MN101EF32D MCU based electronic sphygmomanometer design

Introduction

Blood pressure is a very important health indicators, blood pressure measurement of the accuracy or otherwise directly related to people's health.

Countries the sphygmomanometer classified as compulsory examination and measurement instruments. General Hospital using mercury sphygmomanometer method based on artificial korotkoff sound, this approach has some inherent disadvantages: first is deflated speed have a direct impact on the readings, the international standard on gas speed for 3 ~ 5mmHg per second, while a different doctor placed there fast with slow gas would have an impact on measurement accuracy; second, this approach to human vision, hearing, and the degree of coordination as a main basis, it is difficult to standardize. To this end, the design from the detection of blood pressure, use ribensongxiagongsi high-speed, low-power single-chip, as MN101EF32D sphygmomanometers, control, data read/write, data, display of core, can be accurately use oscillometric (oscillation) measurement of blood pressure.

Working principle

Oscillometric (oscillation) is based on the cuff in decompression process, its pressure oscillation amplitude of wave envelope to determine the changes of blood pressure.

Now compare consensus is when cuff pressure oscillation amplitude of the wave maximum, cuff pressure is the arterial pressure on average. Artery of systolic blood pressure corresponds to amplitude envelope of the first corner, diastolic blood pressure corresponds to the envelope of a second ahead of the curve.

Hardware design

System basic works as shown in Figure 1.

Pressure sensor output voltage signal first lowpass filter, and then by the amplifier circuit will signal to the input signal for SCM, the last analog signal after MN101EF32D of sampling into digital single chip. Procedure for acquisition of data analysis, digital filtering calculated after the blood pressure of the two key indicators of the diastolic pressure "and" systolic blood pressure ", after the MCU to store the data to an external storage, and these important data displayed on the LCD.

Sensor and its peripheral circuit design

The use of sensors for sphygmomanometer MPS-3100-006G Piezoresistive pressure sensors, is made up of four equivalent resistance-bridge consisting of Hui, its output voltage and input pressure proportional, ideally when pressure input resistance values follow change, but in fact the temperature change can also affect the results of its output resistance value.

In addition, because Crystal and circuit design error, coupled with the packaging process, impact, zero offset is zero. It must be applied to individual components of temperature compensation circuit calibration. Its important indicators are as follows:

A determination of the scope: sensor 5.8 ~ 15 PSIG

B, operation temperature range:-40 ~ 85 ° c

C, driving current: 1.5 ~ 3mA

D, driving voltage: 5 ~ 15V

E, zero drift: 25 ~ 25mV

F, resistance temperature coefficient: 0.2%/¡æ

Because blood pressure signal is taken from the arm, measurement of signal vulnerable cuff's location, the arm to move the interference.

According to these professional features, request system with a high input impedance, high gain, high common-mode rejection ratio, low noise, low drift, and other features. As shown in Figure 2, figure of T1 is MPS-3100-006G Piezoresistive pressure sensors. The entire circuit first pressure signal is converted to a voltage signal, and then make amplification filtering. Figure U1, U2 LM324 for active operational amplifier, the input impedance is high. Pressure sensor signal amplifier, and adjust the size of the VR1 to changing operational amplifier-loop gain to regulation in order to adapt to the A/D input voltage range. U1 amplifier circuit used to measure the pressure in the cuff, measurement data to analysis and control for MCU on cuff inflated and deflated. Plus U2 amplifier circuit are separated through C11 capacitance direct AC signal amplification, this loop measurement is human pulse wave. Two loop collection data form the various indicators of important sphygmomanometer calculation parameters.

MN101EF32D properties

MN101EF32D is Matsushita (Panasonic) launched in early 2008, MN101Exx series 8-bit microcontroller peripheral compound a multifunctional, flexible and optimized hardware structure, simple and efficient instruction system, the full realization of economical efficiency and high-speed.

MN101E32D type microcontroller, built-in Flash and RAM 64KB 4KB, with six external interrupt, 20 internal interrupt (NMI), nine timer counter, 3 x serial interface, 8-channel A/D converters, 32 x 4 LCD drive, watchdog timer, single system of data transmission function, automatically synchronizing the output function and the buzzer output and peripheral functions.

Minimum instruction execution time can be up to 64 50ns, encapsulation. pin LQFP This functionality using MN101EF32D sphygmomanometer is as follows:

A and 10-bit A/D sampling for static pressure and the pulse wave measurement.

B, LCD controller, direct drive 23 * 4 LCD monitor, display measurement process and results.

C, timer for scheduled A/D sampling data and calculate auto power off time.

D, using digital signal processing technology on A/D sampling of signal processing, mainly for digital low-pass filter and related calculations.

E, power open hardware control, power off using the control's methods, the shutdown occurs apart from voltage module, chip powered power consumption is very low.

F, measurement can choose mmHg and Kpa as the primary display, measuring accuracy is high, reaching static 1mmHg, dynamic 3mmHg measuring accuracy.

As a result of using ferroelectric memory as storage media, data protectionSave time is very long.

MN101EF32D and external serial fram hardware connection

In the choose the external storage, due to the long term, taking into account the need to repeatedly erase, write the settings work parameters and measured with important information, and save a lot of historical data, you must use the larger SRAM to write as many data information and ensure that the data is not lost after loss.

Because of the design process itself EEPROM. Life limited and write longer, therefore is not ideal for battery-powered systems. Sphygmomanometer need to save data design for systolic blood pressure (2 bytes), diastolic pressure (2 bytes), average pressure (2 bytes), pulse (2 bytes), each record of the time (5 bytes), and so on, each measure 13 bytes needed to store the data. Hypothesis 4 times a day, the need for measuring 13 × 4 = 52 bytes, sphygmomanometer to save seven days of data you need, choose 364 bytes "fram" 24cL04. When you open used sphygmomanometer, MCU in its PA0 port analog out IIC bus SCL and input to the external memory 24cL04 SCL pin, PA1 port and the SDA 24cL04 mouth for data exchange, data will be useful to appear on the LCD. Power processing module and its associated circuit set

This selection 2 section 7 sphygmomanometer, battery as the power input.

In order to achieve a better quality of power supply, in this circuit has chosen the DC/DC boost chip RN5RK331A, 2-line no. 7 1.5 volt battery voltage of 3V rises to around 3.3V, supply system simulation circuit power, but also as a digital circuit is power supply MCU (as shown in Figure 3). Considering the air pump, valve if and analog circuit, digital circuit directly to share power, would introduce greater interference, thereby affecting pressure sensor, amplifier and MCU to work properly, so designed to pump, valve and other devices connected together, directly by the battery.

In addition, an important collection of blood through the amplifier magnified cuff pressure and straight after the pulse wave, because they are small signal amplification, so after A/D conversion of design is also very important.

The system adopts intelligent automatic blood pressure measurement, inflatable, in Buck's process measurement. Due to work pressure valve, power supply is wave, if you are using the system directly to as A/D reference voltage reference, will bring to measurement error. Adoption of National Semiconductor's LM385 as A/D conversion voltage reference connected to the chip's VREF + pin, to ensure that the collection of accurate data conversion.

LCD display module design

As in Figure 4, 5, in order to enable users to more easily, simply by using this system, a LCD display.

Panasonic's MN101EF32D chip built-in LCD driver module, you can direct drive LCD.

To initialize the LCD mode control register 1 (LCDMD), it is an 8-bit registers, used to specify the LCD clock, LCD display ON/OFF, display the duty cycle.

System software design

Software key processes are as follows:

Power-on, the first complete system initialization.

Microcontroller power supply to pump start, allowing rapid inflatable cuff to users about 30mmHg systolic blood pressure above. After the SCM through 1 D started A/road acquisition cuff pressure and cuff pressure drop in speed to control exhaust vent, the cuff band uniform buck (3 ~/s 5mmHg). At the same time, 1-channel A/D start acquisition after every other straight pulse wave. When the amplitude of the pulse wave maximum cuff pressure is the arterial pressure on average. Artery of systolic blood pressure corresponds to amplitude envelope of the first corner, diastolic blood pressure corresponds to the envelope of a second ahead of the curve.

Software major subdivided into the following three major modules:

A) uniform pressure control module

Although valve automatic slow deflated, but in order to enable the rapid inflatable cuff to users about systolic blood pressure above 30mmHg after uniform buck (3 ~ 5mmHg/s), but you cannot use the normal method, because the entire measurement process is vulnerable to external shocks, such as human vibration cuff, trachea of the vibration, the body movement, etc., and tracheal rigidity degree also affects the cuff pressure changes within the Dim.

So the cuff pressure reduction in speed and frequency of the valve switches to a non-linear relationship.

This design uses a PID algorithm to control valve switches to ensure that the cuff to 3 ~ 5mmHg/s speed uniform buck.

By SCM's processing speed and RAM resource limitations, it does not use floating point arithmetic, and all parameters all integer, then divide by 2N (equivalent to shift), made a similar point arithmetic, can greatly improve the speed. Final assignment for the timer to control valve opening time, thus ensuring a constant pressure of speed.

In PID algorithm in three basic parameters of the Kp, Ki, setting and adjustment of Kd is to compare the hard part, in accordance with the principle of these parameters, adjusting method as follows:

1, pressure will soon drop to the target value, but the pressure drop of too much:

A) scaling factors too big;

B) too small differential coefficient;

2. pressure drop to the target value:

A) scaling factors too small;

B) integral coefficients too small;

3, basically can control on the target, but the upper and lower deviation of the larger and frequent fluctuations

a

) Differential coefficient is too small;

B) integral coefficients too large;

2) signal processing module

Ben sphygmomanometers signal 2 road, MPS-3100-006G pressure sensor signal first for treatment of low-pass filtering, excluded due to outside interference caused by signal reading after amplification of the error, as sending AD1; static pressure at direct after zoom sent again as AD2, pulse wave signal.

Since the A/D MN101EF32D 10 bits, so the maximum precision of up to 1/1024. In order to maximize the use of A/D conversion of sampling speed, use interrupt for A/D converted data processing. When A/D conversion completion interrupt program, anti pulse interference moving average method to implement a simple and effective digital filtering, enable measurements more accurate. Specific practices in a scheduled interruption successive 5 times within A/D conversion, removes the maximum and minimum values, the remaining 3 data for the arithmetic mean, the arithmetic mean as the a/d conversion result.

3) calculation of blood pressure modules

Cuff pressure and the pulse wave by signal processing module and then conclude that as shown in Figure 6.

The diagram below for users of pulse wave, above the sphygmomanometer boost and pressure drop in pressure in the cuff. On this basis, analysis signal for systolic, diastolic pressure, average pressure and heart rate calculation. SCM in the measurement process is stored each pulse wave peak, and each pulse interval time.

Systolic pressure criterion to determine the maximum amplitude method, i.e. deflated during pulse wave amplitude envelope rise above, when a pulse wave amplitude Ui and most significant Um (average pressure) than just greater than Ks, you think at this point the corresponding gas pressure for systolic blood pressure cuff.

Ps=P/Ui=Ks*Um

Diastolic pressure criterion determining also use maximum amplitude method to decide, but is in pulse wave amplitude envelope decreases, when a pulse wave amplitude Ui and most significant Um (average pressure) of just less than Kd, it believed that at this point the corresponding gas sleeve pressure is diastolic pressure.

Pd=P/Ui=Kd*Um

First experience parameter Ks = 0.54 and Kd = 0.72, tested for correction.

Heart rate is the pulse wave cycle, also to arithmetic mean.

Conclusions of the

MN101EF32D MCU based sphygmomanometer, takes full advantage of the chip itself, the circuit is simple, low power consumption, power requirements of a single, high precision and practicality, and other characteristics, have broad market prospects.