Wetness sensor for a window of a motor vehicle

• US 5,672,976 A
• Filed: 07/27/1995
• Issued: 09/30/1997
• Est. Priority Date: 07/28/1994
• Status: Expired due to Fees

• Current Assignee: VDO Adolf Schindling AG (Vodafone Group PLC)
• Original Assignee: VDO Adolf Schindling AG (Vodafone Group PLC)
• Inventors: Egger, Armin, Berberich, Reinhold, Busch, Dieter
• Primary Examiner(s): Nguyen, Vinh P.
• Assistant Examiner(s): Solis, Jose M.
• Application Number:
  US08/508,214
• Time in Patent Office: 796 Days
• Field of Search: 318/DIG. 2, 318/483, 361/286, 340/602, 324/687, 324/689, 324/668, 219/203
• US Class Current: 324/668
• CPC Class Codes: B60S 1/0822 characterized by the arrangement or type of detection means
• CPC Class Codes: B60S 1/0829 Oscillator-resonator rain sensor
• CPC Class Codes: G01N 27/223 for determining moisture content, e.g. humidity rain detectors on vehicle windows B60S1/0825
• CPC Class Codes: H05B 1/0236 for vehicles
• CPC Class Codes: H05B 2203/035 Electrical circuits used in resistive heating apparatus
• CPC Class Codes: H05B 3/84 Heating arrangements specially adapted for transparent or reflecting areas, e.g. for demisting or de-icing windows, mirrors or vehicle windshields

Abstract

In a wetness sensor for a window of a motor vehicle, a heating resistor arranged in the window and powered by DC current can be connected to a measuring device for measuring an AC impedance of the heating resistor, in particular the capacitive portion thereof. The impedance varies with the amount of wetness and, thereby, serves as a measure of the wetness. A capacitor is used to bring the sensor near resonance for improved sensitivity.

Claims

1. A wetness sensor for a window of a motor vehicle, comprising:

a heating resistor disposed in the window for removal of frost, said resistor comprising a grid of a plurality of wires extending across the window from a first terminal connected to a first end of each wire to a second terminal connected to a second end of each wire;

a measuring device connected to the heating resistor via said first terminal and said second terminal for measuring a capacitive portion of an impedance of the heating resistor; and

wherein the capacitor portion of the impedance varies in accordance with an amount of wetness of the window, thereby enabling the measuring device to sense the wetness.

2. A wetness sensor according to claim 1, wherein for the measurement of the impedance, the measuring device applies to the heating resistor an AC current and measures a resulting AC voltage developed at the heating resistor, the heating resistor being energizable by a DC current applied between said first terminal and said second terminal.

3. A wetness sensor according to claim 2, further comprising a resonance capacitor connected between said first terminal and said second terminal in parallel to the heating resistor.

4. A wetness according to claim 1, further comprising a choke coil connected to the heating resistor;

and

wherein the measuring device includes a source of direct current connected via the choke coil to the heating resistor for providing said DC current.

5. A wetness sensor according to claim 1, further comprising a coupling capacitor;

and

wherein the heating resistor is connected via one of said first and said second terminals to the measuring device via the coupling capacitor.

6. A wetness sensor according to claim 1, further comprising a transformer;

and

wherein the heating resistor is connected to the measuring device via the transformer.

7. A wetness sensor according to claim 2, wherein the measuring device comprises a computer, and an oscillator and a current source driven by the oscillator to produce the AC current for the impedance measurement, the oscillator being controllable for varying a frequency of the AC current;

and

wherein the frequency is varied in a predetermined range providing that a frequency at which an AC voltage developed across the heating resistor attains a greatest value or changes at a fastest rate is stored and compared by the computer with a stored frequency which was measured with the window in a dry condition.