Linear capacitive pressure transducer system

• US 4,398,426 A
• Filed: 07/02/1981
• Issued: 08/16/1983
• Est. Priority Date: 07/02/1981
• Status: Expired due to Term

• Current Assignee: Kavlico Corporation A Corporation IOF CA
• Original Assignee: Kavlico Corporation (Sensata Technologies Holding Plc)
• Inventors: Park, Kyong, Lee, Chen Y.
• Primary Examiner(s): Woodiel, Donald O.
• Application Number:
  US06/279,938
• Time in Patent Office: 775 Days
• Field of Search: 73/724, 73/701, 73/718, 73/708, 73/753, 361/283, 328/1, 328/5, 307/118, 340/562
• US Class Current: 73/724
• CPC Class Codes: G01L 9/125 with temperature compensating means non electric temperature compensating means G01L19/04
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitive pressure transducer system includes a reference capacitor and a capacitor which varies with pressure; and circuitry which involves alternately charging one of the capacitors to a reference voltage level, and then switching to charge the other capacitor; and circuitry for producing an output voltage which varies as the duty cycles of the two capacitors shift; and the output is therefore substantially proportional to the pressure changes. Feedback is provided to incrementally change the capacitor charging and switching timing or duty cycles by changing the charging current or the reference voltage at which switching occurs, to substantially eliminate deviations from linearity in the voltage versus pressure characteristic.

Claims

1. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a relatively constant capacitance;

circuit means associated with said variable capacitor for charging said variable capacitor with a predetermined current to a predetermined reference voltage level, and for providing a first timing signal when the predetermined reference voltage level is reached;

circuit means associated with said reference capacitor for charging said reference capacitor with a predetermined current to a predetermined reference voltage level and providing a second timing signal when the predetermined voltage level is reached;

a current source;

switching means for directing current from said current source alternatively to said variable capacitor and to said reference capacitor;

bistable circuit means responsive to said first and second timing signals to change state and to reverse the state of said switching means to change current flow;

detection and integrator circuit means for receiving the pulse output from said bistable circuit means and for converting it into a D.C. electrical signal;

means for amplifying said D.C. electrical signal to provide a system output signal which varies substantially linearly with changes in pressure but which when uncompensated has a substantially continuous curvature producing minor departures from linearity;

feedback means connected from said system output voltage amplifying means to at least one of said circuit means associated with said capacitors for incrementally varying the time required for charging at least one of said capacitors, to substantially compensate for the curvature of the system output voltage versus pressure characteristic; and

said feedback means including means for changing the charging current to at least one of said capacitors incrementally as said system output voltage changes.

2. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a relatively constant capacitance;

circuit means associated with said variable capacitor for charging said variable capacitor with a predetermined current to a predetermined reference voltage level, and for providing a first timing signal when the predetermined reference voltage level is reached;

circuit means associated with said reference capacitor for charging said reference capacitor with a predetermined current to a predetermined reference voltage level and providing a second timing signal when the predetermined voltage level is reached;

a current source;

switching means for directing current from said current source alternatively to said variable capacitor and to said reference capacitor;

bistable circuit means responsive to said first and second timing signals to change state and to reverse the state of said switching means to change current flow;

detection and integrator circuit means for receiving the pulse output from said bistable circuit means and for converting it into a D.C. electrical signal;

means for amplifying said D.C. electrical signal to provide a system output signal which varies substantially linearly with changes in pressure but which when uncompensated has a substantially continuous curvature producing minor departures from linearity;

feedback means connected from said system output voltage amplifying means to at least one of said ciruit means associated with said capacitors for incrementally varying the time required for charging at least one of said capacitors, to substantially compensate for the curvature of the system output voltage versus pressure characteristic; and

said feedback means including means for changing the reference voltage level to which at least one of said capacitors is charged, incrementally as said system output voltage changes.

3. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a relatively constant capacitance;

circuit means associated with said variable capacitor for charging said variable capacitor with a predetermined current to a predetermined reference voltage level, and for providing a first timing signal when the predetermined reference voltage level is reached;

circuit means associated with said reference capacitor for charging said reference capacitor with a predetermined current to a predetermined reference voltage level and providing a second timing signal when the predetermined voltage level is reached;

a current source;

switching means for directing current from said current source alternatively to said variable capacitor and to said reference capacitor;

bistable circuit means responsive to said first and second timing signals to change state and to reverse the state of said switching means to change current flow;

detection and integrator circuit means for receiving the pulse output from said bistable circuit means and for converting it into a D.C. electrical signal;

means for amplifying said D.C. electrical signal to provide a system output signal which varies substantially linearly with changes in pressure but which when uncompensated has a substantially continuous curvature producing minor departures from linearity;

feedback means connected from said system output voltage amplifying means to at least one of said circuit means associated with said capacitors for incrementally varying the time required for charging at least one of said capacitors, to substantially compensate for the curvature of the system output voltage versus pressure characteristic; and

said feedback circuit means including a purely resistive path.

4. A linearized capacitive pressure transducer system as defined in claim 3 wherein said reference capacitor and said variable capacitor are mounted on the same support, whereby thermal variations are minimized.

5. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a relatively constant capacitance;

circuit means associated with said variable capacitor for charging said variable capacitor with predetermined current to a predetermined reference voltage level, and for providing a first timing signal when the predetermined reference voltage level is reached;

circuit means associated with said reference capacitor for charging said reference capacitor with a predetermined current to a predetermined reference voltage level and providing a second timing signal when the predetermined voltage level is reached;

means responsive to said first and second timing signals for generating a system output voltage which increases substantially linearly with increases in pressure which increases the capacitance of the variable capacitor and the time required for charging the variable capacitor, said system output voltage having minor departures from linearity;

feedback means connected from said system output voltage generating means to at least one of said circuit means for incrementally varying the time required for charging at least one of said capacitors, to correct for curvature of the system output voltage versus pressure characteristic; and

said feedback means including means for changing the charging current to at least one of said capacitors incrementally as said system output voltage changes.

6. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a relatively constant capacitance;

circuit means associated with said variable capacitor for charging said variable capacitor with predetermined current to a predetermined reference voltage level, and for providing a first timing signal when the predetermined reference voltage level is reached;

circuit means associated with said reference capacitor for charging said reference capacitor with a predetermined current to a predetermined reference voltage level and providing a second timing signal when the predetermined voltage level is reached;

means responsive to said first and second timing signals for generating a system output voltage which increases substantially linearly with increases in pressure which increases the capacitance of the variable capacitor and the time required for charging the variable capacitor, said system output voltage having minor departures from linearity;

feedback means connected from said system output voltage generating means to at least one of said circuit means for incrementally varying the time required for charging at least one of said capacitors, to correct for curvature of the system output voltage versus pressure characteristic; and

said feedback means including means for changing the reference level to which at least one of said capacitors is charged, incrementally as said system output voltage changes.

7. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a substantially constant capacitance or a capacitance which changes by only a relatively small amount with changes in pressure;

capacitor associated circuit means for charging and discharging said variable and said reference capacitors and for providing a variable signal as the capacitance of said variable capacitor changes;

circuit means coupled to said capacitor associated circuit means, for generating an output voltage which varies substantially linearly with changes in pressure, but which when uncompensated has a continuous curvature producing minor departures from linearity;

means for feeding back a portion of the output signal to said capacitor associated circuit to produce an incremental output having a polarity and magnitude to substantially compensate for said minor departures from linearity; and

said feedback means includes means for changing the charging current to at least one of said capacitors incrementally as said system output voltage changes.

8. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a substantially constant capacitance or a capacitance which changes by only a relatively small amount with changes in pressure;

capacitor associated circuit means for charging and discharging said variable and said reference capacitors and for providing a variable signal as the capacitance of said variable capacitor changes;

circuit means coupled to said capacitor associated circuit means, for generating an output voltage which varies substantially linearly with changes in pressure, but which when uncompensated has a continuous curvature producing minor departures from linearity;

means for feeding back a portion of the output signal to said capacitor associated circuit to produce an incremental output having a polarity and magnitude to substantially compensate for said minor departures from linearity; and

said feedback means including means for changing the reference level to which at least one of said capacitors is charged, incrementally as said system output voltage changes.

9. A linearized capacitive pressure transducer system comprising:

a main variable capacitor which has a capacitance which varies with applied pressure;

a reference capacitor which has a substantially constant capacitance or a capacitance which changes by only a relatively small amount with changes in pressure;

capacitor associated circuit means for charging and discharging said variable and said reference capacitors and for providing a variable signal as the capacitance of said variable capacitor changes;

circuit means coupled to said capacitor associated circuit means, for generating an output voltage which varies substantially linearly with changes in pressure, but which when uncompensated has a continuous curvature producing minor departures from linearity;

means for feeding back a portion of the output signal to said capacitor associated circuit to produce an incremental output having a polarity and magnitude to substantially compensate for said minor departures from linearity; and

said feedback circuit means including a purely resistive path.

10. A linearized capacitive pressure transducer system as defined in claim 9 wherein said reference capacitor and said variable capacitor are mounted on the same support, whereby thermal variations are minimized.