Capacitive distance sensor, particularly for acquiring fingerprints

Abstract: The distance sensor has a capacitive element (33, 34) in turn having a first armature (23) which is positioned facing a second armature (18) whose distance is to be measured. In the case of fingerprinting, the second armature is defined directly by the skin surface of the finger being printed. The sensor comprises an inverting amplifier (13), between the input and output of which the capacitive element (33, 34) is connected to form a negative feedback branch. By supplying an electric charge step to the input of the inverting amplifier, a voltage step directly proportional to the distance being measured is obtained at the output.

Description: The present invention relates to a capacitive distance sensor, in particular, a small-distance (micrometric to millimetric) sensor. Small-distance sensors are used, among other things, as pressure, proximity, roughness, mechanical stress and acceleration sensors, for example, in integrated microphony and for acquiring fingerprints. For fingerprint acquisition in particular (to which reference is made herein purely by way of example of a preferred application of the present invention), known sensors include various types, for example: optical, piezoelectric, variable-conductance, thermal, ultrasonic and capacitive, the most promising of which in terms of precision, size, production and cost are capacitive sensors. Capacitive sensors are based on the principle that the capacitance between two armatures is inversely proportional to the distance between them, so that, using the contacting dermal tissue itself as the second armature of the sensor capacitor, and by determining the capacitance, it is possible to locate the ridges and grooves of the fingerprint. This is the principle used in US-A-5,325,442 to Knapp, which relates to a sensor comprising an array of elementary cells, each comprising a sensitive electrode and an electronic switching device. The electrode is coated with dielectric material, such as passivation oxide or a polymer compound, onto which the epidermis is placed. When a cell is selected, a predetermined variation in potential is applied to the electrode to induce at the terminals an appropriate variation in charge, the extent of which depends on the capacitance associated with the electrode, and which is read by amplifying elements connected to the output of the device. To improve efficiency, the above patent suggests a surface grid connected to a reference potential to appropriately bias the skin tissue. In the above known capacitive sensor, the capacitance between the armatures of a capacitor varies in inverse proportion to the distance between the armatures, which therefore poses the problem of normalizing the resulting data. In particular, if the capacitance being measured is very small, as in the application in question, serious difficulty is encountered in detecting the charge and discriminating between the various intermediate charge levels corresponding to different grey levels of the image to be generated in the presence of a low signal/noise ratio. The article "Capacitance-Type Blade-Tip Clearance Measurement System Using a Dual amplifier with Ramp/DC Inputs and Integration", IEEE Transactions on Instrumentation and Measurement, vol. 41, n. 5, 1 October 1992, New York, US, pages 674-678 describes a measuring arrangement generating an output signal proportional to the clearance capacitance or the derivative thereof and not to the distance. US-A-4,958,129 describes a distance measuring circuit including a probe having a central electrode connected to the inverting input of a measure operational amplifier and a guard electrode connected to the non-inverting input thereof; the output of the measure operational amplifier is connected to ground and to the element the position whereof is to be measured. By applying an AC signal to a compensation amplifier having an output connected to the inverting input of the measure operational amplifier, the voltage at the non-inverting input of the operational amplifier is proportional to the distance between the element to gauge and the probe. US-A-3,641,431 teaches a distance measuring circuit including a negatively-fedback amplifier, the input whereof is connected to an AC source and to a probe armature and the output whereof is connected to an element the distance whereof is to be measured. It is an object of the present invention to provide a sensor designed to overcome the drawbacks typically associated with known technology. According to the present invention, there is provided a capacitive distance sensor, as claimed in Claim 1. A preferred, non-limiting embodiment of the present invention will be described .by way of example with reference to the accompanying drawings, in which: Figure 1 shows a sensor device for acquiring fingerprints; Figure 2 shows a detail of a cell of the Figure 1 device; Figure 3 shows the electric equivalent of the Figure 2 cell. Figure 1 shows a sensor device 1, preferably embodied in an integrated chip, comprising a number of cells 2 arranged to form an array 3 and each constituting an elementary sensor. Device 1 also comprises a horizontal scanning stage 5 and a vertical scanning stage 6 for enabling one cell 2 at a time according to a predetermined scanning pattern. Preferably, to read the cells, stages 5, 6 enable the outputs of the cells sequentially, and comprise shift registers or decoders. Device 1 also comprises a supply and logic stage 7, which supplies the components of the device (including cells 2), feeds the necessary reference voltages, and controls the sequence of steps provided for (as explained in detail later on). In particular, Figure 1 shows a voltage source 12 for generating a reference voltage variation ΔV R ; a buffer 8 is connected to the outputs of all the cells 2, and supplies, at output 10 of the device, the signal present at the output of the cell 2 enabled by scanning stages 5, 6. As shown in Figure 2, each cell 2 comprises a low-power inverting amplifier 13 of gain A, in turn presenting an input 16 at input voltage V i , and an output 17 at output voltage V o and defining the output of cell 2; two armatures 23, 24 of equal area, which are positioned facing the skin surface 18 of the finger being printed; a reset switch 19 connected between the input 16 and output 17 of inverting amplifier 13; and an input capacitor 20 connected between input 21 of cell 2 and input 16 of inverting amplifier 13. More specifically, armatures 23 and 24 are respectively connected to output 17 and input 16 of inverting amplifier 13, and are covered with a dielectric layer 25 covering the face of integrated device 1 at array 3 of cells 2. In use, therefore, skin surface 18 forms a second armature facing armatures 23, 24, and defining with them a pair of series capacitors feedback connected between input 16 and output 17 of inverting amplifier 13, so that a contact grid is not required to bias the skin surface at constant voltage. Switch 19 is a controlled switch formed using any known technology (e.g. a MOS switch) and receives a control signal R from supply and logic stage 7. Input 21 of the cell is also connected to supply and logic stage 7 to receive a voltage signal ΔV R as explained below. To acquire fingerprints, skin surface 18 is placed on the surface of integrated device 1, at array 3, to complete the pairs of capacitors forming the feedback loops of amplifiers 13 of all the cells. At the start of the measurement, switches 19 of all the cells are closed, and the voltage level at inputs 21 is constant, so that the input voltage V i of all the cells 2 is brought to the same potential V o as the output, between the supply and ground at a high-gain point of inverting amplifier 13. Subsequently, supply and logic stage 7 opens all the switches 19 in parallel, and supplies all the inputs 21 with a voltage step ΔV R , so that a charge variation ΔQ = C i *ΔV R (where C i is the capacitance of input capacitor 20) is induced at the terminals of input capacitors 20 to permit a reading, as explained below, of the local distance d between armatures 23, 24 and skin surface 18 facing them. Obviously, local distance d varies according to whether the point being measured corresponds to a groove, a ridge or a point between the two. Scanning stages 5, 6 then sequentially enable the reading of cells 2, so that the voltage signal at output 10 of buffer 8 is supplied to a system for representing the distance, in known manner, by means of grey levels, and so providing a three-dimensional display of the skin surface. The way in which local distance d between armatures 23, 24 of each cell 2 and the armature formed by skin surface 18 is detected will now be described with reference to the equivalent electric diagram in Figure 3. Figure 3 shows the equivalent input capacitance 30 and output capacitance 31 of inverting amplifier 13; the charge flow direction (indicated by the arrows) corresponding to the voltage variations at the armatures; and the capacitors 33, 34 formed by armatures 23, 24 and skin surface 18. Assuming C l is the equivalent input capacitance of inverting amplifier 13 (capacitance of capacitor 30); C r is the total capacitance of series capacitors 33 and 34; A is the gain of inverting amplifier 13; ΔQ is the charge variation induced in capacitor 30 by voltage step ΔV R ; ΔQ i is the charge variation stored in equivalent input capacitance 30 as a result of step ΔV R ; ΔQ r is the charge variation in the feedback branch formed by the series connection of capacitors 33, 34; ΔV i is the voltage step at input 16 of inverting amplifier 13; ΔV o is the corresponding voltage variation at output 17 (equal to -A ΔV i ); S is the surface of each armature 23, 24 of capacitors 33, 34; ε o is the electric constant (seeing as, in the application in question, the average distance between the skin and insulating layer 25 - typically 60 µm at the grooves - is greater than the thickness of layer 25 - typically 2 µm) ; and d is the local distance between armatures 23, 24 and skin surface 18 (approximately the same for both armatures 23, 24, in view of the very small size of cells 2 - typically about 45 µm); then total feedback capacitance C r is given by the following equation: C r = Sε 0 2 d Moreover: ΔQ = ΔQ i + ΔQ r = C l ΔV i + C r (ΔV i - ΔV o )= = - Δ V o A ( C l + C r )-Δ V o C r so that: Δ V o = - Δ Q C l A + (1 + 1 A ) C r Substituting (1) in (2) gives: Δ V o = - Δ Q C l A + (1 + 1 A ) Sε 0 2 d = - 2Δ Qd 2C l d A + (1 + 1 A ) Sε 0 Assuming A>>1, (3) becomes: Δ V o = d 2Δ Q Sε 0 Consequently, by virtue of the negative feedback effected by capacitive coupling the output and input of inverting amplifier 13 via the skin tissue, the variation in output voltage as a result of the charge step is directly proportional to the distance between armatures 23, 24 and the skin surface, in turn dependent on the three-dimensional structure of the skin. With appropriate amplification levels (e.g. 1000-2000), it is possible to detect differences in capacitance of about ten fF and hence micrometric distances. The output signals of the device according to the invention are therefore such, when converted into grey levels, as to provide a highly reliable representation of the three-dimensional structure of the skin surface. The advantages of the device according to the invention are as follows. In particular, as stated, it provides for a high degree of precision with no need for complex processing of the output signal; may be produced easily and integrated using current microelectronic technology; and is highly reliable, compact, and cheap to produce. The device according to the invention may also be used to advantage in other applications requiring precise detection of small distances. Moreover, the simple design of each cell enables a large number of cells to be accommodated in array structures for detecting two-dimensional physical quantities. Clearly, changes may be made to the device as described and illustrated herein without, however, departing from the scope of the present invention. In particular, if fabrication techniques enabling the formation of elastic structures (micromachining techniques) are available, the electrode whose distance is being measured may be connected directly to the input or output of inverting amplifier 13 to eliminate one of armatures 23, 24. Moreover, all the components may be replaced by technical equivalents. For example, though an inverter such as inverting amplifier 13 is currently preferred for design and layout reasons, amplifier 13 may be implemented by any inverting or differential amplifier (e.g. an operational amplifier) in a charge amplifier configuration to increase the speed of the output signal.

Claims: A capacitive distance sensor device (1), characterized in that it comprises an array (3) of sensor cells (2), each cell (2) comprising an inverting amplifier (13); a first capacitor plate (24) connected to the input of said inverting amplifier (13); and a second capacitor plate (23) connected to the output of said amplifier (13), said first and second capacitor plates (24, 23) being arranged adjacent to one another and, in use, being positioned facing an armature (18) the distance (d) whereof from said first and second capacitor plates (24, 23) is to be measured, said first and second capacitor plates (24, 23) forming, together with said armature (18), a first and a second capacitive element (33, 34) connected in series to each other and forming a negative feedback branch for said inverting amplifier (13). A sensor device according to claim 1, characterized in that said first and second capacitor plates (24, 23) are coplanar. A sensor device according to claim 1 or 2, characterized in that it is formed as an integrated chip. A sensor device as claimed in any one of the foregoing claims, characterized by a layer of insulating material (25) contacting said capacitor plates (23,24). A sensor device as claimed in any one of the foregoing Claims, characterized by logic means (7, 20) connected to said input (16) of said amplifier (13) for generating an electric charge variation; and output detecting means (8) for detecting a voltage step at said output (17) of said amplifier. A sensor device as claimed in Claim 5, characterized in that said logic means comprises a reference voltage source (12) for generating a voltage step; and a capacitive element (20) interposed between said voltage source and said input (16) of said amplifier (13). A sensor device as claimed in any one of the foregoing Claims, characterized by a switching element (19) connected between said input (16) and said output (17) of said amplifier (13). A sensor device according to any of the preceding claims, wherein the array (3) of sensor cells (2) is connected to input enabling means (5, 6) and to output lines said enabling means comprising horizontal scanning means (5) and vertical scanning means (6) ; and said output lines being connected to an output buffer element (8). A sensor device as claimed in Claims 6 and 8, characterized in that said voltage source means (12) comprises means for generating a reference voltage step supplied in parallel to said sensor cells (2); and in that said horizontal and vertical scanning means (5, 6) comprises means for sequentially enabling said sensor cells (2). A method of detecting the distance between a capacitive sensor device (1) and an armature (18) according to any of the preceding claims, characterized by the steps of: placing the armature (18) on the surface of the sensor device (1) to form, together said first and second capacitor plates (24, 23), a negative feedback branch for said inverting amplifier (13) including a first and a second capacitive element (33, 34) connected in series to each other; applying an electric charge variation to an input (16) of said inverting amplifier (13); and detecting a voltage step at the output (17) of said inverting amplifier (13), said voltage step being directly proportional to the distance to be detected. A method as claimed in Claim 10, characterized in that said step of applying an electric charge variation comprises the steps of: applying a first reference voltage to a capacitive element (20), and then applying, with a sharp variation, a second reference voltage other than said first reference voltage. A method as claimed in Claim 11, characterized in that said step of applying an electric charge variation is preceded by an initializing step comprising the steps of closing a reset switch (19) connected between the input (16) and output (17) of said amplifier (13), and then opening said reset switch.

Inventors: TARTAGNI MARCO, TARTAGNI, MARCO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1996
Main classification: G01B7
IPC codes: G01B7, G06T1, G06K9, G06T7