[MJX] Incorrect accelerations when setting initial joint positions via mjx.Data #1117
Comments
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Looking at your XML, the relevant section is: <body name="target/">
<joint name="target/target_x_slide/" class="/" type="slide" axis="1 0 0" limited="false" damping="1"/>
<joint name="target/target_y_slide/" class="/" type="slide" axis="0 1 0" limited="false" damping="1"/>
<site name="target/target_site" class="target/" type="sphere" rgba="0.71764705882352942 0.33725490196078434 0.34901960784313724 1" size="0.20000000000000001"/>
</body>This body has no geom in it and therefore has no mass or inertia! In any case, in classic MuJoCo you wouldn't add joints to this, but rather just change In the meantime, add some large mass to the body using |
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@saran-t thank you for the reply. I had previously tried to replace the site with a heavy geom, but that does not solve the issue (and thus neither does the addition of an The reason why I added two joints was to more closely mimic the brax reacher example. Previously I indeed got it working by changing the model, but that requires to both change the
Do you have any intuition with respect to this behaviour though? |
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Geom won't work unless you've also excluded contacts with the ground. Try again with the inertial, leave the site alone. |
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Tried both cases but the issue persists. |
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Thanks @driesmarzougui for the bug report with the code example! I'm able to reproduce the issue with this minimal example: xml = "<xml provided above with added <geom size="0.1"/> at the target>"
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)
mujoco.mj_forward(model, data)
renderer = mujoco.Renderer(model)
jit_step = jax.jit(mjx.step)
def get_image(state) -> np.ndarray:
d = mujoco.MjData(model)
mjx.device_get_into(d, state)
mujoco.mj_forward(model, d)
renderer.update_scene(d)
return renderer.render()
# grab a trajectory
images = []
for i in range(100):
mjx_data = jit_step(mjx_model, mjx_data)
images.append(get_image(mjx_data))
if i == 50:
# move the sphere
target_joint_id = model.joint("target/target_x_slide/").id
qpos = mjx_data.qpos
qpos = qpos.at[target_joint_id: target_joint_id + 2].set(
jp.array([1.0, 1.0]))
mjx_data = mjx_data.replace(qpos=qpos)
mjx_data = jax.jit(mjx.forward)(mjx_model, mjx_data)
media.show_video(images)Behavior: after the ball is displaced to (1, 1) at step 50, it slides back to (0, 0) If you add a joint range to the slide joints, the issue goes away, so I expect this is an issue with joint ranges that I think @erikfrey has a fix for |
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@btaba thanks! In any case, should behaviour such as random target positions be implemented through the Model or through the Data? I would prefer to keep things similar to classic MuJoCo and do it via the Model. But in that case, it would be nice to have a function similar to |
Hi
I'm currently trying to get a simple environment working in MJX. The environment looks like this:
The red sphere represents a target to which the robot has to locomote.
Upon environment resetting, I would like to relocate the red sphere on the XY plane randomly. I currently do so by overwriting the
mjx.Data.qposvalues of two sliding joints I added to the sphere's body, followed by anmjx.forwardcall. Doing so, however, currently causes the sphere to receive high and incorrect acceleration values. I have prepared a minimal code example to reproduce this issue:mjx_target_position_issue.zip
What's strange to me is that I do the same thing for the robot as well; This however does not induce the same issue. Furthermore, if I remove the robot from the environment, this problem no longer persists.
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