(71c) Mitigation of Positional-Dependent Limitations of Resonant Sensors and Applications Enabled Thereby

Resonant (LC) sensors are a class of wireless, passive (no battery) sensors; they consist of a simple conductive trace that resonate at a specific frequency which is interrogated with a proximal reader coil. The resonant frequency and magnitude of power reflection or transmission changes with variations in geometric features of the sensor or electrical properties of the environment. The sensor can be designed to transduce changes in temperature, biological growth, enzyme activity, motion, etc. in closed systems by monitoring the resonant features (scattering parameters via VNA). However, this interrogation through coupled electromagnetic fields of sensor and reader coil is also highly sensitive to the relative position between the sensor and reader coil. This limits resonant sensors to static applications where the relative position between the reader and sensor is fixed. We have developed two approaches to mitigate the positional-dependent signal and thereby extend the applications to which LC sensors can be used: 1) an array of sensors that allows for position determination and signal normalization and 2) an inductively-coupled extender (ICE) that is included between the reader coil and sensor to dramatically reduce the positional dependence. These two systems applied to changes in hydration including deep soil moisture sensing.

In the first approach, four square, planar resonators with unique frequency windows were used to form a 2 by 2 array for wireless position determination and normalization of position-dependent, embedded resonant sensors. First, a master table of S­₂₁ magnitude and phase data was collected at 8100 positions. Automated scripts extracted the characteristic magnitude and phase peaks and used cubic interpolation to expand the master table to 7,157,160 unique angle and coordinate positions. An unknown position is then determined by comparing its S₂₁ measurements to this table. To further improve the position accuracy, multiple measurements are collected on linear flyby trajectories. The average and standard deviation of predicted position offset from true value using this method were 3.2 and 2.3 mm, respectively. To test normalization of a position dependent sensor, a spiral resonant sensor was placed underneath the square array. The sensor signal was modulated using varying amounts of water on the sensor surface. A corrected reading was determined using four different flyby trajectories using the position array data to adjust the signal based on position. We found that average errors of the normalized signals were between 0.04 to 0.15 MHz at lower water volume (0.5 mL) and -0.53 to -0.74 MHz at higher water volume (2.0 mL). In its current state, the positional array can be used for asset tracking or feedback control and the sensor normalization can be used to improve the measurement accuracy of embedded sensors. This technique can be further improved by collecting more accurate master calibration data using an automated system.

In the second approach we detail an inductively coupled extender (ICE) and resonant (LC) sensor to monitor soil moisture using a portable reader. Significant advances of this ICE-LC design are extending typical LC sensor read range over a meter and reducing positional alignment sensitivity between reader and sensor. An analytical model validates the working principle and feasibility of the ICE-LC system. Prototypes of the ICE-LC sensor were built and optimized in terms of sensitivity and power transfer (single and four turns for ICE top and bottom coils, respectively). Soil moisture tests validated the ICE-LC improvements on minimized positional alignment sensitivity and extended read range, transducing a decrease in resonant frequency with increasing soil moisture. When calibrating with existing wired approaches, the ICE-LC sensor had a reproducible, linear sensor gain of 4.52 %moisture content/MHz with an R² of 0.745 and RMSE of 2.41%. A smaller, planar form factor of the ICE-LC sensor was also tested and exhibited reduced positional alignment sensitivity between reader and sensor at shorter read ranges. This initial study demonstrates the feasibility of the ICE-LC resonant sensor as a cost-effective method to monitor soil moisture content throughout the growing season at many field locations.