This chapter presents a novel real-time method called “cascaded filtering” that enables wireless estimation of high-frequency changes in capacitance by algebraically manipulating two measurements (the magnitude and the phase of the reflected sensor impedance). Compared to previous estimation techniques, the proposed capacitance estimation algorithm has been developed to obtain the following advantages. First, the proposed algorithm makes it possible to continuously estimate the rapidly varying capacitance of a capacitive sensor as well as static or slowly varying ones. Second, it is quite simple and inexpensive to practically implement the proposed algorithm since it is derived without the need for frequency sweeping operation on the primary circuit.
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