manipulatorRRT

Plan motion for rigid body tree using bidirectional RRT

Description

The manipulatorRRT object is a single-query planner for manipulator arms that uses the bidirectional rapidly exploring random trees (RRT) algorithm with an optional connect heuristic to potentially increase speed.

The bidirectional RRT planner creates two trees with root nodes at the specified start and goal configurations. To extend each tree, the planner generates a random configuration and, if valid, takes a step from the nearest node based on the MaxConnectionDistance property. After each extension, the planner attempts to connect between the two trees using the new extension and the closest node on the opposite tree. Invalid configurations or connections that collide with the environment are not added to the tree.

For a greedier search, enabling the EnableConnectHeuristic property disables the limit on the MaxConnectionDistance property when connecting between the two trees.

Setting the EnableConnectHueristic property to false limits the extension distance when connecting between the two trees to the value of the MaxConnectionDistance property.

The object uses a rigidBodyTree robot model to generate the random configurations and intermediate states between nodes. Collision objects are specified in the robot model to validate the configurations and check for collisions with the environment or the robot itself.

To plan a path between a start and a goal configuration, use the plan object function. After planning, you can interpolate states along the path using the interpolate object function. To attempt to shorten the path by trimming edges, use the shorten object function.

To specify a region to sample end-effector poses near the goal configuration, create a workspaceGoalRegion object and specify it as the goalRegion input to the plan object function. To change the probability of sampling additional goal configurations, specify the WorkspaceGoalRegionBias property.

For more information about the computational complexity, see Planning Complexity.

Creation

Syntax

rrt = manipulatorRRT(robotRBT,{})

rrt = manipulatorRRT(robotRBT,collisionObjects)

Description

rrt = manipulatorRRT(robotRBT,{}) creates a bidirectional RRT planner for the specified rigidBodyTree robot model. The empty cell array indicates that there are no obstacles in the environment.

rrt = manipulatorRRT(robotRBT,collisionObjects) creates a planner for a robot model with collision objects placed in the environment. The planner checks for collisions with these objects.

Properties

MaxConnectionDistance — Maximum length between planned configurations

0.1 (default) | positive scalar

Maximum length between planned configurations, specified as a positive scalar. The object computes the length of the motion as the Euclidean distance between the two joint configurations. During the extension process, this is the maximum distance a configuration can change.

When revolute joints have infinite limits, differences between two joint positions are calculated using the angdiff function.

If the EnableConnectheuristic property is set to true, the object ignores this distance when connecting the two trees during the connect stage.

Data Types: double

ValidationDistance — Distance resolution for validating motion between configurations

0.01 (default) | positive scalar

Distance resolution for validating motion between configurations, specified as a positive scalar. The validation distance determines the number of interpolated nodes between two adjacent nodes in the tree. The object validates each interpolated node by checking for collisions with the robot and the environment.

Data Types: double

MaxIterations — Maximum number of random configurations generated

10000 (default) | positive integer

Maximum number of random configurations generated, specified as a positive integer.

Data Types: double

EnableConnectHeuristic — Directly join trees during connect phase

true or 1 (default) | false or 0

Directly join trees during the connect phase of the planner, specified as a logical 1 (true) or 0 (false). Setting this property to true causes the object to ignore the MaxConnectionDistance property when attempting to connect the two trees together.

Data Types: logical

WorkspaceGoalRegionBias — Probability to sample additional goal state from workspace goal region

0.50 (default) | positive value in the range [0,1)

Probability to sample a goal state from the workspace goal region, specified as a positive value in the range [0,1). The bias defines the probability to add additional goal states to the tree from the workspaceGoalRegion object. When this value is set to zero, the workspaceGoalRegion object still samples a single goal for the planner to plan to.

Increasing this value increases the likelihood of reaching a goal state in the goal region, but may lead to longer planning times because each new goal state adds additional complexity for planning.

Dependency

You must use the goalRegion input when calling the plan object function.

Data Types: double

IgnoreSelfCollision — Ignore self collisions during planning

0 or false (default) | 1 or true

Ignore self collisions during planning, specified as a logical. If this property is set to true, the plan function skips checking between bodies for collisions and only compares the bodies to the environment. By not checking for self-collisions, you may improve the speed of the planning phase.

Data Types: logical

SkippedSelfCollisions — Body pairs skipped for checking self-collisions

"parent" (default) | "adjacent"

Body pairs skipped for checking self-collisions, specified as either "parent" or "adjacent":

• "parent" — Skip collision checking between child and parent bodies.

• "adjacent" — Skip collision checking between bodies on adjacent indices.

See Change Self-Collision Checking Behavior for more information.

Data Types: char | string

Object Functions

plan	Plan path using RRT for manipulators
interpolate	Interpolate states along path from RRT
shorten	Trim edges to shorten path from RRT

Examples

Plan Path for Manipulator Robot Using RRT

Use the manipulatorRRT object to plan a path for a rigid body tree robot model in an environment with obstacles. Visualize the planned path with interpolated states.

Load a robot model into the workspace. Use the KUKA LBR iiwa© manipulator arm.

robot = loadrobot("kukaIiwa14","DataFormat","row");

Generate the environment for the robot. Create collision objects and specify their poses relative to the robot base. Visualize the environment.

env = {collisionBox(0.5, 0.5, 0.05) collisionSphere(0.3)};
env{1}.Pose(3, end) = -0.05;
env{2}.Pose(1:3, end) = [0.1 0.2 0.8];

show(robot);
hold on
show(env{1})
show(env{2})

Create the RRT planner for the robot model.

rrt = manipulatorRRT(robot,env);
rrt.SkippedSelfCollisions = "parent";

Specify a start and a goal configuration.

startConfig = [0.08 -0.65 0.05 0.02 0.04 0.49 0.04];
goalConfig =  [2.97 -1.05 0.05 0.02 0.04 0.49 0.04];

Plan the path. Due to the randomness of the RRT algorithm, set the rng seed for repeatability.

rng(0)
path = plan(rrt,startConfig,goalConfig);

Visualize the path. To add more intermediate states, interpolate the path. By default, the interpolate object function uses the value of ValidationDistance property to determine the number of intermediate states. The for loop shows every 20th element of the interpolated path.

interpPath = interpolate(rrt,path);
clf
for i = 1:20:size(interpPath,1)
    show(robot,interpPath(i,:));
    hold on
end
show(env{1})
show(env{2})
hold off

Plan Path To A Workspace Goal Region

Specify a goal region in your workspace and plan a path within those bounds. The workspaceGoalRegion object defines the bounds on the XYZ-position and ZYX Euler orientation of the robot end effector. The manipulatorRRT object plans a path based on that goal region and samples random poses within the bounds.

Load an existing robot model as a rigidBodyTree object.

robot = loadrobot("kinovaGen3", "DataFormat", "row");
ax = show(robot);

Create Path Planner

Create a rapidly-exploring random tree (RRT) path planner for the robot. This example uses an empty environment, but this workflow also works well with cluttered environments. You can add collision objects to the environment like the collisionBox or collisionMesh object.

planner = manipulatorRRT(robot,{});
planner.SkippedSelfCollisions="parent";

Define Goal Region

Create a workspace goal region using the end-effector body name of the robot.

Define the goal region parameters for your workspace. The goal region includes a reference pose, XYZ-position bounds, and orientation limits on the ZYX Euler angles. This example specifies bounds on the XY-plane in meters and allows rotation about the Z-axis in radians.

goalRegion = workspaceGoalRegion(robot.BodyNames{end}); 
goalRegion.ReferencePose = trvec2tform([0.5 0.5 0.2]);
goalRegion.Bounds(1, :) = [-0.2 0.2];    % X Bounds
goalRegion.Bounds(2, :) = [-0.2 0.2];    % Y Bounds
goalRegion.Bounds(4, :) = [-pi/2 pi/2];  % Rotation about the Z-axis

You can also apply a fixed offset to all poses sampled within the region. This offset can account for grasping tools or variations in dimensions within your workspace. For this example, apply a fixed transformation that places the end effector 5 cm above the workspace.

goalRegion.EndEffectorOffsetPose = trvec2tform([0 0 0.05]);
hold on
show(goalRegion);

Plan Path To Goal Region

Plan a path to the goal region from the robot's home configuration. Due to the randomness in the RRT algorithm, this example sets the rng seed to ensure repeatable results.

rng(0)
path = plan(planner,homeConfiguration(robot),goalRegion);

Show the robot executing the path. To visualize a more realistic path, interpolate points between path configurations.

interpConfigurations = interpolate(planner,path,5);

for i = 1 : size(interpConfigurations)
    show(robot,interpConfigurations(i,:),"PreservePlot",false);
    set(ax,'ZLim',[-0.05 0.75],'YLim',[-0.05 1],'XLim',[-0.05 1],...
        'CameraViewAngle',5)
  
    drawnow
end
hold off

Adjust End-effector Pose

Notice that the robot arm approaches the workspace from the bottom. To flip the orientation of the final position, add a pi rotation to the Y-axis for the reference pose.

goalRegion.EndEffectorOffsetPose = ... 
    goalRegion.EndEffectorOffsetPose*eul2tform([0 pi 0],"ZYX");

Replan the path and visualize the robot motion again. The robot now approaches from the top.

hold on
show(goalRegion);
path = plan(planner,homeConfiguration(robot),goalRegion);

interpConfigurations = interpolate(planner,path,5);

for i = 1 : size(interpConfigurations)
    show(robot, interpConfigurations(i, :),"PreservePlot",false);
    set(ax,'ZLim',[-0.05 0.75],'YLim',[-0.05 1],'XLim',[-0.05 1])
    drawnow;
end
hold off

Tips

Planning Complexity

• When planning the motion between nodes in the tree, a set of configurations are generated and validated. This computation time of the planner is proportional to the number of configurations generated. The number of configurations between nodes is controlled by the ratio of the MaxConnectionDistance and ValidationDistance properties. To improve planning time, consider increasing the validation distance or decreasing the max connection distance.

• Validating each configuration has a complexity of O(mn+m²), where m is the number of collision bodies in the rigidBodyTree object and n is the number of collision objects in worldObjects. Using large numbers of meshes to represent your robot or environment increases the time for validating each configuration.

Infinite Joint Limits

• If your rigidBodyTree robot model has joint limits that have infinite range (e.g. revolute joint with limits of [-Inf Inf]), the manipulatorRRT object uses limits of [-1e10 1e10] to perform uniform random sampling in the joint limits.

References

[1] Kuffner, J. J., and S. M. LaValle. “RRT-Connect: An Efficient Approach to Single-Query Path Planning.” In Proceedings 2000 ICRA. Millennium Conference. IEEE International Conference on Robotics and Automation. Symposia Proceedings (Cat. No.00CH37065), 2:995–1001. San Francisco, CA, USA: IEEE, 2000. https://doi:10.1109/ROBOT.2000.844730.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using MATLAB® Coder™.

Version History

Introduced in R2020b

R2022b: Alter rigid body tree self-collision checking behavior change and new default self-collision checking behavior

You can now specify self-collision checking behavior for a rigid body tree robot model by using the SkippedSelfCollisions property. Specify SkippedSelfCollisions as "parent" or "adjacent":

• "parent" — Collision checking ignores self-collisions between parent and child rigid bodies.

• "adjacent" — Collision checking ignores self -collisions between rigid bodies of adjacent indices.

As of R2022b, the default behavior of collision checking is to ignore self-collisions between parent and child rigid bodies. In previous releases, the default behavior of self-collision checking was to ignore self-collisions between adjacent rigid bodies. To instead ignore self-collisions between rigid bodies of adjacent indices, specify SkippedSelfCollisions as "adjacent".

See Change Self-Collision Checking Behavior for more information.