Guarded Charge Transfer Capacitance Sensing Against Parasitic Noise

Solution Overview

Problem

Capacitance sensors face challenges in effectively detecting measurable capacitance while minimizing the adverse effects of spurious noise signals, which can reduce sensor resolution and increase parasitic effects due to the use of ground planes and other shielding structures.

Innovation Solution

A guarded capacitance detection scheme using switched charge transfer techniques with a plurality of sensing electrodes and at least one guarding electrode, where a charge transfer process involves applying pre-determined voltages and guard voltages to shield the sensor from noise, allowing charge sharing between sensing electrodes and a filter capacitance to determine measurable capacitance.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Object-affected harmful factors

If ground planes and shielding structures are used to shield the sensing regions from external and internal noise signals, then the sensor is protected from spurious noise, but sensor resolution is reduced and parasitic effects are increased

Engineering Contradiction:

Improvespurious noiseVSAvoidsensor resolution

Core Design Contradiction:

Object-affected harmful factorsVSMeasurement precision

Solution Approach 1:

A guarding electrode is introduced as an intermediary element positioned between the sensing electrode and the ground plane. This guarding electrode is driven by a guard signal that tracks the voltage on the sensing electrode, creating an equipotential surface that eliminates electric field lines between the sensing electrode and ground plane, thereby eliminating parasitic capacitance while maintaining noise shielding

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The guard signal is designed to track the voltage on the sensing electrode, making the guarding electrode equipotential with the sensing electrode. This eliminates potential differences between the sensing electrode and surrounding structures, removing parasitic capacitance effects while maintaining electromagnetic shielding against noise

Inventive Principle:

Principle #12Equipotentiality

2Object-affected harmful factors

If ground planes and shielding structures are used to shield the sensing regions from external and internal noise signals, then the sensor is protected from spurious noise, but parasitic effects are increased

Engineering Contradiction:

Improvespurious noiseVSAvoidparasitic effects

Core Design Contradiction:

Object-affected harmful factorsVSObject-generated harmful factors

Solution Approach 1:

A guarding electrode is introduced as an intermediary element positioned between the sensing electrode and the ground plane. This guarding electrode is driven by a guard signal that tracks the voltage on the sensing electrode, creating an equipotential surface that eliminates electric field lines between the sensing electrode and ground plane, thereby eliminating parasitic capacitance while maintaining noise shielding

Inventive Principle:

Principle #24Intermediary (Mediator)

Solution Approach 2:

The guard signal is designed to track the voltage on the sensing electrode, making the guarding electrode equipotential with the sensing electrode. This eliminates potential differences between the sensing electrode and surrounding structures, removing parasitic capacitance effects while maintaining electromagnetic shielding against noise

Inventive Principle:

Principle #12Equipotentiality

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 method enhances sensor performance by efficiently detecting capacitance while reducing the impact of spurious noise, improving resolution, and eliminating the need for additional active electronics, using readily available components like standard ICs and microcontrollers.

Implementation Method 1

a first guard voltage is applied to a guard electrode... a second guard voltage different from the first guard voltage is applied to the guard electrode

Methodology Applied

Scientific EffectElectrostatic shielding: Faraday Cage

Implementation Method 2

charge sharing between the at least one of the plurality of sensing electrodes and a filter capacitance

Methodology Applied

Scientific EffectCharge transfer: Capacitance

Data Source

PatentUS7417441B2Methods and systems for guarding a charge transfer capacitance sensor for proximity detection

Publication Date: 2008.08.26 SYNAPTICS INC

AI summary

Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.