Capacitive Finger Detection Circuit for Fingerprint Sensors
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Solution Overview
Problem
Existing fingerprint sensing systems face challenges in achieving energy-efficient operation while accurately detecting fingers and preventing unwanted activation, such as from conductive objects in pockets or noise signals.
Innovation Solution
The implementation of capacitive sensing elements with dedicated finger detecting structures and circuitry, using threshold values to differentiate between weak and strong capacitive coupling, along with time duration criteria to confirm finger placement, ensures accurate finger detection and activation only when a finger is present, thereby reducing energy consumption and preventing false activations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the fingerprint sensing system operates continuously to ensure accurate finger detection, then detection reliability is improved, but energy consumption increases
Solution Approach 1:
The system performs preliminary finger presence detection using a simplified detection mechanism before activating the full fingerprint sensing system. This preliminary action identifies potential finger placements without requiring full system activation, thereby reducing energy consumption while maintaining detection reliability.
Solution Approach 2:
The system dynamically adjusts its operation mode based on detected finger presence. When no finger is detected, the system operates in a low-power state with minimal sensing activity. Upon detecting finger presence, the system transitions to full operational mode for accurate fingerprint capture, optimizing the balance between reliability and energy consumption.
2Speed
If the sensing device is activated by any capacitive coupling to detect fingers quickly, then detection speed is improved, but false activations increase
Solution Approach 1:
The system performs preliminary screening of capacitive coupling signals to identify potential finger placements before triggering full activation. This preliminary action filters out weak or spurious signals that would cause false activations while maintaining quick response to genuine finger placements.
Solution Approach 2:
The system uses feedback from the capacitive coupling detection to modulate the activation decision. By continuously monitoring the coupling strength and comparing it against threshold criteria, the system provides feedback control that prevents false activations while maintaining fast detection response.
3Measurement precision
If threshold values are set low to detect weak capacitive coupling, then detection sensitivity is improved, but false detections from noise increase
Solution Approach 1:
The system performs preliminary assessment of the capacitive coupling signal characteristics before making detection decisions. This preliminary action evaluates both the strength and temporal patterns of the signal to distinguish genuine finger placements from noise, enabling low threshold operation without excessive false detections.
Solution Approach 2:
The system employs feedback mechanisms that monitor detection outcomes and adjust sensitivity thresholds dynamically. When noise patterns are detected, the system provides feedback to temporarily increase thresholds or apply filtering, maintaining high sensitivity for genuine signals while suppressing noise-induced false detections.
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 enhances the reliability of finger detection, reduces energy consumption, and improves the quality of fingerprint images by ensuring that the sensing device is only activated when a finger is placed on the sensor, minimizing false triggers and maintaining low power usage.
Implementation Method 1
a finger detecting structure (4a-d) connected to finger detecting circuitry (9) for providing a finger detection signal indicative of a capacitive coupling between the finger detecting structure (4a-d) and a finger approaching the sensor surface
Implementation Method 2
The sensing elements may, for example, be capacitive sensing elements, each providing a measure indicative of the capacitive coupling between that particular sensing element and a finger surface touching the sensor surface
Data Source
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AI summary
The present invention relates to a method of sensing a fingerprint pattern of a finger using a fingerprint sensing device comprising an array of sensing elements; an electrically conductive finger detecting structure; and finger detecting circuitry connected to the finger detecting structure for providing a finger detection signal indicative of a capacitive coupling between the finger detecting structure and the finger. The method comprises the steps of: comparing the finger detection signal with a first threshold value indicating a first capacitive coupling, and a second threshold value indicating a second capacitive coupling stronger than the first capacitive coupling; and activating at least a subset of the sensing elements when the finger detection signal changes from a first value indicating a capacitive coupling weaker than the first capacitive coupling to a second value indicating a capacitive coupling stronger than the second capacitive coupling.