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Design and evaluation of a linear servocontrolled pressure generator for studies in respiratory impedance

A very common problem in respiratory impedance measurement is the unreliability of the data in the low frequency range. The use of unbiased estimators is a good alternative to overcome this problem, provided that the generator is linear. This, however, is not the case in the great majority of the generators described so far, because the loudspeakers are non-linear systems. The generator described in this work is designed to overcome this problem by the use of a feedback loop which controls the loudspeaker cone position. The evaluation of the proposed generator has shown an effective decrease of non-linearities. In addition, an improvement in the frequency response was achieved (open loop=23 Hz: closed loop=44 Hz). The closed loop band includes the most common frequency range (2 to 32 Hz) used in the literature.