Assisted breathing machine
(Translated with www.DeepL.com/Translator)
Daniele Tommei - Digitarch Rome Italy
Rome April - 8 - 2020
The aim was to create a device of simple and economic production, with basic characteristics and without ambitions of universal adaptability, but which, although only in the simplest cases, can be useful. This machine, born to help human beings with breathing difficulties, has been designed to be easily built with low cost and easy to find components (the cost of all the necessary parts just over 100 euros). It is necessary to specify, however, that this machine is not a medical device and has not been certified for any of the features listed below.
However, despite the limitations set out above, given the practicality and simplicity of its operation, we have made this document public so that those who wish to do so can make it so that they can keep a device at home that they obviously hope will never have to use.
In this document and its annexes, all the information necessary for the construction of the fan will be included, namely: the list of components and their availability, the construction diagrams (mechanical, electrical and electronic) and the source code of the software for programming the microprocessor used.
The operating principle of this machine is based on the intermittent generation of ventilated air to be inhaled through a nose/mouth mask worn by the patient.
Short video on stand-alone operation
Short movie about the operation in inspiration-controlled mode (triggered)
Composition of the device for assisted ventilation:
The main parts of this device are:
1) a high-pressure centrifugal fan
2) a differential pressure sensor
3. a microprocessor electronic card
4) a parameter settings panel
5) a container for general applications
6) a power supply adapter
7) a flexible hose with nose/mouth mask
Description of respiratory ventilator operation
A physiological human breathing cycle consists of these three phases
- inhalation (approx. 1.3-1.5 seconds duration)
- exhalation (approx. 2.5-3 seconds duration)
- short pause between exhalation and inhalation (approx. 0.5 seconds)
The natural action of inhalation occurs through the enlargement of the chest cavity obtained through the action of the different inspiratory muscles.
In case the patient has difficulty in activating the inspiratory muscles to enlarge the costal arch and thus expand the volume of the lungs to inhale the air, the respiratory ventilator can help by directly introducing the air into the lungs, determining the expansion and thus lightening the load on the inspiratory muscles.
This intake of air must be synchronized with the first spontaneous muscle movement of inspiration by the patient.
Specifically, the mechanism of operation is as follows:
As soon as the patient tries to breath in, a depression, albeit minimal, is generated in the lungs, which is detected by the pressure sensor in the mask. The signal detected by the sensor is transmitted to the ventilation device which immediately generates the pressurized air that is injected into the lungs. Once the air supply phase is complete, the ventilator stops and the patient can automatically exhale air from the lungs in the following seconds. In this second phase it is the lungs themselves which, without any muscular aid by virtue only of their elasticity, expel the air contained in them.
At the end of the exhalation phase, the natural breathing cycle starts again and with it also the action of the ventilator. With this system, the air is introduced into the lungs synchronously following the patient's natural breathing cycle.
The ventilator will work even if the patient does not have the ability to begin the inhalation phase independently. In fact, the ventilator is equipped with a timer that will make it operate in any case, i.e. even if no inhalation attempt signal is detected. In this hypothesis, the ventilator will still supply air into the lungs after the period set in the timer has elapsed.
This concerns the operation of the fan with detection mode activated. However, it is possible to set the fan to autonomous mode.
In this mode, the ventilator will introduce air into the lungs at regular intervals, regardless of whether or not the sensor detects signs that breathing has begun. Of course, this mode will be used in extreme cases where the patient is not in a condition to stimulate assisted breathing.
Control panel: settings and indicators
On the front control panel there are four knobs, a switch, and four LED indicator lights.
- The red knob (Inspiration) allows you to set the inspiration time. The adjustable value ranges from 1 to 2 seconds and determines how long the ventilator will generate the continuous flow of pressurized air to be sent to the patient's mask.
- The red led above this knob indicates that the fan is in action.
- The white knob (Expiration Fixed) determines the duration of the expiration period. The value can be set from 1 to 4 seconds and during this period the ventilator will not produce airflow allowing the patient to exhale the air contained in the lungs.
- The green led above this knob indicates that the fan is stopped and we are in the air exhalation phase.
- The orange-colored knob (Expiration Triggered) determines the maximum waiting period during which the ventilator is still standing waiting for a patient's inspiration. As soon as the ventilator detects an inhalation stimulus (red trigger LED) within this period, ventilation starts immediately. If no inhalation stimulus occurs within the set period (can be set up to 4 seconds) the ventilator will still start and a beep will sound to warn the caregivers that the patient has not been able to inhale on his own.
- If this orange knob is set to zero (off) the ventilator will not wait for the inspiratory stimulus but after the previous expiration phase (the green LED) the ventilator will start immediately.
- - The yellow led above this knob indicates that the fan is waiting for an inspiratory stimulus to activate the fan and generate pressurized air.
- The red Trigger LED sends a light signal when negative air pressure is detected (patient attempt to inhale) during the "Triggered" Expiration period set via the orange knob. If this knob is set to "off" the Trigger led will not be activated.
- The fourth knob, the blue knob (Pressure), determines the maximum air pressure exiting the ventilator. The pressure value can be set from 5 to 35 cm. of H2O (from 0.5 to 3.5 KPa).
- The top right 0/1 switch controls the power on of the device.
Description of parts and operation diagram
This assisted breathing device consists of these four main functional parts:
the fan, the pressure sensor, the microcomputer, the control panel.
The first of these is a high-pressure centrifugal fan.
This is driven by a DC electric motor. The function of this fan is to suck air from outside, pressurize it and send it through a hose to the mask worn by the patient. The fan is called a high pressure fan because it produces significantly more "pressurization" of the air than a normal fan.
The high pressure, although not that high, must be at least about 20 cm. of H2O to ensure that it is sufficient to "inflate the lungs".
To obtain this characteristic, this type of fans have an impeller that rotates at a very high speed, normally higher than 10,000 rpm and for this reason they must be well constructed and balanced to minimize vibrations and noise produced.
In this project, at the moment, the low cost has been privileged and therefore a very cheap fan (less than 10 Euro) easily available because used in a common 12V inflator for inflatable boats has been used. This fan, in the test that we have carried out, comes to produce a pressure (with zero flow) of 35 cmH2O. To obtain this pressure, however, you need a 12V. power supply that can support at least 4A. continuous (the one supplied does not exceed 2A.).
The impeller of this fan can rotate at different speeds depending on the voltage applied to its motor. Since the pressure of the air flow generated by the fan is a function of the rotation speed, adjusting the electrical voltage on the motor will regulate the air pressure generated.
The second functional part, of this breathing machine, is the pressure sensor.
Inside the air transport tube, which goes from the fan to the mask, a second thinner tube has been inserted to measure the pressure of the air sent. This small tube runs along the entire length of the larger tube where the air passes, starting from the fan and ending up at the plug on the mask.
While on the mask side this small tube is left open and not connected to anything, on the fan side it is connected to a differential pressure sensor.
The other input, of the two inputs of the pressure sensor, is left open inside the box and not connected to anything (actually it is not true that it is not connected to anything: it has been left free just to measure the pressure inside the box).
This differential sensor has the task of measuring the pressure difference between its two inputs and transforming it into a signal that will indicate to the microcomputer what is the difference in the air pressure value between the inside of the box (which is actually the same as the ambient pressure) and the air inlet in the mask.
It was considered appropriate to extend the measuring tube up to the mask to improve the accuracy of the air pressure measurement.
If in stationary air conditions the pressure is always the same at every point of the tube, with the air in motion there will be a pressure difference due to the pressure drop between the beginning of the tube, connected to the fan, and the end of the tube connected to the mask. In addition, the proximity of the measurement point to the patient's mouth/nose improves the accuracy and speed of the ventilator's response to the patient's inhalation attempt.
The differential pressure sensor has two functions:
- The first function is to measure the positive pressure of the pressurized air generated by the fan during the patient's inhalation phase, which must remain within the limits set on the control panel. In this phase the microcomputer will adjust the speed of the fan impeller to not exceed these limits.
- the second function is to detect the first inhalation stimulus immediately after the exhalation phase. In a certain period of time, which can be set from the control panel, the sensor will detect if there is a rapid drop in pressure, following the patient's inhalation stimulus, and the microcomputer will then immediately activate the generation of pressurized air to help the patient to inhale.
The sensor used is the MPX5010DP model available from RS-Components or other distributors of electronic components, and has a flow rate up to 10KPa, i.e. 100 cm. of H20, more than enough for the pressures in play that should never exceed 3.5KPa.
The third functional part is the microcomputer.
A Pic16F series micro from Microchip was used for this device. The model chosen is the 16f1718 in DIP version, which already has inside it all the functional modules needed for this assisted breathing device such as CLOCK, ROM, RAM, ADC, PWM, etc..
The microcomputer can be programmed from any PC through a special interface such as PICKIT3 or PICKIT4. The development system is MPLABX and can be downloaded free of charge from the Microchip website. The program has been written directly in machine code (MASM) and is documented among the attachments.
The microprocessor is mounted on a prototype circuit board and does not currently have a dedicated PCB. On the board together with the micro, all other electronic components necessary for operation are assembled, as shown in the attached wiring diagram. For the 12V. power supply of the circuit, the same mains adapter supplied with the inflator has been used, but of course it is advisable to purchase a more robust one such as the one indicated in the list of the main parts needed. It would also be advisable to connect a 12V. buffer battery of at least 10Ah, or alternatively use an uninterruptible power supply for computer to ensure operation even in the event of a power failure.
The fourth and last functional part is the control panel.
The panel simply consists of 4 knobs and 4 LEDs, in addition to the power switch and a buzzer inside the box.
At the moment we have decided to keep to a minimum all the possible options and variables that can be used to adapt the device to more particular cases, favouring simplicity and easy management even by a home healthcare service.
The 4 knobs regulate 4 different potentiometers connected to the microcomputer and set the necessary parameters for the desired operation. The 4 leds, and the acoustic signaller, also connected to the microcomputer, signal the different states of operation to help and monitor the correct adjustment of the parameters.
List of main parts
High pressure inflator, complete with 12V 2A adapter
A plastic container (180x110x90)
- Microprocessor Microchip Pic 16F1718 (DIL 28 pin)
pcb for prototypes
- Pressure differential sensor NXP mod. MPX5010DP
- Potentiometers and knobs
- Programmer for Microchip microprocessor
- Power supply 12 V. 7.5A
- Washing machine extension hose
To separate the fan from the inflator, simply remove the two screws on the rear cover. Then we desolder the electrical connections. From the inflator we also recover the switch and the power connector.
The box we used is very small but still allowed to contain everything you need. Of course it is possible to use larger ones.
After drilling the box both at the bottom and at the side with a 31 mm diameter cup drill, we shape and drill a rectangle of 6-8 mm thick pvc forex to fix it at the bottom of the box.
With the help of the attached template we can drill the holes on the front for the potentiometers, the LEDs and the on/off switch
At this point we can try to insert the fan and adapt the slots if necessary
We assemble the electronic circuit adapting the size of the multi-hole PCB and solder the potentiometers on the bottom side and the LEDs and the rest of the components on the top side
The circuit shown is the prototype and therefore the connections have been made with wires
The board will be fixed on the front side of the box by the same potentiometers.
On the back edge, the card has been fixed directly to the fan through one of the holes already present on the fan.
On the rear edge of the box the power connector has been inserted, while on the front side the On/Off switch has been inserted (both taken from the inflator).
A 6 mm hole has been drilled in the outlet of the fan, into which a tube of a few centimetres is inserted as shown. This is the pressure sensor that will be brought up to the mask. On the inside of the box the small tube, through a 90° connection, will be connected to the pressure sensor on the side indicated, corresponding to pin 6 (the other inlet must be left open). On the external side of the box, a metal tube is inserted into the control line, which will be the connection with the rest of the control line that will go to the mask.
The small tube, passing inside the main 30 mm air tube, will come close to the mask worn by the patient. In this way, the measured pressure will be exactly the pressure that is inhaled by the patient. In addition, the time required to detect the pressure drop induced by the inhalation attempt and thus the subsequent start of the ventilator is reduced to a minimum.
The air hose connected to the respirator is connected to the fan using a piece of common PVC hydraulic hose and heated to widen the internal diameter to about 30.5 mm.
At the other end of the pipe, the connection with the template is obtained by means of a fitting.
A second inlet can also be seen on the mask to which the oxygen tube to the cylinder or to an oxygen concentration machine can optionally be applied if necessary.
Technical notes on future features and improvements
The heart of this device is the pressurized air generation system. A small fan with DC brush motor has been used in this machine to perform this function. This choice was dictated mainly by the easy availability and low cost, but of course at the expense of the features. The critical points of this type of fan are the noise, the not very high pressure and the inertia in reaching the necessary rotation speed, which results in a delay in the generation of pressurized air from the moment the demand is activated.
We have tried different types of commercial fans and are testing others. The best choice will be to use a fan with a small impeller, but with very high speeds above 30,000 rpm. Fans of this type use a brushless motor. Unfortunately these fans, besides being more difficult to find, are more expensive and require additional circuit complexity for their operation. The model that could be a perfect replacement for the current fan is the WM7040 which can reach a pressure of more than 60 cmH2O. This model, equipped with its driver board, can be easily inserted into the current circuit, using the PWM signal that currently arrives at the gate of the power mosfet IRLZ44 as a speed control.
The programming software for the microprocessor has been developed on the MPLABX platform of the Microchip (downloadable free of charge from their website) using a personal computer with Windows. The control program for the uP was written directly in the microchip's native code. For the programming then a five-pole cable is used that directly accesses the micro via a connector on the board. We used the PICKIT4 programmer but any other programming system compatible with this uP family can be used. All source code is included in the attachments of this project.
The code is not very complex and can be easily modified to suit different needs.
Below is the flowchart that summarizes the control mechanism of the uP. on the interconnected hardware.
Description of electrical operation
The circuit is based on an in-line programmable microchip uP with internal clock set at 16 Mhz. The control circuit consists of 4 optical signalling leds and 4 potentiometers that provide 4 values from 0 to 5V. on the inputs of the uP and a piezo acoustic signaller.
The power supply is predisposed for a voltage of 12V. for an absorption of about 4 amperes of current. The voltage of the logic part is 5V. and is generated by a linear stabilizer L7805.
The fan speed is controlled by the IRLZ44 N channel mosfet which is directly driven on the gate by an output of the uP 16F1718. The current of this output is sufficient to guarantee a switching time of less than 2 uS, short enough to use a PWM signal with a cycle of 250 uS (4 Khz.)
IRLZ44 Mosfet Gate signal
In parallel to the motor a fast BYV34 diode is used as "freewheeling" for current recovery. Also in parallel to the motor a P channel mosfet has been used to short circuit and brake the motor itself, reducing the deceleration phase time when the fan must be stopped quickly.
The MPX5010 differential pressure sensor produces a ratiometric voltage ranging from 0 to the supply voltage (5V.) depending on the pressure difference between the two pneumatic inputs. With an equal pressure on the two inputs the output voltage is about 0.5V while with a difference of 10KPa (100 cmH2O) the voltage is about 4.5V It should be noted that the sensor can detect even a slight negative pressure bringing the 0.5V output up to 0V.
The sensor is connected through a small tube to the fan outlet in order to measure the pressure in reference to the ambient pressure, detected by the second sensor input left free.
Pressure value on the sensor output
In the image above we can identify the air intake phase (steep climb) and the exhalation phase (asymptotic descent) followed finally by the negative pressure trigger phase.
All attached documentation (in English) can be found here:
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