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

1. A device for measuring a variation (ΔC X ) of a capacitance (C X ), comprising:

a supply voltage (V CC );

circuitry that charges the capacitance (C X ) on the basis of the supply voltage (V CC );

circuitry that discharges the capacitance (C X ) into a reference capacitance (C S ) in a fixed number of discharges (x);

circuitry that measures a voltage (V S ) across the terminals of the reference capacitance (C S );

circuitry that detects a threshold of voltage (V TH ) across the terminals of the reference capacitance (C S );

circuitry that charges, with current (I C ), the reference capacitance (C S ) on the basis of the supply voltage (V CC ) for a duration (t), after the capacitance (C X ) has been charged and discharged in a fixed number of discharges (x) into the reference capacitance (C S );

a charge resistance (R C ) that calibrates the current (I C ), charging the reference capacitance (C 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 X ) of the capacitance (C 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 X ) of the capacitance (C X ).

3. The device as claimed in claim 2 , wherein the fixed number of discharges (x) of the capacitance (C X ) to the reference capacitance (C S ) is defined by:

x

=

Th

×

Δ

⁢

⁢

t

Δ

⁢

⁢

C

X

×

R

c

with:

Th: the detection threshold

Δt: the temporal resolution

ΔC X : the variation of the capacitance (C X )

R 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 S ) across the terminals of the reference capacitance (C S ) to be equal to the voltage threshold (V TH ) and is equal to:

t

=

-

R

C

×

C

S

×

ln

⁡

(

V

CC

-

V

TH

V

CC

-

V

S

⁡

(

x

)

)

with:

C S : the reference capacitance

V CC : the supply voltage

V TH : the voltage threshold

V S : the voltage across the terminals of the reference capacitance (C S ).

5. The device as claimed in claim 2 , wherein the measurement of the capacitance variation (ΔC X ) is independent of the capacitance (C X ), and is equivalent to:

Δ

⁢

⁢

C

X

=

Th

×

Δ

⁢

⁢

t

R

C

×

x

where x is the number of fixed discharges of the capacitance (C X ) into the reference capacitance (C S ).

6. The device as claimed in claim 1 , wherein the reference capacitance (C S ) exhibits a greater capacitance than that of the capacitance (C 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 X ) as claimed in claim 1 , wherein the capacitance (C X ) whose capacitance variation (ΔC X ) is measured comprises a detection electrode ( 4 ) disposed within said apparatus, the capacitance (C 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 X ) of the capacitance (C X ), using a measurement device as claimed in claim 1 , comprising the steps of:

charging with current (I C ), said current (I C ) being constant and calibrated by passage through a charge resistance (R C ), the reference capacitance (C S ) on the basis of the supply voltage (V CC ) for a duration (t), after the capacitance (C X ) has been charged and discharged in a fixed number of discharges (x) into the reference capacitance (C 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 X ) of the capacitance (C X ).