Add light weight suction gripper with updated multicopter velocity controller

[iche033]

Signed-off-by: Ian Chen ichen@osrfoundation.org

Alternative to #194

In comparison to #194, this PR adds the proposed changes while preserving existing behavior of multicopters with the original suction grippers by

• adding a new mbzirc_suction_gripper_light model with the proposed mass changes
• adding a parameter to multicopter velocity controller plugin to enable/disable rotor thrust computation changes
  • the updated controller with the thrust changes is only enabled if a quad / hexrotor is equipped with the lightweight version of the suction gripper

Some minor issues I noticed when testing the lightweight gripper:

• a quadrotor with light weight suction gripper and no payload drifts slowly upwards in z when set to hover, i.e. linear vel set to zero
• a hexrotor with light weight suction gripper is slight more unstable compared to the original suction gripper.

It's possible that we could tune other params to make the flight behavior more stable but at this stage, we prefer not to make more changes. The original gripper has its own caveats. We just give the users an extra option to choose whichever works best for them.

To test:

1. launch the test environment

   ros2 launch ros_ign_gazebo ign_gazebo.launch.py ign_args:="-v 4 -r test/simple_demo_manip.sdf"

2. Remove a test model in this environment that is not needed

   ign service -s /world/simple_demo/remove --reqtype ignition.msgs.Entity --reptype ignition.msgs.Boolean --timeout 300 --req 'name: "box1kg" type: MODEL'

3. Spawn a quadrotor with the lightweight suction gripper - the new gripper should be green in color

   ros2 launch mbzirc_ign spawn.launch.py name:=quadrotor_1 world:=simple_demo model:=mbzirc_quadrotor type:=uav x:=-0  y:=2.05 z:=1.92 R:=0 P:=0 Y:=0 gripper:=mbzirc_suction_gripper_light

4. Enable suction

   ros2 topic pub --once /quadrotor_1/gripper/suction_on std_msgs/msg/Bool 'data: True'

5. Take off with the blue small box payload

   ros2 topic pub --once /quadrotor_1/cmd_vel geometry_msgs/msg/Twist '{linear: {x: 0.0, y: 0.0, z: 5}, angular: {x: 0.0, y: 0.0, z: 0.0}}'

   the quadrotor should fly upwards in z faster than with the original suction gripper

6. Make the quadrotor hover

   ros2 topic pub --once /quadrotor_1/cmd_vel geometry_msgs/msg/Twist '{linear: {x: 0.0, y: 0.0, z: 0}, angular: {x: 0.0, y: 0.0, z: 0.0}}'

   the quadrotor should hover in place with some small drift