Capacitive Sensor Switch with Synchronous Rectification for EMI Immunity

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Solution Overview

Problem

Capacitive sensor switches face issues with external signal interference and noise, leading to erroneous outputs and unwanted radiation, particularly in environments with high electromagnetic interference.

Innovation Solution

A capacitive proximity sensor design featuring a metallic or metalized electrode connected directly to signal generating and evaluation circuitry, incorporating a bandpass filter and overvoltage protection, along with a microcontroller-based implementation for noise suppression and accurate signal processing, which reduces phase shifts and increases sensitivity.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If an AC signal is fed into the sensor plate to enable capacitive sensing, then the sensor can detect proximity or contact of objects, but external signals and noise may change the amplitude of the AC signal leading to erroneous output signals

Engineering Contradiction:

Improvedetection accuracyVSAvoidexternal signal interference

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent uses periodic AC excitation signals at specific frequencies (e.g., 100 kHz, 200 kHz, or 1 MHz) to drive the sensor electrode. By using periodic signals with defined frequency characteristics, the system can distinguish between the intended sensing signal and random external noise through frequency-selective detection methods

Inventive Principle:

Principle #19Periodic action

Solution Approach 2:

The evaluation circuit continuously monitors the sensor electrode signal and compares it against reference values and thresholds. The circuit provides feedback control by adjusting detection parameters and triggering appropriate outputs based on the measured capacitance changes, enabling the system to adapt to varying environmental conditions and maintain reliable detection despite external interference

Inventive Principle:

Principle #23Feedback

2Reliability

If an AC signal is fed into the sensor plate, then capacitive sensing is enabled, but the signal is radiated or coupled to the environment leading to larger radiation when objects are in proximity

Engineering Contradiction:

Improvesensing capabilityVSAvoidelectromagnetic radiation

Core Design Contradiction:

ReliabilityVSObject-generated harmful factors

Solution Approach 1:

The patent employs high-frequency AC signals (in the range of 100 kHz to 1 MHz) rather than low-frequency signals. This parameter change in frequency allows the system to achieve adequate sensing performance while reducing the wavelength of radiation, thereby minimizing electromagnetic interference and radiation coupling to surrounding objects

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent acknowledges that some radiation is inevitable but converts this potential harm into a benefit by using the radiation characteristics at specific frequencies. The evaluation circuit is designed to detect and measure the capacitive coupling effects that occur through this radiation, transforming the radiated energy from a harmful interference source into a useful sensing mechanism for detecting object proximity

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

3Ease of operation

If a threshold detector and window comparator are used to detect amplitude changes, then control signals can be issued, but the circuit is complex and vulnerable to noise triggering erroneous signals

Engineering Contradiction:

Improvecontrol signal generationVSAvoidevaluation circuit complexity

Core Design Contradiction:

Ease of operationVSDevice complexity

Solution Approach 1:

The patent combines the threshold detection and window comparison functions into an integrated evaluation circuit that processes the sensor signal in a unified manner. By merging these functions, the circuit reduces the number of separate components and interconnections, thereby lowering overall complexity while maintaining the ability to generate appropriate control signals based on capacitance changes

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

The solution provides high immunity to external signals and noise, enhancing accuracy and reducing radiation, allowing the sensor switch to function reliably in industrial environments with high electromagnetic interference levels.

Implementation Method 1

an AC signal is coupled into a sensor plate... a capacitive current flows between the sensor plate and the hand

Methodology Applied

Scientific EffectCapacitive coupling: Capacitance

Implementation Method 2

filtering means, such as an output bandpass filter circuit, to reduce unwanted signal components and therefore to minimize EMI radiation

Methodology Applied

Scientific EffectFrequency filtering: Filter (electronic)

Implementation Method 3

synchronous rectifier... significantly improves background noise suppression... increases accuracy and suppression of external interference

Methodology Applied

Scientific EffectSynchronous detection: Homodyne Detection

Data Source

PatentUS11307056B2Sensor switch with spread spectrum sensing signal and synchronous rectifier

Publication Date: 2022.04.19 CAPTRON ELECTRONICS

AI summary

A capacitive sensor including a sensor electrode connected to a signal generation circuit and a signal evaluation circuit. The signal generation circuit contains a signal generator that is a noise generator or a pseudo-noise generator, the signal evaluation circuit includes a synchronous rectifier. The synchronous rectifier is connected for synchronization to the signal generator.