Add example about using joint control components

[scpeters]

🎉 New feature

Demonstrate issues with joint control

Summary

I was struggling to use both JointVelocityCmd and JointVelocityReset components when controlling joints until I noticed the following comment in src/systems/physics/Physics.cc:

        // Only set joint velocity if joint force is not set.
        // If both the cmd and reset components are found, cmd is ignored.

To demonstrate this interaction I created two example plugins:

• ResetJointVelocityNearPosition: this plugin sets the JointVelocityReset
  component to a fixed value when the measured joint position is near a trigger
  position. This can create a bouncing behavior of a pendulum if the trigger
  position is the bottom stable equilibrium.
• PulseJointVelocityCommand: this plugin applies pulses of joint velocity
  commands as a square wave with a specified time period between pulses.

The example world includes three pendulum models:

• On the far left, the pendulum has only the ResetJointVelocityNearPosition
  with a trigger position at the downward stable eqilibrium point. This exhibits
  a bouncing behavior.
• On the far right, the pendulum has only the PulseJointVelocityCommand.
  It starts out stationary, then alternates between moving at constant velocity
  and swinging passively.
• In the center, the pendulum has both plugins enabled and experiences pulses
  of constant velocity alternating with passive swinging and bouncing at the
  bottom.

Screen capture with dartsim:

example_joint_control_commands_dartsim.mov

Screen capture with dartsim without setting JointForceCmd

If you comment out PulseJointVelocityCommand.cc:67 which sets the JointForceCmd component to 0 during the inactive portion of the pulse, the velocity controller remains active with a velocity command of 0.

example_joint_control_commands_dartsim_without_force_cmd.mov

Bullet

There is an issue with bullet-featherstone; I will file that separately.

Test it

Follow the instructions in the example README to compile and run the examples

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

[j-rivero]

@osrf-jenkins run tests

[j-rivero]

@osrf-jenkins run tests

The CI for Windows is using the new Conda jobs, the -clowin postfix means that we are using the platfrom Conda Legacy Ogre23 (Conda with the legacy environment but using ogre-2.3).