Fingerprint Sensing System With Reference Element For Noise Reduction
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Solution Overview
Problem
Fingerprint sensing systems, particularly capacitive sensors, face challenges in accurately sensing fingerprints from 'difficult' fingers like dry fingers and are susceptible to interference from high-frequency common mode noise, leading to suboptimal performance.
Innovation Solution
A reference sensing structure is configured to face a plurality of friction ridges, allowing for improved signal quality assessment and adaptive acquisition settings, such as correlated double sampling and dynamic resampling, to enhance fingerprint pattern determination and reduce noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If passive capacitive sensing is used to read out capacitance between sensing structures and finger, then sensor protection is improved, but sensing accuracy for difficult fingers deteriorates
Solution Approach 1:
The system performs preliminary actions by injecting a driving signal into the finger before the actual capacitance measurement. This active signaling approach prepares the sensing system to overcome the limitations of passive sensing, enabling accurate detection even for difficult fingers while maintaining sensor protection through controlled signal injection.
Solution Approach 2:
The invention changes the sensing parameter from passive capacitance reading to active capacitance measurement with injected driving signals. By varying the signal frequency and amplitude, the system can adapt to different finger conditions (dry, wet, oily) and achieve accurate fingerprint sensing while protecting the sensor through controlled electrical parameters.
2Measurement precision
If high frequency driving signals are used to improve signal quality, then fingerprint detection accuracy is improved, but susceptibility to common mode noise interference worsens
Solution Approach 1:
The system converts the harmful high-frequency common mode noise into a useful reference signal. By intentionally injecting driving signals at known frequencies and using these same frequencies as reference for correlation processing, the system transforms potential interference into a benefit for enhancing signal detection accuracy through correlated double sampling.
Solution Approach 2:
The invention implements feedback by using the injected driving signal as a reference and correlating it with the sensed signal. This feedback mechanism allows the system to distinguish between actual fingerprint signals and common mode noise, improving detection accuracy while rejecting interference through adaptive signal processing.
3Object-affected harmful factors
If correlated double sampling is used to reduce noise, then noise rejection is improved, but system complexity worsens
Solution Approach 1:
The system achieves multi-functionality by using the same sensing structures and signal processing circuits for both fingerprint detection and noise rejection through correlated double sampling. The reference signal generation and correlation processing are integrated into the existing capacitive sensing architecture, reducing additional complexity while providing robust noise rejection capabilities.
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 results in improved fingerprint sensing accuracy, especially under difficult conditions, by ensuring favorable signal acquisition timing and dynamic range utilization, thereby enhancing the robustness of the fingerprint sensing system.
Implementation Method 1
All capacitive fingerprint sensors provide a measure indicative of the capacitance between several sensing structures and a finger placed on or moved across the surface of the fingerprint sensor
Data Source
Figure 1
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Figure 4a~4b
AI summary
The present invention relates to a method of determining a representation of a fingerprint pattern. The method comprises the steps of acquiring a reference signal indicative of an electric coupling between a hand surface having friction ridges and a reference sensing structure extending across a plurality of the friction ridges;and determining the representation of the fingerprint pattern based on the reference signal and a capacitive coupling between the finger and each of a plurality of sensing elements. The acquired reference signal can, for example, be used for controlling the sensing elements so that the sensing performed by the sensing elements is carried out using favorable timing, when the signal quality is good. Alternatively, or in combination, the acquired reference signal may be used for post-processing, whereby the signals/signal values obtained by the sensing elements are modified depending on the corresponding values of the reference signal.