Capacitive Sensor EMI Robustness via Common Mode Choke
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitive measurement circuits in vehicle applications face challenges with electromagnetic interference (EMI) during Bulk Current Injection (BCI) tests, particularly when using a heating member as an antenna electrode, which disrupts signal measurements and compliance with automotive standards.
Innovation Solution
A capacitive sensor device with a common mode choke and control/evaluation circuit, featuring inductively coupled windings and an EMI filter network, is connected between the heating member and heating current supply to inject a periodic measurement signal and measure impedance, effectively suppressing RF currents during BCI tests by providing a direct low-impedance path to AC ground.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a heating member is used as an antenna electrode for capacitive sensing, then the device can be integrated into vehicle seats with heating functionality, but the circuit becomes susceptible to electromagnetic interference during BCI tests
Solution Approach 1:
A common mode choke is introduced as an intermediary component between the heating member and the measurement circuit. The choke provides magnetic coupling for the measurement signal while blocking common mode RF currents injected during BCI tests, thus mediating between the heating function and the sensing function while protecting against EMI
Solution Approach 2:
The measurement signal path is extracted and coupled inductively through the common mode choke, separating it from the direct DC heating path. This extraction allows the measurement circuit to operate independently while the choke filters out RF interference from the heating circuit
2Reliability
If a common mode choke is used to decouple AC voltage, then EMI robustness is improved, but the device complexity increases
Solution Approach 1:
The common mode choke serves multiple functions simultaneously: it provides inductive coupling for the measurement signal, blocks common mode RF currents during BCI tests, and decouples the AC measurement voltage from the DC heating supply. This multi-functionality reduces the need for additional components
3Reliability
If RF currents are injected during BCI tests, then electromagnetic compatibility is tested, but the signal measurement is disrupted
Solution Approach 1:
The common mode choke converts the harmful RF currents injected during BCI tests into a beneficial filtering effect. The choke's magnetic core provides a low impedance path for common mode RF currents to ground, effectively filtering them out and protecting the measurement circuit while maintaining EMI compliance
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 enhances EMI robustness, ensuring accurate signal measurement and compliance with automotive standards by effectively filtering out injected RF currents during BCI tests, maintaining a compact design and minimizing magnetic stray fields.
Implementation Method 1
a common mode choke having first and second inductively coupled windings... The control and evaluation circuit is configured for injecting a periodic alternating measurement signal into the heating member via a measurement node
Implementation Method 2
an EMI filter network that is electrically connected across the signal input port and the reference input port of the electrical quantity measurement circuit... effectively suppressing RF currents during BCI tests by providing a direct low-impedance path to AC ground
Implementation Method 3
Capacitive sensors and capacitive measurement and/or detection systems employing capacitive sensors have a wide range of applications, and are among others used for the detection of the presence and/or the position of a conductive body in the vicinity of an antenna electrode
Data Source
AI summary
A capacitive sensor device configured for being connected between an electric heating member and a heating current supply and for using the electric heating member as an antenna electrode. The sensor device includes a common mode choke for connecting between the heating member and the current supply, and a control and evaluation circuit for determining an electrical impedance between the electric heating member and a counter electrode via a measurement node. The control and evaluation circuit includes: a third winding inductively coupled to the first and second windings of the common mode choke, a periodic signal voltage source directly connected to the third common mode choke winding, an electrical quantity measurement circuit configured to determine an electrical quantity across the measurement node, and an EMI filter network that is connected across a signal input port of the third winding and a reference input port connected to AC ground potential.