An Automatic Depth Control System for Online Measurement of Spatial Variation in Soil Compaction, Part 4: Improvement of Compaction Maps by using a Proportional Integrative Derivative Depth Controller

The spatial variation in soil compaction is inaccurately measured online (on-the-go, tractor-based continuous measurement) if the measured depth d is not controlled at minimum variations. A depth control system is needed for controlling the depth of the subsoiler used as a tool for online measurement of soil compaction indicated as dry bulk density ρd. A proportional integrative derivative (PID) controller with a filter on the derivative-action that provided the best results in a previous part of this study was implemented. For continuous interpolation of online measured dry bulk density and sensor depth, kriging was performed with Variogram Estimation and Spatial Prediction with ERror (VESPER 1·6) software to develop maps with irregular grid of 0·3 m by 10 m. The VESPER 1·6 software was also used to generate the field maps of dry bulk density with 10 m by 10 m grid. To verify the online measured dry bulk density maps, the 10 m by 10 m online measured dry bulk density maps were compared with maps measured by the conventional core sampler method. Results showed that compaction maps developed with 10 m by 10 m grid were totally different from corresponding maps developed for continuous data of 0·3 m by 10 m grid. The PID controller considerably reduced the variation range in depth of the sensor. In comparison with the conventionally measured soil dry bulk density map, the spatial variation of online measured dry bulk density was improved when the PID controller was implicated. In addition, the PID controller with low soil moisture content variation led to a strong similarity between draught and soil dry bulk density. The automatic depth control system also assisted tracing the tramlines on compaction maps developed by elimination of possible variation in measured soil dry bulk density that could result from variable depth. The PID controller eliminated a partial similarity between compaction and depth maps that occurred without the depth control system.