CN106725474A - A kind of human body impedance detection method - Google Patents

Abstract

A method for detecting human body impedance, the steps comprising: (1) building a test system, (2) based on a standard impedance calibration system, (3) performing a human body impedance test, characterized in that: when building the system in step (1), using The external output impedance R ₀ as small as possible is similar to using constant voltage source excitation to ensure the maximum operating current; in step (2) system calibration, within the measurement range, use enough standard impedance samples to carry out the system Calibration, that is, for each standard impedance sample, measure its corresponding voltage value under a given excitation, and finally fit to establish a one-to-one correspondence between impedance and test voltage.

Description

A method for detecting human body impedance

technical field

The present application relates to a method for detecting human body impedance.

Human body impedance is an important physiological parameter of the human body, and its detection is of great significance in the field of biomedicine. For example, scholars at home and abroad have long discovered that the impedance changes of the human body are closely related to breathing. If the human body impedance is simplified into a resistance, according to the resistance formula R=ρL/A, when the human body breathes, the thorax is changing, and the gas volume and composition in the body are also changing, which is equivalent to the human body resistivity ρ and L/A are both It is changing, that is, the resistance of the human body is changing. Therefore, using the circuit to measure the change of the impedance of the human body also indirectly reflects the breathing situation of the human body. Human body impedance detection is one of the important means of human respiratory signal detection, and it has been widely used in ECG monitors.

Background technique

Human body impedance includes reactance, inductive reactance and capacitive reactance. The inductive reactance of the human body is very small and can be ignored under normal circumstances. The capacitive reactance of the human body is very sensitive to frequency. Based on the capacitive reactance formula X _C =1/2πfc, it can be known that the capacitive reactance is proportional to the frequency l/f. When the frequency increases, the capacitive reactance tends to decrease. Obviously, the impedance of the human body is not a fixed value, but related to frequency. When the human body impedance is tested, there must be a contact impedance between the electrode and the skin surface. Its size is related to factors such as skin condition and contact area, and it also decreases sharply with the increase of the test frequency. Therefore, in order to facilitate the test and analysis of human body impedance data, the human body impedance test system often works at a higher frequency. The impedance of the human body can be regarded as purely resistive; through experiments, the impedance value between the two limbs of the human body is about 1500Ω.

A typical human body impedance test system includes constant voltage source V _S , output internal resistance r _o , external output impedance R _O , measured human body _RB , voltage record processing and impedance demodulation modules, etc. Synchronously record and process two voltage signals, the two signals can be the voltage signal of the two measuring points of the human body and the voltage signal on the known external output impedance R _O , or the voltage signal of the two measuring points of the human body and the output voltage signal of the power supply , it can also be the voltage signal on the known external output impedance R _O and the output signal of the power supply, and then through certain calculations, the complex impedance of the human body can be demodulated. This is the so-called four-wire impedance measurement method, as shown in Figure 2(a) As shown in (b)(c): for a particularly high test frequency, the measured human body impedance R _B can be regarded as purely resistive. At this time, only one voltage signal can be recorded and processed to measure the human body impedance value, which is called Two-wire impedance measurement method, as shown in Figure 3.

No matter what kind of human body impedance test system is used, the basic steps of the human body impedance test method are: (1) build a test system; (2) standard impedance calibration system; (3) human body impedance test. For the sake of convenience, the existing methods are as follows: the first step is to use a large output impedance R _O to change the excitation power supply into a constant current source when building the system, so that the human body impedance and the detection voltage at both ends have a linear relationship, so that it is convenient For the calibration of the system, the second step only needs less standard impedance calibration. The shortcomings of the existing methods are: the output impedance is too large, and the same excitation voltage produces a small working current, resulting in low sensitivity to changes in human body impedance, which is not conducive to reflecting pathological and physiological information that causes changes in human body impedance such as breathing.

references:

[1] TI, ADS 1298R Datasheet, Texas Instruments Incorporated, 2010.

[2] Wang Jianbo, Deng Qinkai, Guo Jinsong, etc. A new type of impedance breathing detection system [J]. Chinese Journal of Medical Devices, 2009, 33(2).

[3] Xi Tao, Yang Guosheng, Tang Chi, etc. Research progress of respiratory signal detection technology. Medical and Health Equipment, 2004, 12: 26-29.

[4] Prutchi, D. and M. Norris. (2004). Design and development of medicalelectronic instrumentation: A practical perspective of the design, construction, and test of medical devices. Hoboken, NJ: Wiley Interscience.

[5] Grenvik, A., Ballou, S., McGinly, E., Cooley, W.L., and P. Safar. (1972). Impedance pneumography: Comparison between chest impedance changes and respiratory volumes in 11 healthy volunteers. Chest, vol 62 , pp. 439-443.

[6] Kelkar, S.P., Khambete, N.D., and S.S. Agashe. (2008). Development of movement artefacts free breathing monitor. Journal of the Instrument Society of India, vol.38, pp.34-43.

[7] Qi Jianxin, Lin Yuan, Bian Zhengzhong. Design of a respiratory signal detection circuit by impedance method [J]. Chinese Journal of Medical Devices, 1998, 22(1): 9-11.

[8] Deng Qinkai. Design Principles of Modern Medical Instruments [M]. Beijing: Science Press, 2004.

Contents of the invention

purpose of the invention.

A high-sensitivity human body impedance detection method is proposed.

Technical solutions.

A method for detecting human body impedance, the steps comprising: (1) building a test system, (2) based on a standard impedance calibration system, (3) performing a human body impedance test, characterized in that: when building the system in step (1), using The external output impedance R _O as small as possible is similar to using constant voltage source excitation to ensure the maximum operating current; in step (2) system calibration, within the measurement range, use enough standard impedance samples to carry out the system Calibration, that is, for each standard impedance sample, measure its corresponding voltage value under a given excitation, and finally fit to establish a one-to-one correspondence between impedance and test voltage. With such a one-to-one correspondence, in the actual detection in step (3), although the voltage value is directly measured, the corresponding human body impedance value can be converted.

The principle of improving the sensitivity of the above human body impedance detection method can be further elaborated. Taking the two-wire impedance measurement method as an example, as shown in Figure 3, assuming that the excitation constant voltage source is V _S , the output internal resistance is r _o , and the external output impedance is R _O , the measured human body impedance R _B is purely resistive. , then the detection sensitivity of human body impedance is:

When R _O ＞＞R _B and R _O ＞＞r _o , the excitation can be regarded as constant current source excitation, then the working current I flowing into the human body is approximated as:

Let the impedance change of the human body be ΔR, then the detected voltage change ΔV is:

ΔV=I*ΔR (3)

From the above derivation, it is not difficult to find that in the traditional constant current source scheme, due to the large external output impedance R _O , the operating current is greatly limited, so that the entire circuit is insensitive to impedance changes, and at the same time due to the actual measurement environment Influenced by various noises, the inspection effect will be very poor.

In this scheme, the approximate constant voltage source is excited, that is, the output impedance R _{O of the circuit is greatly reduced, and the external output impedance R O can} be simply taken as 0. At this time, there is an operating current:

Comparing formula (2) and formula (4), we can easily find that because the external output impedance R _O is removed, the working current I ₁ flowing into the human body at this time is much larger than I. In this way, under the same impedance change, the system built by this scheme will have higher sensitivity than the system built by the traditional constant current source scheme.

Beneficial effect.

According to the two-wire impedance measurement method shown in Figure 3, the circuit under the traditional constant current source scheme and the circuit under this scheme are respectively built, and the sensitivity of the system under the two schemes is detected by using standard resistance. A total of 8 high-precision resistors with different resistance values were selected to simulate the impedance of the human body for testing. Table 1 shows the voltage values at both ends of different standard resistance values measured by this scheme and the constant current source scheme.

Table 1 The measured voltage values corresponding to different resistance values measured by this scheme and the constant current source scheme

To evaluate the sensitivity of the system, we need to compare the change in voltage when the resistance changes the same value. Through the above data, we can know that the above 8 different resistance values divide 910Ω to 2700Ω into 7 sections, so that the resistance change corresponding to each section is ΔR, and the voltage change corresponding to each section is respectively recorded as ΔV ₁ and constant current of this scheme. ΔV ₂ of the source scheme, so that we can find with There are two sets of values for , each set has 7 values. Table 2 shows the voltage change value caused by every 1 ohm change in the resistance of different resistance value segments under the two schemes. The larger the value obtained, the better the sensitivity of the system at this stage.

Table 2 The voltage change value caused by every 1 ohm change in the resistance of different resistance value segments under the two schemes

It can be seen from the above table that the change value of this scheme is different at each stage from 910Ω to 2700Ω, and it shows an upward trend. However, the change value of the traditional constant current source scheme is roughly around 26μV. Through comparison, we found that the sensitivity of this scheme is better than that of the traditional constant current source scheme no matter which resistance value segment the resistance value is in, which is consistent with the derivation results above.

Description of drawings

Figure 1, a block diagram of the basic steps of the human body impedance detection method.

Figure 2, a schematic diagram of a typical four-wire method human body impedance test system.

Figure 3, the schematic diagram of the two-wire method human body impedance testing system.

Fig. 4 is a schematic diagram of the human body impedance detection system in the embodiment.

Fig. 5 is a schematic diagram of the fitting results of the resistance-voltage relationship established by the calibration of this scheme in the embodiment.

Fig. 6 is a diagram of the measured human body impedance data change in the embodiment.

Example

Build the human body impedance detection system as shown in Figure 4, and use it for the actual detection of human body impedance. The system includes acquisition module, control module, upper computer processing module and power supply circuit. The acquisition module adopts the ADS1294R chip produced by TI Company. Through configuration, the chip can generate a 2.4V, 32KHz square wave signal for circuit excitation. This excitation voltage has a certain output internal resistance r _o . Due to the high test frequency, it is a two-wire measurement mode. In addition, the chip integrates a 24-bit ADC and a modulation and demodulation module. The control part adopts C8051F320 single-chip microcomputer. The single-chip microcomputer integrates a USB module and supports SPI communication protocol. It can transmit the information of the host computer to ADS1294R, and at the same time pack the collected signals and send them to the host computer for processing. The upper computer part adopts the graphical programming language LabVIEW of NI Company, which displays and saves the collected data in real time. The power supply circuit supplies power to the entire system to ensure the normal operation of the system.

The whole system is calibrated before the actual measurement of human body impedance. We selected a large number of standard resistors with resistance values ranging from 910Ω to 3000Ω, measured their corresponding voltage values, and fitted the relationship between resistance and voltage, as shown in Figure 5. Of course, there are many ways of fitting, such as interpolation method, least squares method and so on. Through fitting, we can know the voltage value corresponding to a certain resistance or the resistance value corresponding to a certain voltage within a certain range.

Next, the system will be used in human experiments, and the arm will be simply cleaned with alcohol and medical cotton swabs. Configure the ADS1294R according to the requirements of the data sheet, stick the electrodes on the arms at both ends of the human body, run the host computer, sit quietly, measure and record the voltage data for 20s, as shown in Figure 6(a). Assuming that a certain voltage is 100032μV, by looking up the comparison table established before the calibration, it can be found out that the voltage corresponding to 1600Ω is 99122μV, and the voltage corresponding to 1700Ω is 106378μV. Now we assume that the voltage change caused by the change of resistance in this range It is linear, so the corresponding human body impedance value at this time is calculated to be 1612.54Ω. Using the above method, we can obtain the impedance change of the human body within the 20s, as shown in Figure 6(b).

Claims (1)

1. a kind of detection method of human body impedance, step includes that (1) test system building, (2) are demarcated based on normal impedance and are System, (3) carry out human body impedance test, it is characterized in that：When (1st) step builds system, using as far as possible small outside output impedance R₀, it is similar to using constant pressure source forcing, to ensure as far as possible big operating current；In (2nd) step system calibration, in measurement range It is interior, system is demarcated using enough normal impedance samples, i.e., to each normal impedance sample, it is measured in given excitation Under corresponding voltage value, finally fitting sets up one-to-one relationship between impedance and test voltage.