Impedance measurement device

Abstract

An electronic impedance measurement device: a branch, called measurement branch, including an impedance to be measured (Z.sub.m), and; at least one branch, called reference branch, including an impedance (Z.sub.r), called reference impedance; electronics, called detection electronics, configured to provide an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch (104) and of a current (I.sub.m) flowing in the measurement branch; and at least one adjustment structure, changing the current (I.sub.r) in at least one of said reference branches in a manner inversely proportional to a control variable (k).

Claims

1. An electronic impedance measurement device comprising: a branch, called measurement branch, comprising an impedance to be measured (Z.sub.m); and at least one branch, called reference branch, comprising an impedance (Z.sub.r), called reference impedance; electronics, called detection electronics, configured to provide an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch and of a current (I.sub.m) flowing in the measurement branch; and at least one means, called adjustment means, changing the current (I.sub.r) in the at least one reference branch in a manner inversely proportional to a control variable (k); and a charge amplifier connected at a junction point of the measurement branch with the at least one reference branch, between the impedance to be measured (Z.sub.m) and the at least one reference impedance (Z.sub.t), and configured to provide the error signal (V.sub.s) dependent on the algebraic sum of the current (I.sub.m) of the measurement branch and of the current (I.sub.r) of the at least one reference branch.

2. The device according to claim 1, characterized in that, for the at least one reference branch, the adjustment means is configured to change the amplitude of a voltage delivered to said reference branch, in a manner inversely proportional to the control variable (k).

3. The device according claim 2, characterized in that the adjustment means comprises an amplifier a gain of which varies in a manner inversely proportional to the control variable (k), and arranged downstream of a voltage source (V.sub.0).

4. The device according to claim 2, characterized in that the adjustment means comprises: an amplifier; and at least one first digital potentiometer, used as input resistor of said amplifier, and the resistance of which is adjusted proportionally to the control variable (k).

5. The device according to claim 1, characterized in that, for at least one reference branch, the adjustment means is configured to change the reference impedance (Z.sub.r) of the said reference branch, in a manner proportional to the control variable (k).

6. The device according to claim 5, characterized in that the adjustment means comprises: a set of at least two impedances arranged in series, and at least one, in particular several, controllable switch(es), each provided to short-circuit, or not, one of the said impedances.

7. The device according to claim 1, characterized in that it comprises: at least two reference branches comprising reference impedances (Z.sub.r,1,Z.sub.r,2) of different types; and/or at least two reference branches comprising reference impedances (Z.sub.r,1,Z.sub.r,2) of the same type; and supplied by signals in quadrature.

8. The device according to claim 1, characterized in that it also comprises, arranged downstream of the detection electronics, an amplitude demodulator.

9. The device according claim 1, characterized in that it also comprises, arranged downstream of the detection electronics, an amplifier with a gain proportional, and in particular equal, to the control variable (k).

10. The device according to claim 1, characterized in that the reference branch and the measurement branch are supplied by the same source (V.sub.0), by use of a transformer.

11. The device according to claim 1, characterized in that: the impedance to be measured (Z.sub.m) is referenced to a general ground (M), and the current (I.sub.r) of the at least one reference branch is referenced to an alternating electrical potential, called reference potential, different from said general ground (M), at a working frequency; said device comprising an alternating voltage source (V.sub.0) arranged between said reference potential and said general ground (M).

12. The device according to claim 1, characterized in that it is a capacitive impedance measurement device comprising an impedance to be measured (C.sub.m,0-C.sub.m,n) of essentially capacitive type.

13. The device according to claim 12, characterized in that it comprises a reference impedance (C.sub.r) of essentially capacitive type.

14. A method for measuring impedance, in particular capacitive impedance, with an electronic impedance measurement device comprising: a branch, called measurement branch, comprising an impedance to be measured (Z.sub.m); at least one branch, called reference branch, comprising an impedance (Z.sub.r), called reference impedance; a step of obtaining, with detection electronics, an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch and of a current (I.sub.m) flowing in the measurement branch; at least one iteration of a step, called adjustment step, changing the current (I.sub.r) in said at least one reference branch in a manner inversely proportional to a control variable (k); and a charge amplifier connected at a junction point of the measurement branch with the at least one reference branch, between the impedance to be measured (Z.sub.m) and the at least one reference impedance (Z.sub.r), and providing the error signal (V.sub.s) dependent on the algebraic sum of the current (I.sub.m) of the measurement branch and of the current (I.sub.r) of the at least one reference branch.

15. A device for the capacitive detection of an object comprising: at least one electrode (1504.sub.1-1504.sub.n,1506), called measurement electrode, and at least one impedance measurement device according to claim 1, arranged to measure the capacitive impedance formed between said measurement electrode (1504.sub.1-1504.sub.n,1506) and said object.

16. The device according to claim 15, characterized in that an impedance measurement device is common to a plurality of measurement electrodes, said capacitive detection device comprising a polling means connecting said measurement device to each of said measurement electrodes, alternately.

17. An item of equipment for a robot, in particular removable or detachable, equipped with: at least one capacitive detection device according to claim 16; or at least one impedance measurement device including: a branch, called measurement branch, comprising an impedance to be measured (Z.sub.m); and at least one branch, called reference branch, comprising an impedance (Z.sub.r), called reference impedance; electronics, called detection electronics, configured to provide an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch and of a current (I.sub.m) flowing in the measurement branch; and at least one means, called adjustment means, changing the current (I.sub.r) in the at least one reference branch in a manner inversely proportional to a control variable (k).

18. A robot equipped with: an item of equipment according to claim 17; or at least one impedance measurement device including: a branch, called measurement branch, comprising an impedance to be measured (Z.sub.m); and at least one branch, called reference branch, comprising an impedance (Z.sub.r), called reference impedance; electronics, called detection electronics, configured to provide an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch and of a current (I.sub.m) flowing in the measurement branch; and at least one means, called adjustment means, changing the current (I.sub.r) in the at least one reference branch in a manner inversely proportional to a control variable (k).

19. The robot according to claim 18, equipped with a device for the capacitive detection of an object comprising: at least one electrode, called measurement electrode, and at least one impedance measurement device including a branch, called measurement branch, comprising an impedance to be measured (Z.sub.m); and at least one branch, called reference branch, comprising an impedance (Z.sub.r), called reference impedance; electronics, called detection electronics, configured to provide an error signal (V.sub.s) dependent on an algebraic sum of a current (I.sub.r) flowing in the at least one reference branch and of a current (I.sub.m) flowing in the measurement branch; and at least one means, called adjustment means, changing the current (I.sub.r) in the at least one reference branch in a manner inversely proportional to a control variable (k), arranged to measure the capacitive impedance formed between said measurement electrode and said object.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other advantages and characteristics will become apparent on examination of the detailed description of examples that are in no way limitative, and from the attached drawings in which:

(2) FIGS. 1 and 2 are diagrammatic representations of impedance measurement devices of the state of the art;

(3) FIG. 3 is a diagrammatic representation of the principle of an impedance measurement device according to the invention;

(4) FIGS. 4-14 are diagrammatic representations of different embodiment examples of an impedance measurement device according to the invention; and

(5) FIG. 15 is a diagrammatic representation of an embodiment example of a capacitive detection device according to the invention.

(6) It is well understood that the embodiments that will be described hereinafter are in no way limitative. Variants of the invention can in particular 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 can be combined together if there is no objection to this combination from a technical point of view.

(8) In the figures, elements common to several figures keep the same reference.

(9) The embodiments are described using harmonic signals, but it is well understood that the invention can be implemented with all kinds of alternative signals, (square-wave, triangular, etc.).

(10) FIG. 1 is a diagrammatic representation of the principle of a first type of impedance measurement device according to the state of the art.

(11) The impedance measurement device 100, also called impedance measurement bridge, comprises a first branch 102, called measurement branch. The measurement branch 102 comprises an impedance, denoted Z.sub.m, the value of which is to be measured. The measurement branch 102 is excited by an alternating electrical source V.sub.m(t)=V.sub.m cos(ωt), of fixed amplitude V.sub.m, and angular frequency ω.

(12) The impedance measurement device 100 also comprises a first branch 104, called reference branch. The reference branch 104 comprises an impedance, denoted Z.sub.r. This impedance Z.sub.r is preferably of the same type (resistive, capacitive or inductive) as at least a component of the impedance to be measured Z.sub.m. The reference branch 104 is excited by a variable alternating electrical source V.sub.r(t)=V.sub.r cos(ωt+π), of amplitude V.sub.r and in phase opposition with respect to the alternating electrical source V.sub.m(t). The amplitude of the voltage delivered by the reference source V.sub.r is adjusted in a manner proportional to a control variable k, such that V.sub.r=k.Math.V.sub.0, with V.sub.0 an alternating electrical source having a fixed amplitude.

(13) The measurement device 100 comprises detection electronics 106 comprising an amplifier 108 of the transimpedance type (or more simply “transimpedance amplifier”) the “−” terminal of which is connected to a junction point of the measurement 102 and reference 104 branches, between the measurement Z.sub.m and reference Z.sub.r impedances. A feedback impedance Z.sub.cr connects the output of the transimpedance amplifier 108 to the “−” input thereof.

(14) The detection electronics 106 can optionally be combined with a synchronous demodulation unit 110 using as a reference signal an alternating electrical signal of a similar form to the signals to be demodulated, i.e. in the examples presented of type cos(ωt), and with an integration unit 112.

(15) The transimpedance amplifier 108 provides an output signal, or error signal, denoted V.sub.s(t) as a function of time t, proportional to the algebraic sum of the current I.sub.m(t) of the reference branch 102 and of the current I.sub.r(t) of the measurement branch 104. This error signal is demodulated by the unit 110 to extract the amplitude V.sub.s therefrom, and integrated by the integrator 112. The integrator 112 makes it possible to realize a feedback loop of the proportional-integral type (PI) which avoids the effects of lag.

(16) In order to obtain an error signal V.sub.s(t) in phase with the signals of the electrical sources V.sub.m(t) and V.sub.r(t), that is easier to process, it is preferable to use a feedback impedance Z.sub.cr of the same type as the measurement impedance Z.sub.m. In particular, if the measurement impedance Z.sub.m is of a capacitive or essentially capacitive type, it is preferable to utilize a transimpedance amplifier 108 with a feedback impedance Z.sub.cr of a capacitive or essentially capacitive type, in which case the transimpedance amplifier 108 is called “charge amplifier” 108.

(17) The signal originating from the integrator 112 is provided to a control module 114, which can be for example a microcontroller. The control module 114 provides a control variable, denoted k, which will modulate the amplitude V.sub.r of the voltage provided by the variable reference source V.sub.r(t). The objective is to cancel out the algebraic sum of the reference current I.sub.r(t) and of the current I.sub.m(t), and therefore to cancel out the error signal V.sub.s(t) provided by the transimpedance amplifier 108, at any time t.

(18) The measurement branch 102, the reference branch 104, as well as the “+” terminal of the transimpedance amplifier 108 are connected, in the example shown in FIG. 1, at a reference potential 116.

(19) In balance, the currents in the reference 104 and measurement 102 branches verify the following relationship at any time:
I.sub.r(t)+I.sub.m(t)=0
I.e.
V.sub.r(t)/Z.sub.r=−V.sub.m(t)/Z.sub.m
Z.sub.m=−(V.sub.m(t)/V.sub.r(t)).Math.Z.sub.r

(20) Taking account of the fact that the voltage sources V.sub.r(t) and V.sub.m(t) are in phase opposition, and therefore have opposite polarity, this relationship can be written, in amplitudes:
Z.sub.m=(V.sub.m/V.sub.r).Math.Z.sub.r

(21) Thus, for a value k.sub.e of the control variable k corresponding to balance:
Z.sub.m=(V.sub.m/k.sub.e.Math.V.sub.0).Math.Z.sub.r.

(22) It will be easily understood that the impedance Z.sub.m is inversely proportional to the control variable k. As a result, starting from the value k.sub.e, it is necessary to carry out a mathematical inversion operation in order to determine Z.sub.m.

(23) Now, to obtain a value for Z.sub.m with accuracy under n bits after inversion of k, it is necessary to determine k with accuracy of 2 n bits, which is expensive.

(24) A second type of known impedance measurement device exists, an example of which is shown in FIG. 2, which makes it possible to overcome this problem of cost associated with the device 100.

(25) FIG. 2 is a diagrammatic representation of the principle of a second type of impedance measurement device according to the state of the art.

(26) The impedance measurement device 200 comprises all the elements of the device 100 of FIG. 1.

(27) Unlike the device 100 of FIG. 1, in the device 200 of FIG. 2, the source V.sub.r exciting the reference branch 104 is of fixed amplitude and it is the source V.sub.m exciting the measurement branch 102 that has an amplitude that can be varied proportionally to the control variable k, and the control module 114 controls this source V.sub.m and not the source V.sub.r.

(28) In other words, in the device 200 of FIG. 2, V.sub.m=k.Math.V.sub.0, with V.sub.0 an alternating electrical source with fixed amplitude and k the control variable delivered by the control module 114.

(29) In balance condition, still:
I.sub.r(t)+I.sub.m(t)=0
I.e.
V.sub.r(t)/Z.sub.r=−V.sub.m(t)/Z.sub.m

(30) Taking account of the fact that, as above, the voltage sources V.sub.r(t) and V.sub.m(t) are in phase opposition, and therefore have opposite polarity, this relationship can be written, in amplitudes:
Z.sub.m=(V.sub.m/V.sub.r).Math.Z.sub.r

(31) Thus, for a value k.sub.e making it possible to obtain balance:
Z.sub.m=(V.sub.m/k.sub.e.Math.V.sub.0).Math.Z.sub.r.

(32) It is understood that the impedance Z.sub.m is proportional to the control variable k.

(33) As a result, starting from the value k.sub.e, it is not necessary to carry out a mathematical inversion operation in order to determine Z.sub.m, unlike the device 100 of FIG. 1.

(34) The device 200 of FIG. 2 is therefore less expensive than the device 100 of FIG. 1.

(35) However, the device 200 of FIG. 2 cannot be parallelized for a plurality of impedances to be determined because, as the source V.sub.m is located on the measurement branch 102, it cannot be common to all the impedances to be determined.

(36) In addition, in a capacitive detection application the impedances to be determined are purely capacitive impedances between the measurement electrodes and an object to be detected. Now, it is not possible to excite the measurement electrodes at different potentials, because two adjacent measurement electrodes excited at different potentials generate between them a parasitic capacitive coupling and therefore leakage capacitances which interfere with the capacitive detection.

(37) The invention makes it possible to overcome the drawbacks of each of the devices of FIGS. 1 and 2. In particular, the impedance measurement device according to the invention is: less expensive than the device 100 of FIG. 1, and can be parallelized, unlike the device of FIG. 2.

(38) FIG. 3 is a diagrammatic representation of the principle of the impedance measurement device according to the invention.

(39) The impedance measurement device 300 of FIG. 3 comprises all the elements of the device 100 of FIG. 1.

(40) Unlike the device 100 of FIG. 1, in the device 300 of FIG. 3, the value of the current I.sub.r in the reference branch is adjusted in a manner inversely proportional to the control variable k provided by the control module 114, so that, in amplitude, I.sub.r=I.sub.0/k with I.sub.0 a reference current amplitude value.

(41) Under these conditions, the output voltage of the transimpedance amplifier 108, or error signal V.sub.s has the value:
V.sub.s(t)=−Z.sub.cr[i.sub.m(t)+i.sub.r(t)]=−Z.sub.cr[V.sub.m/Z.sub.m+I.sub.0/k]

(42) It is possible to use this equation to determine directly the impedance to be measured Z.sub.m as a function of the error signal V.sub.s and the applied control variable k, with one or more measurements.

(43) It is also possible to seek a balance condition, corresponding to a zero-error signal V.sub.s(t)=0 at any time t. For this, it is necessary to cancel out the sum of the currents:
I.sub.r(t)+I.sub.m(t)=0

(44) In order to be able to cancel out the sum of the currents, it is necessary for them to be in phase opposition or have opposite polarity. In this case, the following may be written, in amplitudes:
I.sub.0/k=V.sub.m/Z.sub.m
Z.sub.m=(k.Math.V.sub.m)/I.sub.0

(45) Thus, for a value k.sub.e making it possible to obtain the balance:
Z.sub.m=(k.sub.e.Math.V.sub.m)/I.sub.0.

(46) It is therefore understood that the impedance Z.sub.m is proportional to the control variable k.

(47) In addition, as the voltage of the source V.sub.m exciting the measurement branch 102 has a fixed amplitude, or more precisely fixed amplitude with respect to the control variable k or independent of this control variable k, the device 300 of FIG. 3 can be parallelizable for measuring a plurality of impedances in turn, without the need to change the source V.sub.m, and without introducing parasitic impedances.

(48) Furthermore, as explained above, the transimpedance amplifier is produced in the form of circuitry corresponding to an operational amplifier with a feedback impedance. Owing to the very principle of the operation of an operational amplifier, it follows that the “+” and “−” inputs of this transimpedance amplifier are at the same potential. As the “+” input is at the reference potential 116, the impedance to be measured Z.sub.m is also electrically connected to the reference potential 116. The voltage of the source V.sub.m is therefore found at the terminals of the impedance to be measured Z.sub.m.

(49) Thus, according to the invention, the voltage V.sub.m at the terminals of the impedance to be measured Z.sub.m and the current I.sub.m=V.sub.m/Z.sub.m, passing through this impedance and more generally the measurement branch are independent of the control variable k and can be kept constant or with a fixed amplitude during the measurements. Moreover, the voltage at the terminals of the impedance to be measured Z.sub.m can be perfectly managed and known, since this may be directly the voltage of the source V.sub.m.

(50) FIG. 4 is a diagrammatic representation of a first embodiment example of an impedance measurement device according to the invention.

(51) The impedance measurement device 400 of FIG. 4 comprises all the elements of the device 300 of FIG. 3.

(52) In the example shown in FIG. 4, the value of the current I.sub.r in the reference branch 104 is adjusted in a manner inversely proportional to the control variable k, by adjusting the amplitude of the voltage supplied to the reference branch 104, in a manner inversely proportional to the control variable k.

(53) To this end, the device 400 utilizes, on the reference branch 104, a variable voltage source V.sub.r delivering a voltage the amplitude of which is inversely proportional to the control variable k.

(54) The voltage source V.sub.r comprises an alternating voltage source with fixed amplitude V.sub.0 combined with an amplification stage 402, the gain of which is inversely proportional to the control variable k.

(55) In particular, the amplification stage 402 comprises an operational amplifier 404 of which: the “+” input is connected to the reference potential 116; and the “−” input is connected to the output thereof by a feedback resistor 406, of value R.sub.1.

(56) In addition, the “−” input is connected to an input resistor 408 the value of which is adjusted in a manner proportional to the control variable k. In particular, the value of the input resistor is equal to k.Math.R.sub.1. The input resistor 408 can be a digital potentiometer the value of which is adjusted in a manner proportional to the control value k.

(57) Under these conditions, the amplification stage 402 carries out a negative gain amplification G, the value of which obeys the following relationship:
G=−R.sub.1/(k.Math.R1)=−1/k

(58) As a result, the voltage provided by the variable voltage source V.sub.r to the reference branch 104 obeys the following relationship:
V.sub.r(t)=−V.sub.0(t)/k

(59) It is deduced therefrom that the current I.sub.r in the reference branch obeys the following relationship:

(60) $I_r\left(t\right)=\frac{V_r\left(t\right)}{Z_r}=\frac{V_0\left(t\right)}{k.Z_r}$

(61) As a result, the current I.sub.r is adjusted in a manner inversely proportional to the control variable k.

(62) In balance, I.sub.r(t)+I.sub.m(t)=0. The following is directly deduced therefrom:

(63) $-\frac{V_0\left(t\right)}{k{.}_e.Z_r}=-I_m\left(t\right)=-\frac{V_m\left(t\right)}{Z_m}$

(64) I.e. in amplitudes:

(65) $Z_m=\frac{k{.}_e.Z_r{V}_m}{V_0}$

(66) Thus, the impedance to be measured Z.sub.m is determined in a manner directly proportional to the control variable k. It is therefore not necessary to carry out an inversion operation.

(67) It will be noted that in this configuration, owing to the presence of the inverter amplification stage 402 used to generate the source V.sub.r of the reference branch, the alternating voltage sources V.sub.0 of the reference branch and V.sub.m of the measurement branch must be in phase or have the same polarity to be able to cancel out the sum of the currents.

(68) In the particular case where all the impedances are purely capacitive impedances, i.e. Z.sub.m=1/C.sub.m and Z.sub.r=1/C.sub.r:

(69) $\frac{1}{C_m}=\frac{k_e.V_m}{V_0}\frac{1}{C_r}$

(70) Within the context of a capacitive detection application, the capacitance C.sub.m represents the capacitance formed by a capacitive measurement electrode and an object. In this case, the distance D separating said measurement electrode from said object is proportional to the impedance value formed by this capacitance C.sub.m according to the following relationship:

(71) $D={\mathrm{.Math.}}_0{\mathrm{.Math.}}_{r}S\frac{1}{C_m}$
with ε.sub.0,ε.sub.r the permittivities of the free space and of the interface material respectively, and S the surface of overlap of the measurement electrode and the object.

(72) It is deduced therefrom that the distance D obeys the following relationship:

(73) $D={\mathrm{.Math.}}_0{\mathrm{.Math.}}_{r}S\frac{k_e.V_m}{V_0}\frac{1}{C_r}$

(74) It can be clearly seen that the distance D is directly proportional to the control variable k. Thus, the distance D is proportional to the value k making it possible to obtain balance.

(75) FIG. 5 is a diagrammatic representation of a second embodiment example of an impedance measurement device according to the invention.

(76) The impedance measurement device 500 of FIG. 5 comprises all the elements of the device 400 of FIG. 4.

(77) Unlike the device 400, in the device 500, the gain stage 402 comprises in addition to the digital potentiometer 408, a second digital potentiometer 502, in series with the first digital potentiometer 408, the value of which can be adjusted in a manner proportional to the control variable k.

(78) In particular, the second digital potentiometer 502 can have a value given by the relationship k.Math.R.sub.2.

(79) The resistances R.sub.1 et R.sub.2 can be chosen such that the adjustment step of the second digital potentiometer is smaller compared with the adjustment step of the first digital potentiometer 408. Thus, it is possible to carry out a more accurate measurement of the impedance Z.sub.m.

(80) In particular, it is possible to carry out a first phase of iterations with a large adjustment step with the first digital potentiometer 408, then a second phase of iterations with a fine adjustment step with the second digital potentiometer 502.

(81) For example R.sub.2=R.sub.1/10, which makes it possible to have an adjustment step ten times smaller, and therefore an accuracy 10 times greater, with the second digital potentiometer 502.

(82) Of course, the invention is not limited to one or two digital potentiometers, and it is possible to use a number of digital potentiometers greater than or equal to 1.

(83) FIG. 6 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(84) The impedance measurement device 600 of FIG. 6 comprises all the elements of the device 300 of FIG. 3.

(85) In the example shown in FIG. 6, the value of the current I.sub.r in the reference branch 104 is adjusted in a manner inversely proportional to the control variable k, by adjusting the value of the reference impedance Z.sub.r of the reference branch 104, in a manner proportional to the control variable k.

(86) To this end, the reference impedance Z.sub.r is formed by a set of eight impedances 602.sub.0-602.sub.7, mounted in series, and of respective values 2.sup.n.Math.Z.sub.0, with n=0 . . . 7. Thus, the reference impedance Z.sub.r is formed by eight impedances 602.sub.0-602.sub.7 having respective values Z.sub.0, 2Z.sub.0, 4Z.sub.0, 8Z.sub.0, 16Z.sub.0, 32Z.sub.0, 64Z.sub.0 and 128Z.sub.0. Each impedance 602 can be short-circuited by a switch, 604.sub.0-604.sub.7 respectively.

(87) The control variable k is a value provided on eight bits and each switch 604.sub.0-604.sub.7 is controlled by one bit of the control variable such that when the bit is equal to 0 the switch is open and when the bit is equal to 1 the switch is closed. The value of k is therefore written in binary on the configuration of the switches 604.sub.0-604.sub.7. Under these conditions, the value of the impedance Z.sub.r is:
Z.sub.r=Z.sub.0.Math.k/2.sup.n

(88) As a result, Z.sub.r varies in a manner proportional to the control variable k.

(89) In balance, I.sub.r(t)+I.sub.m(t)=0. It is directly deduced therefrom that:

(90) $\frac{V_r\left(t\right)}{k_e.\left(Z_0/2^n\right)}=-I_m\left(t\right)=-\frac{V_m\left(t\right)}{Z_m}$

(91) Taking account of the fact as above, that the voltage sources V.sub.r(t) and V.sub.m(t) are in phase opposition, and therefore have opposite polarity, this relationship can be written, in amplitudes:

(92) $Z_m=k_e.Z_0.V_m\frac{1}{V_r{\mathrm{.2}}^n}$

(93) Thus, the impedance to be measured Z.sub.m is determined in a manner directly proportional to the control variable k. It is therefore not necessary to carry out an inversion operation.

(94) In the particular case where all the impedances are purely capacitive impedances, i.e. Z.sub.m=1/C.sub.m and Z.sub.0=1/C.sub.0:

(95) 0$\frac{1}{C_m}=\frac{1}{C_0}\frac{k_e.V_m}{V_r{\mathrm{.2}}^n}$

(96) Within the context of a capacitive detection application, and by using the relationship linking the distance to the capacitance C.sub.m, it is deduced therefrom that the distance D obeys the following relationship:

(97) $D={\mathrm{.Math.}}_0{\mathrm{.Math.}}_{r}S\frac{1}{C_0}\frac{k_e.V_m}{V_r{\mathrm{.2}}^n}$

(98) It can be clearly seen that the distance D is directly proportional to the control variable k. Thus, the distance D is proportional to the value k.sub.e allowing balance to be obtained.

(99) FIG. 7 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(100) The impedance measurement device 700 of FIG. 7 comprises all the elements of the device 300 of FIG. 3.

(101) Unlike the device 300, the device 700 comprises two branches referenced 104.sub.1 and 104.sub.2.

(102) Each reference branch 104.sub.1 and 104.sub.2 is balanced according to any one of the principles described with reference to FIGS. 4-6. Each reference branch 104.sub.1, 104.sub.2 comprises an alternating excitation, V.sub.r,1 and V.sub.r,2 respectively, and a reference impedance Z.sub.r,1 and Z.sub.r,2 respectively.

(103) The device 700 makes it possible for example to measure different components (active and reactive) of the impedance to be measured Z.sub.m. The reference impedances Z.sub.r,1, Z.sub.r,2 each correspond respectively to an impedance component to be measured Z.sub.m. As a result, they introduce a lag between the voltage and the current in their reference branch, which makes it possible to compensate for a component of the current of the measurement branch with a similar lag. For example, if the impedance to be measured comprises resistive and capacitive components, reference impedances Z.sub.r,1, Z.sub.r,2 are used, of a resistive and capacitive type respectively. Similarly, it is possible to measure a component of an inductive type with a reference impedance of inductive type.

(104) With each reference branch 104.sub.1, 104.sub.2 is associated a control module, respectively 114.sub.1 and 114.sub.2, providing to each branch a control variable, k.sub.1 and k.sub.2 respectively, used to adjust the value of the current in each branch, I.sub.r,1 and I.sub.r,2 respectively in an inversely proportional manner such that I.sub.r,1˜1/k.sub.1 and I.sub.r,2˜1/k.sub.2.

(105) With each branch 104.sub.1, 104.sub.2 is associated a synchronous demodulation unit, 110.sub.1 and 110.sub.2 respectively, and an integration unit, 112.sub.1 and 112.sub.2 respectively.

(106) In this configuration, the balance of the bridge is achieved when the algebraic sum of the current I.sub.m of the measurement branch 102 and of the currents of all the reference branches 104.sub.1 and 104.sub.2 is zero at all times:
I.sub.m(t)+I.sub.r,1(t)+I.sub.r,2(t)=0.

(107) When the impedances Z.sub.r,1 and Z.sub.r,2 are of a different type, and correspond to the different components of the impedance to be measured Z.sub.m, the excitations V.sub.r,1 and V.sub.r,2 are in phase.

(108) It is also possible, for detecting different components (resistive and reactive) of the impedance to be measured Z.sub.m, to use impedances Z.sub.r,1 and Z.sub.r,2 of the same type, namely resistive or capacitive or also inductive, and phase-shifted excitation sources V.sub.r,1 and V.sub.r,2 (for example in quadrature), to reproduce the lag of the currents of the reference branches that would be generated by reference impedances of a different type.

(109) In all cases, a phase-shift unit 702 is used to apply a phase shift of Π/2 to the carrier wave for the synchronous demodulation for one of the two branches 104.sub.1 and 104.sub.2, and thus to detect the phase and quadrature components thereof. In the example of FIG. 7, the demodulation unit 702 is used for the branch 104.sub.2.

(110) Of course, the invention is not limited to one or two reference branches, and it is possible to use a number of reference branches greater than or equal to 1.

(111) FIG. 8a and FIG. 8b are diagrammatic representations of impedance measurement devices according to the invention making it possible to measure a plurality of impedances Z.sub.m,1-Z.sub.m,n.

(112) The impedance measurement device 800 of FIG. 8a comprises all the elements of the device 300 of FIG. 3.

(113) Unlike the device 300, the device 800 makes it possible to measure, in turn or sequentially, a plurality of impedances Z.sub.m,1-Z.sub.m,n.

(114) To this end, the device 800 comprises a polling means 802 making it possible to selectively connect the impedances to be measured Z.sub.m,1-Z.sub.m,n to the detection electronics 106 and to the reference branch 104. In the example shown, the source V.sub.m of the measurement branch 102 is common to all the impedances to be measured. The polling means 802 can comprise switches, controllable for example by the control module 114, and making it possible to connect the detection electronics 106 to each of the impedances Z.sub.m,i to be measured in turn.

(115) Preferably, the polling means 802 makes it possible to connect the impedances to be measured Z.sub.m,1-Z.sub.m,n either to the detection electronics 106 or to the reference potential 116. This is particularly important if the impedances to be measured Z.sub.m,i are capacitive, to avoid parasitic couplings.

(116) For each impedance Z.sub.m,i, as above, the balance condition is given by the relationship I.sub.m,i(t)+I.sub.r(t)=0, with I.sub.m,i the current passing through this impedance Z.sub.m,i.

(117) As explained above, the voltage at the terminals of the impedances to be measured Z.sub.m,i is identical and corresponds to the voltage; in the diagram shown, to the voltage source V.sub.m. This property is very important to be able to measure a plurality of impedances Z.sub.m,i without parasitic electrical couplings between them, in particular when they are capacitive impedances.

(118) The device 800 can be combined with each of the devices in FIGS. 4-7.

(119) The impedance measurement device 850 of FIG. 8b comprises all the elements of the device 300 of FIG. 3.

(120) Unlike the device 300, the device 850 makes it possible to measure, in parallel or simultaneously, a plurality of impedances Z.sub.m,i-Z.sub.m,n, with a plurality of independent parallel measurement paths.

(121) To this end, it comprises a plurality of impedance measurement devices 300 each constituting a measurement path, for example according to the embodiments of FIGS. 4-7, operating in parallel.

(122) Each impedance Z.sub.m,i-Z.sub.m,n is connected to separate detection electronics 106.sub.1-106n and constitutes, with a separate reference branch 104.sub.1-104.sub.n, a separate bridge.

(123) The value of the current I.sub.r,i in each reference branch 104.sub.i is adjusted in a manner inversely proportional to a separate control variable k.sub.i provided by a separate control module 114.sub.i (or a common control module 114), so as to determine a balance condition:
I.sub.r,i(t)+I.sub.m(t)=0

(124) According to a particularly advantageous embodiment, the device 850 comprises a source V.sub.m common to all the impedances to be measured. Thus, all the impedances to be measured Z.sub.m,1-Z.sub.m,n are polarized at the same potential, which is essential when for example these impedances to be measured are capacitances between capacitive measurement electrodes and an object.

(125) Furthermore, the device 850 allows measurements that are independent, in parallel, or simultaneous, or asynchronous, of all the impedances Z.sub.m,1-Z.sub.m,n.

(126) Of course, the different measurement paths of the device 850 can be realized in any manner, in particular with separate components and/or common components for each measurement path.

(127) The device 850 can be combined with each of the devices in FIGS. 4-8a.

(128) In particular, certain or all of the measurement paths of the device 850 can comprise a polling means 802 as shown in FIG. 8a for sequentially measuring a plurality of impedances. Furthermore, the device can comprise a source V.sub.m common to all the impedances to be measured, sequentially and/or with parallel measurement paths.

(129) FIG. 9 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(130) The impedance measurement device 900 of FIG. 9 comprises all the elements of the device 300 of FIG. 3.

(131) Unlike the device 300, in the device 900 the synchronous demodulation unit 110, the integration unit 112 and the control module 114 are integrated in a microcontroller 902.

(132) Alternatively, the microcontroller 902 can calculate a value of the impedance to be measured Z.sub.m starting from one or more measurements of an error signal V.sub.s as explained with respect to FIG. 3, without reproducing the operation of a feedback loop.

(133) The device 900 can be combined with each of the devices in FIGS. 4-8.

(134) FIG. 10 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(135) The impedance measurement device 1000 of FIG. 10 comprises all the elements of the device 900 of FIG. 9.

(136) The impedance measurement device 1000 also comprises a linearization amplifier 1002 placed downstream of the detection electronics 106. This linearization amplifier 1002 can receive the alternating signal V.sub.s(t) at the input, as originating from the transimpedance amplifier 108. It can also be implemented in a device with a demodulator 110, such as the device 300, and receive the demodulated amplitude signal V.sub.s at the input.

(137) The linearization amplifier 1002 has a gain G.sub.l which varies in a manner proportional to the control variable k.

(138) To this end, it can be produced in the form of an inverting amplifier with an operational amplifier 1004 with an input resistor 1006 for example having a value R.sub.0 and a feedback resistor 1008 of the digital potentiometer type, the resistance R.sub.p(k) of which varies linearly according to the digital control variable k: R.sub.p(k)=kR.sub.0.

(139) The gain of the linearization amplifier 1002 is then written:
G.sub.i=−k.Math.R.sub.O/R.sub.O=−k

(140) By using the expression of the error signal V.sub.s established with respect to FIG. 3, at the output of the linearization amplifier 1002 a linearized error signal V.sub.sl is obtained:
V.sub.sl(t)=k.Math.Z.sub.cr[V.sub.m(t)/Z.sub.m+I.sub.o(t)/k]=k.Math.V.sub.m(t)Z.sub.cr/Z.sub.m+I.sub.o(t)]

(141) The corresponding amplitude of the linearized error signal, taking account of the fact that the currents in the measurement and reference branches are in phase opposition, or have opposite polarity, is:
V.sub.sl=k.Math.V.sub.mZ.sub.cr/Z.sub.m−I.sub.o

(142) A linearized error signal V.sub.sl is then obtained, which depends linearly on the control variable k. This linearized error signal V.sub.sl can advantageously be used by a microcontroller 902 to calculate a value of the impedance to be measured Z.sub.m starting from one or more measurements of the error signal V.sub.s. It can also be used in closed loop operation (digital or analogue) to ensure rapid convergence.

(143) FIG. 11 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(144) The impedance measurement device 1100 of FIG. 11 comprises all the elements of the device 900 of FIG. 9.

(145) In the device 1100 of FIG. 11, the microcontroller 902 does not carry out a synchronous demodulation.

(146) Instead, the device 1100 comprises in addition a first comparator 1102 arranged between the transimpedance amplifier 108 and the microcontroller 902 and a second comparator 1104 arranged between the voltage source of fixed amplitude V.sub.0 of the reference branch 104 and the microcontroller 902.

(147) The comparator 1102 arranged at the output of the transimpedance amplifier 108 only detects the sign of the alternating measurement signal V.sub.s provided by the transimpedance amplifier 108, namely negative or positive. Depending on the error sign given by the amplitude of the output signal V.sub.s, at the output of the comparator 1102 an alternating signal is obtained in phase or in phase opposition with a reference signal originating from the source V.sub.0, and discretized by the comparator 1104. It is then possible simply to detect this phase condition, for example with the microprocessor 902, and carry out a measurement of the “by successive approximations” type, completely eliminating the need for demodulation of the error signal V.sub.s.

(148) The device 1100 can be combined with each of the devices in FIGS. 4-8.

(149) FIG. 12 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(150) The impedance measurement device 1200 of FIG. 12 comprises all the elements of the device 400 of FIG. 4, with the exception of the measurement source V.sub.m of the measurement branch 102.

(151) Unlike the device 400, in the device 1200 the measurement branch 102 is supplied by the alternating source of fixed amplitude V.sub.0 by means of a transformer 1202, for example inductive.

(152) Conversely, the reference branch 104 can be supplied by an alternating source present in the measurement branch, and a transformer 1202, for example inductive.

(153) The device 1200 can be combined with each of the devices in FIGS. 4-11.

(154) FIG. 13 is a diagrammatic representation of another embodiment example of an impedance measurement device according to the invention.

(155) The impedance measurement device 1300 of FIG. 13 comprises all the elements of the device 400 of FIG. 4, with the exception of the source V.sub.m.

(156) Unlike the device 400, in the device 1300, the impedance to be measured Z.sub.m is connected to an electrical ground M, which can be a general ground such as earth for example, different from the reference potential 116 at the working frequency.

(157) The fixed alternating voltage source V.sub.0 supplies both the reference branch 104 and the measurement branch 102 via the common ground M. The source V.sub.0 is then positioned between the reference potential 116 and the ground M and thus establishes this reference alternating electrical potential 116, which is different from the ground potential M at the working frequency. The other elements of the device 1300 that are shown, including in particular the detection electronics 106, can advantageously be supplied by power supplies referenced to the reference potential 116, in order to avoid the parasitic capacitances with the ground M.

(158) Under these conditions, seen from the alternating electrical potential 116, the electrical ground M oscillates with respect to the reference potential 116. Under these conditions, and seen with respect to the reference potential 116, the measurement branch 102 is seen as being supplied by the fixed source V.sub.0. For the same reasons as explained above, the impedance to be measured Z.sub.m is also polarized at the potential of this source V.sub.0.

(159) The reference branch 104 can also be supplied by the source V.sub.0 associated with the gain stage 402, or by a source derived from the source V.sub.0, for example through a resistive voltage divider or through a transformer as shown in FIG. 12.

(160) The reference branch 104 can also be supplied by a separate source V.sub.r, as shown in FIG. 3, in which case it is the source V.sub.m of the measurement branch that is positioned between the reference potential 116 and the ground M.

(161) The configuration described with reference to FIG. 13 can be implemented in the device 500 of FIG. 5.

(162) In addition, the configuration described with reference to FIG. 13 can be implemented in the device 600 of FIG. 6, by replacing the fixed source V.sub.0 with the fixed source V.sub.r and by using the variable impedance Z.sub.r of FIG. 6.

(163) FIG. 14 gives an embodiment example of such a device, in a configuration in which the measurement branch 102 is supplied by a separate source V.sub.m, positioned between the reference potential 116 and the ground M.

(164) Generally, the device 1300 or 1400 can be combined with each of the devices in FIGS. 4-11.

(165) In the case where the devices according to the invention, in particular the devices 1300 and 1400, are used for the capacitive detection of objects, the potential 116 can be an alternating potential, called guard potential, used to guard the measurement electrodes by guard electrodes. The use of such a guard potential is well known and will not be described in greater detail herein.

(166) FIG. 15 is a diagrammatic representation of an embodiment example of a device for the capacitive detection of object(s).

(167) The device 1500 shown in FIG. 15, is used to measure the capacitive impedance corresponding to each of the capacitances C.sub.m,1-C.sub.m,n, and C.sub.m,0 formed between an object, and in particular a hand 1502 electrically referenced to the ground potential M, and a plurality of capacitive measurement electrodes, 1504.sub.1-1504n and 1506 respectively.

(168) As explained above with reference to FIGS. 4-6, the capacitive impedance Z.sub.m,i=(1/C.sub.m,i) is directly proportional to the distance between the object 1502 and the measurement electrode 1504.sub.i, and the capacitive impedance Z.sub.m,0=(1/C.sub.m,0) is directly proportional to the distance between the object 1502 and the measurement electrode 1506.

(169) The device 1500 comprises a first measurement path with a polling means 802 making it possible to poll measurement electrodes 1504.sub.1-1504n sequentially, as explained in FIG. 8a.

(170) The device 1400 also comprises a second measurement path making it possible to poll a measurement electrode 1506 in parallel, as explained in FIG. 8b.

(171) Of course, the device 1400 can also comprise only one sequential measurement path, or only parallel measurement paths, or a plurality of sequential and parallel paths.

(172) The reference branches 104 of the first and the second measurement path are supplied by a common source V.sub.0 with fixed amplitude, which supplies respectively gain stages 402 controlled by control variables k, as explained with respect to FIG. 4. The reference branches 102 of the first and the second measurement path are supplied by a source V.sub.m positioned between a ground M and the reference potential 116 (which is determined by this source V.sub.m). In practice, the source V.sub.m is connected to the measurement branches 102 via the ground M and the object 1502.

(173) Alternatively, the sources V.sub.0 and V.sub.m of the reference and measurement branches can be realized by one and the same source, directly as shown in FIG. 13, or via a resistive voltage divider or a transformer as shown in FIG. 12.

(174) On the measurement branch 102 of the first measurement path, the polling means 802 is used to measure, in turn, the value of the capacitive impedance 1/C.sub.m,i for each of the measurement electrodes 1504.sub.i, as described with reference to FIG. 8a.

(175) The reference branch 104 of the first and second paths comprises a purely capacitive reference impedance 1/C.sub.r each formed by a capacitance C.sub.r. In addition, the feedback impedance used for the transimpedance amplifier or in this case, charge amplifier 108 of the first and second paths is a purely capacitive impedance 1/C.sub.cr and formed by a capacitance C.sub.cr.

(176) The device 1400 also comprises the microcontroller 902 integrating the synchronous demodulation unit 110, the integration unit 112 and the control module 114 of the first and second paths.

(177) The transimpedance or charge amplifiers 108 and optionally the microcontroller 902 are supplied by power supplies referenced to the reference potential 116, to minimize the leakage capacitances.

(178) It should be noted that, as explained above, all the capacitances to be measured C.sub.m,i are polarized at the same potential as the source V.sub.m, which also corresponds to the reference potential 116 with respect to the ground M, and which can also be used as guard potential as explained with respect to the device 1400 of FIG. 14.

(179) This property is very important because if all the capacitive measurement electrodes 1504.sub.1-1504n and 1506 corresponding to the capacitances to be measured C.sub.m,i are polarized at one and the same reference potential 116, also used as guard potential to polarize elements in proximity to these electrodes according to well known implementations, and optionally also used as reference potential of the power supplies of the electronics as explained above, in this way capacitive detection electronics are obtained with an optimum sensitivity in which all the parasitic coupling capacitances and the leakage capacitances can be eliminated.

(180) Of course, the capacitive detection device 1500 can utilize any combination of the devices described with reference to FIGS. 3-14.

(181) The invention is not limited to the examples that have just been described, and numerous modifications may be made to these examples without exceeding the scope of the invention.