CN115363555A

CN115363555A - System and method for measuring human body impedance

Info

Publication number: CN115363555A
Application number: CN202210868127.XA
Authority: CN
Inventors: 魏明; 李辉; 李勇军; 申德光
Original assignee: Zhixinjian Nanjing Technology Co ltd
Current assignee: Zhixinjian Nanjing Technology Co ltd
Priority date: 2022-07-22
Filing date: 2022-07-22
Publication date: 2022-11-22

Abstract

The invention relates to the technical field of human body impedance measurement, in particular to a system and a method for human body impedance measurement, which comprises a DDS signal generator and a high-precision voltage-current conversion circuit, wherein a voltage signal of the DDS signal generator is used as the input of the high-precision voltage-current conversion circuit; then, the human body impedance can be equivalently converted into a component part by inquiring a voltage table, and the accuracy problem in actual measurement can be better solved.

Description

System and method for measuring human body impedance

Technical Field

The invention relates to the technical field of human body impedance measurement, in particular to a system and a method for measuring human body impedance.

Background

The body impedance is the total impedance including the body skin, blood, muscle, cell tissue and their junctions, which contains resistance and capacitance. The body impedance is one of the parameters that determine and limit the body current. The impedance of the human body varies within a wide range under the influence of various factors such as skin condition, contact voltage, current, contact area, contact pressure, and the like.

The most simple instruments are based on fixed frequency measurements (single frequency bioelectrical impedance analysis or SF-BIA), some instruments use multi-frequency systems (multi-frequency bioelectrical impedance analysis or MF-BIA), and the most complex instruments perform actual spectral measurements (bioimpedance spectroscopy or BIS) over a range of frequencies. The most important of the methods is bioelectrical impedance vector analysis and real-time analysis, but the existing system for measuring the human body impedance is complex, complicated to operate and low in measurement precision, so that the system and the method for measuring the human body impedance are provided.

Disclosure of Invention

It is an object of the present invention to provide a system and method for measuring impedance of a human body to solve the above problems in the prior art.

The technical scheme of the invention is as follows: the system for measuring the human body impedance comprises a DDS signal generator and a high-precision pressure flow conversion circuit, wherein a voltage signal of the DDS signal generator is used as the input of the high-precision pressure flow conversion circuit, the DDS signal generator adopts an AD9833 sub-circuit, the AD9833 sub-circuit comprises two frequency selection registers, a phase accumulator, two phase offset registers and a phase offset adder, the frequency selection registers and the phase accumulator are both 28 bits, and the phase range of a continuous time signal is 0-2 pi.

A method for measuring impedance of human body comprises injecting current frequency of human body into impedance measuring instrument for human body respiration; performing measurements at low and high frequencies, respectively, to obtain ICW values; and finally, carrying out frequency spectrum scanning on the human body impedance to realize human body impedance measurement.

Preferably, the method comprises the steps of providing 25M clocks for the DDS chip, and programming and writing different register configurations through the MCU to obtain different frequency outputs.

Preferably, in the human body respiration impedance measuring instrument, the frequency of the current injected into the human body is 50KHz, and the output frequency of the register is fixed at 50KHz, so that the frequency of the human body excitation signal is reached.

Preferably, the measurements are performed at low and high frequencies: the low frequency measurement is used to estimate the ECW value, the high frequency measurement is used to estimate the TBW value, and the difference between the two results in the ICW value:

ICW＝TBW-ECW；

wherein ICW is intracellular water, ECW is extracellular water, and TBW is total water of human body.

Preferably, the body impedance is spectrally swept at a frequency of DC to 200 KHz.

Preferably, the voltage signal generated by the DDS is input to a high-precision voltage-to-current conversion circuit to obtain a precision voltage-controlled current source, and the current calculation formula is output:

the invention provides a system and a method for measuring human body impedance by improving, compared with the prior art, the following improvements and advantages are provided:

according to the characteristics of the resistance-capacitance network of the human body impedance, one path of controllable frequency current flows through the network to generate a voltage drop proportional to the network impedance, and the voltage drop is measured through an ADC (analog to digital converter), so that the equivalent impedance of the resistance-capacitance network can be converted; then, the human body impedance can be equivalently converted into a component part by inquiring a voltage table, and the accuracy problem in actual measurement can be better solved.

Drawings

The invention is further explained below with reference to the figures and examples:

FIG. 1 is a simplified model diagram of the electrical characteristics of a bioimpedance and measurement instrument according to the present invention;

FIG. 2 is a hardware schematic of the system for body impedance measurement of the present invention;

fig. 3 is a schematic diagram of a precision voltage-to-current conversion circuit of the system for measuring body impedance of the present invention.

Detailed Description

The present invention is described in detail below, and the technical solutions in the embodiments of the present invention are clearly and completely described, and it is obvious that the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.

The bio-impedance is a complex number consisting of a resistance value R (real part) mainly caused by the total amount of water in the human body and a reactance value Xc (imaginary part) mainly caused by capacitance generated by cell membranes. The impedance may also be represented by a vector modulo Z and having a phase angle phi. The phase angle plays a major role in determining the body composition.

Wherein:

Z＝R+jXc；

wherein the resistance R of a conductor having a cross-sectional area S and a length l, and the capacitance C of a parallel plate capacitor having a surface area S and a distance d are given by the following equation:

as can be seen from the above equations, the resistance and capacitance depend on geometrical parameters (length, distance and surface area), which means that they are related to the measurement system and physical parameters employed. Namely: the resistivity ρ and the dielectric constant ε are closely related to the type of material to be measured (in this example, biological tissue). Fig. 1 shows a simplified model of the electrical properties of a bioimpedance and its measuring instrument, where RE considers the resistance of the extracellular fluid, RI stands for the resistance of the intracellular fluid, cm is the capacitance of the cell membrane, the connection between the instrument and the human body is made by means of electrodes placed on the skin, the instrument supplies the electrodes with an excitation voltage and measures the resulting current, the excitation signal is generated by means of a digital-to-analog converter (DAC) connected to a back-end driver, the DAC is programmed by means of a microcontroller, whereby the signal amplitude and frequency can be set. For current measurement, a transimpedance amplifier (TIA) is used, which is connected to a high resolution analog-to-digital converter (ADC) to achieve accurate measurement. The system microcontroller is used to process the collected data and extract the information needed for analysis.

To achieve the object, the present invention provides a system and a method for measuring impedance of a human body by improving, wherein the system for measuring impedance of a human body comprises a DDS signal generator, a high precision voltage-to-current conversion circuit to solve the precision problem encountered in actual measurement.

The technical scheme of the invention is as follows:

the utility model provides a system for human impedance measurement, includes DDS signal generator and high accuracy pressure stream converting circuit, this design adopts AD9833 as waveform generator, can produce sine wave, triangle wave and square wave output, and output frequency and phase place accessible software are programmed, and the adjustment is simple, need not external element, and the frequency register is 28 bits: at a clock rate of 25MHz, a resolution of 0.1Hz can be achieved. The waveform generator writes data through a three-wire serial interface, the hardware schematic of which is shown in fig. 2.

The AD9833 sub-circuit is composed of two frequency selection registers, a phase accumulator, two phase offset registers and a phase offset adder. The main element of the NCO is a 28-bit phase accumulator. The phase of the continuous-time signal ranges from 0 to 2 pi. Outside this range of values, the sine function repeats continuously in a periodic manner. The digital implementation is not different. The accumulator simply expands the range of phase values to a multi-bit number. The phase accumulator in AD9833 is implemented with 28 bits.

Thus, in AD9833, 2 pi =228. Similarly, the Δ Phase term is expanded to this range: the final output frequency is: f = Δ Phase × Fmclk/228; wherein, Δ phase is the average phase count difference in a specified length of time, and Fmclk is the main clock frequency.

In the human respiration impedance measuring instrument, the frequency of current injected into the human body is 50KHz based on the inverse relationship between the measured impedance and the total amount of human moisture (TBW) (the conductive portion of the impedance) which is composed of intracellular moisture (ICW) and extracellular moisture (ECW). This technique may provide good results for test subjects with normal moisture content, while for test subjects with drastically altered moisture content. Aiming at the respiratory signal measurement, the output frequency of the chip register can be fixed at 50KHz, so that the frequency of the human body excitation signal is reached. This fixed frequency has limited ability to assess ICW changes and thus loses its effectiveness.

The limitations overcome by performing measurements at low and high frequencies. Low frequency measurements allow a more accurate estimate of ECW, while at high frequencies an estimate of TBW can be obtained. The ICW is derived from the difference of the two estimates:

ICW＝TBW-ECW。

different frequencies are switched in the MCU, and the frequencies of 20KHz, 40KHz, 60KHz and the like are output through programming, so that different human body impedances under the measurement state are calculated, and the limitation of ICW change limitation can be solved. However, this technique shows error in the estimation of body fluid for elderly patients with disease.

And finally, automatically switching the programming frequency, performing frequency spectrum scanning on the human body impedance from DC to 200KHz, and testing the resistance RE generated by extracellular fluid under the zero frequency of the impedance and the parallel connection of the RE and the RI under the infinite frequency according to a human body model. At these two extreme frequencies, the capacitance created by the cell membrane appears as an open or short circuit. The intermediate frequency measurement provides information about the capacitance value. Under this test, the measurement takes longer and the amount of data will be considerable.

In the design, a precise voltage-current conversion circuit is additionally used, because a voltage signal is generated by the DDS and the human body is an impedance network, precise voltage-current conversion is required to be carried out in order to linearly change the voltage, thereby achieving the purpose of measuring the impedance of the human body; FIG. 3 is a schematic diagram of a precision voltage-current conversion circuit; by inputting the voltage signal generated by the DDS into the circuit, a precise voltage-controlled current source is obtained, and the operational amplifier provides buffering for the reference pin so as to ensure good common-mode rejection performance. The output current calculation formula is as follows:

by modifying the value of the precision resistor R1, the size of the constant current source can be simply changed, and the constant current source has good linearity under the uA condition.

The previous description is provided to enable any person skilled in the art to make or use the present invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other embodiments without departing from the spirit or scope of the invention. Thus, the present invention is not intended to be limited to the embodiments shown herein but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A system for body impedance measurement, characterized by: the DDS circuit comprises a DDS signal generator and a high-precision voltage-current conversion circuit, wherein a voltage signal of the DDS signal generator is used as the input of the high-precision voltage-current conversion circuit, the DDS signal generator adopts an AD9833 sub-circuit, the AD9833 sub-circuit comprises two frequency selection registers, a phase accumulator, two phase offset registers and a phase offset adder, the frequency selection registers and the phase accumulator are both 28 bits, and the phase range of continuous time signals is 0-2 pi.

2. Method of applying a system for impedance measurement of the human body according to claim 1, characterized in that: injecting the current frequency of the human body into a human body respiratory impedance measuring instrument; performing measurements at low and high frequencies, respectively, to obtain an ICW value; and finally, carrying out frequency spectrum scanning on the human body impedance to realize human body impedance measurement.

3. A method for body impedance measurement according to claim 2, wherein: the method comprises the steps of providing 25M clocks for a DDS chip, and programming and writing the 25M clocks into different register configurations through an MCU (microprogrammed control unit) to obtain different frequency outputs.

4. A method for body impedance measurement according to claim 2, wherein: in the human body respiratory impedance measuring instrument, the frequency of current injected into a human body is 50KHz, and the output frequency of the register is fixed at 50KHz, so that the frequency of a human body excitation signal is reached.

5. A method for body impedance measurement according to claim 2, wherein: measurements were performed at low and high frequencies: the low frequency measurement is used to estimate the ECW value, the high frequency measurement is used to estimate the TBW value, and the difference between the two results in the ICW value:

ICW＝TBW-ECW；

wherein ICW is intracellular water content, ECW is extracellular water content, and TBW is total human body water content.

6. A system and method for body impedance measurement according to claim 2, wherein: the body impedance is spectrally swept at a frequency of DC to 200 KHz.

7. A system and method for body impedance measurement as claimed in claim 2, wherein: inputting a voltage signal generated by the DDS into a high-precision voltage-current conversion circuit to obtain a precision voltage-controlled current source, and outputting a current calculation formula: