Method and apparatus for measuring liquid dielectric behavior

• US 6,509,745 B1
• Filed: 09/25/2000
• Issued: 01/21/2003
• Est. Priority Date: 09/25/2000
• Status: Expired due to Fees

• Current Assignee: Detroit Diesel Corporation (Mercedes-Benz Group AG)
• Original Assignee: Detroit Diesel Corporation (Mercedes-Benz Group AG)
• Inventors: Marszalek, Gary A.
• Primary Examiner(s): Oda, Christine
• Assistant Examiner(s): He, Amy
• Application Number:
  US09/669,162
• Time in Patent Office: 848 Days
• Field of Search: 324/658, 324/664, 324/663, 324/665, 324/668, 324/670, 324/676, 324/677, 324/678, 324/698
• US Class Current: 324/663
• CPC Class Codes: G01N 27/228 Circuits therefor measuring capacitance per se G01R27/26
• CPC Class Codes: G01N 33/2888 Lubricating oil characteristics, e.g. deterioration lubricating properties G01N33/30
• CPC Class Codes: G01R 31/64 Testing of capacitors

Abstract

A system and a method for measuring capacitance of a capacitor having a dielectric material, such as a lubricating oil, disposed between the electrodes of the capacitor, where the dielectric material causes a double layer effect within the capacitor. A variable source is provided in series with a resistor to induce a change in a current flowing through the capacitor. A sensor converts the resulting current flow into a current value readable by a processor. The processor records a plurality of current values. An initial value is recorded to correspond to the induced change in the current. A leakage value is recorded after the current has exponentially decayed to approximate steady state. Multiple other current values are also recorded during the exponential decay. An intermediate value is then selected from among the multiple other current values based upon the initial value and the leakage value. The capacitance of the capacitor is then calculated based upon the initial value, intermediate value, leakage value and a resistance of the series resistor.

Claims

1. A method for measuring a capacitance of a capacitor, the method comprising:

inducing a change in a current flowing through the capacitor;

determining a plurality of current values in response to inducing the change in the current flowing through the capacitor, the plurality of current values including an initial value and a leakage value determined while the current is approximately steady state;

selecting an intermediate value from the plurality of current values in response to determining the plurality of current values; and

calculating the capacitance of the capacitor based upon the initial value, the leakage value, the intermediate value, and a time between inducing the change in the current flowing through the resistor and determining the intermediate value in response to selecting the intermediate value.

2. The method of claim 1 further comprising:

delaying at least five time constants after inducing the change in the current flowing through the capacitor to determine the leakage current, where one time constant is defined as a product of a predetermined capacitance of the capacitor and a predetermined resistance.

3. The method of claim 2 further comprising:

periodically inducing the change in the current flowing through the capacitor with a period of greater than ten time constants.

4. The method of claim 1 further comprising:

calculating an ideal value equal to the leakage value offset by approximately 20 percent of a difference between the initial value and the leakage value in response to determining the plurality of current values; and

where the intermediate value is selected to be approximately equal to the ideal value in response to calculating the ideal value.

5. The method of claim 1 further comprising:

calculating a range of interest between the leakage value offset by approximately 15 percent of a difference between the initial value and the leakage value and the leakage value offset by approximately 28 percent of the difference between the initial value and the leakage value in response to determining the plurality of current values; and

where the intermediate value is selected to be within the range of interest in response to determining the range of interest.

6. The method of claim 1 further comprising:

providing a liquid within the capacitor prior to inducing the change in the current in the capacitor; and

calculating a dielectric value of the liquid based upon the capacitance and a known geometry of the capacitor in response to calculating the capacitance.

7. The method of claim 6 further comprising:

calculating a conductivity of the liquid based on the geometry of the sensor, the leakage value, and a strength of an electric field applied across the fluid.

8. The method of claim 1 further comprising:

providing a liquid within the capacitor prior to inducing the change in the current in the capacitor; and

calculating a dielectric dissipation factor for the liquid based upon the initial value and the leakage value in response to determining the plurality of current values.

9. The method of claim 1 further comprising:

providing a liquid within the capacitor prior to inducing the change in the current in the capacitor;

controlling a temperature for the liquid to approximately 100 degrees Celsius prior to determining the leakage value;

changing the temperature of the liquid to approximately 40 degrees Celsius in response to determining the leakage value at 100 degrees Celsius;

determining a second leakage value in response to changing the temperature of the liquid; and

calculating a viscosity ratio of the liquid based upon the leakage value and the second leakage value.

10. A system for measuring a capacitance of a capacitor, the system comprising:

a resistor connected to the capacitor;

a variable source operational to induce a change in a current through the capacitor;

a sensor operational to convert a current flowing through the resistor into a current value; and

a processor operational to record a plurality of current values, the plurality of current values including a initial value and a leakage value determined while the current is approximately steady state, select an intermediate value from the plurality of current values, and calculate the capacitance of the capacitor based upon the initial value, the leakage value, the intermediate value, and a time between inducing the change in the current flowing through the resistor and determining the intermediate value in response to selecting the intermediate value.

11. The system of claim 10 wherein the processor is further operational to delay at least five time constants after the variable source induces the change in the current flowing through the capacitor to determine the leakage current, where one time constant is defined as a product of a predetermined capacitance of the capacitor and a predetermined resistance of the resistor.

12. The system of claim 11 where the variable source is further operational to periodically induce the change in the current flowing through the capacitor with a period of at least ten time constants.

13. The system of claim 10 where the processor is further operational to calculate an ideal value equal to the leakage value offset by approximately 20 percent of a difference between the initial value and the leakage value in response to determining the plurality of current values, and select the intermediate value to be approximately equal to the ideal value.

14. The system of claim 10 where the processor is further operational to calculate a range of interest between the leakage value offset by approximately 15 percent of a difference between the initial value and the leakage value and the leakage value offset by approximately 28 percent of the difference between the initial value and the leakage value, and select the intermediate value to be within the range of interest in response to determining the range of interest.

15. The system of claim 10 having a liquid disposed within the capacitor, and where the processor is further operational to calculate a dielectric value of the liquid based upon the capacitance and a known geometry of the capacitor.

16. The system of claim 15 where the processor is further operational to calculate a conductivity of the liquid based on the geometry of the sensor, the leakage value, and a strength of an electric field applied across the fluid.

17. The system of claim 10 having a liquid disposed within the capacitor, and where the processor is further operational to calculate a dielectric dissipation factor for the liquid based upon the initial value and the leakage value.

18. The system of claim 10 having a liquid disposed within the capacitor, the system further comprising:

a temperature sensor for converting a temperature of the fluid to a temperature value; and

the processor being further operational to calculate a viscosity ratio for the fluid based upon a change in the leakage value induced by a change in the temperature of the fluid between approximately 40 degrees and 100 degrees Celsius.