An FFT based readout scheme for passive LC sensors

A new readout scheme for passive LC sensors based on the Fast Fourier Transform (FFT) method is presented here. The sensor coil of the LC sensor is magnetically coupled to a readout coil. The maximum value of the imaginary part of the impedance of this system occurs at the sensor resonance frequency (fp). In the conventional systems, it is the real part of the impedance which becomes maximum at fp. The existing methods to determine this frequency rely on a frequency sweep approach which is inherently time consuming. Further, to track fp continuously, a complex feedback loop is required to adjust the input excitation frequency. A new method to directly determine the sensor resonance frequency with a simple excitation is presented. A nearly impulse current is sent through the readout coil, by discharging a capacitor that is pre-charged from a dc source. The response current through the readout coil captures the multiple resonance frequencies in the system. The maximum value of the imaginary part of this modified system occurs at fp. The resonance frequency is computed from the FFT output. The proposed method has all the advantages of the conventional readout schemes. Unlike the existing schemes, the exciter part of the readout is very simple, the measurement time is very small and the approach is scalable to have multiple sensor coils, without any additional hardware in the readout unit. A prototype of the proposed system has been made and tested. The worst case error observed in the test was less than 0.5%.