Offset Signal Phase Matching for Capacitive Contact Sensing
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Solution Overview
Problem
Capacitive sensors face challenges in detecting small changes in signals due to contact with objects, making it difficult to accurately determine presence or proximity, especially when the changes in amplitude or phase are minimal.
Innovation Solution
A system that includes a waveform generator, an R-2R ladder circuit with adjustable resistors and capacitors, and a differential amplifier to generate and adjust an offset signal that matches the amplitude and phase of the sensed signal, allowing for improved detection by subtracting and amplifying the difference between the offset and sensed signals.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a capacitive sensor is used to detect object contact, then the sensor can sense contact between an object and a surface, but the changes in amplitude or phase of the signal are minimal making accurate detection difficult
Solution Approach 1:
The patent introduces an intermediary signal processing circuit that includes a reference signal generator, a phase shifter, and a mixer. The reference signal generator creates a reference signal that is a copy of the original excitation signal. The phase shifter adjusts the phase of this reference signal to match the phase of the sensed signal. The mixer then combines the sensed signal with the phase-adjusted reference signal to produce an output signal whose amplitude directly represents the contact detection result. This intermediary processing chain transforms the difficult-to-detect small amplitude/phase changes into a clearly detectable amplitude modulation in the final output signal.
Solution Approach 2:
The patent employs dynamic phase adjustment through a phase shifter that can adaptively change the phase of the reference signal in real-time. This dynamic adjustment ensures that the reference signal remains synchronized with the sensed signal even when phase drift occurs due to temperature changes or other environmental factors. The system continuously monitors and adjusts the phase relationship, transforming a static reference signal approach into a dynamic adaptive system that maintains optimal detection conditions.
2Reliability
If the amplitude and phase of the signal are used to determine contact, then contact information can be obtained, but the signal changes are too small to be reliably detected
Solution Approach 1:
The mixer acts as an intermediary that performs coherent detection by multiplying the sensed signal with the reference signal. When the phases are aligned, the multiplication produces a large DC component plus a double-frequency component. When phases are misaligned, the DC component is reduced. This coherent detection process amplifies the subtle phase and amplitude differences into significant variations in the DC level of the output signal, making reliable detection possible.
Solution Approach 2:
The system applies excessive phase adjustment capability by providing a phase shifter with a wide adjustment range that exceeds what would be minimally required. This ensures that even under varying environmental conditions, the system can always find and maintain the optimal phase alignment point. The over-engineered phase adjustment capability guarantees reliable operation across all expected operating conditions.
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 solution enhances the detection of object contact and proximity by ensuring the offset signal aligns with the sensed signal, allowing for accurate determination of contact or proximity based on amplitude and phase differences, even when changes are small, thereby improving the sensitivity and reliability of capacitive sensing.
Implementation Method 1
an R-2R ladder circuit having a constant output resistance that is coupled to an output of the waveform generator and is configured to generate the offset signal based on the waveform
Implementation Method 2
an adjustable capacitor that is connected between an output of the R-2R ladder circuit and a reference voltage... adjusting amplitude and phase of the offset signal
Implementation Method 3
a capacitive sensor configured to sense contact between an object (e.g., a finger) and a surface, such as a surface of the electronic device
Data Source
AI summary
A system for generating an offset signal for a capacitive sensor includes a waveform generator configured to generate a waveform, an R-2R ladder circuit having a constant output resistance that is coupled to an output of the waveform generator and is configured to generate the offset signal based on the waveform, and an adjustable capacitor that is connected between an output of the R-2R ladder circuit and a reference voltage. The capacitive sensor includes one or more sensing electrodes coupled to the waveform generator through a capacitance and is configured to generate a sensed signal by sensing an object. The R-2R ladder circuit and the adjustable capacitor are configured to adjust amplitude and phase of the offset signal such that the amplitude and the phase of the offset signal respectively coincide with an amplitude and a phase of the sensed signal when the sensing electrodes do not sense the object.