Capacitive Sensor Group Scanning for Low-Power Touch Detection
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Solution Overview
Problem
Low power systems with capacitive sensors face high power consumption due to the need for frequent wake-ups to scan multiple sensors, as current methods do not efficiently minimize the time spent scanning capacitive sensors.
Innovation Solution
A method and apparatus that charge capacitive sensors to a first voltage, transfer electron charge to a sample and hold capacitor, and compare resulting voltages to detect capacitance changes, allowing for efficient scanning and minimizing wake-up time by using a multiplexer, sample and hold capacitor, and analog-to-digital converter.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If the system wakes up to scan all capacitive sensors individually, then capacitance changes can be detected, but the wakeup time and power consumption increase
Solution Approach 1:
The patent merges multiple capacitive sensor measurements into a single group capacitance value by connecting all sensor capacitors in parallel to a common sample-and-hold capacitor. This allows the system to scan multiple sensors simultaneously in one wakeup event rather than individually, reducing wakeup time while maintaining detection capability through comparison of total group capacitance changes between successive scans.
Solution Approach 2:
The patent creates a universal scanning approach where a single measurement circuit scans multiple capacitive sensors by grouping them together. The same hardware infrastructure (multiplexer, sample-and-hold capacitor, ADC) serves all sensors simultaneously, eliminating the need for separate scanning circuits for each sensor and minimizing the time spent in active mode.
2Measurement precision
If the system scans multiple capacitive sensors individually, then each sensor can be monitored, but the scanning time and power consumption increase
Solution Approach 1:
The patent combines multiple individual sensor measurements into a single group measurement by connecting all sensor capacitors in parallel. This merging approach allows the system to monitor all sensors simultaneously with one ADC conversion event, dramatically reducing the time the microcontroller must remain awake and thus reducing overall power consumption while maintaining the ability to detect capacitance changes across the entire sensor array.
Solution Approach 2:
The patent implements periodic scanning where the system wakes up at intervals to perform a batch scan of all capacitive sensors, then returns to sleep mode. By grouping multiple sensors into a single periodic scan event rather than continuous individual scanning, the system minimizes the duration of high-power operation while maintaining sensor monitoring capability through comparison of group capacitance values between periodic scans.
3Use of energy by stationary object
If the system minimizes wakeup time for scanning, then power consumption is reduced, but the ability to detect capacitance changes may be compromised
Solution Approach 1:
The patent merges all capacitive sensor measurements into a single group capacitance measurement taken during each wakeup event. By connecting all sensor capacitors in parallel to a common sample-and-hold capacitor, the system captures the total capacitance state of all sensors simultaneously. This merged measurement approach maintains detection accuracy for capacitance changes while minimizing wakeup time, as the entire sensor array is scanned in one operation rather than requiring multiple sequential scans.
Solution Approach 2:
The patent implements feedback by comparing the group capacitance measurement from the current scan with the measurement from the previous scan. This comparison mechanism allows the system to detect capacitance changes with high accuracy by identifying differences between successive group measurements, enabling reliable touch or proximity detection even though sensors are scanned in batch mode during brief wakeup periods.
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 reduces average power consumption by enabling quick scanning of multiple capacitive sensors while maintaining low power usage, allowing the system to efficiently detect capacitance changes and return to sleep mode.
Implementation Method 1
charging a plurality of capacitive sensors to a first voltage; charging a sample and hold capacitor to a second voltage; individually coupling each one of the plurality of capacitive sensors to the sample and hold capacitor, wherein electron charge from each of the individually coupled ones of the plurality of capacitive sensors may be transferred to the sample and hold capacitor
Data Source
Figure 1(a)
Figure 1(b)
Figure 1(c)
AI summary
Relative capacitance of a plurality of capacitive sensors may be monitored by using only one ADC conversion. A plurality of capacitive sensors individually charges a sample and hold capacitor. After all of the plurality of capacitive sensors have charged the sample and hold capacitor, a digital conversion of the resulting analog on the sample and hold capacitor is made and stored in a memory. This stored digital collective voltage is compared to a previously stored one and if different then a proximity/touch event may have occurred. Therefore, an entire panel of capacitive sensors may be quickly monitored for a change in the "group" capacitance thereof, or portions of the capacitive sensors may be monitored for a change in the "subgroup" capacitance thereof. By knowing which subgroup of capacitive sensors has changed its collective capacitive value, a more focused and selective capacitive sensor measurement can be made that uses less power.