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steering_controllers_library

Library with shared functionalities for mobile robot controllers with steering drives, with so-called non-holonomic constraints.

The library implements generic odometry and update methods and defines the main interfaces.

The update methods only use inverse kinematics, it does not implement any feedback control loops like path-tracking controllers etc.

For an introduction to mobile robot kinematics and the nomenclature used here, see Wheeled Mobile Robot Kinematics.

Execution logic of the controller

The controller uses velocity input, i.e., stamped twist messages where linear x and angular z components are used. 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

  • Bicycle - with one steering and one drive joints;

  • Tricylce - with one steering and two drive joints;

  • Ackermann - with two steering and two drive joints.

Description of controller’s interfaces

References (from a preceding controller)

Used when controller is in chained mode (in_chained_mode == true).

  • <controller_name>/linear/velocity double, in m/s

  • <controller_name>/angular/position double, in rad

Command interfaces

If parameter front_steering == true

  • <front_wheels_names[i]>/position double, in rad

  • <rear_wheels_names[i]>/velocity double, in m/s

If parameter front_steering == false

  • <front_wheels_names[i]>/velocity double, in m/s

  • <rear_wheels_names[i]>/position double, in rad

State interfaces

Depending on the position_feedback, different feedback types are expected

  • position_feedback == true –> TRACTION_FEEDBACK_TYPE = position

  • position_feedback == false –> TRACTION_FEEDBACK_TYPE = velocity

If parameter front_steering == true

  • <front_wheels_names[i]>/position double, in rad

  • <rear_wheels_names[i]>/<TRACTION_FEEDBACK_TYPE> double, in m or m/s

If parameter front_steering == false

  • <front_wheels_names[i]>/<TRACTION_FEEDBACK_TYPE> double, in 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).

Publishers

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