DE10350410A1 - Fingerprint sensor and circuit for capture of measurement parameters has sensor circuits for each of a multiplicity of electrodes surfaces with a sensor memory that is charged and discharged during loading and measurement cycles - Google Patents

Abstract

Fingerprint sensor has a multiplicity of sensor electrode surfaces (1) for detecting papillary bodies (2) on the finger surface (3). Each electrode surface has a sensor circuit (4) with a sensor memory (5) that stores a limited charge amount (6) that depends on the distance between a papillary body and the electrode. The sensor circuit has a charge source (8) with a first switch (SA) that loads the sensor memory over a charging cycle and a second switch (SB) with which a measurement device (22) measures the charge stored in the sensor memory during a measurement cycle. The invention also relates to a corresponding sensor circuit and a method for measuring parameter differences between a multiplicity of sensor electrode surfaces.

Description

The The invention relates to a fingerprint sensor with sensor circuitry for detecting a measurement parameter, wherein the sensor has a plurality of sensor electrode surfaces which corresponds to the papillary body the fingerberry opposite is arranged, wherein a single sensor electrode surface a stores limited amount of charge.

The Amount of charge stored on a single sensor electrode can be, is low and varies with the papillary ridges of the Fingertip. However, to identify a person it is necessary to accurately determine this small amount of charge to a reproducible electronic pattern of the skin papillary body, genetically fixed and individual-specific papillary borders to capture and compare with a stored pattern. One another problem is over the small amount of stored charge per sensor electrode in addition charge differences between to detect and store adjacent sensor electrodes with high precision.

The Detecting such charge quantities usually takes place analogously by measuring the for the individual sensor electrodes required charge currents.

One analog measurement and detection of charge quantities on the sensor electrodes has the disadvantage that compressing the measurement results is expensive, if not impossible is. For need a compression the measured analog values are first digitized and then be evaluated. It may be useful that the image a fingerprint is relatively redundant. The gray values respectively Adjacent pixels are similar namely strong, especially as the Papillarleisten and their spaces in fingerprint images several pixels wide.

From the publication US 5,325,442 For example, a fingerprint sensor is known which comprises a matrix of sensor elements each comprising a sensor electrode and a circuit device which are coupled together. Each of the sensor electrodes is supplied via its switching device with a predetermined potential, wherein a sensor device can be coupled to all sensor elements via their switching device. The sensor device is provided for measuring capacitances due to individual finger surface portions in combination with the sensor electrodes of the sensor elements. In order to detect the small amounts of charge of the sensor electrodes, according to US 35,325,442 measured the charging current and a sensor charge amplifier used to detect the low charging currents after appropriate gain can.

Of the known fingerprint sensor with switching device triggers accordingly the problem of measuring small amounts of charge on the sensor electrodes, by charging current for each sensor electrode measured by means of a sensor charging current amplifier becomes. This method provides analog values, that of data compression first accessible when using the downstream A / D converter, the measured values are digitized. About that In addition, the reproducibility of the detection results of a Such analog method, the charging current per sensor electrode recorded and amplified, extremely critical. An exact and reproducible detection of the amount of charge per sensor electrode is not always guaranteed with the known charging current detection method.

The The object of the invention is a fingerprint sensor with circuit be reproducibly the multiple of the charge quantities on a Sensor electrode detected and in particular charge quantity differences adjacent charged sensor electrodes reliably detected.

These Task is solved with the independent claims. advantageous Further developments of the invention will become apparent from the dependent claims.

According to the invention is a Fingerprint sensor indicated having a plurality of sensor electrode surfaces. These sensor electrode surfaces are opposite a papillary body arranged the finger berry. The fingerprint sensor is with a Sensor circuit equipped for a single sensor electrode surface, as a sensor memory a limited amount of charge as a function of across from arranged cutout of the papillary body stores. For this purpose points the sensor circuit has a charge source with a first switch on, with the charge source over the first switch repeatedly charges the sensor memory in one measuring cycle. A Measuring device has a second switch of the sensor circuit on. This measuring device collects the charge quantities of the sensor memory for one Measuring cycle over the second switch.

This fingerprint sensor has the advantage that it already generates a numerical value as a measurement parameter with the aid of the first and the second switch when detecting the charge amount of a sensor electrode surface, wherein the number of switching operations or the charging exactly the limit te amount of charge reflects, which can accommodate the sensor electrode surface as a sensor memory. Even before the further processing of such numerical values, this measurement parameter can be digitized and strongly compressed, since the gray values of respectively adjacent pixels are very similar and the papillary bars with their spaces in fingerprint images are several pixels wide.

Practically become compression units with the fingerprint sensor of the present invention implemented on the sensors, so on the one hand a saving of transmission bandwidth possible and, on the other hand, data encryption on the sensor reduced encryption performance as a result of the reduced data volumes. The saving of transmission bandwidth, with the fingerprint sensor according to the invention is achieved with sensor circuit is particularly advantageous in a special version of fingerprint sensors, namely the Fingerprint swipe sensors. Such fingerprint strip sensors read at a high speed Picture strip off while the finger over it becomes. The amount of data can increase fifty times, so that any reduction and compression of the output data by the Advantageous fingerprint sensor according to the invention effect.

If even the data compression itself requires a limited computing power, so can still with the fingerprint sensor according to the invention a first step of compression by the associated detection circuit or sensor circuit itself.

In a preferred embodiment invention, the first and second switches form a toggle switch, the high-frequency change between charging and discharging the sensor memory switches. With this embodiment The invention is in high frequency sequence with the sensor electrode surface loaded a limited amount of charge and this limited amount of charge transferred to the measuring device when unloading the sensor memory. The measuring device accumulates or accumulates the limited amounts of charge the sensor memory reaches a predetermined threshold is. The number of changeover circuits required to reaching the threshold is a size for the data stored on the sensor electrode surface limited amount of charge.

is this amount of charge low, so is a high number of changeover circuits required until the threshold is reached, and is this amount of charge high, which is transported every time you load, so will the Number of changeover circuits to be low. Thus, in an advantageous manner Make a numerical measure for the the sensor electrode surface stored limited charge amount determined at each measurement cycle become. The limited amount of charge on the sensor electrode surface is high, when the sensor electrode surface opposite a papillary strip, and the limited amount of charge is small when the sensor electrode surface a Skin groove between two Papillarleisten opposite. Thus, this delivers first embodiment of the Invention an exact numerical value for the localization of Papillarleisten and skin grooves of the papillary body a fingerberry.

Farther For example, the fingerprint sensor may have a charge source, which in turn includes a charging capacitor. This charging capacitor can via a third switch of Sen sorschaltung connected to a charging station his and a storage capacity which receive a multiple of the limited amount of charge of the sensor memory can. The first switch can be switched off after charging the charging capacitor, the amount of charge stored there, in high-frequency sequence to the sensor memory during the loading process transferred to the sensor memory.

These Charge source according to the invention has the advantage that the charging capacitor for each measuring cycle of a sensor electrode surface is a constant high, stored in the charging capacitor amount of charge on to transmit the measuring capacitor is available provides. Thus, this fingerprint sensor comes without measuring the low charging currents and without any amplifier for charging currents off and on simultaneously provides a numerical measurement parameter that is accurate on the sensor electrode surface charged limited amount of charge corresponds.

On The measuring side of the fingerprint sensor is a measuring device to disposal, which is connected to an evaluation circuit. This evaluation circuit measures the number of loads until reaching a threshold and / or measures the achievement a time interval until reaching this threshold in Comparison to the adjacent or next sensor electrode. In this embodiment The invention relates to the fingerprint sensor and its circuit in Advantageously, a numerical difference value between adjacent ones or next Sensor electrodes determined.

With time difference formation, a fingerprint strip sensor can be realized. This takes in time sequence partial images of a fingerprint while the finger is moved over the sensor. From these partial images, the original fingerprint image can be reconstructed, or can be fed to another matching process. In this case, sensor units can be integrated in a matrix like the sensor electrodes, and in each case the differences for the same sensor electrode can be detected between two temporally successive sensor operations.

By Detecting difference values instead of absolute values can already with the help of the sensor circuit of the fingerprint sensor first compression step done. Because with the same gray values of fingerprints same numeric values for the limited amounts of charge of the adjacent sensor electrodes measured their difference value is zero. In this difference of the Evaluation circuit, zero values are not registered. At a sharp, unsteady and / or abrupt transition from a papillae to a skin groove is in contrast a high difference and thus a high numeric value of the Evaluation circuit output.

While these numerical values in the present evaluation circuit capacity differences adjacent or next Sensor electrodes reflect, detects a further embodiment the invention conductivity differences between the sensor electrodes. For this purpose, this embodiment of the invention additionally Resistance device between adjacent or next Sensor electrodes conductivity differences can detect.

With this embodiment The invention has the advantage that impedance differences Combination of capacitive and resistance measurements determined can be, so that distinguished between living and non-living papillary bodies can be. Thus, the evaluation circuit non-living papillary body, such as plaster casts or Paint films that have copies of papillary bodies sort out, which are not recognized by purely capacitive fingerprint sensors.

A Sensor circuit for a fingerprint sensor as described in the above examples is, has following components. At first, it includes at least a sensor electrode which is a section of the papillary body of the Fingerberry opposite is arranged. Further, for the sensor circuit provided a triple changeover switch. This Triple changeover switch has three positions. In a first Position, the sensor electrode is connectable to a charge source. In a second position, the sensor electrode is provided with a measuring device and in a third position, the sensor electrode is a Earth potential connectable.

there the charge source has a plurality of the limited amount of charge, which can be stored on the sensor electrode, on. The triple changeover switch can switch in high frequency sequence between the three positions. For discharging the charge source, one of the limited charge amount the sensor electrode dependent Number of charges of the measuring device until reaching a predetermined threshold value performed. The third position is an intermediate position, with the help of this third position between each recharging the sensor electrode placed at ground potential becomes.

In a further embodiment the sensor circuit, the charge source, a ground potential switch and have a La depotentialumschalter, wherein the two switches connected to capacitor electrodes of a charging capacitor. In this case, in a first position of the charging capacitor to a Supply voltage to earth potential connected and in a second position, the ground potential electrode of the charging capacitor switched to supply potential and the on charge potential electrode of the charging capacitor to a Drain electrode of a source follower connected. The source electrode the source follower will follow with the first position of the triple changeover switch of the sensor electrode connected.

There the triple changeover switch between this first position and the two other positions in high-frequency sequence, is a limited number of circuit changes of the triple changeover switch required until a threshold reaches the measuring device becomes. This number is inversely proportional to the limited Charge amount on the sensor electrode, making it a numerical Value for the amount of charge is present on the sensor electrode. Furthermore, can the measuring device have a measuring capacitor whose first Electrode constantly is at ground potential and its second electrode with a circuit node connected is. From the circuit node, a connection line lead to the second position of the triple changeover switch. simultaneously From the circuit node, a connection line goes to a threshold comparison circuit out. With this circuit variant, the threshold value at the measuring capacitor via the Threshold comparison circuit can be specified.

The threshold comparison circuit in turn has a threshold capacitor. This threshold capacitor has an earth electrode that is constantly at ground potential. The second electrode of the threshold capacitor is connected via a switching node to a subtraction element. In addition, the circuit node can be connected via an on / off switch with the second position of the charging potential switch of the charging capacitor. This connection becomes after Ab circuit of a measuring cycle connected to take over a residual charge amount of the charging capacitor to the threshold capacitor. With this variant of the sensor circuit, it becomes possible to record differences between adjacent sensor electrodes and to output them as a numerical value.

Farther the subtraction element is connected to a reference potential. The difference between reference potential and residual potential becomes then by the evaluation circuit as a threshold for a next Measurement cycle held. The evaluation circuit itself provides the Number of charge volume transports fixed by the next sensor electrode to the measuring capacitor until reaching the threshold for this next Sensor electrode is required. This number is simultaneous the numeric value for the charge quantity difference between two sensor electrode surfaces.

With Help this sensor circuit, it is possible a simple procedure for detecting parameter differences between a plurality of Sensor electrodes and one, the sensor electrodes arranged opposite papillary a fingertip, to identify a person. there the following process steps are carried out. First, the finger berry is the Person on the sensor electrodes of a fingerprint sensor, such as he in the above remarks was described. Then the parameters are terdifferenzen by means of the sensor circuit as explained in the above embodiment, detected. Finally the recorded parameter differences are evaluated and stored.

This Procedure has the advantage that it has a numerical measurement parameter returns exactly the difference between two next to each other or one after the other arranged sensor electrodes corresponds and thus already a first compression of the measured data is possible, this numerical value be converted into a digital number by simple conversion can. Furthermore, for the method involves compression of digital values after digitization the numerical measurement parameters are obtained. Finally, can With this procedure the identification of the person with the help of the detected Parameter differences are made by taking the result of the fingerprint measurement compared with archived values by the fingerprint sensor and a release or blocking takes place. Be as parameter differences the capacitive difference values of a fingerprint with conductivity differences a fingerprint combined, so may additionally fingerprint sensor according to the invention distinguish between living and non-living tissue, so that thus a forgery-proof Method is given to identify the user of the fingerprint sensor.

The The invention will now be explained in more detail with reference to the accompanying drawings.

1 shows a schematic diagram of a fingerprint sensor of a first embodiment of the invention.

2 shows a schematic diagram of a fingerprint sensor of a second embodiment of the invention.

3 shows a schematic diagram of a fingerprint sensor of a third embodiment of the invention.

1 shows a schematic diagram of a fingerprint sensor 100 a first embodiment of the invention. Of the variety of sensor electrode surfaces 1 a fingerprint sensor 100 is in this schematic diagram only a sensor electrode surface 1 shown. This sensor electrode surface 1 is a section of a papillary body in a fingerprint measurement 2 a fingerberry 3 arranged opposite. Thus, the sensor electrode surface 1 a limited amount of charge 6 depending on the arranged opposite section of the papillary body 2 store via a first switch S _A and a second switch S _B to a measuring device 22 submit. A third switch S _C ensures that a multiple of charge quantities of the sensor memory 5 in a charge source 8th kept ready and to the sensor memory 5 or the sensor electrode surface 1 can be delivered.

Through the interaction of the third switch S _C and a charging capacitor C _P , it is provided that a higher voltage than the operating or supply voltage V _{DD is} provided. In each sensor operation, the charging capacitor C _P is charged once and then connected in series with the supply voltage. The high-frequency sequence of the charge delivery to the sensor electrode takes place at 8-bit output values 256 times per sensor operation via the switch S _A , while S _C switches only once per sensor operation.

To each of the sensor electrode surfaces 1 of the fingerprint sensor 100 can be a sensor circuit 4 be clamped with a triple change switch S ₃ . In the schematic diagram shown here 1 First, this changeover switch S _{3 is} fixed to the sensor electrode 10 connected. The triple changeover switch S ₃ has three positions with a first position ( 1S3 ), in which the sensor electrode 1 with a charge source 8th is connectable. About that In addition, the triple changeover switch S _{3 has} a second position 2S3 on, in which the sensor electrode 10 with a measuring device 22 is connectable. Finally, the triple changeover switch S _{3 has} a third position 3S3 on, in which the sensor electrode 10 can be connected to a ground potential EP.

The charge source 8th has a source follower T ₁ , which with its source electrode to the first position 1S3 of the triple changeover switch S ₃ and its drain electrode via a circuit node 23 either with a second position 2S2 a charging potential switch S ₂ via a connecting line 24 is connected and via a connection line 25 is connected to the on position ES4 of an on / off switch S ₄ . The gate electrode of the source follower T ₁ is connected via a circuit node 26 at the supply voltage V _DD .

The source follower T ₁ is here an n-channel transistor. When the potential is on 1S3 is low, so there is a positive voltage across the gate-source path and T ₁ opens, so that a charge in the sensor electrode at the switch position 1S3 flows until the voltage across the gate-source path reaches or falls below the transistor threshold voltage. Thus, the source potential follows the gate potential, and a charge is taken linearly from the charging capacitor C _{P as} a function of the sensor electrode capacitance.

Thus forms the core of the charge source 8th the charging capacitor C _p , with its first electrode C _P1 via a ground potential switch S ₁ in the position 1S1 at ground potential and in its second position 2S1 to the supply voltage V _DD via the circuit node 26 is legible. The second electrode C _{P2 of} the charging capacitor C _P is connected to a charging potential switch S ₂ , which in a first position 1S2 with the supply voltage V _DD via the circuit node 26 and in a second position 2S2 over the circuit node 23 with the drain electrode of the source follower T _{1 is} connectable. With this charge source 8th For example, the drain electrode of the source follower T _{1 can be set} to almost twice the potential 2 × V _{DD of} the supply voltage V _DD .

In 1 is the charge source 8th shown on the left side of the schematic diagram, while the right side of the schematic diagram of the measuring device 22 and their circuit is mastered. The measuring device 22 has a measuring capacitor C _M. The first electrode C _M1 is constantly at ground potential EP. A second electrode C _{M2 of} the measuring capacitor C _M is connected to a circuit node 18 connected via a connection line 27 with the second position 2S3 of the triple changeover switch S ₃ via a connecting line 27 connected is. This measuring capacitor C _M accumulates the limited amount of charge when the triple switch S ₃ from the first position 1S3 to the second position 2S3 is switched.

The number of switching required depends on the one hand on the amount of charge 6 on the sensor electrode 10 is storable per charge, and on the other hand from a threshold for the measuring capacitor C _M by a threshold comparison circuit 19 is given. The threshold value is specified on the one hand by a reference voltage V _ref and on the other hand determined by a state of charge of a threshold capacitor C _SH . The threshold capacitor C _SH is always at ground potential with a first electrode C _SH1 , while the second electrode C _{SH2 of} the threshold capacitor C _{SH is} connected to a circuit node 20 connected is. The circuit node 20 is on the one hand via a connecting line 28 with a subtraction element 21 on the other hand, and is on another connection line 29 with the above-mentioned on / off switch S ₄ in conjunction.

The result of the subtraction element 21 at the circuit node 35 then becomes a circuit component 30 , also called threshold value decision, supplied, which is the result potential of the subtraction element 21 with the potential of the measuring capacitor C _M at the circuit node 18 compares, and an evaluation circuit 9 supplies. This evaluation circuit 9 , Also called accumulator, counts the number of operations of charging and discharging the sensor electrode until the threshold indicates that the signal of the measuring capacitor C _M at the node 18 the result potential of the substrate member 21 exceeds.

With the measuring device 22 in conjunction with the switchable charge source 8th Thus, the difference between the charge quantities present on adjacent sensor electrodes can be determined very accurately. For this purpose, the charging capacitor C _{P is} charged, the earth potential switch S ₁ in the position 1S1 and thus sets the first electrode C _{P1 of} the charging capacitor C _P at ground potential EP. At the same time, the second electrode C _{P2 is} in its first position via the charge potential switch S ₂ 1S2 over the circuit node 26 connected to the supply potential of the supply voltage V _DD .

The charging capacitor C _P is in this position, as well as in 1 shown charged to the predetermined by the supply voltage V _DD charge potential. Thereafter, first the charging potential switch S _{2 is} in its second position 2S2 switched over and only then the Erdpotentialumschalter S ₁ disconnected from the ground potential EP and in the second position 2S1 spent, so that the first electrode C _{P1 of} the charging capacitor C _P now is at the supply voltage potential and the second electrode C _{P2 of} the charging capacitor C _{P is} at almost twice the supply potential 2 × V _DD . This almost twice the supply potential 2 × V _DD lies above the circuit node 23 now also at the drain electrode of the source follower T ₁ . The gate electrode of the source follower T ₁ is still above connection line 31 and via the circuit node 26 at the supply voltage potential V _DD . However, via the source electrode of the source follower T ₁ , only the potential V _DD almost reaches the first position 1S3 of the triple changeover switch S ₃ passed and thus a first limited amount of charge 6 on the sensor electrode 10 saved.

In N-bit, the triple changeover switch S _{3 is} now 2 ^N times rotating in the positions 1S3-2S3-3S3 brought. It is as mentioned above in the first position 1S3 the sensor electrode 10 charged, in the second position 2S3 the measuring capacitor C _M through the sensor electrode 10 charged and in position 3S3 becomes the sensor electrode 10 completely discharged against ground potential EP. Because the potential 1S3 is constant and the sensor electrode 10 at 3S3 is discharged each time, is taken from the charging capacitor C _P each time a charge that is based solely on the capacitance of the sensor electrode 10 depends because all other factors such as the potential at the source electrode of the source follower T ₁ and parasitic influences are constant. Therefore, the voltage across the charging capacitor C _P decreases linearly with the number of operations with one of the capacitance of the sensor electrode 10 certain slope. The evaluation circuit 9 counts the number of processes, starting at -2 ^N-1 +1, until the voltage across the measuring capacitor C _M counts the potential at the negative input of the threshold value discriminator 21 exceeds. The maximum value that can be achieved is 2 ^N-1 .

• – Der Erdpotentialumschalter S₁ wird in Stellung 1S1 gebracht. Das Potential an 2S2 sinkt auf die über den Ladekondensator C_P abfallende Restspannung, die nach der regelmäßigen Ladungsentnahme übrig geblieben ist.
• – Nach dem Sample-and-hold-Prinzip wird mit kurzem Schließen des Ein-/Ausschalters S₄ diese Spannung von dem Schwellenwertkondensator C_SH übernommen. Das Vergleichspotential am Schwellenwertscheider bzw. das Ergebnispotential an dem Subtraktionsglied 21 ist dann V_ref–U_SH.
• – Die Sensorschaltung lässt sich grob an vier Fällen charakterisieren unter der Annahme, dass eine Papillarleiste bzw. ein dunkles Bild des Fingerabdrucks eine hohen Kapazität der Sensorelektrode 10 bewirkt und eine Hautrille bzw. ein helles Bild eine niedrige Kapazität an der Sensorelektrode 10 hervorruft.

For the next counting process and thus for the next measuring cycle, the charging of the charging capacitor C _{P is} started again. In addition, the following preparations are made for the next measurement cycle:

• - The earth potential switch S ₁ is in position 1S1 brought. The potential 2S2 decreases to the voltage drop across the charging capacitor C _P residual voltage that has remained after the regular charge removal.
• - According to the sample-and-hold principle, this voltage is taken over by the threshold capacitor C _SH with a short closing of the on / off switch S ₄ . The comparison potential at the threshold value separator or the result potential at the subtraction element 21 is then V _ref -U _SH .
• The sensor circuit can be broadly characterized in four cases, assuming that a papillary strip or a dark image of the fingerprint has a high capacitance of the sensor electrode 10 and a skin groove or a bright image causes a low capacitance on the sensor electrode 10 causes.

• 1. Bei einem Übergang von dunkel nach dunkel wird der Ladekondensator C_P stark entladen, die Restspannung über C_P ist somit sehr niedrig, und die Schwellenspannung am Knoten 35 ist fast V_ref. Jetzt wird der Messkondensator C_M durch die hohe Sensorelektrodenkapazität des angenommenen dunklen Bildes einer Papillarleiste schnell aufgeladen. Die Dauer bis zum Erreichen der Schwellenwertspannung ist mittelgroß. Die Auswerteschaltung 9 bzw. der Akkumulator bleibt ca. bei Null stehen.
• 2. Bei einem Übergang von dunkel nach hell wird der Ladekondensator ebenfalls stark entladen. Das Schwellenpotential V_SH am Schwellenwertkondensator C_SH ist sehr niedrig und die Schwellenwertspannung ist fast V_ref. Jetzt wird der Messkondensator C_M durch die niedrige Sensorelektrodenkapazität langsam aufgeladen. Die Dauer bis zum Erreichen der Schwellenwertspannung ist lang und die Auswerteschaltung 9 bzw. der Akkumulator bleibt bei einem hohen positiven Wert stehen.
• 3. Bei einem Übergang von hell nach hell wird der Ladekondensator C_P mittelmäßig entladen, das Schwellenwertpotential V_SH am Schwellenwertkondensator C_SH wird nun wirksam, die Schwellenwertspannung ist dann V_ref–V_SH und somit niedrig. Jetzt wird der Messkondensator C_M durch die niedrige Sensorelektrodenkapazität langsam aufgeladen. Die Dauer bis zum Erreichen der Schwellenwertspannung ist mittelgroß und der Akkumulator bzw. die Auswerteschaltung 9 bleibt bei ca. Null stehen.
• 4. Bei einem Übergang von hell nach dunkel wird der Ladekondensator C_P mittelmäßig entladen. Das Schwellenpotential V_SH wirkt sich wieder aus und die Schwellenwertspannung ist nun V_ref–V_SH und somit niedrig. Jetzt wird der Messkondensator C_M durch die hohe Sensorelektrodenkapazität schnell aufgeladen. Die Dauer bis zum Erreichen der Schwellenwertspannung ist sehr kurz. Die Auswerteschaltung 9 bzw. der Akkumulator bleibt bei einem hohen negativen Wert stehen.

These four characteristic cases are in detail:

• 1. In a transition from dark to dark, the charging capacitor C _{P is} heavily discharged, the residual voltage across C _P is thus very low, and the threshold voltage at the node 35 is almost V _ref . Now, the measuring capacitor C _M is quickly charged by the high sensor electrode capacity of the assumed dark image of a papillary strip. The time to reach the threshold voltage is medium. The evaluation circuit 9 or the accumulator stays at about zero.
• 2. In a transition from dark to light, the charging capacitor is also heavily discharged. The threshold potential V _SH at the threshold capacitor C _SH is very low and the threshold voltage is almost V _ref . Now the measuring capacitor C _M is slowly charged by the low sensor electrode capacity. The duration until reaching the threshold voltage is long and the evaluation circuit 9 or the accumulator remains at a high positive value.
• 3. In a bright to light transition, the charging capacitor C _{P is} moderately discharged, the threshold potential V _SH at the threshold capacitor C _SH now becomes active, the threshold voltage is then V _ref -V _SH and thus low. Now the measuring capacitor C _M is slowly charged by the low sensor electrode capacity. The duration until reaching the threshold voltage is medium and the accumulator or the evaluation circuit 9 stays at about zero.
• 4. In a transition from light to dark, the charging capacitor C _{P is} discharged moderately. The threshold potential V _SH has an effect again and the threshold voltage is now V _ref -V _SH and thus low. Now, the measuring capacitor C _M is quickly charged by the high sensor electrode capacity. The time to reach the threshold voltage is very short. The evaluation circuit 9 or the accumulator remains at a high negative value.

2 shows a schematic diagram of a fingerprint sensor 200 a second embodiment of the invention. Components with the same functions as in 1 , are denoted by the same reference numerals and not explained separately.

The principle sketch according to 2 differs from the schematic diagram according to 1 as a result of that 2 several sensor electrodes 10-17 and egg NEN larger section of a papillary body 2 shows that opposite these sensor electrodes 10-17 is arranged. Due to the different acting and different from the sensor electrodes 10-17 spaced papillary ridges 33 and skin grooves 34 become the sensor electrodes 10-17 via the triple changeover switch S ₃ and via the switching matrix 32 from the charge source 8th from, with different high limited charge quantities 6 busy.

For detecting the charge differences between adjacent or subsequent sensor electrodes 10-17 becomes the switching matrix 32 Switched after each measurement cycle and the difference between two consecutive measurement cycles for two adjacent sensor electrodes 10-17 in the evaluation circuit 9 or the accumulator detected. At the fingerprint sensor 200 Thus, only one sensor circuit is required to cover the entire matrix of sensor electrodes 10-17 over the switching matrix 32 to capture for a differential image of a fingertip.

3 shows a schematic diagram of a fingerprint sensor 300 a third embodiment of the invention. Components having the same functions as in the previous figures are identified by the same reference numerals and will not be discussed separately.

The fingerprint sensor of the third embodiment is different from the fingerprint sensor according to FIG 2 in terms of the size ratio of sensor electrodes 10 to n to the papillary ridges 33 as well as the skin grooves 34 , This sensor has a lateral resolution of about 513 dpi. This means that the sensor electrodes 10 to n are arranged at an interval of 50 microns. The edge length of the sensor electrodes 10 to n is about 40 microns. The above arranged Papillarleisten 33 thus extend over a plurality of sensor electrodes and in extreme cases have a width which extends over 3 to 12 pixel intervals of the sensor normally they are between 5 to 8 pixel intervals, wherein below a pixel interval, the step size between two sensor electrodes 10 until n is understood.

The charges are on the surface of the sensor electrodes 10 not only notably stored up there where the papillary ledges 33 the fingerberry 3 the sensor surface almost touched, but to a lesser extent also where the papillary body 2 from the sensor electrode surface 1 , namely in the skin grooves 34 , is removed. This results in a transition in the fingerprint images between the papillary ridges 33 and their respective spaces the skin grooves 34 with increasing distance of the papillary body 2 from the sensor electrode surface 1 , Thus, in this embodiment of the invention, the dependence of the stored charge is no longer the cutout of the papillary body over the sensor electrode crucial, but it is the dependence on the distance profile of the papillary body 2 from the sensor electrode surface 1 crucial.

Furthermore, it is provided in this third embodiment of the invention that for each column shown here in the cutout of a sensor electrode matrix in each case via the switching matrix 32 a sensor circuit is connected, so that the complete sensor operation relative to the in 2 shown second embodiment, especially since the sensor circuits for each column of the matrix can work simultaneously. Thus, in a sensor operation, the sensor circuit only has to analyze as many sensor electrodes as there are rows in the matrix.

The sensor circuit 4 The third embodiment of the invention differs from the sensor circuits of the first and second embodiments in that the threshold comparison circuit 19 no subtraction unit as in the embodiments of 1 and 2 contains. Rather, another on / off switch S5 with the switch positions 1S5 and 2S5 into the threshold circuit 19 introduced. When the switch S ₄ in the 1S4 Position is, and the switch S ₅ in the position 1S5 is, has at the end of the sensor operation of the threshold capacitor C _SH, the potential difference, which is applied across the charging capacitor C _P. Subsequently, the switch S ₅ is initially in the position 2S5 brought and then the switch S ₄ in the position 2S4 connected. The potential 2S5 is now V _ref -V _SH . This achieves the same result as in the circuits of the first and second embodiments with the subtraction unit indicated there. Prerequisite for this circuit is that the input of the threshold decider 30 no load is allowed to represent what by correspondingly small gates as the input of the circuit component 30 can be achieved without further connections.

100

200

300

fingerprint sensor

1

Sensor electrode area

2

papillary

3

fingertip

4

sensor circuit

5

sensor memory

6

limited amount of charge

7

neckline of the papillary body

8th

charge source

9

evaluation or accumulator

10-17

sensor electrodes

18

circuit node

19

Threshold comparison circuit

20

circuit node

21

subtraction or threshold decision maker

22

measuring device

23

circuit node

24

connecting line

25

connecting line

26

circuit node

27

connecting line

28

connecting line

29

connecting line

30

circuit component

31

connecting line

32

switching matrix

33

dermal papillae

34

skin groove

35

circuit node

S _A

first switch

S _B

second switch

S _C

third switch

S ₁

Erdpotentialumschalter

S ₂

Charging potential switch

S ₃

Triple changeover switch

S ₄

Off / On switch

S ₅

On / off switch

1S1

first Position of the ground potential switch

2S1

second Position of the ground potential switch

1S2

first Position of the charging potential switch

2S2

second Position of the charging potential switch

1S3

first Position of the triple changeover switch

2S3

second Position of the triple changeover switch

3S3

third Position of the triple changeover switch

ES4

Attitude of the on / off switch

AS4

Exhibition of the on / off switch

C _P

charging capacitor

C _P1

first Electrode of the charging capacitor

C _P2

second Electrode of the charging capacitor

C _M

measuring capacitor

C _M1

first Electrode of the measuring capacitor

C _M2

second Electrode of the measuring capacitor

C _SH

Threshold capacitor

C _SH1

first Electrode of the threshold capacitor

C _SH2

second Electrode of the threshold capacitor

T ₁

source follower

V _DD

supply voltage

V _ref

reference voltage

V _SH

tension at the threshold capacitor

Claims (18)

Fingerprint sensor comprising a plurality of sensor electrode surfaces ( 1 ), which corresponds to the papillary body ( 2 ) of the fingerberry ( 3 ) is arranged opposite, with a sensor circuit ( 4 ) for a single sensor electrode surface ( 1 ), which serve as sensor memory ( 5 ) a limited amount of charge ( 6 ) in dependence of the opposite section ( 7 ) or removal of the papillary body ( 2 ), wherein the sensor circuit ( 4 ) a charge source ( 8th ) having a first switch (S _A ), which via the first switch (S _A ) the sensor memory ( 5 ) in a measuring cycle several times, and wherein the sensor circuit ( 4 ) a measuring device ( 22 ) with a second switch (S _B ), wherein the measuring device ( 22 ) the charge quantities ( 6 ) of the sensor memory ( 5 ) for one measurement cycle via the second switch (S _B ). Fingerprint sensor according to claim 1, characterized in that the first (S _A ) and the second switch (S _B ) form a changeover switch (S ₃ ) which is in high-frequency alternating between charging and discharging the sensor memory ( 5 ) switches. Fingerprint sensor according to claim 1 or claim 2, characterized in that the charge source ( 8th ) has a charging capacitor (C _P ), which via a third switch (S _C ) of the sensor circuit ( 4 ) is connected to a charging station, and wherein the charging capacitor (C _P ) has a storage capacity which is a multiple of the limited amount of charge ( 6 ) of the sensor memory ( 5 ), and wherein the first switch (S _A ) of the sensor circuit ( 4 ) the amount of charge of the charging capacitor (C _P ) in high-frequency sequence in limited amounts of charge to the sensor memory ( 5 ) transmits during loading. Fingerprint sensor according to one of the preceding claims, characterized in that the measuring device ( 22 ) to an evaluation circuit ( 9 ), which determines the number of charging processes until a threshold value is reached and / or the time interval until a threshold value is reached in comparison to adjacent or next following sensor electrodes ( 10 ) evaluates. Fingerprint sensor according to one of the preceding claims, characterized in that the sensor circuit ( 4 ) Capacitance differences of adjacent or next successive sensor electrodes ( 10 to 17 ) detected. Fingerprint sensor according to one of the preceding claims, characterized in that the sensor circuit ( 4 ) additionally a cons standing measuring device, which between adjacent or next following sensor electrodes ( 10 to 17 ) Conductivity differences detected. Fingerprint sensor according to one of the preceding claims, characterized in that the evaluation circuit ( 9 ) Impedanzdifferenzen taking into account by the sensor circuit ( 4 ) detected conductivity differences and capacitance differences of adjacent or next sensor electrodes ( 10 to 17 ) evaluates. Fingerprint sensor according to one of the preceding claims, characterized in that the supply of the sensor electrode ( 10 ) includes a charge storage, the residual charge or its residual voltage at the end of a sensor operation of the evaluation circuit ( 19 ) is supplied for the next measurement. Sensor circuit for a fingerprint sensor ( 100 . 200 ) according to one of claims 1 to 8, wherein the sensor circuit ( 4 ): - a sensor electrode ( 10 ), which is a section of the papillary body ( 2 ) of the fingerberry ( 3 ) is arranged opposite, - a triple changeover switch (S ₃ ) having a first position ( 1S3 ), in which the sensor electrode ( 10 ) with a charge source ( 8th ), a second position ( 2S3 ), in which the sensor electrode is connected to a measuring device ( 22 ), and a third position ( 3S3 ), in which the sensor electrode ( 10 ) is connectable to a ground potential (EP). Sensor circuit according to claim 9, characterized in that the charge source ( 8th ) has a charging capacitor (C _P ) and a ground potential switch (S ₁ ) and a charging potential switch (S ₂ ), wherein the two switches (S ₁ and S ₂ ) with the two capacitor electrodes (CP ₁ and CP ₂ ) are connected and in one first position ( 1S1 respectively. 1S2 ) connect the charging capacitor (C _P ) to a supply voltage V _DD to earth potential (EP) and in a second position ( 2S1 and 2S2 ) connect the ground potential electrode (C _P1 ) of the charging capacitor (C _P ) to the supply potential V _DD and connect the charge potential electrode (C _P2 ) of the charging capacitor (C _P ) to a drain electrode of a source follower (T1) whose source electrode is at the first position ( 1S3 ) of the triple changeover switch (S ₃ ) of the sensor electrode ( 10 ) connected is. Sensor circuit according to claim 9 or claim 10, characterized in that the measuring device ( 22 ) has a measuring capacitor (C _M ) whose first electrode (C _M1 ) is at ground potential (EP) and whose second electrode (C _M2 ) is connected to a circuit node (C) 18 ), from which a connecting line to the second position ( 2S3 ) of the triple changeover switch (S ₃ ) leads, and from which a further connection line with a threshold comparison circuit ( 19 ) connected is. Sensor circuit according to claim 11, characterized in that the threshold comparison circuit ( 19 ) has a threshold lenwertkondensator (C _SH ) whose ground potential electrode (C _SH1 ) is at ground potential (EP) and via the second electrode (C _SH2 ) via a circuit node (C 20 ) with a subtraction element ( 21 ) and via the circuit node ( 20 ) and an on / off switch (S4) with the second position ( 2S2 ) of the charging potential switch (S ₂ ) of the charging capacitor (C _P ) is connectable to transfer a residual charge amount of the charging capacitor (C _P ) to the threshold capacitor (C _SH ). Sensor circuit according to claim 11, characterized in that the subtraction element ( 21 ) is connected to a reference potential, and the difference between reference potential and residual potential as a threshold value for a next measurement cycle to an evaluation circuit ( 9 ), which determines the number of charge-quantity transports from the next sensor electrode ( 10 to 17 ) to the measurement capacitor (C _M ) until the clamp value for this next sensor electrode ( 10 to 17 ) evaluates. Method for detecting parameter differences between a plurality of sensor electrodes ( 10 to 17 ) and a sensor electrodes ( 10 to 17 ) arranged opposite papillary body ( 2 ) of a finger-berry ( 3 ), for the identification of a person, the method comprising the following steps: - application of the fingertip ( 3 ) of the person on the sensor electrodes ( 10 to 17 ) of a fingerprint sensor ( 100 . 200 ), according to one of claims 1 to 8, - detecting parameter differences with the aid of the sensor circuit ( 4 ), according to one of claims 8 to 12, - evaluating and storing the detected parameter differences. Method according to claim 14, characterized in that that for identification of the person the detected parameter differences are compared with archived values and a release or a blockage takes place. Method according to claim 14 or claim 15, characterized in that the sensor circuit ( 4 ) Impedance differences between sensor select clear ( 10-17 ) and between a living papillary body ( 2 ) and non-living copies of a papillary body ( 2 ) based on the impedance differences and access to stored data for copies of a papillary body ( 2 ) refused. Method according to one of claims 14 to 16, characterized in that for the differential measurement in each sensor process, an analog value is stored in the form of charge, which of the evaluation circuit ( 9 ) is supplied to influence the respective subsequent measurement. Method according to one of claims 14 to 16, characterized in that for the differential measurement in each sensor operation, an analog value is stored in the form of charge, the supply of the sensor electrode ( 10 ) at the respective subsequent measurement.