FR3046677B1 - SENSOR, SYSTEM AND METHOD FOR DETECTING VARIATION OF A CAPACITY OF A MEASURING CAPACITOR - Google Patents

Abstract

The present invention relates to a sensor (10) for detecting capacitance variation of a measurement capacitor (21), comprising a measurement branch (20) and a reference branch (30). The measurement branch (20) comprises the measuring capacitor (21) and charging means (22) for charging the measurement capacitor from a DC voltage source. The reference branch (30) includes a reference capacitor (31) and load means (32) for charging said reference capacitor from said DC voltage source. Said sensor (10) further comprises comparison means (40) configured to compare a measurement voltage, representative of the voltage across the measurement capacitor (21), and a reference voltage representative of the voltage across the capacitor. (31) of reference.

Description

The present invention belongs to the field of measurement sensors, and more particularly to a capacitance detection sensor of a measuring capacitor.

The present invention finds a particularly advantageous, although in no way limiting, application in the field of motor vehicles, in particular for detecting the presence of a user of a motor vehicle in the vicinity of said motor vehicle.

Nowadays, some motor vehicles are indeed equipped with so-called "hands-free" access, for which it is no longer necessary to use a key to unlock the doors and other doors (hood, trunk, etc.).

To unlock a door, for example, a user of the motor vehicle approaches a hand of the door handle, which is equipped with a presence sensor. When the presence sensor detects the hand, it sends a presence detection signal to an electronic calculator ("Electronic Control Unit" or ECU in the English literature).

The electronic calculator checks in parallel that the user is an authorized user. For this purpose, the electronic computer sends, by radio, an identification request to an identification badge worn by the user. The identification badge responds by sending an identification code to the electronic calculator. If the electronic computer recognizes the identification code as that allowing access to the motor vehicle, it triggers the unlocking of the door. If, on the other hand, the electronic computer has not received an identification code, or if the identification code received is incorrect, the door remains locked.

The presence sensor, used to detect the hand of the user, is generally a capacitive sensor having a measuring capacitor having a first electrode located in the handle of the door to be unlocked. A second electrode of the measurement capacitor is connected to the electrical ground. When a hand approaches the first electrode of the measuring capacitor, said hand behaves as a second electrode of the measuring capacitor, which causes a variation of the capacitance of the measuring capacitor, in this case an increase of this, compared to the case where no hand is present near the first electrode. Therefore, to detect the presence of a hand near the door handle of the motor vehicle, it is sufficient to detect a change in the capacitance of the measuring capacitor.

Capacitive sensors implemented for such applications for detecting the presence of a user of a motor vehicle are generally capacitive sensors called "charge transfer".

In a known manner, a capacitive charge transfer sensor comprises two capacitors: a measurement capacitor whose aim is to detect the capacitance variation induced by the presence of a user; a reference capacitor of constant capacitance.

In order to detect the variation of the capacitance of the measurement capacitor, a measurement sequence comprising several charges / discharges of said measuring capacitor is carried out by controlling the switching means of the capacitive sensor. During a load, the switching means are controlled so that the measurement capacitor is charged from a DC voltage source. During a discharge, the switching means are controlled so that said measuring capacitor discharges into the reference capacitor, thereby providing charge transfer from the measurement capacitor to the reference capacitor. The charges / discharges of the measuring capacitor, within a measurement sequence, continue until the voltage between the electrodes of the reference capacitor reaches a predefined fixed threshold value.

The number of charges / discharges of a measurement sequence necessary to reach the threshold value depends on the capacitance of the measurement capacitor and is representative thereof. Therefore, a variation of the capacitance of said measuring capacitor can be detected by comparing the numbers of charges / discharges of the measurement capacitor obtained for successive measurement sequences.

A disadvantage of capacitive charge-coupled sensors is that they are likely to pose problems of electromagnetic compatibility with other equipment of a motor vehicle, in particular because of uncontrolled electromagnetic radiation induced by the large number and high frequency of the charges / discharges of each measurement sequence.

In addition, capacitive charge-coupled sensors have low noise immunity on the voltage supplied by the DC voltage source from which the measurement capacitor is charged, which impacts the accuracy of measurement of the capacitance variation of the capacitor. capacitor.

The present invention aims to overcome all or part of the limitations of the solutions of the prior art, including those described above. For this purpose, and according to a first aspect, the invention relates to a sensor for detecting variation in capacitance of a measurement capacitor, comprising a measurement branch and a reference branch each comprising an input and an output, the respective inputs said measurement branch and said reference branch being interconnected and being intended to be connected to the same DC voltage source, and wherein: • the measurement branch comprises the measuring capacitor, charging means allowing charging said measuring capacitor from the DC voltage source and discharging means for discharging said measuring capacitor, • the reference branch comprises a reference capacitor, charging means for charging said reference capacitor from of the DC voltage source and discharge means for discharging said reference capacitor nce.

In addition, said detection sensor comprises comparison means, connected to the respective outputs of said measurement branch and said reference branch, configured to compare a measurement voltage, representative of the voltage across the measurement capacitor, and a reference voltage representative of the voltage across the reference capacitor.

Thus, the detection sensor comprises a measurement capacitor and a reference capacitor. However, the measuring capacitor and the reference capacitor belong to two different arms, respectively the measuring arm and the reference arm, both connected to the same DC voltage source. As a result, unlike capacitive charge transfer sensors, the reference capacitor is charged directly from the DC voltage source. By "directly loaded" is meant that the reference capacitor is not charged from the measurement capacitor, so that no charge transfer from the measurement capacitor to the reference capacitor is made.

Since the reference capacitor is charged directly from the DC voltage source, it is not necessary to perform several successive charges / discharges of the measurement capacitor within the same measurement sequence. A measurement sequence by means of such a detection sensor can indeed comprise a single charge of the measuring capacitor and the reference capacitor, so that the problems of electromagnetic compatibility are reduced compared with the case where a capacitive transfer sensor load is implemented.

Moreover, since both the measuring capacitor and the reference capacitor are charged by the same DC voltage source, in particular because the reference capacitor is charged directly from the DC voltage source, the effects related to the variation of the capacitance of the measuring capacitor, to detect, and those related to the noise on the voltage supplied by the DC voltage source are dissociated.

More particularly, the effects related to the variation of the capacitance of the measuring capacitor only impact the measurement voltage. The effects of noise on the voltage supplied by the DC voltage source affect both the measurement voltage and the reference voltage. On the other hand, these effects related to noise introduce substantially the same bias on the measurement voltage and on the reference voltage, so that they do not affect the result of the comparison of said measurement voltage with said reference voltage. The detection sensor therefore has a better immunity to noise on the voltage supplied by the DC voltage source than the capacitive charge transfer sensors.

In particular embodiments, the detection sensor may further comprise one or more of the following characteristics, taken individually or in any technically possible combination.

In particular embodiments, the measurement branch comprises, between the input of said measuring branch and the measurement capacitor, a current source adapted to supply a current of continuous intensity to the measurement capacitor, starting from the voltage supplied by the DC voltage source.

In particular embodiments, the reference branch comprises, between the input of said reference branch and the reference capacitor, a current source adapted to supply a current of continuous intensity to the reference capacitor, starting from the voltage supplied by the DC voltage source.

In particular embodiments, the reference branch comprises a resistive component between the input of said reference branch and the reference capacitor.

In particular embodiments, the detection sensor comprises a source of current between the DC voltage source and the respective inputs of the measurement branch and of the reference branch, said current source being adapted to form a current of continuous current from the voltage supplied by the DC voltage source.

In particular embodiments, the measurement branch comprises, between the measurement capacitor and the output of said measuring branch, a sample-and-hold circuit.

According to a second aspect, the present invention relates to a capacitance variation detection system of a measurement capacitor comprising a detection sensor according to any one of the embodiments of the invention. The detection system further comprises a control device configured to: • control the respective load means of the measurement branch and the reference branch so as to charge the measurement capacitor for a predetermined duration and so as to charge the reference capacitor until a reference voltage equal to the measurement voltage obtained after having charged the measuring capacitor during said predetermined time, • measuring a duration, called "crossover duration", representative of the time required to obtain a measurement capacitor. reference voltage equal to the measurement voltage obtained after having charged the measuring capacitor during said predetermined time, • checking the respective discharge means of the measurement branch and the reference branch so as to discharge the measurement capacitor and the reference capacitor after measuring the crossover duration, detecting a variation in the capacitance of the measurement capacitor according to crossing times measured for successive charges of the measurement capacitor and the reference capacitor.

According to a third aspect, the present invention relates to a motor vehicle comprising a detection system according to any one of the embodiments of the invention.

According to a fourth aspect, the present invention relates to a capacitance variation detection method of a measurement capacitor, by means of a detection sensor according to any one of the embodiments of the invention, said method comprising recurrent steps of: • charging the measuring capacitor for a predetermined time, • charging the reference capacitor until a reference voltage is obtained equal to the measurement voltage obtained after charging the measurement capacitor during said period of time predetermined duration, • measurement of a duration, called "crossover duration", representative of the time required to obtain a reference voltage equal to the measurement voltage obtained after charging the measurement capacitor during said predetermined duration, • discharging of the measuring capacitor and reference capacitor after measuring the duration of the ement.

Said detection method also comprises a step of detecting variation in the capacitance of the measurement capacitor as a function of measured crossover durations for successive charges of the measurement capacitor and the reference capacitor.

In particular modes of implementation, said detection method comprises, before each charging step of the reference capacitor, a step of pre-charging said reference capacitor, so that the voltage between the electrodes of said reference capacitor is not zero at the beginning of the charging step of said reference capacitor. The invention will be better understood on reading the following description, given by way of non-limiting example, and with reference to the figures which represent: FIG. 1: a schematic representation of a preferred embodiment of a capacitance variation detection sensor of a measurement capacitor, - figures 2 and 3: schematic representations of alternative embodiments of a capacitance detection sensor of a capacitor of measurement, - figure 4: a diagram illustrating the main steps of a capacitance variation detection method of a measurement capacitor, - FIG. 5: time diagrams illustrating the capacitance variation detection principle of a measurement capacitor, in the case of the capacitance sensor. detection of Figure 1,

In these figures, identical references from one figure to another designate identical or similar elements. For the sake of clarity, the elements shown are not to scale unless otherwise stated.

FIG. 1 schematically represents a preferred embodiment of a sensor 10 for detecting the variation of a capacitance of a measurement capacitor 21.

As illustrated in FIG. 1, the detection sensor 10 comprises a measurement branch 20 and a reference branch 30 each having an input and an output. The respective inputs of said measurement branch 20 and said reference branch 30 are interconnected at a node 11 of the sensor 10. In the nonlimiting example illustrated in FIG. 1, the node 11 is connected to a source 50 of DC voltage.

The measurement branch 20 comprises the measurement capacitor 21 whose variation of the capacitance Cm is to be detected. The measuring capacitor 21 comprises two terminals formed by a first electrode 21a connected to the node 11 and a second electrode 21b connected to the electrical ground.

The measurement branch 20 further comprises charging means for charging said measurement capacitor 21 from the DC voltage source 50 through the node 11. In the nonlimiting example illustrated in FIG. the load means of the measurement branch 20 comprise a switch 22 arranged between the node 11 and the first electrode 21a of the measurement capacitor 21. When the switch 22 is closed, the measurement capacitor 21 is connected to the node 11, through which the DC voltage source 50 loads said measurement capacitor 21. When the switch 22 is open, the measurement capacitor 21 is disconnected from the node 11, so that the DC voltage source 50 does not charge the capacitor 21 measurement.

The measuring branch 20 also comprises discharge means for discharging said measuring capacitor 21. In the nonlimiting example illustrated in FIG. 1, the discharge means of the measurement branch 20 comprise a switch 23 arranged between the first electrode 21a of the measurement capacitor 21 and the electrical ground. When the switch 23 is closed, the measuring capacitor 21 is discharged. When the switch 23 is open, the capacitor 21 of measurement does not discharge.

As illustrated in FIG. 1, the reference branch 30 comprises a reference capacitor 31. The reference capacitor 31 comprises two terminals formed by a first electrode 31a connected to the node 11 and by a second electrode 31b connected to the electrical ground.

The reference branch 30 further includes charging means for charging said reference capacitor 31 from the source 50 of DC voltage via the node 11. In the non-limiting example illustrated in FIG. the charging means of the reference branch 30 comprise a switch 32 arranged between the node 11 and the first electrode 31a of the reference capacitor 31. When the switch 32 is closed, the reference capacitor 31 is connected to the node 11, through which the DC voltage source 50 loads said reference capacitor 31. When the switch 32 is open, the reference capacitor 31 is disconnected from the node 11, so that the DC voltage source 50 does not charge the reference capacitor 31.

The reference branch 30 also includes discharge means for discharging said reference capacitor 31. In the nonlimiting example illustrated in FIG. 1, the discharge means of the reference branch 30 comprise a switch 33 arranged between the first electrode 31a of the reference capacitor 31 and the electrical ground. When the switch 33 is closed, the reference capacitor 31 discharges. When the switch 33 is open, the reference capacitor 31 does not discharge.

The detection sensor 10 further comprises comparison means connected to the respective outputs of said measurement branch 20 and said reference branch. The comparison means are configured to compare a measurement voltage, representative of the voltage Vm across the capacitor 21 of measurement, with a reference voltage representative of the voltage Vr across the capacitor 31 of reference. In the nonlimiting example illustrated in FIG. 1, the measurement voltage corresponds directly to the voltage Vm across the capacitor 21 of measurement and the reference voltage corresponds directly to the voltage Vr across the capacitor 31 of reference.

In the nonlimiting example illustrated in FIG. 1, the comparison means comprise a comparator circuit 40 based on an operational amplifier, comprising: a non-inverting input "+" connected to the first electrode 21a of the capacitor 21 of measuring, • an inverting input "-" connected to the first electrode 31a of the capacitor 31 of reference.

More generally, other types of comparison means can be implemented, and the choice of a particular type of comparison means is only a variant of implementation of the invention.

In the nonlimiting example illustrated in FIG. 1, the measurement branch 20 comprises a current source 24 arranged between the switch 22 and the first electrode 21a of the measurement capacitor 21. The current source 24 is adapted to provide a DC current (i.e. substantially constant) to the measurement capacitor 21 from the voltage supplied by the DC voltage source 50. For example, the source 24 of current is a current mirror type circuit. When the switch 22 is closed, the measurement capacitor 21 is charged with a current of intensity I0 continuous by said source 24 of current, which depends on the value Vcc of the voltage supplied by the source 50 of DC voltage. Consequently, the voltage Vm across the capacitor 21 of measurement gradually increases, linearly, with a slope that depends on the capacitance Cm of said capacitor 21 for measuring, according to the following expression:

Vm (t) = IOt / Cm

In addition, in the nonlimiting example illustrated in FIG. 1, the reference branch 30 comprises a resistive component 34 arranged between the node 11 and the first electrode 31a of the reference capacitor 31. Consequently, when the switch 32 is closed, the voltage Vr across the reference capacitor 31 increases progressively and tends towards the value Vcc of the voltage supplied by the DC voltage source 50, according to the following expression: r _t_ \

Vr (t) - VccX l-e RrxCr v y expression in which: • Rr is the resistance of the resistive component 34, • Cr is the capacitance of the reference capacitor 31.

FIGS. 2 and 3 diagrammatically represent alternative embodiments of the detection sensor 10 shown in FIG.

The detection sensor 10 illustrated in FIG. 2 uses the same elements as those of the detection sensor 10 of FIG. 1, with the exception of the resistive component 34 which is replaced by a current source 35. The current source 35 is arranged between the node 11 and the first electrode 31a of the reference capacitor 31, and is adapted to supply a current of continuous intensity to the reference capacitor 31, from the voltage supplied by the source 50 of the reference capacitor 31. DC voltage. The voltage Vr across the capacitor 31 of reference therefore increases linearly over time, as the voltage Vm across the capacitor 21 measurement.

The detection sensor 10 illustrated in FIG. 3 uses the same elements as those of the detection sensor 10 of FIG. 1, with the exception of the current source 24 of the measurement branch 20 and the resistive component 34 of the branch Reference. On the other hand, the detection sensor 10 of FIG. 3 comprises a source 12 of current between the DC voltage source 50 and the node 11, said current source 12 being adapted to form a DC current from the voltage supplied by the source 50 of DC voltage.

In particular embodiments, the measurement branch 20 may further comprise, between the measurement capacitor 21 and the output of said measurement branch 20, a sample-and-hold circuit. This is advantageous, in particular, if the input impedance of the comparison means is not very high, so that the measurement capacitor 21 is likely to be discharged towards the comparison means by causing a progressive decrease in the voltage measurement. The use of a sample-and-hold circuit, between the measurement capacitor 21 and the comparison means, makes it possible to keep, after having loaded said measurement capacitor 21, a constant measurement voltage at the input of the comparison means during the charge of the capacitor 31 of reference. In addition, the measuring capacitor 21 behaves like an antenna, so that the measuring voltage is also likely to change, after charging said measuring capacitor, under the influence of external noise measured by said measuring capacitor 21 . The use of a sample-and-hold circuit between the measurement capacitor 21 and the comparison means makes it possible to limit the duration during which the measurement voltage is sensitive to external noise, to the charging duration of said capacitor. 21 of measurement.

The detection sensor 10 is intended to be implemented in a detection system, comprising said detection sensor 10, and further comprising a control device (not shown in the figures).

The control device of the detection system is adapted to control the charging means and the discharge means of the measurement branch 20 and the reference branch. The control device is further connected to the comparison means 40 of the detection sensor 10, so that said control device is informed of the result of the comparison between the measurement voltage Vm and the reference voltage Vr.

The control device comprises for example one or more processors and storage means (magnetic hard disk, electronic memory, optical disk, etc.) in which is stored a computer program product, in the form of a set of program code instructions to be executed to implement the different steps of a capacitance variation detection method Cm of the measurement capacitor 21. In a variant, the control device comprises one or more programmable logic circuits (FPGA, PLD, etc.), and / or one or more specialized integrated circuits (ASIC), and / or a set of discrete electronic components, etc., adapted to implement all or part of the steps of the capacitance detection process 60 of the measuring capacitor 21.

In other words, the control device comprises a set of means configured in software (specific computer program product) and / or hardware (FPGA, PLD, ASIC, discrete electronic components, etc.) to implement the steps of the capacitance detection detection method 60 of the measurement capacitor 21.

Figure 4 shows the main steps of a detection method. As shown in FIG. 4, the detection method 60 executes a measurement sequence comprising steps of: • charging the measuring capacitor 21 for a predetermined duration δT. The charge of the reference capacitor 31 up to obtain a reference voltage Vr equal to the measurement voltage Vm obtained after having charged the measurement capacitor 21 for the duration δT, measurement of a duration, called the "duration of crossover >>, representative of the time required to obtain a reference voltage Vr equal to the measurement voltage Vm obtained after charging the measurement capacitor 21 during said duration δT, • 64 discharging the measurement capacitor 21 and the capacitor 31 of reference after measuring the crossing time.

Since the measuring capacitor 21 is charged for a predetermined fixed duration δΤ, the voltage Vm at the terminals of the measurement capacitor 21 (and therefore the measuring voltage), at the end of the charging step 61, depends mainly on the capacity of the measuring capacitor 21 and decreases as said capacitance of the measuring capacitor 21 increases.

The capacity of the reference capacitor 31 is constant over time, so that the crossing time depends mainly on the value of the measurement voltage Vm that the reference voltage Vr must reach, and increases with said measurement voltage Vm. Since the measurement voltage Vm decreases as the capacitance of the measurement capacitor 21 increases, the crossover duration also decreases as said capacitance Cm of the measurement capacitor 21 increases. In other words, an increase (respectively a decrease) in the capacitance Cm of the measurement capacitor 21 causes a decrease (respectively an increase) in the crossing time.

The measurement sequence described above is therefore executed recurrently, considering in each case the same duration δΤ of charge of the capacitor 21 of measurement, in order to measure the evolution over time of the crossing time, in order to be able to detecting variations in said crossing time. The detection method 60 then comprises a step 65 for detecting variation in the capacitance of the measurement capacitor 21 as a function of successive measurements of the crossing time, obtained during successive measurement sequences. A capacitance variation of the measuring capacitor 21 is detected when a predetermined detection criterion is verified. If the detection criterion is checked (reference 650 in FIG. 4), the control device may, for example, interrupt the execution of the detection method 60. In the opposite case (reference 651 in FIG. 4), the control device carries out a new measurement sequence, etc.

FIG. 5 diagrammatically represents time diagrams illustrating the capacitance detection detection principle of the measurement capacitor 21, considering in a nonlimiting manner the case of the detection sensor 10 represented in FIG. 1.

In the remainder of the description, one places oneself in a nonlimiting manner in the case where the detection system is implemented in a motor vehicle (not shown in the figures), to detect the presence of a user's hand. near an equipment of said motor vehicle. For example, the equipment is a handle of a door of the motor vehicle, in which case the first electrode 21a of the measuring capacitor 21 is preferably installed inside said handle.

FIG. 5 represents the evolution over time of the voltage Vm at the terminals of the measurement capacitor 21 (in solid lines) and of the voltage Vr at the terminals of the reference capacitor 31 (in broken lines).

Part a) of FIG. 5 represents a measurement sequence in the case where no hand is present near the handle, while part b) represents a measurement sequence in the case where a hand is present in the vicinity. of said handle.

As illustrated by part a) of FIG. 5, at the beginning of the measurement sequence, the voltage Vm is zero, that is to say that the measurement capacitor 21 is completely discharged. At a time TO, the control device closes the switch 22 (the switch 23 being open), which causes the start of the step 61 of charging the capacitor 21 of measurement, during which the voltage Vm increases by linear way over time. At a time T1 (T1 = ΤΟ + δΤ), the control device opens the switch 22, which causes the end of the charge of the capacitor 21 measurement. At time T1, the voltage Vm has reached a value VO equal to: 70

An expression in which: • I0 is the intensity of the current supplied by the source 24 of current, • Cm is the capacitance of the capacitor 21 of measurement, in the absence of a hand near the handle.

At the beginning of the measurement sequence, the voltage Vr is also zero in the nonlimiting example illustrated in FIG. 5. At the instant T1, the control device closes the switch 32 (the switch 33 being open), this causes the start of the step 62 of charging the capacitor 31 of reference, during which the voltage Vr increases gradually over time until reaching, at a time T2, the value VO.

It should be noted that, in the case of the detection sensor 10 of FIG. 1, the maximum voltage at the terminals of the reference capacitor 31 corresponds to the voltage Vcc supplied by the DC voltage source 50. On the other hand, the voltage at the terminals of the measurement capacitor 21 is theoretically not limited. Therefore, the charging duration δΤ of the measuring capacitor 21 must be previously calibrated so as to ensure that the maximum value of the voltage Vm across the capacitor 21 of measurement, at the end of the charging step 61, is always less than Vcc, so that the voltage Vr across the capacitor 31 of reference can reach the same value as the voltage Vm across the capacitor 21 measurement.

The crossed crossing time measured is equal to ΔΤ and corresponds, in the example illustrated in FIG. 5, to the time elapsed between the beginning of step 61 of charging the capacitor 21 of measurement (instant TO) and the instant at which the Voltage Vr reaches the same value as voltage Vm (instant T2).

It should be noted, however, that other events can be considered for the moment at the beginning of the crossing time, since the same event is considered for each of the measurement sequences. For example, the measured crossover duration may equivalently correspond to the time elapsed between the end of the step 61 of charging the capacitor 21 of measurement (instant T1 = ΤΟ + δΤ) and the instant at which the voltage Vr reaches the same value than the voltage Vm (instant T2).

As indicated above, part b) represents the case where a hand is present near said handle, causing an increase ACm of the capacitance of said capacitor 21 measurement. The capacitance of said capacitor 21 of measurement is therefore equal to (Cm + ACm).

As illustrated by part b) of FIG. 5, at a time TO ', the control device closes the switch 22 (the switch 23 being open), which causes the start of step 61 of charging measuring capacitor 21, during which the voltage Vm increases linearly over time. At a time TT (TT = TO '+ ÔT), the control device opens the switch 22, which causes the end of the charge of the capacitor 21 measurement. At the moment TT, the voltage Vm has reached a value VO 'equal to:

70 '= IOxST (Cm + Δ Cm)

With TO '<TO At the moment TT, the control device closes the switch 32 (the switch 33 being open), which causes the start of the step 62 of charging the capacitor 31 of reference, during which the voltage Vr increases gradually over time until reaching, at a time T2 ', the same value VO' as the voltage Vm.

As illustrated by part b) of FIG. 5, the measured crossing time is equal to ΔΤ ', and is smaller than the crossing time ΔΤ measured in the absence of a hand near the handle and illustrated in part at).

It is therefore possible to detect the presence of the hand according to the successive measurements of the crossing time, and whatever the embodiment considered for the detection sensor 10 implemented.

If the crossing time decreases between two measurement sequences, and if this decrease is important, then it means that a hand has moved closer to said first electrode 21a of the measurement capacitor 21. For example, if we consider two measures ΔΤ and ΔΤ 'of the crossing time, measured for two consecutive measurement sequences at respective times T2 and T2' (T2 '> T2), then the detection criterion is for example checked if (ΔΤ - ΔΤ ')> C1, where C1 is a predetermined positive threshold value. According to another example, the detection criterion can be considered as verified when ΔΤ / ΔΤ '> C2, expression in which C2 is a predetermined positive threshold value, etc.

It should be noted that it is also possible, according to other examples, to consider a detection criterion which only takes into account a measure of the crossing time. Indeed, it is possible, by a prior calibration of the detection sensor 10, to determine the theoretical value of the crossing time in the absence of hand. It is then possible to define a positive threshold value C3 for the crossing time, less than said theoretical value of the crossing time in the absence of a hand, corresponding to a maximum value of the crossing time in the presence of a hand. . Therefore, if the crossing time measured during a measurement sequence is less than the threshold value C3, it means that a hand is close to the handle of the door of the motor vehicle. The step 64 of discharging the measurement capacitor 21 and the reference capacitor 31 (not shown in FIG. 5) consists, in the case of the detection sensors 10 of FIGS. 1 to 3, in closing the switches 23 and 33 (the switches 22 and 32 being open).

In the example illustrated in FIG. 5, the charging step 62 of the reference capacitor 31 starts immediately after the end of step 61 of charging the measurement capacitor 21. However, according to other examples, nothing makes it possible to start the step 62 of charging the reference capacitor 31 at other times, including during step 61 of charging the capacitor 21 for measurement. On the other hand, the event that triggers the start of the charging step 62 of the reference capacitor 31 must be the same from one measurement sequence to another.

It should be noted that it is advantageous to have, during step 62 of charging the capacitor 31 of reference, a voltage Vr which increases slowly at the instant at which said voltage Vr reaches the same value as the voltage Vm at the terminals of the capacitor 21 of measurement. Such arrangements make it possible to detect smaller variations in the capacitance of the measurement capacitor 21. However, the more the increase in the voltage Vr is slow, and the longer the duration of the measurement sequence is important and makes it difficult to quickly detect the presence of a hand near the handle.

In order to reduce the duration of the measurement sequence, to allow faster detection of a hand near the handle, it is possible to pre-charge the reference capacitor 31 before the start of step 62 of charging said capacitor 31 of reference. Thus, the voltage Vr across the capacitor 31 of reference is not zero at the beginning of the charging step 62, so that it will reach more quickly the same value as the voltage Vm across the capacitor 21 of measurement . For example, each measurement sequence may comprise a precharging step (not shown in the figures) of the reference capacitor 31 to a predefined value for the voltage Vr, the pre-charge step being for example executed before the step 61 of the capacitor 21 of measurement. It is also possible, according to other examples, to only partially discharge said reference capacitor 31 during the discharge step 64, to said predefined value for the voltage Vr.

More generally, it should be noted that the modes of implementation and realization considered above have been described by way of non-limiting examples, and that other variants are therefore possible.

In particular, the invention has been described by considering that the measurement capacitor 21 was charged via a direct current source, so that the voltage Vm across said measuring capacitor 21 increases linearly with time. . According to other examples, nothing excludes charging the measurement capacitor 21 other than via a direct current source, for example simply by inserting a resistive component between the node 11 and the first electrode 21a of said source. capacitor 21 of measurement.

Claims (9)

1. Sensor (10) for detecting capacitance variation of a measurement capacitor (21), comprising a measurement branch (20) and a reference branch (30) each having an input and an output, the respective inputs of said measurement branch (20) and said reference branch (30) being connected to one another and being intended to be connected to the same source (50) of DC voltage, and in which: • the measuring branch (20) comprises the measuring capacitor (21), charging means (22) for charging said measuring capacitor from the DC voltage source and discharge means (23) for discharging said measuring capacitor; 30) comprises a reference capacitor (31), charging means (32) for charging said reference capacitor from the DC voltage source and discharge means (33) for discharging said capacitor. reference, said sensor (10) further comprising comparator means (40), connected to the respective outputs of said measurement branch (20) and said reference branch (30), configured to compare a measurement voltage (Vm ), representative of the voltage at the terminals of the measurement capacitor (21), and a reference voltage (Vr) representative of the voltage at the terminals of the reference capacitor (31), said sensor being characterized in that the branch (20) device comprises, between the input of said measurement branch and the measurement capacitor (21), a source (24) of current adapted to supply a current of continuous intensity to the measurement capacitor, starting from the voltage supplied by the source (50) of DC voltage. 2. Sensor (10) according to the preceding claim, wherein the reference branch (30) comprises, between the input of said reference branch and the reference capacitor (31), a current source (35) adapted to provide a DC current to the reference capacitor from the voltage supplied by the DC voltage source (50). The sensor (10) according to claim 1, wherein the reference branch (30) comprises a resistive component (34) between the input of said reference branch and the reference capacitor (31). 4. The sensor (10) according to claim 1, comprising a source (12) of current between the DC voltage source (50) and the respective inputs of the measurement branch (20) and of the reference branch (30). said current source being adapted to form a DC current from the voltage supplied by the DC voltage source. 5. A sensor (10) according to any one of the preceding claims, wherein the measuring arm (20) comprises, between the measuring capacitor (21) and the output of said measurement branch, a sample-and-hold circuit. . 6. System for detecting variation in capacitance of a measurement capacitor (21), characterized in that it comprises a detection sensor (10) according to any one of the preceding claims, and a control device configured for: • control the respective load means of the measuring branch (20) and the reference branch (30) so as to charge the measuring capacitor (21) for a predetermined duration (δΤ) and so as to charge the capacitor ( 31) to obtain a reference voltage (Vr) equal to the measurement voltage (Vm) obtained after charging the measuring capacitor during said predetermined time, • measuring a representative duration, called "crossover duration" the time required to obtain a reference voltage (Vr) equal to the measurement voltage (Vm) obtained after charging the measurement capacitor (21) during said predetermined time, the respective discharge means of the measurement branch (20) and of the reference branch (30) so as to discharge the measuring capacitor (21) and the reference capacitor (31) after measuring the crossover duration, Detecting a variation of the capacitance of the measurement capacitor (21) as a function of measured crossover times for successive charges of the measurement capacitor (21) and the reference capacitor (31). 7. Motor vehicle, characterized in that it comprises a detection system according to claim 6. 8. Method (60) for detecting variation in capacitance of a measurement capacitor (21) by means of a detection sensor (10) according to any one of Claims 1 to 5, characterized in that it comprises recurrent steps of: • (61) charging the measuring capacitor (21) for a predetermined time (δΤ), • (62) charging the reference capacitor (31) until a reference voltage is obtained equal to the measuring voltage obtained after having charged the measuring capacitor (21) during said predetermined duration, • (63) measuring a duration, called "crossover duration", representative of the time required to obtain a reference voltage (Vr) equal to the measurement voltage (Vm) obtained after charging the measuring capacitor during said predetermined time, • (64) discharging the measurement capacitor (21) and the reference capacitor (31) after measuring the duration e), said method (60) further comprising a step (65) for detecting variation in the capacitance of the measurement capacitor (21) as a function of measured crossover durations for successive charges of the measurement capacitor (21). of the capacitor (31) of reference. 9. Method (60) according to claim 8, comprising, before each step (62) of charging the capacitor (31) of reference, a step of pre-charging said capacitor (31) of reference, so that the voltage at the terminals said reference capacitor (31) is non-zero at the beginning of the charging step (62) of said reference capacitor (31).