Capacitive monitor for topological finger print pattern

(54)

Applicant Inventor Representative Designation

9/20

87-02-16

87- 01-02

85-07-01

85-07-01 (11)

A61B 5/10

Publication number 44g ₄qq

Application received as:

Swedish patent application □ completed international patent application with number □ converted European patent application with number

SE (56) (57)

ASEA AB, 721 83 Västerås P. Svedberg .Vällingby Öhman 8 Device for capacitive sensing of a topological pattern

Published publications: - Abstract:

A device for capacitive sensing of. a topological pattern, e.g., a fingerprint, comprises a two-dimensional matrix (SM). The matrix has a signal-giving conductor pattern with a plurality of conductors (LS1-LS5) and a signal-receiving conductor pattern (LM1-LM5), as well as a plurality of conductors. The conductors of the two patterns intersect and are capacitively coupled to each other. The device has a pulse generator (PG) which, for reading the pattern, emits a voltage pulse to each of the conductors in the signal-providing wiring pattern. The pulses are output to one conductor at a time and with a time delay between the pulses. To each of the conductors in the signal receiving wiring pattern, means (A1-A5, SR1-SR5) for detecting and storing the signals transmitted to these conductors are connected.

<img file="SE448408B_D0001.tif" />

• ET

The numbers in brackets indicate international identification code, INID code. Letters in clamps indicate international document code

T

448 408

The present invention relates to a device for capacitive sensing of a topological pattern and comprising a two-dimensional matrix. The matrix has a signal-giving conductor pattern with a plurality of different conductors and a signal-receiving conductor pattern with a plurality of different conductors arranged to intersect the conductors in the signaling conductor pattern.

An example of such a topological pattern for which the sensing device according to the invention is intended is a fingerprint. The finger whose imprint is to be sensed is thereby brought into contact with the matrix, but separated from it by an insulating layer so that a direct conductive contact rate is not obtained between the conductor patterns and the finger. At a point where a ridge of the fingerprint abuts the matrix, the surface of the finger is closer to the matrix than at a point where a valley of the fingerprint is located. The finger affects the capacitive coupling between the two conductor patterns and the degree of influence on the coupling depends on the distance of the finger surface from the 15. matrix. By performing the matrix at a sufficiently high resolution and by reading the degree of the finger's influence on the capacitive coupling at each of the matrix points (the intersection area between the two conductor systems), information defining the fingerprint pattern can be obtained and passed on for registration and / or analysis.

448 408

BACKGROUND OF THE INVENTION

A device of similar kind is previously known from European patent application 41,693. It describes a fingerprint sensor having a conductor pattern with a plurality of capacitively coupled conductors. The proximity or pressure of the surface of a finger tip pressed against the surface of the sensor affects the capacitive coupling between the conductors. The capacitances to be read are connected to the inputs of a multiplexer which connects the capacitances one by one and in turn to a measuring equipment.

A similar sensing principle is described in German Laid-Open No. 2,529,475, where a large number of variable capacitors are read in turn in sequence and one at a time using a demultiplexer and a multiplexer.

To obtain sufficient information about the read pattern, high resolution and thus a large number of measuring points must be read. A reading in the above-described known way of measuring points one by one and in turn takes a long time, which is already in itself a disadvantage. For example, when detecting fingerprints, there is a high risk that the finger may move slightly during the long reading time, which may give rise to a completely incorrect image of the pattern to be read.

The invention is intended to provide a sensing device of the type initially indicated, in which the sensing can be done much faster than with previously known devices, thereby greatly reducing the risk of a movement of the sensed pattern relative to the matrix.

INVENTION

The above-mentioned problem is solved according to the invention by providing signal generating means for applying electrical signals in the form of voltage pulses to the conductors of the signal-giving pattern. Only one conductor at a time is applied one voltage pulse, and between consecutive voltage pulses there is a certain time offset. For each of the conductors in the signal receiving wiring pattern, sensing means are provided for detecting and storing the capacitively transmitted signals to the conductor and in the form of a pulse train, which arise upon activation of the pulse generator and the signal emitting wiring pattern.

<img file="SE448408B_D0002.tif" />

In a preferred embodiment, the sensing means comprises comparative means arranged to compare the signals on the conductors in the signal receiving wiring pattern. with one reference level or reference signal. As a result, for each application or case, a suitable detection threshold can be adjusted.

Furthermore, the reference level can be made automatically variable to compensate for changes in the environment, such as temperature changes, which would otherwise. affect the measurement result. The reference signal · can be generated via fixed, capacitive switching.

The comparative means may conveniently comprise differential amplifiers.

TO Each differential amplifier's input is thereby connected to one of the conductors of the signal receiving pattern and its other input to a common signal source that outputs a reference signal.

In order to store the sensing result, according to a preferred embodiment of the invention, a shift register is provided for each of the conductors in the sigrtal-receiving wiring pattern.

BENEFITS

By means of a device according to the invention, a very fast reading can be obtained, which, among other things, greatly reduces the risk of distortion of the reading result due to relative movement between the pattern and the matrix during the reading time.

.RITNINGSFIGURER

The invention will be described in the following with reference to the accompanying Figures -1 to 4, where Fig. 1 shows an example of conductor pattern and matrix for use in the invention, Fig. 2 shows a detail of the conductor pattern, Fig. 3 shows a block diagram of a device. according to the invention and Fig. 4 shows some of the signals appearing in the device of Fig. 3.

Fig. 5 shows an example of the ratio x between output and input signal as a function of the distance h between the wiring pattern and the finger surface.

EMBODIMENT

Fig. 1 shows an example of a sensor pre-sensing a fingerprint 30 by pressing the fingertip against the sensor. The sensor has a conductor pattern

<img file="SE448408B_D0003.tif" />

448 408 which may, for example, be made as metallisations on an insulating substrate. The sensor conductor pattern consists of a signal emitting conductor pattern consisting of conductors LS1-LSÖ and a signal receiving conductor pattern consisting of conductors LM1-LMn. The conductors of the two groups cross each other and are in the intersection area 'capacitively coupled to each other. The sensor wiring pattern · can be seen as a matrix where the lines are the signal emitting wiring pattern and the columns are defined by the signal receiving wiring pattern.

In a typical example, the number of leaders in each can be. both patterns10. for example, be 50 (n = 50) and the distance between adjacent ones

Conductors can be 50-1Ö0 / Um. However, depending on the desired degree of resolution, etc., both larger and smaller values ​​can, of course, be selected for the division in the management pattern and the number of conductors.

The sensor with its wiring pattern is indicated in Fig. 1 with SM.

To increase the effect of the finger surface on the capacitive coupling between the conductors, each conductor between the conductor intersections may be provided with a wider portion, for example as shown in Fig. 1-1 in the form of a square plate. Thus, the conductor LSI is provided with the plates P 1, P₁₂.... P and the conductor LM1-with the plates

The connections of the signal emitting wiring pattern are denoted by S ,, S ... S

Π and the signal receiving wiring pattern connections with 0 0₂ ___ 0 .

Fig. 2 shows in detail part of the sensor shown in Fig. 1. It is formed on an insulating substrate 1 of, for example, sapphire or glass. Closest to the substrate is a conductor pattern JP, which is grounded and constitutes a ground plane.

This conductor pattern is isolated from the above conductor pattern which constitutes the sensor matrix itself of a layer 2 of, for example, silica. Of the sensor matrix, only the two adjacent plates P ^ are shown. The conductor pattern is protected against mechanical damage and against direct conductive bridging of an insulating layer 3 of, for example, silica.

The plates and are capacitively coupled to each other, which is illustrated in the figure by means of the capacitance C. In addition, the plates are capacitively coupled to earth, which is shown in the figure with the capacitances

448 . With F1 in the figure, the surface of a ridge of the pattern of the finger tip pressed against the sensor is denoted. Finger surface F1 shields due to the high dielectric constant of the skin (= .80 for water) of some of the field lines between the two plates. This has the effect of reducing direct capacitance between the plates. The result is because the capacitive coupling between the plates is reduced when a finger is pressed against the sensor, and this effect on the capacitive coupling becomes considerably stronger when the two plates are under a i.e. in the fingerprint pattern than when they are under a valley, (shown by the dashed line F2 in the fi gure). . .

Fig. 3 shows the sensor matrix SM and a ridge F1 (the dotted surface in the figure) adjacent to the sensor of the fingerprint pattern. For the sake of clarity, the sensor matrix is ​​shown with only five signal emitting and five signal receiving conductors, while in practice the number is preferably substantially larger.

The capacitive couplings between the two wiring patterns are symbolically shown as capacitances, e.g., C_g respectively C_Qi. A pulse generator PG has as many outputs as the number of conductors in the signal emitting wiring pattern, and the outputs are connected to the terminals S S-SS of the conductors in this pattern. The pulse generator is arranged to in turn supply voltage pulses to one of the conductors in this wiring pattern. The pulse generator is controlled by a clock pulse generator CPG which emits clock pulses CP.

To each of the terminals 0, -0_ of the conductors of the signal receiving ib pattern, one of the inputs, of a differential amplifier, A1-A5, is connected. To each amplifier's second input is a reference signal u_c connection, which can be variable with the help of a potentiometer PT. potentiometer

PT is connected via capacitances C signal-C till to the conductors of the signal emitting wiring pattern, thereby achieving any amplitude variation of the output signals. from the encoder PG does not affect the ratio between measured signal (S-S_) and reference signal (u).

I □. c

The output signals v1-v5 from the amplifiers are supplied to shift register SR1-SR5. Each shift register is arranged so that it. can store at least as many digital digits as the number of leads in the signal emitting pattern. Shift registers. content can be read out to a circuit AC for the desired signal processing of the sensed pattern, for example for analysis thereof or for comparison with a known pattern. The function of the shift register is controlled by the clock 488 408 pulse generator CPG such that each shift register is advanced one step for each voltage pulse delivered by pulse generator PG. The device shown in Fig. 3 can conveniently be supplemented with means for starting and stopping the reading, for controlling the reading of the contents of the shift register to the analyzer circuit AC, etc. The signal receiving electronics shown are only schematically indicated. Additional functions can be added to ensure a good signal reception and signal level.

The function of the device described above will now be described in connection with Fig. 4. In this figure a number of signals present in the device are shown as a function of time t. At the top five clock pulses CP are shown. The first clock pulse causes the pulse generator PG to emit a voltage pulse p1 on ni ri in the figure top output, the second clock pulse causes the pulse generator to output one. voltage pulse p2 at its second highest output, etc. Thus, after five clock pulses, each of the conductors in the signal emitting wiring pattern has been supplied with a voltage pulse. This can easily be achieved by, for example, training the pulse generator as a ring counter which is advanced by the clock pulses and whose output signals are supplied to pulse formers to provide the appropriate pulse length and pulse amplitude.

The first voltage pulse p1 is capacitively coupled to the conductors of the signal receiving pattern and the outputs s1-s5 are obtained on these conductors at a level determined by the ratio of the current coupling capacitance to the total capacitance to ground of the signal receiving conductor. The capacitive connection to the three conductors in the left figure is relatively good, but the connection to the two conductors in the figure is lower due to the presence of the finger surface F1. The amplitude of the outputs s4 and s5 is thereby reduced relative to the amplitude of the outputs s1-s3. Fig. 5 shows an example of the ratio x between output and input signal as a function of the distance h between the wiring pattern and the finger surface.

In the figure, reference level u is entered with dashed lines_q. If an output signal exceeds this reference level, the corresponding differential amplifier emits a signal which causes a logical zero to be stored as the first digit in the corresponding shift register, and if the output exceeds the reference level, a logical one is stored as the first digit in the shift register. Thus, after the voltage pulse p1, the binary digits 1, 1, 1,0, 0 are stored as the first digits in the shift register SR1-SR5.

448 408

The second voltage pulse p2, which is delivered to the second top conductor in the figure, has good connection to the two left conductors in the figure but lower connection to the three right conductors in the figure. This causes the second voltage pulse to cause storage as a second digit in the shift registers SR1-SR5 5 - the digital digits 1, 1, 0, 0, 0.

The sensing was continued until voltage pulses were supplied to all conductors (five) in the signal output. wiring pattern. After the fifth clock pulse, the sensing is conveniently interrupted. In the shift registers, there is then stored 1 form of binary word information that defines the degree of connection in each of the twenty-five nodes of the matrix. The contents of, for example, the shift register SR1 are in the described example after the end of the reading 11100 and in the shift register SR5 00111.

The information stored in the shift register, which thus completely defines the sensed pattern, can be supplied to the signal processing circuits immediately after the end of the reading, or at a suitable time, in the figure shown in the form of the analysis circuit AC.

As mentioned above, in Figures 3 and 4, for the sake of clarity, the number of conductors in each conductor pattern is greatly reduced. In practice, a larger number of leaders are usually required in each pattern. Thus, for example, for the detection of fingerprints, 50 conductors per pattern would be a practically appropriate value.

It has been assumed above that the number of conductors in the two patterns is the same (in Figs. 1, 5 in Figs. 3-4). Of course, however, the two conductor patterns can have different numbers of conductors.

The pulse length of the voltage pulses p1-p5 can be readily reduced to, for example, 1yUs, and the space between successive voltage pulses can also be about 15 µs. The read time at a matrix about. 50 x 50 measuring points will then only be 50 · - (1 + 1) = 100 µs. This extremely short sensing time achieved by a device according to the invention means, inter alia, that any movements of the finger during the sensing cannot distort the food result. The extremely rapid sensing and the storing of information that completely defines the sensed pattern thus gives a snapshot of the sensed pattern, and this image can then be analyzed at any rate.

448 408 ⁸

A device according to the invention can be designed in a number of ways other than the exemplary embodiment described above. Thus, for example, both the sensor matrix itself as well as the electronic means for driving and sensing can be designed in a disturbed number of other ways. In the example described above, the output signals from the sensor matrix are compared with a single reference / level u_c- The measure of the amplitude of the output signal is therefore obtained as a single binary digit (1 or 0). Of course, if desired, the amplitude determination can be made; - £ with higher resolution, however, the signal storing means (shift register SR1-SR5) may be carried out with correspondingly larger capacity.

The above has described how a device according to the invention is used for sensing a pattern of the surface of a single body (the line pattern on the surface of a fingertip). However, the device can also be used in the case where the sensed pattern is formed by an assembly of a number of smaller bodies, for example, particles which are incident to the surface of the sensor. The sensor 15 pattern can thus be made very fine-mesh, whereby the sensor can be used, for example, as a detector for organic molecules, which upon impact on the sensor affects the capacitive coupling at the points where the particles hit the sensor. The use of the sensor for sensing surface structure, patterns etc. of surfaces is, of course, not limited to fingerprint / 0 sensing either.