Add doc page about joint kinematics

[christophfroehlich]

I summarized some results of discussions about mimic joints and how I understand that.
Note, this is different than implemented currently with

ros2_control/hardware_interface/src/mock_components/generic_system.cpp

Lines 568 to 576 in 16fbde3

	for (const auto & mimic_joint : info_.mimic_joints)
	{
	for (auto i = 0u; i < joint_states_.size(); ++i)
	{
	joint_states_[i][mimic_joint.joint_index] =
	mimic_joint.offset +
	mimic_joint.multiplier * joint_states_[i][mimic_joint.mimicked_joint_index];
	}
	}

This came from the time where we defined the mimic joints in the ros2_control tag. IMHO, it does not make sense now using the URDF-specification, where the behavior of the mimic joint changes by switching the command interface from position to velocity, because we use the offset and calculate
velocity = multiplier * other_joint_velocity + offset

Also related to ros-controls/gazebo_ros2_control#297 and ros-controls/gz_ros2_control#276

I also included transmission interfaces, to give an overview what they are and what they aren't (they were not mentioned once in the docs).

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[saikishor]

I've reviewed the part of mimic joints and left some comments

[saikishor]

Some comments about the transmission interface

Thanks for the nice doc

[saikishor]

ok 👍

[saikishor]

I'm not sure if I understand this part properly

[christophfroehlich]

This was copied from here :D

ros2_control/transmission_interface/include/transmission_interface/transmission.hpp

Lines 29 to 38 in 16fbde3

	* Mechanical transmissions transform effort/flow variables such that their product (power) remains
	* constant. Effort variables for linear and rotational domains are \e force and \e torque; while
	* the flow variables are respectively linear velocity and angular velocity.
	*
	* In robotics it is customary to place transmissions between actuators and joints. This interface
	* adheres to this naming to identify the input and output spaces of the transformation. The
	* provided interfaces allow bidirectional mappings between actuator and joint spaces for effort,
	* velocity and position. Position is not a power variable, but the mappings can be implemented
	* using the velocity map plus an integration constant representing the offset between actuator and
	* joint zeros.

Any suggestions? Then I'd change that in the docstring, too.

[saikishor]

Well if it is like that in the original docs it's fine.
At least for me the product he is talking about is not clear, I see that point of energy conservation, but within the transmission plugin you transform also position and velocity

[saikishor]

LGTM