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Capacitive sensing device

  • US 5,291,534 A
  • Filed: 06/19/1992
  • Issued: 03/01/1994
  • Est. Priority Date: 06/22/1991
  • Status: Expired due to Term
First Claim
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1. A capacitive sensing device comprising:

  • a sensing capacitor having capacitance variable according to a variation of a force to be measured;

    a measuring pulse signal generating circuit connected to said sensing capacitor for generating a measuring pulse signal with a frequency in accordance with capacitance of said sensing capacitor;

    a reference capacitor having a reference capacitance that is not affected by said force;

    a reference pulse signal generating circuit connected to said reference capacitor, having a construction similar to that of said measuring pulse signal generating circuit and capable of generating a reference pulse signal with a frequency in accordance with said reference capacitance; and

    a signal processing circuit for calculating a value of said force from frequencies of said reference pulse signal and said measuring pulse signal,wherein said measuring pulse signal generating circuit and said reference pulse signal generating circuit have oscillation circuits that provide said measuring pulse signal and said reference pulse signal, respectively, ratios of frequency deviations of said measuring pulse signal and said reference pulse signal to temperature deviations being controlled by varying input voltages applied to said measuring pulse signal generating circuit and said reference pulse signal generating circuit, respectively;

    an input voltage applied to said measuring pulse signal generating circuit adjusted so that a detection sensitivity is nearly constant for a temperature variation; and

    an input voltage applied to said reference pulse signal generating circuit adjusted so that a ratio of a deviation of a frequency difference between said measuring pulse signal and said reference pulse signal to a temperature deviation is nearly constant for a temperature variation.

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