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This Letter proposes a modified method for the dynamic harmonic analysis of signals with non-stationary harmonics, which is based on the previously proposed multiple-resonator observer structure. An optimisation technique is applied to reshape frequency responses of the basic transfer functions. This method, by using a parallel structure with common feedback, is very robust and, in addition, allows a reduction of the computational burden. The proposed method has been investigated for up to 64 harmonics, using LabView software package, under different conditions, and confirmed to be valuable and efficient tool for signal components estimation.

Self-powered sensors are gaining a lot of attention in recent years due to their possible application in the Internet of Things, medical implants and wireless and wearable devices. Human breath detection has applications in diagnostics, medical therapy and metabolism monitoring. One possible approach for breath monitoring is detecting the humidity in exhaled air. Here, we present an extremely sensitive, self-powered sensor for breath humidity monitoring. As a power source, the sensor uses electromagnetic energy harvested from the environment. Even electromagnetic energy harvested from the human body is enough for the operation of this sensor. The signal obtained using the human body as a source was up to 100 mV with an estimated power of 1 nW. The relatively low amount of energy that could be harvested in this way was producing a signal that was modulated by an interdigitated capacitor made out of electrochemically activated aluminum. The signal obtained in this way was rectified by a set of Schottky diodes and measured by a voltmeter. The sensor was capable of following a variety of different respiration patterns during normal breathing, exercise and rest, at the same time powered only by electromagnetic energy harvested from the human body. Everything happened in the normal environment used for everyday work and life, without any additional sources, and at a safe level of electromagnetic radiation.