Low cost high precision sensor

• US 4,951,236 A
• Filed: 11/06/1989
• Issued: 08/21/1990
• Est. Priority Date: 05/05/1986
• Status: Expired due to Term

• Current Assignee: Sensata Technologies Incorporated (Sensata Technologies Holding Plc)
• Original Assignee: Texas Instruments, Inc.
• Inventors: Kawate, Keith W., Sabetti, Anthony J.
• Primary Examiner(s): Lall, Parshotam S.
• Assistant Examiner(s): Ramirez, Ellis B.
• Application Number:
  US07/432,023
• Time in Patent Office: 288 Days
• Field of Search: 364/571.01, 364/558, 364/550, 364/557, 73/780, 73/718, 73/708, 73/724, 361/283
• US Class Current: 702/98
• CPC Class Codes: G01L 27/005 Apparatus for calibrating pressure sensors
• CPC Class Codes: G01L 9/0072 using variations in capacitance
• CPC Class Codes: G01L 9/12 by making use of variations in capacitance , i.e. electric circuits therefor

Abstract

A universal sensor has a selectively scaled condition responsive element such as a capacitive pressure transducer mounted on a common support with signal conditioning means embodied in an integrated circuit to provide a control signal precisely corresponding to a sensed condition. Electrically actuatable means on the support are incorporated in the circuit for low cost, mass producible calibration of the circuit and condition-responsive element relative to each other after assembly on the common support. Preferably, a capacitive pressure transducer and a reference capacitor are arranged in a charge-locked loop relation with a common node while an array of switches cycles the voltage across the transducer and reference capacitors with opposite transitions with a predetermined frequency so change in transducer capacitance results in a differential voltage at the common node. That voltage is amplified to drive a current source-sink-integrate network and adjust sensor output voltage to restore a balanced condition at the common node and maintain sensor output voltage at a level inversely proportional to transducer capacitance. The sensor is adapted for calibration to provide an electrical calibration signal for actuating calibration means such as electrically settable potentiometers, or serially programmed digital-to-analog converters or the like to provide bias and gain and non-linearity calibration in the sensor. In one embodiment, calibration is achieved by adjustment of variable capacitors.

Claims

1. A low-cost high-precision sensor comprising a separate transducer capacitor having a capacitance which varies with changes in a condition being monitored, a separate capacitance-to-voltage converter circuit having input means for receiving a supply voltage and output means for providing an output voltage, a reference capacitor, switch means for selectively connecting the transducer and reference capacitors in the circuit, clock means for operating the switch means in a predetermined sequence to cycle a supply voltage and feedback signal from the output voltage across the transducer and reference capacitors with opposite transitions with a predetermined frequency to establish an unbalance condition in the circuit corresponding to a change in transducer capacitance due to any change in the condition being monitored for adjusting output voltage to correct the unbalanced circuit condition and maintain the output voltage proportional to the capacitance of the transducer capacitor, the transducer capacitor, reference capacitor, and capacitance-to-voltage converter circuit being mounted on a common support for adjusting said application of supply voltage and feedback from the output voltage to the reference and transducer capacitors after mounting of the capacitors and circuit on the support for calibrating the sensor.

2. A sensor according to claim 1 wherein the calibration means comprises a pair of variable resistors disposed in the circuit to vary the respective voltages at which the transducer and reference capacitors are cycles for providing bias and gain calibration of the sensor respectively.

3. A low-cost high-precision sensor comprising a transducer capacitor having a capacitance which varies with changes in a condition being monitored, a reference capacitor arranged in a charged-locked loop relation having a common node with the transducer capacitor, and a capacitance-to-voltage converter circuit having input means for receiving a supply voltage, output means for providing an output voltage, an array of switch means operable in predetermined sequence for cycling a supply voltage and a feedback signal from the output voltage across the transducer and reference capacitors with opposite transitions with a predetermined frequency, and clock means for operating the switch means in said sequence with said frequency to establish a voltage change at the common node during said cycling corresponding to change in capacitance in the transducer capacitor due to change in the condition being monitored and to adjust the output voltage and the voltage at the common node and maintain the output voltage inversely proportional to the capacitance of the transducer capacitor the clock means including a switch connected to the common node alternately operable for resetting the voltage at the common node after said adjustment.

4. A low-cost high-precision sensor comprising a separate capacitor pressure transducer, the transducer embodying a capacitor having a capacitance which varies with changes in a pressure zone to be monitored;

a separate reference capacitor connected in a charged-locked loop relation with the transducer capacitor to have a common node between the capacitors;

a separate capacitance-to-voltage converter circuit, the circuit having input means for receiving a supply voltage, output means for providing an output voltage, and an array of switch means operable in a predetermined sequence for cycling voltage across the transducer and reference capacitors with a supply voltage and a feedback signal from the output voltage across the transducer and reference capacitors with opposite transitions, and clock means for operating the switches in said sequence with a predetermined frequency to establish a voltage change at the common node during said cycling in response to change in capacitance in the transducer capacitor due to change in pressure in the zone to be monitored to drive the output means to adjust output voltage in a manner corresponding to the change in capacitance in the transducer capacitor, the output means having feedback means connected to the array of switches for providing feedback signals corresponding to the output voltage to adjust the voltage at the common node in response to change in output voltage to maintain the output voltage proportional to the capacitance of the transducer capacitor;

a support mounting the capacitor pressure transducer, the reference capacitor and the capacitance-to-voltage converter circuit together; and

calibrating means mounted on the support to be electrically actuatable on the support to adjust the voltages applied across the transducer and reference capacitors on the support relative to the transducer on the support after mounting on the support for calibrating the sensor to provide an output voltage suitable for performing a control function in precise response to occurrence of a pressure condition in the zone to be monitored.

5. A pressure sensor according to claim 4 wherein the circuit comprises an integrated circuit mounted on the support and the calibrating means are incorporated in the integrated circuit and adapted to be calibrated after mounting on the support by electrical input to the integrated circuit.

6. A sensor according to claim 5 wherein the calibrating means in the integrated circuit comprises memory means for receiving data from testing of the sensor to adjust the integrated circuit relative to the capacitive transducer on the support to provide said output signals to perform such control functions in precise test response to the occurrence of said selected condition, and write means for selectively fixing the memory means to fix the tested calibration of the sensor.

7. A sensor according to claim 6 wherein the calibration means in the integrated circuit comprises shift register means for receiving said data, the memory means comprises a non-volatile memory having selectively fusible links, and write means comprising means for selectively fusing fusible metal links in the memory means are provided to fix the calibration of the sensor.

8. A sensor according to claim 7 wherein the shift register means comprises a serial-in, parallel-out register for furnishing a pair of words, and a pair of digitial-to-analog converter means are responsive to the respective words for adjusting the integrated circuit to provide bias and gain calibration for the sensor.

9. A low-cost high-precision pressure sensor comprising a support;

a capacitor pressure transducer mounted on the support, the transducer embodying a capacitor having a capacitance which varies with changes in a pressure zone to be monitored;

a reference capacitor mounted on the support and connected in a charge-locked loop relation with the transducer capacitor to have a common node between the capacitors;

a capacitance-to-voltage converter circuit mounted on the support;

the circuit having input means for receiving a supply voltage, output means for providing an output voltage, and an array of switch means operable in a predetermined sequence for cycling voltage across the transducer and reference capacitors with opposite transitions, and clock means for operating the switches in said sequence with a predetermined frequency to establish a voltage change at the common mode during said cycling in response to change in capacitance in the transducer capacitor due to change in pressure in the zone to be monitored to drive the output means to adjust output voltage in a manner corresponding to the change in capacitance in the transducer capacitor, the output means having feedback means connected to the array of switches for nulling voltage changes at the common node in response to change in output voltage proportional to the capacitance of the transducer capacitor; and

calibrating means mounted on the support to be electrically actuatable on the support to adjust the circuit on the support relative to the transducer on the support after mounting on the support for calibrating the sensor to provide an output voltage suitable for performing a control function in precise response to occurrence of a pressure condition in the zone to be monitored, the circuit comprising an integrated circuit mounted on the support, the calibrating means being incorporated in the integrated circuit and adapted to be calibrated after mounting on the support by electrical input to the integrated circuit, and the integrated circuit being implemented in CMOS technology for permitting improved temperature stability in operation at high temperatures.

10. A low-cost high-precision sensor comprising a first transducer capacitor having a capacitance which varies with changes in a condition being monitored, a variable reference capacitor arranged in a charged-locked loop relation having a common node with the transducer capacitor, and a capacitance-to-voltage converter circuit having input means for receiving a supply voltage, output means for providing an output voltage, means for cycling a supply voltage and a feedback signal from the output voltage across the transducer and variable reference capacitors with opposite transitions with a predetermined frequency to establish a differential voltage at the common node corresponding to change in transducer capacitance due to change in the condition being monitored and to adjust the output voltage to null the differential voltage at the common node and maintain the output voltage inversely proportional to the capacitance of the transducer capacitor, the first capacitor and second reference capacitor being individually variable for providing the sensor with selected characteristics, and means for adjusting application of said supply voltage and feedback signal to the capacitors for providing bias and gain calibration of the sensor respectively.

11. A low-cost high-precision sensor comprising a first transducer capacitor having a capacitance which varies with changes in a condition being monitored, a variable reference capacitor arranged in a charge-locked loop configuration having a common node with the transducer capacitor, and a capacitance-to-voltage converter circuit having input means for receiving a supply voltage, output means for providing an output voltage, means for cycling a supply voltage and a feedback signal from the output voltage across the transducer and variable reference capacitor with opposite transitions with a predetermined frequency to establish a voltage change at the common node comprising to change in transducer capacitance due the change in the condition being monitored and to adjust the output voltage to null the voltage change at the common node and maintain the output voltage inversely proportional to the capacitance of the transducer capacitor, the first capacitor and second reference capacitor being individually variable for providing the sensor with selected characteristics, and means for adjusting application of said supply voltage and feedback signal to the capacitors for providing bias and gain calibration of the sensor respectively, said variable capacitors comprising electrically actuatable on-chip capacitor means, shift register means receive data for adjusting the variable capacitors during testing of the sensor, non-volatile memory means having fusible metal links receive said data from the shift register means, said permanent programming means are selectively operable for permanently programming selected data in the memory means for calibrating the sensor.