WO2010043391A2

WO2010043391A2 - Capacitive null-mode displacement measurement technology

Info

Publication number: WO2010043391A2
Application number: PCT/EP2009/007386
Authority: WO
Inventors: Jose A Carrasco; Francisco Garcia De Quiros
Original assignee: Embedded Instruments And Systems Sl
Priority date: 2008-10-14
Filing date: 2009-01-12
Publication date: 2010-04-22
Also published as: WO2010043391A4; WO2010043391A3; NL1036070C2

Abstract

Technique for sensing the absolute position of an object, in linear or rotary displacement, relative to a stationary object or the speed of displacement, is formed, in one embodiment, by two stationary elements and a moving element able to have a linear or rotary displacement when located between the stationary elements. The stationary elements have metallic coatings on their confronted sides to form eight capacitances whose value change when the moving element is displaced between them. Four pairs of sinusoidal waveforms are injected in the metallic tracks of one stationary element by the processing electronics whose amplitude is controlled to cancel each other in the metallic tracks of the confronted stationary element. The generation of the four pairs of sinusoidal waveforms occurs in a close loop that generates coarse and fine information of the position of the moving object. Such information is offered to a user as analog or digital.

Description

Capacitive null-mode displacement measurement technology

FIELD OF THE INVENTION

The present invention relates generally to position sensors and specifically sensors that use capacitive coupling techniques to measure angular or linear displacement. These sensors are widely used in industry, vehicles and spacecrafts.

BACKGROUND OF THE INVENTION

Measurement of linear or angular displacement is of great interest for several technology fields for the sensing or control of acceleration, velocities or position. Present applications that need extreme endurance for systems and parts are demanding contact-less sensors with low sensitivity to environmental conditions and robust mechanics. Among all technologies for contact-less displacement sensors capacitive techniques are among the most promising for the future.

Capacitive sensors have not been widely used until present because of the low noise electronics needed for signal conditioning since it is necessary to detect tiny variations of capacitance, in the order of the Pico Farads, to attract the interest of users that demand accuracies in the order of tens arc-seconds. However, the simplicity of their manufacturing — they are implemented using standard printed circuit board technology, the insensitiveness to the environment and the advances in low noise and power electronic parts are attracting the interest in this technology.

References to the application of electric coupling for contact- less displacement sensors date back to 1969, where a report from Litton Systems Inc. entitled Development of a Miniature Capacitive Resolver, discloses a device as small as two stacked one Euro coins and able to provide arc-minutes resolution. This type of sensor mimics the behaviour of the very well known resolvers that by making use of magnetic coupling have been widely used in all kind of applications as described in Synchro/Resolver Convertion Handbook, Data Device Corporation, 4th edition, 1994. Capacitive coupled displacement sensors are introduced because they are more simple than resolvers and are made out of real contact-less moving elements.

Following the mentioned 1969 work, we may find in the technical literature several ideas that by using clever combinations of metallic coating shapes in two confronted plates produce resolver outputs after proper amplification and conditioning. U.S. Patent 4,092,579 discloses a device that provides resolver outputs by using the capacitance variation of two sets of non-moving metallic shapes when a dielectric material is rotated in between.

Another example is disclosed in U.S. Patent 4,429,307: two stationary plates excited by two AC voltages are confronted with a moving plate with two conductive areas separated by a sine wave shaped gap. These two conductive areas produce signals whose amplitude is proportional to the sine and cosine of the rotation angle as the stationary plates are excited with two AC signals in phase opposition. The voltages induced in the moving plates are amplified and demodulated to provide resolver outputs. And further, U.S. Patent 6,211,641, presents a contact- less sensor with resolver outputs for angular velocity measurements in space applications.

The use of resolver outputs has the advantage of using the technology already developed for resolvers for this kind of sensors signal conditioning. However, there are other capacitive sensors not providing resolver outputs, which also claim to provide better performances than resolver type sensors. US Patent 6,492,911 describes a contact-less displacement capacitive technology currently under manufacture by Netzer Precision, an Israel based company. Netzer sensor provides a proprietary output that unlike resolver sensors, which produces the modulation of one carrier sinusoidal voltage, provides two different carrier signals proportional to the sinus and cosinus of a reference angular frequency. Netzer technology needs, apparently, proprietary or custom electronics for the conditioning of the sensor. Further, Netzer technology, as well as others, may also be used to measure linear displacements.

US Patent 4,851,835 discloses an idea that combines a resolver type electric coupling sensor for coarse measurement and an electric coupling sensor for fine measurement. The disclosure also describes the full conditioning electronics to provide digital output by generating signals at the fine electric coupling measurement side to calculate the difference between the mechanical and electronic angles of an intervening rotor.

In addition, there are many implementations of capacitive displacement sensors that output proprietary signals that depend on the physical characteristics of the sensor and therefore will need a particular realisation of their conditioning electronics. As practical examples:

US Patent 3,732,553, discloses an electric coupling based contact-less angular sensor that provides one sinusoidal output whose amplitude depends on the angular position of a metallic (conductive) plate between an emitter and a receiver plate electrically coupled. The conditioning electronics for this device is quite simple but only works in the 0 to 180° range. This conditioning circuit has latter been used in many capacitive displacement encoder patents as first stage conditioner, including US Patent 6,492,911. Moreover, the basic ideas of two printed circuit board plates with a metallic rotor in between are present in later patents,

US Patent 5,598,153 presents an invention of a capacitive displacement sensor that provides two AC signals whose outputs are modulated linearly rather than sinusoidally as in the Resolver sensors. This patent seems to evolve the idea presented in US Patent 3,845,377 and US Patent 4,404,560 extending it to a coarse and fine set of signals from which accurate absolute angular positioning may be extracted, For a modern example of a capacitive displacement sensor based on yet another topological variation of the confronted fixed printed circuit board tracks on two confronted printed circuit boards with an intermediate moving plate and clever conditioning, very similar to US Patent 6,211,641 already mentioned, but for angular position measurement instead of velocity we may see US Patent 5,099,386. No example waveforms are given in the patent description for this disclosure; it is only mentioned and described the capacitance variation that the plates produce with no reference to electronics conditioning. We presume, after seeing the details in the description of physical implementation of this capacitive angular displacement sensor, that it is being used by the applicant company, General Scanning Inc., for its production line of optical scanners,

After year 2000, digital techniques are generally introduced together with electric coupling, and contact-less angular sensors incorporate confronted plates with tracks that directly produce digital ouputs or close to digital outputs. As examples of this we may review US Patent 6,483,321, US Patent 6,170,162 and US Patent 7,123,027.

If we consider the operation mode, a reference in sensor studies as Sensors and Signal Conditioning, Ramon Pallas-Areny and John G. Webster, Wiley-Interscience, 2nd edition (November 6, 2000), ISBN- 13 978-0471332329, classifies them as functioning in deflection or null-mode. Deflection sensors produce an output proportional to the physical variable under measurement. All the capacitive sensors in the technological review presented herein belong to this kind of classification. Null-mode sensors create an effect that opposes to the quantity under measurement until a balance is obtained. This kind of behaviour has not been yet found in any of the different capacitive displacement implementations based on electric coupling in our technology search.

The solution we propose for a capacitive sensor belongs to the kind of null-mode sensors in what is the disclosure of this idea to measure angular or linear displacement.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive embodiments of the present invention are described making reference to the drawings listed. Unless otherwise specified, the numerals in the drawings refer to parts of the embodiment through the various figures.

For a better understanding of the present invention, further reference will be made in the Detailed Description of the Invention hereafter, which is to be read in association with the accompanying drawings herein:

FIG. 1 is a system diagram of a possible embodiment of the invention including the realisation of an angular displacement sensor and the block diagram of the electrical circuitry, FIG. 2 shows a front view in perspective of a possible embodiment of angular displacement sensor,

FIG. 3 shows a rear view in perspective of a possible embodiment of angular displacement sensor,

FIG. 4 shows a floor plan of a possible embodiment of the mechanical pieces of an angular displacement sensor,

FIG. 5 shows a block diagram of the computational logic needed for the implementation of the displacement sensor based in microcontroller or Field Programmable Gate Array,

FIG. 6 shows the amplitude of exciting sinusoidal AC voltages that produce voltage cancellation at the top plate metallic coating tracks,

FIG. 7 shows a block diagram of the electrical circuitry of a possible embodiment of the computational logic based in discrete analog and digital electronics,

FIG. 8 shows a front view in perspective of a possible embodiment of linear displacement sensor,

FIG. 9 shows a rear view in perspective of a possible embodiment of linear displacement sensor.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described herein in more detail with reference to the accompanying drawings, which form a part hereof, and which show, by way of illustration, specific exemplary embodiments by which the invention may be practiced. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set so forth herein; rather, this embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Among other things, the present invention may be embodied as methods and devices. Accordingly, the present invention may take the form of a combination of hardware and software embodiments combining software and hardware aspects.

Through the specification and claims, the following terms take the meaning explicitly associated herein, unless the context clearly dictates otherwise. The phrase "in one embodiment" as used herein does not necessarily refer to the same embodiment, though it may.

In FIG. 1, is presented a possible embodiment of an angular displacement sensor based on this technology, in which a computational electronics 10 generates two variable in time digital outputs, 11 and

12, that are converted into two sinusoidal AC waveforms, 15 and 16, whose amplitude is proportional to that digital outputs, by the blocks 13 and 14. These two, 15 and 16, AC voltages are inverted by two amplifiers with unitary gain, 17 and 18, and the four AC voltages 15, 16, 19 and 20 are connected to four electrodes in a bottom stator 1.

In FIG. 2, the four electrodes, 50, in stator 1 , are electrically coupled to one electrode, 61 in FIG. 3, in a top stator 3 and form four capacitances with it. The value of these capacitances is modulated by a rotor 2 that freely rotates between the stators, 1 and 2, attached to a shaft.

An amplifier 21, in FIG. 1 , takes the signal 22 from the electrode in the top stator into a coherence demodulator 23 and a comparator 24 whose output 25 is connected to the computational logic 10. From the output of this comparator the computational logic is able to change the digital words 11 and 12 in a way that the measured voltage 22 is kept to zero.

The complete circuit forms a close voltage loop that produces a null-mode measurement of the capacitances between the electrodes 50, in stator 1, and 61, in stator 3. Since these capacitances are modulated by the angular position of rotor 2 and depend linearly with it, the digital signals 1 1 and 12, or the amplitude of the sinusoidal AC signals 15 and 16, are proportional to the angular displacement of rotor 2. These signals may be used to output the absolute position of rotor 2. In a preferred embodiment of this invention a digital output would be preferred and offered by the computational logic as 26.

In a preferred embodiment of the system the electronics circuitry described is repeated attached to a second set of electrodes, 51 in FIG. 2, at the bottom stator 1 coupled to a second electrode, 62 in FIG. 3, at top stator 3 in order to provide the system with a high accuracy for the detection of rotor 2 absolute angular position.

A second computational logic 30 generates two variable in time digital outputs, 31 and 32, that are converted into two sinusoidal AC waveforms, 35 and 36, whose amplitude is proportional to that digital outputs, by the blocks 33 and 34. These two, 35 and 36, AC voltages are inverted by two amplifiers with unitary gain, 37 and 38, and the four AC voltages 35, 36, 39 and 40 are connected to a set of sixty four electrodes on the bottom stator 1. These sixty-four electrodes, 51 in FIG. 2, in stator 1 are electrically coupled to one electrode, 62 in FIG. 3, in the top stator 3 and form four capacitances with it. The value of these capacitances is modulated by the rotor 2. The amplifier 41 takes the signal 42 from the electrode, 62, in the top stator into a coherence demodulator 43 and a comparator 44 whose output 45 is connected to the computational logic 30. From the output of this comparator the computational logic is able to change the digital words 31 and 32 in a way that the measured voltage 42 is kept to zero. The computational logic produces one output 46 that provides digital information on the amplitudes of signals 31 and 32 and is connected to the computational logic 10.

The complete system behaves as having a coarse absolute angular measurement loop controlled by the computational logic 10 and a fine absolute angular measurement loop controlled by the computational logic 30. From output 46 and the generated words 11 and 12, the computational logic 10 is able to construct a digital word 26 that gives information on the absolute angular position of the rotor 2 with arc- minute precision.

FIG. 2 shows a front view in perspective of a possible embodiment of an angular displacement sensor based on this technology. Bottom stator 1 is preferably implemented by using printed circuit board techniques; a set of tracks for coarse position measurement 50 and fine position measurement 51 are located in its inner side. The rotor 2, made out preferably of a dielectric material, is located between the top rotor 3 and the bottom one. The rotor has several holes on it: one hole 52 over one of the tracks for coarse measurement and one hole for every four holes 53 over the fine position measurement tracks.

FIG. 3 shows a rear view in perspective of a possible embodiment of an angular displacement sensor based on this technology. The top stator 3 is preferably implemented by using printed circuit board techniques. It has two ring tracks in its inner side: one track for coarse position measurement 61 is located over the coarse position measurement tracks of the bottom stator 50, in FIG. 2, and one fine position measurement track 62 is located over the fine position measurement tracks of the bottom stator 51 , in FIG. 2.

FIG. 4 presents a floor plan view of a possible embodiment of the mechanical pieces of an angular displacement sensor based on this technology: the bottom stator 1 inner face, the rotor 2 and the top stator 3 inner face view. Under a possible implementation the bottom stator has a fist set of four metallic coating tracks 71 for coarse measurement that form four capacitances with the inner metallic coating ring on the top stator 61. Also, under a possible implementation, the bottom stator has a second set of sixty four metallic coating tracks 72 — or any other number multiple of four, that form four capacitances with the outer metallic coating ring on the top stator 62 when electrically connected as indicated by the discontinuous lines 73 and the dots 74. Each discontinuous line indicates a connection track, placed at the bottom stator outer face that connects the tracks at the inner face by means of electrical bias, which perforate the substrate and are represented by the dots.

In this possible implementation on the invention lines 15, 16, 19 and 20 in FIG. 1 are connected to tracks 75, 76, 77 and 78 in FIG. 4 and Lines 35, 36, 39 and 40 in FIG. 1 are connected to tracks 79, 80, 81 and 82 in FIG. 4.

In this implementation of the invention line 22 in FIG. 1 is connected to track 61 in FIG. 4 — which is the same connection as 61 in FIG. 3, and line 42 in FIG. 1 is connected to track 62 in FIG. 4 — which is the same connection as 62 in FIG. 3.

FIG. 5 presents a possible embodiment of the computational logic needed for the implementation of an angular displacement sensor based on this technology. The implementation is presented as part of the coarse electronics of the embodiment in FIG. 1 ; the fine electronics of the embodiment uses a similar electronics. Line 25 is the output of a logical comparator that inputs the computational logic 10 through a digital port 91 , in FIG. 5, giving information on the magnitude and phase of the AC voltage presented at line 22 — as rectified by the coherence demodulator 23 — which is intended to be zero. The computational logic may be implemented by an electronics microcontroller or Field Gate Programmable Array and contains a program that calculates two digital outputs, 11 and 12, that after being converted into two AC voltages by two multiplying four quadrant analog-to-digital-converters, 13 and 14, fed by a sinusoidal voltage 27 as reference, result in two AC signals 15 and 16 whose amplitude depends on the absolute angular position of the rotor 2 in FIG. 1 as given in FIG. 6. The microcontroller also drives the coherence demodulator 23 either through a digital or analog port 92 and the signal inverters 28 and 29, which invert or leave non-inverted the output of the analog-to-digital-converters, 13 and 14 at the command of the controller.

In FIG. 5 the digital-to-analog converters may be implemented by an Analog Devices DAC7541 and the multipliers by xl or x(-l) 28 and 29 by a controlled switch, such as Analog Devices AD7592, and an operational amplifier as described in Analog Devices DAC7541 Rev. B datasheet. The rest of the components including the coherence demodulator (see Analog Signal Processing by Ramon Pallas- Areny, ISBN ISBN-IO: 0471125288, for a practical implementation of the coherence demodulator using operational amplifiers and switches) may be implemented with usual operational amplifiers, switches and comparators.

FIG. 6.A curves 100 and 101 represent the dependence of the AC amplitude of signals 15 and 16 in FIG. 1 and FIG. 5, in the ordinates axis 102, versus the absolute angular position of the rotor of the displacement sensor in the abscissas axis 103: The amplitude of AC signal 15 is represented by 101 and the amplitude for AC signal 16 by 100 (a negative amplitude means a phase inversion and Vmax is the amplitude of the sinusoidal voltage used as reference for the multiplying digital-to-analog converters). This dependence is presented in the coarse measurement part of the sensor. The fine measurement part of the sensor, controlled by the computational logic 30 in FIG. 1, has the same dependence amplitude vs. angle but with an angular period given by the number of metallic tracks, sixty four in our particular embodiment, over four, giving to an angular period on twenty two degrees and a half in this particular embodiment. FIG. 6.B represents the dependence of the AC amplitude of signals 35 and 36 in FIG. 1 versus the absolute angular position of the rotor of the displacement sensor in the abscissas axis: The amplitude of AC signal 35 is represented by 104 and the amplitude for AC signal 36 by 105 (a negative amplitude means a phase inversion).

FIG. 7 presents a second possible embodiment of the computational logic 10 in FIG. 1 or FIG. 5 implemented with discrete electronic blocks. Such an implementation is preferred when an application makes difficult the use of microcontrollers or FPGAs. In this possible implementation of the invented technology line 25 is digital (it is the output from a comparison with zero) and drives a 2-bit Johnson up/down counter 1 16. The output of this counter drives the up/down input of a 12-bit counter 117, the clock signal of which is generated from the circuit sinusoidal generator 27 and a squarer 115. The outputs of the 12-bit counter, 109, drive one multiplying digital-to-analog-converters directly, 112, and the other one, 113, after a logic inversion circuit, 110. The multiplying digital-to-analog-converters multiply the sinusoidal reference 1 14 by its digital input and provide signals that may be further inverted or not inverted (multiplied by xl or x(-l)) using analog switches, 118 and 119, controlled by each bit of the outputs of the 2-bit Johnson counter. The final output of this circuit produces four AC sinusoidal voltajes, 15, 16, 19 and 20 that cancel the voltage measured at line 22 by having an amplitude dependence with the angle as described by FIG. 6A for the coarse angle measurement. This schema may be repeated for the fine angle measurement.

In FIG. 7 the digital-to-analog converters may be implemented by an Analog Devices DAC7541 and the multipliers by xl or x(-l) 118 and 119 by a controlled switch, such as Analog Devices AD7592, and an operational amplifier as described in Analog Devices DAC7541 Rev. B datasheet. The logic components are based on logic circuitry available commercially: CD4030 for XOR gates, CD4013 for D-type latches, CD4069 for NOT (and inverters) and three CD4029 for a 12-bit up/down counter. The implementation of a 2-bit Johnson counter is an exercise of basic digital electronic courses by using 2 CD4013 latches and 2 CD4030 XOR gates (although it may be implemented using an CD4017) . The rest of the components including the coherence demodulator (See Analog Signal Processing by Ramon Pallas-Areny, ISBN ISBN-IO: 0471125288, for a practical implementation of the coherence demodulator using operational amplifiers and switches) may be implemented with usual operational amplifiers, analog switches and comparators.

FIG. 8 shows a front view in perspective of a possible embodiment of the technology in a sensor for absolute linear displacement measurements. In a possible embodiment of the invention 120 and 121 are two non-moving pieces and 122 is a moving piece placed between 120 and 121. This pieces work in the same manner and are built using the same techniques that the pieces 1, 2 and 3 in FIG. 2. Further, plates 123, 124, 125 and 126 in FIG. 8, indented for coarse position measurement, are equivalent to plates 75, 76, 77 and 78 in FIG. 4; and plates 126 in FIG. 8, intended for fine position measurement, are equivalent to plates 72 in FIG. 5 and are connected following the same pattern so to add up in four different capacitors. The moving plate 122, made out of a dielectric material, has wholes on it 127 and 128, over the plate 123 for coarse capacitance variation and over the plates 126 for fine capacitance variation that result in a coarse position measurement part and a fine measurement part.

FIG. 9 shows the rear view of a possible implementation of an absolute linear displacement sensor based on the technology being disclosed. Plates 120, 121 and 122 have the same meaning as in FIG. 8. Tracks 130 and 131 on plate 122 are respectively located over the coarse and fine tracks on plate 120 and its functionality is the same as tracks 61 and 62 on FIG. 3. The conditioning electronics for the absolute linear displacement sensor is the same as the one for absolute angular displacement

Claims

CLAIMSWhat is claimed as new and asked to be protected is:

1. A capacitive null-mode displacement measurement technology for absolute sensing and control of angular or linear displacements or velocities of objects formed by two opposed plates defining a space in between where a moving plate is positioned attached to the object to control, the opposed plates presenting multiple coatings on the confronted sides over an insulating material forming capacitors, the capacitance of which varies with the position of the moving plate, the moving plate formed by a dielectric material with holes that cover or uncover the metallic coatings, the capacitances of the metallic coatings over one of the static plates are added on the confronted coating plate so as a set of injected voltage signals on the former are added or subtracted to a null value on the confronted one.

2. A capacitive null-mode displacement measurement technology according to claim 1 , characterised by presenting one, two or more sets of metallic coatings, the number of elements on each set multiple of four, on one static plate confronted each to a metallic coating strip on the other static plate, the number of sets defining the accuracy in which the displacement or velocity is measured.

3. A capacitive null-mode displacement measurement technology according to claim 1, wherein a coarse displacement measurement is performed by the capacitance formed by one set of metallic coatings and its confronted metallic coated strip and a fine displacement measurement is performed by another set of metallic coatings and its confronted metallic coated string, the dependence in capacitance with the moving object being produced by two sets of holes in the moving plate one for coarse measurement located over the coarse metallic coatings and other for fine measurement located over the fine metallic coatings.

4. A capacitive null-mode displacement measurement technology according to claim 1, that makes equal to 0 Volts the voltage on each metallic strip of one of the static plates by injecting appropriate AC voltages in each set of multiple of four metallic coatings on the confronted static plate with independence of the position of the moving plate, the amplitude of the AC signals injected being proportional to the position of the moving plate.

5. A capacitive null-mode displacement measurement technology according to claim 4, that implements an electronic circuitry that continuously calculates and generates the amplitudes of the AC voltages to be injected in each set of metallic coatings on one static plate and measures and amplifies the voltage at each metal coating strip of the confronted static plate to exactly null it either if the moving plate is moving or stationary.

6. A capacitive null-mode displacement measurement technology according to claim 1 , that implements an electronic circuitry according to claim 4, that offers a digital output proportional to the position of the moving plate from the calculations performed and the AC signals generated and injected at every set of multiple of four metallic coatings on one static plate that null the voltage at every metallic coating strip at the confronted static plate.

7. A capacitive null-mode displacement measurement technology, according to claim 1 , wherein the static plates are produced from an insulating material and the metallic coatings from a conductive material and connected among them with conductive tracks.

8. A capacitive null-mode displacement measurement technology according to claim 1 , wherein the electronic circuitry needed to provide the output is implemented over one of the static plates following usual printed circuit board technology.

9. A capacitive null-mode displacement measurement technology according to claim 1 , wherein the moving plate is produced from a dielectric material with a high dielectric constant.

10. A capacitive null-mode displacement measurement technology for absolute sensing and control of angular displacements or velocities of objects formed by two opposed circular stator plates defining a space in between where a circular rotor plate is positioned attached to the object to control, the opposed plates presenting one or more coatings on the confronted sides arranged as to form concentric rings over an insulating material forming capacitors, the capacitance of which varies with the position of the moving plate, the moving plate formed by a circular dielectric material with holes that cover or uncover the metallic coatings, characterized in that, the coatings are so arranged on one of the stators as to form concentric rings with a number of metallic coatings of four or multiple of four, the number of coatings multiple of four are interconnected to form four metallic sets, the metallic coatings to form four capacitances with a metallic coated ring on the confronted stator, the rotor plate produced from dielectric material and having concentric rings of holes under the metal coating rings, the number of holes being equal to the number of metallic coatings on the stator divided by four, the capacitances of the metallic coatings over one of the stators are added on the confronted coating plate so as a set of injected voltage signals on the former are added or subtracted to a null value on the confronted one, the null of the voltage performed by an electronic conditioning and computation logic.

11. A capacitive null-mode displacement measurement technology for absolute sensing and control of linear displacements or velocities of objects formed by two opposed static plates defining a space in between where a moving plate is positioned attached to the object to control or measure, the opposed plates presenting one or more coatings on a linear position on the confronted sides over an insulating material forming capacitors, the capacitance of which varies with the position of the moving plate, the moving plate formed by a dielectric material with holes that cover or uncover the metallic coatings, characterized in that, the coatings are so arranged on one of the moving plates as to form linear dispositions with a number of metallic coatings of four or multiple of four, the number of coatings multiple of four are interconnected to form four metallic sets, the metallic coatings to form four capacitances with one linear metallic coating on the confronted plate, the moving plate produced from dielectric material and having concentric rings of holes under the metal coating rings, the number of holes being equal to the number of metallic coatings on the static plate divided by four, the capacitances of the metallic coatings over one of the static plates are added on the confronted coating plate so as a set of injected voltage signals on the former are added or subtracted to a null value on the confronted one, the null of the voltage performed by an electronic conditioning and computation logic.