Capacitive Fingerprint Sensor Isolation for Clearer Signal Sensing
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive fingerprint sensors face limitations due to restricted drive ring voltage, leading to attenuated capacitance signals and blurry images, and are prone to parasitic coupling from user's body, resulting in variable and unreliable biometric data.
Innovation Solution
The implementation of a capacitive fingerprint sensor system with a sensor pad and drive ring positioned beneath a dielectric, where the drive ring is maintained at system ground voltage while the sensor array is modulated with a higher peak-to-peak voltage, reducing signal attenuation and enhancing image resolution without causing physical sensation to the user.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the drive ring voltage is increased to improve capacitance signal strength, then the capacitance measurement precision is improved, but the user experiences uncomfortable tingling sensations
Solution Approach 1:
The patent introduces a ground ring as an intermediary element between the drive ring and the sensing array. The ground ring is held at system ground voltage while the drive ring operates at higher voltages. This intermediary structure allows the drive ring to achieve higher voltage excitation for improved measurement precision while the ground ring shields the user's finger from direct exposure to high voltage, preventing uncomfortable tingling sensations.
2Reliability
If a thicker dielectric is used between sensor pad and finger, then the electrical isolation and protection are improved, but the capacitance signal is attenuated and fingerprint image becomes blurry
Solution Approach 1:
The patent changes the voltage parameter of the drive ring to compensate for dielectric thickness. By operating the drive ring at higher voltages (e.g., 12V peak-to-peak instead of 4V), the system overcomes the signal attenuation caused by thicker dielectrics. This parameter change allows maintaining both electrical isolation through the dielectric and measurement precision despite the increased thickness.
3Volume of moving object
If the sensor is placed close to other electronic components to reduce device size, then the device compactness is improved, but parasitic coupling from user's body to system ground increases
Solution Approach 1:
The ground ring serves as an intermediary shielding structure that isolates the sensing array from parasitic coupling to system ground. By holding the ground ring at system ground voltage and positioning it between the sensing array and other electronic components, it creates an electrical barrier that prevents variable capacitive coupling paths through the user's body, thereby improving biometric data consistency while allowing compact device design.
4Measurement precision
If the drive ring is modulated with higher voltage to overcome signal attenuation, then the capacitance signal strength is improved, but the user perceives tingling sensations
Solution Approach 1:
The patent segments the voltage application into two separate components: the drive ring operates at higher modulated voltages to generate strong capacitance signals, while the ground ring is held at system ground voltage to provide a safe reference potential near the user's finger. This segmentation allows the system to achieve high measurement precision through drive ring voltage modulation without exposing the user to high voltages that cause tingling sensations.
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 allows for improved fingerprint imaging resolution by compensating for dielectric thickness and minimizing parasitic coupling, providing a more reliable and consistent biometric sensing experience.
Implementation Method 1
Capacitance to the epidermis is measured at each capacitive sensing element... the voltage and/or charge at each capacitive sensing element of capacitive sensing element array 102 varies as drive ring 104 is modulated since finger's 108 voltage potential changes with the modulation of drive ring 104
Implementation Method 2
Another limitation arises when other parts of the user's finger or hand or body may capacitively couple through earth ground to the system, or directly to the system ground when touching other parts of the system. This capacitive coupling from the user to the system may be highly variable depending on how the user is touching the device.
Data Source
AI summary
A capacitive fingerprint sensor that may be formed of an array of sensing elements. Each capacitive sensing element of the array may register a voltage that varies with the capacitance of a capacitive coupling. A finger may capacitively couple to the individual capacitive sensing elements of the sensor, such that the sensor may sense a capacitance between each capacitive sensing element and the flesh of the fingerprint. The capacitance signal may be detected by sensing the change in voltage on the capacitive sensing element as the relative voltage between the finger and the sensing chip is changed. Alternately, the capacitance signal may be detected by sensing the change in charge received by the capacitive sensing elements as the relative voltage between the finger and the sensing chip is changed.