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Details about parameters

This controller uses the generate_parameter_library to handle its parameters. The parameter definition file located in the src folder contains descriptions for all the parameters used by the controller.

List of parameters

joints (string_array)

Joint names to control and listen to

Read only: True

Default: {}

Constraints:

contains no duplicates

command_joints (string_array)

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.

Read only: True

Default: {}

Constraints:

contains no duplicates

command_interfaces (string_array)

Command interfaces provided by the hardware interface for all joints

Read only: True

Default: {}

Constraints:

contains no duplicates

every element is one of the list [‘position’, ‘velocity’, ‘acceleration’, ‘effort’]

Custom validator: joint_trajectory_controller::command_interface_type_combinations

state_interfaces (string_array)

State interfaces provided by the hardware for all joints.

Read only: True

Default: {}

Constraints:

contains no duplicates

every element is one of the list [‘position’, ‘velocity’, ‘acceleration’]

Custom validator: joint_trajectory_controller::state_interface_type_combinations

allow_partial_joints_goal (bool)

Allow joint goals defining trajectory for only some joints.

Default: false

open_loop_control (bool)

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).

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.

Read only: True

Default: false

allow_integration_in_goal_trajectories (bool)

Allow integration in goal trajectories to accept goals without position or velocity specified

Default: false

set_last_command_interface_value_as_state_on_activation (bool)

When set to true, the last command interface value is used as both the current state and the last commanded state upon activation. When set to false, the current state is used for both.

Default: true

action_monitor_rate (double)

Rate to monitor status changes when the controller is executing action (control_msgs::action::FollowJointTrajectory)

Read only: True

Default: 20.0

Constraints:

greater than or equal to 0.1

interpolation_method (string)

The type of interpolation to use, if any

Read only: True

Default: “splines”

Constraints:

one of the specified values: [‘splines’, ‘none’]

start_with_holding (bool)

(Deprecated) If true, start with holding position after activation. Otherwise, no command will be sent until the first trajectory is received.

Default: true

allow_nonzero_velocity_at_trajectory_end (bool)

If false, the last velocity point has to be zero or the goal will be rejected

Default: false

cmd_timeout (double)

Timeout after which the input command is considered stale. Timeout is counted from the end of the trajectory (the last point). cmd_timeout must be greater than constraints.goal_time, otherwise ignored. If zero, timeout is deactivated

Default: 0.0

constraints

Default values for tolerances if no explicit values are set in the JointTrajectory message.

constraints.stopped_velocity_tolerance (double)

Velocity tolerance for at the end of the trajectory that indicates that controlled system is stopped.

Default: 0.01

constraints.goal_time (double)

Time tolerance for achieving trajectory goal before or after commanded time. If set to zero, the controller will wait a potentially infinite amount of time.

Default: 0.0

Constraints:

greater than or equal to 0.0

constraints.<joints>.trajectory (double)

Per-joint trajectory offset tolerance during movement.

Default: 0.0

constraints.<joints>.goal (double)

Per-joint trajectory offset tolerance at the goal position.

Default: 0.0

gains

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

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

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

gains.<joints>.p (double)

Proportional gain \(k_p\) for PID

Default: 0.0

gains.<joints>.i (double)

Integral gain \(k_i\) for PID

Default: 0.0

gains.<joints>.d (double)

Derivative gain \(k_d\) for PID

Default: 0.0

gains.<joints>.i_clamp (double)

Integral clamp. Symmetrical in both positive and negative direction.

Default: 0.0

gains.<joints>.ff_velocity_scale (double)

Feed-forward scaling \(k_{ff}\) of velocity

Default: 0.0

gains.<joints>.normalize_error (bool)

(Deprecated) Use position error normalization to -pi to pi.

Default: false

gains.<joints>.angle_wraparound (bool)

For joints that wrap around (without end stop, ie. are continuous), where the shortest rotation to the target position is the desired motion. 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.

Default: false

An example parameter file

joint_trajectory_controller:
  ros__parameters:
    action_monitor_rate: 20.0
    allow_integration_in_goal_trajectories: false
    allow_nonzero_velocity_at_trajectory_end: false
    allow_partial_joints_goal: false
    cmd_timeout: 0.0
    command_interfaces: '{}'
    command_joints: '{}'
    constraints:
      <joints>:
        goal: 0.0
        trajectory: 0.0
      goal_time: 0.0
      stopped_velocity_tolerance: 0.01
    gains:
      <joints>:
        angle_wraparound: false
        d: 0.0
        ff_velocity_scale: 0.0
        i: 0.0
        i_clamp: 0.0
        normalize_error: false
        p: 0.0
    interpolation_method: splines
    joints: '{}'
    open_loop_control: false
    set_last_command_interface_value_as_state_on_activation: true
    start_with_holding: true
    state_interfaces: '{}'