WO2021104712A1

WO2021104712A1 - Noise reduction sensor for a motor vehicle

Info

Publication number: WO2021104712A1
Application number: PCT/EP2020/076848
Authority: WO
Inventors: Cédric VERGNIERES; Xavier Hourne
Original assignee: Vitesco Technologies GmbH
Priority date: 2019-11-26
Filing date: 2020-09-25
Publication date: 2021-06-03
Also published as: FR3103564B1; FR3103564A1

Abstract

The invention relates to a capacitive sensor (1) for a motor vehicle, the sensor (1) comprising a storage capacitor (Cext) comprising a first branch (B1) and a second branch (B2), a detection capacitor (Ce) comprising a first branch (D1), connected to the second branch (B2) of the storage capacitor (Cext) at a central point (A), and a second branch (D2) intended to be connected to a ground (M), a first switch (S1), a second switch (S2), a third switch (S3), the capacitive sensor (1) comprising a bias resistor (Rpolar) connected, on the one hand, between the central point (A) of the storage capacitor (Cext) and the detection capacitor (Ce) and, on the other hand, to the second branch (S12) of the first switch (S1).

Description

Title of the invention: Anti-noise sensor for motor vehicles

Technical area

The invention relates to the field of sensors for motor vehicles and more particularly a capacitive sensor for a motor vehicle. The invention aims in particular to improve the robustness to low-frequency noise of capacitive sensors for motor vehicles.

State of the art

[0002] In a motor vehicle, it is known to use capacitive sensors to detect the presence of a user. A capacitive sensor can for example be mounted in a vehicle door handle in order to detect the presence of a hand on said handle and unlock the door. A capacitive sensor can for example also be mounted under the trunk of the vehicle in order to detect the presence of a foot nearby and thus allow the opening of said trunk.

[0003] Different types of capacitive sensor technology can be used depending on the application. In particular, in the case of a sensor for detecting the presence of a foot under a vehicle trunk, it is in particular known to use a capacitive sensor based on the principle of measurement of the amplified capacitive divider (Amplified Capacitive Divider). or ACD).

[0004] This measuring principle makes it possible to measure the variation of a capacitance over time. This principle is based on the use of a capacitive voltage divider bridge produced with a so-called “storage” physical capacity (electronic component soldered onto a printed circuit) and a sensitive element in the form of an electrode modeled by a so-called “detection” capacity. As the user's foot approaches the electrode, the capacitance value of the electrode increases.

FIG. 1 shows an example of such a capacitive sensor 100. The sensor 100 comprises a first switch S1 connected on the one hand to the midpoint of the assembly formed by the storage capacitor Cext and the detection capacitor Ce and on the other hand to a supply voltage Vcc. The detection capacitor Ce is also connected to a ground M while the storage capacitor Cext is also connected to the midpoint of an assembly formed by a second switch S2 and a third switch S3, said second switch S2 being of on the other hand connected to the supply voltage Vcc and said third switch S3 being on the other hand connected to ground M. A so-called “parasitic” _capacitor C is connected between ground and the midpoint of the capacitor Cext storage and Ce detection capability. This parasitic capacitance C _dparasitic corresponds to a capacitance representing the parasitic noises of the circuit.

The capacity measurement allowing the detection of the presence or absence of a user near the sensor 100 is carried out in two steps. First of all, an initialization step is carried out by closing the first switch S1 and the second switch S2 and by opening the third switch S3, which has the effect of charging the detection capacitor Ce to the supply voltage Vcc while discharging the Cext storage capacity which is found short-circuited. When the circuit stabilizes, the initialization step is interrupted by opening the first switch S1. Then, a sharing step is carried out by opening the second switch S2 and by closing the third switch S3 so that the so-called “detection” voltage Vc _e defined at the terminals of the detection capacitor Ce decreases to a value defined according to the following equation:

[0007] [Math. 1]

[0008]

The detection voltage Vc _e is amplified and digitized by an analog digital converter ADC (Analog Digital Converter). When a user approaches the foot of the sensor 100, the capacitance of the detection capacitor Ce increases, which increases the value of the detection voltage Vc _e.

FIG. 2 shows the sensor of FIG. 1 coupled to a noise source 200. This noise source 200 is modeled in FIG. 2 using a so-called “BCI disturbance” model ( Bulk Current Injection), known per se, which tests the immunity of the sensor to noise sources coupled and then conducted to the power and communication wire harness connected to the sensor. This model comprises a sinusoidal VDISR voltage source at the frequency of the disturbance and coupled with the sensor, this coupling being modeled by the disturbance capacitor C _disruption connected to the midpoint of the assembly formed by the storage capacitor Cext and the capacitance detection Ce. This voltage source V _DISR is characterized by its peak voltage (hereinafter denoted Vpeak) and its pulsation (denoted hereinafter w) which varies as a function of time (hereinafter denoted t).

There is shown in Figures 3 to 5 examples of behavior of the circuit of Figure 2 in equivalent diagram called "small signals", that is to say for signals whose values do not vary sufficiently so that the characteristics of the components vary little and remain in a linear approximation.

In the presence of this noise source 200, when the first switch S1 is closed, the detection capacity Ce is not impacted by the disturbance, the detection voltage Vc _e is equal to the supply voltage Vcc and the capacity of disturbance C _disruption is connected to ground M, illustrated in FIG. 3, in a small signal equivalent diagram.

In the presence of this noise source 200, when the first switch S1 is open (the second switch S2 being open and the third switch S3 being closed in the sharing step), the detection capacitor Ce is found in parallel of the Cext storage capacity so that the values of these two capacities are added (figure 4).

The resulting detection voltage Vc _e depends on time and is expressed according to the following equation:

[0015] [Math. 2]

[0016]

[0017] where ΔV _disruption is the voltage offset when the average of the additional term V _{DC_added} is not zero.

More specifically, the voltage shift AV _disruption as a function of time t is written:

[0019] [Math. 3]

[0020] ΔV _disruption = Vpeak * (sin (wt) - sin (wt ₀ ))

Where t ₀ corresponds to the opening instant of the first switch S1, V _peak is the peak voltage of the signal and w is the pulse of the signal.

The effect of this voltage shift is illustrated in Figure 5 which describes three examples of the evolution of the detection voltage Vc _e depending on the state of the first switch S1 and the presence or not of a noise source 200 which creates a disturbance.

In the first example (on the left of FIG. 5), the detection voltage Vc _e varies in the absence of a noise source 200. It is noted that, when the first switch S1 goes from the open state (state (I) in figure 5) in the closed state (state (II) in figure 5) and the second switch S2 is open, then the third switch S3 is closed, the detection voltage Vc _e changes to Vcc / 2 (in this simulation, the Ce and Cext capacities are of equal value).

In the second example (middle of Figure 5), the detection voltage Vc _e varies in the presence of a noise source 200 phase-shifted by 90 °. In this example, the disturbance DISR caused by the noise source 200 is at the maximum value when the first switch S1 opens (change from state (I) to state (II) in FIG. 5). On opening, the capacitor Ce becomes floating and a fraction of the disturbance DISR is added to the detection voltage signal Vc _e . In other words, a DC voltage is added to the detection voltage Vc _e . The value of this direct voltage is equal to:

[0025] [Math. 4]

Where V _peak is the peak voltage of the signal in the presence of the disturbance DISR.

In the third example (to the right of FIG. 5), the detection voltage Vc _e varies in the presence of a noise source 200 phase-shifted by 270 °. In this example, the disturbance DISR caused by the noise source 200 is at the minimum value when the first switch S1 opens (change from state (I) to state (II) in FIG. 5). On opening, the capacitor Ce becomes floating and a fraction of the disturbance DISR is added to the detection voltage signal Vc _e. In other words, a DC voltage is added to the detection voltage Vc _e. The value of this direct voltage is equal to:

[0028] [Math. 5]

Thus, we see that the additional term V _{DC_Added} can be written more generally according to the following equation:

[0032] [Math. 6]

[0034] where φ is the phase of the sine of the DISR disturbance at the instant of opening of the first switch S1.

In summary, the capacitive sensor amplifies then measures the value of the detection voltage Vc _e which results from the capacitive divider bridge. If the value of the DC voltage varies as a function of a disturbance, this results in measurement noise. At each measurement, the phasing between the closing of the first switch S1 and the sine of the disturbance has a very high probability of being different, then generating the addition of a _{random direct} voltage V DC_Added to each measurement. 6. During the tests carried out from the noise source 200, this voltage value may be much greater than the voltage variation generated by the approach of a foot thus making it impossible to detect said foot, but may also cause false detections if the signature of the voltage variation due to the disturbance resembles the signature of the passage of a foot.

[0036] There is therefore a need for a simple, reliable and effective solution making it possible to remedy at least in part these drawbacks.

Disclosure of the invention To this end, the invention relates first of all to a capacitive sensor for a motor vehicle, said sensor comprising:

- a storage capacity comprising a first branch and a second branch,

- a detection capacitor comprising a first branch, connected to the second branch of the storage capacity at a midpoint, and a second branch, intended to be connected to a ground,

- a first switch comprising a first branch, intended to be connected to a supply voltage, and a second branch,

- a second switch, comprising a first branch, intended to be connected to the supply voltage, and a second branch,

- a third switch, comprising a first branch, connected to the second branch of the second switch at a midpoint, and a second branch, intended to be connected to ground, the first branch of the storage capacity being connected to said midpoint, said capacitive sensor being remarkable in that it comprises a polarization resistor connected on the one hand between the midpoint of the storage capacitor and of the detection capacitor and on the other hand to the second branch of the first switch .

The sensor according to the invention makes it possible to attenuate or even eliminate the influence of the noises of the engine of the vehicle on the measurements of the sensor so as to reduce or even avoid both non-detections or false detections by the sensor .

Advantageously, the value of the bias resistance is greater than 20 kΩ, or even 50 kΩ.

Preferably, the value of the bias resistance is greater than 65 kΩ.

More preferably, the value of the bias resistance is greater than 100 kΩ or even 150 kΩ.

[0042] According to one aspect of the invention, the sensor comprises a printed circuit on which the bias resistor is mounted.

Advantageously, the storage capacity is mounted on the printed circuit.

The invention also relates to a motor vehicle comprising at least one capacitive sensor as presented above.

[0045] According to one characteristic of the invention, the vehicle comprises an electronic control unit connected to at least one sensor and able to control the first switch, the second switch and the third switch in opening and closing.

Description of figures

Other characteristics and advantages of the invention will appear further on reading the description which follows. This is purely illustrative and should be read in conjunction with the accompanying drawings in which:

[0047] [Fig. 1]: Figure 1 schematically illustrates an embodiment of a prior art sensor.

[0048] [Fig. 2]: FIG. 2 illustrates the sensor of FIG. 1 connected to a noise source.

[0049] [Fig. 3]: FIG. 3 illustrates an equivalent electric diagram of the circuit of FIG. 2 when the first switch is closed.

[0050] [Fig. 4]: FIG. 4 illustrates an equivalent electric diagram of the circuit of FIG. 2 when the first switch is open.

[0051] [Fig. 5]: Figure 5 illustrates three examples of changes in the detection voltage of the circuit of Figure 2 depending on the absence or presence of a disturbance caused by the noise source.

[0052] [Fig. 6]: Figure 6 schematically illustrates an embodiment of a sensor according to the invention.

[0053] [Fig. 7]: FIG. 7 illustrates an equivalent electric diagram of the circuit of FIG. 6 when the first switch is closed.

[0054] [Fig. 8]: FIG. 8 illustrates an equivalent electric diagram of the circuit of FIG. 6 when the first switch is open.

[0055] [Fig. 9]: FIG. 9 illustrates an example of the change in the ratio of the impedance of the first closed switch to the impedance of the first open switch as a function of the value of the polarization resistor R _polar .

Detailed description of at least one embodiment

The capacitive sensor according to the invention is intended to be mounted in a motor vehicle, for example in order to detect the presence of a user, in particular a foot or a hand, near said sensor in order to enable a or more vehicle functions. For example, the capacitive sensor according to the invention can be used to detect the presence of a human foot and unlock a vehicle trunk when said foot passes or is positioned sufficiently close to the sensor, for example less than 10 or 20 cm .

FIG. 6 shows an embodiment of the capacitive sensor 1 according to the invention. The sensor 1 comprises a storage capacitor Cext, an electrode in the form of a detection capacitor Ce, a first switch S1, a second switch S2, a third switch S3 and a polarization resistor R _polar . The sensor 1 is intended to be connected to an electronic control unit of the vehicle (not shown) which is able to control the first switch S1, the second switch S2 and the third switch S3 in opening and closing. The sensor 1 is coupled to a source of noise 2 generated by the vehicle engine. This noise source 200 is modeled in FIG. 6 using a so-called “BCI disturbance” model (Bulk Current Injection) which tests the immunity of the sensor to noise sources coupled and then conducted to the cable bundle of power supply and communication connected to the sensor. This model comprises a source of sinusoidal voltage VDISR at the frequency of the disturbance and coupled with the sensor, this coupling being modeled by the disturbance capacitor C _disruption connected to the midpoint of the assembly formed by the storage capacitor Cext and the capacitance detection Ce.

The storage capacity Cext comprises a first branch B1 and a second branch B2. The detection capacity Ce comprises a first branch D1, connected to the second branch B2 of the storage capacity Cext at a midpoint A, and a second branch D2, connected to a ground M.

The first switch S1 comprises a first branch Sll, connected to a supply voltage Vcc, and a second branch S 12. The second switch S2 comprises a first branch S21, connected to the supply voltage Vcc, and a second branch S22. The third switch S3 comprises a first branch S31, connected to the second branch S22 of the second switch S2 at a midpoint B, and a second branch S32, connected to ground M, the first branch of the storage capacitor Cext being connected to said midpoint B.

The polarization resistor R _polar connected on the one hand between the midpoint A of the storage capacitor Cext and the detection capacitor Ce and on the other hand to the second branch S12 of the first switch S1.

The implementation of the capacitive sensor will now be described with reference to Figures 7 to 9.

First of all, when the first switch Sl is closed, the detection capacitor Ce is located in parallel with the storage capacitor Cext in a small signal equivalent diagram so that the values of these two capacitors are added as illustrated. in FIG. 7, the polarization resistor R _polar being itself in parallel with the detection capacitor Ce and the storage capacitor Cext. In fact, in a small signal equivalent diagram, the polarization resistor R _polar is connected to the supply voltage Vcc which can be considered as the ground M in a small signal equivalent diagram.

In this case, the detection voltage Vc _e at the terminals of the detection capacitor Ce as a function of time t is defined according to the following equation:

[0065] [Math. 7]

[0066]

Where Z1 is the impedance equivalent to the disturbance pulse w of the assembly formed by the polarization resistor R _polar connected in parallel with the sum of the storage capacitor Cext and the detection capacitance Ce and Z _Cdisrupt is the impedance equivalent to the disturbance pulse w of the capacity C _disruption.

Then, when the first switch S1 is open, the polarization resistor R _polar is disconnected and the detection capacitor Ce is located in parallel with the storage capacitor Cext so that the values of these two capacitors are added as shown in figure 8.

In this case, the detection voltage Vc _e at the terminals of the detection capacitor Ce is defined according to the following equation:

[0070] [Math. 8]

Where Z2 is the impedance equivalent to the disturbance pulse w of the set formed by the storage capacitor Cext and the detection capacitor Ce connected in parallel and Z _Cdisrupt is the impedance equivalent to the disturbance pulse w of Capacity C _disruption .

Thus, to prevent the disturbance from adding a DC component to the opening of the first switch S1, the detection voltage Vc _e at the terminals of the detection capacitor Ce must remain constant, that is to say park the same value before and after opening the first switch S1.

Ideally, therefore:

[0075] [Math. 9]

[0076]

The impedance of the polarization resistor R _polar in parallel with the impedance Z2 corresponds to the impedance Z1. Consequently, if the impedance of the polarization resistor R _polar is large compared to Z2 then Z1 will tend towards Z2 and the value of the detection voltage Vc _e will be substantially the same regardless of the state of the first switch S1. It is therefore advisable to choose a polarization resistance value R _polar much greater than Z2 in order to significantly reduce, or even almost cancel, the effects of disturbances caused by a noise source 2.

The terms

[0079] [Math. 10]

[0080]

[0081] and [0082] [Math. 11] [0083]

Are the impedance ratios seen by the disturbance DISR when the first switch S1 is respectively open and closed. To simplify the writing, they will subsequently be called

The ratio [0086] [Math. 12]

[0087]

[0088] can then be analyzed in order to judge the improvement. Without the Rpolar bias resistor, the impedance

[0089] [Math. 13] [0090] is equal to 0 and the best possible case is equality, ie a ratio of 1. Un

a result of 0.5 will mean that the disturbance has been attenuated by 50%, a result of 0.75 that has been attenuated by 75% and so on.

Numerical example

By taking C _disruption = 0.5 pF; Ce + Cext = 50 pF; w = 27π * (100 kHz), we have: [0093] [Math. 14]

The calculation of

[0096] [Math. 15]

[0097]

For different values of the polarization resistance R _polar is given in the following table:

[0099] [Table. 1]

[0100] Figure 9 shows the change in the ratio of the impedance of the first closed switch to the impedance of the first open switch

[0101] [Math. 16]

[0102]

[0103] as a function of the value of the polarization resistor R _polar for the values of the example used previously (Table 1).

Thus, it is noted that a value of the polarization resistance R _polar greater than 65 kΩ makes it possible to eliminate more than 90% of the disturbance phenomenon due to the noise source 2.

[0105] The invention thus makes it possible to drastically reduce the disturbances of the capacitive sensors due to the noise generated by a motor vehicle engine in a simple, reliable and efficient manner.

Claims

[Claim 1] Capacitive sensor (1) for a motor vehicle, said sensor (1) comprising:

- a storage capacity (Cext) comprising a first branch (B1) and a second branch (B2),

- a detection capacitor (Ce) comprising a first branch (D1), connected to the second branch (B2) of the storage capacitor (Cext) at a midpoint (A), and a second branch (D2) , intended to be connected to a mass (M),

- a first switch (S1) comprising a first branch (S11), intended to be connected to a supply voltage (Vcc), and a second branch (S12),

- a second switch (S2), comprising a first branch (S21), intended to be connected to the supply voltage (Vcc), and a second branch (S22),

- a third switch (S3), comprising a first branch (S31), connected to the second branch (S22) of the second switch (S2) at a midpoint (B), and a second branch (S32), intended to be connected to ground (M), the first branch (Bl) of the storage capacity (Cext) being connected to said midpoint (B), the sensor (1) being coupled to a noise source (2) modeled by :

-a sinusoidal voltage source (VDISR) at a frequency of a disturbance and coupled with the sensor (1) by

a disturbance capacitor (Cdisruption), connected to the midpoint of an assembly formed by the storage capacitor (Cext) and the detection capacitor (Ce), said capacitive sensor (1) being characterized in that it comprises a polarization resistor (Rpolar) connected on the one hand between the midpoint (A) of the storage capacitor (Cext) and of the detection capacitor (Ce) and on the other hand to the second branch (S12) of the first switch (S1), the value of the polarization resistance (Rpolar) being chosen such that:

With:

Z2: impedance equivalent to a disturbance pulse of a set formed by the storage capacity Cext and the detection capacity Ce connected in parallel,

ZCdisrupt: impedance equivalent to the disturbance pulse of the disturbance capacitance (Cdisruption),

Z1: impedance equivalent to the disturbance pulse of an assembly formed by the polarization resistor Rpolar connected in parallel with a sum of the storage capacitor Cext and the detection capacitance Ce.

[Claim 2] A sensor (1) according to claim 1, wherein the value of the bias resistance is greater than 20 kΩ.

[Claim 3] A sensor (1) according to the preceding claim, wherein the value of the bias resistance is greater than 50 kΩ.

[Claim 4] A sensor (1) according to the preceding claim, wherein the value of the bias resistance is greater than 65 kΩ.

[Claim 5] A sensor (1) according to the preceding claim, wherein the value of the bias resistance is greater than 100 kΩ.

[Claim 6] A sensor (1) according to the preceding claim, wherein the value of the bias resistance is greater than 150 kΩ.

[Claim 7] Sensor (1) according to one of the preceding claims, said sensor comprises a printed circuit on which the bias resistor is mounted.

[Claim 8] A sensor (1) according to the preceding claim, wherein the storage capacity is mounted on the printed circuit.

[Claim 9] Motor vehicle comprising at least one capacitive sensor (1) according to one of the preceding claims.

[Claim 10] Vehicle according to the preceding claim, said vehicle comprising an electronic control unit connected to the at least one sensor (1) and able to control the first switch (S1), the second switch (S2) and the third switch (S3) in opening and closing.