Breathing Therapy Air Delivery Unit: Simulation, Design and Development
White, David Edward
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Although constant positive airway pressure therapy is currently the most effective form of non-invasive treatment to relieve obstructive sleep apnea symptoms, it has relatively low treatment compliance due to pressure related side effects. Existing commercial continuous positive airway pressure (CPAP) devices rely on the combined airflow characteristics of both the air delivery unit and nasal mask vent to regulate treatment pressure. Fluctuation in mask pressure occurs however, due to patient breathing, presenting an opportunity to develop an alternative breathing therapy device capable of achieving dynamic control of mask pressure. Within this research, a computer model of a proposed patient breathing therapy device, based on characteristics of a prototype system, is developed to determine the breathing system air delivery requirements whilst operating under a simulated patient breathing load. This model initially utilises an idealised, zero order, air delivery unit behaviour, since this system element is yet to be built. A review of different types of air compressors is undertaken and the diaphragm type compressor selected as being best suited for practical implementation within the air delivery unit of the breathing system, based on constraints of air quality, available machining resource and materials. Thermodynamic design of the compressor is undertaken to determine physical dimensions and a range of actuation methods are reviewed, based on force and speed requirements. A speed controlled 3 phase AC induction motor is selected to actuate the compressor. The diaphragm compressor is built and tested under both steady state and dynamic conditions and proven capable of meeting the breathing system air supply for both air pressure and flow requirements. The air delivery unit within the model simulation, previously based on an idealised, zero order element, is characterised with the same dynamic behaviour as the prototype unit built, established during testing, and shown by simulation to meet the breathing system requirements under dynamic patient breathing load. Implementation of the air delivery unit within the completed prototype breathing system shows the mask pressure to fluctuate outside the desire pressure tolerance range; however, to remedy this situation, the compressor requires the development of an appropriate control scheme which is beyond the scope of this work.