抄録
The actuation of hydraulic excavators is a complex and non-intuitive task which requires long and costly training periods, since the qualification of the operator has a significant impact in productivity and safety. Simulation-based training combined with virtual reality, is becoming a competitive alternative to traditional training to reduce costs and risks in the instruction of excavator operators. Several excavator training simulators have been developed, but none of them features a dynamic model of the machine complete enough to simulate all the maneuvers performed in the daily work of real excavators. The authors have applied real-time simulation techniques from multibody system dynamics to develop a full 3D physics-based excavator simulator made up of 14 rigid bodies with 17 degrees of freedom. The simulation engine includes a custom collision detection algorithm and detailed tire force and contact force models. Terrain excavation and bucket loading and unloading are also simulated. The resulting model delivers realistic real-time behavior and can simulate common events in real excavators: slipping on slope terrains, stabilizing the machine with the blade or the outriggers, using the arm for support or impulsion to avoid obstacles, etc. This paper explains several issues related to the development of a simulator of a hydraulic excavator. Excavation and earth loading are the most common tasks for excavators, and therefore they shall be included in the capabilities of a training simulator. The detailed simulation of bucket filling requires models to predict the material flow, a simplified bucket filling model has been developed for real-time purposes. During the excavation process, the bucket penetrates the removable terrain mesh originating viscous contact forces. A ray-casting method is used to compute the intersection area between the bucket admission and the mesh representing the terrain surface; this area is integrated using the velocity component of the bucket normal to the area, in order to compute the volume and weight of earth loaded by the bucket in each time step. In addition, the algorithm diminishes the z-coordinates of the points from the removable terrain mesh that have entered inside the bucket. The algorithm provides a reasonable estimation of the loaded weight and a realistic visualization of the process. The unloading process is simulated in a similar way: when the front of the bucket surpasses a predefined critical angle, a flow of material is cast from it; the location where this flow contacts the terrain mesh or other object (e.g. a truck) is calculated. If this location does not belong to the terrain mesh, a generic flat mesh is created at that position. The flow of material is used to modify the height field of the mesh, increasing its z-coordinates using a Gaussian distribution to distribute the material randomly around the intersection point.
