Capacitive Sensor Impedance Ratio for Living Tissue Detection
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Solution Overview
Problem
Existing biometric authentication systems using capacitive sensors fail to detect living tissue effectively, allowing unauthorized access with fake fingerprints made from materials mimicking tissue conductivity, and require lengthy measurement periods.
Innovation Solution
A method utilizing a capacitive sensor with multiple electrode pairs to measure impedance ratios on mutually perpendicular lines, identifying living tissue by exceeding a predetermined threshold, thereby distinguishing real from fake fingerprints within less than 0.3 seconds.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional capacitive sensors are used for fingerprint recognition, then the device can perform basic authentication, but it cannot detect living tissue effectively and allows fake fingerprints
Solution Approach 1:
The capacitive sensor is divided into multiple electrode pairs arranged in specific patterns, with some electrode pairs oriented in different directions. This segmentation allows the system to measure impedance ratios in different orientations, enabling detection of anisotropic properties of living tissue that conventional uniform sensors cannot detect.
Solution Approach 2:
Different regions of the sensor surface are assigned different functions - some electrode pairs are optimized for measuring impedance in specific orientations. This local differentiation of measurement capabilities allows the system to detect directional variations in tissue properties, a key characteristic of living tissue that distinguishes it from fake fingerprints.
2Reliability
If multiple electrodes are used to measure potential differences for anti-spoofing analysis, then fake detection capability improves, but the measurement period becomes quite long (about 0.5 sec)
Solution Approach 1:
The electrode pairs are pre-positioned in specific orientations during device manufacturing, so that when a finger is placed on the sensor, the system immediately has ready-to-measure impedance paths in multiple directions. This preliminary arrangement eliminates the need for time-consuming electrode positioning or sequential measurement protocols.
Solution Approach 2:
The system performs impedance measurements in parallel across multiple electrode pairs simultaneously rather than sequentially. By continuously measuring impedance ratios in different orientations at the same time, the system achieves both accurate living tissue detection and fast measurement speed (less than 0.3 seconds).
3Measurement precision
If stringent requirements on finger position are imposed (first joint on grounding electrode), then measurement accuracy improves, but user convenience deteriorates
Solution Approach 1:
The sensor is designed so that any region of the finger contacting the sensor surface can be used for authentication, not just a specific position. Multiple electrode pairs are distributed across the sensor, allowing the system to adapt to different finger placement positions while maintaining measurement accuracy through the impedance ratio calculation method.
Solution Approach 2:
The system dynamically determines which electrode pairs are in contact with the finger based on the actual touch location, rather than requiring a fixed finger position. This dynamic adaptation allows the system to maintain measurement accuracy regardless of where the user places their finger on the sensor surface.
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
Enhances security by accurately differentiating living from non-living tissue in real-time, reducing the risk of unauthorized access and improving authentication efficiency.
Implementation Method 1
calculating impedance values of respective parts of the object on the basis of the received electrical signals
Implementation Method 2
determining a ratio of the calculated impedance values for each pair of electrode pairs disposed on mutually perpendicular lines
Data Source
AI summary
The disclosure relates to a method for determining whether to allow user access at an electronic device comprising: detecting a touch by an object on a touch-sensitive area of the electronic device, in which a plurality of electrode pairs are mounted, the plurality of electrode pairs including a first set of electrode pairs and a second set of electrode pairs; determining a subset of the first set of electrode pairs that are in contact with a part of the object; receiving electrical signals from each pair of the determined subset; calculating impedance values of respective parts of the object on the basis of the received electrical signals; determining a ratio of the calculated impedance values for each pair of electrode pairs disposed on mutually perpendicular lines from said the determined subset, and, when a first impedance value for one electrode pair included in electrode pairs disposed on the mutually perpendicular lines is greater than a second impedance value for the other electrode pair in the electrode pairs, the ratio of the calculated impedance values is a ratio of the first impedance to the second impedance; selecting two pairs of electrode pairs disposed on the mutually perpendicular lines with the maximum ratio of the calculated impedance values; and when the maximum ratio exceeds a predetermined threshold, identifying the object as a living tissue object, and allow access on the electronic device.