Implementation of a semi-digital automatic control loop for control of hot wire anemometers particularly for use in mobile spirometry

Mobile spirometry helps to supervise and evalu-ate therapy-accompanying measures of different lung diseases such as asthma and COPD (Chronic Obstructive Pulmonary Disease). For this reason it is essential to record the respiration profile of patients as precisely as possible. The focus of this work was to develop a mass flow rate sensor which is highly flexible respective to its requirements and qualified for the use in mobile spirometry at the same time. The sensor used for mass flow rate detection is based on the principle of constant temperature anemometry. Therefore it is necessary to use an automatic control loop for heating power regulation. In opposition to usual analog designs this automatic control loop was developed on semi-digital basis. The digital control loop is implemented using a freely programmable microcontroller whereby the sensor gains a high degree of flexibility. Thereby it is possible to realize a large number of complex control laws almost irrespective of hardware issues. Moreover a system was developed in order to be able to evaluate the quality of the mass flow rate sensor. It is based on a cylinder-piston-system which is steered by a two stage controlled linear drive to simulate respiration profiles of humans. With such dynamic lung simulations it is possible on the one hand to adapt the software of the mass flow rate sensor according to its requirements and on the other hand to run tests of different sensor types under reproducible site conditions. Different test procedures could confirm the high flexibility of the mass flow rate sensor under reduction of its measuring dynamics. Due to technological advancements within the range of microcontroller and transducer electronics it is possible to improve the measuring accuracy of the sensor system constantly.