# joint_trajectory_controller

Controller for executing joint-space trajectories on a group of joints. Trajectories are specified as a set of waypoints to be reached at specific time instants, which the controller attempts to execute as well as the mechanism allows. Waypoints consist of positions, and optionally velocities and accelerations.

## Trajectory representation

The controller is templated to work with multiple trajectory representations. By default, a spline interpolator is provided, but it’s possible to support other representations. The spline interpolator uses the following interpolation strategies depending on the waypoint specification:

Linear: Only position is specified. Guarantees continuity at the position level. Discouraged because it yields trajectories with discontinuous velocities at the waypoints.

Cubic: Position and velocity are specified. Guarantees continuity at the velocity level.

Quintic: Position, velocity and acceleration are specified: Guarantees continuity at the acceleration level.

## Hardware interface type

The controller is templated to work with multiple hardware interface types. Currently joints with position, velocity and effort interfaces are supported. For position-controlled joints, desired positions are simply forwarded to the joints; while for velocity (effort) joints, the position+velocity trajectory following error is mapped to velocity (effort) commands through a PID loop. Example controller configurations can be found here.

Similarly to the trajectory representation case above, it’s possible to support new hardware interfaces, or alternative mappings to an already supported interface (eg. a proxy controller for generating effort commands).

## Other features

Realtime-safe implementation.

Proper handling of wrapping (continuous) joints.

Robust to system clock changes: Discontinuous system clock changes do not cause discontinuities in the execution of already queued trajectory segments.

## ros2_control interfaces

### References

(the controller is not yet implemented as chainable controller)

### States

The state interfaces are defined with joints and state_interfaces parameters as follows: <joint>/<state_interface>. Supported state interfaces are position, velocity, acceleration and effort as defined in the hardware_interface/hardware_interface_type_values.hpp. Legal combinations of state interfaces are:

• position

• position and velocity

• position, velocity and acceleration

• effort

## Using Joint Trajectory Controller(s)

The controller expects at least position feedback from the hardware. Joint velocities and accelerations are optional. Currently the controller does not internally integrate velocity from acceleration and position from velocity. Therefore if the hardware provides only acceleration or velocity states they have to be integrated in the hardware-interface implementation of velocity and position to use these controllers.

The generic version of Joint Trajectory controller is implemented in this package. A yaml file for using it could be:

controller_manager:
ros__parameters:
joint_trajectory_controller:
type: "joint_trajectory_controller/JointTrajectoryController"

joint_trajectory_controller:
ros__parameters:
joints:
- joint1
- joint2
- joint3
- joint4
- joint5
- joint6

command_interfaces:
- position

state_interfaces:
- position
- velocity

action_monitor_rate: 20.0

allow_partial_joints_goal: false
open_loop_control: true
constraints:
stopped_velocity_tolerance: 0.01
goal_time: 0.0
joint1:
trajectory: 0.05
goal: 0.03


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.

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 using ignoring the states provided by hardware interface and using last commands as states in the next control step. This is useful if hardware states are not following commands, i.e., an offset between those (typical for hydraulic manipulators).

Default: false

If this flag is set, the controller tries to read the values from the command interfaces on starting. If they have real numeric values, those will be used instead of state interfaces. Therefore it is important set command interfaces to NaN (std::numeric_limits<double>::quiet_NaN()) or state values when the hardware is started.

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)

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

$u = k_{ff} v_d + k_p e + k_i \sum e dt + k_d (v_d - v)$

with the desired velocity $$v_d$$, the measured velocity $$v$$, the position error $$e$$ (definition see below), the controller period $$dt$$, and the velocity or effort manipulated variable (control variable) $$u$$, respectively.

gains.<joint_name>.p (double)

Proportional gain $$k_p$$ for PID

Default: 0.0

gains.<joint_name>.i (double)

Integral gain $$k_i$$ for PID

Default: 0.0

gains.<joint_name>.d (double)

Derivative gain $$k_d$$ 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 $$k_{ff}$$ of velocity

Default: 0.0

gains.<joint_name>.normalize_error (bool)

If true, the position error $$e = normalize(s_d - s)$$ is normalized between $$-\pi, \pi$$. Otherwise $$e = s_d - s$$ is used, with the desired position $$s_d$$ and the measured position $$s$$ 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

### ROS2 interface of the controller

~/joint_trajectory (input topic) [trajectory_msgs::msg::JointTrajectory]

Topic for commanding the controller.

~/state (output topic) [control_msgs::msg::JointTrajectoryControllerState]

Topic publishing internal states with the update-rate of the controller manager.

~/follow_joint_trajectory (action server) [control_msgs::action::FollowJointTrajectory]

Action server for commanding the controller.

## Specialized versions of JointTrajectoryController (TBD in …)

The controller types are placed into namespaces according to their command types for the hardware (see general introduction into controllers).

The following version of the Joint Trajectory Controller are available mapping the following interfaces:

• position_controllers::JointTrajectoryController - Input: position, [velocity, [acceleration]] - Output: position

• position_velocity_controllers::JointTrajectoryController - Input: position, [velocity, [acceleration]] - Output: position and velocity

• position_velocity_acceleration_controllers::JointTrajectoryController - Input: position, [velocity, [acceleration]] - Output: position, velocity and acceleration

(Not implemented yet) When using pure velocity or effort controllers a command is generated using the desired state and state error using a velocity feedforward term plus a corrective PID term. (#171)