Single-Sided Capacitive Force Sensing for Touch Interface Linearity

Solution Overview

Problem

Capacitive touch sensors in portable electronic devices cannot effectively detect contact force, relying on costly and non-linear force sensitive resistors, which increase device costs and complicate user interfaces.

Innovation Solution

A capacitive force sensor system using a compliance member and electrode pairs on a substrate to measure changes in capacitance, correlating force with displacement by establishing a gap between electrodes and a reference ground plane, allowing for cost-effective and precise force detection.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If force sensitive resistors are used to detect contact force, then force detection capability is achieved, but device cost increases and response linearity deteriorates

Engineering Contradiction:

Improveforce detection accuracyVSAvoiddevice cost

Core Design Contradiction:

Measurement precisionVSEase of manufacture

Solution Approach 1:

The patent replaces mechanical force sensitive resistors with a capacitive sensing system that uses electrical fields to detect force. The capacitive sensor measures changes in capacitance caused by displacement of a compliant substrate when force is applied, eliminating the need for expensive mechanical force sensitive resistors while maintaining force detection capability and improving response linearity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Measurement precision

If force sensitive resistors are used to detect contact force, then force detection capability is achieved, but response linearity deteriorates

Engineering Contradiction:

Improveforce detection accuracyVSAvoidresponse linearity

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent replaces mechanical force sensitive resistors with a capacitive sensing system that uses electrical fields to detect force. The capacitive sensor measures changes in capacitance caused by displacement of a compliant substrate when force is applied, eliminating the need for expensive mechanical force sensitive resistors while maintaining force detection capability and improving response linearity.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

3Ease of operation

If capacitive touch sensors are used, then touch location detection is achieved, but force detection capability is lost

Engineering Contradiction:

Improvetouch interface functionalityVSAvoidforce detection capability

Core Design Contradiction:

Ease of operationVSMeasurement precision

Solution Approach 1:

The patent merges capacitive touch sensing with force sensing into a single integrated system. The same capacitive electrodes used for touch location detection also measure force by detecting changes in capacitance caused by substrate displacement when force is applied. This combination allows the touch interface to provide both location and force detection capabilities simultaneously.

Inventive Principle:

Principle #5Merging (Combining)

4Adaptability or versatility

If mechanical buttons are replaced with touch sensitive screens, then device sophistication is improved, but force detection capability is lost

Engineering Contradiction:

Improveuser interface sophisticationVSAvoidforce detection capability

Core Design Contradiction:

Adaptability or versatilityVSMeasurement precision

Solution Approach 1:

The patent merges capacitive touch sensing with force sensing into a single integrated system. The same capacitive electrodes used for touch location detection also measure force by detecting changes in capacitance caused by substrate displacement when force is applied. This combination allows the touch interface to provide both location and force detection capabilities simultaneously.

Inventive Principle:

Principle #5Merging (Combining)

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

Enables accurate and cost-effective detection of contact force on touch-sensitive interfaces, eliminating the need for expensive force sensitive resistors and improving user interface reliability by integrating force and position sensing capabilities.

Implementation Method 1

The electrode pair is in electrical communication with a capacitive sensing circuit configured to measure changes in capacitance in response to compression of the compliance member

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

The compliance member establishes a relationship between the applied force and the gap height

Methodology Applied

Scientific EffectElasticity: Elasticity

Data Source

PatentUS7784366B2Single sided capacitive force sensor for electronic devices

Publication Date: 2010.08.31 GOOGLE TECHNOLOGY HOLDINGS LLC

AI summary

A capacitive force sensor (100) includes a substrate (101) having at least one electrode pair (102,103) defining a capacitance disposed thereon. The substrate (101) is fixed relative to a first plate (106). A drive circuit (104) is configured to apply a voltage relative to a circuit ground (105) to the electrode pair (102,103). The first plate (106) is separated from a second plate (107) that is coupled to circuit ground (105) by a compliance member (108,109). The compliance member (108,109) is configured to oppose a compression force (110) while allowing the first plate (106) to physically move relative to the second plate (107). A capacitive detection circuit (111) is then configured to detect a change the capacitance when the compliance member (108,109) is compressed. The compression force (110) is then determined from the change in capacitance and the spring constant of the compliance member (108,109).