Capacitive pressure transducer signal conditioning circuit

• US 4,517,622 A
• Filed: 08/29/1983
• Issued: 05/14/1985
• Est. Priority Date: 08/29/1983
• Status: Expired due to Term

• Current Assignee: VTI Holding Oy
• Original Assignee: United Technologies Corporation (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Male, Barry
• Primary Examiner(s): GRIFFIN, DONALD
• Application Number:
  US06/527,530
• Time in Patent Office: 624 Days
• Field of Search: 361/283, 361/434, 73/718
• US Class Current: 361/283.1
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor

Abstract

Signal conditioning circuitry for a three plate capacitive pressure transducer eliminates the effects of parasitic capacitance in the transducer'"'"'s output, to provide a true indication of sensed pressure magnitude as an electrical signal equivalent of the instantaneous value of only the pressure responsive capacitance of the transducer.

Claims

1. Apparatus, responsive to a capacitive pressure transducer'"'"'s indication of pressure magnitude as the instantaneous value of a pressure responsive capacitance associated with the central electrode-to-diaphragm junction of the transducer in combination with the value of the nonpressure responsive capacitance associated with the central electrode-to-substrate junction of the transducer, for providing a signal indication of the instantaneous value of the pressure responsive capacitance, comprising:

conversion means, connected at an input and output thereof for regenerative response to the transducer'"'"'s central electrode-to-diaphragm junction, for providing thereto an output bistable current signal having alternate high current and low current states in dependence on the alternately increasing and decreasing amplitude of an actual voltage integral signal provided by the pressure responsive capacitance in response to said bistable current signal, said integral signal having an amplitude rate of change dependent on the instantaneous value of said pressure responsive capacitance, said conversion means providing as said signal indication of the instantaneous value of the pressure responsive capacitance the frequency at which said bistable signal alternates between said high and low signal states; and

signal coupling means, responsive to said conversion means bistable current signal for providing an equivalent voltage integral signal to the transducer'"'"'s substrate-to-diaphragm junction, whereby said central electrode-to-substrate junction nonpressure responsive capacitance is presented with a zero voltage potential thereacross to provide a zero integral response.

2. The apparatus of claim 1, wherein said conversion means comprises:

bistable means, having an input with maximum amplitude and minimun amplitude threshold limits responsive to said integral signal, for providing an oscillating bistable voltage signal with alternate high voltage and low voltage signal states and oscillating therebetween at a frequency corresponding to said amplitude rate of change of said integral signal amplitude between said amplitude threshold limits, whereby said oscillating bistable signal frequency provides the apparatus indication of the instantaneous value of the pressure responsive capacitance; and

current source means responsive to said oscillating bistable voltage signal for providing said output bistable current signal with the high current and low current signal states thereof in correspondence with the high voltage and low voltage signal states of said bistable voltage signal.

3. The apparatus of claim 1, wherein said equivalent voltage integral signal provided by said signal coupling means is equal in phase and magnitude to said actual voltage integral signal provided by said conversion means.

4. The apparatus of claim 1, wherein said equivalent voltage integral signal provided to the substrate to diaphragm junction is electrically isolated from said actual voltage integral signal appearing at the central electrode-to-diaphragm junction.

5. The method of providing a signal indication of the instantaneous value of the pressure responsive capacitance of a three element capacitive pressure transducer which provides an output capacitive indication of sensed pressure magnitude in the form of the combined values of the pressure responsive capacitance associated with the central electrode to diaphragm junction of the transducer and the nonpressure responsive capacitance associated with the central electrode to substrate junction of the transducer, comprising the steps of:

providing, in regenerative fashion a bistable current signal having alternate high current and low current states to the transducer'"'"'s central electrode to diaphragm junction, and alternating the signal between states in dependence on the amplitude of the actual integral signal provided by the pressure responsive capacitance in response thereto,

applying an equal phase and magnitude voltage integral signal to the substrate to diaphragm junction of the transducer.

6. A pressure sensor for providing output signal indicia of the magnitude of an applied pressure signal, comprising:

a source of reference pressure signals;

a pair of three plate capacitive pressure transducers for providing an output capacitance indication of sensed pressure magnitude in the form of the combined values of the pressure responsive capacitance associated with the central electrode to the pressure sensitive diaphragm junction of the transducer and the nonpressure responsive capacitance associated with the central electrode to substrate junction thereof, a first one of said transducers being responsive to said source of reference pressure signals and the other one of said transducers being responsive to the applied pressure signal; and

signal conditioning means for converting the capacitive indication of sensed pressure magnitude from each transducer into an electrical signal respresentative of the instantaneous value of the pressure responsive capacitance of each, in each of a successive number of sampling periods associated with each transducer, said signal conditioning apparatus connected at an input and output thereof for regenerative response to each transducer'"'"'s central electrode to diaphragm junction in a related sampling period to provide thereto a source bistable current signal having alternate high current and low current states in dependence on the alternately increasing and decreasing amplitude of a voltage integral signal provided to said input by each transducer'"'"'s pressure responsive capacitance in response thereto, each of said integral signals having an amplitude rate of change dependent on the instantaneous value of the associated transducer pressure responsive capacitance, said signal conditioning apparatus providing said signal representation of the instantaneous value of each transducer'"'"'s pressure responsive capacitance as the frequency at which said bistable signal alternates between said high and low current states, said signal conditioning apparatus further providing in each sample time interval said voltage integral signal to each transducer substrate to diaphragm junction, whereby said central electrode to substrate junction nonpressure responsive capacitance is presented with a zero voltage potential thereacross to provide a zero interval response, the ratio of the signal frequencies in successive sample time interval providing the indicia of the applied pressure signal magnitude as a relative factor of the reference pressure signal magnitude.