is there a 'normal' range used for geometry_msgs::Twist values?

edit

thanks.

My confusion is mostly being a noob. The various turtlesim tutorials were not explicit (or I missed the details) about what was happening and it appeared the example was using the vector3 as a generic speed value rather than as a true velocity.

I was further confused when I had previously read the diff_drive_controller. When it said "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." I missed the latter reference to "units of measure".

Unless otherwise stated, a good assumption to make is that everything that accepts Vectors, Twists, etc will always use SI units. In fact: if you find something that does not conform to this, you should regard it a problem, as it would violate the (implicit) contract.

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

This just means that diff_drive_controller will only consider linear velocities along X, and angular velocities along Z axes. It ignores the rest.

Btw: if you have the chance, try and make your controller ros_control compatible. As soon as you have a hardware_interface implemented for your hw, you can re-use all of the rest of the infrastructure and controllers. Something like diff_drive_controller saves you a lot of work.

long long term, it may be possible to make the code ros_control compliant. I have only been coding ROS for 5 days and this node is for controlling basic analog RC servos using PWM. The target audience is high school students building small project robots for education and competition.

Ok. Some reference material for the future: Dave's ros_control_boilerplate, a Dynamixel hardware_interface example: resibots/dynamixel_control_hw.

Btw: if you feel this question has been answered, could I ask you to mark it as such by ticking the checkbox to the left of the answer? That will clearly show the question as answered when looking at the question listing.

Thanks.