Phase-Locked Loop for Contactless Vital Sign Detection
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Solution Overview
Problem
Existing contactless vital sign monitoring technologies, such as Doppler radar, face challenges in accurately detecting heart and respiratory rates due to the small angle approximation and null points, which are sensitive to subject distance and chest wall movements, and require specific conditions that are difficult to maintain.
Innovation Solution
A method and device using a phase-locked loop to track the phase and frequency variations of the reflected signal, allowing for the detection of heart and respiratory rates without relying on the small angle approximation or avoiding null points, by mixing the reflected signal with a reference signal and adjusting the phase or frequency of the reference signal to lock onto the vital sign signals, thereby providing a demodulated output indicative of the heart and respiratory rates.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If Doppler radar is used for contactless vital sign monitoring, then heart rate and respiratory rate can be detected, but the detection accuracy deteriorates when subject distance or chest wall movements cause violations of the small angle approximation
Solution Approach 1:
The patent employs a phase-locked loop (PLL) that dynamically tracks the phase and frequency of the reflected signal, allowing the system to adapt to varying subject distances and chest wall movements. The PLL continuously adjusts its phase and frequency to remain locked onto the vital sign carrying signal, making the system robust against dynamic changes in subject position and movement that would violate the small angle approximation in conventional Doppler radar.
Solution Approach 2:
The phase-locked loop implements a feedback mechanism where the detected phase and frequency information is fed back to continuously adjust the local oscillator signal. This feedback ensures that the system maintains accurate tracking of the vital sign signals even when subject distance or chest wall movements cause phase shifts that would normally violate the small angle approximation, thereby preserving detection accuracy under varying conditions.
2Device complexity
If conventional Doppler radar operates without phase-locked loop, then system complexity is reduced, but detection reliability deteriorates due to null points and requirement for specific operational conditions
Solution Approach 1:
The phase-locked loop implements a feedback mechanism where the detected phase and frequency information is fed back to continuously adjust the local oscillator signal. This feedback ensures that the system maintains accurate tracking of the vital sign signals even when subject distance or chest wall movements cause phase shifts that would normally violate the small angle approximation, thereby preserving detection accuracy under varying conditions.
Solution Approach 2:
The phase-locked loop acts as an intermediary system between the transmitted signal and the detection process. It mediates the relationship by continuously adjusting the phase and frequency of the local oscillator to match the reflected signal, thereby eliminating the problems of null points and sensitivity to operational conditions while adding manageable complexity to the system.
3Reliability
If phase-locked loop is used to track vital sign signals, then detection robustness against random movements improves, but device complexity increases
Solution Approach 1:
The patent employs a phase-locked loop (PLL) that dynamically tracks the phase and frequency of the reflected signal, allowing the system to adapt to varying subject distances and chest wall movements. The PLL continuously adjusts its phase and frequency to remain locked onto the vital sign carrying signal, making the system robust against dynamic changes in subject position and movement that would violate the small angle approximation in conventional Doppler radar.
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
Enables accurate detection of heart and respiratory rates even in the presence of chest wall random movements and varying subject distances, without the need for specific operational conditions, improving the reliability and robustness of vital sign monitoring.
Implementation Method 1
By the Doppler effect, the radio frequency signal reflected by the moving tissue of the target undergoes a frequency shift proportional to the surface velocity of the tissue.
Implementation Method 2
A method and device using a phase-locked loop to track the phase and frequency variations of the reflected signal, allowing for the detection of heart and respiratory rates
Data Source
AI summary
A method of detecting a vital sign comprising at least one of a heart rate and a respiratory rate of a subject is provided. In one aspect, the method includes transmitting a radio frequency signal towards the subject; and receiving a reflected signal from the subject, wherein the transmitted signal is reflected by the subject and Doppler-shifted due to at least one of the heart rate and the respiratory rate to form the reflected signal. The method also includes mixing the reflected signal with a first reference signal; and providing a vital sign carrying signal based on the mixing to a first input of a phase or frequency comparator. The method further includes generating an adjustable second reference signal and providing the reference signal to a second input of the phase or frequency comparator; and generating an output signal, by the phase or frequency comparator. The method includes varying at least one of a phase and a frequency of the adjustable second reference signal based on the output signal to track a phase or frequency of the vital sign carrying signal.