Capacitive Sensor Using Astable Multivibrator for Permittivity Distribution
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Solution Overview
Problem
Existing methods for determining permittivity distribution in pipelines, such as capacitive tomography, are prone to interference and require complex and expensive analog signal processing, limiting their accuracy and practicality for distinguishing between gas and liquid in multiphase flows.
Innovation Solution
A capacitive sensor system utilizing an astable multivibrator to measure capacitances between electrodes, with a switch-based configuration allowing each electrode to be connected to ground, supply voltage, or the multivibrator, enabling direct digital conversion of capacitance values and simplifying signal processing, thereby reducing interference and complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If operational amplifier circuits or high-frequency signal methods are used to measure capacitance, then capacitance measurement capability is achieved, but the system becomes susceptible to interference and loses sensitivity
Solution Approach 1:
The patent replaces traditional operational amplifier circuits and high-frequency signal methods with a digital counting-based measurement approach. The astable multivibrator generates square wave oscillations whose frequency is directly proportional to the capacitance being measured, and a digital counter directly counts these oscillations to determine capacitance value. This digital substitution eliminates the need for analog signal processing, thereby removing susceptibility to electromagnetic interference and signal loss while maintaining high measurement precision.
2Measurement precision
If operational amplifier circuits and analog-to-digital converters are used for signal processing, then capacitance measurement is enabled, but the system becomes expensive and complex
Solution Approach 1:
The patent eliminates operational amplifier circuits and analog-to-digital converters by using a purely digital measurement approach. The astable multivibrator circuit generates oscillations at a frequency determined by the capacitance value, and a simple digital counter directly counts these oscillations over a fixed time period. This digital-native approach removes entire classes of analog components, dramatically simplifying the signal processing chain while maintaining measurement capability.
Solution Approach 2:
The measurement system uses the capacitance being measured to directly determine the oscillation frequency of the astable multivibrator. The capacitance value itself controls the timing of the oscillations, eliminating the need for external reference signals or complex conversion circuits. The system essentially measures itself using the property being quantified.
3Device complexity
If astable flip-flop is used to measure individual capacitance, then simple measurement is achieved, but the method cannot be transferred to large numbers of electrodes due to mutual interference
Solution Approach 1:
The patent divides the measurement process into discrete time segments where only one capacitance is measured at a time. A multiplexer sequentially connects different electrode pairs to the astable multivibrator and counter, allowing the simple single-capacitance measurement circuit to be applied to multiple electrodes without mutual interference. Each electrode pair gets dedicated measurement time, eliminating cross-talk while maintaining circuit simplicity.
Solution Approach 2:
The patent employs periodic switching of the multiplexer to sequentially present different capacitances to the measurement circuit. By measuring one capacitance at a time in a systematic sequence, the system avoids mutual interference between multiple electrodes while maintaining the simplicity of the astable flip-flop based measurement approach. The periodic nature of the measurement cycle ensures that no two capacitances are measured simultaneously.
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 provides a robust and cost-effective method for determining permittivity distribution, allowing for accurate distinction between gas and liquid phases in pipelines, correcting measurement errors due to liquid components in gas flows and enabling precise flow measurement.
Implementation Method 1
A measuring unit with an astable multivibrator measures capacitances between electrodes, for example different pairs of electrodes. The astable multivibrator generates a type of oscillation, idealized as a square-wave voltage, from whose period the capacitance is determined
Implementation Method 2
The switch connects either to ground, a supply voltage or the astable multivibrator, depending on what role the electrode connected in this way is to play in a given capacitance measurement
Implementation Method 3
The flow rate resulting from this is offset against the pipe cross-section for the flow rate. If, in addition to the pumped gas, there is also liquid such as condensed water in the pipeline, the effective gas-carrying cross-section is reduced by the liquid, and this results in a corresponding error when determining the gas volume flow. It is therefore desirable to determine the ratio of gas to water in the pipe cross-section. A material property that is suitable for distinguishing between gas and water is the permittivity, often also referred to as the dielectric constant, because it applies εr (water) ≈ 80 and εr (gas) ≈ 1
Data Source
Figure 1~2
Figure 3~4
Figure 5~6(d)
AI summary
A capacitive sensor (10) for determining the permittivity distribution in an object (12) is described, wherein the sensor (10) comprises several electrodes (14a-d) for attachment to the object (12), a measuring unit (18, 26) connectable to the electrodes (14a-d) with at least one astable multivibrator (18a-d) for measuring capacitances between electrodes (14a-d), and an evaluation unit (24) for determining the permittivity distribution from the measured capacitances. The measuring unit (18, 26) includes at least one switch (26a-d) for selectively connecting an electrode (14a-d) to ground, a supply voltage (V0), or the astable multivibrator (18a-d).