Touch Sensor Noise Detection Using Digital Mixers

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Solution Overview

Problem

Touch sensor panels face noise interference issues due to high voltage stimulation signals and external noise sources, leading to inaccurate touch event detection and localization, particularly in capacitance-based systems with LCDs and AC adapters.

Innovation Solution

The use of multiple digital mixers for spectrum analysis to identify low noise stimulation frequencies and the application of multiple stimulation frequencies and phases to demodulate signals, allowing for precise detection and localization of touch events by calculating noise magnitude and selecting optimal frequencies for touch sensor panel scans.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Power

If high voltage stimulation signals are used to drive the touch sensor panel, then the stimulation signal strength is sufficient for detection, but noise interference increases due to capacitive coupling from LCD voltage switching and other external sources

Engineering Contradiction:

Improvestimulation signal strengthVSAvoidnoise interference

Core Design Contradiction:

PowerVSObject-affected harmful factors

Solution Approach 1:

The patent segments the frequency spectrum into multiple discrete frequency components. By using multiple digital mixers to analyze different frequency ranges, the system can identify which frequency segments are affected by noise and which are clean, allowing selective use of low-noise frequencies for touch detection

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent changes the stimulation frequency parameter dynamically. Instead of using a fixed high voltage stimulation signal, the system applies stimulation at multiple different frequencies and phases, then uses spectrum analysis to identify which frequencies have the lowest noise levels, adapting the stimulation parameters based on measured noise characteristics

Inventive Principle:

Principle #35Parameter changes

2Measurement precision

If multiple digital mixers are used to perform spectrum analysis and identify low noise frequencies, then measurement precision of noise levels is improved, but device complexity increases

Engineering Contradiction:

Improvenoise magnitude detection accuracyVSAvoidnumber of digital mixers and circuitry

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent makes the sense channels and digital mixers multi-functional. The same sense channels used for normal touch detection are also used for noise measurement during the spectrum analysis phase. The digital mixers serve dual purposes: demodulating touch signals during scanning and performing spectrum analysis during noise measurement, eliminating the need for separate dedicated noise measurement hardware

Inventive Principle:

Principle #6Universality (Multi-functionality)

Solution Approach 2:

The system performs self-diagnosis and self-configuration. The touch sensor panel system automatically measures its own noise characteristics by analyzing the outputs of its sense channels, identifies low-noise frequencies, and configures subsequent touch scans to use those frequencies, without requiring external calibration equipment or manual intervention

Inventive Principle:

Principle #25Self-service

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 effectively reduces noise interference, enhancing the accuracy of touch event detection and localization on touch sensor panels by identifying and mitigating noise sources, thereby improving the reliability of touch-based interactions.

Implementation Method 1

The mixers can be paired up, and each pair of mixers can demodulate the sum of all sense channels using the in-phase (I) and quadrature (Q) signals of a particular frequency.

Methodology Applied

Scientific EffectDemodulation:

Implementation Method 2

touch locations can be identified because the charge injected into the columns due to the stimulation signal is proportional to the amount of touch

Methodology Applied

Scientific EffectCapacitance coupling: Capacitance

Implementation Method 3

the voltage switching required to operate an LCD can capacitively couple onto the columns of the touch sensor panel and cause inaccurate measurements of touch

Methodology Applied

Scientific EffectCapacitive coupling: Capacitance

Data Source

PatentUS8120591B2Detection of low noise frequencies for multiple frequency sensor panel stimulation

Publication Date: 2012.02.21 APPLE INC

AI summary

The identification of low noise stimulation frequencies for detecting and localizing touch events on a touch sensor panel is disclosed. Each of a plurality of sense channels can be coupled to a separate sense line in a touch sensor panel and can have multiple mixers, each mixer using a demodulation frequency of a particular frequency, phase and delay. With no stimulation signal applied to any drive lines in the touch sensor panel, pairs of mixers can demodulate the sum of the output of all sense channels using the in-phase (I) and quadrature (Q) signals of a particular frequency. The demodulated outputs of each mixer pair can be used to calculate the magnitude of the noise at that particular frequency, wherein the lower the magnitude, the lower the noise at that frequency. Several low noise frequencies can be selected for use in a subsequent touch sensor panel scan function.