Detection apparatus for a capacitive proximity sensor
First Claim
1. A capacitance detection apparatus for a capacitive proximity sensor having at least one sensing element, comprising:
- a 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;
a detection means responsive to said output voltage for detecting the capacitance of said sensing element; and
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 produced by said timing signal generator so that the detected capacitance is substantially insensitive to capacitive effects due to moisture on said sensor.
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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.
13 Citations
12 Claims
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1. A capacitance detection apparatus for a capacitive proximity sensor having at least one sensing element, comprising:
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a 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; a detection means responsive to said output voltage for detecting the capacitance of said sensing element; and 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 produced by said timing signal generator so that the detected capacitance is substantially insensitive to capacitive effects due to moisture on said sensor. - View Dependent Claims (2, 3, 4, 5, 6, 7)
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8. A capacitance detection apparatus for a capacitive proximity sensor having at least one sensing element, comprising:
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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; detection means responsive to said output voltage for detecting the capacitance of said sensing element;
said detection means controls a frequency associated with timing signals generated by a timing signal generator and said frequency selected and generated by the timing signal generator is set 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. - View Dependent Claims (9, 10)
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11. A capacitance detection apparatus for a capacitive proximity sensor having a plurality of different sensing elements, comprising:
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a switching means for alternately charging and discharging each of the different sensing elements in succession; an operational amplifier responsive to a detection voltage developed during the charging of each of the different sensing elements to produce an output voltage at an output of the operational amplifier that varies in relation to a capacitance of each of the different sensing elements being alternately charged and discharge; and a detection means includes means for determining absolute value differences in said output voltage among the different sensing elements, 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. - View Dependent Claims (12)
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Specification