Capacitance Sensing Multiphase Procedure for EMI Reduction
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Solution Overview
Problem
Existing capacitance sensing systems for user interface devices, such as touchscreens and trackpads, face inaccuracies due to noise and fluctuations in drive voltages and current sources, leading to inaccurate touch detection and positioning, especially when trying to distinguish between liquid and intentional touches, and they generate significant electromagnetic emissions.
Innovation Solution
A multiphase sensing procedure that simultaneously measures mutual and self capacitances using a combined approach, reducing electromagnetic emissions by a factor of 100 and eliminating the need for separate scanning procedures, allowing for accurate distinction between liquids and intentional touches without additional shielding.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If separate self capacitance and mutual capacitance scanning procedures are used, then measurement completeness is improved, but measurement time and update rate are worsened
Solution Approach 1:
The patent combines self capacitance and mutual capacitance scanning into a single integrated scanning procedure. The same electrode scanning process simultaneously measures both self capacitance (capacitance of each electrode to ground) and mutual capacitance (capacitance between electrode pairs), eliminating the need for separate scanning passes and doubling the update rate without additional hardware complexity.
2Object-affected harmful factors
If additional shielding is added to reduce electromagnetic emissions, then electromagnetic interference is reduced, but device complexity and cost are worsened
Solution Approach 1:
The patent employs periodic alternating polarity scanning, where the polarity of the scanning signal alternates between positive and negative cycles. This periodic action causes electromagnetic emissions to cancel themselves out over complete scan cycles, reducing net electromagnetic interference without requiring additional shielding hardware. The same scanning process that measures capacitance also generates the cancellation effect.
3Productivity
If noise and fluctuations in drive voltages and current sources are present, then measurement speed is maintained, but measurement accuracy is worsened
Solution Approach 1:
The patent measures both self capacitance and mutual capacitance values and uses feedback processing to distinguish actual touch events from noise and fluctuations. By comparing changes in self capacitance (which remain relatively stable) against changes in mutual capacitance (which change during touches), the system can filter out measurement noise and drive voltage fluctuations, maintaining accuracy without sacrificing measurement speed.
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 solution provides accurate and efficient touch detection with reduced electromagnetic interference, enabling reliable operation in applications like vehicles where low emissions are critical, and allows for faster update rates without the need for additional shielding or separate scanning processes.
Implementation Method 1
Capacitance sensing systems function by sensing electrical signals generated on electrodes that reflect changes in capacitance. Such changes in capacitance can indicate a touch event or the presence of a conductive object, such as a finger, near the electrodes.
Data Source
AI summary
A capacitance sensing method includes generating a first set of currents by, for each transmit (TX) electrode of a set of TX electrodes, precharging a self capacitance of the TX electrode and a mutual capacitance between the TX electrode and a receive (RX) electrode of a set of RX electrodes by applying to the TX electrode a first excitation voltage corresponding to the TX electrode to induce a first current of the first set of currents, generating a second set of currents by, for each TX electrode, applying a reference voltage to the TX electrode to induce a second current of the second set of currents, and for each TX electrode, calculating a measure of the self capacitance of the TX electrode based on the second set of currents, and calculating a measure of the mutual capacitance between the TX electrode and each RX electrode based on the first set of currents.