  Simplified models are useful to increase the computational efficiency of a
motion planning algorithm, but their lack of accuracy have to be managed. We
propose two feasibility constraints to be included in a Single Rigid Body
Dynamicsbased trajectory optimizer in order to obtain robust motions in
challenging terrain. The first one finds an approximate relationship between
joint-torque limits and admissible contact forces, without requiring the joint
positions. The second one proposes a leg model to prevent leg collision with
the environment. Such constraints have been included in a simplified nonlinear
nonconvex trajectory optimization problem. We demonstrate the feasibility of
the resulting motion plans both in simulation and on the Hydraulically actuated
Quadruped (HyQ) robot, considering experiments on an irregular terrain.
