Low consumption device for measuring a variation of a capacitance and associated method

Abstract

A device for measuring a variation (ΔC.sub.X) of a capacitance (C.sub.X), includes: elements for charging the capacitance (C.sub.X) on the basis of a supply voltage (V.sub.CC). elements for discharging the capacitance (C.sub.X) into a reference capacitance (C.sub.S) in a fixed number of discharges (x), elements for measuring a voltage (V.sub.S) and for detecting a threshold of voltage (V.sub.TH) across the terminals of the reference capacitance (C.sub.S). elements for charging with current (I.sub.C) the reference capacitance (C.sub.S) on the basis of the supply voltage (V.sub.CC) for a duration (t), after the transfer of charge from the capacitance (C.sub.X) into the reference capacitance (C.sub.S), and elements for measuring the variation between the duration (t) with respect to a previously measured duration so as to estimate the variation (ΔC.sub.X) of the capacitance (C.sub.X).

Claims

1. A device for measuring a variation (ΔC.sub.X) of a capacitance (C.sub.X), comprising: a supply voltage (V.sub.CC); circuitry that charges the capacitance (C.sub.X) on the basis of the supply voltage (V.sub.CC); circuitry that discharges the capacitance (C.sub.X) into a reference capacitance (C.sub.S) in a fixed number of discharges (x); circuitry that measures a voltage (V.sub.S) across the terminals of the reference capacitance (C.sub.S); circuitry that detects a threshold of voltage (V.sub.TH) across the terminals of the reference capacitance (C.sub.S); circuitry that charges, with current (I.sub.C), the reference capacitance (C.sub.S) on the basis of the supply voltage (V.sub.CC) for a duration (t), after the capacitance (C.sub.X) has been charged and discharged in a fixed number of discharges (x) into the reference capacitance (C.sub.S); a charge resistance (R.sub.C) that calibrates the current (I.sub.C), charging the reference capacitance (C.sub.S); and circuitry that: measures the duration (t) with a temporal resolution (Δt), and determines a variation of the duration (t) with respect to a previously measured duration, the variation being representative of the variation (ΔC.sub.X) of the capacitance (C.sub.X).

2. The device as claimed in claim 1, wherein a predetermined threshold of detection (Th) of a number (y) of intervals of the temporal resolution (Δt) over the duration (t) is defined, corresponding to the variation (ΔC.sub.X) of the capacitance (C.sub.X).

3. The device as claimed in claim 2, wherein the fixed number of discharges (x) of the capacitance (C.sub.X) to the reference capacitance (C.sub.S) is defined by: $x=\frac{\mathrm{Th}\times \Delta t}{\Delta C_X\times R_c}$ with: Th: the detection threshold Δt: the temporal resolution ΔC.sub.X: the variation of the capacitance (C.sub.X) R.sub.C: the charge resistance.

4. The device as claimed in claim 1, wherein the duration (t) is defined by the duration required for the voltage (V.sub.S) across the terminals of the reference capacitance (C.sub.S) to be equal to the voltage threshold (V.sub.TH) and is equal to: $t=-R_C\times C_S\times \ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_S\left(x\right)}\right)$ with: C.sub.S: the reference capacitance V.sub.CC: the supply voltage V.sub.TH: the voltage threshold V.sub.S: the voltage across the terminals of the reference capacitance (C.sub.S).

5. The device as claimed in claim 2, wherein the measurement of the capacitance variation (ΔC.sub.X) is independent of the capacitance (C.sub.X), and is equivalent to: $\Delta C_X=\frac{\mathrm{Th}\times \Delta t}{R_C\times x}$ where x is the number of fixed discharges of the capacitance (C.sub.X) into the reference capacitance (C.sub.S).

6. The device as claimed in claim 1, wherein the reference capacitance (C.sub.S) exhibits a greater capacitance than that of the capacitance (C.sub.X).

7. A capacitive sensor for detecting the presence of a user of an apparatus that implements the device for measuring a variation of the capacitance (C.sub.X) as claimed in claim 1, wherein the capacitance (C.sub.X) whose capacitance variation (ΔC.sub.X) is measured comprises a detection electrode (4) disposed within said apparatus, the capacitance (C.sub.X) being measured between said detection electrode (4) and a close environment (M) of said detection electrode (4).

8. The capacitive sensor as claimed in claim 7, wherein the apparatus in which the detection electrode (4) is disposed is a door handle (6) of a vehicle.

9. A motor vehicle comprising a capacitive sensor (3) as claimed in claim 8.

10. A method for measuring a variation (ΔC.sub.X) of the capacitance (C.sub.X), using a measurement device as claimed in claim 1, comprising the steps of: charging with current (I.sub.C), said current (I.sub.C) being constant and calibrated by passage through a charge resistance (R.sub.C), the reference capacitance (C.sub.S) on the basis of the supply voltage (V.sub.CC) for a duration (t), after the capacitance (C.sub.X) has been charged and discharged in a fixed number of discharges (x) into the reference capacitance (C.sub.S); measuring the duration (t) with a temporal resolution (Δt); and computing the variation of this duration (t) with respect to a previously measured duration, the variation being representative of the variation (ΔC.sub.X) of the capacitance (C.sub.X).

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) Other characteristics and advantages of the invention will become apparent on reading the description which follows and on examining the appended drawings in which:

(2) FIG. 1 represents a schematic view, described previously, of a door handle of a vehicle integrating a charge-transfer capacitive sensor,

(3) FIG. 2 represents a schematic view, described previously, of a charge-transfer capacitive sensor according to the prior art,

(4) FIG. 3 represents a schematic view of a charge-transfer capacitive sensor according to the prior art, described in FR 2 938 344 A1,

(5) FIG. 4 represents a schematic view of a charge-transfer capacitive sensor, according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(6) The invention proposes the device, such as illustrated in FIG. 4, for measuring a variation ΔC.sub.X of the capacitance C.sub.X and which comprises, furthermore, a charge resistance R.sub.C, placed between the supply voltage V.sub.CC and the measurement capacitance C.sub.S.

(7) The charge transfer according to the invention breaks down into two phases: acquisition and measurement.

(8) The acquisition phase is identical to that of the prior art. During this acquisition phase, the capacitance C.sub.X of the electrode 4 charges and discharges into the reference capacitance C.sub.S in a fixed number of discharges x, as described previously.

(9) During the measurement phase, according to the invention, a charge current I.sub.C passing through the charge resistance R.sub.C charges the reference capacitance C.sub.S until the voltage V.sub.S across the terminals of the latter attains the threshold value V.sub.TH.

(10) The single step of this measurement phase consists in the charging of the reference capacitance C.sub.S by the charge current I.sub.C passing through the charge resistance R.sub.C. Accordingly, the third switch S3 is closed.

(11) The first and the second switches S1 and S2 are open during this measurement phase. Consequently the capacitance C.sub.X of the electrode 4 is neither charged, nor discharged during this measurement phase.

(12) Charging continues until the voltage V.sub.S across the terminals of the reference capacitance C.sub.S attains the threshold voltage V.sub.TH. The duration t of charging required to reach the threshold V.sub.TH represents an image of the capacitance C.sub.X. The duration t is therefore measured between the closing of the third switch S3 and detection (that is to say the instant corresponding to V.sub.S=V.sub.TH). The charge current I.sub.C charging the reference capacitance C.sub.S is constant and calibrated by passage through the charge resistance R.sub.C. By changing the value of the charge resistance R.sub.C, the intensity of the charge current I.sub.C also changes as well as the duration t of charging into the reference capacitance C.sub.S.

(13) The reference capacitance C.sub.S is thereafter completely discharged by closing the switch S in preparation for the next measurement.

(14) According to the invention, the equations governing the operation of the capacitive sensor 3 at low consumption are the following: during the acquisition phase, the evolution of the voltage V.sub.S across the terminals of the reference capacitance C.sub.S is given by equation (1), during the measurement phase, the duration t of charging required in order that V.sub.S=V.sub.TH is defined.

(15) $V_{\mathrm{TH}}=\left(V_{\mathrm{CC}}-V_S\left(x\right)\right)\times \left(1-e^{-\frac{t}{R_C\times C_S}}\right)+V_S\left(x\right)$
Which is equivalent to:

(16) $\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_S\left(x\right)}=e^{-\frac{t}{R_C\times C_S}}$
Then to:

(17) $-\frac{t}{R_C\times C_S}=\ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_S\left(x\right)}\right)$
And finally:

(18) $\begin{array}{cc}t=-R_C\times C_S\times \ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_S\left(x\right)}\right)& \left(6\right)\end{array}$

(19) The duration t is measured with a time base Δt, which is the temporal measurement precision of a clock of the printed circuit 5. This time base Δt (or temporal resolution) is therefore fixed by the clock of the printed circuit 5. The number of intervals of precision Δt included in the duration t is called y, i.e. y=t/Δt, and we obtain:

(20) $y\times \Delta t=-R_C\times C_S\times \ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_S\left(x\right)}\right)$
Which is equivalent to:

(21) $y\times \Delta t=-R_C\times C_S\times \ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}-V_{\mathrm{CC}}\times \left(1-{\left(1-\frac{C_X}{C_S}\right)}^x\right)}\right)$$\mathrm{Then},y\times \Delta t=-R_C\times C_S\times \ln \left(\frac{V_{\mathrm{CC}}-V_{\mathrm{TH}}}{V_{\mathrm{CC}}\times {\left(1-\frac{C_X}{C_S}\right)}^x}\right)$
Isolating y, we obtain:

(22) $y=-\frac{R_C\times C_S}{\Delta t}\times \left[\ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)-\ln \left({\left(1-\frac{C_X}{C_S}\right)}^x\right)\right]$$\mathrm{Then},y=-\frac{R_C\times C_S}{\Delta t}\times \left[\ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)-x\times \ln \left(1-\frac{C_X}{C_S}\right)\right]$
The finite series expansion of

(23) $\ln \left(1-\frac{C_X}{C_S}\right)\cong -\frac{C_X}{C_S},$
since the value of the reference capacitance C.sub.S is appreciably greater than the value of the capacitance C.sub.X of the electrode 4. We thus obtain:

(24) 0$y=-\frac{R_C\times C_S}{\Delta t}\times \left[\ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)+x\times \frac{C_X}{C_S}\right]$$\mathrm{Then},y=-\frac{R_C}{\Delta t}\times \left[C_S\times \ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)-x\times C_X\right]$
We define Th to be the variation of y giving rise to a detection, hence Th is equivalent to:

(25) $\mathrm{Th}=\frac{R_C}{\Delta t}\times \hspace{1em}\left[-C_S\times \ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)-x\times C_X+C_S\times \ln \left(1-\frac{V_{\mathrm{TH}}}{V_{\mathrm{CC}}}\right)+x\times \left(C_X+\Delta C_X\right)\right]$
i.e. also:

(26) $\mathrm{Th}\times \frac{\Delta t}{R_C}=x\times \Delta C_X$
And finally:

(27) $\begin{array}{cc}\Delta C_X=\frac{\mathrm{Th}\times \Delta t}{R_C\times x}& \left(7\right)\end{array}$

(28) Thus, according to equation (7) of the invention, the variation ΔC.sub.X of the capacitance C.sub.X, is determined by: the fixed number of discharges x of the acquisition cycle, the value of the charge resistance R.sub.C, and the time base Δt (temporal resolution of the printed circuit 5 clock).

(29) It is thus possible to increase the value of the charge resistance R.sub.C so as to improve the variation ΔC.sub.X of the capacitance C.sub.X, that is to say so as to improve the sensitivity of the capacitive sensor 3.

(30) It is also possible, by way of software, to reduce the temporal resolution Δt of the clock of the printed circuit 5 so as to improve the variation ΔC.sub.X of the capacitance C.sub.X.

(31) The advantage of the invention is therefore a reduction in detection time (and therefore a reduction in the consumption of the capacitive sensor 3) with respect to the prior art and/or an increase in detection precision due to the reduction in the variation ΔC.sub.X of the capacitance C.sub.X of the electrode 4 which is measurable by the capacitive sensor 3.

(32) An example of gain in time and precision is illustrated hereinbelow.

(33) Consider a capacitive sensor 3 with the following characteristics:

(34) C.sub.X=35 pF, Th=5, V.sub.TH=1.1V, V.sub.CC=3.3V, C.sub.S=20 nF, x=170.

(35) According to the solution of the prior art, that is to say according to the invention described in FR 2 938 344 A1, by using a measurement capacitance C.sub.M, of minimum value, i.e. C.sub.M=10 pF (residual capacitance), then according to equation (5), the number n of charge transfers until V.sub.S=V.sub.TH, is equal to n=215.

(36) And the total cycle number N=(n+x) to carry out the charge transfer is equal to 385. With a clock having a resolution of Δt=2 μs, a charge transfer lasts 12 μs. The duration of the 385 charge transfers, that is to say the duration of detection is therefore 4.6 ms (385×0.12). And according to equation (4), the variation ΔC.sub.X of the capacitance C.sub.X is equal to ΔC.sub.X=0.3 pF.

(37) According to the invention of the present patent application, by fixing the charge resistance R.sub.C=200 kΩ and a resolution of the clock of the printed circuit 5 identical to that of the previous example (Δt=2 μs), then: V.sub.S(x) according to equation (1) equals V.sub.S(170)=0.849 V, according to equation (6), the duration t is equal to t=430 μs, i.e. 0.43 ms, the duration of the x=170 charge transfers of the capacitance C.sub.X into the reference capacitance C.sub.S is equal to 2.04 ms (170×0.12), the total duration of detection is therefore equal to 2.47 ms (2.04+0.43) i.e. a duration of detection 1.84 times shorter than that of the prior art (4.6 ms).

(38) The charge transfer device according to the invention makes it possible to significantly reduce the measurement time t and consequently the consumption of the capacitive sensor 3.

(39) In another example, by increasing the value of the charge resistance R.sub.C to R.sub.C=800 kΩ so as to decrease the value of the variation ΔC.sub.X of the capacitance C.sub.X, then: according to equation (6), the variable duration t=1.723 ms, and the total duration of detection is equal to 3.76 ms (1.723+2.04),
i.e. a duration 0.82 times shorter than that of the prior art (4.6 ms), and variation ΔC.sub.X of the capacitance C.sub.X, according to equation (7) is equal to ΔC.sub.X=0.073 pF, i.e. a variation ΔC.sub.X divided by four with respect to that of the prior art.

(40) The invention therefore allows faster and/or much more precise detection of approach of the user's hand by the capacitive sensor than the prior art solution described in document FR 2 938 344 A1.

(41) The invention is not limited to the embodiments described. In particular, the invention applies to any device for measuring a variation of a capacitance and is not limited to the detection of the approach of a user's hand to a door handle of a vehicle.