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steering_controllers_library

Library with shared functionalities for mobile robot controllers with steering drive (2 degrees of freedom). The library implements generic odometry and update methods and defines the main interfaces.

Nomenclature used for the controller is used from wikipedia.

Execution logic of the controller

The controller uses velocity input, i.e., stamped or unstamped Twist messages where linear x and angular z components are used. Angular component under Values in other components are ignored. In the chain mode the controller provides two reference interfaces, one for linear velocity and one for steering angle position. Other relevant features are:

support for front and rear steering configurations;
odometry publishing as Odometry and TF message;
input command timeout based on a parameter.

The command for the wheels are calculated using odometry library where based on concrete kinematics traction and steering commands are calculated. Currently implemented kinematics in corresponding packages are:

Bicycle - with one steering and one drive joints;
Tricylce - with one steering and two drive joints;
Ackermann - with two seering and two drive joints.

Description of controller’s interfaces

References (from a preceding controller)

<controller_name>/linear/velocity [double], in m/s
<controller_name>/angular/position [double] # in [rad]

Commands

front_steering == true

<front_wheels_names[i]>/position [double] # in [rad]
<rear_wheels_names[i]>/velocity [double] # in [m/s]

front_steering == false

<front_wheels_names[i]>/velocity [double] # in [m/s]
<rear_wheels_names[i]>/position [double] # in [rad]

States

Depending on the position_feedback, different feedback types are expected

position_feedback == true –> TRACTION_FEEDBACK_TYPE = position
position_feedback == false –> TRACTION_FEEDBACK_TYPE = velocity

front_steering == true

<front_wheels_names[i]>/position [double] # in [rad]
<rear_wheels_names[i]>/<TRACTION_FEEDBACK_TYPE> [double] # in [m] or [m/s]

front_steering == false

<front_wheels_names[i]>/<TRACTION_FEEDBACK_TYPE> [double] # [m] or [m/s]
<rear_wheels_names[i]>/position [double] # in [rad]

Subscribers

Used when controller is not in chained mode (in_chained_mode == false).

<controller_name>/reference [geometry_msgs/msg/TwistStamped] If parameter use_stamped_vel is true.
<controller_name>/reference_unstamped [geometry_msgs/msg/Twist] If parameter use_stamped_vel is false.

Publishers

<controller_name>/odometry [nav_msgs/msg/Odometry]
<controller_name>/tf_odometry [tf2_msgs/msg/TFMessage]
<controller_name>/controller_state [control_msgs/msg/SteeringControllerStatus]

Parameters

This controller uses the generate_parameter_library to handle its parameters.

For an exemplary parameterization see the test folder of the controller’s package.

reference_timeout (double)

Timeout for controller references after which they will be reset. This is especially useful for controllers that can cause unwanted and dangerous behaviour if reference is not reset, e.g., velocity controllers. If value is 0 the reference is reset after each run.

Default: 1.0

front_steering (bool)

Is the steering on the front of the robot?

Read only: True

Default: true

rear_wheels_names (string_array)

Names of rear wheel joints.

Read only: True

Constraints:

length is less than 5

contains no duplicates

parameter is not empty

front_wheels_names (string_array)

Names of front wheel joints.

Read only: True

Constraints:

length is less than 5

contains no duplicates

parameter is not empty

rear_wheels_state_names (string_array)

(Optional) Names of rear wheel joints to read states from. If not set joint names from ‘rear_wheels_names’ will be used.

Read only: True

Default: {}

Constraints:

length is less than 5

contains no duplicates

front_wheels_state_names (string_array)

(Optional) Names of front wheel joints to read states from. If not set joint names from ‘front_wheels_names’ will be used.

Read only: True

Default: {}

Constraints:

length is less than 5

contains no duplicates

open_loop (bool)

Choose if open-loop or not (feedback) is used for odometry calculation.

Default: false

velocity_rolling_window_size (int)

The number of velocity samples to average together to compute the odometry twist.linear.x and twist.angular.z velocities.

Default: 10

base_frame_id (string)

Base frame_id set to value of base_frame_id.

Default: “base_link”

odom_frame_id (string)

Odometry frame_id set to value of odom_frame_id.

Default: “odom”

enable_odom_tf (bool)

Publishing to tf is enabled or disabled?

Default: true

twist_covariance_diagonal (double_array)

diagonal values of twist covariance matrix.

Default: {0.0, 7.0, 14.0, 21.0, 28.0, 35.0}

pose_covariance_diagonal (double_array)

diagonal values of pose covariance matrix.

Default: {0.0, 7.0, 14.0, 21.0, 28.0, 35.0}

position_feedback (bool)

Choice of feedback type, if position_feedback is false then HW_IF_VELOCITY is taken as interface type, if position_feedback is true then HW_IF_POSITION is taken as interface type

Default: false

use_stamped_vel (bool)

Choice of vel type, if use_stamped_vel is false then geometry_msgs::msg::Twist is taken as vel msg type, if use_stamped_vel is true then geometry_msgs::msg::TwistStamped is taken as vel msg type

Default: false