Seat Heater Capacitive Sensor Frequency Compensation

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Solution Overview

Problem

Capacitive occupant detection systems using seat heaters as sensors face inaccuracies due to errors introduced by inductance and measurement drive voltage variations, particularly when using common mode chokes in the heater supply wires.

Innovation Solution

The system performs multiple measurements at different frequencies to isolate and calculate the unknown capacitance using complex current and voltage measurements, reducing errors by employing a formula that accounts for these variations.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If a common mode choke is used in the heater supply wires to isolate the heating circuit from the capacitive detection circuit, then the isolation between heating and sensing functions is improved, but measurement errors are introduced due to inductance and drive voltage variations

Engineering Contradiction:

Improveisolation between heating and sensing circuitsVSAvoidcapacitive detection accuracy

Core Design Contradiction:

ReliabilityVSMeasurement precision

Solution Approach 1:

The patent applies parameter changes by measuring the complex current at two different known carrier frequencies (first known carrier frequency and second known carrier frequency) rather than a single frequency. This frequency variation allows the system to distinguish between inductive effects (which vary with frequency) and capacitive effects (the target measurement), enabling error compensation and improved measurement precision while maintaining circuit isolation.

Inventive Principle:

Principle #35Parameter changes

2Device complexity

If the heating element is used as the antenna electrode for capacitive sensing, then device complexity is reduced by combining functions, but measurement accuracy deteriorates due to inductance and voltage variations in the heating circuit

Engineering Contradiction:

Improvenumber of componentsVSAvoidoccupant detection accuracy

Core Design Contradiction:

Device complexityVSMeasurement precision

Solution Approach 1:

The patent merges the heating element and the antenna electrode into a single component. The heating element serves dual purposes: generating heat through resistive heating and acting as the antenna electrode for capacitive occupancy sensing. This combination reduces device complexity by eliminating the need for separate heating and sensing elements while the multi-frequency measurement technique compensates for the resulting measurement errors.

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent uses parameter changes by performing measurements at two different known carrier frequencies to separate the inductive and capacitive components of the impedance. This allows the system to accurately determine the unknown capacitance (occupant detection signal) even when using the inductive heating element as the sensor, thereby maintaining measurement precision despite the merged design.

Inventive Principle:

Principle #35Parameter changes

3Measurement precision

If multiple measurements at different frequencies are performed to compensate for inductance errors, then measurement precision is improved, but the time required for occupancy detection increases

Engineering Contradiction:

Improvecapacitive detection accuracyVSAvoiddetection time

Core Design Contradiction:

Measurement precisionVSLoss of time

Solution Approach 1:

The patent applies periodic action by performing measurements at two different known carrier frequencies in a sequential manner. The system first measures at a first known carrier frequency, then measures at a second known carrier frequency, using the periodic alternation between frequency measurements to extract both the inductive and capacitive components. This structured periodic measurement approach enables error compensation while keeping the additional time required manageable.

Inventive Principle:

Principle #19Periodic action

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 accuracy of capacitive detection by minimizing errors caused by inductance and measurement drive voltage variations, improving the reliability of occupant presence detection.

Implementation Method 1

a heating element (10) connected between a first (21) and a second (22) node, the heating element having a positive resistance value

Methodology Applied

Scientific EffectResistive heating: Joule Heating

Implementation Method 2

the heating element (10) forms one electrode of a capacitive occupancy sensor

Methodology Applied

Scientific EffectCapacitance: Capacitance

Data Source

PatentUS9332592B2Seat heater and capacitive occupancy sensor combination

Publication Date: 2016.05.03 IEE INT ELECTRONICS & ENG SA

AI summary

In a combined seat heater and capacitive occupancy sensor, the present invention proposes to perform two measurements with two different frequencies. In a first step, the carrier frequency is set to a first known carrier frequency, the complex current through inductance and unknown capacitance and complex drive voltage are measured. In a second step, the carrier frequency is set to a second known carrier frequency, the complex current through inductance and unknown capacitance and complex drive voltage are measured. The measured complex values are used in a formula which yields the unknown capacitance.