Capacitive Sensing Device Using Microcontroller Digital Signal Filtering
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Solution Overview
Problem
Capacitive sensing devices for detecting vehicle occupants face challenges in sensitivity and cost-effectiveness, particularly in distinguishing small changes in capacitance caused by a person's presence, and often require active components like oscillators or transistors.
Innovation Solution
A capacitive sensing device utilizing a microcontroller to generate a sinusoidal alternating reference voltage through low-pass-filtering of a digital signal, combined with a driven shield electrode and a transimpedance amplifier, to enhance sensitivity and reduce costs by eliminating the need for active components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If traditional capacitive sensing devices use active components like oscillators or transistors, then the device can generate and detect electric fields, but the device complexity and cost increase
Solution Approach 1:
The patent extracts the oscillation generation function from separate active components and integrates it into the microcontroller's digital output. The microcontroller generates a square wave signal that is then converted to a sinusoidal reference voltage through passive RC filtering, eliminating the need for dedicated oscillator circuits and transistors while maintaining detection accuracy
Solution Approach 2:
The microcontroller serves multiple functions: it generates the oscillating reference voltage through its digital output, controls the driven shield electrode, and processes measurement signals. This multi-functionality consolidates what would traditionally require separate active components into a single integrated controller, reducing overall device complexity
2Measurement precision
If the sensing device uses separate transmitting and sensing electrodes, then the sensitivity to capacitance changes improves, but the device complexity and cost increase
Solution Approach 1:
The patent introduces a driven shield electrode as an intermediary between the transmitting electrode and the sensing electrode. This driven shield is controlled by the microcontroller and serves to isolate the sensing electrode from parasitic capacitances, thereby improving sensitivity to actual occupant presence while maintaining a manufacturable structure using standard automotive seat heating elements
3Device complexity
If a digital signal is directly used as reference voltage, then the circuit is simplified, but measurement offsets increase due to non-sinusoidal waveform
Solution Approach 1:
The microcontroller generates a periodic square wave signal at a specific frequency (e.g., 100-500 kHz) that matches the expected oscillation frequency of the capacitive sensing system. This periodic digital signal is then filtered through an RC circuit to produce a sinusoidal reference voltage, maintaining both circuit simplicity and measurement accuracy through frequency-matched periodic excitation
Solution Approach 2:
The patent replaces the need for complex active filtering circuits with a simple passive RC low-pass filter. The microcontroller's digital output directly drives the RC filter, which converts the square wave into a sinusoidal reference voltage without requiring additional active components, thus maintaining circuit simplicity while improving measurement precision
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 improves sensitivity to smaller changes in capacitance, reducing measurement offsets and costs, while maintaining accurate detection of occupant presence without inducing undesirable measurement offsets.
Implementation Method 1
an antenna electrode for emitting an alternating electric field in response to an alternating voltage caused in the antenna electrode
Implementation Method 2
a low-pass filter operatively connected to the digital output for generating the alternating reference voltage by low-pass-filtering the digital signal
Implementation Method 3
the sensor comprises at least one sensing electrode at which the influence of an object or living being on the electric field is detected
Data Source
AI summary
A capacitive sensing device includes an antenna electrode for emitting an alternating electric field in response to an alternating voltage caused in the antenna electrode and a control and evaluation circuit configured to maintain the alternating voltage equal to an alternating reference voltage by injecting a current into the antenna electrode and to measure the current. The control and evaluation circuit includes a microcontroller with a digital output for providing a digital signal and a low-pass filter operatively connected to the digital output for generating the alternating reference voltage by low-pass-filtering the digital signal.