Capacitance Measurement Using Segmented Sense Plate and Dual Integrators
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Capacitance measurement devices integrated into mains-operated devices face noise interference from switched mode power supplies and RF noise, leading to measurement errors due to noise coupling between the device's ground potential and the body potential, particularly when measuring capacitance between human body and sensing electrodes.
Innovation Solution
The apparatus employs electrically separated sub-plates for the sense plate, using dual integrators and switching circuitry to perform parallel and simultaneous integrations at different measurement points, canceling time-variant common mode noise and time-invariant errors by interchanging sub-plate connections, and applying different reference voltages to eliminate noise, while grounding sub-plates prior to integration to ensure discharge.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Duration of action of stationary object
If capacitance measurement is performed in a mains-operated device, then the device can be powered continuously, but noise from switched mode power supplies and RF noise couples into the measurement, degrading measurement precision
Solution Approach 1:
The sense plate is divided into two electrically separated sub-plates. Each sub-plate is connected to a separate integrator, allowing independent measurement of capacitance values. This segmentation enables the system to process signals from each sub-plate separately and combine the results to cancel common-mode noise, thereby maintaining measurement precision while operating in a noisy mains-powered environment.
Solution Approach 2:
The system measures capacitance values from both sub-plates and uses the difference between these measurements to cancel common-mode noise. The output signal is generated based on the difference between the two capacitance measurements, which eliminates noise that is common to both measurements. This feedback mechanism continuously compensates for noise interference, maintaining high measurement precision.
2Measurement precision
If the sense plate is divided into electrically separated sub-plates, then noise cancellation capability is improved, but the device complexity increases due to additional integrators and switching circuitry
Solution Approach 1:
The sense plate is divided into two electrically separated sub-plates, each connected to its own integrator. This segmentation allows independent processing of signals from each sub-plate, enabling the system to cancel common-mode noise by comparing the two measurements. The additional complexity of having two integrators and switching circuitry is justified by the significant improvement in noise cancellation capability and measurement precision.
Solution Approach 2:
The system combines the measurements from both sub-plates to generate the final output signal. By merging the capacitance values from the two sub-plates and calculating their difference, the system achieves noise cancellation. The switching circuitry efficiently manages the connections between sub-plates and integrators, allowing the system to combine information from both sub-plates while maintaining a manageable level of complexity.
3Productivity
If parallel and simultaneous integration is performed for both sub-plates, then measurement speed is improved, but the switching circuitry complexity increases
Solution Approach 1:
The switching circuitry periodically switches the connections between sub-plates and integrators at two different measurement points in time. During the first measurement interval, the first integrator is connected to the first sub-plate and the second integrator to the second sub-plate. During the second measurement interval, the connections are swapped. This periodic switching enables parallel and simultaneous integration for both sub-plates, improving measurement speed while keeping the switching circuitry design systematic and manageable.
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 achieves high precision capacitance measurement by effectively canceling noise and compensating for size differences between sub-plates, resulting in improved signal-to-noise ratio and reduced measurement errors.
Implementation Method 1
performing a first parallel and simultaneous integration for electrically separate sub-plates can be used to cancel time-variant common mode noise
Implementation Method 2
performing a (subsequent) second parallel and simultaneous integration with interchanged sub-plates can be used to further cancel time-invariant errors introduced by the spatial separation of the sense plate into sub-plates
Implementation Method 3
a first integrator comprising a first integrator input terminal, a first integrator reference terminal connected to a first reference voltage, and a first integrator output terminal for providing a first output voltage indicative of a capacitance connected to the first integrator input terminal
Data Source
Figure 1~2
Figure 3~5
Figure 4
AI summary
There is described an apparatus for measuring a capacitance formed by a sense plate and a counter plate, wherein the sense plate comprises a first sub-plate and a second sub-plate, the first sub-plate and the second sub-plate being electrically separated. The apparatus comprises (a) a first integrator comprising a first integrator input terminal, a first integrator reference terminal connected to a first reference voltage, and a first integrator output terminal for providing a first output voltage indicative of a capacitance connected to the first integrator input terminal, (b) a second integrator comprising a second integrator input terminal, a second integrator reference terminal coupled to a second reference voltage, and a second integrator output terminal for providing a second output voltage indicative of a capacitance connected to the second integrator input terminal, (c) switching circuitry adapted to, at a first measurement point in time, connect the first integrator input terminal to the first sub-plate and connect the second integrator input terminal to the second sub-plate, and, at a second measurement point in time, connect the first integrator input terminal to the second sub-plate and connect the second integrator input terminal to the first sub-plate, and (d) output circuitry coupled to the first integrator output terminal and the second integrator output terminal and adapted to generate an output signal based on a difference between the first output voltage and the second output voltage, the output signal being indicative of the capacitance formed by the sense plate and the counter plate.. There is also described a fingerprint sensor and a method.