The Design of an Integrated Soil Moisture Sensor for Agriculture
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This thesis details the design and testing of several soil moisture sensors for use in agricultural applications. The thesis concentrates on capacitive sensors measuring water content by the change in capacitance of two electrodes in direct contact with the soil water, although several other sensor types are investigated. Two electrodes placed in the soil will have a characteristic complex impedance that depends on the sensor configuration, soil structure and soil components. The impedance measured is dependant also on the frequency of the signal used for the measurement. As the frequency is lowered the effect due to capacitance and inductance become quite small and the impedance becomes mainly resistive. Conversely at high frequencies(>100MHz) the resistive component becomes less significant. A sensor designed to operate at 10kHz needs to sense small changes in an already small capacitance (pF) while ignoring the very significant resistive component. The use of insulated electrodes allows the separation of resistive and capacitive components but at the expense of placing a capacitance in series with the small soil capacitance. A method was developed to compensate for variations due to manufacturing tolerances in the insulating layer which would have made this solution impractical. Reference electrodes constructed on the same substrate as the sensor electrodes correct errors introduced by environmental and manufacturing effects on the electrode insulation. A model is presented describing the relation between water content and measured capacitance and conductivity. In experimental tests the model is successful in describing some damp to wet soils. It is shown that the small soil capacitance due to the water content is swamped by conduction in the soil. This would seem to prevent practical water content measurement at low frequencies. Experimental work demonstrated that in an insulated electrode sensor operating at low frequencies, quite large variations in measured capacitance still occur with changes in water content. Soil water forms conductive paths terminating on the electrode insulation surface. This capacitance is shown to be unaffected by soil conductivity and varies with water content and so can be used to estimate soil water. An insulated electrode sensor incorporating reference electrodes and operating at 10KHz was designed and tested. It was shown that the reference electrodes enabled correction of manufacturing and environmental variables. It is further demonstrated that contamination of the insulation coating with conductive material from within the soil prevents the practical use of this design. A method is shown for deriving both soil water content and water conductivity by measurement of the complex impedance of the sensor in soil. Solder-less construction of sensor circuitry and electronics using screen printed circuitry on plastic films was demonstrated. A design for a sensor using un-insulated electrodes and operating at 10MHz was constructed and tested. This sensor was not affected by contamination of the sensor surface. Measurements in samples of sand ranging from dry to saturated (20% water by mass) were linearly related( < 6%) to the log of water content. The sensor used only 2 active components and 23 passive components.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Microelectronics Engineering
Item Access Status
Appendices pages 176-238 have not been published.
Integrated soil moisture sensor design
Integrated soil moisture sensor testing