Quadcopter Drone Model with Unity Plugin #2891

kalzoo · 2025-10-07T05:39:03Z

kalzoo
Oct 7, 2025

Intro

Hello all -- I'm working to integrate open-source drone autopilots (like Ardupilot) with Unity for automated flight simulation.

My setup

Unity 6, Mujoco 3.3.5, OSX

My question

there's a "reference" drone architecture provided in the menagerie, which is great. However, this is not importable by the Unity plugin, namely due to the error on motor: it needs a "joint or tendon," and just a site is not enough.

However, no modifications I've used have been sufficient to make the drone respond correctly to the autopilot control. Does anyone have a working example of such an integration or at least a model?

Minimal model and/or code that explain my question

So i've tried two different remedies (with full XML posted below):

Approach 1: joint="root" and gear to represent the force offset from the drone's origin. Intuitively, I might think that using rotational force in this way wouldn't work, and opposite motors might cancel out the torque from one another on the main body, but this does fly in Mujoco's desktop application.

However, in Unity, it just sits and is not affected by the force on the motors. it's not a matter of insufficient force - I've scaled it appropriately.

var data = _mjScene != null ? _mjScene.Data : null;
if (data != null)
{
    int nu = (int)_mjScene.Model->nu;
    double* ctrl = data->ctrl;
    if (ctrl != null && nu >= 4)
    {
        // Direct mapping: ArduPilot motor N → MuJoCo ctrl[N-1]
        // XML motors are defined in ArduPilot Quad X order
        ctrl[0] = a0 * mujocoCtrlScale;  // Motor 1: Front Right
        ctrl[1] = a1 * mujocoCtrlScale;  // Motor 2: Rear Left
        ctrl[2] = a2 * mujocoCtrlScale;  // Motor 3: Front Left
        ctrl[3] = a3 * mujocoCtrlScale;  // Motor 4: Rear Right
    }
}

Full XML

<mujoco model="Drone">
  <compiler autolimits="true" assetdir="assets"/>

  <option timestep="0.01" density="1.225" viscosity="1.8e-5"/>

  <default>
    <default class="drone">
      <geom mass="0"/>
      <motor ctrlrange="0 13"/>
      <mesh scale="0.01 0.01 0.01"/>
      <default class="visual">
        <geom group="2" type="mesh" contype="0" conaffinity="0"/>
      </default>
      <default class="collision">
        <geom group="3" type="box"/>
        <default class="rotor">
          <geom type="ellipsoid" size=".13 .13 .01"/>
        </default>
      </default>
      <site group="5"/>
    </default>
  </default>

  <asset>
    <texture type="2d" file="X2_lowpoly_texture_SpinningProps_1024.png"/>
    <material name="phong3SG" texture="X2_lowpoly_texture_SpinningProps_1024"/>
    <material name="invisible" rgba="0 0 0 0"/>

    <mesh class="drone" file="X2_lowpoly.obj"/>
  </asset>

  <worldbody>
    <body name="drone"  pos="0 0 1" childclass="drone">
      <freejoint name="root"/>
      <camera name="track" pos="-1 0 .5" xyaxes="0 -1 0 1 0 2" mode="trackcom"/>
      <site name="imu" pos="0 0 .02"/>
      <geom material="phong3SG" mesh="X2_lowpoly" class="visual" quat="0 0 1 1"/>
      <geom class="collision" size=".06 .027 .02" pos=".04 0 .02"/>
      <geom class="collision" size=".06 .027 .02" pos=".04 0 .06"/>
      <geom class="collision" size=".05 .027 .02" pos="-.07 0 .065"/>
      <geom class="collision" size=".023 .017 .01" pos="-.137 .008 .065" quat="1 0 0 1"/>
      <geom name="rotor1" class="rotor" pos=".14 .18 .08" mass=".25"/>
      <geom name="rotor2" class="rotor" pos="-.14 -.18 .05" mass=".25"/>
      <geom name="rotor3" class="rotor" pos=".14 -.18 .08" mass=".25"/>
      <geom name="rotor4" class="rotor" pos="-.14 .18 .05" mass=".25"/>
      <geom size=".16 .04 .02" pos="0 0 0.02" type="ellipsoid" mass=".325" class="visual" material="invisible"/>
      <!-- <site name="thrust3" pos=".14 .18 .08"/>
      <site name="thrust1" pos="-.14 -.18 .05"/>
      <site name="thrust4" pos=".14 -.18 .08"/>
      <site name="thrust2" pos="-.14 .18 .05"/> -->
    </body>
  </worldbody>

  <actuator>
    <!-- Motors MUST be in ArduPilot Quad X order for ctrl[] array: front right, back left, front left, back right -->
    <!-- TODO if the torque values are the opposite of what ardupilot expects that may cause loss of control (in yaw). Try both signs -->
    <!-- gear format: [fx fy fz tx ty tz] -->
    <!-- front right -->
    <motor class="drone" name="thrust1" joint="root" gear="0 0 1  0.18  0.14  -.0201" ctrlrange="0 20"/>
    <!-- back left -->
    <motor class="drone" name="thrust2" joint="root" gear="0 0 1  -0.18 -0.14 -.0201" ctrlrange="0 20"/>
    <!-- front left -->
    <motor class="drone" name="thrust3" joint="root" gear="0 0 1  -0.18  0.14 .0201" ctrlrange="0 20"/>
    <!-- back right -->
    <motor class="drone" name="thrust4" joint="root" gear="0 0 1  0.18 -0.14  .0201" ctrlrange="0 20"/>
  </actuator>

  <sensor>
    <gyro name="body_gyro" site="imu"/>
    <accelerometer name="body_accel" site="imu"/>
    <framequat name="body_quat" objtype="site" objname="imu"/>
  </sensor>
</mujoco>

Approach 2: Different drone structure with hinges. This drone does react to physical input, but it flops around out of control and does not appear to react to forces in an intuitive way. The autopilot is not able to control it. While this could be due to the handling of autopilot inputs & outputs within the bridge code, that has been debugged and inspected carefully. I suspect that this file is not configured correctly:

XML

<mujoco model="Drone">
  <compiler autolimits="true" assetdir="assets"/>
  <option timestep="0.01" density="1.225" viscosity="1.8e-5"/>

  <default>
    <geom material="phong3SG"/>
    <mesh scale="0.01 0.01 0.01"/>
  </default>

  <asset>
    <texture type="2d" file="X2_lowpoly_texture_SpinningProps_1024.png"/>
    <material name="phong3SG" texture="X2_lowpoly_texture_SpinningProps_1024"/>

    <mesh name="aditali_mesh" file="X2_lowpoly.obj"/>
  </asset>

  <worldbody>
    <body name="drone" pos="0 0 0.1">
          <geom type="mesh" material="phong3SG" mesh="aditali_mesh" quat="0 0 1 1" mass="1"/>

      <freejoint name="drone_free"/>

      <body name="rotor1" pos="0.1415 0.1767 0.0788">
        <joint name="hinge1" type="hinge" axis="0 0 1"/>
        <geom type="cylinder" size="0.02 0.002" rgba="0.8 0.2 0.2 1"/>
      </body>

      <body name="rotor2" pos="-0.143 0.1878 0.0484">
        <joint name="hinge2" type="hinge" axis="0 0 1"/>
        <geom type="cylinder" size="0.02 0.002" rgba="0.2 0.8 0.2 1"/>
      </body>

      <body name="rotor3" pos="-0.143 -0.1879 0.048">
        <joint name="hinge3" type="hinge" axis="0 0 1"/>
        <geom type="cylinder" size="0.02 0.002" rgba="0.2 0.2 0.8 1"/>
      </body>

      <body name="rotor4" pos="0.1417 -0.1764 0.0792">
        <joint name="hinge4" type="hinge" axis="0 0 1"/>
        <geom type="cylinder" size="0.02 0.002" rgba="0.8 0.8 0.2 1"/>
      </body>
    </body>
  </worldbody>

  <actuator>
    <motor name="thrust1" joint="drone_free" gear="0 0 1  0.1  0.1  0" ctrlrange="0 20"/>
    <motor name="thrust2" joint="drone_free" gear="0 0 1 -0.1  0.1  0" ctrlrange="0 20"/>
    <motor name="thrust3" joint="drone_free" gear="0 0 1 -0.1 -0.1  0" ctrlrange="0 20"/>
    <motor name="thrust4" joint="drone_free" gear="0 0 1  0.1 -0.1  0" ctrlrange="0 20"/>
  </actuator>

  <sensor>
    <framepos name="drone_pos" objtype="body" objname="drone"/>
  </sensor>

</mujoco>

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Quadcopter Drone Model with Unity Plugin #2891

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Quadcopter Drone Model with Unity Plugin #2891

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kalzoo Oct 7, 2025

Intro

My setup

My question

Minimal model and/or code that explain my question

Confirmations

Replies: 0 comments

kalzoo
Oct 7, 2025