Capacitive Occupant Detection with Interference Discrimination
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Solution Overview
Problem
Capacitive occupant detection systems face interference from electronic noise, which can lead to incorrect occupancy readings, potentially triggering or disabling airbags inappropriately.
Innovation Solution
A capacitive occupant detection system that includes an antenna electrode and a sine signal generator, with a current measurement circuit to measure current components in phase and 90°-phase-offset with the sine voltage signal, and a control and evaluation unit to determine occupancy states. The system operates in both interference detection and occupant detection modes, using different amplitude settings to distinguish between interference and actual occupancy signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitive sensing is used to detect occupancy, then occupancy detection capability is achieved, but electronic interference causes incorrect readings
Solution Approach 1:
The system uses periodic amplitude modulation of the sensing signal at a specific frequency (e.g., 1 kHz). By modulating the signal amplitude periodically and detecting the modulated response, the system can distinguish between actual occupancy signals and random electronic noise, thereby improving measurement precision in the presence of interference.
Solution Approach 2:
The system changes the amplitude parameter of the sensing signal dynamically. By varying the signal amplitude and observing the response characteristics, the system can detect changes indicative of occupancy while filtering out constant or random interference patterns, thus maintaining accurate occupancy detection despite electronic noise.
2Measurement precision
If signal amplitude is increased to improve detection range, then detection sensitivity is improved, but susceptibility to interference increases
Solution Approach 1:
Instead of using a constant high-amplitude signal that is more susceptible to interference, the system applies periodic amplitude modulation. This allows the system to achieve detection sensitivity through the modulated signal pattern rather than raw amplitude, reducing susceptibility to broad-spectrum electronic interference while maintaining the ability to detect occupancy.
3Productivity
If continuous occupancy monitoring is performed, then real-time detection is achieved, but false triggers from interference increase
Solution Approach 1:
The system performs continuous monitoring using periodically modulated signals. The periodic nature of the modulation creates a distinctive signal signature that can be easily distinguished from random interference through synchronous detection, allowing real-time monitoring without increasing false triggers from electronic noise.
Solution Approach 2:
The system uses feedback mechanisms where the modulated signal response is continuously monitored and compared against expected patterns. When the modulated response matches the expected occupancy pattern, detection is confirmed; when it deviates (indicating interference), the system can filter out false readings, maintaining both real-time detection and reliability.
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
Effectively differentiates between electronic interference and actual occupancy, ensuring accurate detection and reducing false triggers or disabling of airbags by interlacing interference detection with occupant detection modes and adjusting signal amplitudes to maintain system reliability.
Implementation Method 1
the sine signal generator being operatively coupled to the antenna electrode to apply the sine voltage signal to the antenna electrode. When the sine signal voltage is applied to the antenna electrode, the latter radiates an oscillating electric field into its surroundings
Implementation Method 2
A capacitive sensor generally comprises at least one antenna electrode, to which is applied an oscillating electric signal and which thereupon emits an electric field into a region of space proximate to the antenna electrode, while the sensor is operating. The sensor comprises at least one sensing electrode—which could comprise the one or more antenna electrodes themselves—at which the influence of an object or living being on the electric field is detected.
Implementation Method 3
A current measurement circuit is configured to measure current signals occurring in the antenna electrode or in a separate sensing electrode at the same frequency as the sine signal. The measured current signals may e.g. comprise amplitude (maximum extent of deviation from the mean value of an alternating signal) and phase of the current or a first and a second current component of the current, the first and second current components being out of phase with one another by a phase difference which is not 180° or an integer multiple thereof.
Data Source
AI summary
A capacitive occupant detection system (10) comprises a sine signal generator (12) to apply a sine voltage signal to an antenna electrode (14) and a current measurement circuit (18, 20, 28, 30, 40) to measure current signals, based upon which a control and evaluation unit determines and outputs an occupancy state. The signal generator (12) is coupled to the antenna electrode via an amplitude adjustment stage (13), configured to adjust the amplitude of said sine voltage signal applied to said antenna electrode to an amplitude selected among at least two discrete amplitudes. The control and evaluation circuit selects one of the discrete amplitudes at a time and causes the amplitude adjustment stage to adjust the amplitude of said the voltage signal applied to the antenna electrode accordingly. The control and evaluation circuit carries out an interference detection mode and an occupant detection mode. In interference detection mode, the current signals are measured while a low one of the at least two discrete amplitudes is selected and it is determined whether the antenna electrode is exposed to interference. In occupant detection mode, the current signals are measured while a high one of the at least two discrete amplitudes is selected and occupancy state is determined based upon the current signals thus measured.