From URDF to OCP¶
One of the main challenges that hinders the widespread application of MPC for robotic tasks is the burden of setting up the optimal control problem. OCS2 provides several helper classes for defining some of the commonly-used models, cost, and constraints to mitigate this issue. To this end, OCS2 interfaces to several third party packages such as RobCoGen, CppADCodeGen, Pinocchio, and HPP-FCL. We here focus on our Pinocchio interfaces. All the discussed packages on this page can be found in the meta-package ocs2_pinocchio.
A poly-articulated floating-base system, such as a legged robot, can be modeled as an unactuated 3D rigid body to which is attached a set of fully actuated limbs. Under the mild assumption that one has sufficient control authority in the robot’s joints, it would be justifiable to independently consider centroidal dynamics in the MPC formulation as a simplified template model. This model’s state space comprises the normalized centroidal momentum, the base coordinate, and the joint positions. The input space is the concatenation of all contact wrenches and the joint velocities. To capture the effect of the generalized coordinates’ rate of change on the centroidal momentum, this model uses the centroidal momentum matrix and introduces a correct mapping between base twist and joint velocities. For more details on the implementation, refer to the package ocs2_centroidal_model.
OCS2 provides a Kinematics interface for any named frame in the URDF model based on the Pinocchio library. This interface provides a first-order model of position, orientation error, and velocity for a list of named frames. OCS2 offers two interfaces: PinocchioEndEffectorKinematics, which is based on analytical deviates, and PinocchioEndEffectorKinematicsCppAd, which is based on auto differentiation. The former is often used when one intends to rely on the caching capability of OCS2; Otherwise, the CppAd variant should be used.
Self Collision Avoidance¶
The OCS2 library provides helper classes for defining self-collision avoidance constraints. These constraints are conveniently defined through the URDF model and a user-defined list of collision bodies. This list should be a subset of the collision bodies of the URDF model. This list is used to avoid collision checking in between all the bodies, and to reduce the computation overhead. The collision constraints computation requires the HPP-FCL and the Pinocchio libraries. For more details on the implementation, refer to the package ocs2_self_collision.