  In this paper, we present a minimal torque and time variable trajectory
optimization method for autonomous excavator considering the soil-tool
interaction. The method formulates the excavation motion generation as a
trajectory optimization problem and takes into account geometric, kinematic and
dynamics constraints. To generate time-efficient trajectory and improve the
overall optimization efficiency, we propose a time variable trajectory
optimization mechanism so that the time intervals between the keypoints along
the trajectory subject to the optimization. As a result, the method uses few
keypoints and reduces the total number of optimization variables. We further
introduce a soil-tool interaction force model, which considers the geometric
shape of the bucket and the physical properties of the soil. The experimental
result on a high fidelity dynamic simulator shows our method can generate
feasible trajectories, which satisfy excavation task constraints and are
adaptive to different soil conditions.
