Hardware of the Respiratory Simulator

Improving and stabilizing the lung simulator

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The advanced technology of today has reduced the need of testing medical equipment on human patients. The invention of the lung simulator has made it possible to test ventilators without putting the condition of test patients in jeopardy. The device also allows medical staff to be trained in a safe and more rapid way. Several types of lung simulators are available. The digitally controlled variant is the most versatile one for simulations. This type has easily adjustable static and dynamic properties, which, in contrary to traditional passive mechanical simulators, make it possible to simulate different types of breathing. The available design of a digitally controlled lung simulator, however, has one major drawback. The system uses a motor-ball screw assembly to drive a piston in the air compartment. One of the properties of this assembly is that the motor shaft motion is negligibly affected (not ‘backdrivable’) by air pressure exerted on the piston by an external source. The result of this is a feedback system that needs to realize the entire dynamic response to pressure changes, by active control of the motor. Because of this, the system can easily become unstable for certain settings of the static and dynamic properties. The objective of this thesis is to determine what the best alternative actuator is for the lung simulator, to the end of making it more stable than the current version. By improving the existing lung simulator and improving its range of operation, and its performance, it can be made more effective and accurate, which will result in better healthcare. To reach this goal, firstly the desired requirements for the lung simulator will be obtained from the end user. After this, a literature study about alternative actuators shall be conducted. With the results of the literature study, a ranking of the alternative actuators are determined. From the literature study eight alternative actuators have come up. These are the moving coil actuator, moving iron actuator, permanent magnet synchronous motor (PMSM), ironless core motor, piezoelectric actuator, pneumatic actuator, hydraulic actuator and wax motor. The most important requirements on which the different actuators are evaluated are backdrivability, force linearity, force, precision, speed and stroke length. Of the eight alternative actuators, except for the PMSM and the ironless core actuator, all the others have an insufficient score on one or more of the requirements. A comparison of the PMSM and the ironless core actuator shows that the backdrivability of the ironless core actuator is better, and that it has the best overall score. The conclusion is that the ironless core actuator is the most suitable alternative for the lung simulator. The use of the actuator should increase the stability of the lung simulator. However, no testing was possible because of malfunctioning hardware and time shortage. Therefore no practical confirmation could be obtained about the effects of the new actuator on the system. It is recommended to put the top-ranked actuators to the test in a prototype. From the test results it will be possible to confirm whether the instability of the original lung simulator was indeed caused by the actuator, and whether the ironless core motor is indeed the best suitable alternative actuator for the lung simulator.