Capacitive thickness gauging for ungrounded elements

A system for capacitively gauging proximity to an element wherein the element has a finite impedance to ground comprising:

• a first probe positioned for detecting the capacitance between a region of the element and itself;
• a second probe positioned for detecting the capacitance between the region of the element and itself;
• means for exciting the first probe with an alternating electric signal at a first frequency;
• means for exciting the second probe with an alternating electricl signal at a second frequency;
• the first and second frequencies differing by a small percentage of either frequency whereby the voltage of said element is amplitude modulated at the frequency difference;
• the first and second probes providing respective first and second output signals representative of the gap between the respective probes and the region of the element;
• means for combining the first and second output signals to provide a composite output signal;
• and
• means for detecting the value of the composite output signal when the voltage of the region of said element is at a defined potential in the amplitude modulation thereof which generally minimizes the effect of said finite impedance on said composite signal.

A multiprobe capacitive distance gauging system for gauging the distance to a generally ungrounded element, said system comprising:

• a first probe positioned for detecting the capacitance between itself and a region of the element;
• a second probe positioned for detecting the capacitance between itself and the region of the element;
• the first and second probes being approximately positioned for equal capacitance to the element;
• means for generating an alternating electric potential;
• means for applying the alternating electric potential to the first probe in a first phase;
• means for applying the alternating electrical potential to the second probe in an opposite phase whereby the potential of the element is maintained generally and approximately at the ground potential level at which the effect of impedance between the region of said element and the ground potential on capacitance detected by said first and second probes is generally minimized;
• the first and second probes providing respective first and second output signals representative of the capacitance between the respective probes and the element
• means responsive to the first and second output signals for combining the first and second output signals;
• and
• means responsive to the combined signals for providing a representation of a dimension of said element.

A multiprobe capacitive distance gauging system for gauging the distance to a generally ungrounded element, said system comprising:

• a first probe positioned for detecting the capacitance between itself and the element;
• a second probe positioned for detecting the capacitance between itself and the element;
• means for generating an alternating signal;
• means for applying the alternating signal to the first probe in a first phase;
• means for applying the alternating signal to the second probe in an opposite phase;
• the first and second probes providing respective first and second output signals representative of the capacitance between the respective probes and the element;
• means responsive to the first and second output signals for combining the first and second output signals;
• means responsive to the combined signals for providing a representation of a dimension of said element;
• means for generating a product signal representing the product of the first and second output signals;
• means for generating the control signal representative of the difference between a constant and the ratio of the control signal and the product signal.