Sharing of design skills for low-power and high-precision portable multi-function medical examination meter

Introduction
The accelerated pace of society has led to a significant reduction in the time and energy people have to take care of children (especially infants) and the elderly. A series of diseases such as pneumonia in infants and young children will be reflected in their body temperature, respiratory rate and other indicators (medical research shows that the respiratory rate of pneumonia patients is obviously abnormal), while diseases in the elderly are reflected in indicators such as blood pressure.

In addition, people in modern society pay more and more attention to physical exercise, and more and more people are joining the ranks of sports. However, how to measure the quality of their exercise and their physical condition after exercise requires sports enthusiasts to have a sense of self-care. For example, they need to understand whether their pulse, breathing rate and other physical fitness indicators have returned to normal after a certain period of time after training (under normal circumstances, after the end of exercise, within a specified number of minutes, heart rate and breathing rate should gradually recover).

The above two demands tell us that it is necessary to design some simple and convenient medical electronic systems to assist people in taking care of infants, young children, the elderly and other groups, to monitor the health status of these vulnerable groups, to measure the body temperature and respiratory rate of infants and young children, and to measure the blood pressure of the elderly; at the same time, it is also used to assist in measuring pulse and other sports-related physical indicators, so that sports enthusiasts can monitor their physical condition after exercise.

However, similar medical electronic products on the market are mostly single-function systems, which limits their scope of application. As the development trend of "system integration" progresses step by step, single systems that can only achieve the above-mentioned single functions will gradually be eliminated, and designing systems that can achieve comprehensive functions will become the general trend.

This article is based on the MXT8051 microcontroller independently developed by Beijing Times Minxin Technology Co., Ltd., making full use of its internal resources and rationally selecting peripheral devices. It gives a design scheme for a portable physical examination meter that integrates multiple functions such as blood pressure measurement, body temperature measurement, respiratory rate measurement, and pulse measurement, and attempts to pursue low power consumption, high precision, high speed and other characteristics, which has a certain reference value for the final realization of the product.

Overall system design

MXT8051 Introduction

MXT8051 is a SoC independently developed by Beijing Times Minxin Technology Co., Ltd. with high-speed single instruction cycle 8051 as the core. It integrates a wealth of digital and analog peripherals, such as power-on reset, pulse width modulation (PWM), serial port (UART), watchdog (WDT), real-time clock, internal oscillator and external clock optional modules in the digital part; 4-channel 10-bit AD converter, programmable gain amplifier (PGA), operational amplifier (OP), 4com*36seg LCD driver and other modules in the analog part.

By embedding high-performance digital and analog peripherals into low-power MCUs, it is relatively easy to implement system-on-chip for portable medical electronic devices using MXT8051 as the core device.

System composition

The design of a portable multi-function medical examination meter based on MXT8051 given in this article makes full use of the rich on-chip peripheral resources provided by MXT8051. It can integrate multiple functions such as blood pressure measurement, body temperature measurement, respiratory rate measurement, pulse measurement, etc. into a portable system by relying solely on the cooperation of simple peripheral devices and the collaborative work of software.

The overall design of the system is shown in Figure 1. The entire system consists of five parts. The MXT8051 is the core processing module, and is equipped with a signal acquisition module based on sensors, an output display module based on voice chips and LCDs, system control modules such as power supplies and keyboards, and application expansion modules.

How the system works

Since this system has many functions to be implemented, it is crucial to provide a more complete architecture in the overall system design stage for subsequent software and hardware design.

For blood pressure measurement, the MPS series dedicated blood pressure sensor is used to sense the arm blood flow pressure, and the PWM wave inside the MXT8051 is used to control the motor and the inflation and deflation of the cuff. The collected analog signal is sent to the OP and ADC inside the MXT8051 for analog front-end processing, and then the software algorithm is used for digital signal processing.

For body temperature measurement, the single bus sensor DS18B20 is used to sense the body temperature signal. While strictly following the DS18B20 timing, the converted digital signal is directly sent to the MXT8051 for processing, and algorithms such as software compensation are used to improve the accuracy.

For respiratory rate measurement, two thermistors and two ordinary resistors are used to form a Wheatstone bridge. One thermistor senses the ambient temperature, and the other thermistor senses the temperature of the exhaled gas. This can effectively eliminate the changes in the bridge output caused by different ambient temperatures, so that the bridge pressure difference signal is used to characterize the breathing action and input into the OP and ADC inside the MXT8051 for analog front-end processing, and then processed by software algorithms inside the MXT8051.

For pulse measurement, the wrist pulse sound signal is converted into an electrical signal using a sound sensor, and then input into the OP and ADC inside the MXT8051 for analog front-end processing to achieve amplification, shaping, digitization, etc. Then, the software algorithm is used inside the MXT8051 to perform further signal processing to achieve the purpose of pulse measurement.

There are two design functions for the output display of measurement results. One is to send the measurement value to the LCD display, and the other is to use the voice module to inform the user whether the measurement result is normal or not.

Hardware Design

In order to fully utilize the on-chip resources of MXT8051, this paper comprehensively weighs various solutions and reasonably selects peripheral devices, trying to use the simplest peripheral circuit to ensure functional design. The hardware block diagram of the entire system is shown in Figure 2, and the design of each module will be introduced in detail later.

Design of blood pressure measurement module

For the perception of blood pressure signals, the MPS-2108 sensor from Metrodyne Microsystem can be used. Its wide application in the field of sphygmomanometers, digital pressure gauges and medical equipment has shown that it can perform the functions to be designed in this article. The interface schematic diagram of the MPS-2108 sensor and MXT8051 is shown in Figure 3.

Body temperature measurement module design

For the perception of body temperature signals, the DS18B20 sensor from Dallas Semiconductor is used. It has a unique single bus interface, which only requires one interface pin for communication; it can directly transmit digital signals to the microcontroller, thus eliminating the need for many peripheral circuit designs of the analog front end; and it can reach a resolution of 0.0625°C by configuring the internal registers. These characteristics show that the DS18B20 can already undertake the task of collecting body temperature signals in terms of hardware. The interface schematic diagram of the DS18B20 sensor and the MXT8051 is shown in Figure 4.

Design of respiratory rate measurement module

For the perception of respiratory rate signals, a highly sensitive thermal resistor Pt1000 sensor can be selected. It has high accuracy, good stability, and a wide range of application temperatures. It is not only widely used in industrial temperature measurement, but also made into various standard thermometers for measurement and calibration. The interface schematic diagram of the Pt1000 sensor and the MXT8051 is shown in Figure 5.

Pulse measurement module design

For the perception of pulse signals, a common microphone head MIC can be selected to perceive the pulse sound signal, and the collected analog signal is sent to MXT8051 for processing through the on-chip operational amplifier. The interface schematic diagram of the microphone head and MXT8051 is shown in Figure 6.

Output display module design

In the output module, the voice chip can be selected from Winbond's ISD1730 single-chip high-quality voice recording and playback circuit. It can record and play 100,000 times, can process up to 255 segments of information, and multiple sampling frequencies correspond to multiple recording and playback times. This also provides certain possibilities for system function expansion. The LCD uses the product on the MXT8051 development board provided by Beijing Times Minxin Technology Co., Ltd.

System control module design

In the system control module, the power supply circuit, clock circuit, reset circuit, keyboard control circuit and other required devices can refer to the mature products on the MXT8051 development board provided by Beijing Times Minxin Technology Co., Ltd., and refer to its interface mode and make slight modifications to meet the expected requirements. The slightly modified power supply schematic diagram and clock circuit schematic diagram are shown in Figures 7 and 8.

For blood pressure measurement, the industry generally uses the oscillometric method, and the realization of the final product can be completed by using this type of mature algorithm. Respiratory rate measurement and pulse measurement can be simply completed by using the method of measuring the cycle and then taking the inverse. At the same time, in order to avoid errors, you can consider increasing the number of samples. For body temperature measurement, the initialization timing and read and write timing of DS18B20 must be strictly followed.

Program flow chart

Putting the design scheme mentioned in this article into practice, the function finally realized is body temperature measurement. The software program flow chart of body temperature measurement is shown in Figure 9.

Summarize

The design scheme of the portable multifunctional physical examination meter based on MXT8051 proposed in this paper has certain reference value for the final realization of such integrated medical electronic equipment products. Due to time constraints, the expected functions were not fully realized. At the same time, in addition to the expected functions, corresponding expansions can be made, such as using the on-chip resources of MXT8051-serial port (UART) and computer communication to transmit historical measurement data to the computer for medical comparison and analysis.

Software Design

Algorithm Idea

According to the results of the division of the entire system modules, the analysis found that the most difficult part of the algorithm should be the blood pressure measurement part, while the body temperature measurement, respiratory rate measurement and pulse measurement are relatively easy. Of course, in the body temperature measurement, due to the strict timing requirements of DS18B20, the code needs to be written carefully.