API review

Proposer: Stu

• pr2_gripper_action

Present at review:

• List reviewers

Question / concerns / comments

Stu

• From Kaijen/Matei:
  • The gripper should abort when it stops moving and hasn't reached the desired position.
  • A "min_force" should also be specified. This will be the force necessary to overcome static friction.
  • The action should provide feedback as to the current position and effort.

Gunter

• For grip: admittedly the action interface looks very confusing to me. I assume that's just me????
• Is the goal always a position with max force?
• Can we specify velocity?
• Can we specify just a force, regardless of position? Guess that is 0 or negative position with max force... might be nice to make a little more explicit, but okay.
• I assume there is little point (given the shape of the fingers), to specifying a max opening force?? We are always looking at closing forces only?
• Does this use and of the touch sensors? I'm assuming max force is simply max motor force?
• What units are position in? meters open?

Josh

• This may be out of scope, but it would be nice to have an action for "close until contact", which I assume is what a lot of people care about.

Meeting agenda

To be filled out by proposer based on comments gathered during API review period

• Feedback needs to include the current position
• It should tell me when it's stuck. Should be a guess of some sort. Has it moved in a few timesteps?
• Specify in the request? Specify as a parameter?
• We should have a trajectory for the gripper. - Gunter
• But max force is more important, but this means that we definitely won't track the trajectory.
• Beta grippers may improve the min force.
• There's also different types of friction compensation. Consider these separately.
• Stu will follow up on friction with Gunter (possibly post M3)
• The desired behavior is often "grab with 10N"
• "Move until you touch something and the contact force reaches a certain level"
• Having to set a position for what "closed" means is not necessarily intuitive.
• Could be two actions: close/open and move to position
• Three options: move to position, move at force, move to position with force
• The force level "specifies" the trajectory ("constrains")
• Document: what max_force means don't limit force?
• max_force of 0 could actually have meaning: "move while the touch pad reads 0"
• Side note: if you have a model of what your friction is, you can estimate what your contact force actually is.
• Document: what units are the position in (meters, gap size)
• What does the action do when the gripper gets "stuck"?
• Should be a different action (Josh) because the position action should abort on not reaching the position, but the "just close" action should succeed when stopped.
• Like other setpoint actions, this action will no longer succeed/abort. Information on stuckness will be provided in the feedback.
• Feedback needs: stuck, reached end
• min_force: It's difficult for the controller to report when it's stuck or not if the max_force specified is below the minimum friction.
• Option: max_force should always be above the minimum friction. Should reject
• Really want to know "is the gripper touching something?"
• Best guess of if it's touching something?
• Option: best guess of the threshold, and it thresholds the min force itself
• Option: make the user figure it out.
• Option: have a calibration for determining static friction.
• Option: Test: is out velocity near 0?
• Stalled flag: not moving, not at setpoint, applying max effort
• Stu: head trajectory can abort, so shouldn't the head controller abort too???

Conclusion

• The gripper will be a setpoint-style action: It will never succeed or abort.
• max_force = 0 means you don't move. Negative numbers mean don't limit force
• Provide a min_force, below which the action rejects the goal

feedback:

bool stalled  # Are we stuck?  (likely contact)
bool reached_position  # Did we reach the desired position?
float64 position
float64 effort

Package status change mark change manifest)

• Action items that need to be taken.

• Major issues that need to be resolved