omni_wheel_controller

Omni wheel controllers to accompany ros2_control (unofficial) This package is based on diff_drive_controller.

• This package checked in docker "ros:humble" image. Sample code is here.

Overview

Controller for omni wheel robot. Control is in the form of a velocity command, that is split then sent on the wheels of a omni wheel robot. Odometry is computed from the feedback from the hardware, and published.

Velocity commands

The controller works with a velocity twist from which it extracts the x component of the linear velocity and the z component of the angular velocity. Velocities on other components are ignored.

Hardware interface type

The controller works with wheel joints through a velocity interface.

Other features

• Realtime-safe implementation.
• Odometry publishing
• Task-space velocity, acceleration and jerk limits
• Automatic stop after command time-out

Mathematical Background

The illustration on the below shows the pattern diagram of one wheel in the Omni wheel robot. In this diagram, the following notation applies.

• $V$

  Robot's linear velocity

• $\Omega$

  Robot's angular velocity

• $x_n, y_n$

  n-th omni wheel position.

• $l_n$

  Distance between n-th omni wheel and robot's center. $l_n = \sqrt{x_n^2+y_n^2}$ This package expects same distance $l$.

• $r$ Omni wheel's radius.

• $\theta_n$ n-th omni wheel's direction In this package, the omniwheel is expected to be perpendicular to the direction vector from the center of the robot.

• $\omega_n$ n-th omni wheel's velocity

This controller calculates the n-th omni wheel's velocity $\omega_n$ to: $\omega_n = -\frac{1}{r}(V_x \cos{\theta_n} + V_y \sin{\theta_n} + \Omega l \sin{(\theta_n - \arctan{\frac{y_n}{x_n}})})$

ROS API

Description

The controller main input is a geometry_msgs::Twist topic in the namespace of the controller.

Subscribed Topics

• cmd_vel (geometry_msgs/Twist)

  Velocity command.

Published Topics

• odom (nav_msgs/Odometry)

  Odometry computed from the hardware feedback.

• /tf (tf/tfMessage)

  Transform from odom to base_footprint

• publish_cmd (geometry_msgs/TwistStamped)

  Available when "publish_cmd" parameter is set to True. It is the Twist after limiters have been applied on the controller input.

Parameters

• omni_wheel_names (string | string[...])

  Omni wheel joint names list

• omni_wheel_angle (double | double[...])

  Omni wheel direction $\theta_n$ list

• pose_covariance_diagonal (double[6])

  Diagonal of the covariance matrix for odometry pose publishing

• twist_covariance_diagonal (double[6])

  Diagonal of the covariance matrix for odometry twist publishing

• publish_rate (double, default: 50.0)

  Frequency (in Hz) at which the odometry is published. Used for both tf and odom

• cmd_vel_timeout (double, default: 0.5)

  Allowed period (in s) allowed between two successive velocity commands. After this delay, a zero speed command will be sent to the wheels.

• base_frame_id (string, default: base_link)

  Base frame_id, which is used to fill in the child_frame_id of the Odometry messages and TF.

• linear/x/has_velocity_limits (bool, default: false)

  Whether the controller should limit linear speed or not.

• linear/x/max_velocity (double)

  Maximum linear velocity (in m/s)

• linear/x/min_velocity (double)

  Minimum linear velocity (in m/s). Setting this to 0.0 will disable backwards motion. When unspecified, -max_velocity is used.

• linear/x/has_acceleration_limits (bool, default: false)

  Whether the controller should limit linear acceleration or not.

• linear/x/max_acceleration (double)

  Maximum linear acceleration (in m/s^2)

• linear/x/min_acceleration (double)

  Minimum linear acceleration (in m/s^2). When unspecified, -max_acceleration is used.

• linear/x/has_jerk_limits (bool, default: false)

  Whether the controller should limit linear jerk or not.

• linear/x/max_jerk (double)

  Maximum linear jerk (in m/s^3).

• angular/z/has_velocity_limits (bool, default: false)

  Whether the controller should limit angular velocity or not.

• angular/z/max_velocity (double)

  Maximum angular velocity (in rad/s)

• angular/z/min_velocity (double)

  Minimum angular velocity (in rad/s). Setting this to 0.0 will disable counter-clockwise rotation. When unspecified, -max_velocity is used.

• angular/z/has_acceleration_limits (bool, default: false)

  Whether the controller should limit angular acceleration or not.

• angular/z/max_acceleration (double)

  Maximum angular acceleration (in rad/s^2)

• angular/z/min_acceleration (double)

  Minimum angular acceleration (in rad/s^2). When unspecified, -max_acceleration is used.

• angular/z/has_jerk_limits (bool, default: false)

  Whether the controller should limit angular jerk or not.

• angular/z/max_jerk (double)

  Maximum angular jerk (in m/s^3).

• enable_odom_tf (bool, default: true)

  Publish to TF directly or not

• omni_wheel_distance (double)

  The distance between the wheel and robot's center $l$. It is expected they all have the same size.

• wheel_radius (double)

  Radius of the wheels $r$. It is expected they all have the same size.

• odom_frame_id (string, default: "/odom")

  Name of frame to publish odometry in.

• publish_cmd (bool, default: False)

  Publish the velocity command to be executed. It is to monitor the effect of limiters on the controller input.

• velocity_rolling_window_size (int, default: 10)

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

Controller configuration examples

Minimal description

T.B.D.

Complete description

omni_wheel_controller:
  ros__parameters:
    omni_wheel_names  : ['wheel0_shaft_joint', 'wheel1_shaft_joint', 'wheel2_shaft_joint', 'wheel3_shaft_joint']
    omni_wheel_angle  : [0.785398, 2.356194, -2.356194, -0.785398]

    omni_wheel_distance : 0.49
    wheel_radius : 0.05

    odom_frame_id: odom
    base_frame_id: base_link

    pose_covariance_diagonal : [0.001, 0.001, 1000000.0, 1000000.0, 1000000.0, 1000.0]
    twist_covariance_diagonal: [0.001, 0.001, 1000000.0, 1000000.0, 1000000.0, 1000.0]

    enable_odom_tf: true

    cmd_vel_timeout: 3.0
    publish_limited_velocity: true
    velocity_rolling_window_size: 10

    # limits
    linear.x.has_velocity_limits: true
    linear.x.has_acceleration_limits: false
    linear.x.has_jerk_limits: false
    linear.x.max_velocity: 1.0
    linear.x.min_velocity: -1.0
    linear.x.max_acceleration: 0.4
    linear.x.min_acceleration: -0.4
    linear.x.max_jerk: 0.5
    linear.x.min_jerk: -0.5

    angular.z.has_velocity_limits: true
    angular.z.has_acceleration_limits: false
    angular.z.has_jerk_limits: false
    angular.z.max_velocity: 1.5
    angular.z.min_velocity: -1.5
    angular.z.max_acceleration: 0.8
    angular.z.min_acceleration: -0.8
    angular.z.max_jerk: 0.5
    angular.z.min_jerk: -0.5

Note

"rcppmath/rolling_mean_accumulator.hpp" is added in rcpputils ver 2.2.0

"rcpputils/rolling_mean_accumulator.hpp" is added in rcpputils ver 2.6.0

Note that the files that can be included depend on the version of rcpputils.