Design of an intelligent human heart rate detection device

0 Preface

Heart rate is a very important vital information in the human body, and traditional pulse diagnosis, due to its qualitative and subjective nature, affects the accuracy of heart rate testing, becoming a restrictive factor in the application, development and communication of pulse diagnosis in traditional Chinese medicine. In order to improve the test accuracy of such biomedical signals, it is necessary to combine modern science and technology. At present, there are many instruments used to detect heart rate. Common ones include test devices based on pressure sensors, photoelectric sensors, capacitive sensors and electroacoustic sensors, but there is not much difference in the choice of pulse test sites. There are still relatively few instruments that can accurately measure the pulse at the fingertips. The intelligent human heart rate detection device introduced here can realize non-invasive measurement of human fingertips. The test process is simple, and it can accurately measure the number of heartbeats, realize data display and upper and lower limit alarm functions.

1. Device composition and working principle

The system composition is shown in Figure 1. This design is based on the single-chip microcomputer AT89C2051. The pulse signal is collected by the photoelectric sensor. After passing through the preamplifier circuit, filter circuit, integration and comparison circuit, the pulse signal related to the pulse is obtained. The pulse signal is used as an interrupt signal to be handed over to the single-chip microcomputer to calculate the pulse cycle. Then the number of pulse beats per minute (i.e. heart rate) is obtained and the heart rate is displayed on the digital tube. At the same time, the upper and lower limit alarm functions are realized by software. When the measured data exceeds the normal range (such as greater than 180 times/min or less than 45 times/min), an alarm is issued to remind the doctor to pay attention.

2 Device hardware circuit design

2.1 Sensor and signal processing circuit

Since the fingertips contain a high amount of arterial components and the fingertips are relatively thin compared to other human tissues, the light intensity detected after passing through the fingers is relatively large, so the measurement site of the photoelectric pulse sensor is at the fingertips. A pair of infrared transmitting and receiving probes are placed on both sides of the fingers. When the arteries contract and relax periodically with the heart, and the blood volume of the arteries changes accordingly, the infrared receiving probes receive the arterial pulsation light pulse signals that contract and relax periodically with the heart, thereby collecting the heart pulsation signals.

The sensor for detecting heart rate uses infrared tubes HRl068C-05Y2 and PT331C. Since the physiological signals collected from human fingers are very weak, their amplitude is generally in the order of magnitude of microvolts to millivolts, and a large amount of noise is generated during the test due to body movements and strong power frequency interference. At the same time, the collected pulse signal must be amplified by a preamplifier circuit, which requires the circuit to have high gain and high common mode rejection ratio, at least above 80 dB, that is, the integrated operational amplifier must have a high common mode rejection ratio and extremely low zero drift, etc., and the selected resistance parameters must be as accurate as possible. The amplifier circuit consists of a resistor network and OP07, and the sensor and preamplifier circuit are shown in Figure 2.

Due to factors such as internal and external noise and 50 Hz power frequency interference, even if the circuit has a high common mode rejection ratio, the pulse signal is very weak and submerged in the interference signal. Since the main peak frequency of the pulse signal is around 1 Hz and the component with stronger energy is also below 20 Hz, the upper cutoff frequency of the low-pass filter is designed to be 40 Hz. For power frequency interference, a symmetrical double-T resistor-capacitor active notch filter is used to filter it out. After shaping through the integration and comparison circuit, the standard 0-5 V pulse signal required by the microcontroller can be obtained. The filtering, notch circuit and integration comparison circuit are shown in Figure 3.

2.2 Single chip microcomputer control and display circuit

The single-chip control and display circuit is shown in Figure 4. The dynamic display mode is adopted, and P1.0~P1.6 of the P1 port of the single-chip microcomputer are used as the input of the seven-segment code of the digital tube. P3.0, P3.1, P3.2, and P3.3 are used as the selection signals of the four digital tubes (see Figure 4). The heart rate pulse output from the photoelectric sensor is directly connected to the 9th pin (i.e., T1 end) of the single-chip microcomputer 89C2051 as an interrupt signal. T0 is used for timing and T1 is used for counting. P1.7 outputs the upper and lower limit alarm signals of the heart rate, and the alarm is driven by the diode to alarm. When the heart rate is lower than the lower limit of 45 times/min, the alarm will sound a long alarm. When the heart rate is higher than the upper limit of 180 times/min, the alarm will sound a short alarm. [page]

3 Software Design

The system software flow chart is shown in Figure 5. The thousands, hundreds, tens and units of the heart rate to be displayed are stored in the 41H, 42H, 43H and 44H units inside the 89C2051 microcontroller. Dynamic scanning is adopted to display the thousands, hundreds, tens and units in turn every 5 ms. When there is a rising edge on the 9th pin of the microcontroller, the T1 pin counts once, the T0 timing is 50 ms, the cycle timing is 1 200 times, and the T1 count is the heart rate frequency. Then return to the main program to continue to execute the display program.

4 Circuit debugging and noise analysis

Circuit debugging is mainly to filter and amplify the input pulse signal. The debugging effect is directly related to the accuracy of data acquisition. Through testing, it can be known that there is serious noise interference in the pulse signal, and the design of the pre-stage amplifier circuit is crucial. The DFl405 digital synthetic signal generator of Ningbo Zhongce Electronics Co., Ltd. is used to simulate the pulse signal. The signal frequency is high, and the signal processing circuit has a good attenuation effect on high-frequency signals (about 106 Hz). When the signal frequency is moderate, the signal can be amplified according to the design requirements. The 50 Hz notch filter has a good inhibitory effect on power frequency interference. The pulse signal required by the single-chip microcomputer can be obtained by shaping the pulse signal through the integration and comparison circuit. Through the debugging of the whole machine, the system has met the expected design requirements.

During the measurement process, the pulse signal collected by the sensor is very weak and easily interfered by the external environment. Therefore, it is necessary to analyze the interference noise of the pulse sensor. The main interference of the photoelectric pulse sensor is the measurement of ambient light interference, electromagnetic interference, and motion noise interference during the measurement process. In order to reduce the influence of ambient light on the measurement of pulse signals, and considering the convenience of using the sensor, a sealed finger sleeve packaging method is adopted, and the entire shell adopts opaque medium and color to minimize the influence of external ambient light. The electrical signal containing pulse information obtained by photoelectric conversion is generally weak and easily interfered by external electromagnetic signals. Therefore, the hardware circuit is properly shielded. The pulse signal changes slowly and is particularly susceptible to interference from power frequency signals. The use of a notch filter effectively solves this problem. During the measurement process, making the finger sleeve and the finger in closer contact reduces the relative movement between them and reduces motion noise.

5 Conclusion

The key technology in heart rate detection lies in the production of sensors and the amplification of weak pulse signals. Through actual design and production, the results confirmed the rationality and feasibility of the design, indicating that the scientifically designed transmission sensor can achieve non-destructive detection of finger pulses. However, further research is needed in the field of small signal amplification technology. Compared with other heart rate detectors, this device has a small size, light weight, low cost, easy use, accurate measurement, etc., and has a good application prospect.