Capacitive Blood Level Sensor Using Pulsed Differential Amplification
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive sensors struggle to reliably detect fill levels of conductive media, such as blood, with high conductivity, due to interference and the formation of thin films or adhesions, which can lead to false readings and failure to detect actual fill levels, especially in medical applications where EMC immunity and reliability are critical.
Innovation Solution
A sensor system utilizing inverted and non-inverted short pulses via two identical electrodes and a fast differential amplifier with high common-mode rejection, generating a switching signal based on level-dependent voltage drops, which minimizes reactance and interference, allowing for accurate detection of fill levels even in high conductivity media without earth reference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional capacitive sensors are used for detecting fill levels of high-conductivity media, then the sensor can operate with simple circuitry, but the measurement reliability deteriorates due to interference and false readings from thin films
Solution Approach 1:
The patent employs periodic pulse signals instead of continuous excitation to create time-varying measurement conditions. The pulse width modulation technique varies the duty cycle based on capacitance changes, creating a periodic measurement cycle that distinguishes between different media states (bulk liquid vs. thin films) through temporal analysis of the response
Solution Approach 2:
The patent changes the excitation parameter from continuous AC voltage to periodic pulses with variable duty cycle. This parameter change enables the system to differentiate between thin films and bulk liquid by analyzing how the capacitive response varies during different phases of the pulse cycle, thereby improving reliability while maintaining simplicity
2Measurement precision
If oscillatory methods are used to achieve high sensitivity, then the detection sensitivity improves, but the EMC immunity deteriorates due to high sensitivity to interference in alternating electrical fields
Solution Approach 1:
The patent uses periodic pulse excitation with variable duty cycle rather than continuous oscillation. This creates discrete measurement moments separated by rest periods, allowing the system to achieve high sensitivity through pulse-width analysis while improving EMC immunity by reducing continuous exposure to interference fields
Solution Approach 2:
The patent employs short-duration pulses that rapidly sample the capacitive state and then skip to a rest period. This rushing through the measurement quickly minimizes the time window for interference to affect the measurement, achieving both high sensitivity through precise timing and improved EMC immunity through reduced exposure
3Object-affected harmful factors
If external control methods are used to meet higher EMC requirements, then the EMC immunity improves, but the detection sensitivity deteriorates
Solution Approach 1:
The patent combines periodic pulse excitation with duty cycle modulation to achieve both goals. The periodic nature provides EMC immunity by creating distinct measurement phases, while the variable duty cycle maintains sensitivity by adjusting the active measurement window based on the capacitive state, enabling detection of small changes without continuous exposure
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 reliable detection of fill levels in media with conductivities above 5 mS/cm, reduces interference sensitivity, and ensures consistent measurements regardless of vessel position or touch, meeting high EMC and reliability standards, particularly in medical applications like blood level monitoring.
Implementation Method 1
a pulse generator, which generates inverted and non-inverted exactly in phase opposition short pulses via two equivalent low-impedance resistors
Implementation Method 2
a fast differential amplifier with high common-mode rejection, which picks up a voltage drop at one of the two resistors caused by the level-dependent pulse current
Implementation Method 3
a downstream peak value detector with filtering to obtain a level-dependent DC voltage
Implementation Method 4
a downstream voltage comparator which, by comparing the output voltage of the peak value detector with an adjustable reference voltage, generates a switching signal
Implementation Method 5
The basic physical principle is always based on the plate capacitor or the 'geometrically expanded' plate capacitor. The active area of the sensor corresponds to one plate of the capacitor
Implementation Method 6
The container wall and in particular the medium to be detected have a dielectric constant εr which, as is known, is always significantly greater than that of air. The εr of a medium to be detected thus leads to an increase in the total capacitance of the plate capacitor
Data Source
Figure 1~3
Figure 3a~4b
Figure 5~6
AI summary
The invention relates to a sensor for the contactless detection of the level of a liquid and adhering high-conductivity medium, especially blood, through a non-metal wall of a container. Said sensor comprises a pulse generator (1) which generates short-period pulses which control an electrode (4), mounted on the exterior of the container wall, via a low-value resistor (2), a measuring resistor. A high common mode rejection differential amplifier (6) tapping a voltage drop on the measuring resistor (2) caused by the level-dependent pulsed current amplifies this voltage drop by a fixed factor and supplies the signal to a filtered down-stream peak value detector (7) for obtaining a level-dependent constant voltage and to a down-stream voltage comparator (8) which generates a switch signal by comparing the output voltage of the peak value detector (7) with an adjustable nominal voltage (9). Said switch signal changes from low level to high level as soon as the rising level sufficiently covers the electrode (4) or from high level to low level when the level falls below the electrode (4) or reacts inversely to a change in the level as soon as the polarity of one of the stages pulse generator (1), differential amplifier (6), peak value detector (7) and voltage comparator (8) are interchanged.