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Blood Pressure Monitor Using Mega32




Introduction

Our final project is to design and build a portable blood pressure monitor device that can measure a user’s blood pressures and heart rate through an inflatable hand cuff. The device is consisted of three main parts: external hardwares (such as cuff, motor, valve, and lcd), analog circuit, and microcontroller. The anolog circuit converts the pressure value inside the cuff into readable and usable analog waveforms. The MCU samples the waveforms and performs A/D convertion so that further calculations can be made. In addition, the MCU also controls the operation of the devices such as the button and lcd display. Since we have the word ‘portable’ in our title, for sure all of the components are put together in one package which allows a user to take it anywhere and perform a measurement whenever and wherever he/she wants.

Blood Pressure Monitor

It is undeniable that nowadays people are more aware of the health conditions. One of the most widely used methods to test the health conditions of an individual is to measure his/her blood pressures and heart rate. We, as ones of those who are concerned about their health, decided to work on this subject matter because we would like to build something that is useful and useable in real life.

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High Level Design

1) How blood pressures are measured

           Usually when the doctor measures the patient’s blood pressure, he will pump the air into the cuff and use the stethoscope to listen to the sounds of the blood in the artery of the patient’s arm. At the start, the air is pumped to be above the systolic value. At this point, the doctor will hear nothing through the stethoscope. After the pressure is released gradually, at some point, the doctor will begin to hear the sound of the heart beats. At this point, the pressure in the cuff corresponds to the systolic pressure. After the pressure decreases further, the doctor will continue hearing the sound (with different characteristics). And at some point, the sounds will begin to disappear. At this point, the pressure in the cuff corresponds to the diastolic pressure.

           To perform a measurement, we use a method called oscillometric. The air will be pumped into the cuff to be around 20 mmHg above average systolic pressure (about 120 mmHg for an average). After that the air will be slowly released from the cuff causing the pressure in the cuff to decrease. As the cuff is slowly deflated, we will be measuring the tiny oscillation in the air pressure of the arm cuff. The systolic pressure will be the pressure at which the pulsation starts to occur. We will use the MCU to detect the point at which this oscillation happens and then record the pressure in the cuff. Then the pressure in the cuff will decrease further. The diastolic pressure will be taken at the point in which the oscillation starts to disappear.

3) Hardware diagram

The user will use buttons to control the operations of the whole system. The MCU is the main component that controls all the operations such as motor and valve control, A/D conversion, and calculation, until the measurement is completed. The results then are output through and LCD screen for the user to see.

4) Analog Circuit

           The analog circuit is used to amplify both the DC and AC components of the output signal of pressure transducer so that we can use the MCU to process the signal and obtain useful information about the health of the user. The pressure transducer produces the output voltage proportional to the applied differential input pressure. The output voltage of the pressure transducer ranges from 0 to 40 mV. But for our application, we want to pump the arm cuff to only 160 mmHg (approximately 21.33 kPa). This corresponds to the output voltage of approximately 18 mV. Thus, we choose to amplify the voltage so that the DC output voltage of DC amplifier has an output range from 0 to 4V. Thus, we need a gain of approximately 200. Then the signal from the DC amplifier will be passed on to the band-pass filter. The DC amplifier amplifies both DC and AC component of the signal (it’s just a regular amplifier). The filter is designed to have large gain at around 1-4 Hz and to attenuate any signal that is out of the pass band. The AC component from the band-pass filter is the most important factor to determine when to capture the systolic/diastolic pressures and when to determine the heart rate of the user. The final stage is the AC coupling stage. We use two identical resistors to provide a DC bias level at approximately 2.5 volts. The 47 uF capacitor is used to coupling only AC component of the signal so that we can provide the DC bias level independently.




Hardware Design

1) Pressure Transducer

We use the MPX2050 pressure transducer from Motorola to sense the pressure from the arm cuff. The pressure transducer produces the output voltage proportional to the applied differential input pressure. We connect the tube from the cuff to one of the inputs and we leave another input open. By this way, the output voltage will be proportional to the difference between the pressure in the cuff and the air pressure in the room.

2) DC Amplifier

Since the output voltage of the pressure transducer is very small, we have to amplify the signal for further processing. We use the instrumentation amplifier AD620 from Analog Devices. The resistor R G is used to determine the gain of the amplifier according to the equation . Since we need the gain of approximately 200, we choose the resistor R G to be 240 ohms. This will give us the gain of 206 according to the equation. However, we have measured the gain from the finished circuit, and the measured gain is 213.

3) Band-pass Filter

The band-pass filter stage is designed as a cascade of the two active band-pass filters. The reason for using two stages is that the overall band-pass stage would provide a large gain and the frequency response of the filter will have sharper cut off than using only single stage. This method will improve the signal to noise ratio of the output.

First Band-pass filter :

The lower frequency cutoff is

The higher frequency cutoff is

The mid-band gain of the first filter is

Second Band-pass filter:

The lower frequency cutoff is

The higher frequency cutoff is

The mid-band gain of the first filter is

           Thus for the band-pass filter stage, the overall gain is 399.6. Combining this gain with the gain from the DC amplifier, the total AC gain for the circuit is .The choice of high and low cut-off frequency is good enough to give us very clean AC waveform.

4) AC coupling stage  

           The ac coupling stage is used to provide the DC bias level. We want the DC level of the waveform to locate at approximately half Vdd, which is 2.5 V. The schematic for AC coupling stage is shown in figure 4. Given this bias level, it is easier for us to process the AC signal using the on-chip ADC in the microcontroller.

Blood Pressure Monitor diagram

 The AC output from this stage will be passed on to the analog-to-digital converter in the Mega32 microcontroller. The image from the laboratory bench is shown in figure 5. We can see that it is very nice and clean.

Software Design

1) Design for the operating control

The block diagram for the operating control is consisted of a total of 7 states. We first start at the START state where the program waits for the user to push the white button of the device. Once the white button has been pushed, the measurement process begins by inflating the hand cuff. While the cuff is being inflated, if the user feels very uncomfortable or painful, he/she can push the grey button(emergency button) to stop the motor, quickly deflate the cuff and stop the measurement. This will ensure that the safety of the user is well maintained while using the device. Anyhow, if the cuff-inflating procedure goes smoothly, the air will be pumped into the cuff until the pressure inside the cuff reaches 160 mmHg. After that, the motor will be stopped and the air will be slowly released from the cuff. Again, at this point, the user can abort the process by pressing grey button. Once the MCU has obtained the values of systolic, diastolic and heart rate, the valve will be open to release air from the cuff quickly. Then it will report the result of the measurement by displaying the obtained data on the LCD screen. After that if the black button is pushed the program will return to the START state again waiting for the next measurement. Note that if the emergency button is pushed, the black button needs to be pushed in order to return to the start state.

Parts list:

Components

Quantity

Price

Mega32 Microcontroller

1

$8.00

Custom PC board

1

$5.00

Hand-cuff

1

$16.0

Solder board

1

$2.50

White button

1

$1.31

Grey button

1

$1.31

Black button

1

$1.31

On-Off switch

1

$1.67

LCD

1

$8.00

Pressure Transducer
MPX2050

1

Free, Sample

Instrumentation Amplifier AD620

1

Free, Sample

Op-amp
OPA2277

1

Free, Sample

9V to -5V Voltage regulator

2

Free, Sample

9V to 5V Voltage regulator

1

Free, From lab

Resistors and Capacitors

Free, From lab

LED

1

Free, From lab

Motor and Valve

2

Free, Used

Total

$45.10

For more detail: Blood Pressure Monitor

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