to isolate the two electrodes 12 and 14.

[0056] FIG. 4 plots the manner in which the demodulated receiving signal U_evaries with time as the conductive bar 10 passes beneath the electrodes 12 and 14. The signal U_eincreases to a maximum and then returns to zero, the whole plot tracing the shape of a bell curve.

[0057] At the distance d between the conductive bar 10 and the two electrodes 12 and 14 and at a fixed amplitude of the transmitting signal U_S, the receiving voltage signal U_eshows two key things: (a) the center location of the conductive bar 10 with respect to the electrodes 12 and 14 corresponding to the peak amplitude of the signal U_eand (b) the width w of the conductive bar 10 corresponding to a duration of the signal (e.g., measured at full width half maximum). Accordingly, a distinction can be made between wide and narrow conductive bars, which renders this technology suitable for reading of barcode patterns in which wide and narrow bars of uniform height are used to encode information.

[0058] A plot in FIG. 5 shows a succession of partially overlapping bell-curve-shaped responses of the signal U_ewhich occur when a barcode pattern 40 of both narrow bars 42 and wide bars 44 moves past the electrodes 12 and 14. The closer the conductive bars 42 or 44 lie to one another, the higher the receiving voltage U_eand the less distinction between maxima and minima of the individual response curves associated with each of the bars 42 or 44. When the conductive bars 42 or 44 are too close together, distinctions between the response curves are lost, a condition that limits the resolution of the reader 20. The resolving power of the reader 20 also diminishes with increases in the distance d. Traces 46 and 48 of FIG. 5 contrast resolutions at two different distances d.

[0059] In principle, it is possible to distinguish between wide and narrow conductive bars 42 and 44 and to determine their spacing with respect to one another by observing the pattern of maxima and minima in the receiving signal U_e. However, this assumption breaks down if the distance d between the electrodes 12 and 14 and the passing conductive bars 42 and 44 varies with time. This happens as a result of wobble—the up and down movement of the envelope 30 toward and away from the electrodes 12 and 14, or other forms of unwanted motion such as vibration in a transport device. These act to increase or reduce the receiving signal U_e, depending on the distance between the envelope 30 and the electrodes 12 and 14. The signal U_ealso changes amplitude if the conductive bars 42 or 44 move out of parallel with the aligned electrodes 12 and 14. In addition, cross-coupling between adjacent conductive bars 42 or 44 further obscures signal distinctions between them. All these variations can cause changes in the amplitude of the receiving signal U_e, which can detract from its meaningful interpretation.

[0060] However, we have found ways to distinguish features of barcode patterns and other patterns despite instabilities between electrodes and barcode patterns. One such way illustrated by FIG. 6 features a new sensor 50 having a single transmitting electrode 52 and an array of three receiving electrodes 54, 56, and 58 positioned along a common axis 60 of relative linear motion between an envelope 62 and the sensor 50. The transmitting electrode 52 is aligned with the middle receiving electrode 56 in the same position along the common axis 60. An insert 64 contained within the envelope 62 is imprinted with a conductive ink in the barcode pattern 70, which has a series of narrow and wide conductive bars 72 and 74.

[0061] The insert 64 on which encoded information is printed is preferably paper, which is a dielectric. However, other non-conducting materials including resin films or fabric materials can also be used as substrates for supporting conductive substances or substances with different permittivities. The conductive ink used for printing the barcode pattern 70 can be visible for conveying optically readable information or can be invisible for performing other functions such as those relating to tracking, accounting, or security. The barcode 70 can also be hidden between layers of additional inserts for similar purposes. Examples of conductive inks appropriate for these purposes include the ink used in a Hewlett-Packard desk jet printer (model number 870CSE) and certain iron-based inks used in cold-set printing.

[0062] Separate voltage signals “E_L”, “E_C”, and “E_R” can be detected at the three receiving electrodes 54, 56, and 58. A signal processor 76 in conjunction with a logic processor 78 (e.g., a microprocessor) combines these signals to produce three comparable outputs, namely, (E_L−E_R), E_C, and (E_L+E_R), which are separately plotted in FIG. 7 for an exemplary barcode pattern.

[0063] The difference output (E_L−E_R) varies between positive values when one of the conductive bars 72 or 74 is closer to the electrode 54 and negative values when the same conductive bar 72 or 74 is closer to the electrode 58. The output (E_L−E_R) is zero when the bar 72 or 72 is nearest the electrode 56 at equal distances from the electrodes 54 and 58. Determination of this zero point defines a reference location of the bar 72 or 74 within the electrode array and provides a point of commonality for making further comparisons between the remaining outputs (E_L+E_R) and E_C.

[0064] For one of the narrow bars 72, whose width is comparable to that of the electrodes 54, 56, or 58, the output (E_L+E_R) has a profile that is broader than the output E_Cbut lower in amplitude, particularly at the reference location in alignment with the electrode 56. The effective area of overlap between the narrow bar 72 and the center electrode 56 is greater than the effective area of overlap between the narrow bar 72 and the two electrodes 54 and 58. Accordingly, a narrow bar 72 is detected by the condition E_C>(E_L+E_R) when E_L=E_R.

[0065] For one of the wide bars 74, whose width straddles at least portions of the electrodes 54 and 58, the output (E_L+E_R) has an amplitude that is greater than the amplitude of the output E_Cat the reference location of (E_L=E_R). The effective area of overlap between the wide bar 74 and the two electrodes 54 and 58 is greater than the effective area of overlap between the wide bar 74 and the center electrode 56. Accordingly, a wide bar 74 is detected by the condition E_C<(E_L+E_R) when E_L=E_R.

[0066] The two comparisons E_Lvs. E_Rand E_Cvs. (E_L+E_R) concern only relative values of the voltage signals E_L, E_C, and E_Rand remain valid for distinguishing narrow and wide bars 72 and 74 despite variations having similar instantaneous effects on the voltage signals E_L, E_C, and E_R. Preferably, the distinguishing electrodes 54, 56, and 58 of the sensor 50 are mounted together in close proximity so that any such variation affecting one also affects the others equally. For example, if the distance between the envelope 62 and the sensor 50 varies, the amplitudes of the signals E_L, E_C, and E_Rincrease or decrease. However, the two comparisons E_Lvs. E_Rand E_Cvs. (E_L+E_R) are not affected by the distance variation because the signals E_L, E_C, and E_Rvary together and are compared only to each other.

[0067] An alternative sensor 80 first shown in FIGS. 8 and 9 has an array of three transmitting electrodes 82, 84, and 86 and a single receiving electrode 88. Similar to the receiving electrodes of the sensor 50, the transmitting electrodes 82, 84, and 86 of the alternative sensor 80 are oriented parallel to each other and positioned along a common axis 90 of relative motion between an envelope 92 and the sensor 80. The single receiving electrode 88 is aligned with the middle transmitting electrode 84 in the same position along the common axis 90. An insert 94 contained within the envelope 92 is imprinted with a conductive ink in a barcode pattern 100, which has a series of narrow and wide conductive bars 102 and 104. The transmitting electrodes 82, 84, and 86 are separated by an amount that approximately corresponds to a minimum distance between narrow bars 102.

[0068] The sensor 80 forms a portion of a barcode reader 110 that is shown in FIG. 10. Three oscillators 112, 114, and 116 drive the three transmitting electrodes 82, 84, and 86 at different frequencies “f₁”, “f₂”, and “f₃” with amplitudes of “S₁”, “S₂”, and “S₃”. At the receiving electrode 88, all three of the frequencies f₁, f₂, and f₃are mixed. However, following a buffer 118, three bandpass filters 122, 124, and 126 separate the receiving signal into three individual signals “U_L”, “U_C”, and “U_R” with center frequencies of f₁, f₂, and f₃. The three signals U_L, U_C, and U_Rare demodulated at detectors 132, 134, and 136 and then passed to an A/D converter 128 and microprocessor 130 for simultaneous processing similar to the signals E_L, E_C, and E_R.

[0069] FIGS. 11 and 12 illustrate primary couplings that occur between the transmitting electrodes 82, 84, and 86 and the narrow and wide bars 102 and 104. In FIG. 11, a narrow bar 102 is centered under the transmitting electrode 84 but is coupled to all three transmitting electrodes 82, 84, and 86 through capacitive couplings “C_L”, “C_C”, and “C_R”. In terms of proximity, it is apparent that couplings C_Land C_Rare equal and sum to a value that is less than the coupling “C_C”. Accordingly, the coupled signals U_L, U_C, and U_Rdistinguish the narrow bar 102 by the condition U_C>(U_L+U_R) when U_L=U_R.

[0070] In FIG. 12, a wide bar 104 is centered under the transmitting electrode 84. The capacitive couplings C_Land C_Rare equal and sum to a value greater than the capacitive coupling C_Cbecause of the additional overlap between the transmitting electrodes 82 and 86 and the wide bar 104. Accordingly, the coupled signals U_L, U_C, and U_Rdistinguish the wide bar 104 by the condition (U_L+U_R)>U_Cwhen U_L=U_R.

[0071] FIG. 13 depicts a more complex set of couplings, which relate to the passing of the entire barcode pattern 100 beneath the sensor 80. Couplings are established between each transmitting electrode 82, 84, and 86 and more than one of the bars 102 or 104 as well as between the bars themselves. For example, the center electrode 84 is shown with couplings C_C1, C_C2, and C_C3to one wide bar 104 and to two narrow bars 102. Couplings C_Boccur between the bars 102 or 104.

[0072] A reader 140, similar to the reader 110 but modified to exploit the further coupling possibilities, is shown in FIG. 14. Most components are the same and are referenced with the same numerals. However, the frequency f₂generated by the oscillator 114 is supplied not only to the transmitting electrode 84 but is also supplied with a positive gain factor “K₂” to the other transmitting electrodes 82 and 86 as well. For the receiving voltage U₂, this results in a larger amplitude difference between narrow bars 102 and wide bars 104 because considerably more voltage of frequency f₂is coupled into a wide bar 104 than into a narrow bar 102. Effects of mutual overcoupling between the electrodes are also reduced.

[0073] FIG. 15 provides a plot of the three signals U_L, U_C, and U_Rfor a barcode pattern similar to the barcode pattern of FIG. 5. Evaluation of the signals U_L, U_C, and U_Ris performed at the intersections of the two signals U_Land U_R, which are each marked with an “X”. At these points X, the two transmitting electrodes 82 and 86 have the same distance from a referenced bar 102 or 104, and the center electrode 84 has the shortest distance to the referenced bar 102 or 104. The identification of these points of intersection X triggers simultaneous evaluations of the other relationships between the three signals U_L, U_C, and U_Rthat distinguish narrow and wide bars 102 and 104 (that is, the comparison of U_L+U_Rto U_C).

[0074] The reader 140 can also be modified to supply the frequency f₂to the transmitting electrodes 82 and 86 at 180 degrees out of phase with the signal sent to transmitting electrode 84. This means that the receiving signal U₂is not increased, but is decreased. In the case of the wide bar 104, the signal U₂is decreased considerably more than a narrow bar 102. Also, the signal is not coupled into the neighboring bars 102 and 104 in phase but in phase opposition. Flattening of the receiving signal U₂due to overcoupling from neighboring bars is therefore considerably reduced, i.e., the difference between amplitude maxima and minima becomes much more pronounced (e.g., see FIG. 5).

[0075] FIG. 16 depicts another reader 150 modified to further distinguish the narrow and wide bars 102 and 104. In addition to transmitting the frequency f₂from adjacent electrodes 82 and 86 at 180 degrees out of phase with transmissions from the electrode 84, the frequencies f₁and f₃are similarly transmitted from their adjacent electrodes. The frequency f₁is amplified by a gain factor K1 and combined with the frequency f₂at the transmitting electrode 84, and the frequency f₃is also amplified by a gain factor K1 and combined with the frequency f₂at the transmitting electrode 84. Both frequencies f₁and f₃are transmitted from the center electrode 84 at 180 degrees out of phase with respect to their primary transmissions from the electrodes 82 and 86. The more pronounced shapes of the three signals U_L, U_R, and U_Care apparent from their plot in FIG. 17.

[0076] Another exemplary sensor 160 for making even finer distinctions is shown in FIG. 18, configured by a square array of three transmitting electrodes 162, 164, and 166 and three receiving electrodes 168, 170, and 172. Each of the transmitting electrodes 162, 164, and 166 is paired with one of the three receiving electrodes 168, 170, and 172 in alignment with each other in positions displaced along a common axis 174 of relative motion between the sensor 160 and the barcode pattern 100.

[0077] An exemplary reader 180 shown in FIG. 19 incorporating the sensor 160 includes the three oscillators 112, 114, and 116 of the prior readers for supplying the three transmitting electrodes 162, 164, and 166 with the different frequencies f₁, f₂, and f₃. The signals U_L, U_C, and U_Rpicked up by the three receiving electrodes 168, 170, and 172 are amplified by buffers 182, 184, and 186; separated into different frequencies by filters by 188, 190, and 192; and detected by demodulators 194, 196, and 198. The detected signals U_L, U_C, and U_Rcan be further processed for making relative comparisons as described earlier by the converter 128 and microprocessor 130.

[0078] All three frequencies f₁, f₂, and f₃can be obtained from each receiving electrode 168, 170, and 172 to gather even more information about the precise positioning and features of the barcode 100. The different frequencies can be transmitted from adjacent transmitting electrodes also as discussed in the previous embodiments to provide more precise information.

[0079] A sensor 200 shown in FIG. 20 is especially useful for this latter purpose. Five transmitting electrodes 202, 204, 206, 208, and 210 are variously coupled through the intermediacy of the barcode pattern 100 to three receiving electrodes 212, 214, and 216.

[0080] A reader 220 shown in FIG. 21 incorporating the sensor 200 directs each of the frequencies to groups of three transmitting electrodes. The frequency f₁is supplied directly to the transmitting electrode 204 and is amplified by a gain factor K1 before being supplied 180 degrees out of phase to adjacent transmitting electrodes 202 and 206. The frequency f₂is supplied directly to the transmitting electrode 206 and is amplified by a gain factor K1 before being supplied 180 degrees out of phase to adjacent transmitting electrodes 204 and 208. The frequency f₃is supplied directly to the transmitting electrode 208 and is amplified by a gain factor K1 before being supplied 180 degrees out of phase to adjacent transmitting electrodes 206 and 210. The received signals U_L, U_C, and U_Rare then amplified, filtered, and detected similar to the other embodiments.

[0081] In order to keep the dimension of the conductive markings as small as possible (e.g., the height h of a bar in a barcode as small as possible), it may be necessary to form a sensor arrangement that allows for a detailed view of a large area, especially if the position of the conductive markings is not known. This can be accomplished without any loss of sensitivity by constructing a linear array of sensors as shown in FIG. 22. Three or more pairs of transmitting and receiving electrodes 132aand 134a,132band 134b,and 132cand 134c(not shown) are positioned along an axis 136 that is transverse to a direction of relative movement along an axis 138. The transmitting electrodes 132a,132b,and 132ccan be connected in parallel with each other; and the receiving electrodes 134a,134b,and 134ccan also be connected in parallel with each other both for the purpose of incorporating the electrode pairs into a single reader.

[0082] Although single transmitting and receiving electrodes are depicted in FIG. 22, either the transmitting electrodes or the receiving electrodes or both the transmitting and the receiving electrodes can be replaced by electrodes in a group of three or more according to any of the sensor embodiments described earlier. For those embodiments using three or more receiving electrodes or three or more transmitting electrodes, similar parallel connections can be made without requiring additional electronics. However, in the case of three receiving electrodes, it is recommended that these be decoupled by means of individual buffer stages in order to maintain a sufficiently high input impedance.

[0083] A sensor head 140 shown in FIG. 23, which is representative of the sensors described earlier, can be easily constructed at little cost using a multi-layer printed circuit board 142. A receiving electrode 144 and three transmitting electrodes 146, 148, and 150 are formed as layers of copper exposed on an end face 152 of the board 142. The thicknesses and lengths of the respective copper layers that are flush with the end face 152 form the respective electrode areas. The board also includes an input buffer stage 154, which features an extremely high input impedance and at the same time drives the necessary shield areas. The sensor head 140 can be connected to the required electronic evaluation unit (oscillator, demodulator, A/D converter, microprocessor, etc.) by means of a replaceable plug connector. Depending on requirements of a specific use, different sensor heads can be connected to the same electronic unit.

[0084] In all the above embodiments, the signals induced in the receiving electrodes can be transferred to a microprocessor by way of an A/D converter and can be evaluated by software that performs comparisons between the different signals. The comparisons preferably involve contrasting simultaneously acquired signals or combinations of such signals to each other for reaching conclusions that distinguish features of the conductive patterns. The electrodes themselves are sized and spaced apart relative to the size and spacing of the features being distinguished. Neither variations in the overall amplitude of the signals nor their rate of acquisition (i.e., the relative motion between the sensors and the patterns) necessarily affect these comparisons.

[0085] FIG. 24 shows an in-line system 170 including a transporter 172, such as a belt drive, for moving a train of envelopes 174 past a sensor 176, which can be arranged similar to any of the preceding sensors. Barcode patterns are printed in conductive ink on inserts contained inside each of the envelopes 174. Signal conditioning circuitry 178 connected to the sensor 176 feeds the sensor signals into the microprocessor or computer 180, and the signals are evaluated to distinguish features of the conductive patterns carried inside the envelopes 174. Once distinguished, the patterns can be decoded by standard barcode interpretation software in order to reconstitute the information encoded on the inserts that are hidden inside the envelopes 174.

[0086] The microprocessor or computer 180 can also be used to monitor the speed with which the envelopes 174 are measured to arrive at the sensor 176, and this information can be used to stop or alter the speed of a drive 182 for the transport 172. A variety of additional processing can take place based on the information acquired from the contents of the envelopes 174. For example, the envelopes 174 can be sorted according to their contents, orders or replies can be generated, records can be updated, or the information can be verified. A conventional printer 184 can be controlled to print information on the envelopes' outer surfaces, which can be linked to the information acquired from the contents of the envelopes 174. For example, addresses can be printed to match addresses or other identifying information acquired from the contents of the envelopes 174. Detection of encoded barcode patterns can also reveal whether the correct number of inserts exists in the envelope or whether “double-stuffing” has occurred.

[0087] Instead of printing the address information on the envelopes' outer surfaces, previously printed address information could be read from the envelopes' outer surfaces by a conventional optical reader and compared with the identifying information acquired from their contents. Further processing of the envelopes can be interrupted upon detection of a mismatch between the two addresses, and the mismatch can be corrected. While the information encoded in the envelopes' contents is preferably a conventional barcode, other symbols interpretable in alphanumeric characters could also be used to support the further processing of the envelopes 174. Unique self-defined symbols could also be used.

[0088] In addition to the processing of envelopes with hidden contents, the invention can also be used to read information hidden in other ways such as within packaging or behind labels. For example, conductive ink could be used to print instructional or identifying information between layers of labels or other laminates to control further operations on or with the labels.