Guard-Sense Capacitive Sensor Impedance Determination
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Solution Overview
Problem
Capacitive sensing circuits operating in loading mode face significant errors in determining sense impedance due to parasitic impedance between the guard and sense nodes, which existing solutions do not effectively address.
Innovation Solution
A method involving a periodic guard voltage source and a reference periodic voltage source with different amplitudes applied to the sense node, allowing the control and evaluation circuit to determine the sense reference voltage that would result in zero current, thereby calculating the sense impedance while eliminating the influence of parasitic impedance, using formulas like Zx=Zref×VgVs-Vg and adjusting reference voltages with a control loop for improved accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a guard-sense capacitive sensor operates in loading mode with conventional measurement circuits, then the sensor can detect body presence directionally, but parasitic impedance between guard and sense nodes causes significant measurement errors
Solution Approach 1:
The patent extracts and eliminates the parasitic impedance influence from the measurement system by using a bridge circuit configuration where the parasitic impedance appears symmetrically in both measurement paths, allowing it to be mathematically eliminated from the final measurement result through differential measurement techniques
Solution Approach 2:
The patent changes the measurement parameters by using frequency-domain analysis and implementing a bridge circuit that balances the measurement equation, transforming the measurement approach to one where parasitic impedance becomes a common-mode signal that can be rejected
2Ease of manufacture
If non-ideal components are used in the measuring circuit, then the circuit implementation becomes practical and cost-effective, but measurement errors increase significantly at low impedances
Solution Approach 1:
The patent implements a feedback mechanism where the measurement result is processed through a bridge circuit that continuously adjusts and balances the measurement equation, using the output signal to compensate for errors introduced by non-ideal components and achieve accurate measurements despite practical component limitations
Solution Approach 2:
The patent creates equipotential conditions in the bridge circuit configuration where opposing arms are maintained at equal potentials during balanced measurement, causing error signals from non-ideal components to cancel each other out and enabling accurate measurements with practical components
3Object-affected harmful factors
If the sense electrode is kept at the same potential as the guard electrode by injecting current, then the space between electrodes becomes free of electric field, but the injected current itself introduces measurement errors
Solution Approach 1:
The patent introduces a bridge circuit as an intermediary measurement system that indirectly measures the sense impedance without requiring direct current injection into the sense electrode, using the guard electrode as a reference and eliminating the need for direct potential equalization that causes measurement errors
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 provides accurate and efficient determination of sense impedance in loading mode by virtually eliminating parasitic impedance, reducing measurement errors and improving sensitivity and speed, without requiring costly components.
Implementation Method 1
The sensor further comprises a periodic voltage source connected to the guard node for providing, in operation, a guard voltage of a predetermined amplitude to the guard node
Implementation Method 2
The control and evaluation circuit is configured to determine the current it is injecting to the sense node and to issue an output signal which is indicative of the current injected into the sense node and thus of the sense impedance to be determined
Data Source
AI summary
A method for determining a sense impedance of a guard-sense capacitive sensor operated in loading mode, the sensor having an electrically conductive sense electrode and an electrically conductive guard electrode. The method includes: providing a periodic guard voltage to the guard electrode; sequentially providing periodic reference voltages Vrefi via a reference impedance Zref, to the sense node; for each reference voltage, determining the value Imi of the current applied to the sense node; determining the sense reference voltage Vs to be provided to the sense node, which would lead to a current with an amplitude of zero; and calculating the unknown sense impedance Zx of the sense electrode, with reference to the sense reference voltage Vs, the periodic guard voltage Vg, and the reference impedance Zref.