Capacitive Touchscreen Drive Frequency Adjustment for Noise Interference

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Solution Overview

Problem

Capacitive touchscreen systems face interference from external noise signals, leading to erroneous touch reports and inaccurate finger position detection due to noise harmonics from devices like LCDs and SMPSs.

Innovation Solution

A method is implemented in the touchscreen controller to adjust the drive signal frequency to a range between 10% greater than the fundamental noise frequency and 10% less than the harmonic frequency, or using the formula (n+knoise)×(fundamental noise frequency), to minimize interference from external noise signals.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If the drive signal frequency is set to a fixed value, then the system is simple to operate, but noise interference from external devices (LCD, SMPS) causes erroneous touch reports and inaccurate detection

Engineering Contradiction:

Improvetouch detection accuracyVSAvoidfrequency adjustment mechanism

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The drive signal frequency is made dynamic rather than fixed. The system automatically adjusts the frequency based on detected noise harmonics from external devices like LCDs and SMPS, moving the operating frequency away from interfering frequencies to maintain accurate touch detection despite changing environmental noise conditions

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The system incorporates feedback by continuously monitoring for noise harmonics in the capacitive touchscreen signals and using this information to adjust the drive frequency. The controller detects beat notes or harmonic interference and responds by shifting the operating frequency to avoid these interfering frequencies, creating a closed-loop system that adapts to environmental noise

Inventive Principle:

Principle #23Feedback

2Object-affected harmful factors

If the drive signal frequency is adjusted to avoid noise harmonics, then noise interference is reduced, but the system becomes more complex requiring frequency detection and adjustment mechanisms

Engineering Contradiction:

Improvenoise interferenceVSAvoidfrequency control circuitry

Core Design Contradiction:

Object-affected harmful factorsVSDevice complexity

Solution Approach 1:

The capacitive touchscreen system performs self-diagnosis and self-adjustment by monitoring its own signals for noise harmonics and automatically modifying its operating parameters. The system detects interference patterns in its capacitive measurements and autonomously adjusts the drive frequency without external intervention, eliminating the need for complex external noise filtering hardware

Inventive Principle:

Principle #25Self-service

3Reliability

If multiple frequency adjustments are made to track noise harmonics, then touch detection accuracy is maintained, but the response time increases due to continuous frequency monitoring and adjustment

Engineering Contradiction:

Improvetouch detection accuracyVSAvoidfrequency adjustment response time

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The system performs frequency adjustments periodically rather than continuously. The controller monitors for noise harmonics at specific intervals and only adjusts the drive frequency when interference is detected, allowing normal touch detection operations to proceed without interruption during stable conditions while maintaining accuracy when needed

Inventive Principle:

Principle #19Periodic action

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 adjustment significantly reduces noise interference, ensuring accurate touch detection and reporting by positioning the drive signal frequency between noise harmonics, thereby improving the reliability of capacitive touchscreen systems.

Implementation Method 1

Self-capacitance involves measuring the self-capacitance of a series of electrode pads... Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, the electric fields projecting from the electrodes are altered. As a result, the self-capacitance of the electrode increases.

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

Self-capacitance is typically measured by applying a known voltage to an electrode, and then using a circuit to measure how much charge flows to that same electrode. When external objects are brought close to the electrode, the electric fields projecting from the electrodes are altered.

Methodology Applied

Scientific EffectElectric Field: Electric Field

Implementation Method 3

Methodology Applied

Scientific EffectFrequency modulation: Phase Modulation

Data Source

PatentUS8836666B2Method and device for reducing noise interference in a capacitive touchscreen system

Publication Date: 2014.09.16 PIXART IMAGING INC

AI summary

Disclosed herein are various embodiments of means and methods for reducing noise interference in a capacitive touchscreen system. Second noise signals related to first noise signals generated by an external noise source are provided to a touchscreen controller, which determines fundamental and harmonic frequencies associated with the first noise signals. In response to determining the fundamental and harmonic frequencies associated with the first noise signals, the touchscreen controller may shift the frequency at which drive signals generated thereby are provided to a capacitive touchscreen. As a result, undesired interference between the first noise signals and the drive signals is minimized or avoided.