Capacitive Sensor Circuit With In-Use Synchronous Calibration
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Solution Overview
Problem
Capacitive measuring systems face challenges in measuring large capacitances and electromagnetic compatibility (EMC), particularly with temperature-sensitive components and the inability to calibrate while in use, which affects sensitivity and accuracy.
Innovation Solution
A capacitive measuring system utilizing a MOS semiconductor switch-based synchronous rectifier and a multi-channel analog multiplexer, with a mono-frequency voltage signal and a reference voltage divider for calibration, allowing continuous operation and reduced temperature influence.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If transimpedance amplifier and multiplexer are used for capacitive measurement, then measurement capability is achieved, but temperature sensitivity increases and measurement precision deteriorates
Solution Approach 1:
The patent replaces the transimpedance amplifier-based measurement system with a capacitive voltage divider and synchronous detector system. This substitution eliminates the temperature-sensitive transimpedance amplifier while maintaining measurement capability through voltage division and synchronous detection methods, thereby reducing temperature sensitivity while preserving measurement precision.
Solution Approach 2:
The patent changes the measurement parameter from current (requiring transimpedance amplifier) to voltage (usable with voltage dividers and synchronous detectors). By measuring voltage instead of current and using a capacitive voltage divider with known reference capacitor, the system achieves temperature compensation since both the reference capacitor and measured capacitor are affected similarly by temperature, allowing ratio-based measurement that cancels temperature effects.
2Measurement precision
If calibration is performed outside operational mode, then calibration accuracy is achieved, but productivity decreases due to system interruption
Solution Approach 1:
The patent enables continuous calibration during normal operational mode by incorporating a calibration capacitor that can be switched into the measurement circuit without interrupting the overall system operation. The calibration process occurs concurrently with measurement operations, maintaining both calibration accuracy and system productivity through continuous useful action.
Solution Approach 2:
The patent implements periodic calibration cycles where the calibration capacitor is intermittently switched into the circuit to perform calibration measurements. This periodic calibration approach maintains measurement accuracy over time while minimizing interruption to normal operations, as calibration occurs in scheduled intervals rather than requiring complete system shutdown.
3Measurement precision
If multi-frequency methods with charge pump are used, then large capacitance measurement capability is achieved, but electromagnetic compatibility worsens due to interference emissions
Solution Approach 1:
The patent replaces the multi-frequency charge pump method with a single-frequency capacitive voltage divider and synchronous detector system. This substitution eliminates the complex multi-frequency switching and charge pumping operations that generate electromagnetic interference, while maintaining the ability to measure large capacitances through voltage division and phase-sensitive detection at a single frequency.
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 system achieves high sensitivity and immunity to external interfering influences, enabling accurate capacitance measurement with continuous calibration and improved EMC performance.
Implementation Method 1
a synchronous rectifier, wherein the capacitive sensors are acted on by a mono-frequency voltage signal
Data Source
AI summary
A capacitive measuring system includes capacitive sensors and an evaluation circuit having a multiplexer, a synchronous rectifier, a sinusoidal signal generator, and a reference voltage divider. The capacitive sensors are acted on by a mono-frequency voltage signal generated by the sinusoidal signal generator, output signals of the capacitive sensors are transmitted in alternation to the synchronous rectifier via the multiplexer, and signal amplification of output signals of the synchronous rectifier are calibrated as a function of an activatable reference impedance. The synchronous rectifier is formed by a MOS semiconductor switch. A source-drain section of the MOS semiconductor switch forms a shunt that is controlled by the mono-frequency voltage signal. A channel of the multiplexer is provided for transmitting a calibration signal, generated by the reference voltage divider, to the synchronous rectifier alternately with the output signals of the capacitive sensors.