Impedance Measurement Circuit Using Parallel Reference Modulation
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Solution Overview
Problem
Impedance measurement systems face challenges in calibrating reference impedance without disconnecting the unknown impedance, which is essential for accurate capacitive sensing in applications like occupant detection systems.
Innovation Solution
The system periodically connects and disconnects the reference impedance in parallel to the unknown impedance using low-frequency modulation, allowing for amplitude demodulation to measure the impedance without disconnecting the unknown impedance, employing a transimpedance amplifier and demodulators to extract the impedance values.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the reference impedance is connected in parallel to the unknown impedance for calibration measurement, then the reference impedance can be measured, but the unknown impedance cannot be measured simultaneously without disconnection
Solution Approach 1:
The reference impedance is periodically connected in parallel to the unknown impedance through a switch that alternates between connecting the reference impedance for calibration measurement and disconnecting it to allow continuous measurement of the unknown impedance. This periodic switching enables both measurement goals to be achieved at different time intervals without permanent disruption to the unknown impedance measurement capability
Solution Approach 2:
The system performs preliminary calibration by measuring the reference impedance before proceeding to measure the unknown impedance. By pre-establishing the reference measurement state, the system ensures accurate calibration is in place before unknown impedance measurement begins, improving overall measurement precision while maintaining a structured measurement sequence
2Reliability
If the unknown impedance is disconnected to measure reference impedance, then reference impedance can be calibrated, but continuous monitoring of unknown impedance is interrupted
Solution Approach 1:
The switch periodically connects the reference impedance for calibration while maintaining the capability to continuously monitor the unknown impedance during non-calibration periods. This periodic action ensures reliable calibration is performed at scheduled intervals without permanently interrupting the continuous monitoring productivity of the unknown impedance measurement system
Solution Approach 2:
The system maintains continuous measurement capability by switching between reference impedance calibration and unknown impedance monitoring rather than stopping measurements entirely. The useful action of impedance measurement continues uninterrupted through the switching mechanism, ensuring both calibration reliability and measurement productivity are preserved
3Adaptability or versatility
If a switch is introduced to selectively connect reference impedance, then both reference and unknown impedance can be measured, but device complexity increases
Solution Approach 1:
The switch serves multiple functions by selectively connecting either the reference impedance or the unknown impedance to the measurement circuit. This single component enables both calibration mode and normal measurement mode, providing measurement mode flexibility without requiring separate dedicated switching mechanisms for each function, thereby limiting the increase in device complexity
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 method enables continuous measurement of the unknown impedance while calibrating the reference impedance, ensuring accurate and reliable capacitive sensing without disrupting the system's operation.
Implementation Method 1
This current is usually measured by means of a transimpedance amplifier, which is connected to the sensing electrode and which converts a current flowing into said sensing electrode into a voltage, which is proportional to the current flowing into the electrode
Implementation Method 2
A capacitive sensor generally comprises at least one antenna electrode, to which an oscillating electric signal is applied and which in response emits an electric field into a region of space proximate to the antenna electrode
Implementation Method 3
The object to be sensed modifies the capacitance between the transmit electrode and ground
Implementation Method 4
an oscillating voltage signal is applied to the transmit electrode, building up an electric field to a receive electrode, and the displacement current induced at the receive electrode is measured, whereby the displacement current may be modified by the body being sensed
Data Source
AI summary
In an impedance measurement system, a reference impedance (2) to be measured is periodically connected and disconnected by means of switch (3) in parallel to the unknown impedance (1) to be measured. The thus generated amplitude modulation of the measurement current is demodulated, and the amplitude of the demodulated signal is indicative of the reference impedance to be measured. Using a low frequency modulation of the reference impedance enables to measure the impedance of the reference impedance without disconnecting the unknown impedance.