  We investigate how well a physics-based simulator can replicate a real wheel
loader performing bucket filling in a pile of soil. The comparison is made
using field test time series of the vehicle motion and actuation forces, loaded
mass, and total work. The vehicle was modeled as a rigid multibody system with
frictional contacts, driveline, and linear actuators. For the soil, we tested
discrete element models of different resolutions, with and without multiscale
acceleration. The spatio-temporal resolution ranged between 50-400 mm and 2-500
ms, and the computational speed was between 1/10,000 to 5 times faster than
real-time. The simulation-to-reality gap was found to be around 10% and
exhibited a weak dependence on the level of fidelity, e.g., compatible with
real-time simulation. Furthermore, the sensitivity of an optimized force
feedback controller under transfer between different simulation domains was
investigated. The domain bias was observed to cause a performance reduction of
5% despite the domain gap being about 15%.
