Charge balance network with floating ground capacitive sensing

• US 6,316,948 B1
• Filed: 07/01/1998
• Issued: 11/13/2001
• Est. Priority Date: 07/01/1998
• Status: Expired due to Fees

• Current Assignee: Setra Systems Incorporated (Fortive Corp.)
• Original Assignee: Setra Systems Incorporated (Fortive Corp.)
• Inventors: Briefer, Dennis K.
• Primary Examiner(s): Metjahic, Safet
• Assistant Examiner(s): DEB, ANJAN K
• Application Number:
  US09/108,659
• Time in Patent Office: 1,231 Days
• Field of Search: 324/678, 324/661, 324/660, 324/679, 324/680
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitance measurement system amenable to implementation on an integrated circuit includes a switching network, a voltage driving element, a sensor network, and a feedback network. The sensor network includes a fixed capacitor connected in series to a variable sensing capacitor. The feedback network receives the voltage generated at the junction node between the two capacitors, and the other side of the sensor capacitor is connected to system ground. The voltage driving element, responsive to a signal from the switching network, shifts the local voltage which supplies power to the feedback network. The feedback network generates a feedback signal representative of the difference in the charge stored on the series capacitors from a predetermined value. The switching network includes a control which is operative to cyclically interconnect the feedback signal and reference potentials with the sensor network and the voltage driving element, whereby the average change in the charge stored on the capacitors over each cycle is substantially equal to the predetermined value. In this configuration, the feedback signal is a predetermined function of capacitance of the sensing capacitor.

Claims

1. A system for generating a signal which is a predetermined function of the capacitance of a sensor capacitor, comprising:

A. a sensor network including a reference capacitor and said sensor capacitor coupled in series, said reference capacitor being coupled between an input terminal and a junction terminal, and said sensor capacitor being coupled between said junction terminal and a system ground;

B. a switching network including a switch element having a first state and a second state, and a switch control having means for cyclically alternating said switch network between said first state and said second state;

C. a feedback network including an amplifier coupled by way of a demodulator to an integrator, said amplifier and integrator being coupled between an upper voltage supply terminal and a lower voltage supply terminal, said upper supply terminal being coupled to a first reference potential and said lower voltage supply terminal coupled to a second reference potential, said first reference potential being a fixed, predetermined voltage above said second reference potential and independent of said system ground, said feedback network being coupled to said sensor network and said switching network, and including means for generating a feedback signal at a feedback terminal, said feedback signal being representative of the difference in charge stored on said series capacitors from a predetermined value;

D. a voltage driving element for providing said first reference potential voltage, said driving means being responsive to at least one of a plurality of signals from said switching network;

wherein said switching network is operative to (i) electrically couple, while in said first state, an input of said voltage driving element with said feedback signal and said sensor input terminal with said second reference potential, and (ii) electrically couple, while in said second state, said input of said voltage driving element with said second reference potential, and said sensor input with a third reference potential, whereby an average change in charge stored on said series capacitors over each cycle is substantially equal to said predetermined value, said feedback signal being said predetermined function of the capacitance of said sensor capacitor.

2. A system according to claim 1 wherein said switching network comprises a double pole, double throw switch having a first pole and a second pole, said first pole being electrically connected to said input terminal of said sensor network and said second pole being electrically connected to an input of said voltage driving element, said first pole being switched between terminals maintained at said second and said third reference potentials, and said second pole being switched between said feedback terminal and a terminal maintained at said second reference potential, and

wherein an input of said amplifier is coupled to the junction between said series capacitors.

3. A system according to claim 2 wherein said feedback network is coupled to the junction between said series capacitors by way of an A.C. coupling network.

4. A system according to claim 2 wherein said double pole, double throw switch is a solid state element.

5. A system according to claim 2 wherein said feedback network includes a demodulator coupled to said series capacitor junction and adapted to demodulate the signal produced at said junction.

6. A system according to claim 5 wherein said demodulator is coupled to said junction via a first difference amplifier having a non-inverting input maintained at said first reference potential, and an inverting input coupled to said series capacitor junction.

7. A system according to claim 6 wherein said second potential is a ground potential local to said feedback network.

8. A system according to claim 5 wherein said demodulator is operative in synchronism with said cyclical operation of said switching network.

9. A system according to claim 2 wherein said second potential is a ground potential local to said feedback network.

10. A system according to claim 1 wherein said feedback network includes a first difference amplifier having an inverting input terminal and a non-inverting input terminal, said inverting input terminal being coupled to the junction between said series capacitors.

11. A system according to claim 10 wherein said inverting input terminal is coupled to the junction between said series capacitors by way of an A.C. coupling network.

12. A system according to claim 10 wherein said switching element comprises a double pole, double throw switch having a first pole and a second pole, said first pole being electrically connected to said input terminal of said sensor network and said second pole being electrically connected to an input of said voltage driving element, said first pole being switched between terminals maintained at said second and said third reference potentials, and said second pole being switched between a terminal maintained at said second reference potential and said feedback terminal;

wherein an input of said amplifier is coupled to the junction between said series capacitors.

13. A system according to claim 12 wherein said double pole, double throw switch is a solid state element.

14. A system according to claim 12 wherein said feedback network includes a demodulator coupled to said first difference amplifier and adapted to demodulate the signal produced by said difference amplifier.

15. A system according to claim 14 wherein said demodulator is operative in synchronism with said cyclical operation of said switching network.

16. A system according to claim 1 wherein said series capacitors comprise a differential capacitor element having two fixed plate terminals and a movable plate terminal, and

wherein said switching network comprises a double pole, double throw switch having a first pole and a second pole, said first pole being electrically connected to said input terminal of said sensor network and said second pole being electrically connected to an input of said voltage driving element, said first pole being switched between a terminal maintained at said second and said third reference potentials, and said second terminal being switched between said feedback terminal and a terminal maintained at said second reference potential, and

wherein an input of said amplifier is coupled to said movable plate terminal.

17. A system according to claim 16 wherein said differential capacitor element includes two fixed plates, each being connected to an associated one of said fixed plate terminals, and includes a conductive diaphragm between said fixed plates, said diaphragm being connected to said movable plate terminal.

18. A system according to claim 16 wherein said double pole, double throw switch means is a solid state element.

19. A system according to claim 16 wherein said feedback network includes a demodulator coupled to said movable plate terminal and adapted to demodulate the signal produced at said movable plate terminal.

20. A system according to claim 19 wherein said demodulator is coupled to said movable plate terminal by a first difference amplifier having a non-inverting input terminal and means to maintain said non-inverting input terminal at a reference potential between said first and second reference potentials, and an inverting terminal coupled to said movable plate terminal.

21. A system according to claim 20 wherein said first and second potentials are characterized by opposite polarity, and wherein said non-inverting input terminal is maintained at ground potential.

22. A system according to claim 19 wherein said demodulator is operative in synchronism with said cyclical operation of said switching network.

23. A system for generating a signal which is a predetermined function of the capacitance of a sensor capacitor, comprising:

A. a sensor network including two series connected capacitors, one of said series capacitors being said sensor capacitor having one terminal electrically connected to a system ground;

B. a switching network including a switch element having two states and a switch control having means to cyclically alternate said switch network between said two states;

C. a feedback network having an upper voltage supply terminal coupled to a first reference potential, a lower voltage supply terminal coupled to a second reference potential, said first reference potential being a fixed, predetermined voltage above said second reference potential and independent of said system ground, said feedback network being coupled to said sensor network and said switching network, including means to generate a feedback signal at a feedback terminal, said feedback signal being representative of the difference in charge stored on said series capacitors from a second predetermined value;

D. a voltage-driving element for providing said second reference potential voltage, said driving element being responsive to at least one of a plurality of signals from said switching network;

wherein said switching network is operative to (i) electrically couple, while in said first state, an input of said voltage-driving element with said feedback signal and said sensor input terminal with said second reference potential, and (ii) electrically couple, while in said second state, said input of said voltage-driving element with said second reference potential, and said sensor input with a third reference potential, whereby an average change in charge stored on said series capacitors over each cycle is substantially equal to said predetermined value, said feedback signal being said predetermined function of the capacitance of said sensor capacitor.