diff_drive — Differential-Drive Controller

This package implements ROS nodes to control and monitor a differential-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 ros_controllers, a C++ package which includes a differential-drive controller that is integrated with ros_control and controller_manager. 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

• diff_drive_controller — Converts from twist to wheel velocities for motors.

• diff_drive_odometry — Publishes odometry from wheel encoder data.

• diff_drive_go_to_goal — Moves the robot to a goal position.

• diff_drive_mock_robot — Implements a mock differential drive robot, for testing.

The nodes in this package are designed with these considerations:

• The node and hardware implementing differential 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.

Demo

To see all the nodes in this package in action, you can launch a demo from ROS. There are no dependencies other than the standard ROS packages.

roslaunch diff_drive demo.launch

This launches rviz as part of the demo, and shows the robot position as a small coordinate system on a 0.25m grid. In rviz you can move the robot by clicking the 2D Nav Goal button in the tools panel at the top. Then click and drag within the grid to set the robot goal position and heading. The mock robot will move to that new pose, which you can see by the movement of the robot axes.

In the demo, both forward and backward movement is allowed, so if the goal position is behind the robot, it will move backward. You can force the robot to move foward only by setting the parameter ~forwardMovementOnly to true.

ROS API

1. diff_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

~lwheel_desired_rate (std_msgs/Int32)

Desired left wheel rotation rate, in encoder ticks per second.

~rwheel_desired_rate (std_msgs/Int32)

Desired right wheel rotation rate, 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 (meters).

~rate (int, default: 50)

The rate that the output velocity target messages will be published (Hz).

~timeout (int, default: 0.2)

The maximum number of seconds expected between cmd_vel messages. If cmd_vel is not received before this limit, the controller will assume the commanding node has died and will set the desired wheel rates to zero, to stop the robot.

2. diff_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

~lwheel_ticks (std_msgs/Int32)

Cumulative encoder ticks of the left wheel.

~rwheel_ticks (std_msgs/Int32)

Cumulative encoder ticks of the right wheel.

~lwheel_rate (std_msgs/Float32)

Left wheel rotation rate, in encoder ticks per second.

~rwheel_rate (std_msgs/Float32)

Right wheel rotation rate, in encoder ticks per second.

Parameters

~ticks_per_meter (double)

Number of encoder ticks per meter of travel.

~wheel_separation (double)

Distance between the two wheels (m).

~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.

3. diff_drive_go_to_goal

Listens for new goal poses and computes velocities needed to achieve the goal.

Published Topics

~distance_to_goal (std_msgs/Float32)

Distance to the goal position (meters).

~cmd_vel (geometry_msgs/Twist)

Desired linear and angular velocity to move toward the goal pose.

Subscribed Topics

~goal (geometry_msgs/Pose)

Desired goal pose.

Parameters

~rate (float, default: 10)

Rate at which to publish desired velocities (Hz).

~goal_linear_tolerance (float, default: 0.1)

The distance from the goal at which the robot is assumed to have accomplished the goal position (meters).

~goal_angular_tolerance (float, default: 0.087)

The difference between robot angle and goal pose angle at which the robot is assumed to have accomplished the goal attitude (radians). Default value is approximately 5 degrees.

~max_linear_velocity (float, default: 0.2)

The maximum linear velocity toward the goal (meters/second).

~max_angular_velocity (float, default: 1.5)

The maximum angular velocity (radians/second).

~max_linear_acceleration (float, default: 4.0)

The maximum linear acceleration (meters/second^2).

~forwardMovementOnly (boolean, default: true)

If true, only forward movement is allowed to achieve the goal position. If false, the robot will move backward to the goal if that is the most direct path.

~Kp (float, default: 3.0)

Linear distance proportionality constant. Higher values make the robot accelerate more quickly toward the goal and decelerate less quickly.

~Ka (float: default: 8.0)

Proportionality constant for angle to goal position. Higher values make the robot turn more quickly toward the goal.

~Kb (float: default: -1.5)

Proportionality constant for angle to goal pose direction. Higher values make the robot turn more quickly toward the goal pose direction. This value should be negative, per Autonomous Mobile Robots.

The control law for determining the linear and angular velocity to move toward the goal works as follows. Let d be the distance to the goal. Let a be the angle between the robot heading and the goal position, where left is positive. Let b be the angle between the goal direction and the final pose angle, where left is positive. Then the robot linear and angular velocities are calculated like this:

v = Kp * d
w = Ka*a + Kb*b

See Autonomous Mobile Robots, Second Edition by Siegwart et. al., section 3.6.2.4. In this code, when the robot is near enough to the goal, v is set to zero, and w is simply Kb*b.

To ensure convergence toward the goal, K_p and K_a must be positive, K_b must be negative, and K_a must be greater than K_p. To ensure robust convergence, so that the robot never changes direction, K_a - 5/3*K_b - 2/pi*K_p must be greater than zero.

4. diff_drive_mock_robot

Implements a simulation of perfect differential drive robot hardware. It immediately follows any speed commands received with infinite acceleration, and publishes the wheel encoder values and encoder rates.

Published Topics

~lwheel_ticks (std_msgs/Int32)

Cumulative encoder ticks of the left wheel.

~rwheel_ticks (std_msgs/Int32)

Cumulative encoder ticks of the right wheel.

~lwheel_rate (std_msgs/Float32)

Left wheel rotation rate, in encoder ticks per second.

~rwheel_rate (std_msgs/Float32)

Right wheel rotation rate, in encoder ticks per second.

Subscribed Topics

~lwheel_desired_rate (std_msgs/Int32)

Desired left wheel rotation rate, in encoder ticks per second.

~rwheel_desired_rate (std_msgs/Int32)

Desired right wheel rotation rate, in encoder ticks per second.

Parameters

~cmd_timeout (float, default: 0.2)

The amount of time after the last wheel rate message when the robot should stop automatically (seconds).

~rate (float, default 10.0)

The rate at which the simulated wheel encoder values and rates should be published (Hz).