Capacitive device for detecting an electrically floating object

Abstract

A device for detecting an object with respect to a detection surface including: a measurement electrode; at least one guard electrode, at the same alternating potential as the measurement electrode; at least one module for measuring a first signal with respect to the capacitance (C.sub.eo), called electrode-object capacitance, formed between said measurement electrode and said object; and
at least one electrode, called polarization electrode, placed opposite the object, and polarized at the ground potential (G) so as to polarize the object by capacitive coupling.

Claims

1. A device for detecting an object with respect to a detection surface, said device comprising: at least one measurement electrode; at least one guard electrode, placed opposite to said at least one measurement electrode, said at least one measurement electrode and said at least one guard electrode being polarized at an alternating guard potential (V.sub.g) different from a ground potential (G), and the at least one measurement electrode polarized at either the alternating guard potential or alternating potentials that are identical at at least one given working frequency as the alternating guard potential, and at least one module for measuring a first signal with respect to an electrode-object capacitance (C.sub.eo) formed between said measurement electrode and the object; and at least one polarization electrode placed opposite the object, and polarized at the ground potential (G) so as to electrically polarize the object by capacitive coupling, wherein the at least one polarization electrode is placed: at the same level as the at least one guard electrode, or at the same level as the at least one measurement electrode.

2. The device according to claim 1, wherein said at least one polarization electrode is placed opposite, or at the level of, the detection surface.

3. The device according to claim 1, wherein the at least one guard electrode is larger than the at least one measurement electrode.

4. The device according to claim 1, further comprising a plurality of the measurement electrodes and the guard electrodes, wherein the at least one polarization electrode is placed between two adjacent measurement electrodes, and/or between two adjacent guard electrodes.

5. The device according to claim 1, further comprising an individual polarization electrode of the at least one polarization electrode for each measurement electrode of the at least one measurement electrode.

6. The device according to claim 1, further comprising a common polarization electrode of the at least one polarization electrode forming a ground plane for at least two measurement electrodes of the at least one measurement electrode.

7. The device according to claim 1, wherein a guard electrode of the at least one guard electrode is placed in a hole of a polarization electrode of the at least one polarization electrode.

8. The device according to claim 1, further comprising at least one calculation module arranged to calculate: the electrode-object capacitance (C.sub.eo), formed between the at least one measurement electrode and the object as a function of the first signal; and/or the distance, or a contact, between the object and the detection surface as a function of said first signal or of said electrode-object capacitance (C.sub.eo).

9. The device according to claim 1, wherein the at least one measurement electrode is separated from the at least one guard electrode by a distance that is elastically modifiable by a load or a force exerted by the object on said detection surface.

10. The device according to claim 9, wherein the at least one measurement electrode is separated from the at least one polarization electrode and from the at least one guard electrode, by a layer that is elastically compressible comprising a dielectric material.

11. The device according to claim 9, where the at least one measurement module is arranged to measure a second signal with respect to an inter-electrode capacitance (C.sub.ie), formed between the at least one measurement electrode and the at least one guard electrode, said at least one guard electrode being polarized at a potential different from the alternating potential of the at least one measurement electrode.

12. The device according to claim 11, further comprising at least one calculation module arranged to calculate: the inter-electrode capacitance (C.sub.ie), formed as a function of the second signal; or the load or the force applied by the object on the detection surface as a function of said second signal, or of said inter-electrode capacitance (C.sub.ie).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics will become apparent on examination of the detailed description of non-limitative examples and from the attached drawings in which:

(2) FIG. 1 is a diagrammatic representation of the electrical principle of a first non-limitative embodiment example of a device according to the invention;

(3) FIG. 2 is a diagrammatic representation of the electrical principle of a second non-limitative embodiment example of a device according to the invention;

(4) FIGS. 3a-3b are diagrammatic representations of the electrical principle of a third non-limitative embodiment example of a device according to the invention; and

(5) FIGS. 4-10 are diagrammatic representations of different configurations of electrodes that can be implemented in a device according to the invention.

DETAILED DESCRIPTION

(6) It is well understood that the embodiments that will be described hereinafter are in no way limitative. In particular, variants of the invention may be envisaged comprising only a selection of characteristics described hereinafter, in isolation from the other characteristics described, if this selection of characteristics is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art. This selection comprises at least one, preferably functional, characteristic without structural details, or with only a part of the structural details if this part alone is sufficient to confer a technical advantage or to differentiate the invention with respect to the state of the prior art.

(7) In particular, all the variants and all the embodiments described may be combined together if there is no objection to such combination from a technical point of view.

(8) In the figures, elements that are common to several figures retain the same reference.

(9) FIG. 1 is a diagrammatic representation of the electrical principle of a first non-limitative embodiment example of a detection device according to the invention.

(10) The device 100, shown diagrammatically in FIG. 1, is intended to detect the approach and the contact of a control object 102 with respect to a detection surface 104.

(11) The device 100 comprises at least an electrode 106, called measurement electrode, placed facing the detection surface 104, and therefore the object 102 approaching said surface 104, and an electrode 108, called guard electrode, placed facing the measurement electrode 106, below and at a distance from the measurement electrode 106.

(12) The device 100 also comprises a charge amplifier formed by an operational amplifier (OA) 110, the output of which is looped on its negative input by an impedance 112, which can be a capacitor or a capacitor combined with a resistor. In the example shown, the impedance 112 is formed by a capacitor C.

(13) A digital or analogue module 114, connected to the output of the OA 110, measures a voltage, denoted V.sub.s, on the output of the OA 110 at a working frequency.

(14) The device 100 also comprises an electrical source E, called guard electrical source, supplying an alternating potential, called guard potential, denoted V.sub.g, different from an electrical ground, denoted G and having a frequency equal to the working frequency.

(15) In the example shown in FIG. 1: the measurement electrode 106 is connected to the negative input of the OA 110, and the guard electrode is connected to the positive input of the OA; and the electrical source E is connected to the positive input of the OA 110.

(16) In this configuration, the measurement electrode 106 and the guard electrode 108 are at the same (or substantially the same) alternating potential V.sub.g, supplied by the source E.

(17) Thus, the guard electrode 108 protects the measurement electrode 106 from the unwanted capacitive couplings with the environment and prevents the appearance of leakage capacitances.

(18) Moreover, the detection electronics and in particular the OA 110 are preferably supplied with power by an electric power supply referenced to the guard potential V.sub.g, in order to avoid leakage capacitances at the level of the electronics.

(19) Alternatively, of course, the detection electronics and in particular the OA 110 can be supplied by an electric power supply referenced to the ground potential.

(20) The voltage V.sub.s measured by the measurement module 114 is proportional to the capacitance C.sub.eo, called electrode-object capacitance, formed between the measurement electrode 106 and the object 102. In particular, the voltage V.sub.s measured verifies the following relationship:

(21) $\begin{array}{cc}{V}_s=E\frac{c_{eo}}{c}& \left(1\right)\end{array}$

(22) Thus, it is possible to deduce the capacitance C.sub.eo from the measured signal V.sub.s.

(23) Furthermore, the capacitance C.sub.eo is directly a function of the distance separating the object 102 from the measurement electrode 106, and thus from the detection surface 104. Consequently, it is possible to deduce the distance between the object 102 and the detection surface as a function of the value of the capacitance C.sub.eo, and more generally as a function of the signal V.sub.s.

(24) The device 100 also comprises another electrode 116, called polarization electrode. The polarization electrode 116 is connected to the electrical ground G and makes it possible to polarize the object 102 with the ground potential G, so as to retain the detection sensitivity and performance of the device 100, when the coupling between the object 102 and the ground potential G is degraded or cut off. Such a situation can take place when the object 102 is only in contact with the detection surface 104 or when the object 102 has no direct or indirect contact with earth, other than via the detection surface 104.

(25) In the example shown, the polarization electrode 116 is openwork and the guard electrode 108 is placed in the openwork part of the polarization electrode 116, with no contact with said polarization electrode 116. Thus, the polarization electrode 116 and the guard electrode 108 are placed at the same level.

(26) The polarization electrode 116 is larger than the guard electrode 108, itself larger than the measurement electrode 106.

(27) Moreover, the device 100 comprises a controllable switch 118 making it possible to connect the measurement electrode 106: either to the negative input of the OA 110: in this case the measurement electrode 106 is called “active” and makes it possible to measure a signal with respect to the capacitance C.sub.eo; or to the guard potential V.sub.g, present for example at the positive input of the OA: in this case, the measurement electrode 106 is connected to the same guard potential V.sub.g as the guard electrode 108 and becomes a guard electrode.

(28) Thus it is possible to poll sequentially a plurality of measurement electrodes 106, either individually or by group.

(29) FIG. 2 is a diagrammatic representation of the electrical principle of a second non-limitative embodiment example of a detection device according to the invention.

(30) The device 200, shown in FIG. 2, comprises all the elements of the device 100 in FIG. 1.

(31) In the device 200, unlike the device 100 in FIG. 1, the polarization electrode 116 is not openwork, and is placed below the guard electrode 108, seen from the measurement electrode 106.

(32) FIGS. 3a-3b are diagrammatic representations of the electrical principle of a third non-limitative embodiment example of a detection device according to the invention.

(33) The device 300, shown in FIGS. 3a-3b, is intended to detect the approach, the contact and also the pressure exerted by the control object 102 on the detection surface 104.

(34) The object 102 is on approach and at a distance from the detection surface 104 in FIG. 3a, exerting pressure on the detection surface 104 in FIG. 3b.

(35) The device 300 comprises all the elements of the device 100 in FIG. 1.

(36) Unlike in FIG. 1, the openwork polarization electrode 116 is placed at the same level as the measurement electrode 106. Such an architecture makes it possible to minimize the unwanted capacitances during the measurements, in particular in the case where the polarization electrode 116 is distanced from the measurement electrode 106 by a distance comparable to or greater than the separation between the measurement 106 and guard 108 electrodes. In this case the guard 108 plays the role of separation guard between the measurement electrode 106 and the polarization electrode 116, even if it is not on the same plane.

(37) Furthermore, in the device 300, the measurement 106, guard 108 and polarization 116 electrodes are placed in a layer 302 that is elastically compressible, filled with a dielectric, such as for example foam or plastic or also a liquid dielectric of the oil type, or also a gaseous dielectric of the air or nitrogen type.

(38) In particular, the measurement electrode 106 and the polarization electrode 116 are firmly fixed to the wall of the layer 302 on the side of the detection surface 104, and are for example placed on said wall.

(39) The guard electrode 108 is firmly fixed to the wall of the layer 302 on the side opposite the detection surface 104, and is for example placed on said wall.

(40) Thus, the distance between the measurement electrode 106 and the guard electrode 108 can be elastically modified, locally, by a load exerted by the control object 102 on the detection surface 104. In particular, when a load is applied on the detection surface 104, the measurement electrode 106 moves closer to the polarization electrode 116, as shown in FIG. 3b.

(41) The device 300 also comprises a controllable switch 304 making it possible to connect the guard electrode 108: either to the guard potential V.sub.g, present for example at the positive input of the OA 110: in this case, the guard electrode 108 is polarized at the guard potential V.sub.g; or to the electrical ground potential G.

(42) Under these conditions, the approach and the contact of the object 102 with the detection surface 104 are detected as a function of the value of the capacitance C.sub.eo formed between the measurement electrode 106 and the control object 102. The electrode-object capacitance C.sub.eo is determined as described above, with reference to FIG. 1 for example.

(43) The device 300 also makes it possible to measure the load exerted by the object 102 on the pressure surface 104 by measuring a signal, called second signal, representative of a capacitance C.sub.ie, called inter-electrode capacitance, formed between the measurement electrode 106 and the guard electrode 108. This inter-electrode capacitance C.sub.ie is determined in the following manner: the measurement electrode 106 remains connected to the negative input of the OA 110: in this configuration the measurement electrode 106 is active; and the guard electrode 108 is connected to the electrical ground potential G by means of the switch 304: in this configuration the guard electrode is polarized at the electrical ground.

(44) When the control object 102 comes into contact with the detection surface 104, or into immediate proximity with this surface (semi-contact) the capacitance C.sub.eo reaches a predetermined threshold value C.sub.s (or a range of threshold capacitances). At that moment, the controllable switch 304 is toggled so as to connect the guard electrode 108 to the ground potential G, as shown in FIG. 3b. In this configuration, a second signal V.sub.s is measured at the output of the OA 110. This second signal V.sub.s is representative of a capacitance C.sub.T, called total capacitance, seen by the measurement electrode 106, so that:
V.sub.s=V.sub.gC.sub.T/C  (2)

(45) This relationship (2) makes it possible to deduce the value of the total capacitance C.sub.T from the measured second signal V.sub.s.

(46) Now, the total capacitance C.sub.T corresponds to the sum of: the electrode-object capacitance C.sub.eo, and the inter-electrode capacitance C.sub.ie which appears between the measurement electrode 106 and the second electrode 108 due to their potential difference.

(47) As the object 102 is in contact with the control surface 104, the electrode-object capacitance C.sub.eo no longer varies and is still equal to (or close to) the threshold capacitance C.sub.s. Consequently, the value of the inter-electrode capacitance C.sub.ie, representative of the pressure of the object 102 on the detection surface 104, is obtained by subtraction, according to the following relationship:
C.sub.ie=C.sub.T−C.sub.s  (3),
or
C.sub.ie=C.sub.T−C.sub.eoT  (4),
Where C.sub.eoT is the electrode-object capacitance measured with the object in contact.

(48) In this configuration, it is necessary to verify periodically if the control object 102 is still in contact with the detection surface 104. To this end, the controllable switch 304 is toggled periodically so as to connect the guard electrode 108 to the guard potential V.sub.g in order to measure the electrode-object capacitance C.sub.eo, then to the ground G in order to measure the total capacitance C.sub.T, and thus the inter-electrode capacitance C.sub.ie. These sequential measurements of C.sub.eo and C.sub.T are carried out as long as the electrode-object capacitance is greater than the threshold capacitance (C.sub.eo≥C.sub.s).

(49) By using for example the parallel-plate capacitor law, it is possible to link the electrode-object capacitance C.sub.eo and the inter-electrode capacitance C.sub.ie respectively to a distance between the measurement electrode and the object, and to a distance D between the measurement electrode and the guard electrode. The load can thus be calculated from the variation in the measured thickness D of the dielectric material filling the compressible layer 302, and its modulus of elasticity.

(50) Determination of each of these capacitances from the measured signals can be carried out by the measurement module 114, or by the control module, or even by one or more additional calculation module(s) (not shown).

(51) To this end, it is possible to utilize a synchronous demodulator which carries out multiplication functions of the signal V.sub.s originating from the OA 110 with a carrier signal corresponding to the guard potential V.sub.g, then low-pass filtering.

(52) It is also possible to use an asynchronous demodulator comprising a rectification followed by a low-pass filter.

(53) Of course, the use of the controllable switch 118 makes it possible to poll sequentially a plurality of measurement electrodes 106, either individually or by group.

(54) FIG. 4 is a diagrammatic representation of a configuration of electrodes which can be utilized in a device according to the invention.

(55) In the configuration 400, shown in FIG. 4, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For each measurement electrode 106.sub.1-106.sub.N the device comprises an individual guard electrode, respectively 108.sub.1-108.sub.N.

(56) The device 400 also comprises a single polarization electrode 116, for all of the measurement electrodes 106.sub.1-106.sub.N. The single polarization electrode 116 is placed below the guard electrodes 108.sub.1-108.sub.N.

(57) FIG. 5 is a diagrammatic representation of an alternative configuration of electrodes that can be utilized in a device according to the invention.

(58) In the configuration 500, shown in FIG. 5, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For each measurement electrode 106.sub.1-106.sub.N the device comprises an individual guard electrode, respectively 108.sub.1-108.sub.N.

(59) Furthermore, the device 500 also comprises a single polarization electrode 116, for all of the measurement electrodes 106.sub.1-106.sub.N.

(60) The single polarization electrode 116 is openwork below each measurement electrode, respectively 106.sub.1-106.sub.N and each guard electrode, respectively 108.sub.1-108.sub.N, is placed in the openwork part of the single polarization electrode 116.

(61) FIG. 6 is a diagrammatic representation of a configuration of electrodes that can be utilized in a device according to the invention.

(62) In the configuration 600, shown in FIG. 6, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For each measurement electrode 106.sub.1-106.sub.N the device comprises: an individual guard electrode, respectively 108.sub.1-108.sub.N; and an individual polarization electrode, respectively 116.sub.1-116.sub.N.

(63) Each polarization electrode 116.sub.i is placed below each guard electrode 108.sub.i, seen from the measurement electrode 106.sub.i.

(64) FIG. 7 is a diagrammatic representation of a configuration of electrodes that can be utilized in a device according to the invention.

(65) In the configuration 700, shown in FIG. 7, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For each measurement electrode 106.sub.1-106.sub.N the device comprises: an individual guard electrode, respectively 108.sub.1-108.sub.N; and an individual polarization electrode, respectively 116.sub.1-116.sub.N.

(66) Each polarization electrode 116.sub.i is openwork and each guard electrode 108.sub.i is placed in the openwork part of the polarization electrode 116.sub.i, at the same level as said polarization electrode 116.sub.i.

(67) FIG. 8 is a diagrammatic representation of a configuration of electrodes that can be utilized in a device according to the invention.

(68) In the configuration 800, shown in FIG. 8, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For each measurement electrode 106.sub.1-106.sub.N the device comprises an individual guard electrode, respectively 108.sub.1-108.sub.N.

(69) The device also comprises (N+1) polarization tracks/electrodes 116.sub.1-116.sub.(N+1), placed at the same level as the guard electrodes 108.sub.1-108.sub.N. Each guard electrode 108.sub.i, with 1≤i≤N is inserted between two polarization tracks 116, and 116.sub.(i+1).

(70) According to an alternative configuration, the polarization tracks are placed between the guard electrodes only. In other words, the device does not comprise polarization tracks 116.sub.1 and 116.sub.(N+1).

(71) FIG. 9 is a diagrammatic representation of a configuration of electrodes that can be utilized in a device according to the invention.

(72) In the configuration 900, shown in FIG. 9, the device comprises a plurality of measurement electrodes 106.sub.1-106.sub.N. For all of the measurement electrodes 106.sub.1-106.sub.N, the device comprises only one single guard electrode 108, common to the set of measurement electrodes 106.sub.1-106.sub.N.

(73) The device also comprises (N+1) polarization tracks/electrodes 116.sub.1-116.sub.(N+1), placed at the same level as the measurement electrodes 106.sub.1-106.sub.N. Each measurement electrode 106.sub.i, with 1≤i≤N is inserted between two polarization tracks 116.sub.i and 116.sub.(i+1).

(74) FIG. 10 is a diagrammatic representation of a configuration of electrodes which can be utilized in a device according to the invention.

(75) In the configuration 1000, shown in FIG. 10, the device comprises an array of M rows and N columns of measurement electrodes 106.sub.i,j with 1≤i≤M and 1≤j≤N. For each measurement electrode, respectively 106.sub.1,1-106.sub.M,N, the device comprises: an individual guard electrode, respectively 108.sub.1,1-108.sub.M,N; and an individual polarization electrode, respectively 116.sub.1,1-116.sub.M,N.

(76) Each polarization electrode 116.sub.i,j is openwork and each guard electrode 108.sub.i,j is placed in the openwork part of the polarization electrode 116.sub.i,j, at the same level as said polarization electrode 116.sub.i,j.