CN113100739A - Portable multi-frequency electrical impedance imaging front-end data acquisition and processing method - Google Patents
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Abstract
The invention discloses a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, which comprises the steps of converting a sine signal generated by a DAC into a current signal by using a voltage-controlled constant current source, and carrying out current excitation under the action of a first analog switch; acquiring a reference input signal through a sampling resistor, gating different groups of electrodes through a second analog switch, and performing program control gain by using an inverted T-shaped program control gain amplifier to obtain a measurement input signal; the reference input signal and the measurement input signal are synchronously acquired, and multiple single-point Fourier demodulation is carried out according to the periodicity and the symmetry of the digital signal and the symmetry of the trigonometric function to obtain the phase information of the signal, so that the conventional demodulation method is improved, the required multiplication times are greatly reduced, and the demodulation speed is increased.
Description
Technical Field
The invention relates to the technical field of electrical impedance imaging, in particular to a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method.
Background
Electrical impedance imaging (EIT) includes a process of capacitive imaging (ECT) which is widely used in the industrial field, particularly for the detection of multiphase flow, and resistive imaging (ERT) which is directed to the biomedical field, particularly various tumor examinations. The measuring method is non-invasive examination, and the examination process comprises the steps of arranging electrode sensors in a distributed manner around biological tissues, injecting safe sinusoidal constant-current excitation signals, generating an electric field after the excitation signals pass through the biological tissues, and enabling the internal electric field to change due to the characteristic that different biological tissues or the same biological tissue have different conductivities when the biological tissues are in different physiological states. And then the electric field information is acquired through a high-speed voltage signal acquisition device, and a conductivity distribution image in the tissue is reconstructed through a certain image reconstruction algorithm, so that whether the inside of the biological tissue is diseased or not and the position of the diseased are judged.
The EIT front-end acquisition system is developed for thirty years, changes from an analog circuit to a digital circuit, and changes from large volume, high power consumption and circuit complexity to small volume, low power consumption and simplified circuit on hardware. The main difficulty in the prior art is shown as follows: because the number of the electrodes is large and the positions have large difference, the gain multiples in the detection circuit cannot be unified, and meanwhile, the number of multiplications required by demodulation is too many when the amplitude and the phase of a detection signal are acquired, so that the demodulation speed is reduced.
Disclosure of Invention
The invention aims to provide a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, which improves the demodulation speed.
In order to achieve the above object, the present invention provides a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, comprising the following steps:
a voltage-controlled constant current source is utilized to convert a sine signal generated by the DAC into a current signal, and current excitation is carried out under the action of a first analog switch;
acquiring a reference input signal through a sampling resistor, and gating different groups of electrodes through a second analog switch to perform program control gain to obtain a measurement input signal;
and synchronously acquiring the reference input signal and the measurement input signal, and performing multiple single-point Fourier demodulation to obtain phase information and amplitude information of the signals.
The voltage-controlled constant current source is used for converting a sine signal generated by the DAC into a current signal, and current excitation is carried out under the action of the first analog switch, and the method comprises the following steps:
controlling a DAC to generate a sinusoidal signal based on the MCU, and filtering the sinusoidal signal by using a three-order low-pass filter;
and converting the filtered sinusoidal signal into a current signal by using a voltage-controlled constant current source, and carrying out current excitation on the sample based on the selection of the first analog switch on the electrode array.
Wherein, obtain reference input signal through sampling resistance to carry out programme-controlled gain through the different group of electrode of second analog switch gating, obtain measurement input signal, include:
acquiring a reference input signal of the sample after current excitation through a sampling resistor;
based on the selection of a second analog switch to the electrode array, converting the current signal into a differential voltage signal, and filtering and differentially amplifying the differential voltage signal;
and carrying out program control gain on different groups of electrodes by using an inverted T-shaped program control gain amplifier to obtain a measurement input signal.
The method for synchronously acquiring the reference input signal and the measurement input signal and performing multiple single-point Fourier demodulation to obtain phase information and amplitude information of the signals comprises the following steps:
synchronously acquiring the reference input signal and the measurement input signal, and obtaining a reference signal discrete sequence and a measurement signal discrete sequence based on the acquired sampling number and basic parameters of the reference input signal and the measurement input signal;
and performing multiple single-point Fourier demodulation on the reference signal discrete sequence and the measurement signal discrete sequence to obtain phase information and amplitude information of the reference input signal and the measurement input signal.
Performing multiple single-point fourier demodulation on the reference signal discrete sequence and the measurement signal discrete sequence to obtain phase information and amplitude information of the reference input signal and the measurement input signal, including:
obtaining a corresponding single-point Fourier coefficient according to the frequency of the sinusoidal signal;
optimizing the discrete Fourier coefficients according to the periodicity and symmetry of the digital signal and the symmetry of the trigonometric function;
performing difference on the optimized discrete Fourier coefficients corresponding to the reference signal discrete sequence and the measurement signal discrete sequence to obtain phase information of the reference input signal and the measurement input signal;
and calculating the modulus value of the digital signal to obtain corresponding amplitude information.
Wherein optimizing the discrete Fourier coefficients according to the periodicity and symmetry of the digital signal and the symmetry of the trigonometric function comprises:
sequentially reducing the complex multiplication times of the discrete Fourier coefficients to 10 times according to the periodicity and symmetry of the digital signals;
and respectively converting the products of the reference signal discrete sequence and the measurement signal discrete sequence and the digital signal into multiplication of the reference signal discrete sequence and the measurement signal discrete sequence with the real part of the digital signal and multiplication with the imaginary part of the digital signal, and completing optimization of the Fourier coefficient based on the symmetry of a trigonometric function.
The invention relates to a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, which comprises the steps of converting a sine signal generated by a DAC into a current signal by using a voltage-controlled constant current source, and carrying out current excitation under the action of a first analog switch; acquiring a reference input signal through a sampling resistor, gating different groups of electrodes through a second analog switch, and performing program control gain by using an inverted T-shaped program control gain amplifier to obtain a measurement input signal; the reference input signal and the measurement input signal are synchronously acquired, and multiple single-point Fourier demodulation is carried out according to the periodicity and the symmetry of the digital signal and the symmetry of the trigonometric function to obtain the phase information of the signal, so that the conventional demodulation method is improved, the required multiplication times are greatly reduced, and the demodulation speed is increased.
Drawings
In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings used in the description of the embodiments or the prior art will be briefly described below, it is obvious that the drawings in the following description are only some embodiments of the present invention, and for those skilled in the art, other drawings can be obtained according to the drawings without creative efforts.
Fig. 1 is a schematic step diagram of a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method provided by the present invention.
Fig. 2 is a schematic structural diagram of a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method provided by the invention.
Fig. 3 is a schematic structural diagram of a low-pass filter provided by the present invention.
Fig. 4 is a low-pass filter amplitude-frequency response curve provided by the present invention.
Fig. 5 is a schematic structural diagram of an analog switch provided by the present invention.
Fig. 6 is a schematic structural diagram of the inverted T-shaped resistor network programmable gain amplifier provided by the present invention.
Detailed Description
Reference will now be made in detail to embodiments of the present invention, examples of which are illustrated in the accompanying drawings, wherein like or similar reference numerals refer to the same or similar elements or elements having the same or similar function throughout. The embodiments described below with reference to the drawings are illustrative and intended to be illustrative of the invention and are not to be construed as limiting the invention.
In the description of the present invention, "a plurality" means two or more unless specifically defined otherwise.
Referring to fig. 1, the present invention provides a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, including the following steps:
and S101, converting a sine signal generated by the DAC into a current signal by using a voltage-controlled constant current source, and carrying out current excitation under the action of a first analog switch.
Specifically, as shown in fig. 2, the MCU controls the DAC to generate a sinusoidal signal, the excitation module peripheral circuit mainly includes a low-pass filter, and although the electrical impedance imaging uses a multi-frequency technique, the frequency range is generally 10KHz to 100KHz, so the low-pass filter only needs to be designed to reserve a frequency band below 100KHz, thereby reducing high-frequency signal components. Because the electrical impedance imaging needs to form an electromagnetic field in a human body in a current excitation mode, a voltage signal of a low-pass filter needs to be converted into a current signal through a voltage-controlled constant current source, and the current excitation is carried out on the human body under the action of an analog switch.
The low-pass filter is mainly composed of two stages as shown in fig. 3, wherein the stage a is composed of a first-order low-pass buffer RC plus an operational amplifier, the stage B is composed of a second-order low-pass filter fed back by a Sallen Key by using the operational amplifier, the two stages are composed of a third-order Butterworth low-pass filter, the amplitude-frequency response characteristic curve of the third-order Butterworth low-pass filter is shown in fig. 4, the pass band is 10KHz, the stop band is 100KHz, and the signal is attenuated by 60dB (1000 times) at 100 KHz.
The signal that the low pass filter comes out is the sine wave of voltage type, the excitation signal that the electrical impedance forms images needs the current type signal, the invention adopts the Howland current pump to change the voltage signal into the current signal, this current signal acts on the electrode to stimulate the biological tissue under the analog selection, the analog switch selection circuit is shown as figure 5, U1 analog switch (namely first analog switch) device chooses the x (1-16) electrode as "current excitation +", U3 analog switch device chooses the y (1-16) electrode as "current excitation-", "current excitation-" returns to the system MCU through connecting a sampling resistance (Rg) in series, the voltage of this sampling resistance is input to ADC2 as the reference voltage.
And S102, acquiring a reference input signal through the sampling resistor, and gating different groups of electrodes through the second analog switch to perform program control gain to obtain a measurement input signal.
Specifically, a reference input signal is obtained in a loop of the current through a sampling resistor and is input into the ADC 2. The detection circuit obtains human skin surface signals under the action of the electrodes, different groups of electrodes are gated through the analog switch for differential amplification, a band-pass filter suitable for electrical impedance imaging is designed due to the fact that a direct-current component exists on the surface of the human body before differential amplification, and the band-pass filter extracts electrical impedance imaging signals of 10KHz-100 KHz. Because the system is used for multi-electrode imaging, the amplification factors of different groups of electrodes are difficult to fix, and the inverted T-shaped resistance network operational amplifier is designed to carry out program control gain on the different groups of electrodes.
The U2 analog switch (i.e. the second analog switch) in fig. 5 selects the h (1-16) electrode as "voltage detection +", and the U6 analog switch selects the z (1-16) electrode as "voltage reception-", this pair of differential voltages will include some noises in the environment, especially static electricity and other noises in human skin, the range of useful signal is 10KHz-100KHz, the invention designs a band-pass filter to filter out noise interference, then amplifies this pair of differential signals with the differential amplifier.
The invention designs a program control gain amplifier (PAG) which has larger gain range and smaller gain step and is suitable for the electrical impedance imaging system by adopting an inverted T-shaped resistance network. As shown in fig. 6, the inverted T resistor network programmable gain amplifier is only provided with resistors with three resistance values of R, 2R and 10R, which brings great convenience to design and manufacture of an integrated circuit. Wherein, ViFor input signal, V0For outputting signals, the circuit of the invention is composed of 10 analog switches and operational amplifiers, if d is orderediWhen equal to 0, the second analog switch is grounded (i.e. V of the amplifier)+) And d isiA second 1-mode analog switch connected to the input of the amplifierTerminal V-Then flow into V-The total current of the terminal is
Where formula 1, I formula 2, indicates the current through the resistor. diIndicating the conducting state of the analog switch, 0 indicating non-conduction and 1 indicating conduction.
Under the condition that the feedback resistance of the summing amplifier is equal to 10R, the output voltage is given by equation 3
V in formula 3iFor the input signal of the programmable gain amplifier, V0D is a digital quantity for controlling the analog switch.
S103, synchronously collecting the reference input signal and the measurement input signal, and performing multiple single-point Fourier demodulation to obtain phase information and amplitude information of the signals.
Specifically, when the frequency of the excitation signal is 50KHz and the sampling period of the ADC is 1MHz, 1000 samples are taken for the reference signal and the measurement signal, and then the discrete sequence x1[ n ] of the reference signal and the discrete sequence x2[ n ] of the measurement signal can be obtained, where n is in the range [0,999], and the frequency resolution is given by equation 4.
The 50KHz signal has a Fourier coefficient of X [50], and the single point DFT is given by equation 5.
The complex multiplication times of the above formula are up to 1000 times by using digital signals
The periodicity and symmetry of the trigonometric function can reduce the number of complex multiplications to 10, and further, the 10 complex multiplications are optimized to 8 real multiplications by using the symmetry of the trigonometric function. The method greatly saves the operation time and improves the response time of the system.
In the invention, three times of single-point DFT optimization are performed in total, and the first step utilizes
Periodic, second step using
The third step optimizes 10 complex multiplications to 8 real multiplications using the symmetry of the trigonometric function.
First of all, using the formula 6
Periodically reducing the number of complex multiplications to 1000 to 20, and converting the multiplications into additions by using the same multiplier factor in a polynomial combination mode. N1 in formula 6 is a positive integer.
Equation 7 can be obtained by combining equation 6 with equation 5, and the complex multiplication number of equation 7 is reduced to 20 when equation 7 is compared with equation 5.
Then given by equation 8
The symmetry of (2) reduces the number of complex multiplications to 10, and the same multiplier factor converts the multiplications into additions by means of polynomial combination. In the formula 8, n is a positive integer.
Equation 9 can be obtained by combining equation 8 with equation 7, and the complex multiplication number of equation 9 is reduced to 10 when equation 9 is compared with equation 7.
Due to x [ n ]]Is a real number and is a real number,
is a complex number, so x [ n ]]And
can be converted into x n]And
multiplication of real part and x [ n ]]And
the multiplication of the imaginary part is carried out,
developed according to formula 10, wherein x [ n ]]Representing a discrete sequence of reference signals and a discrete sequence of measurement signals.
The symmetry of cos (x) with sin (x) transforms 10 complex multiplications into 10 real multiplications. The symmetry of cos (x) and sin (x) is given by equations 11 and 12, respectively.
The real part of X [50] obtained by the combination of equations 11, 10 and 9 is equation 13, the temporary variable temp _ X in equation 13 is given by equation 14, and it can be seen from equation 13 that only 4 real multiplications are performed.
The imaginary part of X [50] is expressed by equation 15 in conjunction with equations 12, 10 and 9, and the temporary variable temp _ X in equation 15 is expressed by equation 14, and it can be seen from equation 15 that only 4 real multiplications are performed.
Equations 13 and 15 give the real and imaginary parts of X [50], respectively, for a total of 8 real multiplications. In summary, the complex multiplication which originally needs to be performed 1000 times is simplified into real multiplication of 8 times, X [50] is obtained, and the operation speed of the MCU is greatly improved.
X[50]Is a complex number, the amplitude of 50KHz signal can be obtained by calculating the modulus, the amplitude of the detection signal is given by the formula 16, | vi50KHz represents the amplitude of the detected signal.
The phase difference between X1[50] and X2[50] is used to obtain the phase information of reference signal and measurement signal.
Advantageous effects
1. And simplifying the single-point frequency of the DFT by utilizing the periodicity of the acquired EIT digital signal, and extracting a DFT conversion value of the single-point frequency.
2. 16 EIT electrodes are multiplexed by adopting a 16-channel analog switch, the same electrode can be multiplexed into 'current excitation +', 'current excitation-', 'voltage detection +', and 'voltage detection-', in a time-sharing manner, and the analog switch is controlled by an MCU (microprogrammed control unit).
3. A sampling circuit is designed in the current excitation loop to obtain reference voltage input, and the signal can truly reflect the phase characteristics of biological tissues.
4. The detection circuit adopts an inverted T-shaped resistance network operational amplifier to design a special program control gain amplifier for EIT, and gain multiples can be conveniently adjusted in different human tissues.
5. The existing demodulation method is improved, the required multiplication times are greatly reduced, and the demodulation speed is improved.
The invention relates to a portable multi-frequency electrical impedance imaging front-end data acquisition and processing method, which comprises the steps of converting a sine signal generated by a DAC into a current signal by using a voltage-controlled constant current source, and carrying out current excitation under the action of a first analog switch; acquiring a reference input signal through a sampling resistor, gating different groups of electrodes through a second analog switch, and performing program control gain by using an inverted T-shaped program control gain amplifier to obtain a measurement input signal; the reference input signal and the measurement input signal are synchronously acquired, and multiple single-point Fourier demodulation is carried out according to the periodicity and the symmetry of the digital signal and the symmetry of the trigonometric function to obtain the phase information of the signal, so that the conventional demodulation method is improved, the required multiplication times are greatly reduced, and the demodulation speed is increased.
While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.
Claims (6)
1. A portable multi-frequency electrical impedance imaging front-end data acquisition and processing method is characterized by comprising the following steps:
a voltage-controlled constant current source is utilized to convert a sine signal generated by the DAC into a current signal, and current excitation is carried out under the action of a first analog switch;
acquiring a reference input signal through a sampling resistor, and gating different groups of electrodes through a second analog switch to perform program control gain to obtain a measurement input signal;
and synchronously acquiring the reference input signal and the measurement input signal, and performing multiple single-point Fourier demodulation to obtain phase information and amplitude information of the signals.
2. The portable multi-frequency electrical impedance imaging front-end data acquisition and processing method of claim 1, wherein converting the sine signal generated by the DAC into a current signal by using a voltage-controlled constant current source, and performing current excitation under the action of the first analog switch, comprises:
controlling a DAC to generate a sinusoidal signal based on the MCU, and filtering the sinusoidal signal by using a three-order low-pass filter;
and converting the filtered sinusoidal signal into a current signal by using a voltage-controlled constant current source, and carrying out current excitation on the sample based on the selection of the first analog switch on the electrode array.
3. The portable multi-frequency electrical impedance imaging front-end data acquisition and processing method of claim 1, wherein obtaining the reference input signal through a sampling resistor and obtaining the measurement input signal by gating different sets of electrodes through a second analog switch for programmable gain, comprises:
acquiring a reference input signal of a sample after current excitation through a sampling resistor;
based on the selection of a second analog switch to the electrode array, converting the current signal into a differential voltage signal, and filtering and differentially amplifying the differential voltage signal;
and carrying out program control gain on different groups of electrodes by using an inverted T-shaped program control gain amplifier to obtain a measurement input signal.
4. The portable multi-frequency electrical impedance imaging front-end data acquisition and processing method of claim 1, wherein the synchronous acquisition of the reference input signal and the measurement input signal and the multiple single-point fourier demodulation to obtain phase information and amplitude information of the signals comprises:
synchronously acquiring the reference input signal and the measurement input signal, and obtaining a reference signal discrete sequence and a measurement signal discrete sequence based on the acquired sampling number and basic parameters of the reference input signal and the measurement input signal;
and performing multiple single-point Fourier demodulation on the reference signal discrete sequence and the measurement signal discrete sequence to obtain phase information and amplitude information of the reference input signal and the measurement input signal.
5. The portable multi-frequency electrical impedance imaging front-end data acquisition and processing method of claim 4, wherein performing multiple single-point Fourier demodulation on the discrete sequence of reference signals and the discrete sequence of measurement signals to obtain phase information and amplitude information of the reference input signals and the measurement input signals comprises:
obtaining a corresponding single-point Fourier coefficient according to the frequency of the sinusoidal signal;
optimizing the discrete Fourier coefficients according to the periodicity and symmetry of the digital signal and the symmetry of the trigonometric function;
performing difference on the optimized discrete Fourier coefficients corresponding to the reference signal discrete sequence and the measurement signal discrete sequence to obtain phase information of the reference input signal and the measurement input signal;
and calculating the modulus value of the digital signal to obtain corresponding amplitude information.
6. The portable multi-frequency electrical impedance imaging front-end data acquisition and processing method of claim 5, wherein optimizing the discrete Fourier coefficients according to periodicity, symmetry of digital signals and symmetry of trigonometric functions comprises:
sequentially reducing the complex multiplication times of the discrete Fourier coefficients to 10 times according to the periodicity and symmetry of the digital signals;
and respectively converting the products of the reference signal discrete sequence and the measurement signal discrete sequence and the digital signal into multiplication of the reference signal discrete sequence and the measurement signal discrete sequence with the real part of the digital signal and multiplication with the imaginary part of the digital signal, and completing optimization of the Fourier coefficient based on the symmetry of a trigonometric function.
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