![AN1571
Sensors
Freescale Semiconductor 3
Figure 3. CP Signal at the Output of the Pressure Sensor
Figure 4. Extracted Oscillation Signal at the Output of Amplifier
Referring to the schematic, Figure 5, the MPX5050GP
pressure sensor is connected to PORT D bit 5 and the output
of the amplifier is connected to PORT D bit 6 of the
microcontroller. This port is an input to the on-chip 8-bit
analog-to-digital (A/D) converter. The pressure sensor
provides a signal output to the microprocessor of
approximately 0.2 Vdc at 0 mm Hg to 4.7 Vdc at 375 mm Hg
of applied pressure whereas the amplifier provides a signal
from 0.005 V to 3.5 V. In order to maximize the resolution,
separate voltage references should be provided for the A/D
instead of using the 5 V supply. In this example, the input
range of the A/D converter is set at approximately 0 Vdc to 3.8
Vdc. This compresses the range of the A/D converter around
0 mm Hg to 300 mm Hg to maximize the resolution; 0 to 255
counts is the range of the A/D converter. VRH and VRL are the
reference voltage inputs to the A/D converter. The resolution
is defined by the following:
Count = [(VXdcr - VRL)/(VRH - VRL)] x 255
The count at 0 mm Hg = [(0.2 - 0)/(3.8 - 0)] x 255 ≈ 14
The count at 300 mm Hg = [(3.8 - 0)/(3.8 - 0)] x 255 ≈ 255
Therefore the resolution = 255 - 14 = 241 counts. This
translates to a system that will resolve to 1.24 mm Hg.
The voltage divider consisting of R5 and R6 is connected to
the +5 volts powering the system. The output of the pressure
sensor is ratiometric to the voltage applied to it. The pressure
sensor and the voltage divider are connected to a common
3
2.5
2
1.5
1
0.5
0
0 5 10 15 20 25 30 35 40
Time (seconds)
Vi(Volts)
Oscillation signal is extracted here
3
2.5
2
1.5
1
0.5
0
Vo(Volts)
10 15 20 25 30 35
Time (seconds)
MAP
DBP
SBP
3.5
2.5](https://image.slidesharecdn.com/freescalenibp-150520022328-lva1-app6891/85/rancang-bangun-NIIBP-3-320.jpg)
rancang bangun NIIBP
- 1. © Freescale Semiconductor, Inc., 2005. All rights reserved. AN1571 Rev 1, 05/2005 Freescale Semiconductor Application Note Digital Blood Pressure Meter by: C.S. Chua and Siew Mun Hin, Sensor Application Engineering Singapore, A/P INTRODUCTION This application note describes a Digital Blood Pressure Meter concept which uses an integrated pressure sensor, analog signal-conditioning circuitry, microcontroller hardware/software and a liquid crystal display. The sensing system reads the cuff pressure (CP) and extracts the pulses for analysis and determination of systolic and diastolic pressure. This design uses a 50 kPa integrated pressure sensor (Freescale Semiconductor, Inc.P/N: MPXV5050GP) yielding a pressure range of 0 mm Hg to 300 mm Hg. CONCEPT OF OSCILLOMETRIC METHOD This method is employed by the majority of automated non- invasive devices. A limb and its vasculature are compressed by an encircling, inflatable compression cuff. The blood pressure reading for systolic and diastolic blood pressure values are read at the parameter identification point. The simplified measurement principle of the oscillometric method is a measurement of the amplitude of pressure change in the cuff as the cuff is inflated from above the systolic pressure. The amplitude suddenly grows larger as the pulse breaks through the occlusion. This is very close to systolic pressure. As the cuff pressure is further reduced, the pulsation increase in amplitude, reaches a maximum and then diminishes rapidly. The index of diastolic pressure is taken where this rapid transition begins. Therefore, the systolic blood pressure (SBP) and diastolic blood pressure (DBP) are obtained by identifying the region where there is a rapid increase then decrease in the amplitude of the pulses respectively. Mean arterial pressure (MAP) is located at the point of maximum oscillation. HARDWARE DESCRIPTION AND OPERATION The cuff pressure is sensed by Freescale's integrated pressure X-ducer‰. The output of the sensor is split into two paths for two different purposes. One is used as the cuff pressure while the other is further processed by a circuit. Since MPXV5050GP is signal-conditioned by its internal op- amp, the cuff pressure can be directly interfaced with an analog-to-digital (A/D) converter for digitization. The other path will filter and amplify the raw CP signal to extract an amplified version of the CP oscillations, which are caused by the expansion of the subject's arm each time pressure in the arm increases during cardiac systole. The output of the sensor consists of two signals; the oscillation signal ( ≈ 1 Hz) riding on the CP signal ( ≤ 0.04 Hz). Hence, a 2-pole high pass filter is designed to block the CP signal before the amplification of the oscillation signal. If the CP signal is not properly attenuated, the baseline of the oscillation will not be constant and the amplitude of each oscillation will not have the same reference for comparison. Figure 1 shows the oscillation signal amplifier together with the filter. Figure 1. Oscillation Signal Amplifier C2 +5.0V R2 - + +DC Offset U1a 3 2 1 150k LM324N 0.33 µ Vi R3 1M 114 Vo R1 1k C1 33u
- 2. AN1571 Sensors 2 Freescale Semiconductor The filter consists of two RC networks which determine two cut-off frequencies. These two poles are carefully chosen to ensure that the oscillation signal is not distorted or lost. The two cut-off frequencies can be approximated by the following equations. Figure 2describes the frequency response of the filter. This plot does not include the gain of the amplifier. Figure 2. Filter Frequency The oscillation signal varies from person to person. In general, it varies from less than 1 mm Hg to 3 mm Hg. From the transfer function of MPXV5050GP, this will translate to a voltage output of 12 mV to 36 mV signal. Since the filter gives an attenuation of 10 dB to the 1 Hz signal, the oscillation signal becomes 3.8 mV to 11.4 mV respectively. Experiments indicate that, the amplification factor of the amplifier is chosen to be 150 so that the amplified oscillation signal is within the output limit of the amplifier (5.0 mV to 3.5 V). Figure 3 shows the output from the pressure sensor and Figure 4 illustrates the extracted oscillation signal at the output of the amplifier. 10 -10 -20 -30 -40 -50 -60 -70 -80 10 10010.10.01 Frequency (Hz) Attenuation(dB) f P1 = 1 2πR1C1 f P2 = 1 2πR3C2 0 CP Signal (0.04 Hz) Oscillation Signal (1 Hz)
- 3. AN1571 Sensors Freescale Semiconductor 3 Figure 3. CP Signal at the Output of the Pressure Sensor Figure 4. Extracted Oscillation Signal at the Output of Amplifier Referring to the schematic, Figure 5, the MPX5050GP pressure sensor is connected to PORT D bit 5 and the output of the amplifier is connected to PORT D bit 6 of the microcontroller. This port is an input to the on-chip 8-bit analog-to-digital (A/D) converter. The pressure sensor provides a signal output to the microprocessor of approximately 0.2 Vdc at 0 mm Hg to 4.7 Vdc at 375 mm Hg of applied pressure whereas the amplifier provides a signal from 0.005 V to 3.5 V. In order to maximize the resolution, separate voltage references should be provided for the A/D instead of using the 5 V supply. In this example, the input range of the A/D converter is set at approximately 0 Vdc to 3.8 Vdc. This compresses the range of the A/D converter around 0 mm Hg to 300 mm Hg to maximize the resolution; 0 to 255 counts is the range of the A/D converter. VRH and VRL are the reference voltage inputs to the A/D converter. The resolution is defined by the following: Count = [(VXdcr - VRL)/(VRH - VRL)] x 255 The count at 0 mm Hg = [(0.2 - 0)/(3.8 - 0)] x 255 ≈ 14 The count at 300 mm Hg = [(3.8 - 0)/(3.8 - 0)] x 255 ≈ 255 Therefore the resolution = 255 - 14 = 241 counts. This translates to a system that will resolve to 1.24 mm Hg. The voltage divider consisting of R5 and R6 is connected to the +5 volts powering the system. The output of the pressure sensor is ratiometric to the voltage applied to it. The pressure sensor and the voltage divider are connected to a common 3 2.5 2 1.5 1 0.5 0 0 5 10 15 20 25 30 35 40 Time (seconds) Vi(Volts) Oscillation signal is extracted here 3 2.5 2 1.5 1 0.5 0 Vo(Volts) 10 15 20 25 30 35 Time (seconds) MAP DBP SBP 3.5 2.5
- 4. AN1571 Sensors 4 Freescale Semiconductor supply; this yields a system that is ratiometric. By nature of this ratiometric system, variations in the voltage of the power supplied to the system will have no effect on the system accuracy. The liquid crystal display (LCD) is directly driven from I/O ports A, B, and C on the microcontroller. The operation of a LCD requires that the data and backplane (BP) pins must be driven by an alternating signal. This function is provided by a software routine that toggles the data and backplane at approximately a 30 Hz rate. Other than the LCD, there are two more I/O devices that are connected to the pulse length converter (PLM) of the microcontroller; a buzzer and a light emitting diode (LED). The buzzer, which connected to the PLMA, can produce two different frequencies; 122 Hz and 1.953 kHz tones. For instance when the microcontroller encounters certain error due to improper inflation of cuff, a low frequency tone is alarm. In those instance when the measurement is successful, a high frequency pulsation tone will be heard. Hence, different musical tone can be produced to differential each condition. In addition, the LED is used to indicate the presence of a heart beat during the measurement. The microcontroller section of the system requires certain support hardware to allow it to function. The MC34064P-5 provides an undervoltage sense function which is used to reset the microprocessor at system power-up. The 4 MHz crystal provides the external portion of the oscillator function for clocking the microcontroller and provides a stable base for time based functions, for instance calculation of pulse rate.
- 5. AN1571 Sensors Freescale Semiconductor 5 Figure 5. Blood Pressure Meter Schematic Drawing PC0 PC1 PC2/ECLK PC3 PC4 PC5 PC6 PC7 PD0/AN0 +5.0V +5.0V +5.0V +5.0V +5.0V +5.0V +5.0V +5.0V PressureSensor MPXV5050GP Vout GND 2 3 Vs 9.0VBattery C5 1 1 2 GND 0.33u InputOutput 5.0VRegulator MC78L05ACP Buzzer 0.33u C2 3 1 3 2 R4 R0 10k 24k R3 1M 11 LM324N 4 R1 C1 1k 33u 150k R2 C7 LED R8 100R 100n 100u C8 C6 330u R10 10M X1 4MHz C3 C4 22p 22p PD1/AN1 PD2/AN2 PD3/AN3 PD4/AN4 PD5/AN5 PD6/AN6 PD7/AN7 PLMB PLMA SCLK TDO TCMP2 TCMP1 VDD OSC2 MC68HC05B16CFN OSC1 /RESET /IRQ TCAP1 TCAP2 RD VRH VRL PA0 PA1 PA2 PA3 PA4 PA5 PA6 PA7 PB0 PB1 PB2 PB3 PB4 PB5 PB6 PB7 +5.0V MC34064 ResetInput GND LCD5657 1 17 10 2 1 52 51 20 21 49 48 47 46 45 44 43 42 14 13 12 11 9 5 4 332 33 34 35 36 37 38 39 24 25 26 27 28 29 30 31 7 8 50 23 22 19 18 16 16 23 22 21 20 19 18 17 12 27 26 25 24 15 14 13 CB AG1D DPEF 2 DPLL 3 DP 4 DP1 G1 F1 A1 B1 C1 D1 E1 DP2 G2 F2 A2 B2 C2 D2 E2 37 36 35 34 7 6 5 28 40 1 8 32 31 30 29 11 10 9 G4 F4 A4 B4 C4 D4 E4 L BP BP DP3 G3 F3 A3 B3 C3 D3 E3 3 2 R9 4.7k R6 15k R5 4.7k + 36R
- 6. AN1571 Sensors 6 Freescale Semiconductor SOFTWARE DESCRIPTION Upon system power-up, the user needs to manually pump the cuff pressure to approximately 160 mm Hg or 30 mm Hg above the previous SBP. During the pumping of the inflation bulb, the microcontroller ignores the signal at the output of the amplifier. When the subroutine TAKE senses a decrease in CP for a continuous duration of more than 0.75 seconds, the microcontroller will then assume that the user is no longer pumping the bulb and starts to analyze the oscillation signal. Figure 6 shows zoom-in view of a pulse. Figure 6. Zoom-In View of a Pulse First of all, the threshold level of a valid pulse is set to be 1.75 V to eliminate noise or spike. As soon as the amplitude of a pulse is identified, the microcontroller will ignore the signal for 450 ms to prevent any false identification due to the presence of premature pulse "overshoot" due to oscillation. Hence, this algorithm can only detect pulse rate which is less than 133 beats per minute. Next, the amplitudes of all the pulses detected are stored in the RAM for further analysis. If the microcontroller senses a non-typical oscillation envelope shape, an error message (“Err”) is output to the LCD. The user will have to exhaust all the pressure in the cuff before re- pumping the CP to the next higher value. The algorithm ensures that the user exhausts all the air present in the cuff before allowing any re-pumping. Otherwise, the venous blood trapped in the distal arm may affect the next measurement. Therefore, the user has to reduce the pressure in the cuff as soon as possible in order for the arm to recover. Figure 7 on the following page is a flowchart for the program that controls the system. SELECTION OF MICROCONTROLLER Although the microcontroller used in this project is MC68HC05B16, a smaller ROM version microcontroller can also be used. The list below shows the requirement of microcontroller for this blood pressure meter design in this project. • On-chip ROM space: 2 kilobytes • On-chip RAM space: 150 bytes • 2-channel A/D converter (min.) • 16-bit free running counter timer • LCD driver • On-chip EEPROM space: 32 bytes • Power saving Stop and Wait modes CONCLUSION This circuit design concept may be used to evaluate Freescale pressure sensors used in the digital blood pressure meter. This basic circuit may be easily modified to provide suitable output signal level. The software may also be easily modified to provide better analysis of the SBP and DBP of a person. REFERENCES Lucas, Bill (1991). “An Evaluation System for Direct Interface of the MPX5100 Pressure Sensor with a Microprocessor,” Freescale Application Note AN1305. -7.1-7.3-7.5-7.7-7.9-8.1-8.3-8.5 Time (second) VO(volt) Premature Pulse 1.75 450 ms
- 7. AN1571 Sensors Freescale Semiconductor 7 Figure 7. Main Program Flowchart N Main Program Initialization Clear I/O ports Display "CAL" and output a musical tone Clear all the variables Take in the amplitude of all the oscillation signal when the user has stop pumping Calculate the SBP and DBP and also the pulse rate Repump? Is there any error in the calculation or the amplitude envelope detected? Output a high frequency musical tone Exhaust cuff before repump Exhaust cuff before repump Display pulse rate. Display "SYS" follow by SBP. Display "dlA" follow by DBP. Display "Err" Output a low frequency alarm Y N Y N N Y Y
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