An Automatic Depth Control System for Online Measurement of Spatial Variation in Soil Compaction, Part 3: Design of Depth Control System

To measure the soil compaction online by measuring the draught on a cutting tool or subsoiler, the moisture content should be known and the cutting depth should remain constant. In this paper the depth controller of a subsoiler is designed. The three-point hitch is used as a correction mechanism to adjust the cutting depth by utilising the signal output of a depth sensor, which consisted of a swinging arm metal wheel and a linear variable displacement transducer (LVDT). An extra control loop is constructed around the existing position control loop of the three-point hitch. Based on a semi-analytical model multiplied by a conversion factor, a proportional (P) controller, a lead compensator, a proportional integrative derivative (PID) controller and a PID controller with filtered derivative (D), actions are designed. All controllers provided satisfactory results, with the latter showing the best performance in practice. It could also be demonstrated that, with the hardware used, no further improvement can be achieved by increasing the complexity of the controller.