THREE-TERMINAL CAPACITIVE APPARATUS FOR REMOTELY RESPONDING TO A CONDITION OR DIELECTRIC PROPERTIES OF A MATERIAL

• US 3,774,238 A
• Filed: 12/09/1971
• Issued: 11/20/1973
• Est. Priority Date: 12/09/1971
• Status: Expired due to Term

• Current Assignee: Edward V. Hardway Jr
• Original Assignee: Edward V. Hardway Jr
• Inventors: Hardway, Edward V. Jr.
• Primary Examiner(s): Krawczewicz, Stanley T.
• Application Number:
  US05/206,463
• Time in Patent Office: 712 Days
• Field of Search: 324/61R,61P 73/34C,398C 340/258C,244C 137/392,386
• US Class Current: 324/663
• CPC Class Codes: G01F 23/263 by measuring variations in capacitance of capacitors
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825
• CPC Class Codes: Y10T 137/7306 Electrical characteristic sensing

Abstract

Three-terminal capacitive apparatus are disclosed which provide information concerning a condition of a material by responding to the dielectric properties of the material. A three-terminal capacitor, which may be a cell or probe, may be used, for example, to measure the dielectric constant of a material, to determine the level of a material in a container, to determine the interface between liquid materials in a pipeline, or the proximity of a material to the probe. Each cell or probe includes a conductive driven element connected to a regulated square wave source, and a conductive receptor element connected to the input of a high gain amplifier with a capacitor in the feedback loop which maintains the receptor element at virtual ground. The output of the feedback amplifier is proportional to the feed through capacitance of the capacitive probe or cell. A synchronous demodulator, synchronized by the drive signal or a signal in phase with it, is connected to the feedback amplifier output and is used to produce a DC output signal used for display or control. The cell or probe is connected to associated electronics through shielded cables and may be remotely located from the electronics. Suitable mechanical apparatus is associated with a particular three-terminal capacitor cell or probe for mounting it in position to respond to a particular condition being monitored.

Claims

1. Three-terminal capacitive apparatus responsive to a condition of a material which causes changes in the feed through capacitance of the apparatus, so that such changes may be relatively accurately sensed or measured even in the presence of substantial amounts of stray capacitance, comprising, in combination:

an electrically driven conductive element;

a conductive receptor element;

support means for supporting said driven and receptor elements spaced from and adjacent to each other and in the proximity of such material so that said condition affects the feed through capacitance of said capacitive apparatus;

a source of square wave drive signals having a substantially constant peak amplitude with respect to circuit ground;

first shielded conductive means connecting said source to said driven element to conduct said drive signals thereto, the output impedance of said source being such that any stray capacitance between said driven element and ground is rendered substantially ineffective with respect to said amplitude of said drive signals;

a high gain amplifier including a capacitive feedback loop for maintaining said receptor element at virtual ground while providing an electrical output signal responsive to the feed through capacitance of said capacitive apparatus, and second shielded conductive means connecting said receptor element to said amplifier.

2. The apparatus of cLaim 1 wherein the presence of conductance of the material in proximity to said driven and receptor elements detrimentally affects the accuracy of determination of the feed through capacitance of said apparatus, and further including a synchronous demodulator connected to said amplifier and to said source of drive signals for demodulating the output signals from said amplifier in synchronism with said drive signals to substantially cancel out the effects of any such conductance present in said output signals.

3. The apparatus of claim 1 wherein the condition of said material is the level of such a material in a container and said driven and receptor elements comprise a level probe supported by said support means in such a container so that the feed through capacitance of the probe is responsive to a level of the material in the container.

4. The apparatus of claim 3 wherein said level probe is mounted in the top of a container and said driven and receptor elements extend from the top of the container to adjacent the bottom thereof whereby said feed through capacitance is responsive to the level of the material in the container throughout substantially all of the container.

5. The apparatus of claim 4 further including a threshold detector connected to said amplifier and providing an output signal when the output of said amplifier exceeds a predetermined level, valve means mounted in said container for permitting and stopping the flow of material into the container, and switch means connected to the output of said threshold detector and to said valve means and responsive to the output signal from said threshold detector to actuate said valve means.

6. The apparatus of claim 5 wherein said threshold detector includes means for varying said predetermined level.

7. The apparatus of claim 2 wherein the condition of said material is the level of such a material in a container and said driven and receptor elements comprise a level probe supported by said support means in such a container so that the feed through capacitance of the probe is responsive to a level of the material in the container.

8. The apparatus of claim 1 wherein said condition of said material is its proximity to a particular point, and wherein said driven and receptor elements comprise a probe supported by said support means at said point, wherein the approach of said material to said point produces detectable changes in the feed through capacitance of said probe, and further including means connected to said amplifier and responsive to when such changes in feed through capacitance exceed a predetermined amount to provide a control signal indicating the proximity of said material to said point.

9. The apparatus of claim 2 wherein said condition of said material is its proximity to a particular point, and wherein said driven and receptor elements comprise a probe supported by said support means at said point, wherein the approach of said material to said point produces detectable changes in the feed through capacitance of said probe, and further including means connected to said amplifier and responsive to when such changes in feed through capacitance exceed a predetermined amount to provide a control signal indicating the proximity of said material to said point.

10. The apparatus of claim 1 wherein said condition is the change in dielectric constant of said material, and further including means for providing an output signal in response to the rate of said change.

11. The apparatus of claim 10 wherein said last mentioned means includes at least one threshold detector responding to said output signal when it exceeds a predetermined level to provide a control signal, and a capacitor connected between the output of said output circuit and said threshold detector.

12. The apparatus of claim 2 wherein said condition is the change in dielectric constant of said material, and further including means for providing an output signal in response to the rate of said change.

13. The apparatus of claim 1 wherein said support means supports said driven and receptor elements in a dielectric cell and the condition of said material is the dielectric constant of said material which is proportional to the moisture content or ingredients ratio of the material.

14. The apparatus of claim 2 wherein said support means supports said driven and receptor elements in a dielectric cell and the condition of said material is the dielectric constant of said material which is proportional to the moisture content or ingredients ratio of the material.