Vertical and horizontal stresses from a lightweight autonomous field robot during repeated wheeling

Some agricultural robots differ in design from traditional tractors which can affect load distribution among the wheels and the resulting soil stresses, especially when moving and carrying implements. Accurate measurements of soil stresses from these machines are needed to estimate their potential to reduce risk of compaction. The aim of this experiment was to study the vertical and horizontal stresses during repeated wheeling from a lightweight autonomous robot and to compare the measurements with calculations using the model Terranimo. We installed stress transducers at 10 cm depth in a loamy sand soil with water content close to field capacity. As an example of an agricultural field robot, we used a 55 kW 4WD robot (model ROBOTTI 150 D equipped with 320/65 R16 tyres) in two configurations: unloaded (33.6 kN) and loaded with a sprayer (37.2 kN). We conducted ten passes with the robot on the same wheel tracks over the transducers and measured the contact area, dynamic wheel load, mean ground pressure and maximum vertical and horizontal stresses during each wheel pass. We observed a significant increase in mean ground pressure with repeated wheeling. Measured values of maximum vertical stresses were 32% larger than the ones predicted by the model. On the same wheel and for different numbers of passes, we found differences between maximum and minimum dynamic wheel loads of up to 9.3 kN. Our results suggest that using static loads from this machine as inputs in models can result in inaccurate predictions of soiltyre interface parameters due to the high variability and unknown redistribution of the load under dynamic conditions. The differences between static and dynamic loads also impede the correct on-field adjustment of tyre inflation pressures. These issues can result in an underestimation of the risk of soil compaction of this type of vehicle, especially in the topsoil. This work can help design new robots that aim to minimize soil compaction.