Capacitive Touch Sensing With Analog Water Rejection
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Solution Overview
Problem
Capacitive touch sensing systems face contamination issues, particularly with water, leading to signal shifts and false triggers, as existing solutions either rely on mutual capacitance alone or require resource-intensive software pattern-matching algorithms.
Innovation Solution
A method combining self-capacitance and mutual capacitance measurements, using a capacitive voltage divider and a guard or shield electrode, to differentially cancel ungrounded conductive objects and additively combine signals for grounded objects, thereby eliminating analog signal shifts caused by contamination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If mutual capacitance measurement is used to detect touch, then touch detection capability is improved, but false triggers occur when water is removed from the system due to signal shifts
Solution Approach 1:
The patent combines self-capacitance measurement and mutual capacitance measurement into a single capacitive sensing system. The self-capacitance measurement detects changes in the sensor's own capacitance to ground, while the mutual capacitance measurement detects changes in capacitance between the sensor and a guard electrode. By merging these two measurement types and processing their combined output, the system achieves robust touch detection that is immune to water presence or removal, eliminating false triggers while maintaining high detection precision.
2Measurement precision
If software pattern-matching algorithm is used to differentiate water and finger inputs, then detection accuracy is improved, but computational overhead and complexity increase
Solution Approach 1:
The patent replaces the software-based pattern-matching algorithm with an analog electrical measurement system. Instead of using complex computational algorithms to differentiate between water and finger inputs, the system uses the inherent electrical properties of the capacitive sensor and guard electrode to directly measure and distinguish between different types of contaminants. This substitution of mechanical/computational approach with an electrical measurement approach reduces software complexity and computational overhead while maintaining high differentiation accuracy.
3Reliability
If guard electrode is added to provide capacitive coupling, then water resistance is improved, but device complexity increases
Solution Approach 1:
The patent designs the guard electrode to serve multiple functions simultaneously. It acts as a reference electrode for mutual capacitance measurement, provides electrostatic shielding to reduce noise and interference, and helps define the sensing field geometry. By making the guard electrode multi-functional, the patent achieves improved water resistance and enhanced measurement accuracy without proportionally increasing device complexity, as the same structural element performs multiple critical roles in the capacitive sensing system.
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
This approach effectively differentiates between water and finger inputs without software decoding, ensuring water resistance at the analog level and reducing false triggers, while maintaining system sensitivity.
Implementation Method 1
capacitive sensing
Implementation Method 2
a shield or guard electrode can be arranged in proximity to the capacitive sensor to provide a capacitive coupling
Data Source
Figure 1~2
Figure 3
Figure 4~5
AI summary
Method for performing a touch determination with a capacitive sensor by initiating a self capacitance measurement of a capacitive sensor (150), wherein at the same time a mutual capacitance measurement including the capacitive sensor is performed. Such a method can be performed such that the self capacitance measurement and the mutual capacitance measurement differentially cancel with ungrounded conductive objects approaching or touching the capacitive sensor and additively combine for grounded objects approaching or touching the capacitive sensor.