diff_drive_controller
Controller for mobile robots with differential drive.
As input it takes velocity commands for the robot body, which are translated to wheel commands for the differential drive base.
Odometry is computed from hardware feedback and published.
For an introduction to mobile robot kinematics and the nomenclature used here, see Wheeled Mobile Robot Kinematics.
Other features
Realtime-safe implementation.
Odometry publishing
Task-space velocity, acceleration and jerk limits
Automatic stop after command time-out
Description of controller’s interfaces
References
(the controller is not yet implemented as chainable controller)
Feedback
As feedback interface type the joints’ position (hardware_interface::HW_IF_POSITION
) or velocity (hardware_interface::HW_IF_VELOCITY
,if parameter position_feedback=false
) are used.
Output
Joints’ velocity (hardware_interface::HW_IF_VELOCITY
) are used.
ROS 2 Interfaces
Subscribers
- ~/cmd_vel [geometry_msgs/msg/TwistStamped]
Velocity command for the controller. The controller extracts the x component of the linear velocity and the z component of the angular velocity. Velocities on other components are ignored.
Publishers
- ~/odom [nav_msgs::msg::Odometry]
This represents an estimate of the robot’s position and velocity in free space.
- /tf [tf2_msgs::msg::TFMessage]
tf tree. Published only if
enable_odom_tf=true
- ~/cmd_vel_out [geometry_msgs/msg/TwistStamped]
Velocity command for the controller, where limits were applied. Published only if
publish_limited_velocity=true
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.
- left_wheel_names (string_array)
Names of the left side wheels’ joints
Default: {}
Constraints:
parameter is not empty
- right_wheel_names (string_array)
Names of the right side wheels’ joints
Default: {}
Constraints:
parameter is not empty
- wheel_separation (double)
Shortest distance between the left and right wheels. If this parameter is wrong, the robot will not behave correctly in curves.
Default: 0.0
Constraints:
greater than 0.0
- wheel_radius (double)
Radius of a wheel, i.e., wheels size, used for transformation of linear velocity into wheel rotations. If this parameter is wrong the robot will move faster or slower then expected.
Default: 0.0
Constraints:
greater than 0.0
- wheel_separation_multiplier (double)
Correction factor for wheel separation (TODO(destogl): Please help me describe this correctly)
Default: 1.0
- left_wheel_radius_multiplier (double)
Correction factor when radius of left wheels differs from the nominal value in
wheel_radius
parameter.Default: 1.0
- right_wheel_radius_multiplier (double)
Correction factor when radius of right wheels differs from the nominal value in
wheel_radius
parameter.Default: 1.0
- tf_frame_prefix_enable (bool)
Enables or disables appending tf_prefix to tf frame id’s.
Default: true
- tf_frame_prefix (string)
(optional) Prefix to be appended to the tf frames, will be added to odom_id and base_frame_id before publishing. If the parameter is empty, controller’s namespace will be used.
Default: “”
- odom_frame_id (string)
Name of the frame for odometry. This frame is parent of
base_frame_id
when controller publishes odometry.Default: “odom”
- base_frame_id (string)
Name of the robot’s base frame that is child of the odometry frame.
Default: “base_link”
- pose_covariance_diagonal (double_array)
Odometry covariance for the encoder output of the robot for the pose. These values should be tuned to your robot’s sample odometry data, but these values are a good place to start:
[0.001, 0.001, 0.001, 0.001, 0.001, 0.01]
.Default: {0.0, 0.0, 0.0, 0.0, 0.0, 0.0}
- twist_covariance_diagonal (double_array)
Odometry covariance for the encoder output of the robot for the speed. These values should be tuned to your robot’s sample odometry data, but these values are a good place to start:
[0.001, 0.001, 0.001, 0.001, 0.001, 0.01]
.Default: {0.0, 0.0, 0.0, 0.0, 0.0, 0.0}
- open_loop (bool)
If set to true the odometry of the robot will be calculated from the commanded values and not from feedback.
Default: false
- position_feedback (bool)
Is there position feedback from hardware.
Default: true
- enable_odom_tf (bool)
Publish transformation between
odom_frame_id
andbase_frame_id
.Default: true
- cmd_vel_timeout (double)
Timeout in seconds, after which input command on
cmd_vel
topic is considered staled.Default: 0.5
- publish_limited_velocity (bool)
Publish limited velocity value.
Default: false
- velocity_rolling_window_size (int)
Size of the rolling window for calculation of mean velocity use in odometry.
Default: 10
- publish_rate (double)
Publishing rate (Hz) of the odometry and TF messages.
Default: 50.0
- linear.x
Joint limits structure for the linear
x
-axis. The limiter ignores position limits. For details seejoint_limits
package from ros2_control repository.- linear.x.has_velocity_limits (bool)
Default: false
- linear.x.has_acceleration_limits (bool)
Default: false
- linear.x.has_jerk_limits (bool)
Default: false
- linear.x.max_velocity (double)
Default: std::numeric_limits<double>::quiet_NaN()
- linear.x.min_velocity (double)
Default: std::numeric_limits<double>::quiet_NaN()
- linear.x.max_acceleration (double)
Default: std::numeric_limits<double>::quiet_NaN()
- linear.x.min_acceleration (double)
Default: std::numeric_limits<double>::quiet_NaN()
- linear.x.max_jerk (double)
Default: std::numeric_limits<double>::quiet_NaN()
- linear.x.min_jerk (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z
Joint limits structure for the rotation about
z
-axis. The limiter ignores position limits. For details seejoint_limits
package from ros2_control repository.- angular.z.has_velocity_limits (bool)
Default: false
- angular.z.has_acceleration_limits (bool)
Default: false
- angular.z.has_jerk_limits (bool)
Default: false
- angular.z.max_velocity (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z.min_velocity (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z.max_acceleration (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z.min_acceleration (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z.max_jerk (double)
Default: std::numeric_limits<double>::quiet_NaN()
- angular.z.min_jerk (double)
Default: std::numeric_limits<double>::quiet_NaN()
An example parameter file for this controller can be found in the test directory:
test_diff_drive_controller:
ros__parameters:
left_wheel_names: ["left_wheels"]
right_wheel_names: ["right_wheels"]
wheel_separation: 0.40
wheel_radius: 0.02
wheel_separation_multiplier: 1.0
left_wheel_radius_multiplier: 1.0
right_wheel_radius_multiplier: 1.0
odom_frame_id: odom
base_frame_id: base_link
pose_covariance_diagonal: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
twist_covariance_diagonal: [0.0, 0.0, 0.0, 0.0, 0.0, 0.0]
position_feedback: false
open_loop: true
enable_odom_tf: true
cmd_vel_timeout: 0.5 # seconds
publish_limited_velocity: true
velocity_rolling_window_size: 10
linear.x.has_velocity_limits: false
linear.x.has_acceleration_limits: false
linear.x.has_jerk_limits: false
linear.x.max_velocity: 0.0
linear.x.min_velocity: 0.0
linear.x.max_acceleration: 0.0
linear.x.max_jerk: 0.0
linear.x.min_jerk: 0.0
angular.z.has_velocity_limits: false
angular.z.has_acceleration_limits: false
angular.z.has_jerk_limits: false
angular.z.max_velocity: 0.0
angular.z.min_velocity: 0.0
angular.z.max_acceleration: 0.0
angular.z.min_acceleration: 0.0
angular.z.max_jerk: 0.0
angular.z.min_jerk: 0.0