Apparatus and method for capacitance measurements

Claim 1

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising a periodic voltage source (30) applied to plates (22) immersed in the medium, summing means (32) coupled to the plates (22) for summing current flowing through the medium in response to said periodic voltage, said summed current having both a resistive current component in phase with said periodic voltage and a capacitive current component in phase with a time derivitive of said periodic voltage, sampling means (42) coupled to said summing means (32) for sampling said summed current over a time period in phase with said resistive current component, averaging means (46) coupled to said sampling means for averaging said capacitive current component to zero and for averaging said resistive current to a constant, correcting means (34) responsive to said averaging means for providing said summing means with a selectable current π radians out of phase with said resistance current component and having an amplitude related to said constant thereby nulling said resistive current component from said summed current, and detecting means (44) coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 2

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising a periodic voltage source (30) applied to plates (22) immersed in the medium, summing means (32) coupled to the plates (22) for summing current flowing through the medium in response to said periodic voltage, said summed current having both a resistive current component in phase with said periodic voltage and a capacitive current component in phase with a time derivitive of said periodic voltage, sampling means (42) coupled to said summing means (32) for sampling said summed current over a time period in phase with said resistive current component, averaging means (46) coupled to said sampling means for averaging said capacitive current component to zero and for averaging said resistive current to a constant, correcting means (34) responsive to said averaging means for providing said summing means with a selectable current π radians out of phase with said resistance current component and having an amplitude related to said constant thereby nulling said resistive current component from said summed current, and detecting means (44) coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 3

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising means for applying a periodic voltage to the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a differential of said periodic voltage, current providing means coupled to both said periodic voltage and a control signal for providing a selectable currentπ radians out of phase with said periodic voltage and having an amplitude related to said control signal, current summing means coupled to both the medium and said current providing means for summing said selectable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, averaging means coupled to said sampling means for averaging said capacitive current component to zero and for generating said control signal by averaging said resistive current component minus said selectable current, means for coupling said control signal to said current providing means so that said current providing means provides said selectable current substantially, equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current, and detecting means coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 4

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising means for applying a periodic voltage to the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a differential of said periodic voltage, current providing means coupled to both said periodic voltage and a control signal for providing a selectable currentπ radians out of phase with said periodic voltage and having an amplitude related to said control signal, current summing means coupled to both the medium and said current providing means for summing said selectable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, averaging means coupled to said sampling means for averaging said capacitive current component to zero and for generating said control signal by averaging said resistive current component minus said selectable current, means for coupling said control signal to said current providing means so that said current providing means provides said selectable current substantially, equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current, and detecting means coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 5

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising, means for applying a periodic voltage to the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a time derivitive of said periodic voltage, a current generator coupled to said periodic voltage applying means for generating a negative current proportional to and π radians out of phase with said periodic voltage, a variable resistor coupled to said periodic voltage and having a resistive value proportional to an electronic signal for producing a variable current related to both said periodic voltage and said electronic signal, current summing means coupled to the medium and said current generating means and said variable resistor for summing said negative current and said variable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, averaging means coupled to said sampling means for averaging said capacitive current component to zero and for averaging said resistive current component plus said variable current less said negative current to define said electronic signal, means for coupling said electronic signal to said variable resistor so that said variable current less said negative current is substantially equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current; and detecting means coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 6

An apparatus for measuring the capacitance of a medium independently of the resistivity of the medium, comprising, means for applying a periodic voltage to the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a time derivitive of said periodic voltage, a current generator coupled to said periodic voltage applying means for generating a negative current proportional to and π radians out of phase with said periodic voltage, a variable resistor coupled to said periodic voltage and having a resistive value proportional to an electronic signal for producing a variable current related to both said periodic voltage and said electronic signal, current summing means coupled to the medium and said current generating means and said variable resistor for summing said negative current and said variable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, averaging means coupled to said sampling means for averaging said capacitive current component to zero and for averaging said resistive current component plus said variable current less said negative current to define said electronic signal, means for coupling said electronic signal to said variable resistor so that said variable current less said negative current is substantially equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current; and detecting means coupled to said summing means and in phase with said capacitive current component for detecting said capacitive current component independently of said resistive current component.

Claim 7

An apparatus as claimed in any one of claims 1 to 3 wherein said periodic voltage comprises a triangular wave having zero crossing points every π radians.

Claim 10

A method for measuring the capacitance of a medium independent of the resistivity of the medium, comprising the steps of, applying a periodic voltage to plates immersed in the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a differential of said periodic voltage, providing a selectable current π radians out of phase with said periodic voltage and having an amplitude related to an eletronic signal, summing said selectable current and said resistive current component and said capacitive current component, sampling said summed current over a time period in phase with said resistive current component, averaging said sampled current to cancel said capacitive current component and to generate a constant related to said resistive current component less said selectable current, coupling said electronic signal to said current providing means so that said selectable current amplitude is substantially equal to said resistive current thereby nulling said resistive current component from said summed current, and sampling said capacitive current in phase with said capacitive current component to measure said capacitive current component independently of said resistive current component.

Claim 11

A method for measuring the capacitance of a medium independent of the resistivity of the medium, comprising the steps of, applying a periodic voltage to plates immersed in the medium to generate both a resistive current component flowing through the medium in phase with said periodic voltage and a capacitive current component flowing through the medium in phase with a differential of said periodic voltage, providing a selectable current π radians out of phase with said periodic voltage and having an amplitude related to an eletronic signal, summing said selectable current and said resistive current component and said capacitive current component, sampling said summed current over a time period in phase with said resistive current component, averaging said sampled current to cancel said capacitive current component and to generate a constant related to said resistive current component less said selectable current, coupling said electronic signal to said current providing means so that said selectable current amplitude is substantially equal to said resistive current thereby nulling said resistive current component from said summed current, and sampling said capacitive current in phase with said capacitive current component to measure said capacitive current component independently of said resistive current component.

Claim 12

An apparatus for measuring the alcohol/gasoline fuel mixture of an automobile by measuring the capacitance of the fuel mixture independent of the resistivity of the fuel mixture, comprising, a voltage source applying a periodic voltage across parallel capacitive plates positioned in the mixture to generate both a resistive current component in phase with said periodic voltage and a capacitive current component in phase with a differential of said periodic voltage, a current generator coupled to both said periodic voltage and a control signal for providing a selectable current ] radians out of phase with said periodic voltage and having an amplitude related to said control signal, current summing means coupled to both said parallel plates and said current generator for summing said selectable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, an integrator coupled to said sampling means for integrating said capacitive current component to zero and for integrating said resistive current component plus said selectable current to define said electronic signal, a feedback loop for coupling said control signal to said current generator so that said current generator provides said selectable current substantially equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current, measuring means coupled to said summing means and in phase with said capacitive current component for measuring said capacitive current component independently of said resistive current component, and means for converting said measurement of said capacitive current component into an indication of the alcohol/gasoline fuel mixture.

Claim 13

An apparatus for measuring the alcohol/gasoline fuel mixture of an automobile by measuring the capacitance of the fuel mixture independent of the resistivity of the fuel mixture, comprising, a voltage source applying a periodic voltage across parallel capacitive plates positioned in the mixture to generate both a resistive current component in phase with said periodic voltage and a capacitive current component in phase with a differential of said periodic voltage, a current generator coupled to both said periodic voltage and a control signal for providing a selectable current ] radians out of phase with said periodic voltage and having an amplitude related to said control signal, current summing means coupled to both said parallel plates and said current generator for summing said selectable current and said resistive current component and said capacitive current component, sampling means for sampling said summed current over a time period in phase with said resistive current component, an integrator coupled to said sampling means for integrating said capacitive current component to zero and for integrating said resistive current component plus said selectable current to define said electronic signal, a feedback loop for coupling said control signal to said current generator so that said current generator provides said selectable current substantially equal to said resistive current component and π radians out of phase with said resistive current component thereby nulling said resistive current component from said summed current, measuring means coupled to said summing means and in phase with said capacitive current component for measuring said capacitive current component independently of said resistive current component, and means for converting said measurement of said capacitive current component into an indication of the alcohol/gasoline fuel mixture.