CN105380647A - Weak dynamic impedance detection device and method based on four-electrode half-bridge method - Google Patents

Abstract

The invention discloses a weak dynamic impedance detection device and method based on a four-electrode half-bridge method. The half-arm automatic balancing bridge circuit is composed of a digital potentiometer and a biological impedance; a programmable amplifying circuit is composed of two instrument amplifiers and a The programmable instrument amplifier is composed of one instrument amplifier whose output is the divided voltage V ₁ of the digital potentiometer, and the output of the other instrument amplifier is the divided voltage V ₂ of the biological impedance, and the output of the programmable instrument amplifier is the difference between V ₁ and V ₂ ; The automatic balance adjustment circuit, according to the voltage amplitude of the weak dynamic impedance change signal demodulated by the fast digital phase-locked circuit based on oversampling, is used as negative feedback to control the tap position of the digital potentiometer until the demodulated voltage amplitude enters Within the threshold, the automatic balance adjustment circuit enters into a balanced state. The invention realizes the dynamic impedance detection based on the four-electrode half-bridge method, and has the advantages of high precision, high sensitivity, large dynamic range, simple realization, and real-time continuous measurement.

Description

A weak dynamic impedance detection device and method based on four-electrode half-bridge method

technical field

The invention relates to the field of dynamic impedance detection, in particular to a weak dynamic impedance detection device and method based on a four-electrode half-bridge method.

Background technique

Bioelectrical impedance analysis is a noninvasive, low-cost, and commonly used method for anthropometric and clinical status assessment. With the development and maturity of bio-impedance measurement technology, it has become possible to apply bio-impedance measurement to clinical practice. Bioimpedance can be divided into two parts: static basic impedance and weak dynamic impedance. Weak dynamic impedance changes often carry more physiological information, so the real-time continuous monitoring of weak dynamic impedance changes provides the possibility for clinical application. In the measurement of weak dynamic impedance, it is the key to deduct the influence of static basic impedance.

The inventor found in the process of realizing the present invention that the existing impedance measurement method usually collects the static impedance and the weak dynamic impedance at the same time during the acquisition process, and performs the same degree of amplification. Measurements pose certain difficulties. At the same time, the large static basic impedance will introduce individual differences, which makes it difficult for the impedance measurement method to be applied clinically.

Contents of the invention

The invention provides a weak dynamic impedance detection device and method based on the four-electrode half-bridge method. The invention has the advantages of high precision, high sensitivity, large dynamic range, low power consumption and low cost, and can detect relatively weak dynamic impedance. The impedance change is continuously monitored for a long time, see the following description for details:

A weak dynamic impedance detection device based on the four-electrode half-bridge method, including: an excitation constant current source, and also includes:

Half-arm automatic balancing bridge circuit, composed of digital potentiometer and biological impedance;

The programmable amplifier circuit is composed of two instrument amplifiers and a programmable instrument amplifier. The output of one instrument amplifier is the divided voltage V ₁ of the digital potentiometer, and the output of the other instrument amplifier is the divided voltage V ₂ of the biological impedance. Programmable _The output of the instrumentation amplifier is the difference between V1 and V2 _;

The automatic balance adjustment circuit, according to the voltage amplitude of the weak dynamic impedance change signal demodulated by the fast digital phase-locked circuit based on oversampling, is used as negative feedback to control the tap position of the digital potentiometer until the demodulated voltage amplitude enters the threshold Within, the automatic balance adjustment circuit enters into a balanced state.

The excitation constant current source is an excitation signal of a high frequency sinusoidal constant current source.

The four-electrode configuration circuit is composed of four electrodes, which are configured in a way that the excitation electrodes and the measurement electrodes are separated.

The programmable instrument amplifier is used to amplify the static basic impedance and the weak dynamic variable impedance to different degrees.

A kind of weak dynamic impedance detection method based on four-electrode half-bridge method, described method is a kind of automatic balance half-bridge method, described method comprises the following steps:

The digital potentiometer is expressed as R _S , and the measured biological impedance is expressed as R _Z ;

After being excited by a high-frequency sinusoidal current, the output voltage of the voltage across R _S passing through an instrument amplifier is expressed as V ₁ , and the output voltage of R _Z passing through another instrument amplifier is expressed as V ₂ ;

The output voltage of V ₁ and V ₂ after the programmable instrument amplifier is expressed as V _out , and V _out is converted into a digital signal by an analog-to-digital converter;

The voltage amplitude of the measured impedance is obtained through demodulation and average filtering of the digital signal based on the fast digital lock based on oversampling;

The voltage amplitude of the measured biological impedance obtained by fast digital phase-locking demodulation is used as a negative feedback signal, and the tap position of the digital potentiometer is adjusted through the automatic balancing bridge;

If the absolute value of the deviation is greater than the deviation threshold, the amplitude and the deviation are recalculated until the absolute value of the deviation is less than the deviation threshold, then the bridge balance adjustment is completed.

The beneficial effects of the technical solution provided by the present invention are: the present invention removes the influence of contact impedance through the four-electrode method, and adopts the automatic balancing half-bridge method to achieve the purpose of deducting the larger static basic impedance at the same time, and the static basic impedance is adjusted by the programmable instrument amplifier. Different degrees of amplification are carried out with the dynamic impedance change, and then the data sampled by the ADC is performed based on a fast phase-locking algorithm based on oversampling to demodulate the weak impedance change. The invention can continuously and long-time detect the weak dynamic impedance variation, and has the characteristics of high precision, high sensitivity, large dynamic range, low cost, low power consumption and small volume, and has high application value.

Description of drawings

Fig. 1 is a structural representation of a weak dynamic impedance detection device based on the four-electrode half-bridge method provided by the present invention;

Fig. 2 is the circuit diagram of the weak dynamic impedance detection based on the four-electrode half-bridge method provided by the present invention;

Fig. 3 is the electrode distribution figure provided by the present invention;

Fig. 4 is the flow chart that carries out amplitude automatic balance adjustment to electric bridge;

Figure 5 is the experimental waveform diagram.

In the accompanying drawings, the list of each component is as follows:

1: excitation constant current source; 2: half-arm automatic balancing bridge circuit;

3: Programmable amplifier circuit; 4: Fast digital phase-lock circuit;

5: Automatic balance adjustment circuit; 6: Four-electrode configuration circuit.

detailed description

In order to make the purpose, technical solution and advantages of the present invention clearer, the implementation manners of the present invention will be further described in detail below.

Example 1

A weak dynamic impedance detection device based on the four-electrode half-bridge method, see Figure 1 and Figure 2, the detection device includes: excitation constant current source 1, half-arm automatic balancing bridge circuit 2, programmable amplifier circuit 3, based on the Sampling fast digital phase-locking circuit 4 , automatic balance adjustment circuit 5 and four-electrode configuration circuit 6 .

Wherein, the excitation constant current source 1 is an excitation signal of a high-frequency sinusoidal constant current source.

The bridge arm of the half-arm automatic balancing bridge circuit 2 is composed of a digital potentiometer and biological impedance. The purpose of the half-arm automatic balancing bridge circuit 2 is to deduct the static basic impedance, eliminate the individual differences caused by it, and extract the weak dynamic impedance components to improve dynamic range and measurement accuracy.

Referring to Fig. 2, the programmable amplifier circuit 3 is mainly composed of two instrument amplifiers and a programmable instrument amplifier, wherein the output of the instrument amplifier A1 is the divided voltage V ₁ of the digital potentiometer, and the output of the instrument amplifier A2 is the biological impedance Divide the voltage V ₂ , and the output of the programmable instrumentation amplifier A3 is the difference V _out between V ₁ and V ₂ . The main function of the programmable instrumentation amplifier A3 is to amplify the static basic impedance and the weak dynamic changing impedance to different degrees.

The automatic balance adjustment circuit 5 controls the tap position of the digital potentiometer until the demodulated weak dynamic impedance When the voltage amplitude of the change signal falls within the preset threshold, the automatic balance adjustment circuit 5 enters into a balanced state.

During specific implementation, the high-frequency sinusoidal constant current source excitation signal can be generated by a microprocessor; the fast digital phase-lock circuit 4 based on oversampling can also be realized by a microprocessor; the programmable amplifier circuit 3 and the automatic balance adjustment circuit 5 can also be implemented by microprocessing.

The four-electrode configuration circuit 6 is composed of four electrodes, which are configured in such a way that the excitation electrodes and the measurement electrodes are separated. Its main function is to eliminate the interference introduced by contact impedance, thereby improving the measurement accuracy of the system. Referring to Figure 2 and Figure 3, the four electrodes are a, b, c and d, respectively, where a and d are excitation electrodes, and b and c are two measurement electrodes.

Wherein, all the microprocessors mentioned above are the same microprocessor, which is a processor with high integrated micro power consumption, low cost and simple operation. Any one of MCU, ARM, DSP or FPGA can be used as the microprocessor.

Example 2

The weak dynamic impedance detection device based on the four-electrode half-bridge method in Embodiment 1 is further described below in conjunction with specific device models, see below for details:

In the embodiment of the present invention, the microprocessor used is a low-power consumption microprocessor CY8C3866AXI; the analog-to-digital converter ADC is 11bit, the sampling rate is 200Ksps, and the oversampling multiple is 4.

Amplifying circuit 3 comprises 2 instrument amplifiers AD623, and the programmable gain amplifier on the microprocessor used and the programmable instrument amplifier A3 that operational amplifier constitutes, after automatic balance adjustment circuit 5 reaches balanced state, improve the gain of programmable amplifier to The maximum gain without signal distortion, at this time, weak dynamic impedance change signals can be demodulated. The digital potentiometer in the automatic balance adjustment circuit 5 is specifically a 20k digital potentiometer AD5272 with 1024 taps.

The number of down-sampling points of the average filter in the fast digital phase-locking circuit 4 based on oversampling is 25000, about between 4 ⁶ -4 ⁸ , so the analog-to-digital converter ADC is equivalently increased by 8 bits, and the equivalent resolution of the detection device It reaches 0.822μV/bit. Relatively weak dynamic impedance changes can be accurately measured. The automatic balance bridge makes the signal-to-noise ratio of the system significantly improved.

Example 3

The basic principle of the weak dynamic impedance detection method to realize the four-electrode half-bridge method is described as follows. See the schematic diagram shown in Figure 4 for details. First, this method is an automatic balancing half-bridge method, which can also be called a differential modulation method. The arm modulation bridge is composed of digital potentiometer R _S and biological impedance R _Z.

101: The microprocessor generates a high-frequency sinusoidal current excitation signal expressed as I(t), and its frequency is set to 50kHz;

102: The digital potentiometer is expressed as R _S , and the measured biological impedance is expressed as R _Z ;

103: After being excited by a high-frequency sinusoidal current, the output voltage of the voltage at both ends of R _S passing through the instrument amplifier A1 is expressed as V ₁ , and the output voltage at both ends of R _Z passing through the instrument amplifier A2 is expressed as V ₂ ;

104: The output voltage of V ₁ and V ₂ after passing through the programmable instrument amplifier A3 is expressed as V _out , and V _out is converted into a digital signal V _Dout by the analog-to-digital converter ADC in the microprocessor at a sampling rate of 200Ksps;

105: The microprocessor demodulates and averages the digital signal V _Dout through a fast digital lock based on oversampling, and obtains the voltage amplitude of the measured impedance;

106: The voltage amplitude of the measured biological impedance obtained by fast digital phase-locking demodulation is used as a negative feedback signal, and the tap position of the digital potentiometer is adjusted through an automatic balancing bridge.

In the specific implementation, according to the four-electrode configuration scheme, the excitation electrodes and the measurement electrodes are arranged separately to ensure that the interference introduced by the contact impedance is eliminated. There is an excitation electrode and a measurement electrode at both ends of the impedance to be measured, and the distance between the excitation electrode and the measurement electrode is about It is 2.5cm.

In the embodiment of the present invention, the flow chart for implementing bridge balance adjustment is shown in Figure 4, which includes: start adjustment, set the initial position of the digital potentiometer, calculate the amplitude of the digital signal, calculate the deviation between the amplitude and the target value, and calculate through the deviation Set the tap position and set the tap position of the digital potentiometer, calculate the deviation again, judge whether it reaches the threshold range of the deviation, complete the adjustment of the bridge balance, increase the gain of the programmable amplifier after the balance, and start to measure the dynamic impedance change signal after the program adjustment is completed.

Wherein, the condition for starting the adjustment is that the human body remains still, the system is powered on, and the program is started. Amplitude balance adjustment uses the signal amplitude of V _Dout calculated based on oversampling fast phase-locking as a negative feedback signal. The tap position of the digital potentiometer is determined through the negative feedback signal. By adjusting the tap position of the digital potentiometer, the bridge reach a state of equilibrium.

First, set the initial position of the tap of the digital potentiometer R _S =0, calculate the signal amplitude y of the digital signal V _Dout at this time, and calculate the deviation e between the amplitude y and the adjustment target value w by e=yw, according to the formula u _i =ke+u ₀ Calculate the tap position u _i of the digital potentiometer required for the next adjustment, where k is the adjustment proportional coefficient, u ₀ is the initial position of the digital potentiometer, and reset the digital potentiometer after calculating u _i If the absolute value of the deviation e is greater than the deviation threshold E0, the amplitude y and deviation e need to be recalculated until the absolute value of the deviation e is less than the deviation threshold E0, then the adjustment of the bridge balance is completed. At this time, Increase the gain of the programmable instrument amplifier to the maximum state without distortion, that is, the gain k=max, and the program adjustment is completed. At this time, the output signal V _out is a weak dynamic impedance change signal.

Among them, k is the proportional adjustment coefficient, which is determined by the characteristics of the whole device.

Among them, the adjustment target value w is set to 20mV, and the threshold value E0 of the deviation e between the output amplitude y and the adjustment target value w is set to 10mV. The adjustment condition for judging whether the balance adjustment is completed is that the absolute value of the deviation e is smaller than the deviation threshold E0. After the adjustment is completed, the range of the output signal y is between w-E0 and w+E0.

Among them, the scope of the programmable instrument amplifier gain improvement is 4 to 32 times. Through the adjustment of the above steps, the small change in the biological impedance value can be amplified and detected more easily.

Example 4

The basic principle of the weak dynamic impedance measurement based on the four-electrode half-bridge method in Embodiment 3 will be described in detail below in combination with specific calculation formulas, see below for details:

When the excitation signal generated by the microprocessor is a high-frequency current signal, it can be expressed by formula (1):

I=Asin(wt)+I ₀ (1)

Among them, ω=2πf, is the angular frequency of the excitation signal; I is the excitation current source; t is the time variable; A is the amplitude of the AC current source; I ₀ is the DC bias current. Among them, the output of the instrument amplifier A1 is the voltage across R _S , and its expression is shown in formula (2):

V ₁ ＝IR _S ＝R _S (Asin(wt)+I ₀ )＝AR _S sin(wt)+I ₀ R _S (2)

The output of the instrument amplifier A2 is the voltage across the biological impedance R _Z , and its expression is shown in formula (3):

V ₂ =I(R _Z +ΔZ)=(R _Z +ΔZ)(Asin(wt)+I ₀ )=R _Z (Asin(wt)+I ₀ )+ΔZ(Asin(wt)+I ₀ )( 3)

The output of programmable instrumentation amplifier A3 is the difference between V ₁ and V ₂ , and its expression is shown in formula (4):

V _out =V ₁ -V ₂ =(R _S -R _Z )(Asin(wt)+I ₀ )-ΔZ(Asin(wt)+I ₀ )(4)

Among them, biological impedance includes: static basic impedance and dynamic change impedance, R _Z represents static basic impedance; ΔZ represents weak dynamic impedance change.

If you want to measure the weak dynamic impedance change, you need to deduct the influence of the static basic impedance, so as to improve the signal-to-noise ratio and expand the dynamic range of the signal. Among them, the main function of the digital potentiometer R _S is to deduct the influence of the static basic impedance.

In the specific implementation, when starting to adjust the bridge, set the multiples of all amplifiers to 1, and adjust the tap position of the digital potentiometer until R _S and R _Z are approximately equal. At this time, the output is as shown in formula (5):

V _out =V ₁ -V ₂ =-ΔZ(Asin(wt)+I ₀ )(5)

At the same time, because ΔZ is extremely small, the output is close to 0, so the bridge is close to balance at this time, and the magnification of the programmable instrument amplifier A3 is adjusted to the maximum state without distortion, and the output signal is a signal of weak dynamic impedance variation.

Example 5

The scheme in embodiment 1-4 is carried out feasibility verification below in conjunction with specific example, see the following description for details:

Taking the measurement of respiration as an example, the microprocessor used is CY8C3866 of CYPRESS, the excitation current source I used is generated by IDAC on the microprocessor, the frequency is 50kHz, the half-bridge circuit is composed of digital potentiometer AD5272 and human body impedance, and the digital potentiometer used AD5272, the tap is 1024 taps, the total resistance is 20kΩ, and the resolution of the resistance is 19.5Ω. The amplification part is composed of two instrument amplifiers AD623 and programmable instrument amplifier A3, among which the programmable instrument amplifier A3 is composed of programmable amplifier and operational amplifier on the microprocessor. The ADC sampling rate on the microprocessor used was set to 200ksps. The demodulation algorithm used is a digital fast phase-locking algorithm based on oversampling, and the number of points is 25,000 points. The sampling rate obtained after downsampling is 8 sps, and 480 points are obtained per minute.

Figure 5 shows the respiratory waveform detected within 1 minute, where the abscissa is the number of sampling points, and the ordinate is the output of the system. It can be seen from Figure 5 that the measured respiratory frequency is 16 times/min.

This weak dynamic impedance measurement method based on the four-electrode half-bridge method eliminates individual differences by deducting the static basic impedance, eliminates the interference introduced by contact impedance, improves the measurement accuracy, and can continuously monitor the changes in weak dynamic impedance of different individuals in real time. It has important clinical application value.

In the embodiments of the present invention, unless otherwise specified, the models of the devices are not limited, as long as they can complete the above functions.

Those skilled in the art can understand that the accompanying drawing is only a schematic diagram of a preferred embodiment, and the serial numbers of the above embodiments of the present invention are only for description, and do not represent the advantages and disadvantages of the embodiments.

The above descriptions are only preferred embodiments of the present invention, and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included in the protection of the present invention. within range.

Claims (5)

1. based on a faint motional impedance checkout gear for four electrode half-bridge methods, comprising: excitation constant-current source, also comprises:

Half arm self-balancing bridge circuit circuit, is made up of jointly digital regulation resistance and bio-impedance;

Pga circuit is made up of jointly 2 instrumentation amplifiers and a programmable instrument amplifier, and the output of an instrumentation amplifier is the dividing potential drop V of digital regulation resistance ₁, the output of another instrumentation amplifier is the dividing potential drop V of bio-impedance ₂, the output of programmable instrument amplifier is V ₁and V ₂difference;

Automatic Balance Regulation circuit, according to the voltage magnitude of the faint motional impedance variable signal out of the fast digital phase lock circuitry demodulation based on over-sampling as negative feedback, the tap position of control figure potentiometer, until the voltage magnitude demodulated enters within threshold value, then Automatic Balance Regulation circuit enters poised state.

2. a kind of faint motional impedance checkout gear based on four electrode half-bridge methods according to claim 1, it is characterized in that, described excitation constant-current source is high frequency sinusoidal constant current supply pumping signal.

3. a kind of faint motional impedance checkout gear based on four electrode half-bridge methods according to claim 1, it is characterized in that, described four electrode configuration circuits are made up of four electrodes, and the mode adopting exciting electrode and measurement electrode to separate is configured.

4. a kind of faint motional impedance checkout gear based on four electrode half-bridge methods according to claim 1, is characterized in that, described programmable instrument amplifier is for carrying out amplification in various degree to static basis impedance and faint dynamic change impedance.

5. based on a faint motional impedance detection method for four electrode half-bridge methods, it is characterized in that, described method is a kind of autobalance half-bridge method, said method comprising the steps of:

Digital regulation resistance is expressed as R _s, tested bio-impedance is expressed as R _z;

After high frequency sinusoidal current excitation, R _sthe output voltage of both end voltage after an instrumentation amplifier represents for V ₁, R _zthe output voltage of two ends after another instrumentation amplifier represents for V ₂;

V ₁with V ₂output voltage after programmable instrument amplifier represents for V _out, V _outdigital signal is converted to by analog-digital converter;

By carrying out solution harmonic average filtering based on the fast digital of over-sampling is phase-locked to digital signal, obtain the voltage magnitude of tested impedance;

The voltage magnitude of the tested bio-impedance that fast digital demodulation of phase locking obtains, as negative-feedback signal, is adjusted by the tap position of self-balancing bridge circuit to digital regulation resistance;

If the absolute value of deviation is greater than deviation threshold, then recalculate amplitude and deviation, until the absolute value of deviation is less than deviation threshold, then complete the adjustment of bridge balance.