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PID Controller

PID Controller implementation that uses PidROS implementation from control_toolbox package. The controller can be used directly by sending references through a topic or in a chain having preceding or following controllers. It also enables to use the first derivative of the reference and its feedback to have second-order PID control.

Depending on the reference/state and command interface of the hardware a different parameter setup of PidROS should be used as for example:

reference/state POSITION; command VELOCITY –> PI CONTROLLER
reference/state VELOCITY; command ACCELERATION –> PI CONTROLLER
reference/state VELOCITY; command POSITION –> PD CONTROLLER
reference/state ACCELERATION; command VELOCITY –> PD CONTROLLER
reference/state POSITION; command POSITION –> PID CONTROLLER
reference/state VELOCITY; command VELOCITY –> PID CONTROLLER
reference/state ACCELERATION; command ACCELERATION –> PID CONTROLLER
reference/state EFFORT; command EFFORT –> PID CONTROLLER

Note

Theoretically one can misuse Joint Trajectory Controller (JTC) for the same purpose by sending only one reference point into it. Nevertheless, this is not recommended. JTC should be used if you need to interpolate between trajectory points using linear, cubic or quintic interpolation. PID Controller doesn’t do that. PID term of JTC has different purpose - it enables commanding only velocity or effort interfaces to hardware.

Execution logic of the controller

The controller can be also used in “feed-forward” mode where feed-forward gain is used to increase controllers dynamics. If one type of the reference and state interfaces is used, only immediate error is used. If there are two, then the second interface type is considered to be the first derivative of the first type. For example a valid combination would be position and velocity interface types.

Using the controller

Pluginlib-Library: pid_controller Plugin name: pid_controller/PidController

Description of controller’s interfaces

References (from a preceding controller)

<reference_and_state_dof_names[i]>/<reference_and_state_interfaces[j]> [double] NOTE: reference_and_state_dof_names[i] can be from reference_and_state_dof_names parameter, or if it is empty then dof_names.

Commands

<dof_names[i]>/<command_interface> [double]

States

<reference_and_state_dof_names[i]>/<reference_and_state_interfaces[j]> [double] NOTE: reference_and_state_dof_names[i] can be from reference_and_state_dof_names parameter, or if it is empty then dof_names.

Subscribers

If controller is not in chained mode (in_chained_mode == false):

<controller_name>/reference [control_msgs/msg/MultiDOFCommand]

If controller parameter use_external_measured_states is true:

<controller_name>/measured_state [control_msgs/msg/MultiDOFCommand]

Services

<controller_name>/set_feedforward_control [std_srvs/srv/SetBool]

Publishers

<controller_name>/controller_state [control_msgs/msg/MultiDOFStateStamped]

Parameters

The PID controller uses the generate_parameter_library to handle its parameters.

List of parameters

dof_names (string_array)

Specifies dof_names or axes used by the controller. If ‘reference_and_state_dof_names’ parameter is defined, then only command dof names are defined with this parameter.

Read only: True

Default: {}

Constraints:

contains no duplicates

parameter is not empty

reference_and_state_dof_names (string_array)

(optional) Specifies dof_names or axes for getting reference and reading states. This parameter is only relevant when state dof names are different then command dof names, i.e., when a following controller is used.

Read only: True

Default: {}

Constraints:

contains no duplicates

command_interface (string)

Name of the interface used by the controller for writing commands to the hardware.

Read only: True

Default: “”

Constraints:

parameter is not empty

reference_and_state_interfaces (string_array)

Name of the interfaces used by the controller getting hardware states and reference commands. The second interface should be the derivative of the first.

Read only: True

Default: {}

Constraints:

parameter is not empty

length is less than 3

use_external_measured_states (bool)

Use external states from a topic instead from state interfaces.

Default: false

gains.<dof_names>.p (double)

Proportional gain for PID

Default: 0.0

gains.<dof_names>.i (double)

Integral gain for PID

Default: 0.0

gains.<dof_names>.d (double)

Derivative gain for PID

Default: 0.0

gains.<dof_names>.antiwindup (bool)

Antiwindup functionality. When set to true, limits the integral error to prevent windup; otherwise, constrains the integral contribution to the control output. i_clamp_max and i_clamp_min are applied in both scenarios.

Default: false

gains.<dof_names>.i_clamp_max (double)

Upper integral clamp.

Default: 0.0

gains.<dof_names>.i_clamp_min (double)

Lower integral clamp.

Default: 0.0

gains.<dof_names>.feedforward_gain (double)

Gain for the feed-forward part.

Default: 0.0

gains.<dof_names>.angle_wraparound (bool)

For joints that wrap around (i.e., are continuous). Normalizes position-error to -pi to pi.

Default: false

An example parameter file

An example parameter file for this controller can be found in the test folder (standalone):

test_pid_controller:
  ros__parameters:
    dof_names:
      - joint1

    command_interface: position

    reference_and_state_interfaces: ["position"]

    gains:
      joint1: {p: 1.0, i: 2.0, d: 10.0, i_clamp_max: 5.0, i_clamp_min: -5.0}

or as preceding controller:

test_pid_controller:
  ros__parameters:
    dof_names:
      - joint1

    command_interface: position

    reference_and_state_interfaces: ["position"]

    reference_and_state_dof_names:
      - joint1state