Capacitive Foam Boundary Detection via Dual Comparator PWM Analysis
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Solution Overview
Problem
Existing methods for detecting foam boundaries in containers are not reliable, particularly in liquids prone to foaming, which can lead to inaccurate liquid level detection and contamination during pipetting operations.
Innovation Solution
A capacitive measuring device with a movable sensor that periodically charges and discharges a measuring capacitance, using two comparators to generate PWM signals, allowing for differentiation between liquid and foam based on changes in pulse widths, enabling precise detection of foam boundaries.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional liquid level detection methods are used, then the detection process is simple, but the reliability of detection is insufficient when foam is present
Solution Approach 1:
The detection process is segmented into multiple measurement steps (first measurement signal, second measurement signal, third measurement signal) with different sensor positions. This segmentation allows the system to distinguish between foam and liquid contacts by comparing measurements taken at different depths, thereby improving reliability without requiring a fundamentally more complex device architecture.
Solution Approach 2:
The sensor performs periodic measurements at different positions during its movement through the container. By taking measurement signals at specific intervals (when the sensor is in foam, when transitioning, and when in liquid), the system can reliably distinguish between foam and liquid phases. This periodic sampling approach enhances detection reliability while maintaining relatively simple device complexity.
2Reliability
If the pipetting tip is immersed deeper in the liquid, then air suction is prevented, but contamination with sample liquid increases
Solution Approach 1:
The system uses feedback from the detection device to determine the exact position of the foam-liquid phase boundary. Based on this feedback, the pipetting tip position is automatically adjusted to be just below the liquid surface. This feedback mechanism ensures the tip is immersed sufficiently to prevent air suction while minimizing immersion depth to reduce contamination, thereby improving pipetting accuracy while reducing harmful contamination effects.
3Adaptability or versatility
If the fill level varies greatly between containers, then the pipetting tip must be positioned over large areas, but detection accuracy decreases
Solution Approach 1:
The sensor is designed to move dynamically through the container during the detection process. By continuously moving the sensor and taking measurements at different positions, the system can adapt to varying fill levels across large areas. The evaluation device processes these dynamic measurements to precisely identify the liquid surface position regardless of the container's fill level, thereby maintaining measurement precision while providing adaptability to large positioning ranges.
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 method provides reliable and accurate detection of foam boundaries, ensuring correct data delivery and preventing contamination by distinguishing between foam and liquid contacts, thus enhancing the precision of automated processes.
Implementation Method 1
the gas-liquid phase boundary can also be determined via a change in capacitance
Implementation Method 2
Since a gas and a liquid have significantly different dielectric constants
Data Source
Figure 1
Figure 2A~2B
Figure 3~4
AI summary
Device (100) for detecting a foam boundary in a container comprising the container, a sensor (3) that is movable into and/or out of the container, an input signal generator (E) that can be connected to the sensor (3) to provide an input signal (sin(t)) that induces a charging and discharging process at the sensor (3), a capacitive measuring device (M) that can be connected to the sensor (3) to tap at least one output signal (sout(t)) from the sensor (3) during the charging and discharging process, wherein the sensor (3) can be periodically charged and discharged during the charging and discharging process, the at least one output signal (sout(t)) corresponding to the time course of a charging/discharging curve resulting from the periodic charging and discharging of a total capacitance, the device (100) comprising a circuit module (10, 11) with two comparators designed tofrom the at least one output signal (sout(t)) to provide a first comparator output signal and a second comparator output signal, wherein the two comparators are connected in parallel, and wherein the first comparator is designed to provide the first comparator output signal if the at least one output signal (sout(t)) reaches a first reference voltage, and wherein the second comparator is designed to provide the second comparator output signal if the at least one output signal (sout(t)) reaches a second reference voltage, characterized in that the second reference voltage is greater than the first reference voltage, and wherein the circuit module (10, 11) is designed to relate the first comparator output signal and the second comparator output signal within an evaluation process in order to determine,whether, on the one hand, the output signal of the submerged sensor (3) rises faster in an initial region than the output signal of the submerged sensor (3), and whether, on the other hand, the output signal of the submerged sensor (3) rises more slowly in a region after the initial region than the output signal of the submerged sensor (3), and if these two conditions are met, to output a detection (Kf) of a foam boundary.