Capacitive Stylus Pressure Sensing Circuit

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Solution Overview

Problem

Existing styluses face challenges in accurately discriminating between hover and touch states due to limitations in pressure sensing mechanisms, particularly in capacitive coupling technologies, which affect the precision of input detection on digitizers.

Innovation Solution

The implementation of a stylus with a capacitive pressure sensing mechanism using a variable capacitor that changes capacitance in response to applied pressure, combined with a circuit design that includes a transimpedance amplifier and analog-to-digital converter, allows for improved discrimination between hover and touch states by modulating the signal output based on capacitance changes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a capacitive pressure sensing mechanism is used in the stylus, then the sensitivity and accuracy of pressure detection is improved, but the device complexity increases due to the need for variable capacitor and signal processing circuitry

Engineering Contradiction:

Improvepressure detection accuracyVSAvoidstylus structure complexity

Core Design Contradiction:

Measurement precisionVSDevice complexity

Solution Approach 1:

The patent changes the electrical parameter (capacitance) of the stylus tip in response to pressure changes. A variable capacitor is used where the capacitance value changes based on the applied pressure, allowing the stylus to encode pressure information through parameter variation rather than complex mechanical structures

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent replaces traditional mechanical pressure sensing mechanisms with an electrical capacitive sensing system. Instead of using mechanical springs, levers, or contact pressure systems, the invention uses changes in capacitance values to detect and measure pressure, substituting mechanical detection with electrical field-based detection

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Reliability

If the stylus tip is electrically connected to the housing during touch, then the signal output is enhanced for better detection, but the ability to discriminate between hover and touch states is reduced

Engineering Contradiction:

Improvesignal detection reliabilityVSAvoidhover vs touch discrimination

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The patent makes the electrical connection state dynamic rather than static. The stylus tip's electrical connection to the housing changes dynamically based on the interaction state - during hover the tip is electrically isolated, while during touch it becomes electrically connected. This dynamic state change provides additional information for discriminating between hover and touch events

Inventive Principle:

Principle #15Dynamics

Solution Approach 2:

The patent uses the electrical connection state as an intermediary mechanism to convey information about the interaction type. The variable capacitor and its connection state act as a mediator that translates physical interaction (hover vs touch) into distinguishable electrical signals that the digitizer can interpret

Inventive Principle:

Principle #24Intermediary (Mediator)

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 solution enhances the sensitivity and accuracy of pressure detection, enabling precise differentiation between hover and touch states, and can be adapted for use in various environments, including those without existing constraints.

Implementation Method 1

a capacitive pressure sensing mechanism using a variable capacitor that changes capacitance in response to applied pressure

Methodology Applied

Scientific EffectCapacitance: Capacitance

Implementation Method 2

a circuit design that includes a transimpedance amplifier and analog-to-digital converter

Methodology Applied

Scientific EffectTransimpedance amplification:

Implementation Method 3

a circuit design that includes a transimpedance amplifier and analog-to-digital converter

Methodology Applied

Scientific EffectAnalog-to-digital conversion:

Data Source

PatentUS9874951B2Stylus for operating a digitizer system

Publication Date: 2018.01.23 MICROSOFT TECHNOLOGY LICENSING LLC

AI summary

An apparatus includes a housing, a tip configured to move with respect to the housing based on contact pressure applied the tip, and a circuit. The circuit detects pressure applied on the tip based on movement of the tip. The circuit includes a variable current source configured to drive a sensor, a transimpedance amplifier (TIA) configured to receive current from the sensor and an analog to digital converter configured to sample output from the TIA.