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Research on Realization of a Device to Measure the Blood Pressure

	المؤلفون / Authors الملخص / Abstract الكلمات المفتاحية / Keywords أقسام الملف Introduction Method and material Material realization and results Conclusion References
Research on Realization of a Device to Measure the Blood Pressure
Sidi.Ahmed Taouli
Department of biomedical genius
Faculty of technology, University Aboubekr-Belkaid
sidiahmed.taouli@univ-tlemcen.dz


Abstract
The tensiometer is a device more used in private clinic and home use for measurement of the blood pressure. This, work is centered on the realization of an electronic sphygmomanometer (tensiometer) able to take measurements of the systolic and diastolic pressure with display on a screen LCD. This realization passes in two stages, the hardware stage and software.
The first stage is made up a measuring equipment of the invasive blood pressure. This chain comprises a pear the inflation, cuff, valve, pressure sensor MPX, amplifier, band pass filter, Microcontrolor (MCU) and a display LCD. Another stage, presents display LCD of the systolic and diastolic pressure.
The results, obtained on 20 patients, compare measurements of systolic and diastolic pressure via a tensiometer with auscultator measurements.
Keywords: Systolic blood pressure, diastolic blood pressure, tensiometer, invasive pressure, not- invasive pressure.

Introduction
The tensiometers or sphygmomanometers are among the devices most used in private clinic and home use for the measurement of the blood pressure (BP).
That constitutes an invaluable help with the medical diagnosis for the professionals of health, because the repetition of the catches at various times of the day can facilitate the choice of a treatment in certain cases.
Calculation is carried out on the level of the cuff or the wrist, the inflating and automatically deflating arm-cuff.
The market of the measuring apparatus of the blood pressure is very changeable. To be able to consider a tensiometer to be used clinically, it is necessary to take account of the method of measurement employed. These methods are divided into two parts: not-invasive and invasive.
The methods not-invasive is an indirect measurement technique uses the principle of the occlusive cuff of Riva-Rocci and also to detect the signal corresponding to BP (palpation, sounding, oscillation, Doppler, microphonic, Ultrasounds) [1-2].
In fact of the principle even of the not invasive technique, based on the application of a backpressure by the inflatable pocket of the cuff, it is significant that the surface of application of the latter is adapted to the circumference of the member, where the pressure is measured [3-4].
However, invasive measurement is necessary when the pressure varies quickly and that should be obtained frequent arterial taking away or when not invasive measurement is difficult or impossible.
It makes, it possible to give measurements more precise than the indirect methods.
Direct measurements of the arterial pressure (AP) or invasive measurements are the measurements carried out directly in the artery (aorta, radial, humeral, femoral) following the introduction of a catheter [5-8], connect to a pressure sensor. Thus the signal of pressure is recorded in the form of wave of pressure.
The site of perfusion of the catheter plays a significant role for the measurement of the blood pressures since the shape of the signal of BP varies from a site of perfusion with another [9] [7].
The electronic tensiometers with the arm are very similar to the traditional tensiometers. However, their displaying digital display a value much more precise and reliable that manual tensiometers.
The recent achievements of tensiometers in literatures present a large variety of apparatuses, each one having various functions [10].
Tensiometer Omron M6 Confort, is especially conceived for the patients, the particular cuff allows an easy and fast use. It is particularly automated functions on the principle of oscillometry [10-11].
Omron m3 Confort, is entirely automatic and functioning on the principle of oscillometry, this apparatus measures the blood pressure, by simultaneously posting the values of systole and diastole, thus the pulse [10-11].
In addition, the tensiometer HBP.1300 [10-11], professional is easy to use thanks to its robust design and its handle which makes it easy to transport.It is conceived for a use in professional environment and is validated clinically to provide precise results. Apparatus HBP is compatible with a broad range of cuffs easy to clean, size SS (12-18-cm) with XL (42-50 cm).
It is delivered with an electric adapter like with a refillable battery.
The principal objective of our work is the realization of an electronic tensiometer to the cuff able to take measurements of systolic and diastolic pressure with display on a LCD screen.

Method and material
The various types of the methods used are quoted in order to measure the blood pressure. Method exploited in the present article is that of oscillometry. The air is pumped in the cuff 60 mm Hg in addition to the normal value of the systolic pressure (120 mm Hg), then the air is slowly expelled cuff, causing the fall of pressure in this last. During the deflation of the cuff, it is possible to measure the small oscillations which appear. Indeed, it is during the systolic pressure that these oscillations start to appear. The microcontrolor (MCU) is used to detect the moment to which these oscillations occur and to record the pressure in the cuff. Thereafter, one notes the decrease of the pressure in the cuff until the disappearance of these oscillations: it is the diastolic phase and the value of the pressure in the arm-cuff can be taken again.
The diagram block of figure 1 makes it possible to measure the blood pressure, it comprises:

Figure 1. Diagram block to measure Pa.
a) Pear inflation
The pear of inflation is used to inflate the cuff.
b) Cuff
The cuff is a piece of fabric carried around the arm above the sleeve; it is composed of two distinct parts.
First of all, the cuff itself is made of an extensible air pocket encircled by a no elastic envelope which is used for maintains pocket around the member of the patient.
c) Pressure sensor
We used the pressure sensor MPX2100AP manufactured by Motorola. This sensor produces an output voltage proportional to the pressure applied to the entry i.e. a linear answer. The tube of the cuff is connected at the entry of the sensor.

Figure 2. The linear response of sensor.
The figure 2 summarizes the characteristics of the output:
minimal, maximum and typical. The output is directly proportional to the difference in pressure and primarily a straight line [12].

Figure 3. Test of sensor MPX2100AP.
To test the correct operation of the sensor, it is enough to put a voltmeter between the two terminals two and four tomeasure the differential tension of output (Vout).
At the same time, makes some increase the positive pressure applied to the entry. The Vout tension of the sensor is directly proportional to the pressure applied, then the output voltage of the sensor increases (Figure 3).
d) Amplifier
As the tension produced by the pressure sensor is very small, one needs to amplify this signal for additional samplings. One uses the amplifier of instrumentation AD620AN. Resistance RG is used to determine the gain according to the equation:

RG = 49.4 kΩ / G – 1
e) Microcontrolor
This PIC 16F887A will be used in order to make working and the formatting of the signals of the sensors. Thus starting from a value of tension ranging between 0 and 5 volts a physical size will be obtained of which the unit will be suitable [12-13].
This MCU has many modules making it possible to carry out various applications such asnumerical analogical conversion and the management of display of a screen LCD. So that itmicro controller can carry them out it is necessary to configure the registers i.e. to put the bits of those to 1 or 0 by programming them in language C.
For a normal operation, the MCU claims configuration hardware minimal made up of an oscillator and a circuit reset (Figure 4).

Figure 4.Minimal configuration hardware of MCU.

f) LCD display
After that, we want to display measurements of systolic and diastolic pressure on a LCD of the type JHD16A [12].
To use LCD display, it should of course be fed (+5V, mass), then it will be necessary to regulate the contrast of the screen in order to be able to read the posted contents. To adjust contrast with a suitable value, one varies the tension Vinp between 0 and +5V by using a potentiometer of 10k (Figure 5).
With the powering of the display, the first line should be completely dark, if such were not the case, it will be necessary to regulate the contrast of the display while exploiting the tension of the Vinp pin.
For a value of potentiometer equal to 9.1k one obtains a completely clear displaying.

Figure 5.Test of LCD displayJHD16A.

f) Filter
The stage of the band passfilter is built as a cascade of two activeband pass filters. Two filters are used because the whole of the bad pass filter provides broad gain and the frequential response of the filter will have a cut bandages acuter than a filter oneonly stage. This method improves the signal report/ratio on noise of the exit. We use it integrated circuit OPA TL082CN [12].
Material realization and results
The analogical circuit amplifies components CD and AC of the signal from the pressure sensor so that this signal is usable by the MCU and that one can sample it.The pressure sensor produces a tension proportional to the pressure of the entry.
The output voltage of the pressure sensor is of 38.5mV with 41.5mV.In our application, one wants to inflate the cuff up to 180 mm Hg (roughly 23kPa), which corresponds to a output voltage of 44.4 mV.For that, one chooses to amplify tension CD of 0 with 5V.One then needs a gain roughly equal to 100 times.
The signal outlet of amplifier CD thereafter will pass in a band pass filter. CD Amplifier was amplifies the two components CD, AC and the filter is built to have a broad gain.

Figure 6. Analogical circuit.
The alternate component AC of the signal resulting from band pass filter is the most significant factor making it possible to determine the favorable moment to collect the pressures systolic and diastolic.
The final stage is the coupling AC. One uses a circuit active offset which contains an identical amplifier 741 and resistances to produce leaning (skew) roughly equal to 2.5V.And, the coupling AC of the signal to provide leaning continuous independent(figure 6).
Figure 6 shows the analogical circuit made up of the AD620 and TL082CN. Its function consists to collect the signal of it, to amplify it and limit the frequently band according to our needs to make it ready for the entries of the ADC.
The stage of the band pass filter is built as a cascade of two active band pass filters. Two filters are used because the whole of the band pass filter provides a broad gain and the frequently response of the filter will have a cut bandages acuter than a stage filter only one. This method improves the signal report/ratio on noise of the output.
The transfers function of the filter:
T=-1/2+j.(ω/ωo -ωo/ω) (2)
With ω_o=1/RC
The low cut-off frequency is
Flow= 1/2π.(47uF).(10k) = 0.338Hz.
The high cut-off frequency is
Fhigh = 1/2π.(220nF).(120k) = 6.028Hz.
The semi-band gain of the first filter is
A1 = - 120k/10k = - 12.

The low cut-off frequency is
Flow = 1/2π.(47uF).(10k) = 0.338Hz
The high cut-off frequency is
F high = 1/2π.(22nF).(330k) = 21.92Hz
The semi-band gain of the second filter is
A2 = - 330k/10k = - 33
Therefore, for the stage of the band pass filter, the total gain is of:
A1xA2= (-12) x (-33) =396
By adding this gain to that of amplifier DC, the gain total AC for the circuit is of 4.07x 104.The choice of high and low frequency of cut is adequate to give a very clear wave AC.
The stage of the alternative coupling makes it possible to get the level of leaning continuous. We want that level DCof the wave is equivalent roughly semi-VDD what is equal to 2.5V.While being based on this level of leaning it is easier to sample the signal AC by using the ADC integrated in the MCU.
Before carrying out the development of the program contained in the Pic, it is preferable to establish as a preliminary an algorithm which defines its principle of operation. The flow chart of figure 7 summarily described the organization of the program.


Figure 7. Block diagram of program

It models the effect of a reset, represents the detection and the treatment of the oscillations and their level of amplitude, and finally the principal loop which carries out calculations of the pressures systolic and diastolic.
Our algorithm is presented in figure 7. Intuitively, all the parameters of entries are initialized, and that the cuff is completely deflated.
If the cuff is actually deflated, the system extinguishes the valve in closing. During the deflation of the cuff, once the value of the systolic pressure reached, one notes oscillations with the pin of ADC0. The program counts 4 oscillations (one counts 4 for more precision) and calculates the systolic pressure. Once this calculation carried out, the result is posted on screen LCD.
Once this achieved, the valve continues to function until the cuff completely deflated evening.
After the pump raises the pressure to 180 mm Hg:what is roughly higher than the systolic pressure of a healthy person, the arm-band starts to deflate and programs it between in the stage of systolic measurement. During this stage the program observes the wave AC exit of the ADC1 when the pressure in the arm-band decrease up to a certain value, blood starts to run in the cuff. At this time, if one observes the oscilloscope one can see the beginning of the oscillations. The systolic pressure can then be obtained. The method of our program is the setting of a threshold of tension of 4V for the wave AC. At the beginning, there are no pulsations and the tension with the pin of the ADC1 (figure 8) is constant with roughly 2.5V.
When the pressure in the cuff decrease until the value of the systolic pressure is reached, the oscillation starts and increases. We can then count the number of pulsations which have maximum values in top of tension threshold. If the program goes to four oscillations, it enters the stage of the calculation of the systolic

Figure 8. AC signal ADC1

Figure 9. DC signal inflation deflating

pressure. In this stage, the program records tension DC of the pin ADC0 which presented in (figure 9). Then, it converts this value of tension DC into the pressure in the cuff to determine the systolic pressure of user.
From the characteristics of transfer of the sensor pressure and the measured gain of amplifier DC. We can determine the systolic pressure one observing tension DC of pin ADC0.
Here the explanation of the procedure of conversion:
That is to say "Tension DC." Tension DC. Read of pin ADC0, and "Gain DC." the gain of amplifier DC. Consequently the differential tension from amplifier DC is calculated as follows:
Sensor tension=Tension DC/Gain DC (3)
From the characteristics of transfer of the sensor pressure, we can calculate the pressure based on the tension of sensor.
Slope = 40mV/50kPa = 8 x 10-4 V/kP
Therefore, the pressure in the cuff in unit KPa can be calculated by the equation:
Pressure_kPa = Sensor tension /Slopr (4)
We can then convert the pressure with the unit mm Hg into multiplying by760 mmHg / 101.325 kPa. Thus the pressure in unit mm Hg is expressed as follows:
Pressure_mmHg=Pressure_kPax760mmHg/101.325kPa
One combining all these conversions, we obtain the formula to convert tension DC into the pressure in the arm-band in the following way:
Pressure_mmHg = (Tension DC / Tension gain) x 9375
Once, that the program finishes this calculation, it enters the stage of measurement of the diastolic pressure of the user.
Figures (10-12) show the final realization of our systolic and diastolic pressure measurement circuitry.

Figure 10. Analogical circuit


Figure 11. Test of circuit
The test was carried out on a healthy female subject, its age, its size and its weight was respectively:23 years, 160cm, and 50kg. The results of the test (systolic and diastolic pressure) were compared with those obtained by the auscultator method.

Figure 12. Results of measurement

Results of the Auscultatoire method:

*Systolic Pressure: 11 cmHg.
*Diastolic Pressure: 8 cmHg.

Conclusion
We during, of this work, realized and implemented an electronic device for the measurement of the blood pressure called the tensiometer. This last, made up of several electronic stages in particular, the sensor pressure MPX2100APX, microcontrolor16F887A, LCD display JHD16A, band pass filter, amplifier.In addition, our work carries a calculation algorithm of the systolic and diastolic pressure.The results obtained, shows that direct (auscultator measures it) and indirect (tensiometer) gives similar results.
References
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Research on Realization of a Device to Measure the Blood Pressure