CN115153580B - A skin impedance measurement method and system for fetal electrocardiogram monitoring - Google Patents
A skin impedance measurement method and system for fetal electrocardiogram monitoring Download PDFInfo
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Abstract
本发明公开了一种用于胎儿心电监测的皮肤阻抗测量方法和系统，利用三轴陀螺仪采集孕妇的人体姿态信息，在孕妇处于平静状态下才启动阻抗测量环节，启动交流激励电流源，通过P电极对孕妇皮肤表面施加预置交流电流，通过差分放大器获取P电极和N电极之间的差分信号作为激励响应信号，对激励响应信号进行预处理，得到滤除母胎生物电信号后的激励响应信号，再对滤除母胎生物电信号后的激励响应信号进行离散傅里叶变换，求解出基波频率实部和虚部的复数结果，得到人体皮肤阻抗，解决了现有的皮肤与电极界面处的阻抗测量方法存在的忽略激励响应信号的虚部信息以及采用模拟开关器件带来噪声干扰，导致阻抗测量准确度下降的技术问题。
The present invention discloses a skin impedance measurement method and system for fetal electrocardiogram monitoring. A three-axis gyroscope is used to collect human posture information of a pregnant woman. The impedance measurement link is started only 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 a P electrode. A differential amplifier is used to obtain a differential signal between the P electrode and the N electrode 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. The excitation response signal after filtering out the maternal and fetal bioelectric signals is then subjected to a discrete Fourier transform to solve the complex results of the real part and the imaginary part of the fundamental frequency to obtain the human skin impedance. The technical problems of ignoring the imaginary part information of the excitation response signal and using analog switch devices to cause noise interference, resulting in reduced impedance measurement accuracy, are solved in the existing impedance measurement method at the interface between the skin and the electrode.
Description
Technical Field
The invention relates to the technical field of electrocardiograph monitoring, in particular to a skin impedance measurement method and system for fetal electrocardiograph monitoring.
Background
The non-invasive fetal electrocardiosignal (NI-FECG) equipment can collect maternal and fetal electrocardiosignals through an electrode arranged on the abdomen of a pregnant woman, separate the maternal and fetal electrocardiosignals to obtain the Maternal Electrocardiosignals (MECG) and the Fetal Electrocardiosignals (FECG), and calculate the fetal heart rate and uterine contraction according to the maternal and fetal electrocardiosignals. However, the FECG signal is very weak, usually 10-50 microvolts, and is easy to submerge in interference signals of human bodies, environments and circuits. The low and stable impedance at the interface of the skin and the electrode is the key of the acquired high-fidelity biological signals, and can provide a quality evaluation basis for the maternal-fetal electrocardiosignal and correct the result of separating the maternal-fetal signals. However, on the one hand, the complexity of human skin causes great signal quality change, and on the other hand, when the electrode is attached to the skin, the problems of wrinkling, falling off and the like are easily caused, so that a method for accurately measuring the interface impedance of the skin and the electrode is needed, when a good fetal signal cannot be acquired due to overlarge interface impedance, impedance information can be fed back to a user, and the user is prompted to perform inspection so as to ensure the accuracy of acquiring fetal electrocardiosignals.
The existing impedance measurement method at the interface of the skin and the electrode generally adopts a method of carrying out ohm law conversion on the amplitude of the excitation response signal, solves the impedance information, ignores the imaginary part information of the excitation response signal, cannot effectively combine the capacitive impedance information to calculate the impedance value, easily causes the measurement error to be amplified, is not matched with the impedance model of the human body, and leads to the reduction of the measurement accuracy of the human body impedance. Meanwhile, in the prior art, an analog switching device is adopted in the impedance measurement method at the interface of the skin and the electrode, and signals of the electrode pair are routed to an impedance signal measurement module or an electrocardiosignal measurement module, so that the impedance signal and the electrocardiosignal are distinguished on a physical link, but the analog switching device has thermal noise and is easy to introduce external noise, and is easy to interfere with weak bioelectricity signals, so that the impedance measurement precision is reduced. Therefore, the invention provides a skin impedance measurement method and a skin impedance measurement system for fetal electrocardio monitoring, which are used for solving the technical problems that the accuracy of impedance measurement is reduced due to the fact that the existing impedance measurement method at the interface of skin and an electrode ignores the imaginary part information of an excitation response signal and adopts an analog switching device to bring noise interference.
Disclosure of Invention
The invention provides a skin impedance measurement method and a skin impedance measurement system for fetal electrocardio monitoring, which are used for solving the technical problems that the accuracy of impedance measurement is reduced due to the fact that the existing impedance measurement method at the interface of skin and an electrode ignores the imaginary part information of an excitation response signal and adopts an analog switching device to bring noise interference.
In view of this, a first aspect of the present invention provides a skin impedance measurement method for fetal electrocardiography monitoring, comprising:
Acquiring human body posture information through a triaxial gyroscope arranged on a pregnant woman;
judging whether the pregnant woman is in a calm state or not according to the human body posture information, if so, starting an impedance measurement link, and measuring the skin impedance of the human body;
The impedance measurement link includes:
Starting an alternating current excitation current source, applying preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, and obtaining a differential signal between the P electrode and an N electrode through a differential amplifier to serve as an excitation response signal, wherein the alternating current excitation current source, the P electrode and the N electrode are connected in a ring shape to form a loop, and the P electrode and the N electrode are connected with two input ends of the differential amplifier;
preprocessing the excitation response signal to obtain an excitation response signal after the maternal biological electrical signal is filtered, wherein the preprocessing comprises analog-to-digital conversion processing and filtering processing;
performing discrete Fourier transform on the excitation response signal after the maternal biological electrical signal is filtered, and solving a complex result of a real part and an imaginary part of fundamental frequency to obtain the skin impedance of the human body.
Optionally, the method further comprises:
after the measurement of the skin impedance of the human body is completed, an alternating current excitation current source is turned off, a fetal electrocardiosignal measuring link is started, the maternal-fetal electrocardiosignal is collected through a P electrode, an N electrode and other collecting electrodes, and the maternal-fetal electrocardiosignal is subjected to maternal-fetal electrocardiosignal separation by combining the measured skin impedance of the human body, so that the fetal electrocardiosignal is obtained.
Optionally, acquiring the human posture information by a triaxial gyroscope mounted on the pregnant woman includes:
Acquiring an instantaneous angular velocity value of a human body in a X, Y, Z dimension direction acquired by a triaxial gyroscope arranged on a pregnant woman in real time, and calculating a vector module length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment;
Caching the vector modular length at each moment to obtain a vector modular length time sequence;
carrying out local spectrum transformation of unit window data on the vector modulus long-time sequence by adopting short-time Fourier transformation, and converting the local spectrum data into power spectrum data;
And summing the power spectrum data, calculating a power total value, judging that the current human body posture is in an active state if the power total value exceeds a threshold value, and judging that the current human body posture is in a static state if the power total value exceeds the threshold value.
Optionally, calculating the vector modular length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment includes:
The instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment is represented as a vector space, the Euclidean norm of the vector space at each moment is calculated, and the vector modular length at each moment is obtained.
Alternatively, a square wave excitation current of 24nA at 250Hz is preset as the alternating current.
Optionally, performing discrete fourier transform on the excitation response signal after filtering the maternal-fetal bioelectric signal, and solving a complex result of a real part and an imaginary part of fundamental frequency to obtain skin impedance of the human body, including:
constructing an excitation response signal with the filtered maternal-fetal bioelectric signals into a finite-length complex sequence with the imaginary number of 0, and performing discrete Fourier transform on the finite-length complex sequence with the imaginary number of 0;
The complex information with the fundamental wave frequency component of 250Hz is taken out from the discrete Fourier transform result, and the human skin impedance is calculated by combining the differential amplification factor of the differential amplifier, wherein the calculation formula is as follows:
Wherein 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 the fundamental frequency component of 250Hz extracted from the discrete Fourier transform result, and I 250 is the imaginary value of the complex information with the fundamental frequency component of 250Hz extracted from the discrete Fourier transform result.
The invention provides a skin impedance measurement system for fetal electrocardio monitoring, which comprises 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 driving circuit module, an information interaction interface and an impedance measurement module, wherein the gyroscope data analysis module is used for analyzing the data of the fetus;
the three-axis gyroscope is connected with the gyroscope data analysis module;
the gyroscope data analysis module is connected with an alternating current excitation current source;
The positive end of the alternating current excitation current source is connected with the P electrode, the negative end of the alternating current excitation current source is connected with the N electrode, the P electrode and the N electrode are connected with two input ends of the differential amplifier, the output end of the differential amplifier is connected with the input end of the analog-to-digital converter and the right leg driving circuit module, the right leg driving circuit module comprises a right leg driving circuit and an E electrode, and the triaxial gyroscope, the P electrode and the N electrode are attached to the abdomen of a pregnant woman;
the output end of the analog-to-digital converter is connected with the impedance measurement module, and the output end of the impedance measurement module is connected with the information interaction interface;
When the skin impedance measuring system for fetal electrocardio monitoring is used for measuring the skin impedance of a human body, the three-axis gyroscope collects the human body posture information and sends the human body posture information 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, an impedance measuring link is started, and the skin impedance of the human body is measured;
The impedance measurement link includes:
Starting an alternating current excitation current source, applying preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, and obtaining a differential signal between the P electrode and an N electrode through a differential amplifier to serve as an excitation response signal, wherein the alternating current excitation current source, the P electrode and the N electrode are connected in a ring shape to form a loop, and the P electrode and the N electrode are connected with two input ends of the differential amplifier;
Preprocessing the excitation response signal through an impedance measurement module to obtain an excitation response signal after the maternal biological electrical signal is filtered, wherein the preprocessing comprises analog-to-digital conversion processing and filtering processing;
And performing discrete Fourier transform on the excitation response signal after the maternal biological electrical signal is filtered by an impedance measurement module, and solving complex results of a real part and an imaginary part of fundamental frequency to obtain the skin impedance of the human body.
Optionally, the device also comprises a fetal electrocardiograph measurement module and other acquisition electrodes;
the other acquisition electrodes are connected with the input end of the differential amplifier, the input end of the fetal electrocardiograph measurement module is connected with the output end of the analog-to-digital converter, and is also connected with the impedance measurement module, and the output end of the fetal electrocardiograph measurement module is connected with the information interaction interface;
The fetal electrocardiosignal module is used for measuring fetal electrocardiosignals after finishing the impedance measurement of the skin of a human body;
After the measurement of the skin impedance of the human body is completed, the impedance measurement module sends a control signal to the alternating current excitation current source, so that the alternating current excitation current source is turned off, the maternal-fetal electrocardiosignals are collected through the P electrode, the N electrode and other collecting electrodes, and the maternal-fetal electrocardiosignals are subjected to maternal-fetal electrocardiosignal separation by combining the skin impedance of the human body measured by the impedance measurement module, so that the fetal electrocardiosignals are obtained.
Optionally, the gyroscope data analysis module is specifically configured to:
Acquiring the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction acquired by the triaxial gyroscope in real time, and calculating the vector module length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment;
Caching the vector modular length at each moment to obtain a vector modular length time sequence;
carrying out local spectrum transformation of unit window data on the vector modulus long-time sequence by adopting short-time Fourier transformation, and converting the local spectrum data into power spectrum data;
And summing the power spectrum data, calculating a power total value, judging that the current human body posture is in an active state if the power total value exceeds a threshold value, and judging that the current human body posture is in a static state if the power total value exceeds the threshold value.
Optionally, calculating the vector modular length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment includes:
The instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment is represented as a vector space, the Euclidean norm of the vector space at each moment is calculated, and the vector modular length at each moment is obtained.
From the above technical scheme, the skin impedance measurement method and system for fetal electrocardiograph monitoring provided by the invention have the following advantages:
The skin impedance measurement method for fetal electrocardiosignal monitoring provided by the invention comprises the steps of collecting human body posture information of a pregnant woman by using a triaxial gyroscope, judging whether the pregnant woman is in a calm state currently, starting an impedance measurement link when the pregnant woman is in the calm state, starting an alternating current excitation current source, applying preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, obtaining a differential signal between the P electrode and an N electrode through a differential amplifier as an excitation response signal, preprocessing the excitation response signal to obtain the excitation response signal after filtering the maternal-fetal bioelectric signal, and performing discrete Fourier transform on the excitation response signal after filtering the maternal-fetal bioelectric signal to solve complex results of a fundamental frequency real part and an imaginary part, thereby obtaining the skin impedance of the human body.
The principle and effect of the skin impedance measurement system for fetal electrocardiograph monitoring provided by the invention are the same as those of the skin impedance measurement method for fetal electrocardiograph monitoring provided by the invention, and the skin impedance measurement system for fetal electrocardiograph monitoring is not repeated here.
Drawings
For a clearer description of embodiments of the invention or of solutions according to the prior art, the figures which are used in the description of the embodiments or of the prior art will be briefly described, it being obvious that the figures in the description below are only some embodiments of the invention, from which, without the aid of inventive efforts, other relevant figures can be obtained for a person skilled in the art.
FIG. 1 is a flow chart of a skin impedance measurement method for fetal electrocardiograph monitoring provided in the present invention;
FIG. 2 is a schematic diagram of a skin impedance measurement system for fetal electrocardiograph monitoring according to the present invention;
Fig. 3 is a schematic diagram of a fetal electrocardiograph flow path of a skin impedance measurement system for fetal electrocardiograph monitoring provided in the present invention.
Detailed Description
In order to make the present invention better understood by those skilled in the art, the following description will clearly and completely describe the technical solutions in the embodiments of the present invention with reference to the accompanying drawings, and it is apparent that the described embodiments are only some embodiments of the present invention, not all embodiments. All other embodiments, which can be made by those skilled in the art based on the embodiments of the invention without making any inventive effort, are intended to be within the scope of the invention.
For ease of understanding, referring to fig. 1, an embodiment of a skin impedance measurement method for fetal electrocardiographic monitoring is provided in the present invention, comprising:
step 101, acquiring human body posture information through a triaxial gyroscope arranged on a pregnant woman.
It should be noted that, in the embodiment of the present invention, the three-axis gyroscope on the pregnant woman may be mounted on the abdomen of the pregnant woman in a pasting manner, and the three-axis gyroscope adopts the three-axis gyroscope of the BMI160 inertial navigation component. The sampling rate of the three-axis gyroscope is set to 1600Hz, and the human body posture information of the pregnant woman is acquired through the three-axis gyroscope. Specifically, the triaxial gyroscope can collect the instantaneous angular velocity values of the human body in X, Y, Z three dimensions in real time, which are respectively recorded as g 1、g2 and g 3, and the vector space of the instantaneous angular velocity values of the human body in X, Y, Z three dimensions corresponding to the time t is represented by g t, so that g t corresponding to each time is represented as:
gt＝{g1,g2,g3}
The vector modulo length, l, is calculated by solving the Euclidean norm for three elements in g t:
The vector modulus length l is proportional to the human activity intensity and is used for measuring the instantaneous intensity of human activity at the current time t. In the embodiment of the invention, the vector module length l of the current time t is calculated in real time, and the newly generated vector module length l is continuously cached and combined into an unlimited time sequence, namely a vector module length time sequence d (x):
d(x)={l0,l1,......,∞}
Where l 0 is the vector modulo length of the first cache and l 1 is the vector modulo length of the second cache.
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 three axes X, Y, Z tend to be 0, otherwise, the greater the intensity of irregular movement of the human body posture is, the greater the absolute value of the corresponding angular velocity output by the sensor is.
Because the single measurement result of the gyroscope is an instantaneous value, but the instantaneous value cannot effectively represent the current activity state of the human body, finite-length time sequence data are required to be constructed by sliding a time window or rolling the time window, and the activity state of the human body is estimated by analyzing the data characteristics in each unit time window
At 1600Hz sampling rate, using the buffer 2048 data points as unit window data, adopting Short Time Fourier Transform (STFT) to perform local spectrum transformation of the unit window data according to the time sequence of the vector modulus long time sequence, combining the unit window data with a window function to perform spectrum conversion, and solving the local spectrum of the vector modulus long time sequence d (x). The Short Time Fourier Transform (STFT) formula is defined as:
m∈[0,1,......,∞]
n∈[0,1,......,1024]
Where H is the frame shift length, preferably 512, m is the number of frame shifts, d (mXH+k) is the vector modulo long time series, w (k) is the window function, j is the imaginary unit, and n is the argument of the windowed Fourier transform.
In order to mitigate the influence of spectrum leakage generated in the short-time Fourier transform process, a Hanning window is selected for calculation, the length of the Hanning window is 2048 points, and the window function w (k) is defined as:
When the independent variable m increases, the hanning window function is represented as sliding along the d (x) sequence, the sliding step is h=512 data points, after each sliding of the window function, the unit window data of the subscript corresponding to d (x) is subjected to one-dimensional matrix multiplication, the multiplied sequence result is solved, the multiplied sequence result is converted into a complex sequence with the imaginary number of 0, discrete Fourier transformation is carried out, and the single local spectrum transformation of d (x) is completed.
Compared with the traditional Fourier Transform (FT), the short-time Fourier transform (STFT) can reflect not only the spectrum characteristics of the original signal, but also the time-varying law of the spectrum of the original signal from a higher dimension. Therefore, by means of the frequency domain characteristics and the time domain characteristics of the STFT, the change intensity of the gyroscope data in the window time can be accurately counted, and the time boundary of the human body entering the calm state can be found. And then the activity state of the human body at the current moment is judged.
After the local spectrum transformation is completed, the local spectrum data are converted into power spectrum data, energy corresponding to all frequencies in the power spectrum data is summed, a power total value is calculated, if the power total value exceeds a threshold value, the current human body posture is judged to be in an active state, otherwise, the current human body posture is judged to be in a static state, and the pregnant woman is regarded as being in a calm state. When the current human body posture is judged to be in a static state, the impedance measurement of the human body skin is carried out, otherwise, the new data is cached by the sliding unit window continuously, the local frequency spectrum transformation of the next round of unit window data is carried out, and the corresponding human body posture judgment result is output.
Step 102, judging whether the pregnant woman is in a calm state according to the human body posture information, if so, starting an impedance measurement link, and measuring the skin impedance of the human body.
When the current human body posture is judged to be in a static state according to the total power value in the step 101, namely, when the pregnant woman is in a calm state, an impedance measurement link is started, an alternating current excitation current source is started, preset alternating current is applied to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, a differential signal between the P electrode and an N electrode is obtained through a differential amplifier to serve as an excitation response signal, wherein the alternating current excitation current source, the P electrode and the N electrode are connected in a ring mode to form a loop, the P electrode and the N electrode are connected with two input ends of the differential amplifier, the excitation response signal is preprocessed, the excitation response signal after the maternal biological electrical signal is filtered is obtained, the preprocessing comprises analog-digital conversion processing and filtering processing, discrete Fourier transformation is carried out on the excitation response signal after the maternal biological electrical signal is filtered, and a complex result of a fundamental frequency real part and an imaginary part is solved, so that the human skin impedance is obtained.
In typical human skin impedance measurement applications, the impedance measurement range is usually from the left upper limb to the right lower limb of the human body, most of the excitation signals applied to the human body are alternating current signals with the frequency being more than 50KHz and the current amplitude being 300 microamps, and the alternating current excitation frequency applied to human tissues is high, the relative intensity of the current is high, and the risk of human electrolyte flocculation is caused. When fetal electrocardiographic detection is carried out, the electrode pair is usually attached to the abdomen of a human body, the attaching positions of the electrode pair are similar, and the impedance generated by the skin between the P electrode and the N electrode and the interface between the electrode and the skin is measured. In the embodiment of the invention, a 250Hz low-frequency square wave excitation signal which is not easy to penetrate human skin tissue is adopted, and a 24nA weak current signal which is harmless to the skin tissue is adopted. By applying a constant frequency, constant current square wave excitation current signal to human skin, a signal response is induced by the complex impedance of human skin and the electrode-skin interface impedance characteristics, providing a response signal with complex impedance information for subsequent impedance measurements.
The skin impedance measurement method for fetal electrocardio monitoring provided by the embodiment of the invention utilizes a triaxial gyroscope to acquire human body posture information of a pregnant woman, judges whether the pregnant woman is in a calm state at present, starts an impedance measurement link when the pregnant woman is in the calm state, starts an alternating current excitation current source, applies preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, acquires a differential signal between the P electrode and an N electrode as an excitation response signal through a differential amplifier, preprocesses the excitation response signal to obtain an excitation response signal after filtering a maternal biological electrical signal, and then carries out discrete Fourier transform on the excitation response signal after filtering the maternal biological electrical signal to solve complex results of a real part and an imaginary part of fundamental frequency to obtain the skin impedance of the human body. The current of the alternating current excitation current source flows out from the N electrode after being led into the human skin from the P electrode, and under the condition of constant current, the response signal is caused between the P electrode and the N electrode due to the complex impedance characteristic of the electric signal of the human skin and the impedance characteristic of the gel bonding degree between the electrode pair and the skin, and 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 interface of the electrode and the skin. The skin impedance measurement method for fetal electrocardio monitoring provided by the invention considers the imaginary part information of the excitation response signal, reduces the error of human skin impedance measurement, avoids the risk of thermal noise interference of the analog switch device and external noise interference, improves the accuracy of skin impedance measurement for fetal electrocardio monitoring, and solves the technical problems of ignoring the imaginary part information of the excitation response signal and noise interference caused by the analog switch device in the existing impedance measurement method at the interface of skin and electrode, thereby reducing the accuracy of impedance measurement.
In one embodiment, in the impedance measurement link, an ac excitation current source applies an excitation signal to human skin through a P-electrode pair, thereby causing a response signal. The response signal belongs to an aliasing signal and comprises an excitation response signal and a maternal bioelectric signal, wherein the excitation response signal and the maternal bioelectric signal are caused by complex impedance of human skin to an excitation signal. The aliasing signals are subjected to analog-to-digital conversion, excitation response signals are screened out through a digital band-pass filter, the digital band-pass filter filters 1024 sampling values cached in a rolling window, the conducting range is 240 Hz-260 Hz, and the filtering output result is a one-dimensional real sequence a (r) with the length equal to 1024. Constructing a finite-length complex sequence s (r) with an imaginary number of 0 by using a (r), and performing discrete Fourier transform on the finite-length complex sequence s (r), wherein a formula of the discrete Fourier transform is defined as follows:
where F (q) is the result of the discrete Fourier transform and s (r) is the finite length complex sequence of the input.
The result F (q) of the discrete fourier transform is in complex form, so that each frequency component contains real and imaginary information. Since the frequency of the signal applied to the human body by the ac excitation current source is 250Hz, complex information whose fundamental frequency component is 250Hz is extracted from the spectrum result F (q):
Z250＝R250+jI250
Wherein j is an imaginary unit, R 250 is a real value of complex information with a fundamental wave frequency component of 250Hz extracted from a discrete Fourier transform result, the real value is characterized as an electrical impedance component in the measured human body complex impedance, I 250 is a virtual value of complex information with the fundamental wave frequency component of 250Hz extracted from the discrete Fourier transform result, and the real value is characterized as a capacitance component in the measured human body complex impedance.
Setting the differential amplification factor of the differential amplifier as b, and calculating the final human skin impedance by combining the differential amplification factor as b as follows:
Wherein Z is the impedance of human skin.
The spectrum conversion is carried out on the excitation response signal with limited length, and the impedance calculation is carried out through real number and imaginary number information, so that the accuracy and stability of the system for measuring the impedance of the human skin are improved.
In one embodiment, after the measurement of the skin impedance of the human body is completed, an alternating current excitation current source is turned off, a fetal electrocardiograph link is started, a maternal electrocardiograph signal is collected through a P electrode, an N electrode and other collecting electrodes, and maternal electrocardiograph signals are separated by combining the measured skin impedance of the human body, so that fetal electrocardiograph signals are obtained.
After the impedance measurement of the skin of the human body is completed, the alternating current excitation current source is turned off, the P electrode and the N electrode respectively collect human body potential signals, and the differential signals of the P electrode and the N electrode are mother fetal electrocardiosignals. After the alternating current excitation current source is turned off, the impedance measurement function is turned off, and excitation response signals caused by an excitation source on the electrode pairs do not exist, so that signals output by the analog-to-digital converter only comprise mother tyre mixed bioelectric signals, the mother tyre mixed bioelectric signals are filtered through an IIR digital filter, direct current components and power frequency interference in the signals are removed, and QRST signals of mother tyre electrocardiosignals are extracted. And according to the characteristics of the maternal-fetal electrocardiosignals, completing the maternal-fetal electrocardiosignal separation through a principal component analysis algorithm.
The circuit structure used for the skin impedance measurement method for fetal electrocardio monitoring provided by the embodiment of the invention is shown in figure 2, and comprises a gyroscope data analysis module, a P electrode, an N electrode, a triaxial gyroscope, an alternating current excitation current source, a differential amplifier, an analog-to-digital converter, a right leg driving circuit module, an information interaction interface and an impedance measurement module, wherein the triaxial gyroscope is connected with the gyroscope data analysis module, the gyroscope data analysis module is connected with the alternating current excitation current source, the positive end of the alternating current excitation current source is connected with the P electrode, the negative end is connected with the N electrode, the P electrode and the N electrode are connected with two input ends of the differential amplifier, the output end of the differential amplifier is connected with the input end of the analog-to-digital converter and the right leg driving circuit module, the right leg driving circuit module comprises a right leg driving circuit and an E electrode, the three-axis gyroscope, the P electrode and the N electrode are attached to the abdomen of a pregnant woman, the output end of the analog-to-digital converter is connected with the impedance measurement module, the output end of the impedance measurement module is connected with the information interaction interface, the output end of the impedance measurement module is also connected with the input end of the alternating current excitation current source, and the impedance measurement module sends a control signal to the alternating current excitation current source after completing impedance measurement, so that the alternating current excitation current source is closed. The circuit structure of fig. 2 operates on the principle that:
The gyroscope is used for detecting the posture change of the human body, when the human body posture is detected to be in an active state, impedance measurement is not carried out, and otherwise, the impedance measurement is carried out.
The alternating current excitation current source is used for generating an alternating current excitation signal harmless to a human body.
The system input electrode comprises at least one set of electrode pairs, each pair comprising a P electrode and an N electrode. During impedance measurement, current is applied to human skin from the P electrode and from the N electrode loop. When fetal electrocardio detection is carried out, an alternating current excitation power supply is turned off, and the P electrode and the N electrode respectively collect human body potential signals.
The differential amplifier is used for amplifying differential signals formed between each group of P electrodes and N electrodes. During impedance measurement, the differential signals of the P electrode and the N electrode are excitation response signals due to the complex impedance characteristic of human skin, and during fetal electrocardio detection, the differential signals of the P electrode and the N electrode are mother fetal mixed electrocardio signals.
The analog-to-digital converter is used for carrying out quantization sampling on the signals after differential amplification.
The impedance measurement module comprises a digital band-pass filter and an impedance analysis unit, and the fetal electrocardiograph measurement module comprises an IIR digital filter and a mother fetal electrocardiograph separation module.
The digital band-pass filter is used for filtering when detecting impedance, so that interference of electrocardiosignals of a human body is reduced.
The IIR digital filter is used for filtering when detecting fetal electrocardio, and reducing interference of direct current and high-frequency signals.
The impedance analysis unit is used for analyzing a response signal formed by a human body to the excitation source and calculating an impedance value from the response signal;
The number of the P-electrode and the N-electrode and other collecting electrodes (a plurality of electrodes are used in fetal electrocardiographic detection, including the P-electrode and the N-electrode in fig. 2, and further including other collecting electrodes, and the number of the other collecting electrodes is selected according to an actual monitoring scene, which is not limited herein) are used for collecting maternal electrocardiographic signals in fetal electrocardiographic detection.
The maternal-fetal electrocardiosignal separation module is used for analyzing electrocardiosignals acquired from a human body, separating maternal-fetal electrocardiosignals and extracting fetal electrocardiosignals;
the system comprises a right leg driving circuit, wherein the right leg driving circuit feeds common mode signals to a human body through an E electrode and is used for inhibiting common mode interference caused by human body functions during electrocardiograph measurement.
The information interaction interface is used for issuing the calculated real-time impedance data or fetal electrocardio data outwards. The information interaction interface adopts a wireless mode of BLE low-power consumption Bluetooth to form a man-machine interaction channel.
When measuring the impedance formed by the electrode pair (P electrode and N electrode) and the skin, the impedance measurement is divided into a front-stage system and a rear-stage system, wherein the front-stage system is an alternating current excitation source plus electrode (front input), and the rear-stage system is amplification plus analog-digital conversion plus signal analysis (rear-stage processing). The front stage system introduces an alternating current excitation current source, the excitation current source generates square wave excitation signals with constant on-off frequency and constant current, the excitation signals are applied to the P electrode, and the current flows out of the N electrode after passing through the skin of a human body and forms a current signal loop with the excitation current source. The differential amplifier in the post-stage system is used for amplifying signals of the P electrode and the N electrode, sampling and quantizing are carried out by the analog-to-digital converter, and after the quantized result is filtered by the digital band-pass filter, the impedance analysis unit carries out data analysis on the filtered result. The current flows out from the N electrode after being led into the human skin from the P electrode, and under the condition of constant current, a response signal is caused between the P electrode and the N electrode due to the complex impedance characteristic of the electric signal of the human skin and the impedance characteristic of the gel bonding degree between the electrode pair and the skin, and 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 interface of the electrode and the skin.
The latter stage system carries out differential signal amplification and analog-to-digital conversion sampling on the electrode pair attached to the skin of the human body. The common mode interference of bioelectric signals can be effectively reduced by using a differential input mode, and the differential signals are amplified and then are subjected to signal quantization sampling by using an analog-to-digital converter. According to the rolling window buffer mechanism, a first-in first-out rule is followed, the length of 1024 sampling values are buffered in a rolling way, and the buffered time sequence data not only comprises excitation response signal data caused by the application of excitation current to the skin, but also comprises electrocardiosignal data of a human body.
Typically, the spectral energy range of the human body electrocardiosignal is less than 30Hz, and the spectral energy of the excitation signal applied by the front stage is concentrated at 250Hz and the frequency multiplication thereof. In order to demodulate and separate response data with complex impedance information from time sequence data of the aliasing electrocardiac, in this embodiment, a digital band-pass filter is constructed by adopting a digital signal processing technology, and the response data with complex impedance information is screened out from the sampled aliasing digital signal by the filter. The passband of the digital band-pass filter is 240 Hz-260 Hz.
For ease of understanding, referring to fig. 2 and 3, an embodiment of a skin impedance measurement system for fetal electrocardiograph monitoring is provided in the present invention, which includes a gyroscope data analysis module, a P-electrode, an N-electrode, a tri-axis gyroscope, an ac excitation current source, a differential amplifier, an analog-to-digital converter, a right leg driving circuit module, an information interaction interface, and an impedance measurement module;
the three-axis gyroscope is connected with the gyroscope data analysis module;
the gyroscope data analysis module is connected with an alternating current excitation current source;
The positive end of the alternating current excitation current source is connected with the P electrode, the negative end of the alternating current excitation current source is connected with the N electrode, the P electrode and the N electrode are connected with two input ends of the differential amplifier, the output end of the differential amplifier is connected with the input end of the analog-to-digital converter and the right leg driving circuit module, the right leg driving circuit module comprises a right leg driving circuit and an E electrode, the E electrode is attached to the right leg of a pregnant woman, and the three-axis gyroscope, 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 with the impedance measurement module, and the output end of the impedance measurement module is connected with the information interaction interface;
When the skin impedance measuring system for fetal electrocardio monitoring is used for measuring the skin impedance of a human body, the three-axis gyroscope collects the human body posture information and sends the human body posture information 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, an impedance measuring link is started, and the skin impedance of the human body is measured;
The impedance measurement link includes:
Starting an alternating current excitation current source, applying preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, and obtaining a differential signal between the P electrode and an N electrode through a differential amplifier to serve as an excitation response signal, wherein the alternating current excitation current source, the P electrode and the N electrode are connected in a ring shape to form a loop, and the P electrode and the N electrode are connected with two input ends of the differential amplifier;
Preprocessing the excitation response signal through an impedance measurement module to obtain an excitation response signal after the maternal biological electrical signal is filtered, wherein the preprocessing comprises analog-to-digital conversion processing and filtering processing;
And performing discrete Fourier transform on the excitation response signal after the maternal biological electrical signal is filtered by an impedance measurement module, and solving complex results of a real part and an imaginary part of fundamental frequency to obtain the skin impedance of the human body.
Also included are fetal electrocardiograph modules and other acquisition electrodes (not labeled in fig. 2 and 3);
the other acquisition electrodes are connected with the input end of the differential amplifier, the input end of the fetal electrocardiograph measurement module is connected with the output end of the analog-to-digital converter, and is also connected with the impedance measurement module, and the output end of the fetal electrocardiograph measurement module is connected with the information interaction interface;
The fetal electrocardiosignal module is used for measuring fetal electrocardiosignals after finishing the impedance measurement of the skin of a human body;
After the measurement of the skin impedance of the human body is completed, the impedance measurement module sends a control signal to the alternating current excitation current source, so that the alternating current excitation current source is turned off, the maternal-fetal electrocardiosignals are collected through the P electrode, the N electrode and other collecting electrodes, and the maternal-fetal electrocardiosignals are subjected to maternal-fetal electrocardiosignal separation by combining the skin impedance of the human body measured by the impedance measurement module, so that the fetal electrocardiosignals are obtained.
The gyroscope data analysis module is specifically used for:
Acquiring the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction acquired by the triaxial gyroscope in real time, and calculating the vector module length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment;
Caching the vector modular length at each moment to obtain a vector modular length time sequence;
carrying out local spectrum transformation of unit window data on the vector modulus long-time sequence by adopting short-time Fourier transformation, and converting the local spectrum data into power spectrum data;
And summing the power spectrum data, calculating a power total value, judging that the current human body posture is in an active state if the power total value exceeds a threshold value, and judging that the current human body posture is in a static state if the power total value exceeds the threshold value.
Calculating the vector modular length of each moment according to the instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment, including:
The instantaneous angular velocity value of the human body in the X, Y, Z dimension direction at each moment is represented as a vector space, the Euclidean norm of the vector space at each moment is calculated, and the vector modular length at each moment is obtained.
The short-time Fourier transform formula for carrying out local frequency spectrum transformation of unit window data on the vector modulus long-time sequence by adopting short-time Fourier transform is as follows:
m∈[0,1,......,∞]
n∈[0,1,......,1024]
Where H is the frame shift length, m is the number of frame shifts, d (mxh+k) is the vector modulo long time series, w (k) is the window function, j is the imaginary unit, and n is the argument of the windowed fourier transform.
The preset alternating current is a square wave excitation current of 250Hz24 nA.
Performing discrete Fourier transform on the excitation response signal after the maternal biological electrical signal is filtered, solving complex results of a real part and an imaginary part of fundamental frequency, and obtaining skin impedance of a human body, wherein the method comprises the following steps:
constructing an excitation response signal with filtered maternal-fetal bioelectric signals into a finite-length complex sequence with an imaginary number of 0, and performing discrete Fourier transform on the finite-length complex sequence with the imaginary number of 0, wherein the formula of the discrete Fourier transform is as follows:
wherein F (q) is the result of discrete Fourier transform, s (r) is the input finite-length complex sequence;
The complex information with the fundamental wave frequency component of 250Hz is taken out from the discrete Fourier transform result, and the human skin impedance is calculated by combining the differential amplification factor of the differential amplifier, wherein the calculation formula is as follows:
Wherein 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 the fundamental frequency component of 250Hz extracted from the discrete Fourier transform result, and I 250 is the imaginary value of the complex information with the fundamental frequency component of 250Hz extracted from the discrete Fourier transform result.
The skin impedance measurement system for fetal electrocardio monitoring provided by the invention utilizes the three-axis gyroscope to acquire human body posture information of a pregnant woman, judges whether the pregnant woman is in a calm state at present, starts an impedance measurement link when the pregnant woman is in the calm state, starts an alternating current excitation current source, applies preset alternating current to the skin surface of the pregnant woman through a P electrode connected with the alternating current excitation current source, acquires a differential signal between the P electrode and an N electrode as an excitation response signal through a differential amplifier, preprocesses the excitation response signal to obtain the excitation response signal after filtering the maternal biological electrical signal, and then carries out discrete Fourier transform on the excitation response signal after filtering the maternal biological electrical signal, so as to solve complex results of a real part and an imaginary part of fundamental frequency, and obtain human skin impedance.
The principle of the skin impedance measurement system for fetal electrocardiograph monitoring provided in the embodiment of the present invention is the same as that of the skin impedance measurement method for fetal electrocardiograph monitoring in the embodiment of the skin impedance measurement method for fetal electrocardiograph monitoring, and the skin impedance measurement system for fetal electrocardiograph monitoring is not described in detail herein.
While the invention has been described in detail with reference to the foregoing embodiments, it will be understood by those skilled in the art that the foregoing embodiments may be modified or equivalents may be substituted for some of the features thereof, and that the modifications or substitutions do not depart from the spirit and scope of the embodiments of the invention.
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