With the improvement of people's living standards, obstructive sleep apnea syndrome is more and more common in clinical practice, and its incidence is high, which seriously affects the quality of life and life span of patients. At this point, the portable ventilator is particularly important.
A portable ventilator requires the use of two pressure sensors to measure the pressure on both sides of the baffle in the ventilation pipe, accurately and quickly collect the pressure value and timely adjust the revolution of the fan to provide a comfortable air supply for snoring people, which is the most critical problem.
At the same time, the application direction of the product in people with obstructive sleep apnea syndrome, so the volume of the product is required to be small, convenient to carry on business trips, and the price is also low.
In the 16th century, the Flemish doctor Andreas Vesalius described ways to keep choking animals alive by inserting tubes into their tracheas and blowing air into their lungs. In 1555, it didn't get much traction. But today, Visari's description is believed to be the first description of a mechanical ventilator -- an important therapeutic tool in modern medicine. To understand the value of a ventilator, we need to understand how the respiratory system works.
The human body expands chest breathing by contracting the diaphragm. So air is drawn in to fill the alveoli -- the thousands of little sacs in our lungs. Each of these pellets is surrounded by a network of capillaries filled with blood. Blood takes in oxygen from the inflated alveoli and releases carbon dioxide. When the diaphragm relaxes, carbon dioxide is exhaled along with oxygen and other gases.
This process occurs automatically when our respiratory system is functioning normally, but it can be disrupted by a variety of factors. Sleep apnea, for example, causes the diaphragm muscle to stop contracting. Asthma can cause inflammation in the airways, which can block the flow of oxygen. Pneumonia is often caused by bacterial or viral infections that attack the alveoli themselves.
Foreign pathogens kill lung cells, triggering an immune response that can lead to deadly inflammation and fluid buildup in the lungs. All of these conditions can cause the lungs to fail to function properly. But a mechanical ventilator takes the place of this process, allowing oxygen into the body when the respiratory system is not working properly.
Ventilators can bypass blocked airways and deliver oxygen-rich air to help the damaged lungs diffuse more oxygen. This article mainly describes in detail the portable ventilator system.
Ⅰ Principle of pressure detection of a portable ventilator
According to different ventilation modes, the portable ventilator can be divided into three modes: CPAP continuous positive pressure ventilation, AUTO CPAP automatic positive pressure ventilator and BIPAP double airway pressure ventilator.
CPAP: In this mode, the ventilator delivers a constant flow of air through the mask to the user. This mode is suitable for patients with sleep apnea or snoring.
Auto CPAP: In this mode, the ventilator determines the patient's exhalation condition through the sensor, including breathing movement, whether breathing is suspended, and whether breathing airflow is reduced. According to the patient's breathing condition, the ventilator automatically outputs the changing pressure, so as to achieve the best therapeutic effect with the minimum pressure output.
BIPAP: This is a more comprehensive dual-airway pressure ventilator that produces higher inspiratory pressure and lower respiratory pressure, with the ventilator nearly in sync with the patient's breathing throughout the breathing process.
Ⅱ Ventilator Working Mode Selection
According to the project design requirements, the design of machine main indicators of breathing during sleep is automatic voltage regulator, namely according to patients' respiratory movement of automatic regulation of breathing machine and the output pressure value, this design USES double horizontal ventilation mode, namely BiPAP ventilation mode, breathing machine in the ventilation mode, can output high suction pressure and lower expiratory pressure for specific performance, through the sensor determine the patient's respiratory movement, when patients inhale, breathing machine output normal suction pressure, help patients breathe, when patients on the exhale, adjust the output a lower expiratory pressure, reduce patients expiratory blocked, namely in patients
During the entire breathing process, the ventilator adjustment is close to synchronous with the patient's breathing.
Patients in this ventilation mode in the whole breathing process will be relatively smooth, comfort will be very high.
Ⅲ Technical requirements of ventilator
According to the requirements of the ventilator technical indicators, combined with the requirements of this paper, the design of the technical parameters of the machine is as follows:
① Gas pressure regulation range: 400PA-2500PA.
② Gas flow requirements: when the pipeline pressure is 800Pa, the flow should be greater than 45L/min.
(3) Start and stop mode of the fan: the user exhales after wearing the mask, and the fan starts when the exhalation pressure is greater than the set expiratory trigger pressure threshold. When the mask is removed and the pipeline pressure is less than 60% of the set value for more than 30 seconds, the fan will stop running automatically.
(4) Leakage compensation parameters: when the user wears the mask improperly, or when the mask is loose, the pressure in the ventilation pipe is reduced, resulting in the actual pressure between 65% and 95% of the set pressure value for more than 30 seconds, the air leakage is considered, the fan pressure is adjusted, and the set pressure value is added 50 Pa.
⑤ Data storage: data storage should be more than 30 days.
Ⅳ Design of ventilator hardware circuit
1. Hardware composition
In this design, the main components of the ventilator are the control host, the keyboard, the display screen, the pressure sensor, the mask catheter, the alarm system, the storage system. According to the preset pressure value, the fan continues to output a certain level of positive pressure and flow of airflow, through the pipeline and mask applied to the patient's upper respiratory tract, through the positive pressure airflow to keep the patient's upper respiratory tract unobstructed.
The pipeline pressure is monitored in real-time through the pressure sensor, and the breathing condition of the patient is judged according to the monitoring data of the pressure, so as to trigger the control of the fan and realize the function of automatic pressure regulation and air leakage compensation. During the operation of the ventilator, the pressure value is sampled in real-time and the real-time dynamic curve is drawn on the display screen. At the same time, the sampled data is stored in the SD card to provide corresponding data support for the treatment of the patient.
2. LDO circuit design
In the ventilator design, the driving power of the fan is 9V, the power required by the pressure sensor and the buzzer is 5V, and the power required by the MCU controller and the peripheral circuit is 3.3V. Therefore, the design adopts the DC 5V regulated power supply to supply power to the motherboard of the ventilator. The 9V regulated power supply produces 5V and 3.3V voltages after two depressurization respectively. 9V to 5V circuit uses of the Ricoh's LDO voltage regulator chip R1191X.
3. MCU circuit design
According to the functional requirements of the design, the master controller needs to have peripheral functions, be stable, be economical, and be able to support the embedding of the operating system. In this program, we choose the high-cost performance of Renesas MCU, its model is R5F101AG. The main controller is mainly completed with the sensor drive and communication, UATA communication, data analysis and processing.
4. SM9541 pressure sensor circuit design
Airway pressure detection plays an important role in the ventilator to the user in the process of ventilation, through the real-time detection of pressure, delivered to the controller, the controller can determine the user's breathing status, and then adjust the fan speed, make the corresponding action processing. Therefore, pressure detection is very important as an eye of the ventilator control system. Normally, pressure sensors are nonlinear devices with zero and temperature drift, which will cause the signal output to exceed or fall below the calculated value according to the scale. Therefore, the sensor with good linearity and less temperature drift is an ideal choice for pressure signal detection.
In this design, the SMI company's sensor is used for real-time pressure acquisition. According to the manual of SM9541, the pressure detection circuit is designed as shown in the figure:
SM9541 series is a ceramic substrate mounted high stability piezoresistive pressure sensor chip, using ceramic substrate film resistance for zero correction, zero temperature compensation and sensitivity temperature compensation, using constant voltage source drive.
5. Fan driven
The work of the ventilator is to supply air to the patient through the rotation of the fan combined with the ventilation mask. In use, it is not only necessary to ensure the ventilation pressure and flow, but also to minimize the noise during the operation, otherwise, it will disturb the rest of the user. Therefore, it is necessary to choose a fan with the characteristics of "compact, quiet and efficient", and at the same time, it needs to meet the power supply requirements, which can reach more than 4000Pa, and the noise does not exceed 60 decibels.
This design needs to achieve automatic pressure regulation function, specific performance is when the user inhales, the fan runs normally, output the required pressure and flow when the user exhales, the fan runs at a low speed, to prevent the user exhalation difficulties. So the operation of the fan will be a dynamic operation process. The fan control functions required by this machine include forward, operation enabling and closed-loop speed control. We use the Milacore core MLX90283 chip, which is designed with mixed-signal CMOS technology. The device integrates a Hall sensor with dynamic offset cancellation, control logic and a full bridge output driver. Single-chip solution for single-coil brushless DC vibration motor.
6. Operation parameter storage
Because the design requirements need to record the user's respiratory pressure parameters during the operation of the ventilator, view the data stored in the ventilator, judge their own conditions, and provide data support for the treatment of patients. Therefore, the storage time of pressure in this design is designed to be more than 6 months, so the system needs a large storage space. The flash inside R5F101AG can only be used to store control procedures and parameters, so external memory is needed.
7. Clock circuit
RTC clock circuit is designed using EPSON RX-8010SJ clock chip. RX-8010SJ clock chip is low power consumption, digital I2C interface, with calendar and clock data RAM, access by BCD code clock/calendar chip, it can provide year, month, day, hour, minute, second and other clock calendar data. Epson's RX-8010SJ combines 32.768kHz crystal and clock IC in one package, with factory clock precision adjustment, eliminating the need for circuit matching and clock adjustment, and greatly reducing PCB circuit area due to the built-in crystal.
According to the data of the RX-8010SJ clock chip, the design of the RTC clock circuit is shown in the following figure.
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