50 lines
1.6 KiB
Markdown
50 lines
1.6 KiB
Markdown
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# Frequently Asked Questions
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## How do I attach a hand to an arm?
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In Menagerie, the convention is to add a site called `"attachment_site"` to each
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arm. This site can subsequently be used to rigidly attach an end-effector,
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like a gripper, to the arm.
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If you are using
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[PyMJCF](https://github.com/google-deepmind/dm_control/tree/main/dm_control/mjcf),
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you can use the following code snippet to do this. Any `qpos` or `ctrl`
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keyframes defined in the arm will be automatically modified to account for the
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added degrees of freedom.
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```python
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def attach_hand_to_arm(
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arm_mjcf: mjcf.RootElement,
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hand_mjcf: mjcf.RootElement,
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) -> None:
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"""Attaches a hand to an arm.
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The arm must have a site named "attachment_site".
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Args:
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arm_mjcf: The mjcf.RootElement of the arm.
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hand_mjcf: The mjcf.RootElement of the hand.
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Raises:
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ValueError: If the arm does not have a site named "attachment_site".
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"""
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physics = mjcf.Physics.from_mjcf_model(hand_mjcf)
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attachment_site = arm_mjcf.find("site", "attachment_site")
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if attachment_site is None:
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raise ValueError("No attachment site found in the arm model.")
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# Expand the ctrl and qpos keyframes to account for the new hand DoFs.
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arm_key = arm_mjcf.find("key", "home")
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if arm_key is not None:
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hand_key = hand_mjcf.find("key", "home")
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if hand_key is None:
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arm_key.ctrl = np.concatenate([arm_key.ctrl, np.zeros(physics.model.nu)])
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arm_key.qpos = np.concatenate([arm_key.qpos, np.zeros(physics.model.nq)])
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else:
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arm_key.ctrl = np.concatenate([arm_key.ctrl, hand_key.ctrl])
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arm_key.qpos = np.concatenate([arm_key.qpos, hand_key.qpos])
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attachment_site.attach(hand_mjcf)
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```
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