FAQ.md

Frequently Asked Questions

How do I attach a hand to an arm?

In Menagerie, the convention is to add a site called "attachment_site" to each arm. This site can subsequently be used to rigidly attach an end-effector, like a gripper, to the arm.

If you are using PyMJCF, you can use the following code snippet to do this. Any qpos or ctrl keyframes defined in the arm will be automatically modified to account for the added degrees of freedom.

def attach_hand_to_arm(
    arm_mjcf: mjcf.RootElement,
    hand_mjcf: mjcf.RootElement,
) -> None:
  """Attaches a hand to an arm.

  The arm must have a site named "attachment_site".

  Args:
    arm_mjcf: The mjcf.RootElement of the arm.
    hand_mjcf: The mjcf.RootElement of the hand.

  Raises:
    ValueError: If the arm does not have a site named "attachment_site".
  """
  physics = mjcf.Physics.from_mjcf_model(hand_mjcf)

  attachment_site = arm_mjcf.find("site", "attachment_site")
  if attachment_site is None:
    raise ValueError("No attachment site found in the arm model.")

  # Expand the ctrl and qpos keyframes to account for the new hand DoFs.
  arm_key = arm_mjcf.find("key", "home")
  if arm_key is not None:
    hand_key = hand_mjcf.find("key", "home")
    if hand_key is None:
      arm_key.ctrl = np.concatenate([arm_key.ctrl, np.zeros(physics.model.nu)])
      arm_key.qpos = np.concatenate([arm_key.qpos, np.zeros(physics.model.nq)])
    else:
      arm_key.ctrl = np.concatenate([arm_key.ctrl, hand_key.ctrl])
      arm_key.qpos = np.concatenate([arm_key.qpos, hand_key.qpos])

  attachment_site.attach(hand_mjcf)