[JTC] Update trajectory documentation (#714) (#741)

joint_trajectory_controller/doc/parameters.rst

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+:github_url: https://github.com/ros-controls/ros2_controllers/blob/{REPOS_FILE_BRANCH}/joint_trajectory_controller/doc/parameters.rst
+
+.. _parameters:
+
+Details about parameters
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+joints (list(string))
+  Joint names to control and listen to.
+
+command_joints (list(string))
+  Joint names to control. This parameters is used if JTC is used in a controller chain where command and state interfaces don't have same names.
+
+command_interface (list(string))
+  Command interfaces provided by the hardware interface for all joints.
+
+  Values: [position | velocity | acceleration] (multiple allowed)
+
+state_interfaces (list(string))
+  State interfaces provided by the hardware for all joints.
+
+  Values: position (mandatory) [velocity, [acceleration]].
+  Acceleration interface can only be used in combination with position and velocity.
+
+state_publish_rate (double)
+  Publish-rate of the controller's "state" topic.
+
+  Default: 50.0
+
+action_monitor_rate (double)
+  Rate to monitor status changes when the controller is executing action (control_msgs::action::FollowJointTrajectory).
+
+  Default: 20.0
+
+allow_partial_joints_goal (boolean)
+  Allow joint goals defining trajectory for only some joints.
+
+  Default: false
+
+allow_integration_in_goal_trajectories (boolean)
+  Allow integration in goal trajectories to accept goals without position or velocity specified
+
+  Default: false
+
+interpolation_method (string)
+  The type of interpolation to use, if any. Can be "splines" or "none".
+
+  Default: splines
+
+open_loop_control (boolean)
+  Use controller in open-loop control mode:
+
+  * The controller ignores the states provided by hardware interface but using last commands as states for starting the trajectory interpolation.
+  * It deactivates the feedback control, see the ``gains`` structure.
+
+  This is useful if hardware states are not following commands, i.e., an offset between those (typical for hydraulic manipulators).
+
+  .. Note::
+     If this flag is set, the controller tries to read the values from the command interfaces on activation.
+     If they have real numeric values, those will be used instead of state interfaces.
+     Therefore it is important set command interfaces to NaN (i.e., ``std::numeric_limits<double>::quiet_NaN()``) or state values when the hardware is started.
+
+  Default: false
+
+allow_nonzero_velocity_at_trajectory_end (boolean)
+  If false, the last velocity point has to be zero or the goal will be rejected.
+
+  Default: true
+
+constraints (structure)
+  Default values for tolerances if no explicit values are states in JointTrajectory message.
+
+constraints.stopped_velocity_tolerance (double)
+  Default value for end velocity deviation.
+
+  Default: 0.01
+
+constraints.goal_time (double)
+  Maximally allowed tolerance for not reaching the end of the trajectory in a predefined time.
+
+  Default: 0.0 (not checked)
+
+constraints.<joint_name>.trajectory (double)
+  Maximally allowed deviation from the target trajectory for a given joint.
+
+  Default: 0.0 (tolerance is not enforced)
+
+constraints.<joint_name>.goal (double)
+  Maximally allowed deviation from the goal (end of the trajectory) for a given joint.
+
+  Default: 0.0 (tolerance is not enforced)
+
+gains (structure)
+  Only relevant, if ``open_loop_control`` is not set.
+
+  If ``velocity`` is the only command interface for all joints or an ``effort`` command interface is configured, PID controllers are used for every joint.
+  This structure contains the controller gains for every joint with the control law
+
+  .. math::
+
+     u = k_{ff} v_d + k_p e + k_i \sum e dt + k_d (v_d - v)
+
+  with the desired velocity :math:`v_d`, the measured velocity :math:`v`, the position error :math:`e` (definition see below),
+  the controller period :math:`dt`, and the ``velocity`` or ``effort`` manipulated variable (control variable) :math:`u`, respectively.
+
+gains.<joint_name>.p (double)
+  Proportional gain :math:`k_p` for PID
+
+  Default: 0.0
+
+gains.<joint_name>.i (double)
+  Integral gain :math:`k_i` for PID
+
+  Default: 0.0
+
+gains.<joint_name>.d (double)
+  Derivative gain :math:`k_d` for PID
+
+  Default: 0.0
+
+gains.<joint_name>.i_clamp (double)
+  Integral clamp. Symmetrical in both positive and negative direction.
+
+  Default: 0.0
+
+gains.<joint_name>.ff_velocity_scale (double)
+  Feed-forward scaling :math:`k_{ff}` of velocity
+
+  Default: 0.0
+
+gains.<joint_name>.normalize_error (bool)
+  If true, the position error :math:`e = normalize(s_d - s)` is normalized between :math:`-\pi, \pi`.
+  Otherwise  :math:`e = s_d - s` is used, with the desired position :math:`s_d` and the measured
+  position :math:`s` from the state interface. Use this for revolute joints without end stop,
+  where the shortest rotation to the target position is the desired motion.
+
+  Default: false

joint_trajectory_controller/doc/trajectory.rst

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+:github_url: https://github.com/ros-controls/ros2_controllers/blob/{REPOS_FILE_BRANCH}/joint_trajectory_controller/doc/trajectory.rst
+
+.. _joint_trajectory_controller_trajectory_representation:
+
+Trajectory Representation
+---------------------------------
+
+Trajectories are represented internally with ``trajectory_msgs/msg/JointTrajectory`` data structure.
+
+Currently, two interpolation methods are implemented: ``none`` and ``spline``.
+By default, a spline interpolator is provided, but it's possible to support other representations.
+
+.. warning::
+  The user has to ensure that the correct inputs are provided for the trajectory, which are needed
+  by the controller's setup of command interfaces and PID configuration. There is no sanity check and
+  missing fields in the sampled trajectory might cause segmentation faults.
+
+Interpolation Method ``none``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+It returns the initial point until the time for the first trajectory data point is reached. Then, it simply takes the next given datapoint.
+
+.. warning::
+  It does not deduce (integrate) trajectory from derivatives, nor does it calculate derivatives.
+  I.e., one has to provide position and its derivatives as needed.
+
+Interpolation Method ``spline``
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The spline interpolator uses the following interpolation strategies depending on the waypoint specification:
+
+* Linear:
+
+  * Used, if only position is specified.
+  * Returns position and velocity
+  * Guarantees continuity at the position level.
+  * Discouraged because it yields trajectories with discontinuous velocities at the waypoints.
+
+* Cubic:
+
+  * Used, if position and velocity are specified.
+  * Returns position, velocity, and acceleration.
+  * Guarantees continuity at the velocity level.
+
+* Quintic:
+
+  * Used, if position, velocity and acceleration are specified
+  * Returns position, velocity, and acceleration.
+  * Guarantees continuity at the acceleration level.
+
+Trajectories with velocity fields only, velocity and acceleration only, or acceleration fields only can be processed and are accepted, if ``allow_integration_in_goal_trajectories`` is true. Position (and velocity) is then integrated from velocity (or acceleration, respectively) by Heun's method.
+
+Visualized Examples
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+To visualize the difference of the different interpolation methods and their inputs, different trajectories defined at a 0.5s grid and are sampled at a rate of 10ms.
+
+* Sampled trajectory with linear spline if position is given only:
+
+.. image:: spline_position.png
+  :alt: Sampled trajectory with splines if position is given only
+
+* Sampled trajectory with cubic splines if velocity is given only (no deduction for interpolation method ``none``):
+
+.. image:: spline_velocity.png
+  :alt: Sampled trajectory with splines if velocity is given only
+
+* Sampled trajectory if position and velocity is given:
+
+.. note::
+  If the same integration method was used (``Trajectory`` class uses Heun's method), then the ``spline`` method this gives identical results as above where velocity only was given as input.
+
+.. image:: spline_position_velocity.png
+  :alt: Sampled trajectory if position and velocity is given
+
+* Sampled trajectory with quintic splines if acceleration is given only (no deduction for interpolation method ``none``):
+
+.. image:: spline_acceleration.png
+  :alt: Sampled trajectory with splines if acceleration is given only
+
+* Sampled trajectory if position, velocity, and acceleration points are given:
+
+.. note::
+  If the same integration method was used (``Trajectory`` class uses Heun's method), then the ``spline`` method this gives identical results as above where acceleration only was given as input.
+
+.. image:: spline_position_velocity_acceleration.png
+  :alt: Sampled trajectory with splines if position, velocity, and acceleration is given
+
+* Sampled trajectory if the same position, velocity, and acceleration points as above are given, but with a nonzero initial point:
+
+.. image:: spline_position_velocity_acceleration_initialpoint.png
+  :alt: Sampled trajectory with splines if position, velocity, and acceleration is given with nonzero initial point
+
+* Sampled trajectory if the same position, velocity, and acceleration points as above are given but with the first point starting at ``t=0``:
+
+.. note::
+  If the first point is starting at ``t=0``, there is no interpolation from the initial point to the trajectory.
+
+.. image:: spline_position_velocity_acceleration_initialpoint_notime.png
+  :alt: Sampled trajectory with splines if position, velocity, and acceleration is given with nonzero initial point and first point starting at ``t=0``
+
+* Sampled trajectory with splines if inconsistent position, velocity, and acceleration points are given:
+
+.. note::
+  Interpolation method ``none`` only gives the next input points, while the ``spline`` interpolation method shows high overshoot to match the given trajectory points.
+
+.. image:: spline_wrong_points.png
+  :alt: Sampled trajectory with splines if inconsistent position, velocity, and acceleration is given
+
+.. _joint_trajectory_controller_trajectory_replacement:
+
+Trajectory Replacement
+---------------------------------
+*Parts of this documentation were originally published in the ROS 1 wiki under the* `CC BY 3.0 license <https://creativecommons.org/licenses/by/3.0/>`_. [#f1]_
+
+Joint trajectory messages allow to specify the time at which a new trajectory should start executing by means of the header timestamp, where zero time (the default) means "start now".
+
+The arrival of a new trajectory command does not necessarily mean that the controller will completely discard the currently running trajectory and substitute it with the new one.
+Rather, the controller will take the useful parts of both and combine them appropriately, yielding a smarter trajectory replacement strategy.
+
+The steps followed by the controller for trajectory replacement are as follows:
+
+  + Get useful parts of the new trajectory: Preserve all waypoints whose time to be reached is in the future, and discard those with times in the past.
+    If there are no useful parts (ie. all waypoints are in the past) the new trajectory is rejected and the current one continues execution without changes.
+
+  + Get useful parts of the current trajectory: Preserve the current trajectory up to the start time of the new trajectory, discard the later parts.
+
+  + Combine the useful parts of the current and new trajectories.
+
+The following examples describe this behavior in detail.
+
+The first example shows a joint which is in hold position mode (flat grey line labeled *pos hold* in the figure below).
+A new trajectory (shown in red) arrives at the current time (now), which contains three waypoints and a start time in the future (*traj start*).
+The time at which waypoints should be reached (``time_from_start`` member of ``trajectory_msgs/JointTrajectoryPoint``) is relative to the trajectory start time.
+
+The controller splices the current hold trajectory at time *traj start* and appends the three waypoints.
+Notice that between now and *traj start* the previous position hold is still maintained, as the new trajectory is not supposed to start yet.
+After the last waypoint is reached, its position is held until new commands arrive.
+
+.. image:: new_trajectory.png
+  :alt: Receiving a new trajectory.
+
+|
+
+The controller guarantees that the transition between the current and new trajectories will be smooth. Longer times to reach the first waypoint mean slower transitions.
+
+The next examples discuss the effect of sending the same trajectory to the controller with different start times.
+The scenario is that of a controller executing the trajectory from the previous example (shown in red),
+and receiving a new command (shown in green) with a trajectory start time set to either zero (start now),
+a future time, or a time in the past.
+
+.. image:: trajectory_replacement_future.png
+  :alt: Trajectory start time in the future.
+
+|
+
+.. image:: trajectory_replacement_now.png
+  :alt: Zero trajectory start time (start now).
+
+|
+
+Of special interest is the last example, where the new trajectory start time and first waypoint are in the past (before now).
+In this case, the first waypoint is discarded and only the second one is realized.
+
+.. image:: trajectory_replacement_past.png
+  :alt: Trajectory start time in the past.
+
+|
+
+.. [#f1] Adolfo Rodriguez: `Understanding trajectory replacement <http://wiki.ros.org/joint_trajectory_controller/UnderstandingTrajectoryReplacement>`_