Capacitive Sensor Switch with Synchronous Rectifier Noise Immunity
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Solution Overview
Problem
Existing capacitive sensor switches face issues with immunity to external signals and noise, radiation of unwanted signals, complexity, high power requirements, and susceptibility to electromagnetic interference, making them unsuitable for industrial environments and compact sensor switch housings.
Innovation Solution
A capacitive proximity sensor design with direct connection of the sensor electrode to signal generating and evaluation means, using an integrating synchronous rectifier, a large sensor electrode covering the housing surface, and a compact form factor to reduce phase shifts and enhance noise suppression, along with filtering and overvoltage protection to minimize interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If an AC signal is fed into the sensor plate to detect proximity, then the sensor can detect capacitive changes, but external signals and noise may change the amplitude of the AC signal leading to erroneous signals
Solution Approach 1:
The patent uses periodic square wave signals instead of continuous AC signals. The signal generator produces periodic square wave signals that are applied to the sensor plate, and the evaluation circuit measures amplitude changes during specific phases of these periodic cycles. This periodic action allows the system to distinguish between actual capacitive changes and random noise by comparing measurements taken at different times within each cycle.
Solution Approach 2:
The patent introduces an intermediary evaluation circuit that processes the raw sensor signals before producing output. This circuit includes amplitude measurement means that specifically measures the amplitude of the square wave signal, and comparison means that compares measured amplitudes against reference values. This intermediary processing stage filters out external noise and signals before they can trigger erroneous outputs.
2Measurement precision
If a complex spread spectrum capacitive sensor is used to detect presence, then detection capability is improved, but the device becomes too expensive and requires large components that do not fit in normal sensor switch housing
Solution Approach 1:
The patent extracts only the essential elements needed for capacitive sensing from complex spread spectrum systems. Instead of using full spread spectrum modulation and demodulation circuits, the invention uses a simple square wave generator and basic amplitude measurement circuitry. This extraction of core functionality maintains detection capability while dramatically reducing complexity and component size.
Solution Approach 2:
The patent employs inexpensive, simple components throughout the circuit design. The signal generator uses basic oscillators, the evaluation circuit uses simple comparators and amplitude detectors, and no expensive microprocessors or complex ICs are required. This approach creates a cost-effective sensor switch suitable for mass production while maintaining adequate detection performance.
3Measurement precision
If an AC signal is used in the sensor plate, then proximity detection is enabled, but the signal is radiated or coupled to the environment leading to larger radiation when a hand or conductive object is placed nearby
Solution Approach 1:
The patent uses periodic square wave signals with rapid transitions instead of continuous sinusoidal AC signals. The square wave nature creates a more contained electromagnetic field that radiates less energy into the environment. The periodic switching also allows the system to measure capacitive effects during specific time windows when radiation is minimized.
Solution Approach 2:
The patent changes the signal parameter from continuous AC voltage to periodic square wave voltage with controlled amplitude and frequency. By optimizing the square wave frequency and amplitude parameters, the system achieves effective capacitive sensing while minimizing electromagnetic radiation. The signal-forming means specifically shapes the square wave to balance detection sensitivity with radiation reduction.
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, reduces radiation, and increases accuracy and reliability in industrial environments, allowing for a compact and cost-effective integration into small sensor switch housings.
Implementation Method 1
An AC signal is coupled into a sensor plate... An evaluation circuit measures the amplitude of the signal... a capacitive current flows between the sensor plate and the hand
Implementation Method 2
The evaluation circuit comprises a synchronous rectifier which synchronously detects a signal from the sensor electrode
Implementation Method 3
there may be filtering means, like an output bandpass filter, to reduce unwanted signal components and therefore to minimize EMI radiation at the sensor electrode
Data Source
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AI summary
A capacitive sensor comprises a sensor electrode connected to a signal generation circuit and a signal evaluation circuit. The signal generation circuit comprising a signal generator being a noise generator or a pseudo-noise generator, the signal evaluation circuit comprising a synchronous rectifier. The synchronous rectifier is connected for synchronization to the signal generator.