Abstract:
The contents of sealed envelopes are accessed by detecting and digitizing a change in capacitance produced when a pattern of dielectric or conductive ink is passed by a sensitive capacitance sensor. The capacitance changes are converted into corresponding electrical patterns for further processing.

Description:
This application claims the benefit of U.S. Provisional application No. 60/044,073, filed on Apr. 17, 1997, which provisional application is incorporated by reference herein. 
    
    
     TECHNICAL FIELD 
     The invention relates to the acquisition of encoded information from the contents of sealed envelopes or other layered structures that conceal the information from view. 
     BACKGROUND 
     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 from the separated envelopes must be linked to information from their contents. 
     Outgoing mail, which may be passed through inserters, is also subject to sorting and other processing errors that are difficult to detect; because once sealed, the contents are concealed from view. Various attempts have been made to “see through” the envelopes to read their contents without opening them, but problems plague each. 
     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 are intended to 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. 
     U.S. Pat. No. 5,288,994 to Berson uses 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. 
     Except for differences in wavelength, these prior art attempts are analogous to shining a flashlight through one side of an envelope in the hope of reading darker printed matter through the envelope&#39;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. 
     U.S. patent application 08/778,077, filed Jan. 2, 1997, by two of the present co-inventors Verschuur and Mitchell, uses microwave heating of conductive or dielectric patterns and subsequent infrared detection of the thermal image conducted to the surface of the envelope to determine the information content inside a sealed envelope. 
     The two present co-inventors along with a third co-inventor have developed another approach to reading through envelopes—the subject of this application, which is independent of any wavelength of radiation, either for seeing through or detecting emission from the surface of an envelope. Instead, a transducer measures changes in capacitance of a localized region beneath the surface of the envelope, such as can be produced by conductive inks or inks with a dielectric constant different from the paper upon which it is printed. This technique shares common elements with other inventions, particularly those for authenticating lottery tickets, documents, and currency, yet is substantially different in both function and purpose. 
     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 are used to 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. 
     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. 
     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. 
     SUMMARY OF THE INVENTION 
     Our invention takes a different approach to accessing information from the contents of sealed envelopes or other layered structures by making use of localized capacitance changes introduced onto a substrate, such as a paper insert inside an envelope, by conductive or dielectric ink used to print encoded information such as a bar-code. The information obtained by the capacitance measurements can be meaningfully interpreted and used to affect further processing of the envelopes or other layered structures. 
     The encoded information concealed behind a cover, such as an envelope, can be printed on a substrate in a pattern using an electrically conductive ink. The substrate and cover are moved past a capacitance sensor at a rate that permits successive portions of the pattern to be measured at points of approximately equal proximity to the capacitance sensor. Variations in capacitance associated with the pattern of the conductive ink are detected as a function of the relative position of the capacitance sensor along the covered substrate and are compared to stored information about similar patterns for reading the encoded information. 
     The invention is particularly useful for processing a succession of envelopes having processing information encoded in their contents. The processing information is recorded in patterns of contrasting permittivity. The envelopes are transported together with their encoded contents past a capacitance sensor. Measured variations in the capacitance associated with the patterns of contrasting permittivity are deciphered into recognized units of information. Subsequent processing among envelopes is distinguished on the basis of the processing information obtained from their contents. 
     For example, the processing information can be arranged to 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&#39; contents can be used to print corresponding address information on the exterior of the envelopes. Orders for further processing can also be read from the envelopes&#39; contents. 
     A system for carrying out the invention can include a transporter that conveys a succession of covered substrates imprinted with conductive ink in patterns representing encoded information visibly obscured by a cover. A capacitance measuring device through which the succession of covered substrates are transported is sensitive to variations in capacitance associated with the patterns of the conductive ink as a function of the transported positions of the covered substrates through the capacitance measuring device. A processor matches the measured variations in capacitance to stored information about similar patterns for reading the encoded information, and a sorter distinguishes subsequent processing of the covered substrates based on the encoded information imprinted on the substrates and obscured by the cover. 
     The capacitance variations are preferably sensed by a transducer comprising (a) parallel plates of a capacitor placed side-by-side on one side of the covered substrates, (b) one pair of such side-by-side plates on each side of the covered substrates, (c) a pair of such plates on one side of the covered substrates and a ground plane on the other side of the covered substrates, or (d) a pair of plates aligned end-to-end on one side of the covered substrates. An output signal is detected by means of a circuit that converts what amounts to an impedance change into a voltage change which can then be used to drive an A/D converter with its output fed to a computer where the signal can be processed to retrieve information and control other processes. 
     Well-known pattern recognition programs can be used to interpret the detected signals for controlling subsequent operations. For example, orders and customer-identifying codes from return mail can be read for processing orders. Inside addresses or other contents of outgoing mail can be verified or used as a basis for printing information including address information on the outside of the envelopes. 
    
    
     DRAWINGS 
     FIG. 1 illustrates a basic layout of the capacitive hidden character recognition system. 
     FIG. 2 is a schematic of the electrode placements making up the transducer that can be utilized in a capacitive measuring device. 
     FIG. 3 is a block diagram representation of exemplary components of an electronic circuit for processing capacitance variations. 
     FIG. 4 shows a wiring diagram of key electronics used for processing the capacitance variations. 
     FIG. 5 an alternative transducer design that facilitates mapping encoded information other than bar-codes. 
     FIG. 6 shows how the computer interprets the signals to control further data processing and/or mechanical actions. 
     FIG. 7 is a schematic diagram showing an alternative layout of a transducer for measuring capacitance. 
     FIG. 8 is a schematic diagram showing another alternative layout of a transducer for measuring capacitance. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1, an envelope  10  containing an insert (substrate)  11  on which encoded information is printed, in this example in the form of a bar-code  12 , passes between a set of parallel plate capacitors (also referred to as a transducer)  13  which are connected to an amplifier  14  whose output is then examined by a computer  47 . The bar-code  12  is printed using either a conductive ink or a dielectric ink. When either passes between the plates  13  of the transducer, the capacitance changes. 
     In a practical embodiment of the invention, the envelope is passed between two parallel sets of plates as shown in FIG.  2 . When no envelope is present between plates  20 - 21  and  22 - 23  of the transducer  13 , the capacitance is C 0  and this is made part of a resonant circuit built into the amplifier  14 , whose frequency is established by a suitable choice of inductance L. When an envelope  10  passes the probe (transducer  13 ), the capacitance changes to C 1 . A further change to C 2  is sensed when the conductive or dielectric ink, with a permittivity other than that of the insert  11  within the envelope  10 , passes between the plates  20 - 21  and  22 - 23 . 
     The inserts  11  on which encoded information is printed are preferably paper, which is a dielectric. However, other nonconducting 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 bar-code  12  can be visible for conveying additional optically readable information on the inserts  11  or can be invisible for performing other functions such as those relating to tracking, accounting, or security. The bar-code  12  can also be hidden between layers of the inserts  11  for similar purposes. An example of a conductive ink appropriate for these purposes is used in a Hewlett-Packard desk jet printer, model number 870CSE. 
     In the block diagram of FIG. 3, an oscillator circuit  32  is used to generate a fixed frequency based on a Colpitts-crystal modified circuit. A frequency monitor  31  allows the operation of the instrument to be monitored for test purposes while also improving the frequency stability. 
     The output of the oscillator  32  is fed through a buffer amplifier  33 , which is also part of the oscillator circuit  32 . The buffer amplifier  33  is a voltage source which together with an impedance  34  acts as a constant AC current generator to supply a high-Q resonator crystal  35  in turn connected to a floating ground  36 . The crystal  35  is specified to operate at a frequency f 2  only slightly different from f 1  at which the oscillator  32  functions. 
     The transducer  13  in parallel with the crystal  35  modifies the impedance seen by the current generator (consisting of the buffer amplifier  33  in combination with the impedance  34 ), thus generating a variable voltage which reflects the capacitance sensed by the transducer  13 . 
     This modulated, high-frequency signal at f 1  is amplified by a buffer amplifier  38 , which is also used to separate the high-Q resonator crystal  35  from the low impedance side of a detection-rectifying circuit  39 . This circuit  39  will demodulate the signals introduced by the buffer amplifier  38  as a result of impedance changes at the transducer  13  produced by the passage of the information printed on inserts inside sealed envelopes. The rectifying circuit  39  uses a thermal compensation technique for stability. 
     The output of the rectifying circuit  39 , which is current amplified, is fed to a level shifting amplifier  40 , which together with a feedback integrator  41  produces a signal that is forced to a level close to that of the floating ground  36  and cancels the undesirable “common mode” that would otherwise affect the final amplification in a voltage amplifier  42 . 
     The feedback integrator  41  acts as a high-pass filter that allows rapidly varying signals, introduced at the transducer  13  by the passage of the encoded information inside the envelope  10 , to be amplified by amplifiers  40  and  42  to the level required by an A/D converter  43 . More slowly varying signals, such as are produced by temperature changes or localized permittivity variations along the length of the rapidly passing envelopes  10  due to structure in the envelope or insert material, are canceled by the action of the feedback integrator  41 . 
     Further undesirable signals that may pass through what is essentially a high-pass filter (integrator  41 ) can be discriminated against by initially adjusting a threshold level of the amplifier  42  using a voltage source  44 . 
     A second input to the A/D converter  43  is produced by an amplitude-duration converter  45 , which offers a pulse train representation of the signals introduced at the transducer  13 . These signals will have an amplitude that is a function of the width of the lines of the bar-code  12 , for example. The lines of the bar-code  12  have a variety of widths; and the change in impedance sensed by the transducer  13  depends on this width, which is equivalent to changing the plate area of the effective capacitor created by the conductive or dielectric lines of the bar-code  12  as they pass by the transducer  13 . 
     A LED bipolar display  46  is used in conjunction with the threshold level adjustment control of voltage source  44  to set initial conditions for the amplifier  42 . The display  46  indicates a range of voltages from negative to positive values with respect to a zero-centered reference representing the floating ground  36 . 
     The output of the A/D converter  43  is sent to a computer  47 , which allows the signals produced at the transducer  13  to be decoded. This information from the computer  47  can also be used to control the speed at which envelopes are sent past the transducer  13 , and the same information can be used to drive a feedback loop to drive the voltage source  44  to remove undesirable signals such as can be produced by accumulated static charges. 
     In FIG. 4, a circuit  50  acts as a buffer to the frequency monitor  31 , which also forms part of the oscillator circuit  32  with a capacitance  60  introduced into the emitter of a main oscillator transistor  61 . High stability (10 −7 ) in frequency is obtained in the oscillator circuit  32  by using a crystal  62  (f 31 =4.93152 MHz) in combination with a phase shifter  63 . The combined effect of capacitance  60  and those of the base-collector of the transistor  61  and an element  64  will increase the capacitance of an element  65  by an amount sufficient to sustain an oscillation. In this way, buffer amplifier  33  acts as a buffer and amplifier for an unusual output tapped across element  65  of the oscillator circuit  32 . 
     The high-speed buffer amplifier  33  will act as an emitter-follower generating a powerful signal to the series configuration of the impedance  34  and the crystal  35 , which has an intrinsic resonant frequency f 2 =5.0000 MHz. 
     No adjusting device is needed inside the oscillator circuit  32  because of the use of two buffers ( 50  and  64 ) with their own capacitive effect. This adds not only simplicity but also considerable size reduction over previous techniques, which generally incorporate numerous trimmers and screens. 
     The combination of the impedance  34  and the crystal  35  will act as a voltage divider. The impedance  34  is constant while that presented by the crystal  35  varies in response to the modifying capacitance sensed by the transducer  13  as the desired signal. 
     In the absence of a trace at the transducer carrying encoded information, only the paper permittivity will affect the transducer capacitance. This creates the highest impedance seen across the crystal  35 , that is, the highest level of voltage produced across the crystal  35  based on the divider effect. Any additional capacitance (for example, when a conductive or a dielectric trace is present between the electrodes; see FIG. 2) will decrease the impedance presented by the resonant circuit formed by crystal  35  in parallel with the transducer  13 . It is well known in the art that increasing capacitance will decrease the resonant frequency of a parallel resonant configuration. Note that the buffer amplifier  33  is injecting a constant amplitude and frequency signal (f 31 ) into the divider, which is therefore forced to resonate at this frequency (f 31 ). The highest impedance presented by the crystal  35  occurs at f 2 , a frequency that is never found in our circuit because of the existence of the parallel capacitance introduced by the transducer  13 . This means that the crystal  35  will never exhibit its maximum impedance value. To the contrary, any additional capacitance in the transducer side will decrease the resonant frequency even further and thus reduce the equivalent impedance presented by the crystal  35  as part of the divider circuit consisting of the impedance  34  and the crystal  35 . The decreased resulting signal will be amplified by the bootstrapped buffer amplifier  38  and supplied to the rectifying circuit  39 . 
     Note that the signal collected from the transducer  13  (the same as the signal collected across crystal  35 ) will sharply decrease for very small capacitive changes of 10 −2  to 10 −3  pF (values corresponding to those expected for coded information printed with conductive inks as independently determined using a professional LCR reference meter) because of the high Q of the resonator crystal  35  and the high impedances and low capacitances on either side of the crystal  35 . 
     Following the rectifying circuit  39 , the signal is processed by the level-shifting amplifier  40  and the feedback integrator  41 . The signal at the emitter of a transistor  67  is above the floating ground in a positive voltage domain, creating an undesirable “common mode” for an amplifier  69 . A feedback loop is therefore included which consists of an integrator  70  and a transistor  75 , which will adjust the inverting input voltage of the amplifier  69  to be the same as the level of the non-inverting input of the same amplifier. 
     The output signal of the amplifier  69  will be forced in this way to the level of the floating ground  36  taking into account that the integrator  70  is self-creating a zero voltage between its inputs. The integrator  70  time-constant, which is determined by the product of elements  71  and  72 , is much larger than the expected pulse duration of the signal produced at the transducer  13  but shorter than the time scale of expected temperature variations that can affect the signal. Previous art (U.S. Pat. No. 5,231,359) has used a similar technique to modify the reactance of the resonator crystal  35  by altering a variable capacitance diode in parallel with the resonator. This requires many additional electronic circuit elements and does not negate the temperature effects suffered by the integrator  70  itself on the resonator side upon which it acts. In our circuit, all summed temperature effects are canceled at the level shifter, thus not affecting the performance of the resonator. Another advantage is using the amplifier  69  as a real amplifier (with gain  30  here) as a first amplification stage. 
     A switch  74  together with a potentiometer  76  allows for initial calibration when the integrator loop is disconnected. 
     The measured parameters for the crystals  62  and  35  indicate a quality factor of over 500 could be achieved at 5 MHz with parallel equivalent capacitance of 3.335 and 3.257 pF respectively. It is with respect to these capacitances that the very small additional capacitance produced at the transducer (1 fF) has to be detected. High selectivity resonant curves of the crystals offer the possibility to operate in the MHz frequency range instead of the GHz range adopted by others in previous work together with all the unwanted problems introduced by operation at such high frequencies (e.g., screening, interference, reflection, etc.). 
     Because of high sensitivity of the crystal circuits, only two stages of amplification are used (amplifiers  40  and  42 ). Amplifier  42  is an inverting amplifier, and its feedback network is adjusted to filter the signals sensed at the transducer. The adjustment control (voltage source)  44  will set the threshold level for the signals that have to be processed by the A/D converter  43 . 
     The amplitude-duration converter  45  will translate with a sample-and-hold circuit combined with a linear sweep integrator the peak amplitude of every signal pulse into a pulse train consisting of equal amplitude pulses, but with a width of individual pulses corresponding to their initial amplitudes. Since the initial amplitudes are indicative of the widths of the lines in the bar-code  12 , the converter  45  allows a visual display of the original bar-code to be exhibited on a video monitor (not shown) driven by the computer  47 . 
     FIG. 5 represents a refinement of the transducer  13  that will allow more complex structure than simple bar-codes to be mapped and hence interpreted. The transducer plates  20 - 21  and  22 - 23  are terminated in flattened points above and below the envelope  10 . A permanent capacitance between plates  20 - 21  as well as  22 - 23  will be modified by the presence of conductive or dielectric ink of the bar-code  12  on the insert(s)  11  enclosed in the envelope  10 . By arranging an array of such transducers  13  to cover the width of an envelope passing the array, an image of the contents of the envelope can be built up within the computer  47 , making use of the translation of the envelope past the transducers as the time base. 
     FIG. 6 shows a train of envelopes  10  being driven past the transducer  13  by means of a conventional envelope transport mechanism such a belt drive  80 . The signals amplified by amplifier  14  are fed into the computer  47  and interpreted to suit the needs of a particular use. For example, conventional recognition programs can be run to interpret the information pattern of the bar-code  12 . The computer  47  can also be used to monitor the speed with which the envelopes  10  are measured to arrive at the transducer  13 , and this information can be used to alter the speed of a drive  82  for the transport belt  80 . 
     A variety of further processing can take place based on the information acquired from contents  83  of the envelopes  10 . For example, the envelopes  10  can be sorted according to their contents  83 , orders or replies can be generated, records can be updated, or the information can be verified. In the in-line system of FIG. 6, a conventional printer  81  is controlled to print information on the envelopes&#39; outer surfaces (exteriors), which is linked to the information acquired from the contents  83  of the envelopes  10 . For example, addresses can be printed to match address or other identifying information acquired from the contents  83  of the envelopes  10 . 
     Instead of printing the address information on the envelopes&#39; outer surfaces, previously printed address information could be read from the envelopes&#39; 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&#39; contents  83  is preferably a conventional bar-code, other conventional symbols interpretable in alphanumeric characters could also be used to support the further processing of the envelopes  10 . Unique self-defined symbols could also be used. 
     The remaining two drawing figures illustrate other arrangements of the transducer  13 . In FIG. 7, the plates of transducer  13  are arranged parallel to each other and oriented normal to a transport direction of the envelope  10 . In FIG. 8, the plates of the transducer  13  are aligned end-to-end and oriented normal to the transport direction of the envelope  10 . The latter arrangement is preferred for measuring linear bar-code patterns, although spatial resolution may be less. 
     Further to either arrangement, the transducers  13  are preferably mounted either above or below the envelopes within a shielded housing (not shown) with one or more grounds. A ground plate (not shown) can also be mounted on the opposite side of the envelope  10 , a variation on a principle used in non-contact capacitive displacement measurement devices such as a Micro-Epsilon System E+H unit. A Faraday shield or cup (also not shown) can also be mounted opposite to the transducer to guard against entry of stray radiation. 
     The plates of transducer  13  can be made using printed circuit board technology by etching away a conductive surface except in a region where the plates are created. The circuit board (not shown) can be cut or drilled between adjacent plates to create a localized air gap. 
     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.