Plural-Frequency Capacitive Occupancy Sensing
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Solution Overview
Problem
Capacitive occupancy sensing systems face challenges in maintaining robustness against environmental changes such as temperature, humidity, and moisture, which affect the accuracy of detecting seat occupancy conditions, particularly distinguishing between an empty seat and a seat occupied by a child restraint system or a lighter person.
Innovation Solution
The system uses a capacitive occupancy sensing system with a detection circuit that drives the antenna electrode with two different frequencies to measure conductance, susceptance, resistance, reactance, and capacitance, allowing for a threshold capacitance determination based on the difference between these values, enabling reliable occupancy state detection by comparing capacitance values at different frequencies.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-frequency capacitive sensing system is used, then the device complexity is reduced, but the measurement precision deteriorates due to inability to distinguish environmental changes from occupancy changes
Solution Approach 1:
The patent segments the measurement process by performing capacitive measurements at multiple distinct frequencies (e.g., first frequency f1 and second frequency f2). This allows the system to separate the occupancy signal from environmental interference signals, as different frequency components respond differently to various physical phenomena. The segmented frequency approach enables accurate occupancy detection while maintaining system simplicity.
Solution Approach 2:
The patent changes the measurement parameter from a single frequency to multiple frequencies. By measuring capacitance at different frequencies and analyzing the differential response, the system can distinguish between occupancy-induced changes and environmental changes. This parameter change strategy improves measurement precision without significantly increasing device complexity, as it involves adding frequency modulation to the existing capacitive sensing architecture.
2Measurement precision
If multi-frequency measurement is implemented, then the measurement precision is improved, but the use of energy increases due to multiple signal generation and processing requirements
Solution Approach 1:
The patent implements periodic switching between different measurement frequencies rather than continuous multi-frequency excitation. The system alternates between measuring at the first frequency and the second frequency in periodic intervals, which reduces the overall energy consumption compared to simultaneous multi-frequency operation. This periodic action maintains measurement precision while significantly lowering power requirements.
Solution Approach 2:
The patent uses partial action by selectively measuring at multiple frequencies only when occupancy detection is required, rather than continuously. The system can switch to single-frequency mode during periods when occupancy status is stable, thereby reducing energy consumption while maintaining adequate measurement precision when needed.
3Reliability
If environmental compensation is added to improve reliability, then the occupancy detection reliability is improved, but the device complexity increases due to additional sensors and processing circuits
Solution Approach 1:
The patent makes the capacitive sensor multi-functional by using it for both occupancy detection and environmental condition sensing. The same sensor that detects occupancy can also detect environmental changes (temperature, humidity, moisture) by analyzing the frequency-dependent capacitive response. This eliminates the need for separate environmental sensors, thereby improving reliability without increasing device complexity.
Solution Approach 2:
The system performs self-compensation for environmental changes by using the multi-frequency measurement data to automatically distinguish between occupancy-induced capacitance changes and environmental-induced changes. The processing circuit analyzes the differential response at different frequencies and automatically compensates for environmental effects, eliminating the need for external compensation mechanisms or additional sensors.
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 approach enhances the system's reliability in distinguishing between occupied and unoccupied states, even under varying environmental conditions, by using a threshold capacitance that depends on the frequency-dependent measurements, reducing errors caused by temperature and humidity.
Implementation Method 1
An oscillator applies on oscillating voltage signal to the antenna electrode, whereby a minute electric field is produced around the antenna electrode
Implementation Method 2
the measurement circuit determines the current flowing into the one or more antenna electrodes in response to an oscillating voltage being applied to them. The relationship of voltage to current yields the complex impedance or admittance of the one or more antenna electrodes
Data Source
AI summary
A plural-frequency capacitive occupancy sensing system comprises an antenna electrode and a detection circuit, which is configured to drive the antenna electrode at least with a first and a second signal at a first and a second frequency, respectively, so as to obtain at least a first and a second measurement value indicative of at least one of conductance, susceptance, resistance, reactance and capacitance between the antenna electrode and a reference node, at the first frequency and the second frequency, respectively. The detection circuit compares the capacitance between the antenna electrode and the reference node with a threshold capacitance, the threshold capacitance being derived from a difference between the first and second measurement values and/or the capacitance between the antenna electrode and the reference node being corrected based upon the difference between the measurement values. The detection circuit outputs an occupancy state signal depending on the comparison.