Capacitance Imaging Sensor for Fingerprint Detection
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Solution Overview
Problem
Existing fingerprint imaging technologies face challenges in accurately capturing high-quality images under conditions of poor acoustic impedance matching and contamination, such as dry skin, and require efficient means to distinguish energy reflections for reliable imaging.
Innovation Solution
A capacitance-based imaging sensor using a sensing array with capacitance transducers, peak detectors, and charge amplifiers, where a dielectric material configured as a capacitor generates charge in response to varying distances, allowing for pixel-level peak detection and amplification to create detailed images of objects like fingerprints or styluses.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If ultrasonic scanning systems are used for fingerprint imaging, then imaging capability is provided, but reliability deteriorates under poor acoustic impedance matching conditions such as dry skin
Solution Approach 1:
The patent replaces the ultrasonic acoustic field with an electrical field-based capacitance sensing system. Instead of using piezoelectric transducers that send longitudinal wave energy through transmitting media, the invention uses an array of capacitance sensors that directly measure the dielectric properties of the skin surface, eliminating the acoustic impedance matching problem entirely.
Solution Approach 2:
The patent changes the detection parameter from acoustic impedance to electrical capacitance. By measuring the capacitance variations caused by the friction ridge details at different distances from the sensor surface, the system achieves reliable imaging regardless of acoustic impedance conditions. The capacitance measurements are converted to digital values representing signal strength, creating a contour map of the object surface.
2Measurement precision
If capacitance sensors are used for fingerprint imaging, then accuracy is improved by being immune to contaminants, but device complexity increases due to the need for solid state arrays and readout circuits
Solution Approach 1:
The patent divides the sensing system into a two-dimensional array of discrete capacitance sensor elements, each independently measuring the local capacitance at its position. This segmentation allows the system to build up a complete fingerprint image by reading out the array elements in a systematic manner, managing the complexity through structured organization rather than a single complex sensor.
Solution Approach 2:
The patent employs a solid state array that serves multiple functions: it acts as both the addressing mechanism and the readout device for the capacitance imaging system. This universal solid state array structure simplifies the overall system by combining what would otherwise be separate components, reducing device complexity while maintaining measurement precision.
3Reliability
If ink and paper methods are used for fingerprint collection, then simplicity is maintained, but reliability deteriorates due to sensitivity to grease, dirt, paint, ink and other contaminants
Solution Approach 1:
The patent replaces the mechanical ink-and-paper friction ridge capture method with an electrical field-based capacitance sensing system. The capacitance sensors measure the dielectric properties of the skin surface directly, eliminating the need for ink application and paper contact, thereby making the system immune to contaminants such as grease, dirt, paint, and ink that would interfere with traditional methods.
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
The solution enables reliable and high-quality imaging by accurately capturing capacitance variations across the skin surface, immune to contaminants and improving image resolution through local or shared peak detection and charge amplification across a matrix of pixels.
Implementation Method 1
capacitance is a function of the distance between capacitance plates, i.e., the TFT (Thin Film Transistor) input pad and the skin of the finger
Implementation Method 2
a dielectric material configured as a simple capacitor whose voltage is sensitive to the varying distances from the sensing surface to the skin
Implementation Method 3
Ultrasonic scanning systems often employ a piezoelectric transducer that sends longitudinal wave energy through transmitting media
Data Source
AI summary
An imaging system disposed on a TFT array employing a capacitance input sensing stage into a diode peak detecting circuit used as a single pixel on a semiconductor or TFT array that is capable of accessing the generated charge across the capacitor, peak detecting it, amplifying it and then distributing it to an external system via row and column addressing. The resulting imaging system is suitable for fingerprint imaging or as a computer touchpad input device.