A method and system for measuring skin impedance for fetal electrocardiogram monitoring

Abstract

The present invention discloses a skin impedance measurement method and system for fetal electrocardiogram monitoring. The method uses a three-axis gyroscope to collect the body posture information of a pregnant woman, and only starts the impedance measurement link when the pregnant woman is in a calm state. An alternating current excitation current source is started, and a preset alternating current is applied to the skin surface of the pregnant woman through the P electrode. The differential signal between the P electrode and the N electrode is obtained through a differential amplifier as the excitation response signal. The excitation response signal is preprocessed to obtain the excitation response signal after filtering out the maternal and fetal bioelectric signals. Then, the discrete Fourier transform is performed on the excitation response signal after filtering out the maternal and fetal bioelectric signals, and the complex results of the real part and the imaginary part of the fundamental frequency are solved to obtain the human skin impedance, thus solving the technical problems existing in the existing impedance measurement method at the skin-electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by using analog switch devices, resulting in a decrease in the accuracy of impedance measurement.

Description

Technical Field

[0001] The present invention relates to the technical field of electrocardiogram monitoring, and particularly to a skin impedance measurement method and system for fetal electrocardiogram monitoring. Background Art

[0002] Non-invasive fetal electrocardiogram (NI-FECG) devices can collect maternal electrocardiogram signals through electrodes arranged on the abdomen of a pregnant woman, separate the maternal electrocardiogram signals into maternal electrocardiogram signals (MECG) and fetal electrocardiogram signals (FECG), and calculate the fetal heart rate and uterine contractions therefrom. However, since the signal of FECG is very weak, mostly 10 - 50 microvolts, it is easily submerged in the interference signals of the human body, the environment, and the circuit. The low and stable impedance at the skin-electrode interface is the key to high-fidelity biological signals collected, which can provide a basis for quality assessment of maternal electrocardiogram signals and correct the results of separating maternal-fetal signals. However, on the one hand, due to the complexity of human skin, the signal quality varies greatly; on the other hand, when the electrode is attached to the skin, problems such as wrinkles and detachment are likely to occur. Therefore, a method for accurately measuring the impedance at the skin-electrode interface is needed. When a good fetal signal cannot be collected due to excessive interface impedance, the impedance information can be fed back to the user to prompt the user to check to ensure the accuracy of collecting fetal electrocardiogram signals.

[0003] Existing impedance measurement methods at the skin-electrode interface usually use the method of converting the amplitude of the excitation response signal according to Ohm's law to solve the impedance information, ignoring the imaginary part information of the excitation response signal, and unable to effectively combine the capacitive reactance information to calculate the impedance value, which easily causes the measurement error to be amplified and does not match the human impedance model, resulting in a decrease in the accuracy of human impedance measurement. At the same time, existing technologies use analog switch devices for the impedance measurement method at the skin-electrode interface to route the signal of the electrode pair to the impedance signal measurement module or to the electrocardiogram signal measurement module, thereby physically separating the impedance signal from the electrocardiogram signal. However, the analog switch device has thermal noise and is prone to introducing external noise, which easily interferes with weak bioelectric signals, resulting in a decrease in impedance measurement accuracy. Therefore, the present invention proposes a skin impedance measurement method and system for fetal electrocardiogram monitoring to solve the technical problems existing in the existing impedance measurement methods at the skin-electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by using analog switch devices, resulting in a decrease in impedance measurement accuracy. Summary of the Invention

[0004] The present invention provides a skin impedance measurement method and system for fetal electrocardiogram monitoring to solve the technical problems existing in the existing impedance measurement methods at the skin-electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by using analog switch devices, resulting in a decrease in impedance measurement accuracy.

[0005] In view of this, a first aspect of the present invention provides a skin impedance measurement method for fetal electrocardiogram monitoring, including:

[0006] Collecting human body posture information through a three-axis gyroscope installed on a pregnant woman;

[0007] Judging whether the pregnant woman is in a calm state according to the human body posture information. If so, start the impedance measurement link to measure the human skin impedance;

[0008] The impedance measurement link includes:

[0009] Starting an alternating current excitation current source, applying a preset alternating current to the skin surface of the pregnant woman through a P electrode connected to the alternating current excitation current source, and obtaining a differential signal between the P electrode and the N electrode as an excitation response signal through a differential amplifier. Among them, the alternating current excitation current source, the P electrode and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to two input ends of the differential amplifier;

[0010] Preprocessing the excitation response signal to obtain an excitation response signal after filtering out the maternal-fetal bioelectric signal. The preprocessing includes analog-to-digital conversion processing and filtering processing;

[0011] Performing a discrete Fourier transform on the excitation response signal after filtering out the maternal-fetal bioelectric signal, solving the complex result of the real part and the imaginary part of the fundamental wave frequency, and obtaining the human skin impedance.

[0012] Optionally, it further includes:

[0013] When the human skin impedance measurement is completed, turn off the alternating current excitation current source, start the fetal electrocardiogram measurement link, collect the maternal electrocardiogram signal through the P electrode, the N electrode and other acquisition electrodes, and perform maternal-fetal electrocardiogram separation on the maternal electrocardiogram signal in combination with the measured human skin impedance to obtain the fetal electrocardiogram signal.

[0014] Optionally, collecting human body posture information through a three-axis gyroscope installed on a pregnant woman includes:

[0015] Real-time obtaining the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions collected by the three-axis gyroscope installed on the pregnant woman, and calculating the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment;

[0016] Caching the vector modulus length at each moment to obtain a vector modulus length time series;

[0017] Performing a local spectral transformation on the vector modulus length time series for unit window data by using the short-time Fourier transform, and converting the local spectral data into power spectral data;

[0018] Sum the power spectrum data to calculate the total power value. If the total power value exceeds the threshold, determine that the current human body posture is in the active state; otherwise, determine that the current human body posture is in the static state.

[0019] Optionally, calculate the vector norm length at each moment based on the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions, including:

[0020] Represent the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment as a vector space, calculate the Euclidean norm of the vector space at each moment, and obtain the vector norm length at each moment.

[0021] Optionally, preset a square wave excitation current with an alternating current of 250 Hz and 24 nA.

[0022] Optionally, perform a discrete Fourier transform on the excitation response signal after filtering out the maternal-fetal bioelectric signal, and solve the complex results of the real part and the imaginary part of the fundamental wave frequency to obtain the human skin impedance, including:

[0023] Construct a finite-length complex sequence with an imaginary part of 0 for the excitation response signal after filtering out the maternal-fetal bioelectric signal, and perform a discrete Fourier transform on the finite-length complex sequence with an imaginary part of 0;

[0024] Extract the complex information with a fundamental wave frequency component of 250 Hz from the discrete Fourier transform result, and combine it with the differential amplification factor of the differential amplifier to calculate the human skin impedance. The calculation formula is:

[0025]

[0026] where Z is the human skin impedance, b is the differential amplification factor of the differential amplifier, R 250 is the real value of the complex information with a fundamental wave frequency component of 250 Hz extracted from the discrete Fourier transform result, and I 250 is the imaginary value of the complex information with a fundamental wave frequency component of 250 Hz extracted from the discrete Fourier transform result.

[0027] The second aspect of the present invention provides a skin impedance measurement system for fetal electrocardiogram monitoring, including a gyroscope data analysis module, a P electrode, an N electrode, a three-axis gyroscope, an alternating current excitation current source, a differential amplifier, an analog-to-digital converter, a right leg drive circuit module, an information interaction interface, and an impedance measurement module;

[0028] The three-axis gyroscope is connected to the gyroscope data analysis module;

[0029] The gyroscope data analysis module is connected to the alternating current excitation current source;

[0030] The positive terminal of the AC excitation current source is connected to the P electrode, and the negative terminal is connected to the N electrode. The P electrode and the N electrode are connected to the two input terminals of the differential amplifier. The output terminal of the differential amplifier is connected to the input terminal of the analog-to-digital converter and the right leg drive circuit module. The right leg drive circuit module includes a right leg drive circuit and an E electrode. The three-axis gyroscope, the P electrode, and the N electrode are attached to the abdomen of the pregnant woman;

[0031] The output terminal of the analog-to-digital converter is connected to the impedance measurement module, and the output terminal of the impedance measurement module is connected to the information interaction interface;

[0032] When the skin impedance measurement system for fetal electrocardiogram monitoring is used to measure the human skin impedance, the three-axis gyroscope collects the human body posture information and sends it to the gyroscope data analysis module. The gyroscope data analysis module judges whether the pregnant woman is in a calm state according to the human body posture information. If so, it starts the impedance measurement link to measure the human skin impedance;

[0033] The impedance measurement link includes:

[0034] Start the AC excitation current source, apply a preset AC current to the surface of the pregnant woman's skin through the P electrode connected to the AC excitation current source, and obtain the differential signal between the P electrode and the N electrode as the excitation response signal through the differential amplifier. Among them, the AC excitation current source, the P electrode, and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to the two input terminals of the differential amplifier;

[0035] Preprocess the excitation response signal through the impedance measurement module to obtain the excitation response signal after filtering the maternal-fetal bioelectric signal. The preprocessing includes analog-to-digital conversion processing and filtering processing;

[0036] Perform a discrete Fourier transform on the excitation response signal after filtering the maternal-fetal bioelectric signal through the impedance measurement module, solve the complex results of the real part and the imaginary part of the fundamental frequency, and obtain the human skin impedance.

[0037] Optionally, it further includes a fetal electrocardiogram measurement module and other acquisition electrodes;

[0038] The other acquisition electrodes are connected to the input terminal of the differential amplifier. The input terminal of the fetal electrocardiogram measurement module is connected to the output terminal of the analog-to-digital converter and is also connected to the impedance measurement module. The output terminal of the fetal electrocardiogram measurement module is connected to the information interaction interface;

[0039] The fetal electrocardiogram measurement module is used to measure the fetal electrocardiogram signal after completing the measurement of the human skin impedance;

[0040] After completing the measurement of the human skin impedance, the impedance measurement module sends a control signal to the AC excitation current source to turn it off, and collects the maternal fetal electrocardiogram signal through the P electrode, N electrode and other acquisition electrodes. The maternal fetal electrocardiogram signal is separated from the maternal fetal electrocardiogram by combining the human skin impedance measured by the impedance measurement module to obtain the fetal electrocardiogram signal.

[0041] Optionally, the gyroscope data analysis module is specifically used for:

[0042] Obtain in real time the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions collected by the three-axis gyroscope, and calculate the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment;

[0043] Cache the vector modulus length at each moment to obtain a vector modulus length time series;

[0044] Perform local spectral transformation of the unit window data on the vector modulus length time series by using short-time Fourier transform, and convert the local spectral data into power spectral data;

[0045] Sum the power spectral data to calculate the total power value. If the total power value exceeds the threshold, it is determined that the current human body posture is an active state; otherwise, it is determined that the current human body posture is a static state.

[0046] Optionally, calculating the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment includes:

[0047] Represent the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment as a vector space, and calculate the Euclidean norm of the vector space at each moment to obtain the vector modulus length at each moment.

[0048] It can be seen from the above technical solutions that the skin impedance measurement method and system for fetal electrocardiogram monitoring provided by the present invention have the following advantages:

[0049] The skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention uses a three-axis gyroscope to collect the body posture information of a pregnant woman, determines whether the pregnant woman is in a calm state at present, and only starts the impedance measurement link when the pregnant woman is in a calm state. An alternating current excitation current source is started, and a preset alternating current is applied to the skin surface of the pregnant woman through the P electrode connected to the alternating current excitation current source. The differential signal between the P electrode and the N electrode is obtained through a differential amplifier as the excitation response signal. The excitation response signal is preprocessed to obtain the excitation response signal after filtering out the maternal and fetal bioelectric signals. Then, the excitation response signal after filtering out the maternal and fetal bioelectric signals is subjected to discrete Fourier transform to solve the complex results of the real part and the imaginary part of the fundamental frequency, and the human skin impedance is obtained. The skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention considers the imaginary part information of the excitation response signal, reduces the error of human skin impedance measurement. At the same time, the skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention does not need to use analog switch devices, avoids the thermal noise interference of the analog switch devices themselves and the risk of introducing external noise interference, improves the accuracy of skin impedance measurement for fetal electrocardiogram monitoring, and solves the technical problems existing in the existing impedance measurement methods at the skin and electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by using analog switch devices, resulting in a decrease in impedance measurement accuracy.

[0050] The skin impedance measurement system for fetal electrocardiogram monitoring provided by the present invention is used to execute the skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention. Its principle and effect are the same as those of the skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention, and will not be elaborated here. Brief Description of the Drawings

[0051] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art. Obviously, the following drawings are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other related drawings can also be obtained based on these drawings.

[0052] Figure 1 It is a schematic flowchart of a skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention;

[0053] Figure 2 It is a schematic structural diagram of a skin impedance measurement system for fetal electrocardiogram monitoring provided by the present invention;

[0054] Figure 3 It is a schematic diagram of the principle of the fetal electrocardiogram measurement process of a skin impedance measurement system for fetal electrocardiogram monitoring provided by the present invention. Detailed Embodiments

[0055] To enable those skilled in the art to better understand the solution of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts belong to the scope of protection of the present invention.

[0056] For ease of understanding, please refer to Figure 1 , an embodiment of a skin impedance measurement method for fetal electrocardiogram monitoring provided in the present invention includes:

[0057] Step 101: Collect human body posture information through a triaxial gyroscope installed on a pregnant woman.

[0058] It should be noted that in the embodiments of the present invention, for the triaxial gyroscope on the pregnant woman, the triaxial gyroscope can be installed on the abdomen of the pregnant woman by means of pasting, and the triaxial gyroscope adopts the triaxial gyroscope of the BMI160 inertial navigation component. Set the sampling rate of the triaxial gyroscope to 1600 Hz, and collect the human body posture information of the pregnant woman through the triaxial gyroscope. Specifically, the triaxial gyroscope can collect the instantaneous angular velocity values of the human body in the X, Y, and Z three-dimensional directions in real time, which are respectively denoted as g 1 , g 2 and g 3 . Use g t to represent the vector space of the instantaneous angular velocity values of the human body in the X, Y, and Z three-dimensional directions corresponding to the t moment. Then, g t corresponding to each moment is expressed as:

[0059] g t ={g 1 , g 2 , g 3}

[0060] Calculate the vector modulus length l by solving the Euclidean norm of the three elements in g t :

[0061]

[0062] The vector modulus length l is proportional to the human body activity intensity and is used to measure the instantaneous intensity of human body activity at the current t moment. In the embodiments of the present invention, the vector modulus length l at the current moment t is calculated in real time, and the newly generated vector modulus length l is continuously cached to form an unlimited time series, that is, the vector modulus length time series d(x):

[0063] d(x)={l 0 , l1 ,......,∞}

[0064] where l 0 is the vector norm of the first cache, and l 1 is the vector norm of the second cache.

[0065] According to the sensor characteristics of the three-axis gyroscope, when the human body posture is in a calm state, the angular velocity values of the sensor on the X, Y, and Z axes tend to 0. Conversely, the greater the intensity of the irregular activities of the human body posture, the greater the absolute value of the corresponding angular velocity output by the sensor.

[0066] Since the result of a single measurement of the gyroscope is an instantaneous value, but the instantaneous value cannot effectively characterize the current activity state of the human body. It is necessary to construct finite-length time series data through a sliding time window or a rolling time window, and evaluate the human activity state by analyzing the data characteristics within each unit time window.

[0067] At a sampling rate of 1600 Hz, taking 2048 data points cached as the unit window data, the short-time Fourier transform (STFT) is used to perform a local spectral transform on the unit window data of the vector norm time series arranged in time. The unit window data is combined with the window function for spectral conversion to solve the local spectrum of the vector norm time series d(x). The short-time Fourier transform (STFT) formula is defined as:

[0068]

[0069] m ∈ [0, 1,......, ∞]

[0070] n ∈ [0, 1,......, 1024]

[0071] where H is the frame shift length, preferably 512, m is the number of frame shifts, d(m×H + k) is the vector norm time series, w(k) is the window function, j is the imaginary unit, and n is the independent variable of the windowed Fourier transform.

[0072] To reduce the influence of spectral leakage generated during the short-time Fourier transform process, the window function w(k) is selected as the Hanning window for calculation. The length of the used Hanning window is 2048 points, and the window function w(k) is defined as:

[0073]

[0074] When the independent variable m increases, the Hann window function slides along the d(x) sequence with a sliding step of H = 512 data points. After each slide of the window function, the unit window data corresponding to the subscript of d(x) is multiplied by a one-dimensional matrix, and the resulting sequence after multiplication is solved. Then, the resulting sequence after multiplication is converted into a complex number sequence with an imaginary part of 0, and a discrete Fourier transform is performed to complete a single local spectrum transformation of d(x).

[0075] Compared with the traditional Fourier transform (FT), the short-time Fourier transform (STFT) can not only reflect the spectral characteristics of the original signal but also reflect the law of the spectral change of the original signal over time from a higher dimension. Therefore, relying on the frequency-domain and time-domain characteristics of the STFT is conducive to accurately counting the change intensity of gyroscope data within the window time and finding the time boundary when the human body enters a calm state. Furthermore, the activity state of the human body at the current moment can be determined.

[0076] After completing the local spectrum transformation, the local spectrum data is converted into power spectrum data, and the energies corresponding to all frequencies in the power spectrum data are summed to calculate the total power value. If the total power value exceeds the threshold, it is determined that the current human body posture is in an active state; otherwise, it is determined that the current human body posture is in a static state, that is, it is considered that the pregnant woman is in a calm state. When it is determined that the current human body posture is in a static state, the human skin impedance is measured; otherwise, the unit window continues to slide to cache new data, and the local spectrum transformation of the unit window data in the next round is performed, and the corresponding human body posture determination result is output.

[0077] Step 102: Determine whether the pregnant woman is in a calm state according to the human body posture information. If so, start the impedance measurement link to measure the human skin impedance.

[0078] It should be noted that when it is determined that the current human body posture is in a static state according to the total power value in step 101, that is, when the pregnant woman is in a calm state, the impedance measurement link is started: an AC excitation current source is started, and a preset AC current is applied to the skin surface of the pregnant woman through the P electrode connected to the AC excitation current source. The differential signal between the P electrode and the N electrode is obtained through a differential amplifier as the excitation response signal. Among them, the AC excitation current source, the P electrode, and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to the two input ends of the differential amplifier. The excitation response signal is preprocessed to obtain the excitation response signal after filtering out the maternal-fetal bioelectric signal. The preprocessing includes analog-to-digital conversion processing and filtering processing. The discrete Fourier transform is performed on the excitation response signal after filtering out the maternal-fetal bioelectric signal, and the complex result of the real part and the imaginary part of the fundamental frequency is solved to obtain the human skin impedance.

[0079] In typical human skin impedance measurement applications, the impedance measurement range usually extends from the left upper limb to the right lower limb of the human body. For the excitation signal applied to the human body, most commonly, an alternating current signal with a frequency greater than 50 KHz and a current amplitude of 300 microamperes is used. Since the applied AC excitation frequency is high and the relative current intensity is large, it may pose a risk of causing electrolyte disorders in the human body. During fetal electrocardiogram detection, the electrode pair is usually attached to the human abdomen. The electrode pair is attached at a close position, and the impedance generated between the skin between the P electrode and the N electrode and the electrode-skin interface is measured. In the embodiments of the present invention, a 250 Hz low-frequency square wave excitation signal that is not easily penetrable through human skin tissue and a 24 nA weak current signal that is harmless to skin tissue are used. By applying a square wave excitation current signal with constant frequency and constant current to the human skin, a signal response is caused by the complex impedance of the human skin and the impedance characteristics of the electrode-skin interface, providing a response signal with complex impedance information for subsequent impedance measurement.

[0080] The skin impedance measurement method for fetal electrocardiogram monitoring provided in the embodiments of the present invention utilizes a three-axis gyroscope to collect the human body posture information of the pregnant woman, determines whether the pregnant woman is in a calm state, and only starts the impedance measurement link when the pregnant woman is in a calm state. An AC excitation current source is started, and a preset AC current is applied to the skin surface of the pregnant woman through the P electrode connected to the AC excitation current source. A differential signal between the P electrode and the N electrode is obtained through a differential amplifier as the excitation response signal. The excitation response signal is preprocessed to obtain an excitation response signal after filtering out the maternal and fetal bioelectric signals. Then, a discrete Fourier transform is performed on the excitation response signal after filtering out the maternal and fetal bioelectric signals, and the complex results of the real part and the imaginary part of the fundamental frequency are solved to obtain the human skin impedance. The current of the AC excitation current source is introduced into the human skin from the P electrode and flows out from the N electrode. Under the condition of constant current, due to the complex impedance characteristics of the electrical signals of the human skin and the impedance characteristics existing due to the gel bonding degree between the electrode pair and the skin, a response signal is caused between the P electrode and the N electrode. The response intensity of the signal is inversely proportional to the complex impedance of the human skin and inversely proportional to the impedance of the electrode-skin interface. The skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention takes into account the imaginary part information of the excitation response signal, reducing the error in human skin impedance measurement. At the same time, the skin impedance measurement method for fetal electrocardiogram monitoring provided by the present invention does not require the use of analog switch devices, avoiding the risk of self-heat noise interference of analog switch devices and introducing external noise interference, improving the accuracy of skin impedance measurement for fetal electrocardiogram monitoring, and solving the technical problems existing in the existing impedance measurement methods at the skin-electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by the use of analog switch devices, resulting in a decrease in impedance measurement accuracy.

[0081] In one embodiment, in the impedance measurement process, the AC excitation current source applies the excitation signal to the human skin through the P electrode pair, thereby generating a response signal. The response signal is an aliased signal, including the excitation response signal caused by the complex impedance of the human skin to the excitation signal and the maternal-fetal bioelectric signal. After the aliased signal undergoes analog-to-digital conversion processing, the excitation response signal is screened out by a digital band-pass filter. The digital band-pass filter filters the 1024 sampled values cached in the rolling window, with a conduction range of 240 Hz to 260 Hz, and the filtered output result is a one-dimensional real number sequence a(r) with a length equal to 1024. A finite-length complex sequence s(r) with an imaginary part of 0 is constructed from a(r), and the discrete Fourier transform is performed on the finite-length complex sequence s(r). The formula definition of the discrete Fourier transform is:

[0082]

[0083] where F(q) is the result of the discrete Fourier transform, and s(r) is the input finite-length complex sequence.

[0084] The result F(q) of the discrete Fourier transform is in complex form, so each frequency component contains real and imaginary part information. Since the signal frequency applied by the AC excitation current source to the human body is 250 Hz, the complex information with a fundamental frequency component of 250 Hz is extracted from the spectrum result F(q):

[0085] Z 250 =R 250 +jI 250

[0086] where j is the imaginary unit, R 250 is the real value of the complex information with a fundamental frequency component of 250 Hz extracted from the discrete Fourier transform result, representing the resistance component in the measured human complex impedance, and I 250 is the imaginary value of the complex information with a fundamental frequency component of 250 Hz extracted from the discrete Fourier transform result, representing the capacitive reactance component in the measured human complex impedance.

[0087] Assuming the differential amplification factor of the differential amplifier is b, the final human skin impedance calculated in combination with the differential amplification factor of b is:

[0088]

[0089] where Z is the human skin impedance.

[0090] By performing spectrum conversion on the finite-length excitation response signal and calculating impedance through real and imaginary information, it is beneficial to improve the accuracy and stability of the system for measuring human skin impedance.

[0091] In one embodiment, after the measurement of the human skin impedance is completed, the AC excitation current source is turned off, and the fetal electrocardiogram measurement process is started. The maternal electrocardiogram signal is collected through the P electrode, the N electrode, and other acquisition electrodes, and the maternal electrocardiogram signal is separated from the maternal electrocardiogram signal by combining the measured human skin impedance to obtain the fetal electrocardiogram signal.

[0092] It should be noted that after the measurement of the human skin impedance is completed, the AC excitation current source is turned off. The P electrode and the N electrode respectively collect the human body potential signals, and the differential signal of the P electrode and the N electrode is the maternal electrocardiogram signal. After the AC excitation current source is turned off, the impedance measurement function is turned off, and there is no excitation response signal caused by the excitation source on the electrode pair. Therefore, the signal output by the analog-to-digital converter only contains the maternal-fetal mixed bioelectric signal. The maternal-fetal mixed bioelectric signal is filtered by an IIR digital filter to remove the DC component and power frequency interference in the signal, and the QRST signal of the maternal electrocardiogram is extracted. According to the characteristics of the maternal electrocardiogram signal, the maternal electrocardiogram signal is separated by the principal component analysis algorithm.

[0093] The circuit structure used in the skin impedance measurement method for fetal electrocardiogram monitoring provided in the embodiment of the present invention is as Figure 2 shown, including a gyroscope data analysis module, a P electrode, an N electrode, a three-axis gyroscope, an AC excitation current source, a differential amplifier, an analog-to-digital converter, a right leg drive circuit module, an information interaction interface, and an impedance measurement module. The three-axis gyroscope is connected to the gyroscope data analysis module, the gyroscope data analysis module is connected to the AC excitation current source, the positive terminal of the AC excitation current source is connected to the P electrode, the negative terminal is connected to the N electrode, the P electrode and the N electrode are connected to the two input terminals of the differential amplifier, the output terminal of the differential amplifier is connected to the input terminal of the analog-to-digital converter and the right leg drive circuit module. The right leg drive circuit module includes a right leg drive circuit and an E electrode. The E electrode, the three-axis gyroscope, the P electrode, and the N electrode are attached to the abdomen of the pregnant woman. The output terminal of the analog-to-digital converter is connected to the impedance measurement module, the output terminal of the impedance measurement module is connected to the information interaction interface, and the output terminal of the impedance measurement module is also connected to the input terminal of the AC excitation current source. After the impedance measurement module completes the impedance measurement, it sends a control signal to the AC excitation current source to turn off the AC excitation current source. Figure 2 The working principle of the circuit structure is as follows:

[0094] The gyroscope is used to detect the posture change of the human body. When it detects that the human body posture is in an active state, impedance measurement is not performed. Otherwise, impedance measurement is performed.

[0095] The AC excitation current source is used to generate an AC excitation signal that is harmless to the human body.

[0096] The system input electrode includes at least one set of electrode pairs, and each pair of electrode pairs includes a P electrode and an N electrode. During impedance measurement: current is applied from the P electrode to the human skin and returns through the N electrode. During fetal electrocardiogram detection: the AC excitation power supply is turned off, and the P electrode and the N electrode respectively collect the human body potential signals.

[0097] The differential amplifier is used to amplify the differential signal formed between each set of P electrodes and N electrodes. During impedance measurement: due to the complex impedance characteristics of the human skin, the differential signal between the P electrode and the N electrode is the excitation response signal; during fetal electrocardiogram detection: the differential signal between the P electrode and the N electrode is the maternal-fetal mixed electrocardiogram signal.

[0098] The analog-to-digital converter is used to perform quantization sampling on the differentially amplified signal.

[0099] The impedance measurement module includes a digital band-pass filter and an impedance analysis unit, and the fetal electrocardiogram measurement module includes an IIR digital filter and a maternal electrocardiogram separation module.

[0100] The digital band-pass filter is used for filtering during impedance detection to reduce the interference of the human electrocardiogram signal.

[0101] The IIR digital filter is used for filtering during fetal electrocardiogram detection to reduce the interference of DC and high-frequency signals.

[0102] The impedance analysis unit is used to analyze the response signal formed by the human body to the excitation source and calculate the impedance value from the response signal;

[0103] The P electrode and the N electrode and other acquisition electrodes (when performing fetal electrocardiogram detection, there are multiple electrodes used, including Figure 2 the P electrode and the N electrode in it, and also including other acquisition electrodes. The number of other acquisition electrodes is selected according to the actual monitoring scenario and is not limited here), are used to collect the maternal electrocardiogram signal when performing fetal electrocardiogram detection.

[0104] The maternal electrocardiogram separation module is used to analyze the electrocardiogram signal collected from the human body, separate the maternal electrocardiogram, and extract the fetal electrocardiogram signal;

[0105] The system includes a right leg drive circuit. The right leg drive circuit feeds the common mode signal to the human body through the E electrode, which is used to suppress the common mode interference caused by the human body function during electrocardiogram measurement.

[0106] The information interaction interface is used to publish the calculated real-time impedance data or fetal electrocardiogram data externally. The information interaction interface forms a human-computer interaction channel in a wireless manner using BLE low-power Bluetooth.

[0107] When measuring the impedance formed by the measurement electrodes (P electrode and N electrode) and the skin, the impedance measurement is divided into a front-end system and a back-end system. The front-end system: an AC excitation source + electrode (front-end input). The back-end system: amplification + analog-to-digital conversion + signal analysis (back-end processing). The front-end system introduces an AC excitation current source, which generates a square-wave excitation signal with a constant on-off frequency and current. The excitation signal is applied to the P electrode, and the current flows out from the N electrode after passing through the human skin, forming a current signal loop with the excitation current source. The differential amplifier in the back-end system is used to amplify the signals of the P electrode and the N electrode, and is sampled and quantized by an analog-to-digital converter. After the quantization result is filtered by a digital band-pass filter, the impedance analysis unit performs data analysis on the filtered result. When the current is introduced into the human skin from the P electrode and flows out from the N electrode, under the condition of constant current, due to the complex impedance characteristics of the electrical signals of the human skin and the impedance characteristics caused by the gel fitting degree between the electrode pair and the skin, a response signal is generated between the P electrode and the N electrode. The response intensity of the signal is inversely proportional to the complex impedance of the human skin and inversely proportional to the impedance of the electrode fitting the skin interface.

[0108] The back-end system performs signal differential amplification and analog-to-digital conversion sampling on the electrode pair attached to the human skin. Using the differential input method can effectively reduce the common-mode interference of bioelectric signals. After the differential signal is amplified, an analog-to-digital converter is used for signal quantization sampling. Through the rolling window caching mechanism, following the first-in-first-out rule, the rolling window caches 1024 sampling values. In the cached timing data, it not only contains the excitation response signal data caused by the application of the excitation current to the skin, but also contains the electrocardiogram signal data of the human body.

[0109] Generally, the spectral energy range of the human electrocardiogram signal is less than 30 Hz, while the spectral energy of the excitation signal applied in the front-end is concentrated at 250 Hz and its multiples. In order to be able to demodulate and separate the response data with complex impedance information from the timing data mixed with the electrocardiogram, in this embodiment, digital signal processing technology is adopted to construct a digital band-pass filter. Through the filter, the response data with complex impedance information is screened out from the sampled mixed digital signals. The used digital band-pass filter has a passband of 240 Hz to 260 Hz.

[0110] For easy understanding, please refer to Figure 2 and Figure 3 In the present invention, an embodiment of a skin impedance measurement system for fetal electrocardiogram monitoring is provided, including a gyroscope data analysis module, a P electrode, an N electrode, a three-axis gyroscope, an AC excitation current source, a differential amplifier, an analog-to-digital converter, a right leg drive circuit module, an information interaction interface, and an impedance measurement module;

[0111] The three-axis gyroscope is connected to the gyroscope data analysis module;

[0112] The gyroscope data analysis module is connected to the AC excitation current source;

[0113] The positive terminal of the AC excitation current source is connected to the P electrode, and the negative terminal is connected to the N electrode. The P electrode and the N electrode are connected to the two input terminals of the differential amplifier. The output terminal of the differential amplifier is connected to the input terminal of the analog-to-digital converter and the right leg drive circuit module. The right leg drive circuit module includes a right leg drive circuit and an E electrode. The E electrode is attached to the right leg of the pregnant woman, and the three-axis gyroscope, the P electrode, and the N electrode are attached to the abdomen of the pregnant woman;

[0114] The output terminal of the analog-to-digital converter is connected to the impedance measurement module, and the output terminal of the impedance measurement module is connected to the information interaction interface;

[0115] When the skin impedance measurement system for fetal electrocardiogram monitoring is used to measure the human skin impedance, the three-axis gyroscope collects the human body posture information and sends it to the gyroscope data analysis module. The gyroscope data analysis module determines whether the pregnant woman is in a calm state according to the human body posture information. If so, the impedance measurement link is started to measure the human skin impedance;

[0116] The impedance measurement link includes:

[0117] Start the AC excitation current source, apply a preset AC current to the skin surface of the pregnant woman through the P electrode connected to the AC excitation current source, and obtain the differential signal between the P electrode and the N electrode as the excitation response signal through the differential amplifier. Among them, the AC excitation current source, the P electrode, and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to the two input terminals of the differential amplifier;

[0118] Preprocess the excitation response signal through the impedance measurement module to obtain the excitation response signal after filtering out the maternal and fetal bioelectric signals. The preprocessing includes analog-to-digital conversion processing and filtering processing;

[0119] Perform discrete Fourier transform on the excitation response signal after filtering out the maternal and fetal bioelectric signals through the impedance measurement module, solve the complex result of the real part and the imaginary part of the fundamental wave frequency, and obtain the human skin impedance.

[0120] It also includes a fetal electrocardiogram measurement module and other acquisition electrodes ( Figure 2 and Figure 3 not marked in

[0121] The other acquisition electrodes are connected to the input terminal of the differential amplifier. The input terminal of the fetal electrocardiogram measurement module is connected to the output terminal of the analog-to-digital converter and is also connected to the impedance measurement module. The output terminal of the fetal electrocardiogram measurement module is connected to the information interaction interface;

[0122] The fetal electrocardiogram measurement module is used to measure the fetal electrocardiogram signal after completing the measurement of the human skin impedance;

[0123] After completing the measurement of human skin impedance, the impedance measurement module sends a control signal to the AC excitation current source to turn it off, and collects the maternal fetal electrocardiogram signal through the P electrode, N electrode and other acquisition electrodes. The maternal fetal electrocardiogram is separated from the maternal fetal electrocardiogram signal by combining the human skin impedance measured by the impedance measurement module to obtain the fetal electrocardiogram signal.

[0124] The gyroscope data analysis module is specifically used for:

[0125] Real-time obtain the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions collected by the three-axis gyroscope, and calculate the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment;

[0126] Cache the vector modulus length at each moment to obtain the vector modulus length time series;

[0127] Perform local spectral transformation of the unit window data on the vector modulus length time series by using short-time Fourier transform, and convert the local spectral data into power spectrum data;

[0128] Sum the power spectrum data to calculate the total power value. If the total power value exceeds the threshold, it is determined that the current human body posture is in the active state; otherwise, it is determined that the current human body posture is in the static state.

[0129] Calculating the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment includes:

[0130] Represent the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment into a vector space, and calculate the Euclidean norm of the vector space at each moment to obtain the vector modulus length at each moment.

[0131] The short-time Fourier transform formula for performing local spectral transformation of the unit window data on the vector modulus length time series by using short-time Fourier transform is:

[0132]

[0133] m∈[0,1,......,∞]

[0134] n∈[0,1,……,1024]

[0135] Among them, H is the frame shift length, m is the number of frame shifts, d(m×H + k) is the vector modulus length time series, w(k) is the window function, j is the imaginary unit, and n is the independent variable of the windowed Fourier transform.

[0136] The preset AC current is a square wave excitation current of 250Hz 24nA.

[0137] Perform a discrete Fourier transform on the excitation response signal after filtering out the maternal-fetal bioelectric signal, solve the complex results of the real and imaginary parts of the fundamental frequency, and obtain the human skin impedance, including:

[0138] Construct a finite-length complex sequence with an imaginary part of 0 from the excitation response signal after filtering out the maternal-fetal bioelectric signal, and perform a discrete Fourier transform on the finite-length complex sequence with an imaginary part of 0. The formula for the discrete Fourier transform is:

[0139]

[0140] where F(q) is the result of the discrete Fourier transform, and s(r) is the input finite-length complex sequence;

[0141] Extract the complex information with a fundamental frequency component of 250 Hz from the discrete Fourier transform result, and combine it with the differential amplification factor of the differential amplifier to calculate the human skin impedance. The calculation formula is:

[0142]

[0143] where Z is the human skin impedance, b is the differential amplification factor of the differential amplifier, and R 250 is the real value of the complex information with a fundamental frequency component of 250 Hz extracted from the discrete Fourier transform result, and I 250 is the imaginary value of the complex information with a fundamental frequency component of 250 Hz extracted from the discrete Fourier transform result.

[0144] The skin impedance measurement system for fetal electrocardiogram monitoring provided in the present invention uses a three-axis gyroscope to collect the body posture information of a pregnant woman, determines whether the pregnant woman is in a calm state at present, and only starts the impedance measurement link when the pregnant woman is in a calm state. An AC excitation current source is started, and a preset AC current is applied to the skin surface of the pregnant woman through the P electrode connected to the AC excitation current source. A differential signal between the P electrode and the N electrode is obtained through a differential amplifier as an excitation response signal. The excitation response signal is preprocessed to obtain an excitation response signal after filtering out the maternal and fetal bioelectric signals. Then, the excitation response signal after filtering out the maternal and fetal bioelectric signals is subjected to a discrete Fourier transform to solve the complex result of the real part and the imaginary part of the fundamental frequency, and the human skin impedance is obtained. The skin impedance measurement system for fetal electrocardiogram monitoring provided in the present invention takes into account the imaginary part information of the excitation response signal, reduces the error of human skin impedance measurement. At the same time, the skin impedance measurement system for fetal electrocardiogram monitoring provided in the present invention does not need to use an analog switch device, avoids the thermal noise interference of the analog switch device itself and the risk of introducing external noise interference, improves the accuracy of skin impedance measurement for fetal electrocardiogram monitoring, and solves the technical problems existing in the existing impedance measurement method at the skin-electrode interface, such as ignoring the imaginary part information of the excitation response signal and the noise interference caused by using an analog switch device, resulting in a decrease in impedance measurement accuracy.

[0145] The skin impedance measurement system for fetal electrocardiogram monitoring provided in the embodiments of the present invention is used to execute the skin impedance measurement method for fetal electrocardiogram monitoring in the foregoing embodiments of the skin impedance measurement method for fetal electrocardiogram monitoring. Its principle is the same as that of the skin impedance measurement method for fetal electrocardiogram monitoring in the foregoing embodiments of the skin impedance measurement method for fetal electrocardiogram monitoring, and will not be elaborated here.

[0146] As mentioned above, the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that: they can still modify the technical solutions recorded in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A skin impedance measurement method for fetal electrocardiogram monitoring, characterized in that, it includes: collecting human body posture information through a triaxial gyroscope installed on a pregnant woman; judging whether the pregnant woman is in a calm state according to the human body posture information, if so, starting the impedance measurement link to measure the human skin impedance; The impedance measurement link includes: starting an AC excitation current source, applying a preset AC current to the skin surface of the pregnant woman through a P electrode connected to the AC excitation current source, and obtaining a differential signal between the P electrode and the N electrode as an excitation response signal through a differential amplifier, wherein the AC excitation current source, the P electrode and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to two input ends of the differential amplifier; preprocessing the excitation response signal to obtain an excitation response signal after filtering out the maternal and fetal bioelectric signals, and the preprocessing includes analog-to-digital conversion processing and filtering processing; performing a discrete Fourier transform on the excitation response signal after filtering out the maternal and fetal bioelectric signals, solving the complex results of the real part and the imaginary part of the fundamental wave frequency, and obtaining the human skin impedance; performing a discrete Fourier transform on the excitation response signal after filtering out the maternal and fetal bioelectric signals, solving the complex results of the real part and the imaginary part of the fundamental wave frequency, and obtaining the human skin impedance, including: constructing a finite-length complex sequence with an imaginary part of 0 for the excitation response signal after filtering out the maternal and fetal bioelectric signals, and performing a discrete Fourier transform on the finite-length complex sequence with an imaginary part of 0; extracting the complex information with a fundamental wave frequency component of 250 Hz from the discrete Fourier transform result, and combining the differential amplification factor of the differential amplifier to calculate the human skin impedance, and the calculation formula is: Among them, Z is the human skin impedance, b is the differential amplification factor of the differential amplifier, and R 250 is the real value of the complex number information obtained by extracting the fundamental frequency component of 250 Hz from the discrete Fourier transform result, and I 250 is the imaginary value of the complex number information obtained by extracting the fundamental frequency component of 250 Hz from the discrete Fourier transform result.

2. The skin impedance measurement method for fetal electrocardiogram monitoring according to claim 1, characterized in that, it further includes: when the human skin impedance measurement is completed, turning off the AC excitation current source, starting the fetal electrocardiogram measurement link, collecting the maternal electrocardiogram signal through the P electrode, the N electrode and other collecting electrodes, and separating the maternal electrocardiogram from the maternal electrocardiogram signal in combination with the measured human skin impedance to obtain the fetal electrocardiogram signal.

3. The skin impedance measurement method for fetal electrocardiogram monitoring according to claim 1, characterized in that, collecting human body posture information through a triaxial gyroscope installed on a pregnant woman, including: real-time obtaining the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions collected by the triaxial gyroscope installed on the pregnant woman, and calculating the vector modulus length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment; caching the vector modulus length at each moment to obtain a vector modulus length time series; performing a local spectrum transformation on the unit window data of the vector modulus length time series by using the short-time Fourier transform, and converting the local spectrum data into power spectrum data; summing the power spectrum data, calculating the total power value, if the total power value exceeds the threshold, determining that the current human body posture is an active state, otherwise, determining that the current human body posture is a static state.

4. The skin impedance measurement method for fetal electrocardiogram monitoring according to claim 3, characterized in that, Calculating the vector norm length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimensional directions, including: Representing the instantaneous angular velocity values of the human body in the X, Y, and Z dimensional directions at each moment as a vector space, calculating the Euclidean norm of the vector space at each moment, and obtaining the vector norm length at each moment.

5. The skin impedance measurement method for fetal electrocardiogram monitoring according to claim 1, characterized in that, a square wave excitation current with an alternating current of 250 Hz and 24 nA is preset.

6. A skin impedance measurement system for fetal electrocardiogram monitoring, characterized in that, it includes a gyroscope data analysis module, a P electrode, an N electrode, a three-axis gyroscope, an alternating current excitation current source, a differential amplifier, an analog-to-digital converter, a right leg drive circuit module, an information interaction interface, and an impedance measurement module; the three-axis gyroscope is connected to the gyroscope data analysis module; the gyroscope data analysis module is connected to the alternating current excitation current source; the positive end of the alternating current excitation current source is connected to the P electrode, the negative end is connected to the N electrode, the P electrode and the N electrode are connected to the two input ends of the differential amplifier, the output end of the differential amplifier is connected to the input end of the analog-to-digital converter and the right leg drive circuit module, and the right leg drive circuit module includes a right leg drive circuit and an E electrode, and the three-axis gyroscope, the E electrode, the P electrode, and the N electrode are attached to the abdomen of the pregnant woman; the output end of the analog-to-digital converter is connected to the impedance measurement module, and the output end of the impedance measurement module is connected to the information interaction interface; when the skin impedance measurement system for fetal electrocardiogram monitoring is used to measure the human skin impedance, the three-axis gyroscope collects the human body posture information and sends it to the gyroscope data analysis module, and the gyroscope data analysis module judges whether the pregnant woman is in a calm state according to the human body posture information. If so, the impedance measurement link is started to measure the human skin impedance; the impedance measurement link includes: starting the alternating current excitation current source, applying a preset alternating current to the skin surface of the pregnant woman through the P electrode connected to the alternating current excitation current source, and obtaining the differential signal between the P electrode and the N electrode as the excitation response signal through the differential amplifier. Among them, the alternating current excitation current source, the P electrode, and the N electrode are connected in a loop, and the P electrode and the N electrode are connected to the two input ends of the differential amplifier; preprocessing the excitation response signal through the impedance measurement module to obtain the excitation response signal after filtering the maternal and fetal bioelectric signals. The preprocessing includes analog-to-digital conversion processing and filtering processing; performing a discrete Fourier transform on the excitation response signal after filtering the maternal and fetal bioelectric signals, and solving the complex results of the real part and the imaginary part of the fundamental wave frequency to obtain the human skin impedance; performing a discrete Fourier transform on the excitation response signal after filtering the maternal and fetal bioelectric signals, and solving the complex results of the real part and the imaginary part of the fundamental wave frequency to obtain the human skin impedance, including: constructing the excitation response signal after filtering the maternal and fetal bioelectric signals into a finite-length complex sequence with an imaginary part of 0, and performing a discrete Fourier transform on the finite-length complex sequence with an imaginary part of 0; Extract the complex information with a fundamental wave frequency component of 250 Hz from the discrete Fourier transform result, and combine it with the differential amplification factor of the differential amplifier to calculate the human skin impedance. The calculation formula is as follows: Among them, Z is the human skin impedance, b is the differential amplification factor of the differential amplifier, and R 250 is the real value of the complex number information obtained by extracting the fundamental frequency component of 250 Hz from the discrete Fourier transform result, and I 250 is the imaginary value of the complex number information obtained by extracting the fundamental frequency component of 250 Hz from the discrete Fourier transform result.

7. The skin impedance measurement system for fetal electrocardiogram monitoring according to claim 6, characterized in that, it further includes a fetal electrocardiogram measurement module and other acquisition electrodes; The other acquisition electrodes are connected to the input end of the differential amplifier. The input end of the fetal electrocardiogram measurement module is connected to the output end of the analog-to-digital converter and is also connected to the impedance measurement module. The output end of the fetal electrocardiogram measurement module is connected to the information interaction interface; The fetal electrocardiogram measurement module is used to measure the fetal electrocardiogram signal after completing the measurement of the human skin impedance; After completing the measurement of the human skin impedance, the impedance measurement module sends a control signal to the AC excitation current source to turn off the AC excitation current source, and collects the maternal electrocardiogram signal through the P electrode, N electrode and other acquisition electrodes. The maternal electrocardiogram signal is separated from the maternal electrocardiogram by combining the human skin impedance measured by the impedance measurement module to obtain the fetal electrocardiogram signal.

8. The skin impedance measurement system for fetal electrocardiogram monitoring according to claim 6, characterized in that, The gyroscope data analysis module is specifically used for: Obtain the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions collected by the three-axis gyroscope in real time, and calculate the vector norm length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment; Cache the vector norm length at each moment to obtain a vector norm length time series; Perform local spectral transformation of the unit window data on the vector norm length time series by using short-time Fourier transform, and convert the local spectral data into power spectral data; Sum the power spectral data to calculate the total power value. If the total power value exceeds the threshold, it is determined that the current human body posture is an active state, otherwise, it is determined that the current human body posture is a static state.

9. The skin impedance measurement system for fetal electrocardiogram monitoring according to claim 8, characterized in that, Calculating the vector norm length at each moment according to the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment includes: Represent the instantaneous angular velocity values of the human body in the X, Y, and Z dimension directions at each moment as a vector space, and calculate the Euclidean norm of the vector space at each moment to obtain the vector norm length at each moment.

Images