Bridged capacitor sensor measurement circuit

• US 6,720,777 B2
• Filed: 02/15/2002
• Issued: 04/13/2004
• Est. Priority Date: 02/15/2002
• Status: Active Grant

• Current Assignee: Rosemount Incorporated (Emerson Electric Company)
• Original Assignee: Rosemount Incorporated (Emerson Electric Company)
• Inventors: Wang, Rongtai
• Primary Examiner(s): Le, N.
• Assistant Examiner(s): Teresinski, John
• Application Number:
  US10/077,402
• Publication Number:
  US 20030155936A1
• Time in Patent Office: 788 Days
• Field of Search: 324/680, 324/658, 324/664, 324/686
• US Class Current: 324/680
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A pair of sensing capacitors, each having a capacitance C₁ and C₂ respectively, based on a process variable, are coupled to a bridge node which is coupled to a summing node. A reference capacitor, coupled to the summing node, has a capacitance C_REF greater than an expected maximum difference between the capacitances of the pair of sensing capacitors. Switches selectively couple the sensing capacitors and the reference capacitor to at least first and second voltages to derive charges representative of C₁-C₂ and C_REF. In one embodiment the sensing capacitors are operated to charge and discharge during respective first and second phases of first cycles and the reference capacitor is operated to charge and discharge during respective first and second phases of second cycles. In another embodiment, the reference capacitor is operated to charge and discharge during alternate phases of the first and second cycles and the sensing capacitors are operated to charge and discharge during respective first and second phases of all cycles.

Claims

1. Apparatus for measuring a process variable comprising:

a first sensing capacitor having a capacitance, C₁, based on the process variable;

a second sensing capacitor having a capacitance, C₂, based on the process variable;

a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor;

an excitation source supplying at least first and second voltage levels;

a reference capacitor having a capacitance, C_REF, greater than an expected maximum difference between the capacitances of the first and second sensing capacitors;

a summing node coupled to a first side of the reference capacitor and the bridge node; and

a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C₁-C₂ at the bridge node and selectively coupling a second side of the reference capacitor to the excitation source to derive first and second charges at the summing node during mutually exclusive cycles.

2. Apparatus according to claim 1, wherein the switch circuit comprises:

a first switch selectively coupling the second side of the first sensing capacitor to the first and second voltage levels,

a second switch selectively coupling a second side of the second sensing capacitor to the first and second voltage levels, and

a switch control coupled to operate the first and second switches during a first phase to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level, and to operate the first and second switches during a second phase to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level.

3. Apparatus according to claim 1, wherein the switch circuit comprises:

a first switch selectively coupling the second side of the first sensing capacitor to the first and second voltage levels,

a second switch selectively coupling a second side of the second sensing capacitor to the first and second voltage levels, and

a third switch selectively coupling a second side of the reference capacitor to one of two voltage levels of the excitation source.

4. Apparatus according to claim 3, further including a switch control coupled to the first, second and third switches and so disposed and arranged to

operate the third switch to couple the reference capacitor to one voltage level during the first phase of a first cycle and during the second phase of a second cycle, and couple the reference capacitor to another voltage level during the first phase of the second cycle and during the second phase of the first cycle,

operate the first and second switches to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of each cycle, and

operate the first and second switches to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of each cycle.

5. Apparatus according to claim 4, wherein the switch control operates the first, second and third switches through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_A-N_B}{N_A+N_B}.$

6. Apparatus according to claim 3, further including a switch control coupled to the first, second and third switches and so disposed and arranged to

operate the first and second switches to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of a first cycle,

operate the first and second switches to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of the first cycle, and

operate the third switch to couple the reference capacitor to one voltage level during the first phase of a second cycle, and couple the reference capacitor to another voltage level during the second phase of the second cycle.

7. Apparatus according to claim 6, wherein the switch control operates the first, second and third switches through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_B}{N_A}.$

8. Apparatus according to claim 3, wherein the excitation source includes a programmable voltage source, the programmable voltage source being programmable to supply the two voltage levels to the reference capacitor.

9. Apparatus according to claim 3, wherein the reference capacitor comprises an array of a plurality of capacitors, and a fourth switch selectively couples capacitors of the array in parallel.

10. Apparatus according to claim 1, further including a switch control coupled to the switch circuit and so disposed and arranged to

operate the switch circuit to couple the reference capacitor to one voltage level during the first phase of a first cycle and during the second phase of a second cycle, and couple the reference capacitor to another voltage level during the first phase of the second cycle and during the second phase of the first cycle,

operate the switch circuit to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of each cycle, and

operate the switch circuit to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of each cycle.

11. Apparatus according to claim 10, wherein the switch control operates the switch circuit through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_A-N_B}{N_A+N_B}.$

12. Apparatus according to claim 1, further including a switch control coupled to the switch circuit and so disposed and arranged to

operate the switch circuit to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of a first cycle, and

operate the switch circuit to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of the first cycle,

operate the switch circuit to couple the reference capacitor to one voltage level during the first phase of a second cycle,

operate the switch circuit to couple the reference capacitor to another voltage level during the second phase of the second cycle.

13. Apparatus according to claim 12, wherein the switch control operates the switch circuit through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_B}{N_A}.$

14. An industrial process control transmitter arranged to be coupled to a central station by a two-wire communication link, the transmitter including:

a transceiver coupled to the communication link for transmitting information to the central station and for receiving information from the central station;

a processor coupled to the transceiver for processing information;

a process variable sensor comprising;

a first sensing capacitor having a capacitance, C₁, based on the process variable

a second sensing capacitor having a capacitance, C₂, based on the process variable, and

a bridge node coupling a first side of the first sensing capacitor to a first side of the second sensing capacitor;

a reference capacitor having a capacitance, C_REF, greater than an expected maximum difference between the capacitances of the first and second sensing capacitors;

a summing node coupled to a first side of the reference capacitor and the bridge node;

an excitation source supplying at least first and second voltage levels;

a switch circuit selectively coupling a second side of each of the first and second sensing capacitors to the excitation source to derive a representation of C₁-C₂ at the bridge node and selectively coupling a second side of the reference capacitor to the excitation source to derive first and second charges at the summing node during mutually exclusive cycles; and

a delta sigma converter coupled to the bridge node for supplying digital information signals to the processor representative of the difference between the capacitances of the first and second capacitors.

15. The industrial process control transmitter according to claim 14, wherein the switch circuit comprises:

a first switch selectively coupling the second side of the first sensing capacitor to the first and second voltage levels,

a second switch selectively coupling a second side of the second sensing capacitor to the first and second voltage levels, and

a switch control coupled to operate the first and second switches during a first phase to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level, and to operate the first and second switches during a second phase to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level.

16. The industrial process control transmitter according to claim 14, wherein the switch circuit comprises:

a first switch selectively coupling the second side of the first sensing capacitor to the first and second voltage levels,

a second switch selectively coupling a second side of the second sensing capacitor to the first and second voltage levels, and

a third switch selectively coupling a second side of the reference capacitor to one of two voltage levels of the excitation source.

17. The industrial process control transmitter according to claim 16, further including a switch control coupled to the first, second and third switches and so disposed and arranged to

operate the third switch to couple the reference capacitor to one voltage level during the first phase of a first cycle and during the second phase of a second cycle, and couple the reference capacitor to another voltage level during the first phase of the second cycle and during the second phase of the first cycle,

operate the first and second switches to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of each cycle, and

operate the first and second switches to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of each cycle.

18. The industrial process control transmitter according to claim 17, wherein the switch control operates the first, second and third switches through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_A-N_B}{N_A+N_B}.$

19. The industrial process control transmitter according to claim 16, further including a switch control coupled to the first, second and third switches and so disposed and arranged to

operate the first and second switches to couple the first sensing capacitor to the first voltage level and the second sensing capacitor to the second voltage level during a first phase of a first cycle,

operate the first and second switches to couple the first sensing capacitor to the second voltage level and the second sensing capacitor to the first voltage level during a second phase of the first cycle, and

operate the third switch to couple the reference capacitor to one voltage level during the first phase of a second cycle, and couple the reference capacitor to another voltage level during the second phase of the second cycle.

20. The industrial process control transmitter according to claim 19, wherein the switch control operates the first, second and third switches through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_B}{N_A}.$

24. The industrial process control transmitter according to claim 14, further including a switch control coupled to the switch circuit to operate the switch circuit through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_A-N_B}{N_A+N_B}.$

25. The industrial process control transmitter according to claim 14, further including a switch control coupled to the switch circuit to operate the switch circuit through N_A first cycles and N_B second cycles such that an integrated charge supplied by the first and second sensing capacitors balances an integrated charge supplied by the reference capacitor, and $\frac{C_1-C_2}{C_{\mathrm{REF}}}=\frac{N_B}{N_A}.$

21. A process for measuring a process variable comprising steps of:

applying the process variable to first and second sensor capacitors to derive first and second capacitances, C₁ and C₂ respectively, based on the process variable;

providing a reference capacitor having a reference capacitance, C_REF, larger than a difference between the capacitances of the first and second capacitors;

deriving a first charge representative of C₁-C₂;

deriving a second charge representative of C_REF; and

integrating the first and second charges.

22. The process of claim 21, wherein the first and second and capacitors are coupled together, and an excitation source supplies a plurality of voltages, the process further comprises steps of:

defining first and second phases of each cycle,

applying a first voltage to the first capacitor and a second voltage to the second capacitor during the first phase of each cycle,

applying the second voltage to the first capacitor and the first voltage to the second capacitor during the second phase of each cycle,

applying a voltage to the reference capacitor during the first phase of the first cycle and during the second phase of the second cycle, and

applying another voltage to the reference capacitor during the first phase of the second cycle and during the second phase of the first cycle.

23. The process of claim 21, wherein the first and second and capacitors are coupled together, and an excitation source supplies a plurality of voltages, the process further comprises steps of:

defining first and second phases of each cycle,

applying a first voltage to the first capacitor and a second voltage to the second capacitor during the first phase of a first cycle,

applying the second voltage to the first capacitor and the first voltage to the second capacitor during the second phase of the first cycle,

applying a voltage to the reference capacitor during the first phase of the second cycle, and

applying another voltage to the reference capacitor during the second phase of the second cycle.