Wrist-worn PPG Sensor Asymmetric Gap Design
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Solution Overview
Problem
Wrist-worn bio-signal measurement devices face challenges in accurately detecting Photoplethysmography (PPG) signals due to low signal strength and noise interference from user movement, particularly when measuring oxygen saturation, which requires higher signal quality.
Innovation Solution
The device optimizes the arrangement of light-emitting and receiving devices with a specific separation distance and structural design, including a light barrier and lens, to enhance signal quality and accuracy by minimizing noise and maximizing light reception from the wrist area, where the light source and receiver are positioned over the radial side and separated by distinct gaps.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the light source and light receiving unit are placed close together to reduce device size, then device complexity is reduced, but measurement precision deteriorates due to increased noise interference
Solution Approach 1:
The patent applies asymmetry by setting different gap distances: the light source is positioned with a first gap (0.5-2.0mm) from the cover, while the light receiving unit is positioned with a second gap (2.0-5.0mm) from the cover. This asymmetric arrangement optimizes light emission efficiency while maximizing noise rejection at the receiving end, resolving the contradiction between compact structure and measurement precision.
Solution Approach 2:
The patent implements local quality by creating distinct optical environments for the light source and light receiving unit. The light source area allows closer proximity to the body surface for efficient light emission, while the light receiving unit area maintains a larger gap to reduce noise interference. This localized optimization of gap distances improves measurement precision without significantly increasing device complexity.
2Measurement precision
If the light receiving unit is positioned closer to the body surface to increase signal strength, then measurement precision improves, but object-affected harmful factors worsen due to increased movement noise
Solution Approach 1:
The patent introduces the cover as an intermediary element between the light receiving unit and the body surface. The cover maintains a controlled second gap (2.0-5.0mm) that serves as an optimal distance: close enough to capture sufficient light signal for measurement precision, but far enough to reduce movement-induced noise. This intermediary spacing resolves the contradiction between signal strength and noise reduction.
3Measurement precision
If the first gap is made larger to reduce light absorption by tissue, then measurement precision improves, but use of energy worsens due to reduced light intensity reaching the detector
Solution Approach 1:
The patent uses asymmetric gap design where the first gap (light source to cover) is smaller (0.5-2.0mm) than the second gap (light receiving unit to cover) (2.0-5.0mm). This asymmetry optimizes energy efficiency at the emission side while prioritizing noise reduction at the reception side. The smaller first gap ensures sufficient light intensity reaches the body and returns to the detector, while the larger second gap filters noise, resolving the energy-quality contradiction.
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 configuration improves the quality of PPG signal measurements by 7 to 10 times compared to conventional devices, enabling more accurate detection of oxygen saturation and other bio-signals, even in low signal conditions.
Implementation Method 1
A PPG signal, which is detected based on optical characteristics of a measuring device, shows a heart rate synchronized with cardiac impulses. A pressure generated by cardiac impulses makes blood flow through blood vessels.
Implementation Method 2
when a light source of a PPG measuring sensor emits a light into a part of body, some of the light is absorbed into blood, bone, and/or tissue while the other of the light is penetrated or reflected so as to enter into a light receiving device
Implementation Method 3
a first optical structure extending from the light source toward the light receiving unit, the first optical structure guiding the light from the light source toward the light receiving unit
Data Source
AI summary
An apparatus for measuring a bio-signal includes: a light source emitting a predetermined amount of light into a human body; a light receiving unit receiving at least some of the predetermined amount of light; and a cover, which is touchable by the human body, protecting the light source and the light receiving unit. The light source and the cover are arranged such that the light source and the cover are separated by a first gap, the light receiving unit and the cover are arranged such that the light receiving unit and the cover are separated by a second gap, and the first gap is less than the second gap.