Code Used in this Tutorial Available

Code can be found at moveit_tutorials repository in doc folder. Use kinetic-devel branch.

Planning Scene Tutorial

The PlanningScene class provides the main interface that you will use for collision checking and constraint checking. In this tutorial, we will explore the C++ interface to this class.

Setup

The PlanningScene class can be easily setup and configured using a RobotModel or a URDF and SRDF. This is, however, not the recommended way to instantiate a PlanningScene. The PlanningSceneMonitor is the recommended method to create and maintain the current planning scene (and is discussed in detail in the next tutorial) using data from the robot’s joints and the sensors on the robot. In this tutorial, we will instantiate a PlanningScene class directly, but this method of instantiation is only intended for illustration.

robot_model_loader::RobotModelLoader robot_model_loader("robot_description");
robot_model::RobotModelPtr kinematic_model = robot_model_loader.getModel();
planning_scene::PlanningScene planning_scene(kinematic_model);

Collision Checking

Self-collision checking

The first thing we will do is check whether the robot in its current state is in self-collision, i.e. whether the current configuration of the robot would result in the robot’s parts hitting each other. To do this, we will construct a CollisionRequest object and a CollisionResult object and pass them into the collision checking function. Note that the result of whether the robot is in self-collision or not is contained within the result. Self collision checking uses an unpadded version of the robot, i.e. it directly uses the collision meshes provided in the URDF with no extra padding added on.

collision_detection::CollisionRequest collision_request;
collision_detection::CollisionResult collision_result;
planning_scene.checkSelfCollision(collision_request, collision_result);
ROS_INFO_STREAM("Test 1: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");

Change the state

Now, let’s change the current state of the robot. The planning scene maintains the current state internally. We can get a reference to it and change it and then check for collisions for the new robot configuration. Note in particular that we need to clear the collision_result before making a new collision checking request.

robot_state::RobotState& current_state = planning_scene.getCurrentStateNonConst();
current_state.setToRandomPositions();
collision_result.clear();
planning_scene.checkSelfCollision(collision_request, collision_result);
ROS_INFO_STREAM("Test 2: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");

Checking for a group

Now, we will do collision checking only for the right_arm of the PR2, i.e. we will check whether there are any collisions between the right arm and other parts of the body of the robot. We can ask for this specifically by adding the group name “right_arm” to the collision request.

collision_request.group_name = "right_arm";
current_state.setToRandomPositions();
collision_result.clear();
planning_scene.checkSelfCollision(collision_request, collision_result);
ROS_INFO_STREAM("Test 3: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");

Getting Contact Information

First, manually set the right arm to a position where we know internal (self) collisions do happen. Note that this state is now actually outside the joint limits of the PR2, which we can also check for directly.

std::vector<double> joint_values;
const robot_model::JointModelGroup* joint_model_group = current_state.getJointModelGroup("right_arm");
current_state.copyJointGroupPositions(joint_model_group, joint_values);
joint_values[0] = 1.57;  // hard-coded since we know collisions will happen here
current_state.setJointGroupPositions(joint_model_group, joint_values);
ROS_INFO_STREAM("Current state is " << (current_state.satisfiesBounds(joint_model_group) ? "valid" : "not valid"));

Now, we can get contact information for any collisions that might have happened at a given configuration of the right arm. We can ask for contact information by filling in the appropriate field in the collision request and specifying the maximum number of contacts to be returned as a large number.

collision_request.contacts = true;
collision_request.max_contacts = 1000;
collision_result.clear();
planning_scene.checkSelfCollision(collision_request, collision_result);
ROS_INFO_STREAM("Test 4: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");
collision_detection::CollisionResult::ContactMap::const_iterator it;
for (it = collision_result.contacts.begin(); it != collision_result.contacts.end(); ++it)
{
  ROS_INFO("Contact between: %s and %s", it->first.first.c_str(), it->first.second.c_str());
}

Modifying the Allowed Collision Matrix

The AllowedCollisionMatrix (ACM) provides a mechanism to tell the collision world to ignore collisions between certain object: both parts of the robot and objects in the world. We can tell the collision checker to ignore all collisions between the links reported above, i.e. even though the links are actually in collision, the collision checker will ignore those collisions and return not in collision for this particular state of the robot.

Note also in this example how we are making copies of both the allowed collision matrix and the current state and passing them in to the collision checking function.

collision_detection::AllowedCollisionMatrix acm = planning_scene.getAllowedCollisionMatrix();
robot_state::RobotState copied_state = planning_scene.getCurrentState();

collision_detection::CollisionResult::ContactMap::const_iterator it2;
for (it2 = collision_result.contacts.begin(); it2 != collision_result.contacts.end(); ++it2)
{
  acm.setEntry(it2->first.first, it2->first.second, true);
}
collision_result.clear();
planning_scene.checkSelfCollision(collision_request, collision_result, copied_state, acm);
ROS_INFO_STREAM("Test 5: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");

Full Collision Checking

While we have been checking for self-collisions, we can use the checkCollision functions instead which will check for both self-collisions and for collisions with the environment (which is currently empty). This is the set of collision checking functions that you will use most often in a planner. Note that collision checks with the environment will use the padded version of the robot. Padding helps in keeping the robot further away from obstacles in the environment.*/

collision_result.clear();
planning_scene.checkCollision(collision_request, collision_result, copied_state, acm);
ROS_INFO_STREAM("Test 6: Current state is " << (collision_result.collision ? "in" : "not in") << " self collision");

Constraint Checking

The PlanningScene class also includes easy to use function calls for checking constraints. The constraints can be of two types: (a) constraints chosen from the KinematicConstraint set: i.e. JointConstraint, PositionConstraint, OrientationConstraint and VisibilityConstraint and (b) user defined constraints specified through a callback. We will first look at an example with a simple KinematicConstraint.

Checking Kinematic Constraints

We will first define a simple position and orientation constraint on the end-effector of the right_arm of the PR2 robot. Note the use of convenience functions for filling up the constraints (these functions are found in the utils.h file from the kinematic_constraints directory in moveit_core).

std::string end_effector_name = joint_model_group->getLinkModelNames().back();

geometry_msgs::PoseStamped desired_pose;
desired_pose.pose.orientation.w = 1.0;
desired_pose.pose.position.x = 0.75;
desired_pose.pose.position.y = -0.185;
desired_pose.pose.position.z = 1.3;
desired_pose.header.frame_id = "base_footprint";
moveit_msgs::Constraints goal_constraint =
    kinematic_constraints::constructGoalConstraints(end_effector_name, desired_pose);

Now, we can check a state against this constraint using the isStateConstrained functions in the PlanningScene class.

copied_state.setToRandomPositions();
copied_state.update();
bool constrained = planning_scene.isStateConstrained(copied_state, goal_constraint);
ROS_INFO_STREAM("Test 7: Random state is " << (constrained ? "constrained" : "not constrained"));

There’s a more efficient way of checking constraints (when you want to check the same constraint over and over again, e.g. inside a planner). We first construct a KinematicConstraintSet which pre-processes the ROS Constraints messages and sets it up for quick processing.

kinematic_constraints::KinematicConstraintSet kinematic_constraint_set(kinematic_model);
kinematic_constraint_set.add(goal_constraint, planning_scene.getTransforms());
bool constrained_2 = planning_scene.isStateConstrained(copied_state, kinematic_constraint_set);
ROS_INFO_STREAM("Test 8: Random state is " << (constrained_2 ? "constrained" : "not constrained"));

There’s a direct way to do this using the KinematicConstraintSet class.

kinematic_constraints::ConstraintEvaluationResult constraint_eval_result =
    kinematic_constraint_set.decide(copied_state);
ROS_INFO_STREAM("Test 9: Random state is " << (constraint_eval_result.satisfied ? "constrained" : "not constrained"));

User-defined constraints

User defined constraints can also be specified to the PlanningScene class. This is done by specifying a callback using the setStateFeasibilityPredicate function. Here’s a simple example of a user-defined callback that checks whether the “r_shoulder_pan” of the PR2 robot is at a positive or negative angle:

bool userCallback(const robot_state::RobotState& kinematic_state, bool verbose)
{
  const double* joint_values = kinematic_state.getJointPositions("r_shoulder_pan_joint");
  return (joint_values[0] > 0.0);
}

Now, whenever isStateFeasible is called, this user-defined callback will be called.

planning_scene.setStateFeasibilityPredicate(userCallback);
bool state_feasible = planning_scene.isStateFeasible(copied_state);
ROS_INFO_STREAM("Test 10: Random state is " << (state_feasible ? "feasible" : "not feasible"));

Whenever isStateValid is called, three checks are conducted: (a) collision checking (b) constraint checking and (c) feasibility checking using the user-defined callback.

bool state_valid = planning_scene.isStateValid(copied_state, kinematic_constraint_set, "right_arm");
ROS_INFO_STREAM("Test 10: Random state is " << (state_valid ? "valid" : "not valid"));

Note that all the planners available through MoveIt! and OMPL will currently perform collision checking, constraint checking and feasibility checking using user-defined callbacks.

The entire code

The entire code can be seen here in the moveit_pr2 github project.

Compiling the code

Follow the instructions for compiling code from source.

The launch file

The entire launch file is here on github. All the code in this tutorial can be compiled and run from the moveit_tutorials package that you have as part of your MoveIt! setup.

Running the code

Roslaunch the launch file to run the code directly from moveit_tutorials:

roslaunch moveit_tutorials planning_scene_tutorial.launch

Expected Output

The output should look something like this, though we are using random joint values so some things may be different:

[ INFO] [1385487628.853237681]: Test 1: Current state is not in self collision
[ INFO] [1385487628.857680844]: Test 2: Current state is in self collision
[ INFO] [1385487628.861798756]: Test 3: Current state is not in self collision
[ INFO] [1385487628.861876838]: Current state is not valid
[ INFO] [1385487628.866177315]: Test 4: Current state is in self collision
[ INFO] [1385487628.866228020]: Contact between: l_shoulder_pan_link and r_forearm_link
[ INFO] [1385487628.866259030]: Contact between: l_shoulder_pan_link and r_shoulder_lift_link
[ INFO] [1385487628.866305963]: Contact between: l_shoulder_pan_link and r_shoulder_pan_link
[ INFO] [1385487628.866331036]: Contact between: l_shoulder_pan_link and r_upper_arm_link
[ INFO] [1385487628.866358135]: Contact between: l_shoulder_pan_link and r_upper_arm_roll_link
[ INFO] [1385487628.870629418]: Test 5: Current state is not in self collision
[ INFO] [1385487628.877406467]: Test 6: Current state is not in self collision
[ INFO] [1385487628.879610797]: Test 7: Random state is not constrained
[ INFO] [1385487628.880027331]: Test 8: Random state is not constrained
[ INFO] [1385487628.880315077]: Test 9: Random state is not constrained
[ INFO] [1385487628.880377445]: Test 10: Random state is feasible
[ INFO] [1385487628.887157707]: Test 10: Random state is not valid

Open Source Feedback

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