KR102444685B1 - Apparatus and method for determining a distance for measuring vital sign based on coherency between magnitude and phase - Google Patents

Abstract

The present invention relates to an apparatus and method for determining a vital sign measurement distance based on coherence between magnitude and phase. According to the present invention, there is provided a method for determining a distance for measuring vital signs, the method comprising: receiving a reflected signal through a radar; determining a magnitude signal and a phase signal based on the reflected signal; determining a coherency between the magnitude signal and the phase signal; and determining a distance for the vital sign measurement based on the coherency.
According to the present invention, magnitude phase coherence to quantify coherence between magnitude and phase in each range bin based on a mathematical model and experimental observation that the fluctuations in magnitude and phase are highly correlated in range bins where vital signs are present. By utilizing the herency index, if the range bin having the highest index value is selected, more accurate vital signs can be measured compared to conventional methods.

Description

Apparatus and method for determining a distance for measuring vital sign based on coherency between magnitude and phase

The present invention relates to an apparatus and method for measuring vital signs, and more particularly, to an optimal relationship between a radar and a target in order to measure vital signs using a Frequency Modulated Continuous Wave (FMCW) radar. It relates to an apparatus and method for determining a distance.

The FMCW radar can obtain both the range and displacement information of the target. Information about range and displacement resides in the spectral components of the demodulated radar signal and can be simply extracted using spectral analysis such as discrete Fourier transform (DFT). In addition, since the FMCW radar uses millimeter (mm) waves, it has the advantage of reducing power consumption and reducing the size of the pavement. Due to these characteristics, FMCW radar has been widely applied in various fields where there is a need for analysis of range and displacement, such as automobile control systems, gesture recognition, structure monitoring, and water level measurement. In particular, in the biomedical field, FMCW is used for monitoring non-contact vital signs such as respiration and heart rate. Non-contact vital sign monitoring can operate continuously without causing any inconvenience to users, making it useful for elderly home health care, prevention of sudden infant death syndrome, driver monitoring systems, and survivor searches.

The general process of extracting biosignals from FMCW radar signals mainly consists of spectral decomposition and selection of range bins. After spectral decomposition using DFT, magnitude and phase are obtained at each range bin or frequency bin. The magnitude and phase of each range bin, called a range profile, contains displacement information. To extract vital signs from a range profile, it is necessary to select a specific range bin in which vital information exists. For example, breathing is measured in a range near the abdomen or chest, and heart rate is measured in a range with thin skin, such as the neck. According to the existing method, a range bin is selected using magnitude or phase. The conventional method of selecting the range bin with the largest phase shift is vulnerable to phase noise. According to the conventional method of integrating range bins within a specific target range, the accuracy of vital signs may be low. Incorrect selection of range bins can lead to inaccurate monitoring of vital signs. Therefore, the selection of range bins is very important for accurate detection of vital signs.

The technology that is the background of the present invention is disclosed in Republic of Korea Patent Registration No. 10-1895324 (2018.09.05. Announcement).

An object of the present invention is to provide an apparatus and method for determining an optimal distance between a radar and a target in order to measure vital signs using a frequency modulated continuous wave (FMCW) radar.

According to an embodiment of the present invention, a method for determining a distance for measuring vital signs includes: receiving a reflected signal through a radar; determining a magnitude signal and a phase signal based on the reflected signal; determining coherency between the magnitude signal and the phase signal; and determining a distance for measuring vital signs based on the coherency.

The coherence between the magnitude signal and the phase signal may be determined based on the magnitude signal and the phase signal within a time window.

The coherence between the magnitude signal and the phase signal may be determined based on an absolute value of the sum of the product of the magnitude signal and the phase signal within a time window, a standard deviation of the magnitude signal, and a standard deviation of the phase signal. have.

The magnitude signal and the phase signal may be determined based on a combined signal of a signal transmitted through the radar and the reflected signal.

The magnitude signal and the phase signal may be determined from an intermediate frequency signal generated by applying a low-pass filter to the combined signal.

The magnitude signal and the phase signal may be determined from a discrete Fourier transform signal generated by performing a discrete Fourier transform on the intermediate frequency signal.

The magnitude signal may be a signal indicating the magnitude of the discrete Fourier transformed signal according to a distance, and the phase signal may be a signal indicating the phase of the discrete Fourier transformed signal according to a distance.

The distance for measuring the vital sign may be determined as a distance when the coherency is the highest.

According to an embodiment of the present invention, an apparatus for determining a distance for measuring vital signs includes: a radar receiving a reflected signal; a coherency determining unit that determines a magnitude signal and a phase signal based on the reflected signal, and determines coherency between the magnitude signal and the phase signal; and a distance determiner configured to determine a distance for measuring vital signs based on the coherency.

The coherency determiner may determine coherency between the magnitude signal and the phase signal based on the magnitude signal and the phase signal within a time window.

The vital sign measuring distance determining device further comprises a mixer for generating a combined signal by combining the signal transmitted through the radar and the reflected signal, the magnitude signal and the phase signal may be determined based on the combined signal have.

The vital sign measuring distance determining device may further include a filter configured to generate an intermediate frequency signal by low-pass filtering the combined signal, and the magnitude signal and the phase signal may be determined from the intermediate frequency signal.

The vital sign measuring distance determining device may further include a discrete Fourier transform unit configured to generate a discrete Fourier transform signal by performing a discrete Fourier transform on the intermediate frequency signal, and the magnitude signal and the phase signal may be determined from the discrete Fourier transform signal.

According to the present invention, magnitude phase coherence to quantify coherence between magnitude and phase in each range bin based on a mathematical model and experimental observation that the fluctuations in magnitude and phase are highly correlated in range bins where vital signs are present. By utilizing the herency index, if the range bin having the highest index value is selected, more accurate vital signs can be measured compared to conventional methods.

1 shows a block diagram of an apparatus for determining a vital sign distance according to an embodiment of the present invention.
2 shows an arrangement of an FMCW radar and a target according to an embodiment of the present invention.
3 shows a method of operating an apparatus for measuring vital signs according to an embodiment of the present invention.
4 shows a scene in which a biosignal is obtained from a single subject according to an embodiment of the present invention.
5 shows the magnitude and phase of a respiration measurement signal measured from a single subject according to an embodiment of the present invention.
6 shows the magnitude and phase of a heartbeat signal measured from a single subject according to an embodiment of the present invention.
7 is a graph showing the MPC of the respiratory measurement signal for the range bin according to an embodiment of the present invention.
Figure 8 shows the magnitude and phase of the respiration measurement signal according to an embodiment of the present invention.
9 is a graph showing the MPC of a heartbeat signal for a range bin according to an embodiment of the present invention.
10 shows the magnitude and phase of a heartbeat signal according to an embodiment of the present invention.
11 is a view showing average error and accuracy according to an embodiment of the present invention compared with a conventional method.

Then, with reference to the accompanying drawings, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. However, the present invention may be implemented in several different forms and is not limited to the embodiments described herein. And in order to clearly explain the present invention in the drawings, parts irrelevant to the description are omitted, and similar reference numerals are attached to similar parts throughout the specification.

The following describes an apparatus for determining a vital sign distance according to an embodiment of the present invention with reference to the drawings.

1 shows a block diagram of an apparatus for determining a vital sign distance according to an embodiment of the present invention.

As shown in FIG. 1 , the vital sign distance determination device 100 includes an FMCW radar 110 , a mixer 120 , a filter 130 , a discrete Fourier Transformer (DFT) 140 , and a magnitude phase. It includes a coherency (Magnitude-phase coherency, MPC) determiner 150 , a distance determiner 160 , and a vital sign measurer 170 . The vital sign distance determining device 100 shown in FIG. 1 may be referred to as a vital sign measuring device.

The FMCW radar 110 transmits a linearly modulated frequency signal and the FMCW radar 110 receives a signal reflected from the target.

The mixer 120 generates a combined signal by mixing the transmitted signal and the received signal.

The filter 130 generates an intermediate frequency (IF) signal by applying a low-pass filter to the combined signal.

The discrete Fourier transform unit 140 discrete Fourier transforms the intermediate frequency signal to generate a discrete Fourier transformed signal that can be expressed as in Equation (3).

The magnitude-phase coherency determining unit 150 obtains a magnitude signal and a phase signal from the discrete Fourier-transformed signal, and determines coherency between the magnitude signal and the phase signal.

The distance determiner 160 determines a distance value when coherency between magnitude and phase is the highest as an optimal distance for measuring vital signs.

The vital sign measuring unit 170 measures vital signs from the distance determined by the distance determining unit 160 using the FMCW radar 110 .

2 shows an arrangement of an FMCW radar and a target according to an embodiment of the present invention.

만큼 변한다.As shown in FIG. 3 , the FMCW radar 110 is placed at a distance of r(t) from the target. Regarding the measurement of heart rate, r(t) is the microdisplacement with heart rate change.

change as much

3 shows a method of operating an apparatus for measuring vital signs according to an embodiment of the present invention.

First, the FMCW radar 110 transmits a linearly modulated frequency signal (S101).

Next, the FMCW radar 110 receives a signal reflected from the target (S103).

The mixer 120 generates a combined signal by mixing the transmitted signal and the received signal (S105).

The filter 130 generates an intermediate frequency (IF) signal x(t,n) by applying a low-pass filter to the combined signal (S107).

The intermediate frequency signal x(t,n) may be expressed as in Equation (1).

As shown in Equation 1, the intermediate frequency signal x(t,n) is a beat frequency (f _r ) corresponding to the distance r, and a magnitude function of the signal reflected from the distance r M(t,r) , can be expressed as a phase function P(t,r) corresponding to the time delay of the radio signal reflected from the distance r. t denotes a scan index, and n denotes a sample index in a chirp signal.

The vibration frequency f _r corresponding to the distance r may be expressed as in Equation (2).

는 처프 신호의 길이(duration)를 나타내며,

는 샘플링 주파수를 나타낸다.In Equation 2, BW represents the bandwidth, c represents the speed of light,

represents the duration of the chirp signal,

represents the sampling frequency.

The discrete Fourier transform unit 140 discrete Fourier transforms the intermediate frequency signal x(t,n) to generate a discrete Fourier transformed signal X(t,r _k ) that can be expressed as in Equation 3 (S109) ).

here,

The magnitude phase coherency determiner 150 obtains the magnitude signal M(t, r _k ) and the phase signal P(t, r _k ) from the discrete Fourier transformed signal X(t,r _k ) ( S111).

The magnitude signal M(t, r _k ) and the phase signal P(t, r _k ) may be obtained according to Equation (4).

When an object exists in r _k , the magnitude function M(t, r _k ) and the phase function P(t, r _k ) may be modeled as in Equation 5.

In Equation 5, M ₀ is the magnitude of a transmission signal, and fc represents a center frequency.

로 움직인다고 가정하여, 거리 함수 r(t)를 수학식 6에 따라 정의할 수 있다.The object at r _k has a small displacement

Assuming that it moves to , the distance function r(t) may be defined according to Equation 6.

If r _k in Equation 5 is replaced with r(t) in Equation 6, Equation 5 may be changed as in Equation 7.

임을 가정하면, 수학식 7의 M(t, r_k)는 Taylor series에 의해 수학식 8과 같이 근사될 수 있다.

Assuming that , M(t, r _k ) in Equation 7 can be approximated as Equation 8 by the Taylor series.

에 존재하는 미세 변위

는 크기 함수 M(t, r_k)와 위상 함수 P(t, r_k)에 모두 반영되며, 크기 함수 M(t, r_k)와 위상 함수 P(t, r_k)는 미세 변위

에 선형적으로 비례한다. 이처럼, 크기 함수 M(t, r_k)와 위상 함수 P(t, r_k)를 이용하여 미세 변위

가 측정될 수 있다. 도 3에 도시된 바와 같이, 활력 징후 측정과 관련하여

는 호흡과 심박 등과 같은 신체 활동에 의해 발생하는 몸의 변위에 해당한다.As can be seen from Equation 7 and Equation 8,

microscopic displacements present in

is reflected in both the magnitude function M(t, r _k ) and the phase function P(t, r _k ), and the magnitude function M(t, r _k ) and the phase function P(t, r _k ) have a fine displacement

is linearly proportional to As such, fine displacement using the magnitude function M(t, r _k ) and the phase function P(t, r _k )

can be measured. As shown in Figure 3, with respect to vital sign measurement

corresponds to the displacement of the body caused by physical activities such as breathing and heart rate.

The following describes the magnitude and phase in relation to breathing and heartbeat extraction with reference to FIGS. 4 to 6 .

4 shows a scene in which a biosignal is obtained from a single subject according to an embodiment of the present invention.

As shown in FIG. 4 , a heart rate sensor 10 and a respiration sensor 20 are attached to the subject, and the FMCW radar 110 is placed at a distance r away from the subject. The heart rate sensor 10 is used to measure a reference heart rate signal, and the respiration sensor 20 is used to measure a reference respiration signal.

5 shows the magnitude and phase of a respiration measurement signal measured from a single subject according to an embodiment of the present invention.

5 , the signals of a respiration reference, magnitude and phase at r=0.700 m, magnitude and phase at r=0.550 m, magnitude and phase at r=1.100 m are shown as a function of time t . In the magnitude and phase graph, the solid line indicates the magnitude, and the dotted line indicates the phase.

As shown in FIG. 5 , the magnitude signal and phase signal at r=0.700 m have a high correlation with the reference respiration signal. However, the magnitude signal and phase signal at r=0.550 m and r=1.100 m are not associated with the reference respiration signal.

6 shows the magnitude and phase of a heartbeat signal measured from a single subject according to an embodiment of the present invention.

6 , the signals of a heartbeat reference, magnitude and phase at r=1.200 m, magnitude and phase at r=1.025 m, magnitude and phase at r=1.325 m are shown as a function of time t lose In the magnitude and phase graph, the solid line indicates the magnitude, and the dotted line indicates the phase.

As shown in FIG. 6 , the magnitude signal and phase signal at r=1.200 m have a high correlation with the reference heartbeat signal. However, the magnitude signal and phase signal at r=1.025m and r=1.325m are not associated with the reference respiration signal.

As shown in FIGS. 5 and 6 , the measured magnitude signal and phase signal have different accuracy depending on the distance between the radar and the measurement target. Therefore, a suitable range bin selection is required.

Fig. 3 will be described again.

을 결정하기 위하여, 크기 위상 코히어런시 결정부(150)는 수학식 9에서 보여지는 바와 같은 M(t, r)과 P(t, r) 간의 코히어런시를 결정한다(S113). 수학식 7, 수학식 8, 도 5, 및 도 6에서 보여지는 바와 같이, 정확한 호흡 또는 심장 박동이 측정되는 범위 빈에서의 크기 M(t, r) 및 위상 P(t, r)은 호흡 또는 심장 박동에 의한 변위에 비례한다. r_k에서의 크기 M(t, r)과 위상 P(t, r)는 서로 높은 상관 관계를 가지며, 크기 M(t, r)과 위상 P (t, r) 사이의 코히어런시 또한 높을 것으로 예상될 수 있다. 크기 M(t, r)과 위상 P (t, r) 사이의 코히어런시를 정량화하기 위해 수학식 9와 같은 크기-위상 코히어런시(Magnitude-phase coherency, MPC) 지수가 사용될 수 있다.Appropriate distance for breathing and heart rate

To determine , the magnitude-phase coherency determiner 150 determines the coherency between M(t, r) and P(t, r) as shown in Equation 9 (S113). As shown in Equation 7, Equation 8, Figure 5, and Figure 6, the magnitude M(t, r) and phase P(t, r) in the range bin where the correct respiration or heart rate is measured is the respiration or It is proportional to the displacement caused by the heartbeat. The magnitude M(t, r) and the phase P(t, r) at r _k have a high correlation with each other, and the coherence between the magnitude M(t, r) and the phase P (t, r) is also high. can be expected to In order to quantify the coherency between magnitude M(t, r) and phase P (t, r), a magnitude-phase coherency (MPC) index as in Equation 9 may be used. .

과

은 각각 M(t,r)과 P(t,r)의 시간에 대한 표준편차이다.At this time, the time averages of M(t,r) and P(t,r) were removed in advance,

class

are the standard deviations with respect to time of M(t,r) and P(t,r), respectively.

As shown in Equation 9, the MPC index is the absolute value of the sum of the products of the magnitude signal and the phase signal within the window between time tt ₀ and time t, the standard deviation of the magnitude signal and the standard deviation of the phase signal. can be determined based on

The distance determiner 160 determines a distance value when coherency between magnitude and phase is the highest as an optimal distance for measuring vital signs ( S115 ). The optimal range bin with the highest MPC index may be determined as shown in Equation (10).

The vital sign measuring unit 170 measures the vital signs from the distance determined by the distance determining unit 160 using the FMCW radar 110 ( S117 ).

Next, with reference to FIGS. 7 to 10 , it is verified that the range bin determined according to Equation 10 is optimal in relation to breathing and heart rate extraction.

7 is a graph showing the MPC of the respiratory measurement signal for the range bin according to an embodiment of the present invention.

7, the MPC index of the respiration measurement signal has a relatively large value when the distance r is r _R1 or r _R2 , and has a relatively small value when the distance r is r _R3 or r _R4 . Specifically, the MPC index of the respiration measurement signal is 0.985 when the distance r is r _R2 , 0.962 when the distance r is r _R1 , 0.022 when the distance r is r _R3 , and 0.010 when the distance r is r _R4 .

Figure 8 shows the magnitude and phase of the respiration measurement signal according to an embodiment of the present invention.

8 , the respiration reference, magnitude and phase at r=r _R2 and MPC=0.985, magnitude and phase at r=r _R1 and MPC=0.962, r=r _R3 and MPC=0.022 A signal of magnitude and phase, r=r _R4 and MPC=0.010, is shown as a function of time t. In the magnitude and phase graph, the solid line indicates the magnitude, and the dotted line indicates the phase.

As shown in FIG. 8 , when the MPC is 0.985, the highest value, that is, when the distance r is r _R2 , the magnitude signal and the phase signal have a high correlation with the reference respiration signal. However, when the value of the MPC index is small, the magnitude signal and the phase signal have a low correlation with the reference respiration signal.

9 is a graph showing the MPC of a heartbeat signal for a range bin according to an embodiment of the present invention.

9 , the MPC index of the heartbeat signal has a relatively large value when the distance r is r _R2 or r _R4 , and has a relatively small value when the distance r is r _R1 or r _R3 . Specifically, the MPC index of the heartbeat signal is 0.887 when the distance r is r _R4 , 0.872 when the distance r is r _R2 , 0.038 when the distance r is r _R1 , and 0.016 when the distance r is r _R3 .

10 shows the magnitude and phase of a heartbeat signal according to an embodiment of the present invention.

In FIG. 10 , the heartbeat reference, magnitude and phase at r=r _R4 and MPC=0.887, magnitude and phase at r=r _R2 and MPC=0.872, and r=r _R1 and MPC=0.038. A signal of magnitude and phase, r=r _R3 and MPC=0.016, is shown as a function of time t. In the magnitude and phase graph, the solid line indicates the magnitude, and the dotted line indicates the phase.

As shown in FIG. 10 , when the MPC is 0.887, the highest value, that is, when the distance r is r _R4 , the magnitude signal and the phase signal have a high correlation with the reference respiration signal. However, when the value of the MPC index is small, the magnitude signal and the phase signal have a low correlation with the reference respiration signal.

As shown in Figures 7-10, it can be seen that the accuracy of the extracted vital signs is highly dependent on the selection of range bins. Therefore, it is possible to increase the accuracy of vital signs by selecting the range bin with the maximum MPC as the range bin for the measurement of vital signs.

The following describes the performance of an embodiment of the present invention in comparison with three conventional methods with reference to FIG. 11 .

11 is a view showing average error and accuracy according to an embodiment of the present invention compared with a conventional method.

To obtain the data of Figure 11, 18 volunteers with normal respiratory range and heart rate were selected. All subjects were well acquainted with the entire procedure of the experiment and rested at least 5 minutes before taking important measurements. 61 GHz was used as the center frequency (f _C ), 6 GHz was used as the bandwidth (BW), 128 us was used as the chirp duration ( _{T C} ), 2 MHz was used as the sampling frequency (FS ), and the scan An interval of 50 ms was used. Measurements lasted 2 minutes for each subject.

To obtain the data of Fig. 11, as three conventional methods, J.-M. The method described in the paper titled "Fmcw-radar-based vital-sign monitoring of multiple patients" published in IEEE by Munoz-Ferreras et al. (hereinafter referred to as the first conventional method), and by M. Alizadeh et al. in IEEE in 2019 The method described in the paper titled "Remote monitoring of human vital signs using mm-wave fmcw radar" (hereinafter referred to as the first conventional method) published in The method described in the paper titled "sign sensing algorithm for multiple subjects based on 24-ghz fmcw doppler radar" (hereinafter referred to as the third conventional method) was used.

As shown in FIG. 11 , compared to the three conventional methods, the measurement method according to the embodiment of the present invention shows a low average error and high accuracy in most subjects. In addition, compared to the three conventional methods, the average error and accuracy of the results measured by the measurement method according to the embodiment of the present invention have less variability depending on the subject.

Although the present invention has been described with reference to the embodiment shown in the drawings, which is only exemplary, those of ordinary skill in the art will understand that various modifications and equivalent other embodiments are possible therefrom. Accordingly, the true technical protection scope of the present invention should be defined by the technical spirit of the appended claims.

10: heart rate sensor 20: breathing sensor
100: vital sign measuring device 110: FMCW radar
120: mixer 130: filter
140: discrete Fourier transform unit 150: magnitude phase coherency measurement unit
160: distance determination unit 170: vital sign measurement unit

Claims (13)

A method for determining a distance between a radar for measuring vital signs and a target object, the method comprising:
receiving a signal reflected from the target after transmission through the radar;
determining a magnitude signal and a phase signal based on the received signal;
determining coherency between the magnitude signal and the phase signal; and
determining a distance between the radar and the target object for vital sign measurement based on the coherency;
The coherence between the magnitude signal and the phase signal is
A vital sign determined by the equation below based on the absolute value of the sum of the product of the magnitude signal and the phase signal within the time window between tt ₀ and t, the standard deviation of the magnitude signal, and the standard deviation of the phase signal How to determine the measuring distance:

here,

is the coherency,

is the time,

silver street,

is the magnitude signal,

is the phase signal,

is the standard deviation of the magnitude signal,

denotes the standard deviation of the phase signal. delete delete According to claim 1,
generating a combined signal by combining the signal transmitted through the radar and the received signal through a mixer; and
Low-pass filtering the combined signal through a low-pass filter further comprising the step of generating an intermediate frequency (intermediate frequency, IF) signal,
wherein the magnitude signal and the phase signal are determined from the intermediate frequency signal. delete 5. The method of claim 4,
The magnitude signal and the phase signal are determined from a discrete Fourier transform signal generated by discrete Fourier transform of the intermediate frequency signal. 7. The method of claim 6,
The magnitude signal is a signal indicating the magnitude of the discrete Fourier transform signal according to a distance,
wherein the phase signal is a signal representing a phase of the discrete Fourier transform signal according to a distance. According to claim 1,
The distance for measuring vital signs,
A method for determining a vital sign measurement distance, wherein the distance is determined as a distance when the coherency is highest. An apparatus for determining a distance between a radar for measuring vital signs and a target object, the apparatus comprising:
Radar for receiving a signal reflected from the target after transmitting the radar signal;
a coherency determining unit that determines a magnitude signal and a phase signal based on the received signal, and determines coherency between the magnitude signal and the phase signal; and
a distance determining unit for determining a distance between the radar for measuring vital signs and the target object based on the coherency,
The coherency determining unit,
the absolute value of the sum of the product of the magnitude signal and the phase signal within a time window between tt ₀ and t, the standard deviation of the magnitude signal, the standard deviation of the phase signal, based on the A vital sign measuring distance determining device for determining herency by the following equation:

here,

is the coherency,

is the time,

silver street,

is the magnitude signal,

is the phase signal,

is the standard deviation of the magnitude signal,

denotes the standard deviation of the phase signal. delete 10. The method of claim 9,
a mixer for generating a combined signal by combining the signal transmitted through the radar and the received signal; and
Further comprising a low-pass filter for generating an intermediate frequency (intermediate frequency, IF) signal by low-pass filtering the combined signal,
wherein the magnitude signal and the phase signal are determined from the intermediate frequency signal. delete 12. The method of claim 11,
Further comprising a discrete Fourier transform unit for generating a discrete Fourier transform signal by performing a discrete Fourier transform on the intermediate frequency signal,
wherein the magnitude signal and the phase signal are determined from the discrete Fourier transform signal.

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