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Physical quantity sensor

  • US 7,628,073 B2
  • Filed: 11/01/2007
  • Issued: 12/08/2009
  • Est. Priority Date: 03/06/2007
  • Status: Active Grant
First Claim
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1. A physical quantity sensor comprising:

  • a substrate;

    a movable element supported on the substrate with a spring, wherein the movable element is displaceable in accordance with a physical quantity along a displacement direction;

    a pair of fixed elements, each of which is spaced apart from the movable element by a predetermined distance in the displacement direction;

    a carrier wave application element for applying a pair of carrier waves having a pulse shape to the pair of fixed elements in such a manner that one carrier wave to be applied to one fixed element has a phase opposite to the other carrier wave to be applied to the other fixed element;

    a signal application element for applying a middle voltage to the movable element, wherein the middle voltage is almost a half of a peak voltage of the carrier waves; and

    a detection circuit for detecting the physical quantity based on a capacitance change between the movable element and the pair of fixed elements, whereinthe signal application element further applies a first self diagnosis signal having a pulse shape to the movable element,the first self diagnosis signal has a first maximum voltage different from the middle voltage,the detection circuit executes a first self diagnosis process based on the capacitance change between the movable element and the pair of fixed elements when the signal application element applies the first self diagnosis signal to the movable element,the first self diagnosis signal has a first frequency, which provides to obtain a resonant magnification equal to or larger than 1.1 times with respect to a resonant frequency of the movable element, so that the movable element is resonated and almost contacts or press contacts one of the pair of fixed elements,the detection circuit determines whether a sticking phenomenon occurs based on the capacitance change between the movable element and the pair of fixed elements when the signal application element applies the first self diagnosis signal to the movable element,the signal application element further applies a pair of first self diagnosis waves having a pulse shape to the pair of fixed elements when the detection circuit executes the first self diagnosis process in such a manner that one first self diagnosis wave to be applied to one fixed element has a phase opposite to the other first self diagnosis wave to be applied to the other fixed element,each first self diagnosis wave has a maximum voltage equal to the peak voltage,each first self diagnosis wave has a frequency equal to the first frequency of the first self diagnosis signal,the detection circuit determines whether the sticking phenomenon occurs based on the capacitance change between the movable element and the pair of fixed elements when the signal application element applies the first self diagnosis signal to the movable element and applies the pair of first self diagnosis waves to the pair of fixed elements, andthe signal application element further applies a second self diagnosis signal to the movable element,the second self diagnosis signal has a second frequency, which provides to obtain a resonant magnification lower than the resonant magnification of the first self diagnosis signal, andthe detection circuit determines whether a sensitivity of the sensor is disposed in a predetermined range based on the capacitance change between the movable element and the pair of fixed elements when the signal application element applies the second self diagnosis signal to the movable element.

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