Noise source evaluation of a real-time soil sensor. II. Dynamic noise sources

Research was conducted which examined the effects of dynamic elevation changes on a real-time soil organic matter sensor. Such soil sensors are intended to provide quick and economical methods of measuring soil properties. These soil properties are then useful in determining the proper chemical and fertilizer application rates for small sections of land. During the mapping process for organic matter content, it was found that the signal from the soil organic matter sensor contained several high-frequency components. A study was conducted to examine the relationship between dynamic elevation changes and the high-frequency components of the sensor signal. A finite element model of the sensor housing was created to obtain estimates of the first three natural frequencies of the system. These frequency values were then used to design laboratory experiments and to set data acquisition speeds for field tests. Controlled tests were performed at Herrick Laboratories, Purdue University, where the only noise source to the sensor system was dynamic changes in the sensor elevation relative to the soil surface. It was found that the level of error produced in the laboratory was significant enough to warrant study during field use. Field experiments were conducted in which acceleration inputs to the sensor assemble and the signal from the sensor were recorded by a portable data acquisition system. The relationship between the acceleration inputs to the sensor signal were examined using both single and multiple coherence analysis methods. The results of the coherence analysis indicated a stronger relationship between the acceleration inputs near the natural frequency of the sensor housing than at other frequencies. This suggests that great care should be taken to determine the resonant frequency of any such sensor housing and that components of the signal above this frequency should be removed via filters prior to using the data for analysis. For this sensor housing, no frequency component above 10 Hz should be used for the analysis of soil organic matter content. If higher sampling rates are desired, subsequent sensor housings should be designed such that the housing's natural frequency is raised.