Method and apparatus for detecting a change in capacitance of a capacitive proximity sensor

• US 20030071639A1
• Filed: 05/10/2002
• Published: 04/17/2003
• Est. Priority Date: 10/17/2001
• Status: Active Grant

• Current Assignee: Strattec Power Access LLC (Strattec Security Corporation)
• Original Assignee: Delphi Technologies, Inc. (BorgWarner Incorporated)
• Inventors: Pasiecznik, John Jr., Haag, Ronald Helmut
• Granted Patent: US 6,777,958 B2
• Time in Patent Office: Days
• US Class Current: 324/674
• CPC Class Codes: E05F 15/46 responsive to changes in electrical capacitance
• CPC Class Codes: E05Y 2600/40 Mounting location; Visibility of the elements
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2017/9602 characterised by the type or shape of the sensing electrodes

Abstract

A method for detecting a change in capacitance of a capacitive sensing element having a nominal capacitance value is disclosed. In an exemplary embodiment, the method includes coupling the sensing element to a first oscillator, the first oscillator generating a first frequency dependent upon the capacitance value of the sensing element. The first frequency is compared to a reference frequency generated by a second oscillator. The change in capacitance from the nominal capacitance value is detected if the first frequency differs from said reference frequency by a determined frequency value.

Claims

1. A method for detecting a change in capacitance of a capacitive sensing element having a nominal capacitance value, the method comprising:

coupling the sensing element to a first oscillator, said first oscillator generating a first frequency dependent upon the capacitance value of the sensing element; and

comparing said first frequency to a reference frequency, said reference frequency being a second frequency generated by a second oscillator;

wherein the change in capacitance from the nominal capacitance value is detected if said first frequency differs from said reference frequency by a determined frequency value.

2. The method of claim 1, further comprising:

configuring said first oscillator and said second oscillator such that said first frequency and said reference frequency are offset from one another by a fixed offset value when the capacitance of said capacitive sensing element is equal to the nominal capacitance value.

3. The method of claim 2, further comprising:

inputting said first frequency and said reference frequency into a mixer; and

inputting an output of said mixer into a low pass filter;

wherein an output of said low pass filter is an intermediate frequency (IF), said intermediate frequency corresponding to the difference between said first frequency and said reference frequency.

4. The method of claim 3, wherein said intermediate frequency (IF) is in the range of about 0 to about 15 MHz.

5. The method of claim 4, wherein said first frequency and said reference frequency are at least 900 MHz.

6. The method of claim 3, further comprising:

transmitting said intermediate frequency to a control module; and

comparing said intermediate frequency to said fixed offset value, said fixed offset value being stored with in said control module;

wherein an increase in capacitance from the nominal capacitance value is detected if said intermediate frequency exceeds said fixed offset value.

7. An apparatus for detecting a change in capacitance of a capacitive sensing element having a nominal capacitance value, comprising:

a first oscillator coupled to the sensing element, said first oscillator generating a first frequency dependent upon the capacitance value of the sensing element;

a second oscillator generating a reference frequency; and

a device for comparing said first frequency to said frequency;

wherein the change in capacitance from the nominal capacitance value is detected if said first frequency differs from said reference frequency by a determined frequency value.

8. The apparatus of claim 7, wherein said first frequency generated by said first oscillator and said reference frequency generated by said second oscillator are offset from one another by a fixed offset value when the capacitance of said capacitive sensing element is equal to the nominal capacitance value.

9. The apparatus of claim 8, further comprising:

a mixer, said mixer having said first frequency as a first input thereto and said reference frequency as a second input thereto; and

a low pass filter, said low pass filter having an output of said mixer as an input thereto;

wherein an output of said low pass filter is an intermediate frequency (IF), said intermediate frequency corresponding to the difference between said first frequency and said reference frequency.

10. The apparatus of claim 9, wherein said intermediate frequency (IF) is in the range of about 0 to about 15 MHz.

11. The apparatus of claim 10, wherein said first frequency and said reference frequency are at least 900 MHz.

12. The apparatus of claim 11, wherein said fixed offset value is about 3 MHz.

13. The apparatus of claim 9, further comprising:

a control module;

a signal line for transmitting said intermediate frequency to said control module; and

said intermediate frequency being compared to said fixed offset value, said fixed offset value further being stored with in said control module;

wherein an increase in capacitance from the nominal capacitance value is detected if said intermediate frequency exceeds said fixed offset value.

14. The apparatus of claim 13, further comprising:

a bias “

T”

arrangement, configured between said low pass filter and said signal line, said bias “

T”

arrangement enabling said signal line to simultaneously transmit said intermediate frequency to said control module and a DC power signal from said control module.

15. The apparatus of claim 14, wherein said signal line comprises a twisted pair of conductors.

16. A capacitive sensing element, comprising:

a capacitive strip having a nominal capacitance value; and

a capacitance detection circuit integrated within said capacitive strip, said capacitance detection circuit further comprising;

a first oscillator coupled to the capacitive strip, said first oscillator generating a first frequency dependent upon the capacitance value of the capacitive strip; and

a second oscillator generating a reference frequency, said reference frequency being offset from said first frequency by a fixed offset value when the capacitance of said capacitive strip equals said nominal capacitance value;

wherein a change in capacitance of the capacitive strip from said nominal capacitance value is detected if said first frequency differs from said reference frequency by more than said fixed offset value.

17. The capacitive sensing element of claim 16, wherein said capacitance detection circuit further comprises:

a mixer, said mixer having said first frequency as a first input thereto and said reference frequency as a second input thereto; and

a low pass filter, said low pass filter having an output of said mixer as an input thereto;

wherein an output of said low pass filter is an intermediate frequency (IF), said intermediate frequency corresponding to the difference between said first frequency and said reference frequency.

18. The capacitive sensing element of claim 17, further comprising:

a bias “

T”

arrangement, said bias “

T”

arrangement simultaneously allowing said intermediate frequency to be transmitted out from said capacitance detection circuit and a DC power signal to be transmitted into said capacitance detection circuit.

19. The apparatus of claim 18, wherein said intermediate frequency (IF) is in the range of about 0 to about 15 MHz.

20. The apparatus of claim 19, wherein said first frequency and said reference frequency are at least 900 MHz.

21. The apparatus of claim 20, wherein said fixed offset value is about 3 MHz.