Closely related to the Contact Algorithm is the Collision Algorithm. This algorithm calculates the discontinuous jump in generalized velocities resulting from inelastic collisions of the multibody system with its environment. We model the collision of the swing leg as it makes contact with the ground in this manner. The recursive algorithm determines first the instantaneous impulsive force experienced by the system, then propagates it throughout all the links of the multibody system.
In several of our experiments, we introduce an impulsive force along the axis of the leg about to lift off of the ground thereby aiding the biped in maintaining its forward momentum. The Collision Algorithm can also be used to determine the resulting jump in the generalized velocities. The magnitude of the impulsive force becomes an additional control parameter which must be included into the energy criteria to be discussed in Section . Figure 2 displays both the influence of the contact forces on the biped model and the introduction of an impulsive liftoff force.