Design of a Basic Parameter Tester Based on Single Chip Microcomputer

1 Introduction

The research is about a basic human parameter tester that can measure parameters such as body temperature, pulse and breathing interval. These parameters and records are the most widely used basic nursing technical operations, and the technologies are relatively mature. However, most ordinary portable devices have single functions, low accuracy, and can only be used for temporary measurements, and cannot track the entire treatment process of patients: large medical equipment used in hospitals can provide high accuracy and comprehensive functions, but their overly professional usage methods and high prices lead to low market demand. In view of these shortcomings, the research of this system has the following two meanings: ① Through one instrument, various human parameters are concentrated together for real-time measurement, thereby improving the integration and convenience of the measuring instrument. ② The measurement adopts a fully automatic method. By setting thresholds for various parameters, relevant instructions can be automatically given for parameters that exceed the threshold after measurement. This portable, accurate and recordable human parameter tester has high scientific value and social significance.

2 Comparative analysis of design schemes

2.1 Temperature measurement

Solution 1: Use a digital temperature sensor. The digital temperature sensor integrates a temperature sensor and an analog-to-digital converter, which can directly convert the temperature into a digital value and send it to the microprocessor.

Solution 2: Use an analog temperature sensor. That is, use an analog temperature sensor with a continuously variable output voltage, and then convert the analog voltage into a digital value through a high-precision A/D converter.

Solution 1 is relatively simple to implement, but it is costly to achieve high-precision temperature measurement. In Solution 2, since the output voltage of the sensor can change continuously, the accuracy of temperature measurement can be greatly improved by simply improving the accuracy of the A/D converter, so the system adopts Solution 2.

2.2 Breathing interval measurement

Solution 1: The rise and fall of the human chest can be reflected by the expansion and contraction of the elastic material. The expansion and contraction of the elastic material drives the slider to slide on the resistance wire, changing the resistance at both ends. Through the method of resistor voltage division, the change of voltage is made consistent with the change of chest rise and fall, and finally the measurement of breathing interval is realized. This solution has strict requirements on elastic materials, resistance wires, and binding methods.

Solution 2: Use a pressure sensor. The device is shown in Figure 1. The belt in the figure is made of wire with very small length deformation. A pressure sensor is fixed on the belt. The rise and fall of the chest causes the pressure on the pressure sensor to change. By collecting the electrical signal output by the sensor and capturing the trend of the chest rise and fall on the time axis, the breathing interval time can be measured. This device is simple and more suitable for portable devices.

To sum up, the system selects solution 2.

2.3 Pulse measurement

Solution 1: Use photoelectric sensors. Place the fingertip between the light source and the photosensitive device. When there is a beating pulse in the finger, the light transmittance of the blood changes, and the light intensity received by the photosensitive device changes accordingly. Thus, an electrical signal that changes according to the pulse beat law is obtained, but the extracted signal is very weak.

Solution 2: Use a piezoelectric sensor. Install a piezoelectric sensor with high sensitivity on the wrist to convert the pressure signal generated by the beating pulse into an electrical signal, thereby measuring the pulse. However, the cost is relatively high.

Solution 3: Use an electret microphone. Put the microphone close to the pulse, and the beating pulse signal can be converted into a corresponding electrical signal through this process. The electrical signal extracted by this solution is very obvious, the measurement accuracy is very high, and the cost is very low.

To sum up, the system adopts solution 3.

3. System Overall Design

The system is controlled by a single-chip microcomputer, and consists of sensors for extracting body temperature, pulse rate and breathing interval, and corresponding signal conditioning modules, amplification and shaping modules, power modules, keyboard control modules, and LCD display modules. The system also includes a centralized monitoring machine that can communicate serially with independent monitoring single-chip microcomputers for network management of instruments and equipment. The specific implementation block diagram of the system is shown in Figure 2.

4 Theoretical Analysis

4.1 Analysis of pulse measurement errors

The human pulse frequency is generally 60 to 100 times/min. The pulse signal is adjusted and shaped as the threshold. The standard pulse fo is counted at the first rising edge and the second rising edge stops counting. The count value is N. This filling count within the threshold has errors. The smaller the pulse frequency, the larger the count value, and the smaller the impact of the inherent error of this method. When the pulse frequency f=100 times/min, the error is the largest. Take fo=1 kHz, then theoretically N=fo(60/100)=600 times, but the actual value may be 599 or 601. According to the formula f=(60xfo)/N, we can get f1=100.169 times/min and 99.833 times/min. △fmax≤0.2 times/min, with high accuracy. [page]

4.2 Analysis of temperature measurement data processing methods

There are many factors that affect body temperature measurement. To improve measurement accuracy, the number of measurements is appropriately increased, and compensation methods are used to reduce the impact of random errors. In order to obtain the most reliable results, the least squares method is used, and the measured value is the most reliable under the condition that the sum of squares of the residual errors is minimized.

The measured temperature and the output voltage V are in a linear relationship, that is, V=b+aT, so the error equation of the linear parameter is

Where: vi is the error between each test value and the true value.

In equal precision measurement, the least squares condition should be satisfied.

By substituting the standard temperature values ​​T1, T2...Tn into the equation group, the values ​​of a and b can be solved. In this way, the process of linear calibration of load and voltage using the least squares method is completed.

5 Functional circuit design

5.1 Temperature sensor and post-amplifier circuit

This circuit uses the LM35 of National Semiconductor as the temperature sensor. Due to its small output voltage, a common-mode amplifier circuit is added in the latter stage, and the operational amplifier uses the LTCl047 with ultra-low noise and offset voltage. As shown in Figure 3.

      5.2 Pulse signal extraction circuit

The circuit consists of a microphone and an instrumentation amplifier AD620. The microphone converts the pulse signal into an electrical signal. The instrumentation amplifier AD620 is powered by a single power supply. Potentiometer R3 is used to adjust the DC bias of pin 2 to make it the same as the DC bias in the input signal of pin 3, thereby achieving differential amplification. The reference pin 5 is added with a DC bias through potentiometer R4, and the input signal can be amplified within a larger range as needed.

AD620 has small offset voltage and offset current, and high common mode rejection ratio. Its maximum offset voltage is 50μV, maximum offset current is 10 nA, and minimum common mode rejection ratio is 100 dB. Therefore, it has excellent quality in processing weak signals, that is, amplification and noise elimination. It is widely used in instrument amplification, such as simple ECG, pressure sensor, ultrasound instrument, etc. AD620 also has a bias terminal, which can output biased signals, and can also be used in occasions such as program-controlled amplification. The most typical application of AD620 is ECG signal detection. Figure 4 shows the AD620 circuit in the ECG signal detection circuit, and its input terminal (pins 2 and 3) is connected to a pair of differential signals.

Figure 4 AD620 typical application

6 System Software Design

The program is the control core of the microcontroller. It completes the basic functions. It should have a good user interface and good adaptability and versatility to facilitate modification and adjustment in the debugging stage to the greatest extent. Therefore, in the compilation of the program, we should pay attention to structured design and hierarchical design. The program flow chart is shown in Figure 5.

Figure 5 Software Flowchart

7 Conclusion

The human body parameter tester is small in size, has high accuracy in measuring human body temperature, pulse, and breathing interval, is easy to operate, and is powered by a single battery. It can be used as a portable instrument. The relative independence of each parameter is taken into consideration during the design. Each measurement module in the physical object is relatively independent and very easy to disassemble. The system also has the function of voice prompts and explanations. The system is integrated with a serial interface, which can form a network with other related instruments and equipment at any time for centralized measurement and management. The research on this system has important social value and practical significance.