PrPy

PrPy is a Python library used by the Personal Robotics Laboratory at Carnegie Mellon University. This library includes robot-agnostic utilities that make it easier to use OpenRAVE in Python scripts. This includes a high-level planning pipeline, helper functions, and visualization tools.

Planning Pipeline

There are a large array of motion planners that have complementary strengths and weaknesses. PrPy provides a planning pipeline in the prpy.planning namespace that makes it easy plan with multiple planners in parallel on a single problem. Additionally, the planning pipeline takes advantage of the dynamic nature of Python to mix-and-match planners with heterogeneous capabilities.

Every planner used in the PrPy planning pipeline extends the prpy.planning.base.Planner class. Typically, a planner will extend one of two subclasses:

1. prpy.planning.base.BasePlanner: implements or wraps a motion planner
2. prpy.planning.base.MetaPlanner: combines the output of multiple motion planners, each of which is a BasePlanner or another MetaPlanner

Each planner has one or more planning methods, annotated with the @PlanningMethod decorator, that look like ordinary functions. However, unlike an ordinary function, calling a planning method clones the robot's environment into a planning environment associated with the planner. Planning occurs in the cloned environment to allow PrPy to run multiple planners in parallel and to paralellize planning and execution.

For example, the following code will use OMPL to plan robot's active DOFs from their current values to to the goal_config configuration:

planner = OMPLPlanner('RRTConnect')
output_path = planner.PlanToConfiguration(robot, goal_config)

First, robot.GetEnv() is cloned into the the planner.env planning environment. Next, planning occurs in the cloned environment. Finally, the output path is cloned back into robot.GetEnv() and is returned by the planner.

See the following sub-sections for more information about the built-in planners provided with PrPy, information about writing your own planner, and several more complex usage examples.

Built-In Planners

PrPy provides wrappers for several existing planning libraries:

• CBiRRTPlanner: Constrained Bi-directional Rapidly-Exploring Random Tree (CBiRRT), requires the CoMPs suite
• CHOMPPlanner: Covariant Hamiltonian Optimization for Motion Planning (CHOMP), requires or_cdchomp
• OMPLPlanner: wrapper for randomized planners implemented in the Open Motion Planning Library, requires or_ompl
• OpenRAVEPlanner: wrapper for OpenRAVE planners that implement the PlannerBase interface
• SBPLPlanner: wrapper for the Search-Based Planning Library (SBPL), requires or_sbpl

Additionally, PrPy provides several simple planners of its own:

• MKPlanner: Jacobian pseudo-inverse controller for executing straight-line workspace trajectories
• SnapPlanner: attempts to execute a straight-line joint-space trajectory to the goal
• GreedyIKPlanner: follows a workspace path by greedily picking IK solutions
• VectorFieldPlanner: follows any custom cspace vector field until a custom termination

Finally, PrPy provides several meta-planners for combining the above planners:

• Sequence: sequentially queries a list of planners and returns the result of the first planner in the list that succeeds.
• Ranked: queries a list of planners in parallel and returns the solution first planner in the list that returns success
• IKPlanner: plan to an end-effector pose by sequentially planning to a list of ranked IK solutions
• NamedPlanner: plan to a named configuration associated with the robot

See the Python docstrings the above classes for more information.

Common Planning Methods

There is no formal list of @PlanningMethods or their arguments. However, we have found these methods to be useful:

• PlanToConfiguration(robot, goal_config): plan the robot's active DOFs from the robot's current configuration to the goal_config configuration.
• PlanToConfigurations(robot, goal_configs): plan the robot's active DOFs from the robot's current configuration to any of the elements in the goal_configs list.
• PlanToEndEffectorPose(robot, goal_pose): plan the robot's active manipulator's end-effector to goal_pose.
• PlanToEndEffectorOffset(robot, direction, min_distance, max_distance): plan the robot's active manipulator in a straight line in the direction specified by the direction unit vector for [ min_distance, max_distance ] meters.
• PlanToTSR(robot, tsrchains): plan with the start, goal, and/or constraints specified by a list of TSR chains.
• PlanToBasePose(robot, goal_pose): plan with the robot's affine DOFs in the plane to a desired base pose.

Most planners that implement these methods accept a timelimit parameter, in seconds, for which to plan before raising a PlanningError. Additionally, many of these methods accept planner-specific keyword arguments.

Writing a Custom Planner

Implementing a custom planner requires extending the BasePlanner class and decorating one or more methods with the @PlanningMethod decorator. Extending the BasePlanner class constructs the planning environment self.env and allows PrPy to identify your planner as a base planner class, as opposed to a meta-planner. The @PlanningMethod decorator handles environment cloning and allows meta-planners to query the list of planning methods that the planner supports (e.g. to generate docstrings).

Please obey the following guidelines:

• Assume that the cloned environment is locked during the entire call.
• Subclass constructor must call BasePlanner.__init__.
• A @PlanningMethod must not call another @PlanningMethod.
• Each @PlanningMethod must accept the first argument robot, which is a robot in the cloned environment.
• Each @PlanningMethod must accept **kwargs to ignore arguments that are not supported by the planner.
• Each @PlanningMethod must return a Trajectory which was created in the cloned environment.
• When possible, use one of the defacto-standard @PlanningMethod names listed below.
• Raise a PlanningError to indicate an expected, but fatal, error (e.g. timeout with no collision-free path).
• When applicable, raise a context-specific subclass of PlanningError to indicate the nature of the error (e.g. StartCollisionPlanningError)
• Raise a standard Python exception, e.g. ValueError, to indicate an unexpected error has occurred (e.g. argument out of range).
• Raise a UnsupportedPlanningError to indicate that the planning operation is fundamentally not supported (e.g. constraint type is not implemented).

Examples

Trajectory optimizers, like CHOMP, typically produce higher quality paths than randomized planners. However, these algorithms are not probabilistically complete and can get stuck in local minima. You can mitigate this by using the Sequence planner to first call CHOMP, then fall back on RRT-Connect:

planner = Sequence(CHOMPPlanner(), OMPLPlanner('RRTConnect'))
path = planner.PlanToConfiguration(robot, goal)

Unfortunately, this means that RRT-Connect does not start planning until CHOMP has already failed to find a solution. Instead of using Sequence, we can use the Ranked meta-planner to plan with both planners in parallel. Just as before, the meta-planner will immediately return CHOMP's solution if it returns success. However, RRT-Connect will have a head-start if CHOMP fails:

planner = Ranked(CHOMPPlanner(), OMPLPlanner('RRTConnect'))
path = planner.PlanToConfiguration(robot, goal)`

In other cases, a meta-planner can be used to combine the disparate capabilities of multiple planenrs. For example, SBPL is currently the only planner included with PrPy that supports planning for affine DOFs. We can use a meta-planner to combine OMPL's support for PlanToConfiguration with SBPL's support for PlanToBasePose:

planner = Sequence(OMPLPlanner('RRTConnect'), SBPLPlanner())
path1 = planner.PlanToConfiguration(robot, goal)
path2 = planner.PlanToBasePose(robot, goal_pose)

Environment Cloning

Cloning environments is critical to enable planning with multiple planners in parallel and parallelizing planning and execution. PrPy provides two utilities to simplify environment cloning in OpenRAVE: the Clone context manager and the Cloned helper function.

Clone Context Manager

PrPy adds a prpy.clone.Clone context manager to manage temporary environment clones; e.g. those used during planning. This context manager clones an environment when entering the with-block and destroys the environment when exiting the block. This code is careful to lock the source and destination environments during cloning correctly to avoid introducing a race condition.

In the simplest case, the Clone context manager creates an internal, temporary environment that is not re-used between calls:

with Clone(env) as cloned_env:
    robot = cloned_env.GetRobot('herb')
    # ...

The same context manager can be used to clone into an existing environment. In this case, the same target environment can be used by multiple calls. This allows OpenRAVE to re-use the environments resources (e.g. collision tri-meshes) and can dramatically improve performance:

clone_env = openravepy.Environment()

with Clone(env, clone_env=clone_env):
    robot = cloned_env.GetRobot('herb')
    # ...

Often times, the cloned environment must be immediately locked to perform additional setup. This introduces a potential race condition between Clone releasing the lock and the code inside the with-block acquiring the lock. To avoid this, use the lock argument to enter the with-block without releasing the lock:

with Clone(env, lock=True) as cloned_env:
    robot = cloned_env.GetRobot('herb')
    # ...

In this case, the cloned environment will be automatically unlocked when exiting the with-statement. This may be undesirable if you need to explicitly unlock the environment inside the with-statement. In this case, you may pass the unlock=False flag. In this case, you must explicitly unlock the environment inside the with-statement:

with Clone(env, lock=True, unlock=False) as cloned_env:
    robot = cloned_env.GetRobot('herb')
    env.Unlock()
    # ...

Cloned Helper Function

It is frequently necessary to find an object in a cloned environment that refers to a particular object in the parent environment. This code frequently looks like this:

with Clone(env) as cloned_env:
    cloned_robot = cloned_env.GetRobot(robot.GetName())
    # ...

The prpy.clone.Cloned helper function handles this name resolution for most OpenRAVE data types (including Robot, KinBody, Link, and Manipulator). This function accepts an arbitrary number of input parameters---of the supported types---and returns the corresponding objects in Cloned environment. For example, the above code can be re-written as:

with Clone(env) as cloned_env:
    cloned_robot = Cloned(robot)
    # ...

If multiple Clone context managers are nested, the Cloned function returns the corresponding object in the inner-most block:

with Clone(env) as cloned_env1:
    cloned_robot1 = Cloned(robot) # from cloned_env1
    # ...
    
    with Clone(env) as cloned_env2:
        cloned_robot2 = Cloned(robot) # from cloned_env2
        # ...
    
    # ...
    cloned_robot3 = Cloned(robot) # from cloned_env1

The Cloned function only works if it is called from the same thread in which the Clone context manager was created. If this is not the case, you can still use the Cloned helper function by explicitly passing an environment:

with Clone(env) as cloned_env:
    def fn(body, e):
        cloned_robot = Cloned(body, clone_env=e)
        # ...

    thread = Thread(target=fn, args=(body, cloned_env))
    thread.start()
    thread.join()

Finally, as a convenience, the Cloned function can be used to simultaneously resolve multiple objects in one statement:

    with Clone(env) as cloned_env:
        cloned_robot, cloned_body = Cloned(robot, body)
        # ...

Concurrent Execution

PrPy has native support for futures and coroutines to simplify concurrent programming. A future encapsulates the execution of a long-running task. We use the concurrency primitives provided by the trollius module, which is a Python 2 backport of the asyncio module from Python 3.

We can use these primitives to parallelize planning and execution:

@coroutine
def do_plan(robot):
    # Plan to goal1 and start executing the trajectory.
    path1 = yield From(robot.PlanToEndEffectorPose(goal1, execute=False))
    exec1_future = robot.ExecutePath(path1)
    
    # Plan from goal1 to goal2.
    robot.SetDOFValues(GetLastWaypoint(path1))
    path2 = yield From(robot.PlanToEndEffectorPope(goal2, execute=False))
    
    # Wait for path1 to finish executing, then execute path2.
    exec1 = yield From(exec1_future)
    exec2 = yield From(robot.ExecutePath(path2))
    
    raise Return(path1, path2)

loop = trollius.get_event_loop()
path = loop.run_until_complete(do_plan(robot))

Method Binding

Finally, PrPy offers helper functions for binding custom methods on (i.e. monkey patching) OpenRAVE data types, including: KinBody, Robot, Link, and Joint.

This may appear trivial to accomplish using setattr. However, this is actually quite challenging to implement because OpenRAVE's Python bindings for these classes are automatically generated as Boost.Python bindings that are managed by a shared_ptr. Each instance of a shared_ptr returned by C++ is wrapped in a separate Python object. As a result, the following code does not work as expected:

robot = env.GetRobot('herb')
setattr(robot, 'foo', 'bar')
robot_ref = env.GetRobot('herb')
robot_ref.foo # raises AttributeError

PrPy provides the prpy.bind.InstanceDeduplicator class to work around this issue. This class takes advantage of the user data attached to an OpenRAVE environment to de-duplicate multiple Python shared_ptr instances that reference same object. This is implemented by overriding __getattribute__ and __setattribute__ to defer all attribute queries to a single canonical instance of the object.

Canonical Instances

An object is flagged for de-duplication using the InstanceDeduplicator.add_canonical function. The above example can be modified to work as follows:

robot = env.GetRobot('herb')
InstanceDeduplicator.add_canonical(robot)

setattr(robot, 'foo', 'bar')
robot_ref = env.GetRobot('herb')
robot_ref.foo # returns 'bar'

Subclass Binding

Frequently we wish to extend an OpenRAVE object with several tightly coupled attributes, properties, and methods. This can be achieved by creating subclass of appropriate OpenRAVE data type (e.g. Robot) and dynamically changing that instance's __class__ at runtime. PrPy provides a prpy.bind.bind_subclass helper function that calls add_canonical, changes __class__, and calls the subclass' __init__ function.

This functionality is most frequently used with the generic PrPy subclasses provided in the prpy.base module. For example, the following code adds the capabilities of prpy.base.Robot to an existing robot:

robot = env.GetRobot('herb')
bind_subclass(robot, prpy.base.Robot)

See the docstrings on the classes defined in prpy.base for more information.

Cloning Bound Subclasses

OpenRAVE is not aware of methods, attributes, or properties that are added to objects in Python; e.g. using add_canonical. As a result, these attributes are not duplicated when the OpenRAVE environment is cloned. PrPy provides the limited capability to clone these attributes when: (1) the class was extended using the bind_subclass function and (2) the clone was created using the PrPy Clone function. If these two conditions hold, PrPy will call the CloneBindings() function on your custom subclass the first time the cloned object is referenced.

See the classes in prpy.base for example implementations of CloneBindings.

Dependencies

To run prpy, you will need to have installed the pacakge enum34. To do that, go to a local directory and run sudo pip install enum34

License

PrPy is licensed under a BSD license. See LICENSE for more information.

Contributors

PrPy is developed by the Personal Robotics Lab in the Robotics Institute at Carnegie Mellon University. This library was originally developed by Michael Koval, with some code copied from the earlier prrave library developed by Chris Dellin.

This is a non-exhaustive list of contributors:

• Chris Dellin
• Evan Shapiro
• Jen King
• Michael Koval
• Pras Velagapudi
• Shervin Javdani
• Sidd Srinivasa
• Stefanos Nikolaidis