Differential capacitive transducer

• US 5,804,736 A
• Filed: 01/27/1997
• Issued: 09/08/1998
• Est. Priority Date: 06/11/1996
• Status: Expired due to Term

• Current Assignee: Siemens Industry Incorporated (Siemens AG)
• Original Assignee: Moore Products Co. (Siemens AG)
• Inventors: O'Brien, Christopher J., Moore, James O., Klauder, Philip R.
• Primary Examiner(s): Oen, William L.
• Application Number:
  US08/790,253
• Time in Patent Office: 589 Days
• Field of Search: 73/706, 73/708, 73/718, 73/724, 361/283.1, 361/283.3
• US Class Current: 73/724
• CPC Class Codes: G01F 1/363 with electrical or electro-mechanical indication G01F1/37 and G01F1/38 take precedence
• CPC Class Codes: G01L 19/0015 using switching means
• CPC Class Codes: G01L 19/02 Arrangements for preventing, or for compensating for, effects of inclination or acceleration of the measuring device; Zero-setting means for aneroid barometers G01L7/14
• CPC Class Codes: G01L 27/005 Apparatus for calibrating pressure sensors
• CPC Class Codes: G01L 9/0019 of a semiconductive element
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm
• CPC Class Codes: G01L 9/065 with temperature compensating means non electric temperature compensating means G01L19/04
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04

Abstract

A pressure transducer having a redundant fluid pressure sensor can be used to determine if the pressure transducer needs to be recalibrated. The pressure transducer comprises a fluid pressure sensor for measuring differential pressure. The fluid pressure sensor includes a pair of substantially identical pressure sensors for providing a variable electrical output as a function of a differential fluid pressure and a reference sensor for providing an electrical output independent of the differential fluid pressure. A conditioning circuit is provided for generating a frequency-based signal whose frequency is a function of the electrical output for each fluid pressure sensor. The electrical output of the first and second pressure sensors can be compared to determine whether the pressure transducer needs to be calibrated. In one embodiment, the pressure sensors comprise variable capacitors. The first and second capacitors each have one fixed plate and one movable plate responsive to differential fluid pressure across the movable plate. The reference capacitor has two fixed plates. The first variable, second variable, and reference capacitors are all located in proximity to one another and in communication with the differential fluid pressure. The pressure transducer also includes a switching network connected to the capacitors for selectably connecting at least one of the capacitors to an electronic circuit.

Claims

1. A pressure transducer comprising:

a capacitive sensor having first and second variable capacitors for providing a variable capacitance as a function of a differential fluid pressure and a reference capacitor for providing a fixed capacitance independent of the differential fluid pressure, the first and second capacitors each having one fixed plate and one movable plate responsive to differential fluid pressure across the movable plate, the reference capacitor having two fixed plates, the first variable, second variable, and reference capacitors all being located in proximity to one another and in communication with the differential fluid pressure; and

a switching network connected to the capacitors for selectably connecting at least one of the capacitors to an electronic circuit via the switching network for generating a frequency-based signal whose frequency is a function of the capacitance of the at least one capacitor connected to the electronic circuit through the switching network.

2. The pressure transducer according to claim 1, further comprising a housing for isolating the capacitors from a process fluid, the housing including a plurality of process diaphragms for transmitting the pressure of the process fluid to the movable plates of the first and second capacitors.

3. The pressure transducer according to claim 1, wherein the movable plate of the first variable capacitor, the movable plate of the second variable capacitor, and one of the fixed plates of the reference capacitor are formed from a semiconductor substrate which forms a common capacitor plate.

4. The pressure transducer according to claim 3, wherein the fixed plate of the first variable capacitor, the fixed plate of the second variable capacitor, and the other fixed plate of the reference capacitor are located on a first dielectric plate.

5. The pressure transducer according to claim 4, further comprising a second dielectric plate, the common capacitor plate being interposed between the first and second dielectric plates.

6. The pressure transducer according to claim 1, wherein the second variable capacitor is substantially identical to the first variable capacitor.

7. The pressure transducer according to claim 1, further comprising a three valve manifold in fluid communication with the capacitive sensor for selectively applying a zero-pressure differential to the capacitive sensor.

8. The pressure transducer according to claim 1, wherein the electronic circuit comprises an oscillator for generating the frequency-based signal.

9. The pressure transducer according to claim 1, wherein the switching network connects the first variable capacitor only, and the reference capacitor only, to the electronic circuit in a preselected sequence, and the electronic circuit generates a first signal whose frequency is a function of the capacitance of the first variable capacitor and a second signal whose frequency is a function of capacitance of the reference capacitor.

10. The pressure transducer according to claim 1, wherein the switching network connects the first variable capacitor only, the reference capacitor only, and both the first variable capacitor and the reference capacitor to the electronic circuit in a preselected sequence, and the electronic circuit generates a first signal whose frequency is a function of the capacitance of the first variable capacitor, a second signal whose frequency is a function of capacitance of the reference capacitor, and a third signal whose frequency is a function of the capacitance of both the first capacitor and the reference capacitor.

11. The pressure transducer according to claim 1, wherein the switching network connects the only first variable capacitor, only the reference capacitor, and only the second variable capacitor to the electronic circuit in a preselected sequence, and the electronic circuit generates a first signal whose frequency is a function of the capacitance of the first variable capacitor, a second signal whose frequency is a function of capacitance of the reference capacitor, and a third signal whose frequency is a function of the capacitance of the second variable capacitor.

12. A capacitive sensor comprising:

first and second variable capacitors for providing a variable capacitance as a function of a differential fluid pressure applied thereto and a reference capacitor for providing a fixed capacitance independent of differential fluid pressure applied thereto, the first and second variable capacitors each having one fixed plate and one movable plate responsive to differential fluid pressure across the movable plate, the reference capacitor having two fixed plates with respect to the first and second variable capacitors, the first variable, second variable, and reference capacitors all being located in proximity to one another and in communication with the differential fluid pressure.

13. The capacitive sensor according to claim 12, wherein the second capacitor is substantially identical to the first capacitor.

14. The capacitive sensor according to claim 13, wherein the movable plate of the first variable capacitor, the movable plate of the second variable capacitor, and one of the fixed plates of the reference capacitor are formed from a semiconductor substrate which forms a common capacitor plate.

15. The capacitive sensor according to claim 14 wherein the fixed plate of the first variable capacitor, the fixed plate of the second variable capacitor, and the other fixed plate of the reference capacitor are located on a first dielectric plate.

16. The capacitive sensor according to claim 15 further comprising a second dielectric plate, the common capacitor plate being interposed between the first and second dielectric plates.

17. The capacitive sensor according to claim 16, further comprising a base plate formed from a semiconductor substrate and attached to the second dielectric plate.