Linearized capacitance sensor system

• US 5,194,819 A
• Filed: 08/10/1990
• Issued: 03/16/1993
• Est. Priority Date: 08/10/1990
• 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): Harvey, Jack B.
• Application Number:
  US07/565,929
• Time in Patent Office: 949 Days
• Field of Search: 324/684, 324/678, 324/676, 324/661, 324/662, 324/725, 324/679, 324/680, 73/718, 73/724, 341/118
• US Class Current: 324/684
• CPC Class Codes: G01D 5/2417 by varying separation
• CPC Class Codes: G01G 7/06 by electrostatic action
• CPC Class Codes: G01L 1/144 with associated circuitry G01L1/146 and G01L1/148 take precedence
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitive sensing circuit and system includes a variable capacitance sensing element as an active element in a circuit to measure changes in capacitance of the sensing element. A switching network in alternate cycles energizes a capacitor to develop a charge which effectively linearizes the sensor'"'"'s capacitance response. In a push-pull circuit with two active sensing elements, a multi-channel switching network energizes two fixed correction capacitors to provide third or higher order correction, significantly extending the effective scale of the sensor. In a preferred embodiment, a single feedback signal is applied to all capacitors in different switching cycles. Examples include weight and pressure sensing systems.

Claims

1. A capacitive sensing circuit comprising

a first and a second capacitor with one end of each capacitor connected to a junction, at least said second capacitor being a sensing capacitor having a capacitance which varies in accordance with physically-induced change in plate position and which has a non-linear capacitance characteristic,

first switching means for applying signals to non-common ends of said first and second capacitors in alternate switching cycles for developing a flow of charge through said capacitors such that the voltage present at the junction varies in functional dependence on the capacitance values of said first and second capacitors,

a third capacitor having one end coupled to said junction,

second switching means for coupling a signal to the non-common end of said third capacitor to develop a charge effective to substantially null the charge variation due to said non-linear capacitance characteristic of the sensing capacitor, and

feedback means responsive to the voltage at said junction for providing a feedback signal to said first switching means to charge at least said second capacitor and maintain stationary the voltage present at the junction.

2. A circuit according to claim 1, further comprising

a fourth capacitor having one end coupled to said junction, and

third switching means for coupling a signal to another end of said fourth capacitor to develop a charge effective to substantially null the charge due to the non-linear capacitance characteristic of said first capacitor.

3. A circuit according to claim 2, wherein said first and second capacitors are both sensing capacitors, each having a non-linear capacitance characteristic, and wherein said environmental condition induces substantially opposite changes in capacitance in said first and in said second capacitors.

4. A circuit according to claim 3, wherein said first, second and third switching means are CMOS analog switches.

5. A circuit according to claim 4, wherein said feedback means develops a feedback signal which is applied to each of said first, second, third and fourth capacitors.

6. A circuit according to claim 2, wherein said first and second capacitors are formed by opposing sides of a sensing structure having a common diaphragm which moves in response to force or pressure to produce said changes in capacitance.

7. A circuit according to claim 1, wherein said feedback means includes means for sampling said junction voltage at a predetermined time after operation of said switching means.

8. A circuit according to claim 1, wherein said third capacitor has a value selected to compensate for leakage of said second capacitor.

9. A circuit according to claim 2, wherein said third and fourth capacitors have values to compensate for leakage of said second and first capacitors, respectively.

10. A circuit according to claim 2, wherein said feedback means provides a feedback signal which nulls the voltage present at the junction.

11. A circuit according to claim 1, wherein the feedback means develops a signal which is applied to each of said first, second and third capacitors by the respective switching means.

12. A circuit according to claim 1, wherein the third capacitor has a value approximately equal in magnitude to a leakage capacitance, and further comprising means for applying a fractional portion of said feedback signal to the third capacitor.