Capacitance measurement

• US 5,136,251 A
• Filed: 01/31/1991
• Issued: 08/04/1992
• Est. Priority Date: 09/23/1988
• Status: Expired due to Term

• Current Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Original Assignee: John Fluke Mfg. Co., Inc. (Fortive Corp.)
• Inventors: Beckert, Richard D., Bacon, Glade B., George, Richard E.
• Primary Examiner(s): Wieder, Kenneth A.
• Assistant Examiner(s): Solis, Jose M.
• Application Number:
  US07/648,277
• Time in Patent Office: 551 Days
• Field of Search: 341/150, 341/172, 324/678, 324/607, 324/677, 324/676
• US Class Current: 324/678
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

The capacitance of an unknown capacitor is measured with multimeter instrumentation employing a dual slope analog-to-digital converter. The initial voltage across the capacitor is measured and the capacitor is cyclically charged until the capacitor reaches a predetermined proportion of possible charge. The final voltage is measured. The voltage across the charging resistance is integrated over successive charging cycles to provide a value proportional to the charge delivered to the capacitor and this value is divided by the difference between the initial and final voltages.

Claims

1. A method for measuring the value of a capacitor comprising the steps of:

at least partially discharging said capacitor and measuring the initial voltage across said capacitor,

cyclically charging said capacitor over a sequence of cycles during each of which a separate charging current is supplied to said capacitor until over the sequence of cycles said capacitor reaches a predetermined proportion of possible charge,

integrating the currents supplied to said capacitor during said sequence of cycles to provide a value proportional to the actual charge delivered to said capacitor,

measuring the final voltage on said capacitor, and

dividing said value proportional to actual charge on said capacitor by the difference between the final and initial voltages across said capacitor to provide a measure of the capacitance of said capacitor.

2. A method for measuring the value of a capacitor comprising the steps of:

at least partially discharging said capacitor and measuring the initial voltage across said capacitor,

cyclically charging said capacitor through a reference resistance over a sequence of minor cycles during which charging current is supplied to said capacitor until said capacitor reaches a predetermined proportion of possible charge,

integrating the voltage drop across said reference resistance during said minor cycles to provide a value proportional to the actual charge delivered to said capacitor, and

dividing said value proportional to actual charge on said capacitor by a quantity corresponding to the difference between the final and initial voltages across said capacitor to provide a measure of the capacitance of said capacitor.

3. The method according to claim 2 wherein cyclically charging said capacitor is accomplished in synchronism with operation of a dual slope, integrating, analog-to-digital converter coupled across said reference resistance, and wherein integration of the voltage drop across said reference resistance to provide a value proportional to actual charge comprises summing the deintegration periods of said converter.

4. The method according to claim 2 wherein cyclically charging said capacitor is accomplished in synchronism with operation of a dual slope, integrating, analog-to-digital converter connected to measure the voltage across said reference resistance, and wherein integration of the voltage drop across said reference resistance to provide a value proportional to actual charge comprises summing minor cycle counts generated during deintegration periods of said converter multiplied by a constant.

5. The method according to claim 4 wherein said predetermined proportion of possible charge is determined when the number of minor cycle counts during a last minor cycle fall to a predetermined proportion of the number of minor cycle counts during a previous cycle.

6. The method according to claim 5 wherein said previous cycle comprises a first cycle.

7. The method according to claim 4 wherein the initial partial discharging of said capacitor is carried out employing a dual slope, integrating, analog-to-digital converter connected across said capacitor until the minor cycle count generated during a deintegration period of said converter drops below a predetermined value.

8. The method according to claim 7 including continuing minor cycles until a predetermined number of minor cycles is reached whereupon a higher capacitance range is automatically selected, said predetermined number being variable with range and higher for higher capacitance ranges.

9. A method for measuring the value of an unknown capacitor comprising the steps of:

charging said unknown capacitor incrementally during separate periods while during each of said periods providing current to a storage capacitor,

between said periods discharging said storage capacitor at a controlled rate, and

summing the times required for discharging said storage capacitor.

10. A method for measuring the value of an unknown capacitor comprising the steps of:

charging said unknown capacitor incrementally during separate periods while during each of said periods providing current to a storage capacitor, wherein said current is provided to said storage capacitor in response to voltage drop across a charging resistor in series with said unknown capacitor,

between said periods discharging said storage capacitor at a controlled rate, and

summing the times required for discharging said storage capacitor.

11. A method for measuring the value of an unknown capacitor comprising the steps of:

measuring the initial voltage value across said unknown capacitor,

charging said unknown capacitor incrementally during separate periods while during each of said periods providing current to a storage capacitor,

between said periods discharging said storage capacitor at a controlled rate,

summing the times required for discharging said storage capacitor,

measuring the final voltage value across said unknown capacitor,

determining the difference between said final and initial voltage values, and

dividing a function of the sum of times for discharging said storage capacitor by said difference.

12. A method for measuring the value of an unknown capacitor comprising the steps of:

measuring the initial voltage value across said unknown capacitor,

charging said unknown capacitor while simultaneously providing current to a storage capacitor,

discharging said storage capacitor at a controlled rate,

measuring the time required to discharge said storage capacitor,

measuring the final voltage value across said unknown capacitor,

determining the difference between said final and initial voltage values, and

dividing a function of said discharge time by said difference.

13. Apparatus for measuring the value of an unknown capacitor, comprising:

a charging circuit,

means for cyclically connecting said charging circuit to said unknown capacitor during separate time periods,

a storage capacitor,

means for providing current to said storage capacitor during said separate time periods and discharging said storage capacitor, and

means for summing the times required for discharging said storage capacitor.

15. The apparatus according to claim 13 further including means for ascertaining initial and final voltage values across said unknown capacitor and dividing a function of the sum of times required for discharging said storage capacitor by the difference between said initial and final voltage values.

14. Apparatus for measuring the value of an unknown capacitor, comprising:

a charging circuit, wherein said charging circuit comprises a reference resistor in series with said unknown capacitor,

means for cyclically connecting said charging circuit to said unknown capacitor during separate time periods,

a storage capacitor,

means for providing current to said storage capacitor during said separate time periods and discharging said storage capacitor, said means for providing current to said storage capacitor supplying such current in proportion to the voltage drop across said reference resistor, and

means for summing the times required for discharging said storage capacitor.

16. Apparatus for measuring the value of an unknown capacitor, comprising:

a charging circuit,

means for cyclically connecting said charging circuit to said unknown capacitor during separate time periods,

a storage capacitor,

means for providing current to said storage capacitor during said separate time periods and discharging said storage capacitor, and

means for summing the times required for discharging said storage capacitor,

wherein said storage capacitor, means for providing current to said storage capacitor and means for discharging said storage capacitor comprise a dual slope, integrating, analog-to-digital converter.

17. Apparatus for measuring the value of an unknown capacitor, comprising:

a charging circuit comprising a first reference resistor and a second reference resistor,

a dual slope, integrating, analog-to-digital converter coupled to said first and said second reference resistors and adapted to measure the voltage thereacross during successive minor cycles each including an integrate phase and a deintegrate phase, said converter including a counter responsive to deintegrate phase duration,

means for alternatively connecting said first and said second reference resistors of said charging circuit to said unknown capacitor during charging phases corresponding to integrate phases of said analog-to-digital converter, and

means for summing the counter outputs during successive deintegrate phases of said converter.

18. The apparatus in accordance with claim 17 further including means for connecting said converter for measuring an initial voltage across said unknown capacitor and a final voltage across said unknown capacitor, said means for summing also being effective for dividing a function of said sum by a quantity corresponding to the difference between final and initial voltage across said capacitor multiplied by a circuit constant.

19. The apparatus in accordance with claim 18 further including means for substantially discharging said unknown capacitance before measuring said initial voltage.

20. The apparatus in accordance with claim 18 wherein said means for summing is responsive to counts during successive deintegrate phases for discontinuing said charging phases when the counter output for a particular deintegrate phase drops to a predetermined proportion of a first deintegrate phase.

21. A method for measuring the value of a capacitor comprising the steps of:

selecting a first reference resistance,

at least partially discharging said capacitor and measuring the initial voltage across said capacitor,

cyclically charging said capacitor through the reference resistance over a sequence of minor cycles during which charging current is supplied to said capacitor,

determining whether said capacitor has reached a proportion of possible charge and if said capacitor has not reached said proportion of possible charge, determining whether a predetermined number of minor cycles has occurred and if a predetermined number of minor cycles has not occurred, continuing said cyclical charging steps and said determining step until said capacitor has reached said proportion of possible charge within the predetermined number of minor cycles, but if said predetermined number of minor cycles has occurred and the capacitor has not reached said proportion of possible charge, selecting a second reference resistance and repeating said discharging and measuring step and said cyclical charging steps until said capacitor reaches a proportion of possible charge through the second reference resistance,

integrating the voltage drop across the last selected reference resistance during said minor cycles to provide a value proportional to the actual charge delivered to said capacitor, and

dividing said value proportional to actual charge on said capacitor by a quantity corresponding to the difference between final and initial voltages across said capacitor to provide a measure of the capacitance of said capacitor.

22. The method according to claim 21 wherein cyclically charging said capacitor is accomplished in synchronism with operation of a dual slope, integrating, analog-to-digital converter coupled across said selected reference resistance, and wherein integration of the voltage drop across the selected reference resistance to provide a value proportional to actual charge comprises summing the deintegration periods of said converter.

23. The method according to claim 21 wherein cyclically charging said capacitor is accomplished in synchronism with operation of a dual slope, integrating, analog-to-digital converter connected to measure the voltage across the selected reference resistance, and wherein integration of the voltage drop across the selected reference resistance to provide a value proportional to actual charge comprises summing minor cycle counts generated during deintegration periods of said converter multiplied by a constant.