Detection apparatus for a capacitive proximity sensor

• US 20080246495A1
• Filed: 04/05/2007
• Published: 10/09/2008
• Est. Priority Date: 04/05/2007
• Status: Active Grant

• Current Assignee: Delphi Technologies Ip Limited (BorgWarner Incorporated)
• Original Assignee: Delphi Technologies, Inc. (BorgWarner Incorporated)
• Inventors: Zarabadi, Seyed R., Haag, Ronald Helmut
• Granted Patent: US 7,616,011 B2
• Time in Patent Office: Days
• US Class Current: 324/678
• CPC Class Codes: H03K 17/955 using a capacitive detector

Abstract

A switched capacitance detection circuit is responsive to changes in the fringing capacitance of a capacitive proximity sensor having at least one capacitive sensor element. In cases where the sensor has a single sensor element, the switching frequency of the detection circuit is controlled to maintain measurement accuracy in the presence of sensor moisture while minimizing power consumption and electromagnetic radiation. In cases where the sensor has multiple sensor elements, the capacitance values for each sensor element are differenced, absolute-valued and summed to form an output in which common-mode effects due to sensor moisture, temperature and sensor aging are canceled out.

Claims

1. Capacitance detection apparatus for a capacitive proximity sensor having at least one sensing element, comprising:

switching means for alternately charging and discharging said sensing element;

an operational amplifier responsive to a detection voltage developed during the charging of said sensing element for developing an output voltage that varies in relation to a capacitance of said sensing element; and

detection means responsive to said output voltage for detecting the capacitance of said sensing element, such that the detected capacitance is substantially insensitive to capacitive effects due to moisture on said sensor.

2. The capacitance detection apparatus of claim 1, where:

said sensing element is charged in series with an offset capacitor; and

said detection voltage is developed at a junction between said sensing element and said offset capacitor so that the output voltage of said operational amplifier varies in relation to a difference between the capacitance of said sensing element and a capacitance of said offset capacitor.

3. The capacitance detection apparatus of claim 1, further comprising:

a timing signal generator for developing timing signals for timing the alternate charging and discharging of said sensing element, where said detection means controls a frequency of the timing signals developed by said timing signal generator so that the detected capacitance is substantially insensitive to capacitive effects due to moisture on said sensor.

4. The capacitance detection apparatus of claim 3, where:

said detection means detects moisture on said sensor when the detected capacitance changes with the frequency of said timing signals.

5. The capacitance detection apparatus of claim 4, where:

said detection means periodically changes the frequency of said timing signals to determine if there is moisture on said sensor.

6. The capacitance detection apparatus of claim 4, where:

said detection means controls the frequency of said timing signals to a first calibrated value that makes the detected capacitance substantially insensitive to the capacitive effects of moisture in response to the detection of moisture on said sensor.

7. The capacitance detection apparatus of claim 6, where:

said detection means detects an absence of moisture on said sensor when the detected capacitance is not substantially affected by the frequency of said timing signals, and controls the frequency of said timing signals to a second calibrated lower than said first calibrated value.

8. The capacitance detection apparatus of claim 6, where:

said detection means periodically suspends the alternate charging and discharging of said sensing element after detecting the capacitance of said sensing element with said timing signals controlled to said first calibrated value.

9. The capacitance detection apparatus of claim 1, where:

said capacitive proximity sensor has a plurality of different sensing elements; and

said switching means alternately charges said discharges each of the different sensing elements in succession so that the output voltage of said operational amplifier varies in relation to a capacitance of the sensing element that is being alternately charged and discharged.

10. The capacitance detection apparatus of claim 9, where:

said detection means includes means for determining absolute value differences in said output voltage among the different sensing elements of said sensor, and for summing said absolute value differences to form a combined output voltage that is substantially insensitive to capacitive effects due to moisture on said sensor.