Multi-Phase Scanning for Capacitive Fingerprint Sensors
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive fingerprint sensors face challenges in acquiring and processing fingerprint images due to their small active area, fewer sensor elements, and weaker signal strength compared to typical capacitive touch-screen sensors, requiring extremely sensitive and low-noise circuitry, which existing techniques do not adequately address.
Innovation Solution
The use of multi-phase scanning techniques with fully-differential zero-sum excitation sequences for capacitive fingerprint sensor arrays, involving interdigitated or intersecting TX and RX electrodes, and hardware baseline compensation circuits to improve signal dynamic range and noise cancellation, allowing for effective fingerprint image reconstruction.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitive sensing techniques are used for fingerprint sensors, then the sensor can operate with standard circuitry, but the signal strength is too weak (2-3 orders of magnitude smaller than touch-screen sensors) to capture usable fingerprint images
Solution Approach 1:
The patent applies multi-phase scanning where sensor elements are periodically excited with different phase sequences (e.g., 3-phase, 4-phase, or 5-phase excitation). This periodic excitation allows the system to accumulate signal information over multiple scan cycles, effectively amplifying the weak fingerprint signal without requiring proportionally more sensitive hardware. The periodic action enables signal integration that overcomes the inherent weakness of small sensor element signals.
Solution Approach 2:
The patent changes the excitation parameters by using fully-differential zero-sum excitation sequences that alternate phases across multiple scan cycles. By varying the excitation phase and using differential measurement techniques, the system extracts maximum signal information from each sensor element. This parameter change approach transforms the measurement process to enhance signal strength while maintaining hardware simplicity.
2Area of stationary object
If the active area of capacitive fingerprint sensors is kept small (100 times smaller than touch-screen sensors), then the device form factor is maintained, but the number of sensor elements and signal strength are reduced
Solution Approach 1:
The patent segments the fingerprint sensing process into multiple phases and scan cycles. Instead of attempting to capture the complete fingerprint image in a single scan, the system divides the measurement into sequential phases where different sensor elements are excited and measured in turns. This segmentation allows small sensor elements to collectively cover the entire fingerprint area over time, maintaining small active area while achieving complete fingerprint capture through temporal multiplexing.
Solution Approach 2:
The patent adds the time dimension to the spatial measurement process by implementing multi-phase scanning across multiple cycles. While the sensor array remains small in spatial dimensions, the temporal extension of the scanning process allows each small sensor element to contribute to multiple positions in the final fingerprint image through phase-coded excitation. This transforms a spatial limitation into a temporal solution.
3Measurement precision
If hardware baseline compensation circuits are added to improve signal dynamic range and noise cancellation, then fingerprint image quality improves, but device complexity and cost increase
Solution Approach 1:
The patent replaces complex hardware baseline compensation circuits with software-based signal processing techniques. By using multi-phase excitation sequences and differential measurement methods, the system achieves baseline compensation and noise cancellation through computational algorithms rather than additional hardware components. This substitution maintains signal dynamic range improvement while avoiding the complexity and cost of extra hardware circuits.
Solution Approach 2:
The patent introduces phase-coded excitation sequences as an intermediary between the simple sensor elements and the final fingerprint image. These excitation sequences act as a mediator that enables signal differentiation and baseline compensation without requiring complex hardware. The intermediary phase coding allows the system to extract clean fingerprint signals from noisy measurements through correlation processing.
4Measurement precision
If multi-phase scanning with fully-differential zero-sum excitation sequences is implemented, then noise is reduced and baseline capacitances are eliminated, but the scanning process becomes more complex
Solution Approach 1:
The patent uses periodic multi-phase excitation sequences where sensor elements are systematically cycled through different excitation phases. This periodic action creates a structured measurement pattern that enables noise cancellation through correlation processing. The regular periodicity of the excitation allows the system to distinguish true fingerprint signals from random noise and baseline capacitances through frequency-domain separation.
Solution Approach 2:
The patent applies different excitation phases to different sensor elements or different time periods, creating local variations in the measurement process. By using fully-differential zero-sum excitation sequences where adjacent or paired elements are excited with opposite phases, the system achieves local noise cancellation while maintaining overall measurement accuracy. This local quality approach enables targeted noise reduction without complicating the entire scanning process.
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 fingerprint image reconstruction by reducing noise, eliminating baseline capacitances, and improving image quality, while enabling the use of lower-cost sensing hardware without compromising performance.
Implementation Method 1
Capacitive sensing typically involves scan operations that periodically measure changes in capacitance associated with the capacitive sensor elements to determine presence, position, and/or other characteristics of a conductive object
Implementation Method 2
The use of multi-phase scanning techniques with fully-differential zero-sum excitation sequences for capacitive fingerprint sensor arrays, involving interdigitated or intersecting TX and RX electrodes, and hardware baseline compensation circuits to improve signal dynamic range and noise cancellation
Data Source
AI summary
Techniques for multi-phase scanning based on pseudo-random sequences in capacitive fingerprint applications are described herein. In an example embodiment, a method performed by a processing device comprises: receiving measurements that are representative of a portion of a finger on a capacitive fingerprint sensor array, where the measurements are obtained from sensor elements of the capacitive fingerprint sensor array that are scanned in a multi-phase mode based on an excitation vector generated from a pseudo-random sequence; and generating a fingerprint image for the portion of the finger based on the measurements.