Capacitance Measuring Circuit Virtual Ground Stray Capacitance
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Solution Overview
Problem
Existing electrostatic capacitive displacement meters face limitations in measuring small electrostatic capacitance and have a short measurable distance, making them ineffective for precise distance measurement and object detection, especially when faced with stray capacitance errors and surface roughness issues.
Innovation Solution
A capacitance measuring circuit with a negative feedback unit that virtually grounds the input of the amplifier, allowing for the measurement of small electrostatic capacitance by eliminating stray capacitance influence and enabling the use of larger electrodes for high sensitivity, which extends the measurable distance to over 1000 mm.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Length of stationary object
If conventional electrostatic capacitive displacement meter is used, then object detection capability is provided, but measurable distance is limited to 8 mm maximum
Solution Approach 1:
The measurement system is segmented into multiple independent components: a first electrode connected to a first amplifier, a second electrode connected to a second amplifier, and separate feedback capacitors for each channel. This segmentation allows each electrode-amplifier pair to independently measure capacitance changes, enabling extended measurement distance while maintaining precision through distributed sensing points
Solution Approach 2:
Feedback capacitors are introduced as intermediary elements in the amplifier circuits to stabilize the virtual ground potential and compensate for stray capacitance effects. These feedback capacitors act as mediators that enable accurate measurement of small capacitance changes over extended distances by canceling out parasitic capacitance influences
2Length of stationary object
If larger electrodes are used to extend measurement distance, then measurable distance increases, but stray capacitance influence increases
Solution Approach 1:
Feedback capacitors are connected from the output to the inverting input of each amplifier to create a virtual ground at the electrode connections. This feedback mechanism stabilizes the potential at the electrode terminals and compensates for stray capacitance effects, allowing larger electrodes to be used without proportionally increasing measurement errors
Solution Approach 2:
The circuit design converts the harmful effect of stray capacitance into a beneficial measurement mechanism. By using feedback capacitors to create virtual grounds, the stray capacitance between electrodes and environment is transformed into a controllable parameter that can be compensated for, actually improving measurement stability over extended distances
3Measurement precision
If conventional capacitance measuring circuit is used, then circuit simplicity is maintained, but small electrostatic capacitance cannot be measured accurately
Solution Approach 1:
Feedback capacitors are introduced as intermediary elements between the amplifier output and input to enable accurate measurement of small capacitance values. These feedback capacitors act as reference standards that allow the circuit to measure minute capacitance changes by comparing them against known feedback capacitor values, significantly improving measurement precision
Solution Approach 2:
The conventional direct measurement approach is replaced with an operational amplifier-based virtual ground system. This substitution uses electronic feedback mechanisms instead of direct electrical connections, enabling the circuit to measure extremely small capacitance values that would be impossible to detect with simple direct measurement methods
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution enables accurate measurement of capacitances as low as 1 fF and allows for the detection of larger objects, improving the range and sensitivity of electrostatic capacitive displacement meters while reducing the impact of stray capacitance and ambient noise.
Implementation Method 1
signal detection means including a negative feedback unit that has a feedback capacitance and applies a negative feedback from an output of the amplifier to an input of the amplifier
Implementation Method 2
measuring an electrostatic capacitance formed between a first conductor that receive an AC signal and a second conductor
Data Source
AI summary
A capacitance measuring circuit measures an electrostatic capacitance formed between a first conductor that receive an AC signal and a second conductor. The capacitance measuring circuit includes an amplifier including an input and an output; signal detection means including a negative feedback unit that has a feedback capacitance and applies a negative feedback from an output of the amplifier to an input of the amplifier, wherein an input of the amplifier is connected to the second conductor and is virtually grounded by the negative feedback unit and an AC signal of an amplitude in a functional relation with the electrostatic capacitance is output; and measuring means that is connected to an output of the signal detection means and has a function of measuring at least an amplitude of an AC signal output of the signal detection means.