Planning motions to grasp an object in cluttered and uncertain environments is a challenging task, particularly when a collision-free trajectory does not exist and objects obstructing the way are required to be carefully grasped and moved out. This paper takes a different approach and proposes to address this problem by using a randomized physics-based motion planner that permits robot-object and object-object interactions. The main idea is to avoid an explicit high-level reasoning of the task by providing the motion planner with a physics engine to evaluate possible complex multi-body dynamical interactions. The approach is able to solve the problem in complex scenarios, also considering uncertainty in the objects pose and in the contact dynamics. The work enhances the state validity checker, the control sampler and the tree exploration strategy of a kinodynamic motion planner called KPIECE. The enhanced algorithm, called p-KPIECE, has been validated in simulation and with real experiments. The results have been compared with an ontological physics-based motion planner and with task and motion planning approaches, resulting in a significant improvement in terms of planning time, success rate and quality of the solution path.
Physics-based motion planning is a step further towards physical realism in the planning process, and therefore useful in manipulation tasks. It extends kinodynamic motion planners (i.e., those planners that consider kinematic and dynamics constraints) by incorporating the physics-based constraints, and by allowing the robot-object and object-object interactions during planning. These interactions are modeled based on rigid body dynamics. Typically, sampling-based kindoynamic motion planners (particularly tree-based planners) are used to sample the states and to construct the solution path. The propagation step is performed using a physics engine such as ODE. Within this framework, approaches are proposed for problems like motion planning among collisionable obstacles @cite_4 , rearrangement planning @cite_13 @cite_7 , object placement @cite_14 , object sorting @cite_23 or bin picking @cite_24 . This paper will tackle the clutter grasping problem using a physics-based motion planner able to cope with uncertainty in the initial state of the system and in the poses of the objects as a result of robot-object or object-object interactions.
@cite_14 @cite_4 @cite_7 @cite_24 @cite_23 @cite_13
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Planning motions to grasp an object in cluttered and uncertain environments is a challenging task, particularly when a collision-free trajectory does not exist and objects obstructing the way are required to be carefully grasped and moved out. This letter takes a different approach and proposes to address this problem by using a randomized physics-based motion planner that permits robot-object and object-object interactions. The main idea is to avoid an explicit high-level reasoning of the task by providing the motion planner with a physics engine to evaluate possible complex multibody dynamical interactions. The approach is able to solve the problem in complex scenarios, also considering uncertainty in the objects' pose and in the contact dynamics. The work enhances the state validity checker, the control sampler, and the tree exploration strategy of a kinodynamic motion planner called KPIECE. The enhanced algorithm, called p-KPIECE, has been validated in simulation and with real experiments. The results have been compared with an ontological physics-based motion planner and with task and motion planning approaches, resulting in a significant improvement in terms of planning time, success rate, and quality of the solution path. Physics-based motion planning is a challenging task, since it requires the computation of the robot motions while allowing possible interactions with (some of) the obstacles in the environment. Kinodynamic motion planners equipped with a dynamic engine acting as state propagator are usually used for that purpose. The difficulties arise in the setting of the adequate forces for the interactions and because these interactions may change the pose of the manipulatable obstacles, thus either facilitating or preventing the finding of a solution path. The use of knowledge can alleviate the stated difficulties. This paper proposes the use of an enhanced state propagator composed of a dynamic engine and a low-level geometric reasoning process that is used to determine how to interact with the objects, i.e. from where and with which forces. The proposal, called κ-PMP can be used with any kinodynamic planner, thus giving rise to e.g. κ-RRT. The approach also includes a preprocessing step that infers from a semantic abstract knowledge described in terms of an ontology the manipulation knowledge required by the reasoning process. The proposed approach has been validated with several examples involving an holonomic mobile robot, a robot with differential constraints and a serial manipulator, and benchmarked using several state-of-the art kinodynamic planners. The results showed a significant difference in the power consumption with respect to simple physics-based planning, an improvement in the success rate and in the quality of the solution paths. Motion planning for mobile agents, such as robots, acting in the physical world is a challenging task, which traditionally concerns safe obstacle avoidance. We are interested in physics-based planning beyond collision-free navigation goals, in which the agent also needs to achieve its goals, including purposefully manipulate non-actuated bodies, in environments that contain multiple physically interacting bodies with varying degrees of controllability. Physics-based planning is computationally hard due to the large number of continuous motion actions and to the difficulty in accurately modeling the rich interactions of such controlled, manipulatable, and uncontrolled, potentially adversarial, bodies. We contribute an efficient physics-based planning algorithm that uses the agent's high-level behaviors to reduce its motion action space. We first discuss the general physics-based planning problem. We then introduce Tactics and Skills as a model for infusing goal-driven, higher level behaviors into a randomized motion planner. We present a physics-based state and transition model that employs rigid body simulations to approximate real-world interbody-dynamics. We introduce and compare two variations of our tactics-driven, physics-based planning algorithm, namely Behavioral Kinodynamic Balanced Growth Trees and Behavioral Kinodynamic Rapidly-Exploring Random Trees. We tested our physics-based planners in a variety of rich domains and show results in simulated domains where the agent manipulates an object in a dynamic non-adversarial and adversarial environment, namely in a robot minigolf and robot soccer domain, respectively. Robotic manipulation involves actions where contacts occur between the robot and the objects. In this scope, the availability of physics-based engines allows motion planners to comprise dynamics between rigid bodies, which is necessary for planning this type of actions. However, physics-based motion planning is computationally intensive due to the high dimensionality of the state space and the need to work with a low integration step to find accurate solutions. On the other hand, manipulation actions change the environment and conditions further actions and motions. To cope with this issue, the representation of manipulation actions using ontologies enables a semantic-based inference process that alleviates the computational cost of motion planning. This paper proposes a manipulation planning framework where physics-based motion planning is enhanced with ontological knowledge representation and reasoning. The proposal has been implemented and is illustrated and validated with a simple example. Its use in grasping tasks in cluttered environments is currently under development.