OpenRAVE Documentation

Trajectory Concepts

Reference: Trajectory

A trajectory is a path between a set of configuration space points. It performs smoothing and filtering on this path.

A trajectory holds waypoints of different pieces of data and their corresponding interpolation method. A waypoint can specify robot joints, timestamps, body transformations, events, etc. Subclasses of trajectories have differnet way of executing and interpolating the waypoints. A trajectory is just the executor, it is not responsible for re-timing and re-adjusting its path, which is a planner’s job. A trajectory cannot be executed without timestamps being present.

Usage Cases

  • Planner fills the trajectory class with timestamps, interpolation methods, and waypoints. It should be up to user to select trajectory class that will best execute this data.
  • Users can create, append, retime, and serialize trajectories both in C++ and Python.
  • Store arbitrary animations of an environment. Support for any configuration space similar to PlannerParameters so that multiple bodies and affine transformations are supported. For example, a robot opening door requires the robot joints and the door to move together.
  • Sample a trajectory at any time. Easily set the scene as the trajectory dictates it. How would this affect robot controllers?
  • It should be possible to play back the trajectory classes without loading any robots or any environment data. A real controller, on robot side, can link with openrave-core and use the trajectory class for interpolating incoming trajectory data.

Details

Every trajectory has a ConfigurationSpecification that defines what is stored in each point. This allows points to hold any type of custom information and to synchronize better with the configuration spaces of the planners. Use the Trajectory.GetConfigurationSpecification() to get the details of what data is in the trajectory and its dimensions.

Playback

Every point should have a deltatime value that specifies the time it takes to go to it given the previous point, and velocity information for each data point. The first point’s deltatime should be 0. In order to playback a set of waypoints within the robot’s velocity and acceleration limits, use the planningutils.RetimeActiveDOFTrajectory() or planningutils.RetimeAffineTrajectory(). If your configuraiton already has time data, but not velocities, you can specify hastimestamps=True to the parameters.

Trajectory Serialization

The file format for exporting the trajectory is in XML:

<trajectory type="string">
  <configuration>
    <group name="string" offset="#OFF1" dof="#D1" interpolation="string"/>
    <group name="string" offset="#OFF2" dof="#D2" interpolation="string"/>
  </configuration>
  <data count="#N">
    1 2 3 4 5
  </data>
  <readable>
    <usercustomclasses/>
  </readable>
  <description>My trajectory
  </description>
</trajectory>

configuration tag

First tags specified. The configuration describes one type of data inside the trajectory and how it is stored and processed. See ConfigurationSpecification for details on the individual attributes. In trajectories a group name=”deltatime” should be present if the trajectory is to be executed. The deltatime values always need to be positive.


data tag

The raw data of the trajectory, there is count x dof values. Multiple data tags can be present

attributes:

  • count - number of waypoints. although the number can be deduced from the size of the data, this is a precaution if the user wants to specify a subset.

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