Capacitance Sensor

• US 20090243633A1
• Filed: 12/05/2008
• Published: 10/01/2009
• Est. Priority Date: 12/06/2007
• Status: Active Grant

• Current Assignee: Infineon Technologies AG
• Original Assignee: Infineon Technologies AG
• Inventors: Werth, Tobias, Zangl, Hubert, Bretterklieber, Thomas, Hammerschmidt, Dirk, Brasseur, Georg, Holler, Gert
• Granted Patent: US 7,944,216 B2
• Time in Patent Office: Days
• US Class Current: 324/681
• CPC Class Codes: G01D 5/2405 by varying dielectric

Abstract

A capacitance sensor, having at least one electrode is disclosed. A frequency spread signal generation circuit is coupled to the at least one electrode to apply a frequency spread signal based on a spread sequence. A periodic signal with a frequency is coupled to the at least one electrode. A frequency controller is coupled to the frequency spread signal generation circuit to vary the frequency of the periodic signal. A receiver circuit is coupled to the at least one or a further electrode to receive a version of the frequency spread signal which depends on a capacitance applied to the at least one electrode. An evaluation circuit is coupled to the receiver circuit to determine a capacitance value based on the received, dependent version of the frequency spread signal.

Claims

1. A capacitance sensor, comprising:

at least one electrode;

a frequency spread signal generation circuit coupled to the at least one electrode to apply a frequency spread signal based on a spread sequence and a periodic signal with a frequency to the at least one electrode;

a frequency controller coupled to the frequency spread signal generation circuit to vary the frequency of the periodic signal;

a receiver circuit coupled to the at least one or a further electrode to receive a version of the frequency spread signal which depends on a capacitance applied to the at least one electrode; and

an evaluation circuit coupled to the receiver circuit to determine a capacitance value based on the received dependent version of the frequency spread signal.

2. The capacitance sensor according to claim 1, wherein the frequency spread signal generation circuit comprises a spread code generator.

3. The capacitance sensor according to claim 2, wherein the spread code generator is implemented to generate a periodic spread code with a period duration.

4. The capacitance sensor according to claim 1, wherein the frequency spread signal generation circuit comprises a controllable oscillator coupled to the frequency controller to adjust the frequency of the periodic signal of the controllable oscillator.

5. The capacitance sensor according to claim 3, wherein the frequency controller is implemented to change the frequency after each integer multiple of the period duration of the periodic spread code.

6. The capacitance sensor according to claim 5, further comprising a plurality of transmit electrodes and wherein the integer multiple of the period duration of the periodic spread code is determined by a number of the transmit electrodes.

7. The capacitance sensor according to claim 1, wherein symbols of the spread sequence comprise a symbol duration, and wherein the frequency controller is implemented to change the frequency after an integer multiple of the symbol duration.

8. The capacitance sensor according to claim 7, further comprising a plurality of transmit electrodes, and wherein the integer multiple of the symbol duration is determined by a number of the transmit electrodes.

9. The capacitance sensor according to claim 8, wherein each transmit electrode of the plurality of transmit electrodes is controlled with a phase-shifted version of the spread sequence.

10. The capacitance sensor according to claim 1, wherein the frequency spread signal generation circuit comprises a modulator and a controllable oscillator coupled to a modulator to modulate the periodic signal of the controllable oscillator with a spread code.

11. The capacitance sensor according to claim 10, wherein the modulator is implemented to modulate a phase of the periodic signal depending on the spread code.

12. The capacitance sensor according to claim 10, wherein the receiver circuit comprises a demodulator to combine a signal received from the at least one electrode or the further electrode with the periodic signal of the controllable oscillator to acquire a demodulated signal.

13. The capacitance sensor according to claim 1, wherein a functioning of the frequency spread signal generation circuit depends on a spread code and the receiver circuit comprises a correlator to subject a signal coming from the at least one or the further electrode to a correlation with the spread code.

14. The capacitance sensor according to claim 1, further comprising a plurality of transmit electrodes and a multiplexer for selecting one or several of the plurality of transmit electrodes, which is connected between the transmit electrodes and the frequency spread signal generation circuit.

15. The capacitance sensor according to claim 14, wherein the plurality of transmit electrodes is controlled in a time multiplex method so that all transmit electrodes with a first carrier frequency are controlled within a first measurement interval and all transmit electrodes with a second carrier frequency which is different from the first carrier frequency are controlled within a second measurement interval.

16. The capacitance sensor according to claim 1, further comprising a plurality of receive electrodes and a multiplexer for selecting one or several of the plurality of receive electrodes, which is connected between the plurality of receive electrodes and the receiver circuit.

17. The capacitance sensor according to claim 1, wherein the spread sequence comprises spread sequence symbols with a spread sequence symbol duration and wherein the evaluation circuit is implemented to determine at least one capacitance value or a value derived from the same based on the received version of the frequency spread signal during the spread sequence symbol duration.

18. The capacitance sensor according to claim 1, wherein the receiver circuit comprises a switched capacitor circuit.

19. A capacitance sensor, comprising:

at least one electrode;

a frequency spread signal generation circuit coupled to the at least one electrode to apply a frequency spread signal phase-modulated according to a spread symbol sequence to the at least one electrode;

a receiver circuit coupled to the at least one or a further electrode to receive a version of the phase-modulated frequency spread signal which depends on a capacitance applied to the at least one electrode; and

an evaluation circuit which is coupled to the receiver circuit to determine a capacitance value based on the received, dependent version of the phase-modulated frequency spread signal.

20. A capacitance sensor, comprising:

a plurality of electrodes;

a frequency spread signal generation circuit which is coupled to the plurality of electrodes and which is implemented to control the plurality of electrodes in time division multiplex so that, for a controlled electrode, a frequency spread signal based on a spread symbol sequence with a spread symbol rate and a periodic signal is applied for a spread symbol duration;

a receiver circuit coupled to the plurality of electrodes or a further electrode to receive a version of the frequency spread signal which depends on a capacitance applied to the plurality of electrodes; and

an evaluation circuit coupled to the receiver circuit to determine a capacitance value based on sampling values of the received dependent version of the frequency spread signal which are determined by a sampling rate which is equal to or higher than the spread symbol rate.

21. The capacitance sensor according to claim 20, further comprising a frequency controller which is coupled to the frequency spread signal generation circuit to vary a frequency of the periodic signal after an integer multiple of the spread symbol duration.

22. A capacitance sensor, comprising:

at least one electrode;

means for applying a frequency spread signal to the at least one electrode, wherein the frequency spread signal is based on a spread sequence and a periodic signal with a frequency;

means for varying the frequency of the periodic signal;

means for receiving a version of the frequency spread signal which depends on a capacitance applied to the at least one electrode; and

means for determining a capacitance value or a signal that may be derived from the capacitance value based on the received dependent version of the frequency spread signal.

23. A capacitance sensor, comprising:

at least one electrode, wherein a capacitance may be measured between the at least one electrode and a further electrode or a reference potential;

a circuit implemented to apply a periodic signal with a variable frequency to the at least one electrode and comprising a spread sequence generation circuit to change the periodic signal depending on a spread sequence generated by the spread sequence generation circuit;

a circuit implemented to detect a signal dependent on the capacitance and the periodic signal at the at least one or the further electrode, wherein the circuit is coupled to the spread sequence generation circuit so that detection depends on the spread sequence; and

an evaluation circuit coupled to the circuit for detecting to determine a sensor output signal dependent on the capacitance from the dependent signal.

24. A method for detecting a capacitance or a capacitance change, comprising:

applying a frequency spread signal to at least one electrode, wherein the frequency spread signal is based on a spread sequence and a periodic signal with a frequency;

varying the frequency of the periodic signal;

receiving a version of the frequency spread signal which depends on a capacitance applied to the at least one electrode; and

determining the capacitance or a value which may be derived from the capacitance from the received dependent version of the frequency spread signal.

25. A computer program for performing a method for detecting a capacitance or a capacitance change, comprising:

applying a frequency spread signal to at least one electrode, wherein the frequency spread signal is based on a spread sequence and a periodic signal with a frequency;

varying the frequency of the periodic signal;

receiving a version of the frequency spread signal which depends on a capacitance applied to the at least one electrode; and

determining the capacitance or a value which may be derived from the capacitance from the received dependent version of the frequency spread signal,

when the computer program is executed on a computer and/or microcontroller.