Capacitive Proximity Switch With Reference Sensing for Stable Actuation
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Solution Overview
Problem
Capacitive proximity switches face challenges in reliably detecting actuation due to aging processes and changes in ambient conditions such as humidity and temperature, leading to unreliable output signals.
Innovation Solution
Incorporating a reference sensor surface to generate a reference signal that accounts for ambient conditions, with the sensor and reference sensor surfaces on a common substrate, and using a clock signal to determine capacitance changes, allowing for early detection of actuation and reduced plausibility queries in output signal evaluation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a capacitive proximity switch uses a simple sensor surface without reference compensation, then the device complexity is low, but the reliability of actuation detection deteriorates due to aging processes and ambient condition changes
Solution Approach 1:
A reference sensor surface is introduced as an intermediary element that measures ambient conditions and aging effects without being actuated by the user. This reference surface serves as a mediator to compensate for environmental influences on the main sensor surface, improving detection reliability without requiring complex external compensation systems.
Solution Approach 2:
The reference sensor surface is designed as a copy of the main sensor surface in terms of its physical and electrical characteristics, but it is electrically isolated from the front side of the cover plate. This copying approach allows the reference surface to track ambient condition changes and aging effects that would otherwise affect the main sensor, enabling compensation while maintaining simple device architecture.
2Measurement precision
If the reference sensor surface is placed close to the sensor surface on the same substrate, then the reference signal accurately represents parasitic capacitances, but the risk of intentional or unintentional actuation of the reference surface increases
Solution Approach 1:
The sensor system is segmented into two functionally distinct surfaces: the main sensor surface that detects user actuation and the reference sensor surface that measures ambient conditions. By spatially separating their evaluation paths and using different electrical connections, the system achieves precise reference measurement while preventing harmful actuation of the reference surface.
Solution Approach 2:
The air layer between the substrate and the front side of the cover plate acts as an electrical intermediary that insulates the reference sensor surface from charges at the front side of the cover plate. This intermediary layer prevents user-induced actuation of the reference surface while allowing it to accurately measure parasitic capacitances through its proximity to the main sensor surface.
3Speed
If a high frequency clock signal is used for capacitance measurement, then the interrogation frequency is sufficient for reliable actuation detection, but the requirement for separate frequency generator components increases device complexity
Solution Approach 1:
The microprocessor is designed to serve multiple functions: it controls the semiconductor switch, processes sensor signals, and generates the high-frequency clock signal required for capacitance measurement. By making the microprocessor universal and capable of generating the clock signal internally, the system achieves high interrogation frequency without adding separate frequency generator components, thus maintaining low device complexity.
Solution Approach 2:
The clock signal generation function is merged with the microprocessor's existing output capabilities. Rather than using a separate frequency generator, the system combines the clock signal generation with the microprocessor's digital output functions, reducing the total number of components while maintaining sufficient interrogation frequency for reliable actuation detection.
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 ensures reliable detection of actuation by accounting for environmental influences and aging effects, improving the quality of output signals and reducing the need for additional components, while ensuring the appliance is automatically switched off if the proximity switch's integrity is compromised.
Implementation Method 1
The sensor surface forms a plate of an open-type capacitor, the capacitance of which is dependent on the distance from a second plate (e.g. ground), which distance is altered by the proximity of a finger of the user for example.
Implementation Method 2
a reference sensor surface for generating a reference signal for determining an actuation state of the proximity switch, signal components of the reference signal being proportional to the capacitance of a reference capacitor formed with the reference sensor surface, and the capacitance of the reference capacitor being determined by ambient conditions of the sensor surface
Data Source
AI summary
A capacitive proximity switch has an electrically conductive sensor surface, which is covered by an electrically non-conductive covering plate and which serves as a part of a capacitor with a capacitance that varies with proximity. The proximity switch includes a reference sensor surface for generating a reference signal for determining an actuation state of the proximity switch. Signal portions of the reference signal are proportional to the capacitance of a reference capacitor formed with the reference sensor surface, and the capacitance of the reference capacitor is determined by surrounding conditions of the sensor surface.