METHOD OF COMPENSATING TEMPERATURE INFLUENCE IN CAPACITIVE MEASUREMENTS

Abstract

A method of operating a capacitive measurement system for compensation of temperature influence is described. The capacitive measurement system includes at least one capacitive sensor member in an installed state and a capacitive measurement circuit for determining a complex impedance of an unknown capacitance from a complex sense current through the at least one capacitive sensor member. In the method, a calibration measurement is carried out to obtain temperature characteristics of both the real part and the imaginary part of determined impedances. In following impedance measurements of the unknown capacitance at a current temperature, the real part and the imaginary part of the measured impedance is determined, and based on the real part determined at the current temperature and the determined temperature characteristics of both the real part and the imaginary part, the imaginary part of the impedance determined at the current temperature is corrected.

Claims

1. A method of operating a capacitive measurement system for compensation of temperature influence, the capacitive measurement system including at least one capacitive sensor member being in an installed state and a capacitive measurement circuit that is configured for determining a complex impedance of an unknown capacitance from a complex sense current through the at least one capacitive sensor member, the method comprising at least the following steps: carrying out a calibration measurement by: determining (50) a plurality of complex impedances at a plurality of different temperatures, at least one complex impedance at each temperature, wherein a range of the plurality of different temperatures includes a reference temperature (T.sub.ref), and determining (52) temperature characteristics of both the real part and the imaginary part of the determined impedances, carrying out (56) an impedance measurement of the unknown capacitance at a current temperature (T.sub.curr) and determining (58) the real part and the imaginary part of the measured impedance, based on the real part determined at the current temperature (T.sub.curr) and the determined temperature characteristics of both the real part and the imaginary part, correcting (60) the imaginary part of the impedance determined at the current temperature (T.sub.curr).

2. The method as claimed in claim 1, wherein the step of correcting (60) the determined imaginary part includes obtaining a current temperature (T.sub.curr) from the real part of the impedance determined at the current temperature (T.sub.curr) and the determined temperature characteristic of the real part, and using the obtained current temperature (T.sub.curr) and the determined imaginary part temperature characteristic for determining a correction amount for the imaginary part.

3. The method as claimed in claim 1, wherein the step of correcting (60) the determined imaginary part of the determined impedance is carried out according to linear transfer function
Y.sub.ref=Y.sub.curr+X.sub.curr.Math.a.sub.1+a.sub.0 wherein Y.sub.ref denotes the corrected imaginary part of the impedance related to the reference temperature T.sub.ref, Y.sub.curr denotes the uncorrected imaginary part of the impedance determined at the current temperature T.sub.curr, X.sub.curr denotes the real part of the impedance determined at the current temperature T.sub.curr, and a.sub.0 and a.sub.1 are constant numerical values obtained by the steps (50, 52) of carrying out the calibration measurement.

4. The method as claimed in claim 1, wherein the step of determining (52) the temperature characteristics includes applying a fitting procedure on the real parts of the determined impedances and applying a fitting procedure on the imaginary parts of the determined impedances to obtain a closed formula for describing the respective temperature characteristic.

5. The method as claimed in claim 4, wherein the closed formula obtained by the fitting procedure is formed as a polynomial of the temperature.

6. The method as claimed in claim 1, wherein the step of determining (52) the temperature characteristics includes partitioning the range of the plurality of different temperatures into a plurality of intervals and calculating a slope of the temperature characteristic for each interval of the partitioning, and wherein the step of correcting (60) is based on the slopes of the intervals between the current temperature (T.sub.curr) and the reference temperature (T.sub.ref) and a width of these intervals.

7. A capacitive measurement system including: at least one capacitive sensor member being in an installed state, a capacitive measurement circuit that is connected to the at least one capacitive sensor member and that is configured for determining a complex impedance of an unknown capacitance from a complex sense current through the at least one capacitive sensor member, an evaluation and control unit that is connected to the capacitive measurement circuit for receiving data signals and that is configured: to control the capacitive measurement circuit to carry out an impedance measurement of the unknown capacitance at a current temperature (T.sub.curr) and to determine the real part and the imaginary part of the measured impedance, and based on the real part determined at the current temperature (T.sub.curr) and the determined temperature characteristics of both the real part and the imaginary part, to correct the imaginary part of the impedance determined at the current temperature (T.sub.curr).

8. The capacitive measurement system as claimed in claim 7, wherein the evaluation and control unit is configured to automatically execute steps of a method comprising: carrying out a calibration measurement by: determining (50) a plurality of complex impedances at a plurality of different temperatures, at least one complex impedance at each temperature, wherein a range of the plurality of different temperatures includes a reference temperature (T.sub.ref), and determining (52) temperature characteristics of both the real part and the imaginary part of the determined impedances, carrying out (56) an impedance measurement of the unknown capacitance at a current temperature (T.sub.curr) and determining (58) the real part and the imaginary part of the measured impedance, based on the real part determined at the current temperature (T.sub.curr) and the determined temperature characteristics of both the real part and the imaginary part, correcting (60) the imaginary part of the impedance determined at the current temperature (T.sub.curr); wherein the step of correcting (60) the determined imaginary part includes obtaining a current temperature (T.sub.curr) from the real part of the impedance determined at the current temperature (T.sub.curr) and the determined temperature characteristic of the real part, and using the obtained current temperature (T.sub.curr) and the determined imaginary part temperature characteristic for determining a correction amount for the imaginary part.

9. The capacitive measurement system as claimed in claim 7, being configured to be installed in a vehicle for a vehicular application.

10. A non-transitory computer-readable medium for controlling automatic execution of steps of the method of claim 1, wherein method steps are stored on the computer-readable medium as a program code, and wherein the computer-readable medium comprises a part of the capacitive measurement system or of a separate control unit and is executable by a processor unit of the capacitive measurement system or of the separate control unit.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0052] Further details and advantages of the present invention will be apparent from the following detailed description of not limiting embodiments with reference to the attached drawing, wherein:

[0053] FIG. 1 schematically illustrates a capacitive measurement system in accordance with the invention in an embodiment as a seat occupation detection and classification system for a vehicle seat,

[0054] FIG. 2 schematically illustrates an embodiment of a capacitive measurement system for Hands Off detection in a vehicle steering wheel,

[0055] FIG. 3 shows a plot of a temperature characteristic of the relative permittivity of a PUR foam material employed e.g. in the steering wheel pursuant to FIG. 2,

[0056] FIG. 4 shows results of a calibration measurement of real parts and imaginary parts of determined impedances across a temperature range,

[0057] FIG. 5 is a flow chart of a method in accordance with the invention of operating the capacitive measurement system pursuant to FIG. 1 or FIG. 2 for compensation of temperature influences, and

[0058] FIG. 6 shows the effect of applying the method pursuant to FIG. 5 on the imaginary part of determined impedances.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0059] FIG. 1 schematically illustrates a capacitive measurement system 10 in accordance with an embodiment of the invention. The capacitive measurement system 10 is configured to be installed in a vehicle seat 32 (shown in FIG. 1 in a side view) of a vehicle for the purpose of seat occupation detection and classification.

[0060] The vehicle seat 32 is designed as a driver's seat of a passenger car and includes a seat structure (not shown) by which it is erected on a passenger cabin floor of the passenger car, as is well known in the art.

[0061] The vehicle seat 32 further includes a seat base 34 supported by the seat structure and configured for receiving a seat cushion 36 for providing comfort to a seat occupant. The seat cushion 36 of the vehicle seat 32 comprises a seat foam member made from a foam material and a fabric cover, which has been omitted in FIG. 1. The seat base 34 and the seat cushion 36 are provided for supporting a bottom of the seat occupant. A backrest 38 of the vehicle seat 32 is provided as usual for supporting a back of the seat occupant.

[0062] The capacitive measurement system 10 includes a capacitive sensor member 12 that is installed in the vehicle seat 32, a capacitive measurement circuit 14 and an evaluation and control unit 20. The capacitive sensor member 12 is located on the A-surface of the seat cushion 36, underneath the fabric cover. The capacitive measurement circuit 14 and the evaluation and control unit 20 are installed in the vehicle, remote from the vehicle seat 32. An output port 26 of the evaluation and control unit 20 is connected to an airbag control unit 40 of the vehicle.

[0063] FIG. 2 shows an embodiment of the invention in an Hands Off detection system. FIG. 2 shows in particular a partial sectional view of a steering wheel comprising a capacitive sensor member 12 integrated therein. The capacitive sensor member 12 is integrated in a foam material, e.g. a polyurethane (PUR) foam, of the steering wheel 62 between a leather trim 64 and the metal steering wheel frame 66. In some cases, a wheel heating system is integrated, too. In such a case, the capacitive sensor member 12 is arranged above the heater (i.e. between the heater and the leather trim).

[0064] The capacitive sensor member 12 comprises e.g. a sensing electrode 68 and a shielding electrode 70 and is configured for detecting or measuring the capacitance between the sensing electrode 68 and the car chassis. The sensing electrode 68 and the shielding electrode 70 extend along the circumference of the steering wheel 62. Both the sensing electrode 68 and the shielding electrode 70 are electrically connected to the capacitive measurement circuit 14. The capacitive measurement circuit 14 is configured to keep the sensing electrode 68 and the shielding electrode 70 at the same AC potential, in terms of amplitude and phase. It follows that at any point in time, the electric field between the sensing electrode 68 and the shielding electrode 70 substantially cancels and the sensitivity of the sensing electrode 68 is, consequently, directed only in the direction away from the shielding electrode 70, i.e. into the detection space. When the driver grasps the steering wheel 62, capacitive coupling between the sensing electrode 68 and the vehicle chassis is increased compared to the situation, in which the driver has no hand on the steering wheel 62.

[0065] The polyurethane (PUR) material employed in the seat cushion 36 of the vehicle seat 32 or the rim of the steering wheel 62 exhibits a variation of its electromagnetic properties with temperature. FIG. 3 shows a plot of a temperature characteristic of the relative permittivity of a PUR material as used e.g. in a vehicle steering rim. With the capacitive sensor member 12 being embedded in the foam material and therefore in in close proximity to the PUR material, an output signal of the capacitive sensor member 12 would be affected by the varying relative permittivity of the seat cushion material if no measures for compensation of temperature influences were taken. Similarly a temperature effect on the electrical properties of the sensor material itself may affect the measurement without temperature compensation.

[0066] The capacitive measurement circuit 14 includes an impedance measurement circuit 16 and a signal processing unit 18. The impedance measurement circuit 16 of the capacitive measurement circuit 14 is connected to the capacitive sensor member 12 and is configured for determining a complex impedance of an unknown capacitance from a complex sense current through the capacitive sensor member 12. The unknown capacitance represents a position of an object relative to the capacitive sensor member 12. The temperature influence by the variation of the permittivity of the PUR seat cushion material or steering wheel material would be detectable in both the real part and the imaginary part of a measured complex impedance and thus, without a compensation of temperature influences, would potentially be leading to a large measurement error.

[0067] The impedance measurement circuit 16 includes a signal voltage source (not shown) that is configured for providing, with reference to an AC ground potential 28, a periodic electrical measurement signal at an output port, and sense current measurement means that are configured to measure complex sense currents with reference to a reference voltage. The sense current measurement means may be formed as a transimpedance amplifier, which is connected to an antenna electrode of the capacitive sensor member 12 and which converts a current flowing into the antenna electrode of the capacitive sensor member 12 into a voltage, which is proportional to the current flowing into the antenna electrode. In principle, any other sense current measurement means could be employed that appears to be suitable to those skilled in the art.

[0068] The evaluation and control unit 20 is connected to the signal processing unit 18 of the capacitive measurement circuit 14 for receiving data signals. The evaluation and control unit 20 is configured to control the capacitive measurement circuit 14 to carry out an impedance measurement of the unknown capacitance at a current temperature and determine the real part and the imaginary part of the measured (complex) impedance.

[0069] In the following, an embodiment of a method of operating the capacitive measurement system 10 pursuant to FIG. 1 or FIG. 2 for compensation of temperature influences will be described. A flowchart of the method is provided in FIG. 6. In preparation of operating the capacitive measurement system 10, it shall be understood that all involved units and devices are in an operational state and configured as illustrated in FIG. 1 or FIG. 2.

[0070] In order to be able to automatically carry out the method, the evaluation and control unit 20 comprises a software module 30 (FIG. 1 or 2). The method steps to be conducted are converted into a program code of the software module 30. The program code is implemented in a non-transitory computer-readable medium such as a digital data memory unit 22 of the evaluation and control unit 20 and is executable by a processor unit 24 of the evaluation and control unit 20. Alternatively, the software module 30 may as well reside in and may be executable by a control unit of the vehicle, for instance by the airbag control unit 40, and established data communication means between the evaluation and control unit 20 and the airbag control unit 40 of the vehicle would be used for enabling mutual transfer of data.

[0071] In a calibration part of the method, a calibration measurement is carried out. In the calibration measurement, a plurality of complex impedances is determined at a plurality of different temperatures in a first step 50, wherein one complex impedance is determined at each temperature. A range of the plurality of different temperatures includes a reference temperature T.sub.ref (FIG. 4).

[0072] FIG. 4 shows results of the calibration measurement of real parts and imaginary parts of determined impedances while the temperature is lowered from about +20° C. to −40° C. and is then ramped up across a temperature range between −40° C. and about +85° C. The temperature range includes the reference temperature T.sub.ref of +20° C. The real part X of the measured impedance, i.e. the resistance, exhibits an essentially linear decrease with increasing temperature for temperatures up to about +40° C., and a non-linearity above this temperature. The imaginary part Y of the measured impedance, i.e. the capacitance, exhibits a linear decrease at a temperature variation from +20° C. to −40° C., and an essentially linear increase during a temperature variation from −40° C. to about +60° C., with a non-linear portion above +60° C.

[0073] In a next step 52 of the calibration measurement, temperature characteristics 42, 44 of both the real part X and the imaginary part Y of the determined impedances are determined by applying a fitting procedure on the real parts of the determined impedances and by applying a fitting procedure on the imaginary parts of the determined impedances to obtain a closed formula for describing the respective temperature characteristic 42, 44.

[0074] The closed formulas obtained by the fitting procedure are formed by linear functions in temperature, which in a temperature range between −40° C. and about +60° C. is a sufficient approximation. By determining the temperature characteristics 42, 44 of both the real part X and the imaginary part Y of the determined impedances, the calibration measurement is completed. The closed formulas representing the temperature characteristics 42, 44 are stored in the digital data memory unit 22 of the evaluation and control unit 20 in a final step 54. The closed formulas representing the temperature characteristics 42, 44 can of course be used in other, identically designed and identically installed capacitive measurement systems 10.

[0075] In next steps of the method, the capacitive measurement circuit 14 is controlled 56 by the evaluation and control unit 20 to carry out an impedance measurement of the unknown capacitance at a current temperature T.sub.curr and to determine 58 the real part and the imaginary part of the measured impedance. In another step 60 of the method, the imaginary part of the impedance determined at the current temperature T.sub.curr is corrected by the evaluation and control unit 20 based on the real part determined at the current temperature T.sub.curr and the determined temperature characteristics 42, 44 of both the real part and the imaginary part.

[0076] The step 60 of correcting the imaginary part of the impedance determined at the current temperature T.sub.curr can be carried out in multiple ways. In this specific embodiment, the step 60 of correcting the determined imaginary part of the determined impedance is carried out according to linear transfer function

Y.sub.ref=Y.sub.curr+X.sub.curr.Math.a.sub.1+a.sub.0

[0077] wherein Y.sub.ref denotes the corrected imaginary part of the impedance related to the reference temperature T.sub.ref, Y.sub.curr denotes the uncorrected imaginary part of the impedance determined at the current temperature T.sub.curr, X.sub.curr denotes the real part of the impedance determined at the current temperature T.sub.curr, and a.sub.0 and a.sub.1 are constant numerical values obtained by carrying out the steps 50, 52 of the calibration measurement.

[0078] Obviously, constant numerical value a.sub.1 is related to a ratio of the slope of the temperature characteristic 44 of the imaginary part of determined complex impedances to the slope of the temperature characteristic 42 of the real part of determined complex impedances. Both the slopes can readily be obtained from the linear functions 46, 48 representing the temperature characteristics 42, 44 determined during the calibration measurement (FIG. 4). Constant numerical value a.sub.0 serves to adjust an offset between the temperature characteristics 42, 44 of the real part and the imaginary part of determined complex impedances, respectively.

[0079] FIG. 6 shows the effect of applying the method pursuant to FIG. 5 on the imaginary part of the impedance Y determined at the current temperature T.sub.curr, which is corrected to the corrected imaginary part of the impedance Y.sub.ref and is thereby related to the reference temperature T.sub.ref.

[0080] In an alternative embodiment, the step of determining the temperature characteristics may include applying a fitting procedure to obtain closed formulas that are formed as polynomials of degree 2 in temperature for better adaptation to the measured real parts and imaginary parts of the measured impedance. The polynomials may be selected as orthogonal polynomials, for instance as Chebyshev polynomials or as Legendre polynomials. If a higher precision for representing the temperature characteristics is required, the degree of the polynomial may be increased by one, and the respective coefficient may be calculated in an extension of the fitting procedure, without having to repeat the complete fitting procedure.

[0081] In such embodiments, the step of correcting the determined imaginary part may include obtaining a current temperature T.sub.curr from the real part of the impedance determined at the current temperature T.sub.curr and the determined temperature characteristic of the real part. This can be accomplished by searching the roots (i.e. temperatures) of an equation, in which the polynomial of degree 2 yields the real part of the determined impedance. Further, the obtained current temperature T.sub.curr and the determined imaginary part temperature characteristic can be used for determining a correction amount for the imaginary part.

[0082] In another alternative embodiment, the step of determining the temperature characteristic may include partitioning the range of the plurality of different temperatures into a plurality of intervals and calculating a slope of the temperature characteristic for each interval of the partitioning. In other words, the temperature characteristic is approximated by a polygon. Further, the step of correcting is based on the slopes of the intervals between the current temperature T.sub.curr and the reference temperature T.sub.ref, and a width of these intervals.

[0083] While the invention has been illustrated and described in detail in the drawings and foregoing description, such illustration and description are to be considered illustrative or exemplary and not restrictive; the invention is not limited to the disclosed embodiments.

[0084] Other variations to be disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality, which is meant to express a quantity of at least two. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. Any reference signs in the claims should not be construed as limiting scope.