WO1990010848A1

WO1990010848A1 - Liquid level sensor

Info

Publication number: WO1990010848A1
Application number: PCT/US1990/001296
Authority: WO
Inventors: Kyong Park
Original assignee: Kavlico Corporation
Priority date: 1989-03-10
Filing date: 1990-03-09
Publication date: 1990-09-20
Also published as: US4924702A

Abstract

A capacitive probe (10) is constructed to eliminate electrical conduction through a liquid (36) being measured. The electrical conduction in the liquid (36) occurs between the plates of the reference and measuring capacitors (52, 54). The electrodes (12, 14, 16) which are spaced from the common electrode (12) to form the reference and measuring capacitor (52, 54) are of unitary construction and formed from anodized aluminum. The anodization forms an insulated surface which prevents conduction between the electrodes (12, 14, 16) through the liquid (36). A ratio taken between the measuring capacitor (54) and reference capacitor (52) is therefore indicative of the level of the liquid (36) within the gap (28) of the measuring capacitor (54) and is independent of the conductance of the liquid (36) and its dielectric constant. Such a probe (10) is particularly useful in applications where the liquid (36) undergoes indeterminable changes in conductivity such as a lubricant which becomes contaminated by metal particulates (38).

Description

LIQUID LEVEL SENSOR

Background of the Invention

Field of the Invention

The present invention relates generally to liquid level sensors and more particularly to a capacitive liquid level sensor for measuring the level of the liquid which undergoes changes in conductivity.

Description of the Related Art

Capacitive probes for measuring the level of a liquid within a reservoir are well known. Such probes generally include a reference capacitor adapted to be fully submerged in the liquid and a measuring capacitor. As the level of the liquid elevationally varies within the space between the plates of the measuring capacitor, the capacitance changes due to the change of the dielectric constant between the plates. By taking the ratio of the capacitance between the two capacitors, the level of the liquid may then be ascertained. The ratio of the capacitances compensates for temperature dependency.

A class of such capacitive probes has been developed to measure the level of a conductive or lossy dielectric liquid, especially where the conductivity of the liquid is initially unknown and also varies during measurement. When the probe is inserted in a conductive liquid, conductance between the plates of the reference and measuring capacitors causes errors in measuring the capacitive ratio whereby the probe becomes inaccurate.

Attempts have been made to correct the above problems in capacitive probes by providing phase discrimination between the resistive and capacitive voltages. The resistive voltage leads to the inaccuracies due to the conduction between the two capacitors. Since the resistive and capacitive voltages are 90° out of phase with each other, phase discrimination should be able to eliminate the resistive component. However, these prior art attempts have not met with a great degree of success. Phase discrimination permits a portion of the resistive voltage to add or subtract from the desired phase discriminated output current due to unavoidable circuit phase shifts. Since the resistive current may be generally very large, indicating a virtual short circuit between the two capacitors, the errors thus produced can be many times greater than the capacitance signal current. The results is that such a capacitive system may be disadvantageously inaccurate.

In the prior art, this inaccuracy has been compensated for by the use of resistive bridge circuit wherein the parasitic conductance through the liquid forms a first resistor of a balancing bridge and a potentiometer is provided in another leg of the bridge to compensate, thereby removing the parasitic resistance from the capacitive ratio. However, the conductance of a liquid may be highly dependent upon temperature, causing the parasitic resistor to change value relative to the potentiometer setting.

Complex electromechanical systems have been proposed for the in situ adjustment of the potentiometer based upon means provided to sense the conductivity of the liquid, for example as described in U.S. Patent No. 3,114,262. In the '262 patent, a feedback circuit is designed to have a negative resistance temperature coefficient to match the temperature coefficient of the conductance of the liquid being measured such that applier gain of the bridge circuit is cancelled. A limitation of the described system is that the resistive bridge circuit and the feedback circuit must be accurately matched to the liquid being measured.

In U.S. Patent No. 3,515,000 an attempt to overcome the above disadvantage of matching the bridge impedance and the resistivity of the liquid being measured is described. A probe is provided wherein the measuring electrode and the common electrode are in the input circuit of an operational amplifier, the reference electrode and the common electrode are in the feedback circuit of the amplifier. The impedance in the input circuit between the measuring electrode and the common electrode will be a function of the level of the liquid in the tank in which the probe is immersed. The feedback impedance will be a function of the characteristics of the same liquid since the reference electrode is immersed in the same liquid as the measuring electrode. The gain of the operational amplifier connected in this manner will be equal to the ratio of the reference impedance to the measuring impedance. Since both impedances are products of the electrical impedance of the same liquid, the ratio will be independent of the electrical impedance of the liquid and it will merely indicate the ratio of the surface areas of the measuring and reference probes, hence the capacitive ratio, immersed in the liquid. A limitation of the capacitive probe described in the '000 patent is that conductance of the liquid may be occur between the measuring and the reference electrodes. If this conductance becomes large enough, the operational amplifier will be shorted out and hence always have unity gain independent of the level of the liquid to be measured.

Summary of the Invention

It is a feature of the present invention to provide a capacitive probe which is insensitive to temperature or dielectric changes due to contamination or aging of the liquid being measured.

According to the present invention, a capacitive probe for measuring the level of a liquid in a reservoir wherein the liquid undergoes variation and contamination in electrical conductivity during the measuring comprises three electrodes. The first electrode is conductive and has a first face region and a second face region elevationally displaced from the first region. The second electrode is of unitary construction and has a conductive portion and an insulative portion. The insulative portion is in the facing relationship to the first face regions of the first electrode to form a first gap of one capacitor. The third electrode also of unitary construction includes a conductive portion and an insulative portion, its insulative portion being in a facing relationship to the second face region of the first electrode to form a second gap and a second capacitor. The probe is positioned within the reservoir wherein the liquid entirely fills the first gap, the reference capacitor, and the liquid elevationally varies in the second gap commensurately with the level of liquid in the reservoir. It is a feature of the present invention that the insulative portion of each of the second and third electrodes electrically isolates each conductive portion from electrical conduction through the liquid between the second and third electrodes.

These and other objects, advantages and features of the present invention will become apparent to those skilled in the art from a study of the following description of the preferred embodiment of the invention and that in conjunction with the attached drawings and appended claims.

Brief Description of the Drawings

Fig. 1 is an exploded view of a capacitive probe constructed according to the principles of the present invention;

Fig. 2 is a cross-section of an assembled probe of Fig. l;

Fig. 3 is a cross-section of the probe of Fig. 2 taken along line 3-3;

Fig. 4 is a cross-section of the probe of Fig. 2 taken along line 4-4;

Fig. 5 is a cross-sectional view of the probe of Fig. 2 taken along line 5-5;

Fig. 6 is a cross-section of the probe of Fig. 2 taken along line 6-6 and

Fig. 7 is a cross-section of the probe of Fig. 2 taken along line 7-7.

Description of the Preferred Embodiment

Referring now to Figs. 1 and 2, there is shown a capacitive probe 10 constructed according to the principles of the present invention. The probe 10 includes a first conductive electrode 12, a second electrode 14 and a third electrode 16.

The first electrode 12 has a first face region 18 and a second face region 20. As best seen Fig. 2, the second face region 20 is elevationally displaced from the first face region 18.

The second electrode 14 is of unitary construction and includes a conductive inner portion 22 and an insulative surface portion 24. The insulative portion 24 is disposed in a facing relationship to the first face region 18 of the first conductive electrode 12 to form a first gap 26.

Third electrode 16 is also of unitary construction and includes conductive inner portion 28 and an insulative surface portion 30. The insulative surface portion 30 of the third electrode 16 is disposed in a facing relationship to the second face region 20 of the first conductive electrode 12 to form a second gap 32.

The probe 10, as best, seen in Fig. 2, is adapted for positioning within a reservoir 34 wherein a liquid 36 entirely fills the first gap 26 and elevationally varies in the second gap 32 commensurately with the level of the liquid 36 in the reservoir 34. The insulative surface portions 24, 30 of each of the second electrode 14 and the third electrode 16 electrically isolates each of their respective conductive inner portions 22, 28 from electrical conductance through the liquid 36. For example the liquid 36 may become contaminated with metal particulates 38, such as lubricating oil within machinery or hydraulic fluid within a transmission. These particulates 38 may then precipitate out and deposit on the second electrode 14 and third electrode 16, and even clog the first gap 26 and second gap 28. In the prior art, this precipitation would cause increased conductance in the liquid, eventually shorting out the electrodes in the prior art probe, although any conductance may cause inaccuracies in the operation of the prior art probe. According to the present invention, the insulative surface portions 24, 28 prevent any conductance or short circuit between the second electrode 14 and the third electrode 16 through the liquid 36.

In one embodiment of the present invention, the first electrode 12 is constructed from an elongated first tube 39 which has an inner surface 40 and an outer surface 41. Each of the first and second face regions 18, 20 extends axially along the inner surface 40 and has a predetermined height. The first tube 39 may be constructed from any suitable conductive material such as aluminum.

The second electrode 14 is constructed from an elongated second tube 42 axially dimensioned commensurately with the predetermined height of the first face region 18. The second tube 42 is coaxially disposed within the first tube 39. The insulative surface portion 24 of the second electrode 14 is formed an outer surface 44 of the second tube 42. An inner surface 45 of the second tube 42 is also insulative. For example, the second tube 42 may be constructed from a conductive material which is then oxidized to form the insulative surfaces.

Third electrode 16 is constructed from an elongated third tube 46 axially dimensioned commensurately with the predetermined height of the second face region 20. The third tube 46 is coaxially disposed within the first tube 39. The insulative surface portion 30 of the third electrode 16 is formed on an outer surface 48 of the third tube 46. An inner surface 49 of the third tube 46 is also insulative. The third tube may also be constructed from a conductive material which is then oxidized to form the insulative surfaces. For example, the second and third tube may be formed from black anodized aluminum.

As best seen in Fig. 1, Fig. 3 and Fig. 6, the first tube 39 has a plurality of openings 50 extending between its inner surface 40 and outer surface 41. Some of the openings 50 communicating the first gap 26 externally of the first tube 39 (Fig. 3) and the remaining openings 50 communicating the second gap 32 externally of the first tube 39 (Fig. 6) . Apparent from the above description of the probe 10 is that the first tube 39 and the second tube 42 form the electrodes of a first capacitor 52 which includes the first gap 26. Similarly, the first tube 39 and the third 46 form the electrodes of a second capacitor 54 which includes the second gap 32.

The probe 10 further includes means 56 for mounting the second tube 42 and the third tube 46 coaxially within the first tube 39. The mounting means 56 includes a first spacer 58. the spacer 58 has a circumferential portion 60 in sliding engagement with the inner surface 40 of the first tube 39 and a frustoconical section 62 extending from a top surface 64 of the first spacer 48. The frustoconical section 62 is with a lower edge portion 66 of the second tube 42. Accordingly, the lower edge portion 66 of the second tube 42 may be inwardly bevelled to receive the frustoconical edge 58. First tube 39 includes first annular flange 68 extending inwardly from it, inner surface 40 at a lower edge portion 70 of the first tube 39. A lower surface 72 of the first spacer 58 is seated on the flange 68.

Mounting means 56 further includes a second spacer 74 disposed between the second tube 42 and the. third tube 46. The second spacer 74 has a circumferential edge portion 76 in sliding engagement with the inner surface 40 of the first tube 39 and a lower frustoconical surface 78 and an upper frustoconical surface 80. The lower frustoconical surface 78 is dimensional to about an upper edge portion 82 of the second tube 42. The upper edge portion 82 of the second tube 42 may also be inwardly bevelled. The upper frustoconical surface 80 is dimensioned to abut with a lower edge portion 84 of the third tube 46 which may also be inwardly bevelled.

Mounting mean 56 further includes a third spacer 86 having a circumferential edge 68 in sliding engagement with inner surface 40 of the first tube 39 and a lower frustoconical surface 90. The lower frustoconical surface 90 of the third spacer 86 is dimensioned to abut an upper edge portion 92 of the third tube 46, which may also be inwardly bevelled.

The third spacer 86 further includes a channel 92 disposed in the circumferential edge 88. Mounting means 46 further includes an O-ring 94 pressingly engaged between the inner surface 40 of the first tube 39 and the third spacer 86 within the channel 92.

Mounting means 56 further includes an end cap 96 dimensioned to be slidingly engaged with the inner surface 40 in an upper end portion 98 of the first tube 39. A lower edge 99 of the end 96 cap abuts an upper surface 100 of the third spacer 86. The first tube 39 further includes a second annular flange 102 extending inwardly from its inner surface 40 at an upper edge portion 104 of the first tube 39. The second annular flange 102 exerts an axially inward force on an upper edge 106 of the end cap 96 to pressingly mount the second tube 42 and the third tube 46 against the first spacer 58, the second spacer 74 and the third spacer 86.

Referring now to Fig. 5, the second spacer 74 includes a plurality of arcuate openings 108 disposed at the circumferential edge thereof. The arcuate openings 108 communicate the first gap 26 with the second gap 32. The second spacer 74 also includes a plurality of openings 110 to allow the liquid 36 to drain from the interior of the probe 10.

In alternative embodiment of the present invention, each of first, second and third spacers 58, 74 and 86 includes a coaxial bore therethrough. Mounting means 46 may further include an elongated bolt (not shown) having a flattened head portion and a rod portion. The rod portion is received through each of the coaxial bores of the first spacer 48 and the second spacer 56. Furthermore, the rod has a threaded portion threadingly engaging the bore of the third spacer. The head portion then engages the annular flange 68 of the first tube 39.

Probe 10 further includes electronic circuit means 112 for determining the capacitive ratio between the first capacitor 52 and the second capacitor 54 and for developing a signal proportional to the ratio. Circuit means 112 may include an integrated circuit chip 114 mounted on the upper surface 100 of the third spacer 86. The third spacer 86 may include a recess 116 dimensioned to receive the chip 114. The capacitive ratio may, for example, be obtained as a ratio of the difference between the first and second capacitance to the sum of the first and second capacitance.

Probe 10 may further include conduction means 118 operatively connecting the chip 114 to each of the first tube 39, the second tube 42 and the third tube 46. A feed through plate 120 has openings 122 for electrical wiring to extend from the chip 114 externally of the probe 10. The feed through plate 120 is mounted on any annular flange 124 within the end cap 96. To further protect the circuit chip 114, potting (not shown) fills the space between the third spacer 86 and the feed through plate 120.

Conduction means further includes a first disc shaped member 126 formed unitarily with second tube 42 and a second disc shaped member 128 formed unitarily with third tube 46. As best seen in Fig. 4 and Fig. 7, first disc member 126 includes a tapped bore 130 and second disc member 128 includes a tapped bore 132. The tapped bores 130, 132 are formed after oxidation to electrically contact the respective conductive inner portions 22, 28 of the second and third electrodes 14, 16.

A mounting bracket may be provided with a bore 136 to receive probe 10 in slidable engagement therein. The annular flange 68 abuts an annular lip 138 within the bore 136. The bracket 134 is designed to mount within the reservoir 34.

There has been hereinabove described preferred embodiments of a capacitive probe constructed according to the principles of the present invention. Those skilled in the art may now make numerous uses of and modifications to the above-described embodiments of the present invention without departing from the inventive concepts described herein. Accordingly, the present invention is to be defined solely by the scope of the appended claims.

Claims

CLAIMSWhat Is Claimed Is;

1. A capacitive probe for measuring a level of a liquid in a reservoir wherein said liquid undergoes variation in contamination and electrical conductivity during such measuring, said probe comprising: a first conductive electrode having a first face region and a second face region elevationally displaced from said first region; a second electrode of unitary construction including a conductive portion and an insulative portion, said insulative portion being in a facing relationship to said first face region to form a first gap; and a third electrode of unitary construction including a conductive portion and an insulative portion, said insulative portion of said third electrode being in a facing relationship to said second face region to form a second gap; said probe being adapted for positioning within said reservoir wherein said liquid entirely fills said first gap and said liquid elevationally varies in said second gap commensurately with said level of said liquid in said reservoir, said insulative portion of each of said second electrode and said third electrode electrically isolating each said conductive portion from electrical conduction through said liquid between said second electrode and said third electrode.

2. A probe as set forth in Claim 1 wherein said first electrode includes an elongated first tube having an inner surface, each said face region being disposed on said inner surface.

3. A probe as set forth in Claim 2 wherein each said face region extends axially along said inner surface and has a predetermined height.

4. A probe as set forth in Claim 2 wherein said second electrode includes an elongated tube axially dimensioned commensurately with said predetermined height of said first face region and co-axially disposed within said first tube, said insulative portion of said second electrode forming an outer surface of said second tube.

5. A probe as set forth in Claim 4 wherein said first tube has at least one opening communicating said first gap externally of said first tube.

6. A probe as set forth in Claim 2 wherein said third electrode includes a third elongated tube axially dimensioned commensurately with said predetermined height of said second face region and coaxially disposed within said first tube, said insulative portion of said third electrode forming an outer surface of said third tube.

7. A probe as set forth in Claim 6 wherein said first tube has at least one opening communicating said second gap externally of said first tube.

8. A capacitive probe comprising: a conductive first tube having an inner surface, an outer surface, a first axially extending face region, a second axially extending face region, and a plurality of openings extending between said inner surface and said outer surface; a second tube having an insulative outer surface and a conductive inner surface, said second tube being coaxially disposed within said first tube, said insulative outer surface being in a facing relationship to first face region, said first face region and said first tube forming a first capacitor having a first gap; a third tube having an insulative outer surface and a conductive inner surface, said third tube being coaxially disposed within said first tube, said insulative outer surface of said third tube being in a facing relationship to said second face region, said second axially extending portion and said third tube forming a second capacitor having a second gap; means for mounting said second tube and said third tube within said first tube, said openings communicating said first gap and said second gap externally of said first tube; and means for electrically contacting said first tube and said inner surface of each said second tube and said third tube to electrically determine a value of capacitance for each of said first capacitor and said second capacitor.

9. A probe as set forth in Claim 8 wherein said mounting means includes: a first elastomeric spacer having a circumferential edge sealingly engaged in said inner surface of said first tube at said first end portion thereof and a frustoconical section extending from one surface of said first spacer and dimensioned to seal one end portion of said first tube; a second elastomeric spacer disposed between said second tube and said third tube, said second spacer having a circumferential edge engaged in said inner surface of said first tube and a pair of frustoconical surfaces, each of said surfaces engaging an end portion of a respective one of said second tube and said third tube; and a third elastomeric spacer having a circumferential edge sealingly engaged in said inner surface of said first tube at said second end portion thereof and a frustoconical surface engaging and end portion of said third tube.

10. A probe as set forth in Claim 9 wherein said third spacer has a channel in said circumferential edge, said mounting means further including an O-ring pressingly engaged between said inner surface of said first tube and said third spacer within said channel.

11. A probe as set forth in Claim 10 wherein said probe further includes electronic circuit means for determining a capacitive ratio between said first and said second capacitor and for developing a signal proportional to said ratio.

12. A probe as set forth in Claim 11 wherein said electronic circuit means includes an integrated circuit chip mounted on an upper surface of said third spacer, said third spacer including a recess dimensioned to receive said chip, said conduction means operatively connecting said chip to each of said first tube, said second tube and said third tube.

13. A probe as set forth in Claim 12 wherein said mounting means further includes an end cap dimensioned for being slidingly fitted in said upper end portion of said first tube, said end cap abutting said upper surface of said third spacer.

14. A probe as set forth in Claim 13 wherein said end cap includes an interior annular flange, said mounting means further including a feed through plate mounted on said flange, said feed through plate having an opening for electrical wiring to extend from said chip externally of said probe.

15. A probe as set forth in Claim 14 wherein said first tube has a first crimped edge portion and a second crimped edge portion, said first crimped edge portion exerting a compressive force on said first spacer, said second crimped edge portion exerting a counter compressive force on said end cap.

16. A probe as set forth in Claim 9 wherein said second end portion of each of said second tube and said third tube have a bevelled edge dimensioned for engagement with said frustoconical surface of a respective one of each said spacer.

17. A probe as set forth in Claim 15 wherein said probe further includes potting received in said second end portion of said first tube above said feed through plate.

18. A probe as set forth in Claim 8 wherein each said gap is 0.14 inches (0.36 cm.).

19. A probe as set forth in Claim 9 wherein said second spacer includes a plurality of semi-circular openings disposed at said circumferential edge thereof, said semi-circular openings communicating said first gap with said second gap.

20. A probe as set forth in Claim 9 wherein said first spacer and said second spacer each include a plurality of openings to allow liquid to be communicated there through.

21. A probe as set forth in Claim 9 wherein each of said second tube and said third tube have an interior disc shaped member formed unitarily therewith, each said member including a tapped bore to electrically contact said inner conductive portion of each said tube.

22. A probe as set forth in Claim 9 wherein each said spacer includes a coaxial board therethrough, said mounting means further comprising an elongated bolt having a flattened head portion engaging said lower end portion of said first tube and a rod portion, said rod portion being received through said coaxial bores of said first spacer and said second spacer and threadingly engaging said bore of said third spacer.