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School of Engineering and Mathematical Sciences. Development of a Compact Patient Ventilator Based on Novel Compressor Technology
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1 School of Engineering and Mathematical Sciences Development of a Compact Patient Ventilator Based on Novel Compressor Technology by Mario G. Bejarano M. Project Report for the Degree of MSc in Clinical Engineering with Healthcare Technology Management Supervisor: Dr. Justin Philips Co-supervisor: Dr. Keith Pullen London 14 th of September 2012
2 Table of Contents Abstract Introduction Theory Medical Background Treatment of OSA with Non-invasive ventilation BiPAP, CPAP and APAP devices architecture Market Devices State of the art in CPAP and APAP ResMed S9 TM series Philips System One Transcend Sleep Apnea Therapy System TM Common problems using compact ventilators Problem Identification Methodologies and Techniques Raspberry Pi Python language Motor controller Sensors and ADC Compressor Work done Motor controller design Temperature sensor Pressure sensor Analogue to digital converter ADC Range of operation with the sensor Interfacing sensor, motor control and RasPi I 2 C protocol communication for the temperature sensor SPI communication protocol PWM pin for motor control Algorithm implementation II
3 CPAP Algorithm APAP Algorithm BiPAP Algorithm User interface design with Python Results Relation speed of the motor pressure in close loop Calibration curve of the device Pressure sensor error margin between datasheet and calibration line Range of operation of the device Air flow range Maximum pressure Compressor noise levels Running temperature Power supplies Discussion of results Testing the compact ventilator Raspberry Pi as a tool for projects in clinical engineering Conclusion Suggestions for further work References Appendix A ResMed S9 Series Specifications... A-1 Appendix B CPAP market comparison... B-1 Appendix C APAP market comparison... C-1 Appendix D Schematic design of the compact ventilator... D-1 Appendix E CPAP Algorithm flowchart... E-1 Appendix F APAP Algorithm flowchart... F-1 Appendix G BiPAP Algorithm flowchart... G-1 III
4 Table of Figures Figure 1 CPAP machine with patient interface (16)... 7 Figure 2 Components of a CPAP APAP device... 8 Figure 3 ResMes S9 Series with Humidifier Figure 4 Philips System One with Humidifier Figure 5 Transcend II CPAP Figure 6 Raspberry Pi Model B Figure 7 Raspberry Pi Communication Port schema (32) Figure 8 Half H-Bridge drive configuration for a DC Brush Motor Figure 9 Modified portable vacuum used in the project as compressor Figure 10 MOSFET IRF9Z34N symbol and packing Figure 11 MOSFET STP16NF06FP symbol and packing Figure 12 Half H-Bridge design and components Figure 13 Schematic of the half H-Bridge with circuit protection Figure 14 Pin configuration of temperature sensor TC Figure 15 Schematic temperature sensor Figure 16 Pressure sensor 40PC001B and pin configuration Figure 17 Graph expected linearity pressure sensor Figure 18 MCP3008 Pin configuration Figure 19 Schematic of the pressure sensor - ADC interface Figure 20 Relation between PWM register and output in Volts Figure 21 CPAP mode window Figure 22 APAP mode window Figure 23 BiPAP Mode window Figure 24 Main menu options available Figure 25 Temperature reading mode Figure 26 Pressure reading mode Figure 27 Configuration window Figure 28 Pressure meter RIGEL BP- SiM Figure 29 Relation PWM register versus converted values from pressure sensor Figure 30 Calibration line using external reference Figure 31 Graph real calibration versus expected calibration IV
5 Figure 32 Flow meter Certifier FA Plus Figure 33 Flow graph versus speed of the motor Figure 34 Thermometer Kane-May KM Figure 35 Relation flow versus pressure Figure 36 Setting for testing the compact ventilator's accuracy Figure 37 Error comparison in CPAP mode Figure 38 Figure Error comparison in APAP mode V
6 Table of tables Table 1 American APAP vs CPAP vs BiPAP consumer preference (17).. 10 Table 2 Ideal operation of a half H-Bridge Table 3 Electrical characteristics of the motor Table 4 MOSFET IRF9Z34N features Table 5 MOSFET STP16NF06FP features Table 6 Features temperature sensor TC74A Table 7 Performance characteristics of the pressure sensor 40PC001B.. 30 Table 8 Main features ADC MCP Table 9 Operation range oh the ADC with the pressure sensor Table 10 Equivalent of temperature in HEX values Table 11 SPI Pin assignment on RasPi and ADC Table 12 Correlation calibration line versus pressure sensor expected values Table 13 Test results in CPAP mode Table 14 Test results in APAP mode VI
7 Symbols and abbreviation C: Degree Celsius µa: Micro Ampers A: Amperes ACK: Acknowledge ADC: Analogue to Digital Converter AHI: Apnea/hypopnea index APAP: Automatic Positive Airway Pressure ARM: Advanced RISC Machine BiPAP: Bilevel Positive Airway Pressure BJT: Bipolar Junction Transistor cmh 2 O: Centimetres of Water CO 2 : Carbon dioxide COPD: Chronic Obstructive Pulmonary Disease CPAP: Continuous Positive Airway Pressure CPU: Central Process Unit CS: Chip Select VII
8 CSA: Central Sleep Apnoea CSV: Coma-separated values db: Decibels DC: Direct Current D IN : Slale Serial Data In D OUT : Slave Serial Data Out EPR: Expiratory Pressure Relief GND: Ground GPIO: General Purpose Input/Output GPU: Graphics Processing Unit HDMI: High Definition Multimedia Interfac HEX: Hexadecimal I 2 C: Inter Integrated Circuit IC: Integrated Circuit L/min: Litres per minute LCD: Liquid crystal display VIII
9 LSB: Least Significant Bit ma: mili Amperes MHz: Megahertz MISO: Master Input MMC: Multi Media Card mmhg: Millimetres of Mercury MOSFET: Metal oxide semiconductor Field-effect transistor MOSI: Master Output ms: mili seconds MSB: Most Significant Bit NIV: Non-Invasive Ventilation OS: Operating System OSA: Obstructive Sleeping Apnoea pf: Picofarad PWM: Pulse Width Modulation PWV: Pharyngeal Wall Vibration IX
10 RasPi: Raspberry Pi RCA: Radio Corporation America REM: Rapid Eye Movement RJ: Registered Jack SCL: Serial Clock SCLK: Serial Clock SD: Secure Digita SDA: Serial Data Line SDB: Sleep Disordered Breathing SDIO: Secure Digital Input Output SDRAM: Synchronous Dynamic Random-Access Memory SoC: System on Chip SPI: Serial Peripheral Interface Bus Sps: Smaples per second t e : Expiratory Time t i : Inspiratory Time X
11 UA: Upper Airway UART: Universal Asynchronous Receiver/Transmitter USB: Universal Serial Bus V: Volt(s) XI
12 Project Title: Development of a Compact Patient Ventilator Based on Novel Compressor Technology Student: Mario G. Bejarano M. Supervisor: Dr. Justin Philips Co-supervisor: Dr. Keith Pullen 14 th of September 2012 Abstract There are three different types of compact ventilators for hospital and home users for the treatment of sleep disordered breathing (SDB). These devices are called continuous positive airway pressure (CPAP), automatic positive airway pressure (APAP) and bilevel positive airway pressure (BiPAP). These three types of machines shared common characteristics but in the market there is not a device capable to run the three modes in one machine with portable capabilities. This project endeavours to explore the design and development of a compact ventilator capable of running the three modes in one sole device using a novel compressor and the new Raspberry Pi (RasPI). For this an in depth analysis of the medical background and the current market was performed and some problems are identified for improvement. As a result of this analysis, a prototype for a compressor was designed using the RasPi as the heart of the project and including interfaces to control an external compressor and data acquisition attached to it. The project endeavoured to use a novel compressor but at the moment a scale model is still under development. Therefore, a portable vacuum cleaner was used as a compressor obtaining interesting results which show that this device can run with an accuracy of up to 0.5% for APAP mode. The data collected from the finalised project as a whole gives an interesting platform from which to develop the novel compressor under the suggestions given in section 8. 1
13 1. Introduction There are several illnesses related to sleeping disorders. These diseases are classified as sleep disordered breathing (SDB) (1). If a disorder in sleep time breathing is caused by upper airway (UA) collapse, it is called obstructive sleep apnoea (OSA), but if caused by a lack of neural input from the central nervous system to the diaphragm it is called central sleep apnoea (CSA) (2). There are statistics which show that this illness may affect 4% of males and 2% of females between the ages of 30 to 60 years world-wide (1; 2). Most of the patients must be treated with non-invasive ventilation (NIV) during their sleep time. The length of the treatment will change according to the patient s pathology but it is a long term treatment. The most common medical device used for this purpose is a CPAP device (3). It has been shown that patients do not always require the same level of pressure every night time. As a result, APAP machines have been developed which adjust the pressure value according to the obstruction gravity using intelligent algorithms. Finally, there are some patients who require two different levels of ventilation during the inspiratory-expiratory process, for these cases a BiPAP device is used (4; 5; 6). Modern ventilator devices are designed for home use but there are some common problems that the patients face every night. Including the lack of portability, and the unconformity when using the device. In addition, there is not a machine capable of offering the three types of ventilation in one device, when patients may benefit from such a device and sometimes switch therapies in their treatments (7). The main objective of this project is the design of a compact ventilator capable of running the three different modes of ventilation CPAP, APAP and BiPAP for the treatment of patients with OSA. The compact ventilator should be able to run an external compressor and read the pressure produced from the positive air flow being given to the patient. This design is intended to be used in the future with a novel compressor technology created and designed by Dr. Keith Pullen. This novel compressor has been designed initially for the automotive industry as part of a turbo 2
14 charger engine. However, a new design could be done making the compressor smaller with the characteristics required to operate as a medical device as NIV that could fill the gap available in the market for portable ventilation devices capable to run the three modes in one device. The secondary objective of this project is to evaluate the use of the RasPi as an academic tool for the application in the clinical engineering field. For this, a group of interfaces that connect sensors and an external motor to the RasPi have been created. All this hardware together will be the base to integrate the future compressor designed by Dr. Keith Pullen. The results obtained using the RasPi were quite impressive, considering the cost-benefit relationship of the device. During the development process of this device, an electronic motor control to operate a common brushed 12 volts (V) direct current (DC) motor was designed as well as the electronics required to acquire data from temperature and pressure sensors. In addition, from the software point of view, it was required to develop all the algorithms for the adjustment of the speed of the motor to deliver positive pressure according to the type of treatment. All the algorithms were implemented with programing language Python which is run by the RasPi. Once completed, the device was tested in the workshop of the Medical Equipment Management at King s College Hospital where data was collected to help find the characteristics of the compact ventilator such as range of pressure, air flow, noise levels, temperature and efficiency. With all this collected data, a recommendation for the future design and implementation of Dr. Pullen s compressor is done at the end of the report. 3
15 2. Theory 2.1. Medical Background OSA is mainly characterized by persistent episodes of obstruction of the upper airways during sleep due to pharyngeal wall collapse. When this obstruction occurs the air exchange is reduced completely or partially and most of the time is followed by pharyngeal wall vibration (loud snoring - PWV) (8; 3; 9). This obstruction has physiological effects on the human body. Some of the common symptoms associated with OSA are PWV, daytime drowsiness, fatigue, heart problems and systemic hypertension (2). Abnormal breathing during sleeping can also contribute to the development of hypercapnia 1 when one is awake (3). There are other significant risks associated with OSA such as hypertension, congestive heart failure, coronary artery disease, stroke and arrhythmias (10; 4). In the case of CSA, the apnea is originated mainly by neuronal causes during the rapid eye movement (REM) sleep cycle. REM is associated with sleep alterations especially inhibition of alpha and gamma motor neurons (3). This means that during this cycle the rib cage muscles, intercostal muscles, postural muscles and respiratory muscles reduce their contribution to ventilation. Hence, causing hypotonia or diminished muscle tone. As a consequence, the respiration during REM sleep cycle becomes highly dependent on the diaphragm s effort and not on the action of the rib cage and intercostal muscles (3). In addition, the lack of activity of the ventilation muscles means that they cannot maintain the end-expiratory pressure to prevent small airway closure. As a result, the ventilation-perfusion relationship is reduced, worsening gas exchange in the patient and efficiency of breathing. Hypoventilation is the common term to describe the lack of gas exchange caused either by obstruction of the upper airways or by hypotonia of the ventilation muscles. This in turn causes an increase of carbon dioxide (CO 2 ) 1 Condition indicating that there is too much carbon dioxide (CO 2 ) in the blood. 4
16 (hypercapnia) in the body during REM sleep. This level of CO 2 could be detected and monitored continuously during the evening using a pulse oximeter or transcutaneous methods (3). The development of technology has helped to better our understanding of breathing during sleep. The following devices have contributed to the development of the research of sleeping disorders. The use of the pulse oximeter has helped to continuously monitor the level of oxygen during the sleeping cycle. The development of a nasal mask interface that is comfortable and acceptable has been an effective, non-invasive way to treat breathing abnormalities. Lastly, yet another example of devices contributing to the research of sleeping disorders has been the development of portable ventilation devices that can even be used at home (3). Some patients who are candidates to use non-invasive ventilation (NIV) technologies for the treatment of their illnesses include patients with any of the following: Kyphoscoliosis: musculoskeletal disorder caused by abnormal curvature of the spine which causes chronic under ventilation due to low chest compliance. Cystic fibrosis: Autosomal recessive genetic disorder compromising mainly the lungs but also in some patients can affect the pancreas, liver and intestine (11). It has been recognized that the use of nocturnal ventilation shows beneficial effects in these kinds of patients. It must be pointed out that low CO 2 retention is a consequence of this disease because of the chronic hyperventilation that the patient suffers. As a consequence, if oxygen is also applied during therapy it could promote CO 2 retention which is beneficial in these patients. (3; 12). Duchene muscular dystrophy: It is a genetic illness which could lead to muscle degeneration, difficulty in breathing and walking. It has been shown that the use of long-term non-invasive ventilation could help to stabilise pulmonary function. Hence, prolonging life expectancy of the patient (13). 5
17 Chronic obstructive pulmonary disease: COPD is the limitation of the flow of air from/to the lungs by narrowing of the bronchi and bronchioles. This disease can affect the respiratory airways, the lungs, parenchym 2 and the pulmonary circulation (14). The use of noninvasive nocturnal ventilation has shown beneficial results mainly in patients with severe hypercapnic COPD by blowing off the extra CO2 and optimising gas exchange (3; 15). Motor neurone disease: is a neurological disorder that affects the motor neurones. The use of NIV is especially useful when respiratory insufficiency occurs in the late manifestation of this disorder because respiratory muscles and global peripheral weakness decrease the effectiveness of the patients breathing which is then improved with the NIV therapy (3). Obesity hypoventilation syndrome: This condition presents in overweight people. People with this condition usually fail to breathe deeply or quickly enough to maintain a healthy level of oxygen in the blood. The use of NIV has been shown to give a rapid and positive response in this group of patients (3) Treatment of OSA with Non-invasive ventilation The treatment of OSA with NIV therapy is a ventilation technique whereby positive pressure is applied to the lungs through the upper airways, without the need for an endotracheal or tracheostomy tube. The aim of this therapy is to improve the gas exchange by supporting the inspiratory effort and reducing the labour of breathing (3; 14). This device is manly used to manage acute or chronic upper airways obstructive disorders especially when the patient is sleeping. The use of this type of device during sleep prevents hypoventilation and may help to restore sensitivity to CO 2 and hence increase the drive to breath (3; 14). There are two different types of NIV systems: volume-preset and pressurepreset (3). The volume-preset ventilators deliver a fixed tidal volume 2 It refers to specializes organ tissue in the respiratory system 6
18 regardless of the airway pressure generated using a time-cycled flow generator. In contrast, the pressure-preset systems work delivering continuous positive pressure to the patient to the pre-set value. There are three types of these devices: the CPAP, APAP and BiPAP devices. The BiPAP devices deliver positive pressure at two different levels. Tidal volume will vary according to the set inspiratory pressure, the difference between inspiratory-expiratory pressure and chest wall and lung compliance of the patient (3). The CPAP/APAP delivers compressed air continuously to the patient s pharyngeal airway at a preset or sense pressured value. This value is set with the aim to act as a pneumatic splint which opposes airway collapse (2; 10; 5). The most common interface used between the device and the patient is a corrugated PVC tube and a specially designed mask (See Figure 1). This is why this method of ventilation is also known as mask ventilation (3). Most of the problems with ventilation are related to the mask, due to leaks or patient discomfort and hence non-compliance with the treatment. If the mask does not fit properly and leaks occur around the mask, this will reduce the effectiveness of the treatment (10). Figure 1 CPAP machine with patient interface (16) 7
19 2.3. BiPAP, CPAP and APAP devices architecture Nowadays, BiPAP, CPAP and APAP are the most common treatments of OSA and hypopnea. It has been demonstrated that this therapy relieves daytime sleepiness, improves driving performance, mood and the patient s overall quality of life (9; 10; 4). BiPAP machines deliver positive pressure at two different levels, inspiratory and expiratory. As a result, tidal volume varies according to the set inspiratory pressure, the difference between inspiratory-expiratory pressure and chest wall and lung compliance of the patient during sleep (3) On the other hand, CPAP machines continuously deliver the therapeutic pressure while the patient is sleeping with the aim of keeping the UA open. However, it has been proved that patients do not always require the same level of pressure every night time. As a result, APAP algorithms have been developed to deliver an auto-set pressure value according to the obstruction severity (4; 5; 6). User control User Interface Compressor Storage CPU AC/DC Converter Pressure Sensors External or Internal power pack External Battery Pack Figure 2 Components of a CPAP APAP device 8
20 All the NIV devices have the following architecture in common (See Figure 2). Including firstly, a central process unit (CPU) which is the brain of the device and performs the control and arithmetic calculations of the algorithms required to adjust the pressure needed by the patient. Some devices also include a storage port, which usually is a secure digital (SD) card. Secondly, a compressor which generates the pressure necessary for the treatment by air flow regulation is required. Thirdly, the analogue to digital converter (ADC) which is the interface between the sensors (temperature, flow and/or pressure) and the CPU is also part and parcel of the typical NIV architecture. As part of the device s set up, modern compact ventilators include user s interface. More modern devices now include data analysis and for this, storage devices have been added to the design. Most of them use SD storage cards for recording events and patients history Market Devices The market of compact ventilators is very competitive. In basic terms there are two segments, trusts and consumer markets. In the UK most of the equipment is bought by the trusts and some are given to the patients and others are used in respiratory wards. However, in the American market consumers tend to buy their own devices. The companies which traditionally have developed and designed these devices are: ResMed (ResMed, Bella Vista, NSA, Australia), Philips Respironics (Philips Electronics, Murrysville, Pennsylvania, USA) and Fisher and Paykel (Fisher and Paykel, East Tamaki, Auckland, New Zealand). However, new companies have been developing new economical devices that are popular between American consumer segments. This is the case of PMI Probasics (PMI Probasics, Marlboro, New Jersey USA), Somnetics (Somnetics International Inc., New Brighton, Minnesota, USA), DeVilbiss (DeVilbiss Healthcare, Somerset, Pennsylvania, USA) and Viasys (CareFusion Corporation, San Diego, California, USA). 9
21 Rank Individual Machine % of Orders Q Average Price Q % of Orders Q Average Price Q % Share Change 1 CPAPs 52.3% $ % $ % 2 APAPs 45% $ % $ % 3 BiPAPs 2.7% $ % $ % Table 1 American APAP vs CPAP vs BiPAP consumer preference (17) Table 1 shows the American consumer preference for CPAP vs APAP vs BiPAP over third quarter Q3 (Jul-Aug 2012) (17). In this table it can be appreciated that consumers are selecting more CPAP machines than APAP and the percentage of change is not very considerable in comparison to the Q2. According to the analysis done by cpap.com the lower price of CPAP machines may contribute to this trend. However, it suggests also that the device Trascend II (Somnetics International Inc., New Brighton, Minnesota, USA) could have influenced this trend because of its innovative size and portability. What can be also noticed on the table is the price difference between the three devices. BiPAPs are considerably more expensive than APAPs (about 200% more expensive). In comparison to the APAPs which are on average 187% more expensive than common CPAPs State of the art in CPAP and APAP The traditional companies are the ones that are always innovating and bringing new features to its products. The following analysis shows what are latest state of the art devices and the unique characteristics of each device. The smallest device available in the market will also be analysed. The Appendix B shows a comparison of the CPAP market and Appendix C shows a market comparison for the APAP. 10
22 ResMed S9 TM series The S9 TM series is the latest model of CPAP and APAP released by ResMed in Europe in February 2010 (18). The main key points of the device are: optimal climate control (temperature compensation for the pressure), reduced noise levels with the Easy-Breathe motor technology and also adaptation to the patient s breathing cycle with the enhanced Expiratory Pressure Relief (EPR) technology. The model S9 Autoset differentiates between obstructive and central apnoea events and adapts accordingly. It has been designed with simplified and intuitive interface. This model can store information, notably a summary of compliance and efficacy data for up to 1 year on an SD-Card, including mask leak, apnea/hypopnea index (AHI) and central apnea statistics. The temperature and humidity are controlled by five different sensors achieving optimal humidification automatically when it is used in conjunction with the humidifier H5i (18). There are three different models available on the market. The model S9 Escape TM is a CPAP device focused mainly to treat OSA. The model S9 Elite TM is also a CPAP machine which treats OSA but includes additional features such as EPR. The S9 AutoSet TM is an APAP device that adjusts pressure throughout the night, this device is recommended for patients with OSA or CSA. It includes all the high end characteristics such as EPR (19). For full specifications of this device see Appendix A. Figure 3 ResMes S9 Series with Humidifier 11
23 Philips System One Due to the acquisition of Respironics by Philips in 2007 the product family is stopping to be branded as Respironics (20). The new family of products that Philips is offering is called System One sleep therapy platform. The main characteristics of this product are: use of advanced software algorithms which track 30 days of sleep apnoea progress and adapts to patient s needs keeping the patient informed. This is not an APAP device because it fixes the pressure of therapy and adjusts every 30 hours. Philips also claims that is the quietest device on the market with the WhisperSmart technology (21). In terms of humidification, it provides two types of humidification, auto-controlled and patient controlled. The system provides the technology System One Resistance Control which instructs the device to compensate any variable resistance characteristic related to different masks (22). It also includes C-Flex Pressure Relief technology for patient s breathing comfort; this is an adaptive control system which relieves pressure at the beginning of exhalation and returns to therapeutic pressure just before inhalation. This is possible due to Philip s patented Digital Auto-Track & Sensitivity algorithm (23). Figure 4 Philips System One with Humidifier As part of the software innovation, the System One provides the robust REMstar Auto algorithm which determines proactive performance for optimal therapy levels, comprising of: leak management, detection of obstructive apnea or hypopnea, flow limitation where the algorithm analyses changes in 12
24 roundness, shape, peak and flatness, as well as, snore detection. This last feature has been recognized as having the best reaction to changes of flow in the market (24) Transcend Sleep Apnea Therapy System TM This is one new competitor on the market that has visible differences compared to the rest of the market. This is a CPAP device that has been catalogued as the smallest CPAP in the world and an innovative humidifying system. Some of the key benefits of this device are that its size makes it highly portable, the water free humidifier is hassle free and the use of an external battery power pack makes it very convenient for outdoors travellers. In fact its low weight makes it highly portable. The technology behind the CPAP does not show any innovations related to pressure relief while breathing out or algorithms that improve the therapy or compliance (25). However, it includes software that must be installed in a computer to see the previous night s treatment, potential problems with apnea events and mask leaks. The only technological advances that this product offers compared to the leaders is the fact that it is highly portable due to extra power options available that can run from 5 8 hours (26). The only differentiator that this device offers is the innovative waterless humidification system. To achieve this, they developed a heat moisture exchange (HME) technology. This is a breakthrough in humidification technology because it does not require water, electrical heating elements and it is disposable. Figure 5 Transcend II CPAP 13
25 2.6. Common problems using compact ventilators Most of the problems related to CPAP/APAP/BiPAP treatment are related to the mask. A poor fit of the mask on the patient could cause a low compliance in the treatment. Mayo Clinic has identified the 10 most common problems related to these devices and possible solutions (7). The wrong CPAP mask size or style and no tolerance to the mask at the beginning of the treatment are the most common problems when using CPAP machines especially with new patients. It is recommended to find the most suitable mask for the patient and start to use the device in common activities such as watching TV or cooking. Mayo clinic recommends the use of a full face mask that covers the mouth and nose attached with straps for good grip. The nasal pillows are also recommended by Mayo clinic, these feature a nostril fitting with straps around the forehead to secure it (3; 7). The patient s ability to tolerate the forced air as well as the difficulty in sleeping whilst using this device are the second and seventh most common problems respectively. For this, it is recommended to use the ramp function that is included in most of the machines or use bilevel machines. The ramp function usually starts at a low level pressure and the pressure increases in small steps during certain periods of time until it reaches the therapeutic pressure (7). Philips and ResMes have solved this problem by adding C-Flex and EPR algorithms to their CPAP devices (18; 23). The fourth problem recognized by Mayo is a dry or stuffy nose. Overcoming this problem could be done by adding humidification. Most of the devices have an optional humidifier to be attached to their machines (See Appendix A CPAP market comparison) (7). The fifth problem is claustrophobia or panic attacks. Some of the masks fit around the nose and mouth making patients feel claustrophobic which can worsen when patients are given high pressure air flow. This can be solved by selecting the most appropriate mask to the patient or changing the patient to bilevel treatment (7; 27). 14
26 The sixth common difficultly is leaky masks, skin irritation or pressure sores. First of all, a leaky mask indicates that the patient is not getting the full treatment. Secondly, it could release air into the patient s eyes, causing irritation or conjunctivitis. This is one of the most distressing difficulties to patients. This can be solved by changing the mask to another one with a better fit or the use of cloth masks which alleviate this problem (7; 27). A dry mouth is the eighth most common problem. This usually happens when patients fall sleep with their mouth open and air comes in. A chin strap or a full face mask could help to cure this problem. Another recommendation is the use of an oral appliance which helps to lower the severity of the upper airways occlusion (7; 27). The ninth common problem is accidentally removing the CPAP/APAP machine while sleeping. During night time some patients tend to move a lot, as a consequence they can unintentionally remove their masks. There is not a solution for this issue, sooner or later this will occur during the patient s treatment. However, Mayo clinic recommends setting alarms in the night to remind the patient to check if the mask is still in position (7; 27). The last common problem related to the use of CPAP/APAP/BiPAP is when patients or their partners are annoyed by the noise of the device. Nowadays, most of the machines are very quiet with noise levels less than 30 dba. Nevertheless, if a filter is blocked it might cause the machine to overrun and make more noise. This can be solved by regularly servicing the machine (7; 27) Problem Identification According to the information and the background found, the following details to be improved can be deduced. 1. All the manufactures claim that their products are portable. However, there is no truly portable equipment that can run with internal batteries. Most of the commercial equipment require between 3.0 and 5.0 amperes (A) to run its motors. The only equipment that could fulfil this 15
27 requirement is the Transcend II but this device uses an external battery pack. 2. It is clear that the Transcend II is for now fulfilling the niche in the market for a portable device and it is becoming popular according to the statistics shown. Nonetheless, in the APAP and BiPAP field there is no such device that could bring the portability of the Transcend II and the software algorithm to create a smart device. 3. It can be seen that there are 3 different products to cover the market of compact ventilators. There is not a product capable to fulfil CPAP/APAP/BiPAP functions in one machine. This means that there is an increase in costs of hospitals and home patients when they need to swap to a different ventilation mode. This research has pointed out that there are still some areas of improvement in the NIV market of compact ventilators. The need and niche in the market of a portable device capable to work in all three modes CPAP, BiPAP and APAP has been highlighted. In the following sections the design of a prototype capable of meeting this niche market and suggestions to solve the portability issue will be discussed in detail. 16
28 3. Methodologies and Techniques A wide range of electronic tools and software were used in the design and development of this project. The basic components of the compact ventilator designed were based on the parts described in Figure 2 of section 2.3 (BiPAP, CPAP and APAP devices). Creating a compact ventilator capable of running CPAP/APAP/BiPAP modes was divided into different sections where hardware and software were primarily required. The hardware design included motor controllers, ADC and interfaces to transmit data/commands in and out the RasPi. The software implementation was aimed to process the data collected from the sensors and then control the speed of the motor according to the pressure set. Once the theory of all the components integrated in the design of this project have been explained, the operation of all the components together including the implementation of the circuits and algorithms will be described in section 4. (Work done) Raspberry Pi The RasPi is an ultra-low-cost credit-card size Linux computer (28) created mainly with purpose of teaching children how to program computers. This device was developed by the Raspberry Pi foundation (Raspberry Pi Foundation, Cambridge, UK, Registered charity Number ) which is a charity which aims to promote computer science and related topics particularly at school level. Figure 6 Raspberry Pi Model B 17
29 There are two different models of RasPi available in the market currently, the model A and model B. For the present project model B was used, this device has a price of (29) and can only be bought through Farnell and RS- Components websites. This device and the particular model were selected for the development of the project because it had all the tools required to achieve the objectives of the assignment. The RasPi includes peripherals such as communication ports with standard electronics protocols and high level programming language capable to execute instructions required by the algorithms. This device requires additional components that must be bought separately such as: an SD card, keyboard and mouse, external mini USB adapter, and cables HDMI or video composite RCA. This device comes equipped with the following characteristics (30): SoC Broadcom BCM2835 (CPU, GPU, DSP, and SDRAM) CPU: 700 MHz ARM1176JZF-S core (ARM11 family) Videocore 4 GPU Memory (SDRAM) Onboard storage/ Storage via: SD, MMC, SDIO card slot 10/100 Ethernet RJ 45 on-board network Another advantage of using this device is that it can run Linux and the programming language Python. This means that the device is able to perform several mathematical calculations and give results practically instantly due to its 700 MHz processor. The image of the operating system used in the assignment was Debian Linux with kernel 3.2 ( r1 built on ) compiled by Chris Boot (31). This version of kernel was selected over the official Raspbian Wheezy release, because it natively supports the communications protocol inter integrated circuit (I 2 C), serial peripheral interface bus (SPI) embedded in the chip and the pulse width modulation (PWM) module. Another advantage of using an open-source operating system and programming languages is that continuously new tools and libraries are being 18
30 developed which could continuously improve the device. For instance, a web server could be installed in the device making the device a candidate for telemedicine projects. However, one of the disadvantages of using the RasPi was that is still a new product and not much academic information or books are available. Most of the information found was on the internet and come from forums or hobbyists making projects with it. Consequently, it was quite time consuming trying to tune in the RasPi for this particular development. Figure 7 Raspberry Pi Communication Port schema (32) The Raspberry Pi has a 26-pin general purpose input/output (GPIO) expansion header (See Figure 7) (32). It provides 8 GPIO pins including access to I 2 C (GPIO 0 GPIO 1), SPI (GPIO 7, GPIO 8, GPIO 9, GPIO 10 and GPIO 11), universal asynchronous receiver/transmitter (UART GPIO 14 and GPIO 15) as well as +3.3 V (PIN 1), +5 V (PIN 2) and ground (GND 19
31 PIN 6) supply. A special protection circuit was required to shield the RasPi from short-circuits or over-charges because its GPIO does not support voltages over +3.3 V and can just supply a few millivolts per pin. In addition, the RasPi does not include an ADC to connect sensors. For this reason, an external ADC operating at the same voltage level was required. In addition to the special communication options explained previously, the GPIO 18 also supports an alternative function such as pulse width modulation (PWM). This option was particularly useful during the project because it allowed the RasPi to directly control the speed of the motor using the PWM; section 4.1 will go into more depth as to how this function was used Python language As previously mentioned, one of the advantages of using a RasPi is that it runs the Linux operating system (OS) using an SD card. There are different versions of this OS with different types of add-ins that could be used for any specific application. It also runs different types of high level programming language running over Linux. RasPi has tested several programming languages such as Python, Java, PHP and Groovy. It also expects to work other several options of programming language (33). Python was selected as the programming language to implement the algorithms. This programing language is similar to C++ but it is easier to learn and implement. This programming language is free to use because of its open source license (34). There are two different versions available of Python the 2.6 and 3.0. This project is based on the version 2.6 because it has several libraries available on the internet and it is also the most stable version Motor controller The motor controller is one of the most active parts of the circuit. This requires a group of transistors switching constantly and working in the saturation region. As a result, the use of transistors capable of handling a high volume of current and dissipation of energy was required. 20
32 There are as many different types of motor controllers as there are motors (35). The portable vacuum cleaner is a common 12 V DC brushed motor with a maximum consumption of 3.2 A. To generate positive airflow the motor only needs to rotate in one direction. Thus, a half H-Bridge circuit was the most suitable circuit to be implemented (Figure 8). Enable Q1 Vsupply Motor PWM Q2 Figure 8 Half H-Bridge drive configuration for a DC Brush Motor As can be seen in Figure 8, the transistor Q2 is using a PWM signal as a voltage regulator. The operation mode for the half H-Bridge using transistors can be explained as follows: firstly, to operate the motor it is necessary to apply voltage to the positive pin of the motor. For this purpose, the transistor Q1 which is connected to the positive power supply is activated by applying a voltage to the input pin. Secondly, the motor requires a differential of potential between its two terminals to control the voltage required by the motor. For this reason, the input of the transistor Q2 is connected to a PWM generator. When the PWM signal is generated the voltage output of the transistor Q2 will vary according to the voltage applied to the input. If a voltage equivalent to Vsupply is applied to the input of Q2, the output will be Vsupply not allowing current to go through the motor. However, if a voltage less than Vsupply is applied to the input, this will create a differential of potential in the motor which will activate the operation of the motor. 21
33 In theory, a half H-Bridge would work ideally according to Table 2. Yet, a brush motor needs a minimum voltage to activate the motor. In the development of the project, the minimum voltage to operate the motor as well as the equations required to operate the motor were found. Q1 Q2 Result 0 0 Motor not enabled Vsupply/2 Motor enabled to operate. 0% of speed of the motor. Motor enabled operating 50% of maximum speed. 1 Vssupply Motor enabled operating maximum speed. Table 2 Ideal operation of a half H-Bridge 3.4. Sensors and ADC The design of a compact ventilator requires different types of sensors according to the application or the complexity of the device. The most common sensors used are temperature, pressure, flow and tachometer. The temperature sensor is commonly used to read the ambient temperature and perform adjustments in the pressure sensor according to the readings obtained. The pressure and flow sensor are constantly sensing the output of the compact ventilator and the air flow coming into the compressor. With this information it is possible to determine whether the therapy its being properly delivered or if there is a leak or fault in the system. Tachometers are used to constantly check the function of the motor in the compressor. If there is any mismatch between the pressure and the speed of the motor, this information could be used to identify possible faults in the device. All the features mentioned previously cannot be possible with a proper method to read these physical variables and making adjustments when the device is in use. This is only possible by incorporating an ADC to the system. All the analogue signals from the different sensors are converted into digital signals using this type of integrated circuit. All this data collected is then 22
34 processed by the main processor to make adjustments or verify the optimal operation of the compact ventilator. The use of the ADC in the project will be further explained in section (Analogue to digital converter) Compressor The compressor is one of the key features when designing a compact ventilator. The compressors available in the market for CPAPs/APAPs/BiPAPs are high-tech devices that can deliver high air flow and hence a great pressure can be created in the patient s UA. Modern ventilators include very quiet motors that can run in the order of decibels (dba), some of them run relatively cold and deliver therapeutic pressure from 4 20 centimetres of water (cmh 2 O). The energy consumption of these devices varies from A according to their power supplies with an input voltage of VDC. In the inspection of the awarded motor of the S9 Elite CPAP Dr. Keith Pullen pointed out some improvements that can be done in terms of the design of the compressor. Dr. Pullen s compressor is being implemented in other nonautomotive applications; this project looks forward to implement a scale model into the medical industry. At the end of this document, the recommendations done of the desirable characteristics of the motor for a compact ventilator will be described. Due to not having the prototype of Dr. Pullen s compressor at this stage; a commercial compressor was used in the project. Different types of compressors were tested but the only one capable to produce the air flow required to induce enough pressure for testing was a portable vacuum cleaner that runs at 12 VDC as shown in Figure 9. This compressor as expected is not designed to generate positive flow, therefore modifications in its case were performed such as sealing all the vents and inserting an air flow outlet on the side. The results obtained from this 9 device were truly 23
35 amazing and provided valuable information for the design of a future compressor 3. Figure 9 Modified portable vacuum used in the project as compressor 3 Commercial compressor bought on ebay in the following link m1439.l2649#ht_4440wt_
36 4. Work done The application of the theories and circuits explained before will be described in the present section. The Appendix D Schematic design of the compact ventilator shows the full schematic of the device. The development of the project was divided into the following sections: 1. Hardware design, selection of components and sensors to be used within the device. 2. Raspberry Pi tuning and programing of the algorithms using Python. 3. Calibration and results Motor controller design As explained in section 3.3 Motor controller, a half H-Bridge was used in the design. The motor characteristics were used to determine the right components to be used. The packaging of the vacuum did not have any electrical description of the motor but the operating voltage was described on the boxing. However, it was necessary to find the current consumption of the device which was not described. This value was essential for the selection of the proper transistors. Consequently, to find this value an experiment was run using a switched power supply with an ammeter display on the front. The motor was connected directly to the power supply and the 12 VDC were applied directly. Using the current limitation knob, this was adjusted until the maximum consumption of current was found. The following table describes the electrical characteristics of the motor. Characteristic Voltage Maximum current Value obtained 12 VDC 3.2 A Table 3 Electrical characteristics of the motor As portrayed in Table 3, the design of the half H-Bridge should be able to drive at least 3.2 A at 12 VDC. The most suitable device to run over this high 25
37 demand of current is a metal oxide semiconductor field-effect transistor (MOSFET) (35). Another advantage of using this type of transistor is that the gate is driven by voltage and not by current, such as with the common bipolar junction transistor (BJT) which will also protect the RasPi in case of any overload or short-circuit. The MOSFET selected for this purpose was the p-channel type, which acts as the enable gate (Table 4 and Figure 10 illustrates the most significant features). On the schematic of Figure 12 this can be identified as the Q1. The transistor Q2 (As on Figure 12) is an n-channel STP16NF06FP 4 which will be doing the switching coming from the PWM. The features, symbol and packing are shown in Table 5 and Figure 11. Symbol Parameter Max Units I p Continuous Drain Current -19 A V GS Gate to Source Voltage ±20 V V DSS Drain to Source Breakdown Voltage -55 V Table 4 MOSFET IRF9Z34N features Figure 10 MOSFET IRF9Z34N symbol and packing Symbol Parameter Max Units I p Continuous Drain Current 16 A V GS Gate to Source Voltage ±20 V V DSS Drain to Source Voltage 60 V Table 5 MOSFET STP16NF06FP features 4 Datasheet available at 26
38 Figure 11 MOSFET STP16NF06FP symbol and packing As part of the design capacitors (10 picofarad pf) were required for filtering the back noise coming from the motor. Diodes (1N4005) were added on the drains-source pins to improve the response of the MOSFET. Figure 12 presents the schematic of the design and components of the half H-Bridge. 1N pf Q1 IRF9Z34N 12 VDC Motor Q2 STP16NF06FP 1N pf Figure 12 Half H-Bridge design and components Figure 13 Schematic of the half H-Bridge with circuit protection 27
39 A buffer was required for additional protection against short-circuiting or overloading on the RasPi. For this purpose a buffer microchip TC4428A 5 was used as shown on the schematic above. This integrated circuit (IC) works as an amplifier by increasing the voltage coming from the RasPi and putting it on the level of the power supply Temperature sensor The temperature reading is required in the device to compensate variations that this physical variable could cause to the pressure measured. A temperature sensor that works over protocol I 2 C was used in the circuit design. The reason why this type of sensor was selected is because it includes its own 8 bits ADC and directly sends the readings of the temperature over the protocol. Figure 14 Pin configuration of temperature sensor TC74 The temperature sensor used is the microchip TC74A5 6 (See Figure 14). As can be seen from Table 6, this device operates in a range of 5 V and gives and acceptable accuracy of 2%, enough for the application requirements. The resolution is 1 degree Celsius ( C) and the conversion rate is a nominal 8 samples per second (sps). Symbol Parameter Max Units V DD Supply Voltage +6 A I DD Operating Current 350 µa 5 Datasheet available at 6 Datasheet link 28
40 Temperature to bits converter T ERR Temperature Accuracy TC47A +2 C CR Conversion rate 8 Sps Table 6 Features temperature sensor TC74A5 Detailed explanations of how I 2 C, protocol and addresses are set to obtain the readings from the sensor are explained in section I 2 C protocol communication. The schematic of the pin connection to the RasPi are shown in the following figure. Figure 15 Schematic temperature sensor 4.3. Pressure sensor The compact ventilator requires an operation pressure in the range of 0 20 cmh 2 O which have been found to be the most common therapeutic pressures. For this reason, using a pressure sensor to work within this pressure specification was required. The pressure sensor selected was the Honeywell 40PC001B 7. This sensor provides a low consumption and a quick response required for the design. It also provides a wide operation range 7 Datasheet link 29
41 from -50 millimetres of mercury (mmhg) to +50 mmhg and the zero point is centred at the middle of the scale. All the details are shown in the table below. Characteristic Min. Typ. Max. Units Operating pressure mmhg Sensitivity 40 mv/mmhg Zero point offset Output at -50 mmhg 0.5 V Output at +50 mmhg 4.5 Current consumption 10 ma Response time 1 ms Table 7 Performance characteristics of the pressure sensor 40PC001B Figure 16 Pressure sensor 40PC001B and pin configuration According to the information found on the datasheet and shown in Table 7, a calibration line can be plotted taking as reference the voltage output at -50 mmhg (0.5 V), +50 mmhg (4.5 V) and 0 mmhg (2.5 V). According to this, it is expected that the sensor would have an ideal calibration line as shown in Figure 17. It is apparent form the graph below that a linear response is expected from the pressure sensor. 30
42 mmhg y = 25x R² = Volts Pressure sensor calibration line Linear (Pressure sensor calibration line) Figure 17 Graph expected linearity pressure sensor In order to identify the characteristic equation of the calibration line, the linear regression of the graph was performed. Thereby, calculating Equation 1 which represents the response of pressure versus voltage. Equation 1 Calibration line of the pressure sensor It must be noted that the characteristics given by the datasheet are represented on the scale of mmhg. However, the scale used in clinical terms is cmh 2 O. For a better understanding of the project all the values will be expressed in both scales were cmh 2 O should be the pressure to be considered for clinical tests Analogue to digital converter To process the readings coming from the pressure sensor, it was necessary to convert the analogue signals into digital values for further processing in the RasPi. The ADC selected for the project was the Microchip MCP This IC is an ADC operated by SPI serial interface (modes 0,0 and 1,1) which can be connected directly to the SPI pins of the RasPi as is explained in section SPI communication protocol. 8 Datasheet available at 31
43 Some of the highlighted features of the device are that it has 8 channels available and it has a 10-bit resolution which is enough to connect the pressure sensor to it. Within the specifications described on the datasheet, it was also found that the ADC use a sample rate of 75 ksps which is quick enough for the application of the compact ventilator. Table 8 shows the general characteristics of the ADC. Symbol Characteristic Min. Typ. Max. Units t conv Conversion Time 10 Clock cycles t SAMPLE Analogue Input Sample Time 1.5 Clock cycles Resolution 10 Bits V DD Operating voltage V I DD Operating Current µa f CLK Clock frequency MHz Table 8 Main features ADC MCP3008 Figure 18 MCP3008 Pin configuration The ADC was powered to the +3.3V of the RasPi which provided enough current and voltage to operate de ADC. The following figure shows the schematic of how the pressure sensor and the ADC were connected and its interface to the RasPi. 32
44 Figure 19 Schematic of the pressure sensor - ADC interface ADC Range of operation with the sensor The ADC was set to use as voltage references the lines +3.3V and GND. With these levels of reference the resolution of the system can be determined. Finding, the resolution of the ADC with these values is possible by using Equation 2. Equation 2 Resolution ADC The minimum resolution of the ADC is 3.2 mv which is enough to detect any change in the pressure sensors. According to the datasheet of the ADC (see Table 7) the sensitivity of the pressure sensor is 40 mv/mmhg. This means that the ADC will be able to detect any changes as small as 0.08 mmhg (0.11 cmh 2 O) per change in the least significant bit (LSB) according to the following equation. Equation 3 Minimum pressure resolution As can be seen in the equation above, this result shows that the sensor with the ADC provides a good response to minimum changes of pressure. It is very sensitive for the application of the compact ventilator. 33
45 The range of operation of the pressure sensor can be found by delimiting the maximum capability of the ADC conversion. For this, the operational range of the positive pressure of the sensor is found by the following assumptions. Firstly, the pressure sensor is powered to a 5V source as shown in Table 7. It is expected that the zero point is found at 2.5 V as per calibration line in Figure 17. The compact ventilator will be delivering positive pressure to the patient; this means that the effective range will be in the positive side of the calibration line. In other words, the operation range will be between 2.5 V (0 mmhg) and 4.5 V (50 mmhg). However, the ADC is using as voltage references +3.3V and GND. This means that the operational range will be limited to a maximum of +3.3V of the input of the pressure sensor. With these values it is possible to find the operational range of the sensor with the ADC. From Equation 1, we can find which will be the maximum pressure read by the system at +3.3V. Equation 4 Maximum pressure that the system can read According to this, the range of the ADC with the sensor is from 0 20 mmhg ( cmh 2 O). The Table 9 illustrates the whole range of values found in different scales. From this information, it can be deduced that the pressure sensor configuration is more than optimum for the application of the compact ventilator. Characteristic V mmhg cmh 2 O ADC value (DEC) Maximum Reading Zero Point Table 9 Operation range oh the ADC with the pressure sensor Now the theoretical digital output value produced by the ADC connected to the pressure sensor is a function between the voltage produce by the transducer in the sensor and the voltage of reference of the circuit. Equation 34
46 5 found in the ADC s datasheet provides an illustration of how to find the expected decimal values from the pressure read from the pressure sensor. Equation 5 Digital output code equation from ADC According to this, the digital range when the sensor is reading from 2.5 V to 3.3 V is 776 and 1024 in decimal values respectively. The Table 9 shows the sensor range and its representation in decimal value after the ADC s conversion Interfacing sensor, motor control and RasPi With all the data transformed to digital signals, now it is possible to process the information and implement the algorithms required for the compact ventilator. However, all the information captured by the sensors must be retrieved to the RasPi and must also control the speed of the motor according to the pressure feedback. As was previously explained, the temperature sensor operates over I 2 C bus and the ADC over SPI bus. The RasPi incorporate these functions on the chip and also provides PWM for motor controlling. In the following section how these buses work and are connected to the external hardware will be explained I 2 C protocol communication for the temperature sensor The I 2 C is a two wires serial single-ended computer bus invented to connect electronic peripherals. This system was developed by Philips. It also includes a protocol to interchange data between a master and slave. The RasPi which is the master of the circuit includes in its chip 2 pins which support I 2 C. These are the GPIO 0 (Pin 3) acting as serial data line (SDA) and the GPIO 1 (Pin 5) as serial clock (SCL). The thermometer sensor which is the slave of the circuit is connected to pin2 (SDA) and pin 4 (SCL). All transfers of information take place under control of the master, as previously mentioned the RasPi as master will provide the clock signal for all 35
47 transfers. The communication takes place when the master changes the start bit followed by the 7-bit address of the thermometer IC and followed by an 8- bit which indicates weather it wants to write (0) to or read (1). The data bytes are sent serially by representing either 1 or 0 according to the bit read, starting with the most significant bit (MSB) and the slave responses with and acknowledge (ACK) signal that the data was received successfully. The master and slave continue in either receive or transmit mode according to the 8-bit sent and the slave responds accordingly. Finishing the transition is achieved by a stopping sequence where the SDA changes from low-to-high while the SCL is high. In this project the thermometer sensor was allocated by default to the hexadecimal (HEX) address 0X4D ( b). Reading a temperature value from the sensor is performed by sending the HEX command 0x00 ( b). As a result, the sensor will respond with a HEX value according to the temperature read with a sensitivity of 1 C, this means that all decimals are rounded up. In the following table there is an example of how the temperature is expressed by the sensor. Actual Temp. Registered Temp. Binary HEX C Table 10 Equivalent of temperature in HEX values The full support of I 2 C and SPI in the RasPi was achieved by using the kernel 3.2 ( r1 built on ) compiled by Chris Boot (31). However, SMBus was the only library in Python available to run the RasPi s I 2 C module. 36
48 SPI communication protocol This is another communication method between ICs. It was developed by Motorola as a synchronous serial data link that operates in full duplex mode. This operates as the I 2 C establishing a master and slave in the circuit. The main difference compared to the I 2 C bus is that SPI uses 4 wires instead of 2. It also allows full duplex mode because the input and output use different wires. The interface specifies 4 logic signals that can be identified on the RasPi and the connection to the ADC MCP3008 as follows: Term Abbreviation Pins on RasPi Pins on ADC SCLK Serial Clock GPIO 11 PIN 13 MOSI Master Output GPIO 10 MISO Master Input GPIO 9 D OUT Serial Data Out PIN 12 D IN Serial Data In PIN 11 CS Chip Select GPIO 8-7 PIN 10 Table 11 SPI Pin assignment on RasPi and ADC At the moment of the implementation of the project, there was not support for the SPI module on the RasPi. As a result, an algorithm to communicate using this protocol was required using the same pins as the SPI on the RasPi and using GPIO 25 as chip select (CS) line. This pin is the first associated with data transmission, when its state is changed to 0 by the RasPi this means that data transmission will start. Then the SPI clock is sent by the master to receive and send data to and/or from the slave. One advantage of SPI is that is not just limited to words of 8-bit that is why transmitting the 10-bit words form the ADC is more convenient with this protocol than the I 2 C.. 37
49 Volts (V) PWM pin for motor control The PWM embedded in the RasPi is one of the functions which allows the motor to run. The PWM works by giving an equivalent voltage of 0V to 3.3V when a decimal value is written on its register value. The PWM works at 100 MHz (36) which is quick enough to create any noise caused by the rotation of the motor on the audible spectrum. The PWM works with a 10-bit register. It operates by assigning a value to the PWM register equivalent to the duty cycle of the signal. For instance, if 0 is assigned to the register the duty cycle will be 0, hence the effective voltage is 0. However, if the value assigned to the register is 512 this will deliver 50% of the duty cycle which is equivalent to 1.65 V and so on. Equation 6 shows the average voltage output when a value from is assigned to the PWM register. Equation 6 Relation voltage output on PWM pin with PWM register value y = x - 6E-16 R² = PWM Register Value Figure 20 Relation between PWM register and output in Volts Figure 20 illustrates the relation between the decimal value written in RasPi PWM register and its output in volts register. Performing the linear regression 38
50 of the graph, an equation expressing the relation between the two values can be found. Equation 7 Relation PWM realtion with the output of the signal Interconnecting the PWM pin of the RasPi with the half H-Bridge was performed using the buffer explained in section 4.1 and the schematic showed in Figure 13. The minimum voltage to start to operate the motor was found when the PWM register had a value of 850. By using Equation 6 it can be found that the starting voltage to kick off the motor is 2.73 V Algorithm implementation The implementation of the algorithms was accomplished using the programming language Python 2.6. Different libraries were needed to enable the functionalities such I 2 C and in general the handling of the RasPi s expansion port. There are three different libraries available on Python to use the expansion port on the RasPi. However, there is not a library capable of handling all the IO functionalities like input/output control, I 2 C, SPI and PWM. For the present project, two libraries were used for this purpose. The library wiringpi (37) developed by Gordon Drogon which enables Python to control the input/output ports including the PWM pin. Nonetheless, this library does not support I 2 C yet. Therefore, the library SMBus was used instead which access directly the I 2 C module embedded on the RasPi. For the SPI port, as explained before there was not a library capable to handle this module, hence a program was implemented to read the data coming from the ADC circuit. In terms of general programming the following libraries were used with the aim of providing different features to the algorithms. curses library: This library is used to develop the user interface. It provides support for working with windows. 39
51 sys library: access some variables used by Python interpreter. time library: It allows to use delays on the algorithms. os library: Enable python to execute Linux commands directly to the bash. multiprocessing library: used to execute two tasks at the same time. Especially useful to print graphics while reading sensors. decimal library: allows Python working with decimal point values. Is used to make the mathematical calculations of the algorithms. xml.etree.celementtree library: The configuration files were written using XML files. This library permits Python to create, read and write XML files. SMBus library: supports the use of the I 2 C ports on the RasPi CPAP Algorithm The CPAP algorithm was designed with the goal to provide positive air pressure during the whole treatment of the patient. The first step is to set the therapeutic pressure on the XML configuration file using a scale of cmh 2 O. There are two variables required to be set before running this mode: CPAP_static_pressure and CPAP_Delay. The first variable stores the pressure of the therapy. This means the value that the device should keep during the whole treatment. The second variable CPAP_Delay is used to refresh the readings from the sensors on the screen. Finding the relation between the speed of the motor and the pressure in closed loop was performed using the methodology explained in section 5.1. Using Equation 8 the value required to be applied to the PWM register can be found to apply a set positive pressure. The algorithm flowchart of the CPAP mode describing all the methodology used can be found in Appendix E CPAP Algorithm flowchart. Figure 21 illustrates the information shown when this mode is running. The top left window shows the readings from the sensors and the window messages illustrates the PWM values set and the readings from the ADC. It also 40
52 includes a graphic gauge that displays the level of pressure from 0 to 15 cmh 2 O. Figure 21 CPAP mode window APAP Algorithm This algorithm is aimed to keep the pressure at a certain level of aperture. This means that if the level of occlusion is bigger the motor will kick off harder to try to achieve the pressure goal. However, if the occlusion level of the UA is wide, the device will try to adjust to this new pressure by adjusting the pressure of the motor. This program uses three variables loaded from the XML configuration file: APAP_Pressure which is the value of the pressure to be set, APAP_Starting_Motor where the starting value of the PWM is stored when the algorithm starts, APAP_Delay for the update time and APAP_motor_limit_speed which stores the value when the motor reaches the maximum speed and the maximum leak is occurring. Finding the values where the APAP mode should auto adjust to the pressure store in the variable APAP_Pressure was performed by constantly reading the sensors and making adjustment within a margin of ±0.1 cmh 2 O of the pressure read. If the speed of the motor was out of the limits of the variable APAP_motor_limir_speed for a certain period of time the device will alarm that there is a leak in the system. 41
53 Figure 22 APAP mode window The full algorithm flowchart explaining the operation of this mode can be found in Appendix F APAP Algorithm flowchart. The windows layout showing the information from the APAP mode is similar to the CPAP mode BiPAP Algorithm The BiPAP algorithm is based similar to the CPAP mode but this mode uses two different levels of positive pressure during the inspiratory and expiratory cycle during a period of time. The algorithm also uses the methodology and equations from section 5.1 to find the values of the inspiratory pressure and the expiratory pressure stored in the variables of the XML configuration file. There are 4 variables that are adjusted in this mode such as expiratory pressure (variable BiPAP_max_pressure), inspiratory pressure (variable BiPAP_min_pressure), inspiratory time (t i variable BiPAP_ti) and expiratory time (t e variable BiPAP_te). Assigning the values of the PWM register for minimum and maximum pressure was done by using Equation 8. The values wrote in the variables are transformed to PWM register values. Then the system is constantly reading the values of the pressure sensor to find if the device is working according to the set pressures. 42
54 Figure 23 BiPAP Mode window The flowchart of this algorithm can be found in Appendix G BiPAP Algorithm flowchart where the details of how this function works is explained in more depth User interface design with Python Using the Python library curses a windows system was created to design a simple user interface. On the main screen there are 7 options as follows. Figure 24 Main menu options available 43
55 1. Temperature graph: This mode allows the system to check the function of the temperature. It is continuously running the sensor and plotting every second the temperature is read. Figure 25 Temperature reading mode 2. Pressure graph: As well as the temperature function, this mode plots the readings from the sensor. This function continuously runs the pressure sensor and checks the function of it. It plots the decimal ADC value. Figure 26 Pressure reading mode 44
56 3. CPAP Mode: This mode executes the algorithm in section It also shows an information window with the actual reading of the pressure and the messages received from the sensor in case of any error. For an example of how this mode looks like refer to Figure 21 CPAP mode window. 4. APAP Mode: This mode executes the algorithm in section As in the CPAP Mode it shows the actual reading of the pressure and the messages received from the sensor in case of any error. It also shows a pressure bar indicating the level of pressure on a scale of 0 to 20 cmh 2 O. See Figure 22 APAP mode window. 5. BiPAP Mode: This mode runs the algorithm as per section and the screen look can be seen in Figure 23 BiPAP Mode window. This mode shows the pressure graph with a scale from 0 20 cmh 2 O. On the information window, the pressure readings are shown for expiratory and inspiratory processes. 6. Configuration: This option executes an external program to read/write the value of the configuration files written on XML. There are three files generated for each mode. Modifying these options will give a new operation mode of the device. Figure 27 Configuration window 45
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text:
Barcelona defender Gerard Pique says Real Madrid star Cristiano Ronaldo deserved to win the Ballon d'Or. The pair were teammates at Manchester United.
"I like the fact that Ronaldo won and cried. It really mattered to him," Pique told Jot Down.
"He had suffered a bit and that's why he reacted the way he did. He's known as a tough character, for being superior in both a good and bad way.
"If I always speak well about Cristiano it's because I like him. People can draw their own conclusions from the gestures and comments, but they don't know. I must add that he's changed a bit recently. He helped me a lot when I joined United."
Pique added: "Cristiano is the perfect player, a born trainer who always wants to improve. He can shoot with his right foot, left foot, from free-kicks, penalties and with his head. He's the perfect player. He's the most demanding with himself."

subset: commoncrawl