An Efficient Readout Scheme for Simultaneous Measurement from Multiple Wireless Passive $lC$ Sensors

A new readout scheme for multiple passive LC sensors based on the impulse response is presented here. A precharged capacitor is discharged through a readout coil which is magnetically coupled to multiple sensor coils. Fast Fourier transform of the resultant oscillatory current through the readout coil is taken, from which the equivalent impedance of the system at different frequencies is computed. The maximum value of the imaginary parts of this impedance occurs at the resonant frequencies of the sensor coils that are coupled. The conventional measurement schemes rely on a frequency sweep approach to determine the resonance frequencies of the sensor coils; that process is inherently time-consuming. A simple, low-cost readout scheme with a fast update rate is proposed. A method to measure the quality factor Q of the sensor coil system is also presented here. Output of the proposed scheme is independent of the variation in the coupling factor k. The measurement resolution of the proposed scheme has been improved using a Gaussian curve fitting approach, which also reduces the effect of noise in the final output. A prototype of the proposed system has been built with a readout coil and three sensors coupled to it and tested. The worst case error observed in the test was less than 0.5%, when the prototype system was tested with a variable capacitor (47 to 345 pF) in the sensor coil. The proposed readout scheme is useful in applications requiring simultaneous wireless monitoring of multiple physical parameters.