Capacitive Sensing With Electret Bias Charge
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Solution Overview
Problem
Capacitive sensing signals are often distorted by static charges and motion artifacts, resulting in low-quality signals due to fluctuations in electrical charge and varying distances between the object and the sensor.
Innovation Solution
A capacitive sensing system that includes an electrical charge providing unit, such as an electret or a voltage source with a conductive foil, to maintain a permanent electrical charge at the sensing site, reducing signal distortion and enhancing sensitivity to mechanical motions, thereby improving signal quality and increasing the signal-to-noise ratio.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-generated harmful factors
If an insulating separation material with matching tribo-electric property is used to reduce static charge generation, then static charge generation on the sensor is reduced, but the sensing signal is still distorted by static charges on the body and motion artifacts
Solution Approach 1:
An electret material is introduced as an intermediary component between the sensor electrode and the body. This electret material carries a permanent electrical charge that creates a stable electric field, serving as a mediator to enhance the sensing of mechanical motions while compensating for the instability caused by body static charges and motion artifacts
Solution Approach 2:
The electrical charge state of the sensing system is changed from a passive state (relying on body capacitance only) to an active state with a permanent bias charge provided by the electret. This parameter change in the electrical field configuration stabilizes the sensing signal and improves the signal-to-noise ratio for detecting mechanical motions
2Adaptability or versatility
If the distance between the object and the capacitive sensor varies during motion, then mechanical motions can be sensed, but motion artifacts distort the sensing signal
Solution Approach 1:
The electret material acts as a charge reservoir that maintains a stable electric field despite variations in distance. This intermediary charge distribution compensates for the effects of changing capacitance due to motion, allowing mechanical motion sensing while reducing signal distortion from distance variations
Solution Approach 2:
A permanent electrical charge is pre-established in the electret material before sensing begins. This preliminary charging creates a stable reference electric field that remains relatively constant during motion, providing a baseline against which mechanical motion signals can be detected without being overwhelmed by distance-related artifacts
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 system generates a larger, higher-quality sensing signal primarily caused by mechanical motions, allowing for effective monitoring of physiological parameters like heart rate and respiration with reduced noise interference.
Implementation Method 1
the electrical charge providing unit comprises an electret for being attached to the object for providing the electrical charge
Implementation Method 2
a capacitive sensor (2) comprising a sensing electrode (5) for generating a sensing signal by capacitively sensing the object (3)
Implementation Method 3
by providing a permanent electrical charge at the sensing site of the object, the bias between the object and the sensing electrode of the capacitive sensor is intentionally preferentially made large, thereby increasing the sensitivity towards mechanical motions
Data Source
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AI summary
The invention relates to capacitive sensing system (1) for sensing an object. The capacitive sensing system (1) comprises an electrical charge providing unit (4) like an electret foil for providing a permanent electrical charge at a sensing site (6) of the object (3) and a capacitive sensor (2) comprising a sensing electrode (5) for generating a sensing signal by capacitively sensing the object (3) at the sensing site (6) of the object (3). By providing a permanent electrical charge at the sensing site (6) of the object (3), the bias between the object (3) and the sensing electrode (5) of the capacitive sensor (2) is intentionally preferentially made large, thereby increasing the sensitivity towards mechanical motions. The resulting sensing signal substantially caused by these mechanical motions between the object (3) and the sensing electrode (5) is generally larger than a signal generated substantially by an electrophysiological field.