Capacitive Force Sensing Architecture with Differential Noise Rejection
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Solution Overview
Problem
Touchscreen devices face interference from display noise, making it difficult to discern force signals from noise, especially in multi-touch scenarios where the magnitude of display noise is greater than the force signals.
Innovation Solution
An electronic device with a capacitive force sensor system that includes a first pair of sensing elements with an air gap and a second pair with a deformable element, allowing for the detection of applied forces through changes in the air gap and deformation, and a conductive border to reduce noise interference.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a capacitive force sensing device is used to detect force inputs on a touchscreen, then force measurement capability is provided, but display noise electrically couples to the sensing device and interferes with operation
Solution Approach 1:
The patent introduces a differential sensing architecture with two capacitive sensing devices: a first device that senses both force signals and display noise, and a second device that senses only display noise. The difference between these two sensing outputs provides a cleaned force signal with display noise rejected. This intermediary approach uses an additional sensing element as a reference to mediate and eliminate the harmful noise interference.
Solution Approach 2:
The system continuously monitors the output of both capacitive sensing devices and dynamically processes the differential signal to reject display noise. The feedback mechanism allows the system to adapt to varying display noise conditions and maintain accurate force sensing by continuously comparing the two sensing channels and adjusting the differential calculation accordingly.
2Productivity
If display noise is present in the capacitive sensing system, then the system can still operate, but the magnitude of display noise is much greater than the magnitude of force signals, making it difficult to discern force signals
Solution Approach 1:
The second capacitive sensing device acts as an intermediary reference that measures only the display noise component. By subtracting this reference measurement from the first sensing device's output (which contains both force and noise), the system isolates the pure force signal. This intermediary reference channel enables the system to maintain operation while achieving precise force measurement despite large noise magnitudes.
3Device complexity
If a single capacitive sensing device is used, then the device structure is simpler, but it cannot distinguish between force signals and display noise
Solution Approach 1:
The patent segments the sensing function into two distinct capacitive sensing devices with different sensing characteristics. The first device is positioned to sense both force inputs and display noise, while the second device is configured to sense only display noise. This segmentation of sensing functions into separate channels enables the system to distinguish and separate force signals from noise through differential processing.
Solution Approach 2:
Each capacitive sensing device is positioned and configured with specific local characteristics: the first sensing device is located to capture both force and noise signals, while the second sensing device is positioned to capture only noise. This local quality differentiation in sensing configuration allows the system to achieve signal discrimination despite using relatively simple capacitive sensing elements.
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 effectively distinguishes and measures force inputs by utilizing the air gap and deformable element to correlate force with deflection, improving the accuracy of force sensing despite display noise interference.
Implementation Method 1
a first capacitive sensor comprising a first pair of sensing elements having an air gap therebetween and configured to determine a first amount of applied force on the user input surface that results in a collapse of the air gap
Implementation Method 2
a second capacitive sensor below the first capacitive sensor comprising a second pair of sensing elements having a deformable element therebetween and configured to determine a second amount of applied force on the user input surface that results in a deformation of the deformable element
Data Source
AI summary
An electronic device with a force sensing device is disclosed. The electronic device comprises a user input surface defining an exterior surface of the electronic device, a first capacitive sensing element, and a second capacitive sensing element capacitively coupled to the first capacitive sensing element. The electronic device also comprises a first spacing layer between the first and second capacitive sensing elements, and a second spacing layer between the first and second capacitive sensing elements. The first and second spacing layers have different compositions. The electronic device also comprises sensing circuitry coupled to the first and second capacitive sensing elements configured to determine an amount of applied force on the user input surface. The first spacing layer is configured to collapse if the applied force is below a force threshold, and the second spacing layer is configured to collapse if the applied force is above the force threshold.