Passive Impedance-Loaded Surface Acoustic Wave (SAW) Sensor for Soil Condition Monitoring

Jian Chu^1^, Ioana Voiculescu^2^, Ziqian Dong^1^, Fang Li^1\*^

Background

Nowadays, Human activities, chemical agriculture over-use, and climate changes show threats to soil fertilization. The changing condition of soil has a significant influence on human development. The monitor system for physical soil conditions needed for modern agriculture. The current practice of soil property measurement relies heavily on taking samples for laboratory testing, which is costly and time consuming for implementation over large areas. A low maintenance autonomous sensing system for in-situ soil nutrient and contaminant measurement is highly desired. SAW sensors are well suited for this application due to their compact structure, high reliability, the capability of operation wirelessly and passively, and high sensitivity to various physical variables. The objective of this research is to investigate the use of an integrated passive impedance-loaded SAW sensor for soil nutrient sensing.

Statement of Contribution/Methods

The impedance-loaded SAW sensor consists of an interdigital transducer (IDT), which converts electrical signals to surface acoustic waves and multiple reflectors (i.e.: sensing reflector and reference reflectors). The sensing reflector is loaded by an external impedance sensor, which can detect soil nutrients, such as nitrate. The reflectors reflect the surface acoustic waves back to the IDT. The signals reflected from the sensing reflector are determined by the external impedance loading. The sensor can operate passively and wirelessly when working with an antenna and a GPR unit.

 

In this research, we performed a theoretical analysis of the SAW sensor device. , The Coupling of Mode (COM) method combined with the Finite Element Method (FEM) was used to model the SAW propagation parameters. The piezoelectric substrate and IDT and reflector parameters (i.e.: IDT and reflector finger number, aperture, electrode materials, thickness, and spacing between IDT and reflectors) were carefully designed. We also simulated the effects of external impedance loadings on the sensor signals. SAW sensors with a central frequency of 250MHz were then fabricated using the E-beam technique. The impact of external impedance loading on the sensor signals was tested experimentally, and the results are agreeable with the simulation results.  

Results

The SAW sensors have been successfully designed, fabricated, and tested. The S~11~ spectrum in the time domain shows very clear pulses correspond to the reflection of each reflector. The time delay of each pulse is agreeable with the theoretical modeling results. Through the theoretical analysis, it has been found that with the increase in the resistance of external loading, the strength of the signal of external loading increases. The signal changes rapidly with the external loading resistance when the resistance is smaller than 100 ohms, and the signal becomes stable when the resistance is more extensive than 100 ohms. Experimental results also show the signal strength increases with the increase in resistance of the external loading.


Passive Impedance-Loaded Surface Acoustic Wave (SAW) Sensor for Soil Condition Monitoring

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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