Development of Low Cost Microcontroller Based Spirometer

Spirometer is an instrument which is used to measure the volume and capacity of lung. It collects the air expired by the patient to measure the lung volume or capacity. The spirometer has a device attached to it which will measure the movement of gas in and out of chest and this device is called as Spirograph. And in some spirometers spirographs are replaced by printer called Spirogram. In most of the computerized system the spirographs or the spirogram will display the predicted value next to the observed value.

Using spirometer various tests are carried out to determine whether the patient is having any respiratory disorder or not.Some of the tests carried out are-

• Tidal Volume (TV)-The amount of air that is breathed in and out by an individual during normal respiration.
• Forced Vital Capacity (FVC)-The amount of air that is maximally forced out of the lung after maximum inspiration.
• Forced Expiratory Volume In 1sec (FEV1)- The amount of air that can be maximally forced out of the lung in the first second of forced expiratory manuever.

1.2 OBJECTIVE OF THE PROJECT

To design a low cost microcontroller based spirometer and carry out the various lung functioning tests.

To develop a program for the microcontroller to measure the lung volume and display the result digitally on a LCD interfaced with it.

CHAPTER 2: LITERATURE SURVEY

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AUTHORS: Agarwal.V ,Ramachandran N.C.S,2008,[1]

TECHNIQUES USED: The patient exhales through the mouthpiece where the gas exchange is being acquired as the voltage signal by the transducer(sensor).

This signal from the sensor is amplified by an instrumentation amplifier(INA 114).

Then the amplified signal is filtered using a low-pass-filter which is then given to a ADC(ADS7812) for digitisation.

After digitisation it is given to the microcontroller(Intel) from which the data is displayed on the LCD screen.

For further interface for communication on internet ETHERNET device is used.

RESULTS: The output voltage from the amplifier is digitised using ADC which corresponds to the pressure difference across mouthpiece is computed by the microcontroller and displayed on the LCD screen.

LIMITATIONS/FUTURE VISION: In future the system is expected to be inexpensive medical care for thousands of patient.

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AUTHORS: Nagaraja K.A,Nanda.S, 2007,[2]

TECHNIQUES USED: The patient exhales the air through the mouthpiece which consists of a flow sensor, differential pressure sensor, instrumentation amplifier,low pass filter and ADC.

The flow sensor relates the velocity and the pressure of air blown if there is no air flow through the sensor the velocity is converted to pressure and this pressure difference is sensed by the difference flow sensor which inturn provide the electrical quantity of the air flow.

Inorder to remove noise and amplify the signal an instrumentation amplifier is used.

Which is then fed to the band pass filter of the range 0.05Hz to 1.3Hz,where the spirometer lies in the range of 0.1Hz to 12Hz.

The filtered signal is digitised using ADC and this data is read by a microcontroller which is then interfaced to a computer.

RESULTS: Various tests are performed and FVC,FEV1 and their ratio are calculated and monitored.Depending on these values we determine whether its normal (or) restrictive (or) obstructive.

LIMITATIONS/FUTURE VISION: Not portable

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AUTHORS: Dr.David Van Sickle et.al ,2009,[3]

TECHNIQUES USED: Initially the patient is allowed to expire through the spirometer.

The spirometer consists of a pressure sensor(HW 24) which gives a DC voltage as output.

This DC voltage is propotional to the pressure drop between the leads which is recorded by the spirometer.

This analog data is then converted to digital using ZMD 31014 “iLite” chip.

Microcontroller(PIC18f13k50) is used to convert the digital data to USB.

Using mathematical algorithm implemented in the computer it calculates the volume and displays on the screen.

RESULTS: Graphical display of flow vs volume is evaluated and monitored from which the following values can be calculated :PEF,FVC, FEV(t) and FEV1/FVC ratio.

LIMITATIONS/FUTURE VISION: Highly expensive.

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AUTHORS: Ovlyaguli et.al,2001[4]

TECHNIQUES USED: The test is performed using wedge bellow type spirometer the FVC and FEV1 data collected are investigated where the test is performed on different patient and are grouped accordingly.

Then a mathematical modeling process is carried out based on fuzzy values obtained from the flow graphs.

These values for the subject with COPD is compared with healthy subject belonging to same age,sex and height.

Then normalized for fuzzy labels ‘Very low’,’low’,’Normal’,’High’,’Very High’which is the generated on rule base and simulated.

RESULTS: Provide the interrelationship between characteristic constants obtained from the curves and degree of disease and FEV1 values are measured.

LIMITATIONS/FUTURE VISION: In future it eliminates the error factors and helps in accurate diagnosis.

CHAPTER 3

RESPIRATORY DISORDER: SYMPTOMS & CAUSES

3.1 RESPIRATORY DISORDER

Respiratory disorder deals with the diseases of the respiratory system which include diseases of the lung, pleural cavity and bronchial tube. Respiratory diseases range from mild to life-threatening such as bacterial pneumonia or pulmonary embolism which may lead to illness and death. The study of respiratory disease is known as pulmonology. A doctor who specializes in respiratory disease is known as a pulmonologist.

Some of the common respiratory diseases are:

• Inflammatory lung disease.
• Obstructive lung disease.
• Restrictive lung disease.
• Pleural cavity diseases.

3.2 OBSTRUCTIVE LUNG DISEASE

Obstructive lung diseases are diseases of the lung in which the bronchial tubes become narrowed.

COPD

Two examples for obstructive lung disease are:

Chronic Obstructive Pulmonary Disease (COPD) is a disease in which the airways become damaged and hence making them narrow. Asthma causes obstruction to the airflow out of the lungs.And the obstruction is reversible. 

3.2.1 SYMPTOMS

• Persistent cough.
• Sputum or mucus production.
• Wheezing.
• Chest tightness.
• Tiredness.

3.2.2 CAUSES

• Smoking.
• Occupational exposures.
• Air pollution.
• Genetics.
• Autoimmune disease.

3.3 RESITRICTIVE LUNG DISEASE

Restrictive lung disease is also known as restrictive ventilatory which restricts the lung expansion and there by decreases the lung volume and increases work of breathing.

SYMPTOMS

Shortness of breath, cough

3.3.2 CAUSES

• Asbestos is caused by long-term exposure to asbestos dust.
• Radiation fibrosis(radiation used for cancer treatment).
• Rheumatoid arthritis.

3.4 DIAGNOSIS OF ABNORMALITY OF RESPIRATORY SYSTEM USING FEV1/FVC RATIO

In restrictive lung disease, both FEV1 and FVC are reduced so the FEV1/FVC ratio is normal.

In obstructive lung disease FEV1 is reduced while the FVC is normal thus the FEV/FVC ratio is lower

CHAPTER 4: PRINCIPLE OF WORKING

The spirometer consists of a turbine flow sensor containing LED(source) on one side and a photo-diode(detector) on the opposite side.Inbetween the LED and the detector there is a rotor.The rotor is a fan like structure with 3 fins.The above arrangement is present inside a cylindrical case which is opened on both the sides.The LED and the photo-diode are molded with a light weight plastic design inside the case.A mouth piece is fitted on one side and the opposite side is kept open.When the patient blows air inside the mouth piece a pressure difference is created due to the atmospheric air(atmospheric pressure) that enters from the opposite side.A power supply circuit is designed to provide power supply to the spirometer.When the patient exhales through the mouth piece the rotor rotates.Whenever the rotor is in-between the source and the detector the light from the source is not detected by the detector and when the rotor is not in-between them then the light from the source is detected by the detector.Each pulse will be detected only if 7ml of gas has been exhaled by the patient.These pulses are then given to the comparator which will give binary output only if the patient is exhaling.And when the patient is not exhaling there is no output from the comparator.This ouput is then given to a microcontroller which is programmed in such a way that it will count the number of pulses from the comparator and these pulses are converted into total number of rotations by dividing them(no of pulses) by 3(since the rotor has 3 fins).These rotations are then used to measure the lung volume.The lung volume is then displayed on a LCD which is interfaced with the microcontroller as digital output.

CHAPTER 5: SYSTEM DESCRIPTION

5.1 ELECTRONIC CIRCUIT

5.1.1 POWER SUPPLY CIRCUIT

A 12V and a 5V power supply circuit was implemented.

5.1.1.1 IC 7812 & IC 7805

The two ICs are used to provide 12V and 5V supply respectively.

12V from IC7812 is given to the turbine flow sensor.

5V from IC7805 is given to LM324 & Microcontroller.

5.1.1.2 STEP-DOWN TRANSFORMER

Transformer being used is a 230V-step down transformer.

It converts AC to AC without any change in frequency.

BRIDGE RECTIFIER

The diode used is IN4007 which acts as bridge rectifier and converts the given AC current to DC.

5.1.2 SENSOR

The sensor used here is FT-330 Turbine flow sensor.The input to the sensor is the air blown by the patient.

5.1.2.1 FEATURES

• Can withstand temperature upto 80 degree celcius.
• Molded with electronics and a light weight plastic design.

5.1.2.2 ADVANTAGES

• Highly accurate.
• Very compact.
• Reliable.

5.1.2.3 POTENTIAL DIVIDER AND ZENER DIODE

• The output of the turbine flow sensor is given to the potential divider.
• The potential divider will reduce 12V from the turbine flow sensor and allows a reduced voltage to zener diode.
• The zener diode is used to fix a threshold i.e it allows only voltage within 5V to the comparator.

5.1.2.4 COMPARATOR

LM324 acts as a comparator which gives a voltage within 5V(square wave) when the patient exhales through the sensor.

And when the patient does not exhale, the comparator output gets reduced to 0V.The output of the comparator is given to the microcontroller.

5.1.2.4.1 FEATURES

Internally frequency compensated for unity gain.

Large DC voltage gain of 100 dB.

Wide bandwidth (unity gain) 1 MHZ.

Wide power supply range:

Single supply 3V to 32Vor dual supplies ±1.5V to ±16V.

5.1.2.4.2 ADVANTAGES

• Eliminates need for dual supplies.
• Four internally compensated op amps in a single package.
• Power drain suitable for battery operation.

5.1.3 MICROCONTROLLER

The microcontroller used here is 16F877A.

The microcontroller receives input from LM324.

The microcontroller is programmed in such a way that it will count the number of pulses from the comparator and these pulses are converted into total number of rotations by dividing them(no of pulses) by 3(since the rotor has 3 fins).These rotations are then used to measure the lung volume(TV,FVC,FEV1).

5.1.3.1 FEATURES

RAM=368 bytes

EEPROM=256bytes

PROGRAM MEMORY=8Kilo bytes

I/O PINS=33

5.1.3.2 ADVANTAGES

• This IC can be reprogrammed and erased up to 10,000 times.
• It is very cheap. It can also be very easily assembled.
• Additional components needed to make this IC work is a 5V power supply , 20MHz crystal oscillator and two 22pF capacitors.

5.1.3.3 LCD DISPLAY

The output(TV,FVC,FEV1)from the microcontroller is displayed on the LCD according to the program.

5.2 MECHNICAL DESIGN( MATERIALS USED):

5.2.1.TURBINE FLOW SENSOR

5.2.1.1 MODEL

FT-330 Turbine flow sensor

5.2.1.2 ADVANTAGE

Highly accurate, compact and reliable.

5.2.1.3 FEATURES

Can withstand temperature upto 80 degree celcius.

Molded with electronics and a light weight plastic design.

5.2.2. MOUTH PIECE

Made up of plastic with a diameter of 2cm.

CHAPTER 6: CIRCUIT DESCRIPTION

6.1 OVERVIEW OF CIRCUIT WORKING

Initially the patient is allowed to blow through the sensor and the output voltage from the sensor is given as input to the potential divider circuit which limits the current to zener diode and which inturn reduces 12V supply from the potential divider o/p and allows only 5V to be given to the inverting terminal of LM324(comparator). Reference voltage is given to pin 3 which gets the feedback from LM324 output. Hence when the patient doesn’t blow, a high voltage is received by pin 2(inverting terminal) which is compared with the reference voltage and in this case the i/p voltage >reference voltage and hence we get a low voltage(0V) at the output and no pulse is seen and when the patient blows, i/p voltage < reference voltage and we get a high voltage at the output and a train of pulses can be seen. Then the LM324 output is given to the microcontroller(pin 33) and the final values are displayed on the LCD screen.

CHAPTER 7: EXPERIMENTAL STUDY

7.1 ALGORITHM

1. Initialize the count to 0.
2. When the patient exhales, the number of pulses from LM324 will be counted i.e.; the count gets incremented.
3. This count is converted into rotations since the rotor has 3 fins, if we get 3 pulse, it is considered as 1 rotation. So by dividing the count by 3 we will get the total number of rotations.
4. The LM324 will give 1 pulse only if 7 ml of gas is exhaled . So the total number of rotations is multiplied by 7 to obtain the lung volume.
5. Initially the flag will be 0.When the patient exhales TV will be displayed.
6. When the reset is done the flag will be 1.Now when the patient exhales FVC will be displayed.
7. When the reset is done again the flag will be 2.Now when the patient exhales FEV1 will be displayed .Again when the reset is done and the patient exhales then TV will be displayed.

7.2 INFERENCE

Thus the tests results obtained from the designed spirometer matches with the tests results obtained from the laboratory spirometer.

Minor differences in the value is due to effect of surrounding environment and moving air.

CHAPTER 8: CONCLUSION

The objective of the project was to design a low cost spirometer for the rural health care centers,private clinicians and primary function laboratories.In order to substantiate the objective of our work, we compared the market price of currently available spirometer(MIR SpiroDoc Spirometer),the starting price of which is around $1974.50.

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Thus the tests results of four subjects obtained from the designed spirometer was compared with the tests results obtained from HELIOS Pulmonary Function test.The values had some minor differences due to effect of surrounding environment and moving air.But they were of acceptable accuracy.Future work involves interfacing the device with PC and implementation of the product in Telemedicine application for distance monitoring of respiratory parameter.

APPENDIX 1

1.1 PIN CONFIGURATION OF LM324(COMPARATOR)

Fig 1.1 Ref:www.voltage current.info

1.2 PIN CONFIGURATION OF PIC16F877A

Fig 1.2 Ref:http://www.best-microcontroller-projects.com/schematic.drawing-program.html

1.3 OVERVIEW OF PIC 16F877A

Fig 1.3 Ref:http://www.best-microcontroller-projects.com/schematic.drawing-program.html