mecanum_drive

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= mecanum_drive -- Mecanum-Drive Controller :imagesdir: ./images

This package implements ROS nodes to control and monitor a mecanum-drive robot.

The package is intended as a lighter-weight solution than the ROS controller framework, albeit with lower performance since it is written in Python. If you need tight, real-time control, you may want to look at link:http://wiki.ros.org/ros_controllers[ros_controllers]. Those controllers are designed to integrate with hardware in the same process, rather than using topics. Instead, this package expects to publish the desired motor speeds using standard ROS messages.

== Supplied Nodes

• mecanum_drive_controller -- Converts from twist to wheel velocities for motors.
• mecanum_drive_odometry -- Publishes odometry from wheel encoder data.

The nodes in this package are designed with these considerations:

• The node and hardware implementing mecanum drive should deal only in encoder ticks.
• Conversions to and from physical coordinates should happen within the nodes in this package.
• This package should integrate cleanly with the navigation stack, perhaps with remappings.
• Nodes should use standard topic and parameter names used by the navigation stack, but should allow remapping.

== ROS API

=== 1. mecanum_drive_controller

Listens for desired linear and angular velocity, and publishes corresponding wheel velocities, in encoder ticks per second, required to achieve those velocities.

==== Published Topics

~wheels_desired_rate (std_msgs/Int16MultiArray):: Desired [front left, front right, rear left, rear right] wheel rotation rates, in encoder ticks per second.

==== Subscribed Topics

~cmd_vel (geometry_msgs/Twist):: Desired linear and angular velocity.

==== Parameters

~ticks_per_meter (double):: Number of encoder ticks per meter of travel.

~wheel_separation (double):: Distance between the two wheels on the same axis (meters).

~wheel_separation_length (double):: Distance between the front and rear axis (meters).

~rate (int, default: 50):: The rate that the output velocity target messages will be published (Hz).

~timeout_ticks (int, default: 2):: The number of velocity target messages that will be published after the last twist message is received.

=== 2. mecanum_drive_odometry

Listens for wheel movement and rates and publishes the transform between the odom frame and the robot frame.

==== Published Topics

~odom -- (nav_msgs/Odometry):: The robot odometry -- the current robot pose.

~tf:: The transform between the odometry frame and the robot frame.

==== Subscribed Topics

~wheel_ticks (std_msgs/Int16MultiArray):: Cumulative encoder ticks of the [front left, front right, rear left, rear right] wheels.

==== Parameters

~ticks_per_meter (double):: Number of encoder ticks per meter of travel.

~wheel_separation (double):: Distance between the two wheels (m).

~wheel_separation_length (double):: Distance between the front and rear axis (meters).

~rate (double, default 10.0):: The rate at which the tf and odom topics are published (Hz).

~timeout (double, default 0.2):: The amount of time to continue publishing desired wheel rates after receiving a twist message (seconds). If set to zero, wheel velocities will be sent only when a new twist message is received.

~base_frame_id (string, default: "base_link"):: The name of the base frame of the robot.

~odom_frame_id (string, default: "odom"):: The name of the odometry reference frame.

~encoder_min (int, default: -32768)::

~encoder_max (int, default: 32768):: The min and max value the encoder should output. Used to calculate odometry when the values wrap around.

~wheel_low_wrap (int, default: 0.3 * (encoder_max - encoder_min + 1) + encoder_min)::

~wheel_high_wrap (int, default: 0.7 * (encoder_max - encoder_min + 1) + encoder_min):: If a reading is greater than wheel_high_wrap and the next reading is less than wheel_low_wrap, then the reading has wrapped around in the positive direction, and the odometry will be calculated appropriately. The same concept applies for the negative direction.