MuJoCo PID plugin

[SteMuc91]

Intro

Hi!

I am a researcher, and I'm working to simulate the motion of the finger of an hand. I'm using the mujoco PID plugin and it works fine. However, I'm not able to initialize the 2 joints/actuators position but only one.

My setup

`

        <instance name="pid_medium">
            <config key="kp" value="2.3" />
            <config key="ki" value="0.6" />
            <config key="kd" value="0.05" />
        </instance>
    </plugin>
</extension>


<actuator>
    <plugin joint="J1_index" plugin="mujoco.pid" instance="pid_index" actdim="1" />
    <plugin joint="J1_medium" plugin="mujoco.pid" instance="pid_medium" actdim="1" />
</actuator>

`

and this is the assignment in the Python code:

--- INDEX ---

joint_id_index = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_JOINT, 'J1_index')
qpos_idx_index = model.jnt_qposadr[joint_id_index]
act_id_index = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_ACTUATOR, "J1_index")

data.qpos[qpos_idx_index] = 0.7156
data.ctrl[act_id_index] = 0.7156

--- MEDIUM ---

joint_id_medium = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_JOINT, 'J1_medium')
qpos_idx_medium = model.jnt_qposadr[joint_id_medium]
act_id_medium = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_ACTUATOR, "J1_medium")

data.qpos[qpos_idx_medium] = 0.7156
data.ctrl[act_id_medium] = 0.7156

mujoco.mj_forward(model, data)

My question

Are there any errors during the initialization? Only one actuator initializes its position at 0.7156 rad, the other remains at 0 rad.

Thank you very much for the help!

Stefano

Minimal model and/or code that explain my question

If you encountered the issue in a complex model, please simplify it as much as possible (while still reproducing the issue).

Model:

minimal XML

<mujoco>
  <worldbody>
    <light pos="0 0 1"/>
    <geom type="sphere" size="1" rgba="1 0 0 1"/>
  </worldbody>
</mujoco>

Code:

import mujoco
import mediapy as media
model = mujoco.MjModel.from_xml_string(xml)
data = mujoco.MjData(model)

with mujoco.Renderer(model) as renderer:
  mujoco.mj_forward(model, data)
  renderer.update_scene(data)

  media.show_image(renderer.render())

Confirmations

• I searched the latest documentation thoroughly before posting.
• I searched previous Issues and Discussions, I am certain this has not been raised before.

[identity-wael]

There is a concrete naming problem in the actuator XML shown. joint="J1_index" connects an actuator to that joint; it does not give the actuator the name J1_index. Both of your actuators are currently unnamed.

Consequently, these actuator lookups can both return -1, and NumPy interprets data.ctrl[-1] as the last actuator's control, so both assignments target the same actuator. MuJoCo documents the -1 result for missing names here.

Give the actuators explicit names:

<actuator>
  <plugin name="index_pid" joint="J1_index"
          plugin="mujoco.pid" instance="pid_index" actdim="1" />
  <plugin name="medium_pid" joint="J1_medium"
          plugin="mujoco.pid" instance="pid_medium" actdim="1" />
</actuator>

Then use those names and check every lookup before indexing:

for joint_name, actuator_name in [
    ("J1_index", "index_pid"),
    ("J1_medium", "medium_pid"),
]:
    jid = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_JOINT, joint_name)
    aid = mujoco.mj_name2id(model, mujoco.mjtObj.mjOBJ_ACTUATOR, actuator_name)
    assert jid >= 0 and aid >= 0, (joint_name, jid, actuator_name, aid)
    data.qpos[model.jnt_qposadr[jid]] = 0.7156  # scalar hinge joint
    data.ctrl[aid] = 0.7156

mujoco.mj_forward(model, data)

This fixes the control-indexing error. It doesn't by itself explain a qpos value being zero immediately after assignment; check those values before the first simulation step as well. The minimal XML at the bottom only contains a sphere, so a full runtime reproduction needs the two joints and plugin definitions too.