Differential windshield capacitive moisture sensors

• US 5,801,307 A
• Filed: 01/10/1997
• Issued: 09/01/1998
• Est. Priority Date: 07/12/1995
• Status: Expired due to Fees

• Current Assignee: Tamar Sensors Ltd.
• Original Assignee: Millennium Sensors Ltd.
• Inventors: Netzer, Yishay
• Primary Examiner(s): Williams, Hezron E.
• Assistant Examiner(s): POLITZER, JAY L
• Application Number:
  US08/782,194
• Time in Patent Office: 599 Days
• Field of Search: 73/335.05, 73/170.17, 324/61, 324/671
• US Class Current: 73/170.17
• CPC Class Codes: B32B 17/10036 comprising two outer glass sheets
• CPC Class Codes: B32B 17/10174 Coatings of a metallic or dielectric material on a constituent layer of glass or polymer
• CPC Class Codes: B60S 1/0822 characterized by the arrangement or type of detection means
• CPC Class Codes: B60S 1/0825 Capacitive rain sensor
• CPC Class Codes: B60S 1/0829 Oscillator-resonator rain sensor
• CPC Class Codes: B60S 1/0877 at least part of the sensor being positioned between layers of the windshield
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825
• CPC Class Codes: G01N 27/226 Construction of measuring vessels; Electrodes therefor
• CPC Class Codes: H03K 2217/960775 Emitter-receiver or "fringe" type detection, i.e. one or more field emitting electrodes and corresponding one or more receiving electrodes

Abstract

A differential capacitive moisture sensor, relying for its operation on the time-varying couplings in two moisture-sensing regions to provide indication of presence of moisture, with selectivity of sensing surface provided by a shielding electrode, and with temperature information provided by a simultaneously-fabricated resistive temperature sensor. Various electrode connections and sensor electronics may be used. An improved differential capacitive moisture sensor has equal dry capacitances, with unequal moisture sensitivities, for providing a single-polarity output signal. An ice detection method and apparatus is also provided.

Claims

1. A fringing-field differential capacitive moisture sensor, comprising

(a) first and second fringing-field capacitances formed between first and second pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having first and second pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having first and second effective ratios of A/d, respectively, said first and second effective ratios of A/d of said first and second fringing-field capacitances being substantially equal, whereby said first and second fringing-field capacitances having substantially equal values of capacitance;

and,

(b) a dielectric having a dielectric thickness and first and second surfaces,

said first and second pairs of capacitive electrode plates in contact with said first surface of said dielectric,

said first and second pairs of capacitive electrode plates having first and second ratios of effective capacitive separation distance, d, to dielectric thickness, respectively;

said first ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said first fringing-field capacitor extends beyond said second surface of said dielectric,

said second ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said second fringing-field capacitor is substantially totally contained within said dielectric,

whereby, the fringing-field differential capacitive moisture sensor having substantially equal dry first and second fringing-field capacitances, resulting in a substantially zero dry-condition sensor output signal, thereby providing insensitivity of a resulting sensor wet-condition output signal to variations in said dielectric versus temperature and stress.

2. A fringing-field differential capacitive moisture sensor as in claim 1, wherein said first pair of capacitive electrode plates have said first value of d comparable to said dielectric thickness;

and said second pair of capacitive electrode plates have said second value of d smaller than said dielectric thickness.

3. A fringing-field differential capacitive moisture sensor as in claim 1, further comprising

(a) a second dielectric;

and,

(b) a shield electrode located in a plane parallel to said capacitive electrode plates and said second dielectric, said capacitive electrode plates separated by said second dielectric from said shield electrode.

4. A fringing-field differential capacitive moisture sensor as in claim 1, further comprising

(a) a flexible substrate;

(b) said capacitive electrode plates fabricated on said substrate; and

,

(c) said substrate having an extension serving as a ribbon cable.

12. A fringing-field differential capacitive moisture sensor as in claim 1, wherein said capacitive electrode plates are fabricated by deposition of a substantially transparent conductive thin film on a substrate.

5. A fringing-field differential capacitive moisture sensor assembly for sensing moisture on a second surface of a glass window having first and second surfaces, comprising:

(a) a first plastic laminate having first and second surfaces;

(b) at least three non-reference active plates on said first surface of said first plastic laminate;

(c) a shield electrode on said second surface of said first plastic laminate, said shield electrode serving to provide directionality,

said sensor assembly placed against the first surface of the glass window, with said first surface of said first plastic laminate toward the first surface of the glass window;

said at least three non-reference active plates on said first surface of said first plastic laminate providing first and second pairs of capacitive electrode plates,

(a) first and second fringing-field capacitances formed between said first and second pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having first and second pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having first and second effective ratios of A/d, respectively, said first and second effective ratios of A/d of said first and second fringing-field capacitances being substantially equal, whereby said first and second fringing-field capacitances having substantially equal values of capacitance; and

,

(b) the glass window being a dielectric having a dielectric thickness and first and second surfaces,

said first and second pairs of capacitive electrode plates in contact with said first surface of said dielectric,

said first and second pairs of capacitive electrode plates having first and second ratios of effective capacitive separation distance, d, to dielectric thickness, respectively;

said first ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said first fringing-field capacitor extends beyond said second surface of said dielectric,

said second ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said second fringing-field capacitor is substantially totally contained within said dielectric,

whereby, the fringing-field differential capacitive moisture sensor having substantially equal dry first and second fringing-field capacitances, resulting in a substantially zero dry-condition sensor output signal, thereby providing insensitivity of a resulting sensor wet-condition output signal to variations in said dielectric versus temperature and stress.

6. A fringing-field differential capacitive moisture sensor assembly as in claim 5, said glass window further comprising

first and second glass laminates, each said first and second glass laminate being first and second dielectrics, respectively, and having a first surface and a second surface, said first surfaces of said first and second glass laminates oriented toward each other,

said first plastic laminate sandwiched between said first glass laminate and said second glass laminate,

(a) said sensor assembly placed against said first surface of said first glass laminate,

(b) said first surface of said first plastic laminate toward said first surface of said first glass laminate.

7. A fringing-field differential capacitive moisture sensor assembly as in claim 6, further comprising

(a) a second plastic laminate having first and second surfaces,

(b) at least three non-reference active plates on said first surface of said second plastic laminate;

and

(c) a shield electrode on said second surface of said second plastic laminate, said shield electrode serving to provide directionality;

said first plastic laminate and said second plastic laminate sandwiched between said first glass laminate and said second glass laminate,

said first surface of said first plastic laminate toward said first surface of a said first glass laminate, and said first surface of said second plastic laminate coward said first surface of said second glass laminate;

said at least three non-reference active plates on said first surface of said second plastic laminate providing third and fourth pairs of capacitive electrode plates, thereby providing a second fringing-field moisture sensor, including

(a) third and fourth fringing-field capacitances formed between said third and fourth pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having third and fourth pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having third and fourth effective ratios of A/d, respectively, said third and fourth effective ratios of A/d of said third and fourth fringing-field capacitances being substantially equal, whereby said third and fourth fringing-field capacitances having substantially equal values of capacitance; and

,

(b) said second glass laminate being said second dielectric having a dielectric thickness and first and second surfaces,

said third and fourth pairs of capacitive electrode plates in contact with said first surface of said second dielectric,

said third and fourth pairs of capacitive electrode plates having third and fourth ratios of effective capacitive separation distance, d, to second dielectric thickness, respectively;

said third ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said third fringing-field capacitor extends beyond said second surface of said second dielectric,

said fourth ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said fourth fringing-field capacitor is substantially totally contained within said second dielectric,

whereby, said second fringing-field differential capacitive moisture sensor having substantially equal dry third and fourth fringing-field capacitances, resulting in a substantially zero dry-condition second sensor output signal, thereby providing insensitivity of a resulting second sensor wet-condition output signal to variations in said second dielectric versus temperature and stress.

8. A fringing-field differential capacitive moisture sensor assembly as in claim 7, wherein said second surfaces of said first and said second plastic laminates oriented toward each other, said second surfaces of said first and second plastic laminates sharing a common shield electrode common to said second sides of said first and second plastic laminates, thereby acting as a shield between said non-reference active plates of said first and second plastic laminates.

9. A method for detecting presence of ice on a second surface of a dielectric, the dielectric having first and second surfaces, comprising

(a) providing a fringing-field capacitive moisture sensor including

(1) first and second fringing-field capacitances formed between first and second pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having first and second pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having first and second effective ratios of A/d, respectively, said first and second effective ratios of A/d of said first and second fringing-field capacitances being substantially equal, whereby said first and second fringing-field capacitances having substantially equal values of capacitance;

and,

(2) a dielectric having a dielectric thickness and first and second surfaces,

said first and second pairs of capacitive electrode plates in contact with said first surface of said dielectric,

said first and second pairs of capacitive electrode plates having first and second ratios of effective capacitive separation distance, d, to dielectric thickness, respectively,

said first ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said first fringing-field capacitor extends beyond said second surface of said dielectric,

said second ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said second fringing-field capacitor is substantially totally contained within said dielectric,

whereby, the fringing-field differential capacitive moisture sensor having substantially equal dry first and second fringing-field capacitances, resulting in a substantially zero dry-condition sensor output signal, thereby providing insensitivity of a resulting sensor wet-condition output signal to variations in said dielectric versus temperature and stress;

(b) exciting said moisture sensor alternately with alternating current of at least two frequencies

(c) detecting resulting alternating current signal output of said capacitive moisture sensor at each said frequency, said detecting of each said alternating current signal output of said capacitive moisture sensor at each of said least two frequencies resulting in DC outputs corresponding to the signal coupling through said sensor at each said frequency; and

,

(d) processing said DC outputs to determine whether ice is present on the first surface of the dielectric.

10. A device for detecting presence of ice on a second surface of a dielectric having first and second surfaces, comprising

(1) a fringing-field capacitive moisture sensor including

(a) first and second fringing-field capacitances formed between first and second pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having first and second pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having first and second effective ratios of A/d, respectively, said first and second effective ratios of A/d of said first and second fringing-field capacitances being substantially equal, whereby said first and second fringing-field capacitances having substantially equal values of capacitance;

and,

(b) a dielectric having a dielectric thickness and first and second surfaces,

said first and second pairs of capacitive electrode plates in contact with said first surface of said dielectric,

said first and second pairs of capacitive electrode plates having first and second ratios of effective capacitive separation distance, d, to dielectric thickness, respectively;

said first ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said first fringing-field capacitor extends beyond said second surface of said dielectric,

said second ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said second fringing-field capacitor is substantially totally contained within said dielectric,

whereby, the fringing-field differential capacitive moisture sensor having substantially equal dry first and second fringing-field capacitances, resulting in a substantially zero dry-condition sensor output signal, thereby providing insensitivity of a resulting sensor wet-condition output signal to variations in said dielectric versus temperature and stress; and

,

(2) instrumentation including,

(a) a dual-frequency oscillator for providing two output frequencies for excitation of said moisture sensor;

(b) a clock oscillator providing a low-frequency clock output, for control of said dual-frequency oscillator;

(c) a first mixer for synchronously detecting said moisture output signal at each said output frequency, providing an alternating, single-polarity, two-level, first mixer output signal;

(d) a second mixer for synchronously detecting said first mixer output signal with respect to said low-frequency clock output, thereby transforming said first mixer output signal to a substantially DC, ground-referenced, second mixer output signal; and

,

(e) a processor for determining whether ice is present, based on said DC outputs corresponding to each said moisture output signal at each said excitation frequency.

11. A device for detecting presence of ice on a second surface of a dielectric having first and second surfaces, comprising

(a) a fringing-field capacitive moisture sensor including

(1) first and second fringing-field capacitances formed between first and second pairs of capacitive electrode plates, respectively, said pairs of capacitive electrode plates having first and second pairs of values of A, effective capacitive plate area, and d, effective capacitive separation distance between the two plates of each said pair of capacitive electrode plates, respectively, said fringing-field capacitances having first and second effective ratios of A/d, respectively, said first and second effective ratios of A/d of said first and second fringing-field capacitances being substantially equal, whereby said first and second fringing-field capacitances having substantially equal values of capacitance;

and,

(2) a dielectric having a dielectric thickness and first and second surfaces,

said first and second pairs of capacitive electrode plates in contact with said first surface of said dielectric,

said first and second pairs of capacitive electrode plates having first and second ratios of effective capacitive separation distance, d, to dielectric thickness, respectively;

said first ratio of effective separation distance, d, to dielectric thickness sufficiently large that the fringing field of said first fringing-field capacitor extends beyond said second surface of said dielectric,

said second ratio of effective separation distance to dielectric thickness sufficiently small that the fringing field of said second fringing-field capacitor is substantially totally contained with said dielectric,

whereby, the fringing-field differential capacitive moisture sensor having substantially equal dry first and second fringing-field capacitances, resulting in a substantially zero dry-condition sensor output signal, thereby providing insensitivity of a resulting sensor wet-condition output signal to variations in said dielectric versus temperature and stress;

said fringing-field differential capacitive moisture sensor for excitation by at least two excitation frequencies,

said fringing-field differential capacitive moisture sensor for providing a moisture output signal at each said excitation frequency;

(b) at least two oscillators for providing respectively at least two said excitation frequencies for excitation of said moisture sensor;

(c) at least two mixers for synchronously detecting each said moisture output signal at each said excitation frequency, each said mixer for providing a DC output corresponding to each said moisture output signal at each said excitation frequency; and

,

(d) a processor for determining whether ice is present, based on said DC outputs corresponding to each said moisture output signal at each said excitation frequency.