Upon launching robot.launch a number of subscribers and publishers are created. Some of those are created natively by the ROSRider board, and some are created by other ROS software, such as the rosrider_diff_drive package.

ROSRider board creates the following subscribers and publishers on the system:

Subscribers

Subscriber	Description	Data Type	Range
/left_wheel/control_effort	Left Motor PWM Control	Float32	0-1023
/right_wheel/control_effort	Right Motor PWM Control	Float32	0-1023

Notice: These subscriber have timeouts, generally twice the update period. For publishing to these subscribers, use -r 20 option, to repetitively publish command at the subscriber. It is build this way, so if your program crashes, the robot stops.

Publishing data to subscriber using rostopic:

Execute the following command:

rostopic pub /left_wheel/control_effort std_msgs/Float64 "data: 200.0" -r 20

This command will turn the left wheel.

Tip: Use TAB Key: After typing rostopic pub /left_wheel/cont, hit TAB, it will auto complete the topic part. Press space, hit TAB again, it will autocomplete the message type Press space, hit TAB again, it will autocomplete the data part. Do not forget to add the -r 20 part.

Deadzone Compensation:

Each motor has a deadzone, when the applied PWM is below a certain limit, the motor will not turn. So, given a PWM value between 0 and 1023, DEADZONE value is added to PWM, after PWM is scaled down. Below is the formula used:

if(pwm > 0) { pwm = DEADZONE + (pwm * ((1023.0f - DEADZONE) / 1023.0f)); }

Publishers

Tip: rostopic info /topic_name will return data type of topic, and publisher and subscriber info

Publisher	Description	Unit
/left_wheel/state	Velocity of Left Wheel	RPM
/left_wheel/position	Position of Left Wheel	Ticks
/right_wheel/state	Velocity of Right Wheel	RPM
/right_wheel/position	Position of Right Wheel	Ticks
/rosrider/diagnostics	System Diagnostics Message	rosrider/diagnostics

See Diagnostics for more information about system diagnostics message.

Tip: rostopic hz /topic_name will measure the frequency of which the topic is published

Echoing data using rostopic:

rostopic echo /left_wheel/state

You will see a stream of data depicting RPM for the wheel such as:

data: 0.0
---
data: 0.0
---
data: 0.0
---
data: 0.0
---
data: 0.0
---
data: 0.0