A recursive frequency tracking method for passive telemetry sensors

A new method for passive telemetry sensor using inductive coupling to measure a broadband signal has been proposed. The method uses multiple excitation frequencies to directly estimate the capacitance of the capacitive sensor, instead of receiving the modulated sensor signal from the sensor that is powered wirelessly via inductive coupling. Recursive least square estimation with a forgetting factor is used for the estimation. Since the proposed system does not use any active elements in the sensor side, there is no bandwidth limitation. Frequency tracking method is developed to make the system more robust to noise, thereby increasing the accuracy of estimation. Model for recursive least squares estimation is derived from the model of inductive coupling, and used with a forgetting factor to estimate time-varying parameters. Simple experiments were done to verify the model, and a static capacitance value was estimated using a least squares estimation. Simulations were done to show the advantage of using the frequency tracking method, which reduces the effect of noise and enables the system to estimate a higher bandwidth signal.