Guarded Charge-Transfer Capacitance Sensing Without Ground-Plane Parasitics
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Solution Overview
Problem
Capacitive sensors face challenges in effectively detecting measurable capacitance while minimizing the adverse effects of spurious noise signals, which can reduce sensor resolution and increase parasitic effects due to the use of ground planes and other shielding structures.
Innovation Solution
A guarded capacitance detection scheme using switched charge transfer techniques with a plurality of sensing electrodes and at least one guarding electrode, where a pre-determined voltage is applied to the sensing electrodes, and guard voltages are applied to the guarding electrode to shield the sensor from undesired electrical coupling, allowing charge sharing with a filter capacitance to determine measurable capacitance.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If ground planes or shielding structures are used to shield the sensing regions from external and internal noise signals, then noise shielding is improved, but sensor resolution is reduced and parasitic effects are increased
Solution Approach 1:
A guarding electrode is introduced as an intermediary element between the sensing electrode and the noise source. This guarding electrode is driven by a guard signal that tracks the voltage on the sensing electrode, creating an equipotential surface that shields the sensing region from noise without requiring traditional ground planes that would increase parasitic capacitance and reduce resolution.
Solution Approach 2:
The voltage on the guarding electrode is dynamically changed to track the voltage on the sensing electrode throughout the charge transfer process. By matching the guard voltage to the sensing electrode voltage, the electric field between them is minimized, preventing noise coupling while maintaining sensor resolution and avoiding parasitic effects associated with static ground planes.
2Object-affected harmful factors
If ground planes are used to prevent spurious signals, then noise interference is reduced, but parasitic capacitance is increased
Solution Approach 1:
The guarding electrode serves as an intermediary that actively manages the electric field between the sensing electrode and surrounding structures. By driving this intermediary electrode with a tracked voltage signal, noise interference is prevented without introducing the fixed parasitic capacitance that would result from using traditional ground planes.
3Reliability
If shielding structures are used to protect from spurious signals, then signal protection is improved, but sensor performance is reduced
Solution Approach 1:
The guarding electrode acts as a protective intermediary that dynamically adapts to the sensing electrode's voltage state. This active shielding approach provides reliable signal protection while maintaining sensor performance, unlike static shielding structures that would degrade performance through increased parasitic effects.
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 enhances sensor performance by efficiently detecting capacitance while reducing noise interference, allowing for precise proximity detection of objects such as fingers or styli, and can be implemented using standard ICs and microcontrollers, improving sensor resolution without increasing parasitic effects.
Implementation Method 1
a first guard voltage is applied to a guard electrode... a second guard voltage different from the first guard voltage is applied to the guard electrode... to shield the sensor from undesired electrical coupling
Implementation Method 2
charge is shared between the at least one of the plurality of sensing electrodes and a filter capacitance... to determine the measurable capacitance
Data Source
AI summary
Methods, systems and devices are described for determining a measurable capacitance for proximity detection in a sensor having a plurality of sensing electrodes and at least one guarding electrode. A charge transfer process is executed for at least two executions. The charge transfer process includes applying a pre-determined voltage to at least one of the plurality of sensing electrodes using a first switch, applying a first guard voltage to the at least one guarding electrode using a second switch, sharing charge between the at least one of the plurality of sensing electrodes and a filter capacitance, and applying a second guard voltage different from the first guard voltage to the at least one guarding electrode. A voltage is measured on the filter capacitance for a number of measurements equal to at least one to produce at least one result to determine the measurable capacitance for proximity detection.