Novel modulation method for exciting LED to detect oxyhemoglobin saturation

Abstract

The invention discloses a frequency division modulation and demodulation method for two paths of signals, which comprises the following steps of: generating a path of square wave signal and a path of direct current signal through a microprocessor, modulating a signal to be measured, then collecting the modulated signal, and obtaining the two paths of signals by using a demodulation algorithm disclosed by the invention. The modulation method is a special and simpler frequency division multiplexing method, the frequency of one path of signal is set to be 0 to become a direct current signal, the ringing effect in the circuit is weakened, and the detection precision of the signal is improved. The demodulation method is also simple, the signal processing speed is improved, and the requirement for the performance of a microprocessor is reduced. According to the method, the dynamic spectrum theory is combined at the same time, the modulation and demodulation method is applied to detection of the oxyhemoglobin saturation, influences of external measurement environments including ambient light and dark current and individual differences are eliminated, the signal-to-noise ratio of detection signals is increased, meanwhile, a measurement circuit is simplified, and cost is reduced.

Description

Technical Field

[0001] The present invention relates to the field of photoelectric technology and signal processing, and in particular to a blood oxygen saturation detection technology based on a new modulation method. Background Art

[0002] Hypoxia is a common pathological process in clinical practice and an important cause of death caused by many diseases. Accurately assessing the oxygen level of blood and tissues is of great significance to the diagnosis, treatment and prognosis of diseases. Blood oxygen saturation can characterize the oxygen-carrying capacity of blood and directly reflect the state of oxygen supply to various tissues of the human body. Therefore, it is a very important physiological indicator. In order to improve the detection accuracy of blood oxygen saturation, the patent with announcement number CN113749655A discloses a method for processing PPG detection signals using a morphological filtering algorithm. The morphological filtering algorithm can reduce the complexity of signal processing, improve the signal processing effect, and thus improve the accuracy of blood oxygen saturation detection. The patent with announcement number CN112472079A discloses a method for determining the blood oxygen saturation of the area to be detected based on signal characteristic information, which overcomes the defect that the PPG signal collected in the prior art often contains a large amount of noise, thereby improving the accuracy of blood oxygen saturation detection. However, the above methods, like the existing technologies, all use the traditional empirical formula, that is, detecting the photoelectric volumetric pulse wave signals of red light and infrared light and extracting their respective alternating current (AC) and direct current (DC) components, calculating the light absorption ratio Q in the two wavelengths based on these four components, and then calculating the blood oxygen saturation.

[0003] Dynamic spectroscopy [1] It is a new method for non-invasive detection of blood components, which can be applied to the detection of blood oxygen saturation. The so-called dynamic spectrum refers to the spectrum composed of the absorbance of the corresponding pulsating arterial blood extracted from the photoelectric volumetric pulse wave corresponding to the incident light of multiple wavelengths. This method only measures the absorbance change of the pulsating arterial blood, removes the influence of tissues such as skin and muscle, and can suppress the influence of individual errors and measurement conditions on the detection results. In theory, the pulse blood oxygen saturation measurement accuracy using the dynamic spectrum method is much higher than the traditional pulse blood oxygen saturation measurement accuracy. The patent with announcement number CN102319075A proposes a blood oxygen saturation measurement device based on dynamic spectroscopy, which uses a microprocessor to output square waves of different frequencies and a 2-fold ratio to drive the light-emitting diode. The method uses square wave frequency division modulation and digital demodulation, which not only improves the measurement accuracy of blood oxygen saturation, but also simplifies the circuit. However, this modulation method has the problem of multiple square wave signal levels jumping at the same time, which will produce a large ringing phenomenon in the circuit, seriously affecting the accuracy of the signal.

[0004] References

[0005] [1] Li G, Wang Y, Lin L, Liu Y, Li X, Lu S. Dynamic Spectrum: aBrand-new Non-invasive Blood Component Measure Method. Conf Proc IEEE Eng MedBiol Soc. 2005;2005:1960-3. Summary of the invention

[0006] In view of the shortcomings of the above methods, the present invention provides a new frequency division modulation and demodulation method for dual-channel signals. The modulation method is a special and simpler frequency division multiplexing method, which sets the frequency of one signal to 0 to become a DC signal, and uses AC and DC signals to modulate the light source. Compared with the method in the above patent, this method has simple demodulation, fast processing speed, simple circuit, and also weakens the ringing phenomenon of the circuit, ensuring the accuracy of blood oxygen saturation detection.

[0007] The blood oxygen saturation detection system designed by the present invention includes a carrier signal generation module, a modulation module, a sampling module, a demodulation module, and a calculation module. The carrier signal generation module is composed of a microcontroller, which generates a square wave signal and a DC signal as a carrier signal. The modulation module loads the measured signal onto the carrier signal. The sampling module is used to collect the modulated signal. The demodulation unit demodulates the collected signal. The calculation module includes two parts: dynamic spectrum extraction and blood oxygen saturation calculation.

[0008] Since the present invention is based on dynamic spectrum theory, this theory can in principle suppress individual errors and the influence of external measurement environment including ambient light and dark current. Therefore, the light source modulation module does not need to use a constant current source, and the signal acquisition module does not need high-pass and low-pass filtering circuits, which greatly simplifies the circuit required for detection.

[0009] The carrier signal generating module generates a square wave signal and a DC signal by a microcontroller. Compared with the double frequency square wave modulation method, the modulation method of the present invention enables at most one IO port of the microcontroller to undergo a level jump at each moment, thereby reducing the ringing phenomenon and improving the stability of the circuit and the accuracy of the detection signal.

[0010] The sampling frequency of the sampling module is 2 times the frequency of the square wave carrier signal. m times ( m =1,2,3…), and to ensure the simplicity of the demodulation algorithm, the generation of the sampling signal is synchronized with the generation of the square wave carrier signal.

[0011] When the demodulation module performs demodulation, the sampling signal frequency is 2 times the square wave signal frequency. mtimes, a total of 2 m data. m >1, the 2 collected in each cycle m data, first perform down sampling, and then m Data and m Each data is downsampled into 1 data, and then the two data are demodulated and calculated. i The two data of the cycle are and , the specific demodulation process is:

[0012]

[0013]

[0014] in, and The demodulation algorithm averages the two data of each cycle, which can improve On the other hand, the demodulation process only includes addition and subtraction operations, and compared with the double frequency square wave modulation method, the amount of calculation of the method of the present invention is greatly reduced, and the calculation speed is improved.

[0015] The beneficial effects of the technical solution provided by the present invention are:

[0016] 1. The modulation and demodulation method proposed in the present invention reduces the ringing phenomenon of the circuit, improves the accuracy of the signal, and at the same time greatly reduces the amount of calculation in the demodulation process, improves the processing speed, and reduces the requirements for microprocessor performance.

[0017] 2. The present invention is based on dynamic spectrum theory, which weakens the influence of individual differences and external measurement environment including ambient light and dark current, and simplifies the circuit required by the system while improving the accuracy of blood oxygen saturation detection.

[0018] 3. The modulation and demodulation method is not limited to the detection of blood oxygen saturation, but is also applicable to the detection of other signals, providing a new idea for the collection and detection of dual-channel signals. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 Flowchart of implementation of the present invention.

[0020] Figure 2 Specific circuit for blood oxygen saturation detection.

[0021] Figure 3 Timing diagram of carrier signal and output signal. Implementation

[0022] In order to make the objectives, technical solutions and advantages of the present invention more clear, the specific implementation modes of the present invention will be further described below in conjunction with the accompanying drawings.

[0023] According to Lambert-Beer's law and dynamic spectrum theory, the absorbance change of pulsating arterial blood in the blood is

[0024]

[0025] in, This is the maximum value of the emitted light intensity, and the amount of blood in the artery is the least at this time. It is the minimum value of the output light intensity, at which time the amount of blood in the artery is the largest.

[0026] Measure the absorbance changes at two wavelengths respectively and , the Q value can be calculated, and then the blood oxygen saturation can be calculated.

[0027]

[0028] The specific process of measuring blood oxygen saturation is as follows Figure 1 As shown, the following steps are included:

[0029] 101: The microprocessor outputs a square wave signal and a DC signal to modulate two light-emitting diodes. The timing diagram of the microprocessor output signal is as follows: Figure 3 shown.

[0030] 102: The light emitted by the light emitting diode is received by the photosensitive device after passing through the finger and converted into a current signal. The current signal is converted into a voltage signal within a preset amplitude range by a transimpedance amplifier.

[0031] 103: Use ADC to sample the voltage signal and then transmit it to the microprocessor. The sampling frequency is 2 times the frequency of the square wave carrier signal. m times ( m =1, 2, 3…).

[0032] 104: The square wave signal collects 2 m data, when m >1, the 2 collected in each cycle m data, first perform down sampling, and then m Data and m The data are downsampled into 1 data respectively. Then the two data of each cycle are demodulated and calculated to obtain the photoelectric volume pulse waves measured by the two light emitting diodes respectively.

[0033] 105: Extract the dynamic spectrum from the photoplethysmogram and calculate the Q value.

[0034] 106: The blood oxygen saturation is obtained by calculating the Q value. The relationship between the Q value and the blood oxygen saturation value can be obtained by collecting multiple samples for modeling.

[0035] Figure 2 For the specific connection method of the measurement circuit. Figure 1 and Figure 2 The blood oxygen saturation measuring device of the present invention includes a carrier signal generating module, a light source modulation module, a signal acquisition module, a sampling module, a demodulation module, and a calculation module.

[0036] in, Figure 2 The embodiments shown are only part of the embodiments of the present application, not all of the embodiments. The specific form of each module is set according to the needs of actual application, and the embodiments of the present invention do not limit this.

[0037] The specific parameters of the circuit elements are set according to the needs in actual applications, and the embodiments of the present invention do not limit this in specific implementation.

[0038] The microprocessor may be any one of MCU, ARM, FPGA, DSP, etc., and the embodiment of the present invention does not impose any limitation on this in specific implementation.

[0039] In summary, the present invention proposes a special and simpler frequency division modulation and demodulation method. The square wave plus DC modulation method weakens the ringing effect in the circuit and improves the detection accuracy of the signal. The demodulation method is simple, improves the processing speed, and reduces the demand for microprocessor performance. At the same time, combined with dynamic spectrum theory, the modulation and demodulation method is applied to the detection of blood oxygen saturation, eliminating the influence of factors such as ambient light and dark current, simplifying the measurement circuit, and reducing the detection cost.

[0040] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A new modulation method excites LED to detect blood oxygen saturation, characterized in that: The method includes five parts: carrier signal generation module, modulation module, sampling module, demodulation module and calculation module. The carrier signal generation module is used to generate two carrier signals, one is a square wave signal and the other is a DC signal; the modulation module is used to load the signal to be tested on the carrier signal; the sampling module is used to generate a trigger signal and collect the modulated signal; The demodulation module is used to separate the two signals; The calculation module is used to obtain blood oxygen saturation, including two parts: dynamic spectrum extraction and blood oxygen saturation calculation.

2. According to claim 1, a new modulation method is used to stimulate LED to detect blood oxygen saturation, characterized in that: The method is based on the principle of dynamic spectroscopy and is used to eliminate the influence of individual differences and external measurement environments including ambient light and dark current.

3. The modulation method according to claim 1, characterized in that: It is a special and simpler frequency division multiplexing method. A square wave signal and a DC signal are generated by a microprocessor to modulate the signal to be measured.

4. The trigger signal according to claim 1, characterized in that: The trigger signal is the sampling signal. The generation of the sampling signal is synchronized with the generation of the square wave carrier signal. The sampling frequency is twice the square wave frequency. m times, a total of 2 m data.

5. The demodulation algorithm according to claim 1, characterized in that: when m >1, down sampling is required before demodulation. m data, you need to m Data and m Each data is downsampled into 1 data and then demodulated.

6. The dynamic spectrum extraction according to claim 1, characterized in that: After the demodulation module separates the two signals, it is necessary to extract the dynamic spectrum from the two signals and then calculate the blood oxygen saturation.

Images