Review of research progress on soil moisture sensor technology

Soil moisture is directly related to the amount of irrigation in agriculture and influences the yield of crops. Accordingly, a soil moisture sensor is an important tool for measuring soil moisture content. In this study, the previous research conducted in recent 2-3 decades on soil moisture sensors was reviewed and the principles of commonly used soil moisture sensor and their various applications were summarized. Furthermore, the advantages, disadvantages, and influencing factors of various measurement methods employed were compared and analyzed. The improvements were presented by several scholars have established the major applications and performance levels of soil moisture sensors, thereby setting the course for future development. These studies indicated that soil moisture sensors in the future should be developed to achieve high-precision, low-cost, non-destructive, automated, and highly integrated systems. Also, it was indicated that future studies should involve the development of specialized sensors for different applications and scenarios. This review research aimed to provide a certain reference for application departments and scientific researchers in the process of selecting soil moisture sensor products and measuring soil moisture

A Self-powered And Autonomous Fringing Field Capacitive Sensor Integrated Into A Micro Sprinkler Spinner To Measure Soil Water Content

We present here the design and fabrication of a self- powered and autonomous fringing field capacitive sensor to measure soil water content. The sensor is manufactured using a conventional printed circuit board and includes a porous ceramic. To read the sensor, we use a circuit that includes a 10 kHz triangle wave generator, an AC amplifier, a precision rectifier and a microcontroller. In terms of performance, the sensor's capacitance ( measured in a laboratory prototype) increases up to 5% when the volumetric water content of the porous ceramic changed from 3% to 36%, resulting in a sensitivity of S = 15.5 pF per unity change. Repeatability tests for capacitance measurement showed that the theta(v) sensor's root mean square error is 0.13%. The average current consumption of the system ( sensor and signal conditioning circuit) is less than 1.5 mu A, which demonstrates its suitability for being powered by energy harvesting systems. We developed a complete irrigation control system that integrates the sensor, an energy harvesting module composed of a microgenerator installed on the top of a micro sprinkler spinner, and a DC/ DC converter circuit that charges a 1 F supercapacitor. The energy harvesting module operates only when the micro sprinkler spinner is irrigating the soil, and the supercapacitor is fully charged to 5 V in about 3 h during the first irrigation. After the first irrigation, with the supercap fully charged, the system can operate powered only by the supercapacitor for approximately 23 days, without any energy being harvested.17

Flexible Electronics for Large Area Sensing and Stimulation

Advancements in soft materials and hybrid flexible electronics have enabled developments in flexible circuits and wearables. Where rigid electronics are extremely precise over small physical areas, flexible electronics have the capability to sense over large curved areas. From the onset of epidermal electronics and flexible transistors, there have been great advancements in sensing over soft curved objects, such as human skin or brain tissue. This thesis focuses on hybrid flexible electronics to sense and stimulate over large areas. The aim of the systems presented is to provide insight into complex navigation and sensor processing systems. In addition to the design, fabrication, and characterization of each device, several important characteristics of each device are investigated: material choice, curvature limits, and device sensitivity. The first device presented in this thesis uses strain gauges to track the bending of neurosurgery navigation stylets for catheter placement. The strain gauge fabrication and characterization is presented. Adhesive testing, stylet bending modeling, and noise techniques are also discussed as they were found to be critical components of the system. The device's limit of detection is 1 mm tip displacement. The purpose of the second set of devices presented is to gain object information from curved or edged robotic structures. Three sensing modes were explored: piezoelectric, strain, and capacitive. The piezoelectric sensor was founded to have a 6.7 times increase in sensitivity when an open-cell foam compliant layer is used. The strain sensor was found to have a gauge factor of 2.83 on a silicone layer and 1.5 on a polymer layer. The combination of the piezoelectric and strain sensing modes is presented. The capacitive sensor is able to detect object shape using inverse problem mathematical techniques. The third device and system presented is a flexible electrode array for stimulating the electroreceptors of electric fish. The spatial and temporal control of a conformal stimulation array enables the decoding of motor signals in the brain. The array fabrication and system development is presented. Surface modification of the electrode array successfully altered the surface energy of the array to match that of the fish for the optimal array-fish interface. In summary, the development and integration of these flexible electronic devices has been achieved. It was found that the interface between the flexible electronic devices and binding objects is critical to device sensitivity and reliability

Capacitive imaging technique for non-destructive evaluation (NDE)

This thesis describes the development and characterization of a novel NDE methodthe Capacitive Imaging (CI) technique. The CI technique employs a pair of (or multiple) electrodes to form a co-planar capacitor, and uses the fringing quasi-static electric field established across the electrodes to investigate specimens of interest. In general, the CI probe is sensitive to surface and hidden defects in insulating materials, and surface features on conducting materials. The CI technique is advantageous for its non-contact and non-invasive nature, and the capacitive coupling allows the CI technique to work on a wide variety of material properties. The theoretical background to the CI technique has been developed. It is shown that in the frequency range of operation (10 kHz to 1 MHz), the quasi-static approximation is valid and the Maxwell’s Equations describing the general electromagnetic phenomena can be simplified. The practical implementation of the CI system is based on this analysis, and it is shown that the CI technique has features that can complement techniques such as eddy current methods that are already established in NDE. The design principles of the CI probes that are required for an optimum imaging performance have been determined, by considering the key measures of the performance including the depth of penetration, the measurement sensitivity, the imaging resolution and the signal to noise ratio (SNR). It has been shown that the operation frequency is not an influential factor - the performance of the CI probe is determined primarily by the geometry of the probe (e.g. size/shape of the electrodes, separation between electrodes, guard electrodes etc.). Symmetric CI probes with triangular-shaped electrodes were identified as a good general purpose design. Finite Element (FE) models were constructed both in 2D and 3D in COMSOLTM to predict the electric field distributions from CI probes. Effects of thickness of specimen, liftoff distance and relative permittivity value etc were examined using the 2D models. The sensitivity distributions of different CI probes were obtained from the 3D models and were used to characterize the imaging ability of the given CI probes. The fundamental concepts of the CI technique have been experimentally validated in a series of scans where the defects were successfully imaged in insulating (Perspex) and conducting (e.g. Aluminium, Steel and carbon fibre composite) specimens. The detection of corrosion under insulation (CUI) has also been demonstrated. The imaging abilities were assessed by investigating various standard specimens under different situations. The CI technique was then successfully applied to various practical specimens, including glass fibre laminated composites and sandwich structures, laminated carbon fibre composites, corroded steel plate and pipe, and concrete specimens. Further measurements were also conducted using modified CI probes, to demonstrate the wide range of applications of the CI technique

A Pipe-Embeddable Impedance Sensor for Monitoring Water Leaks in Distribution Networks: Design and Validation

Water leakage is one of main problems of distribution infrastructures, reaching unacceptable peaks of 50% of water lost in old networks in several countries. In order to address this challenge, we present an impedance sensor able to detect small water leaks (below 1 L of released volume). The combination of real-time sensing and such a sensitivity allows for early warning and fast response. It relies on a set of robust longitudinal electrodes applied on the external surface of the pipe. The presence of water in the surrounding medium alters its impedance in a detectable way. We report detailed numerical simulations for the optimization of electrode geometry and sensing frequency (2 MHz), as well as the successful experimental proof in the laboratory of this approach for a pipe length of 45 cm. Moreover, we experimentally tested the dependence of the detected signal on the leak volume, temperature, and morphology of the soil. Finally, differential sensing is proposed and validated as a solution to reject drifts and spurious impedance variations due to environmental effects

The non-invasive assessment of the condition of paved areas using electrical resistivity techniques

This work to detect wet areas below a paved surface is part of a 10-year research programme “ATU” (Assessing the Underworld), sponsored by EPSRC to assess the condition of the underworld by various techniques [1]. A low-frequency (5 kHz to 15 kHz) non invasive electrical resistivity technique was applied to a paved surface to assess the condition of upper layers of the asphalt. Generally, the paved surface (e.g. an asphalt pavement) is constructed using a base layer, a sub-base layer and a sub-grade layer or the natural soil foundation. The thickness of the upper typically ranges from 18 mm to 40 mm according to the standards for highway construction in the UK [2]. The electrical resistivity technique applied in this research has been implemented using a capacitive-coupled resistivity (CCR) system. Four square-plate electrodes C1, P1, P2 and C2, of large dimensions compared to the electrode spacing, are operated as inline capacitive sensors: electrodes C1 and C2 act as current sources coupling electrical current into the pavement; while electrodes P1 and P2 operate as receiver sensors to measure the voltage in the media. By this method, the quantity of electrical impedance could be estimated. It is assumed that as the moisture content of the wearing layer increases, its resistivity decreases and its dielectric constant increases. It is believed that this is the first time that this capacitive-coupled resistivity technique has been researched and applied to the condition assessment of asphalt pavement. The separation between the electrode plate and the ground surface affects the capacitance value, and hence, the coupling of the electrical signal into the ground. Thus, the surface roughness of the wearing layer could reasonably be expected to influence the capacitance. Surface roughness effects on capacitors have been studied in the nanoscale integrated circuit (IC) application area. However, no research has been published about the impact of the pavement surface roughness on the impedance measurements, obtained by the capacitive-coupled resistivity technique. In this research a laser profiling instrument with an along-track resolution of 0.125 mm and a typical height resolution of 50 μm has been utilised to measure the asphalt pavement surface roughness height distribution of the areas surveyed using the capacitive coupled resistivity technique. These surface roughness height distribution data will be incorporated into the measurement data processing as correction factors, to achieve more accurate survey results. The moisture content of the wearing layer of asphalt pavements will be estimated using the Cole-Cole model for these corrected results. According to the experiments conducted within the laboratory and field surveys on asphalt pavements in different locations, it is concluded that the measured reactance is an indication of surface roughness (with a typical maximum peak-peak roughness of 1.2 cm for the results presented within this thesis); while the real part of the measured impedance indicates the moisture content of the wearing layer of the asphalt pavement. A larger measured reactance indicates a rougher pavement surface. A smaller real part of the measured impedance indicates a higher moisture content within the sub-surface layer