Actuator thermal plugin for mujoco

[vidurvij-apptronik]

Add Thermal Plugin for Actuator Temperature Modeling

Summary

This PR introduces a new thermal plugin for MuJoCo that models the temperature dynamics of actuators.
The plugin simulates heat generation from electrical resistance, heat dissipation to the
environment, and temperature-dependent motor characteristics.

Description

Physics Model

The thermal plugin implements a first-order lumped thermal model with the following components:

1. Temperature-Dependent Torque Constant

The motor torque constant varies linearly with temperature:
Kt(T) = Kt25 + ((Kt130 - Kt25) / (403.15 - 298.15)) × (T - 298.15)
where:

• Kt25: Torque constant at 25°C (298.15 K)
• Kt130: Torque constant at 130°C (403.15 K)
• T: Current temperature in Kelvin

This can be replace by a constant value in the future

2. Motor Current Calculation

From the actuator force and torque constant:
I = F / (Kt(T) × G)
where:

• F: Actuator force (N)
• G: Gear ratio
• I: Motor current (A)

3. Temperature-Dependent Electrical Resistance

The electrical resistance increases with temperature:
R(T) = RNorm × (1 + α × (T - 298.15))
where:

• RNorm: Nominal resistance at 25°C (Ω)
• α: Temperature coefficient of resistance (1/K)

4. Thermal Dynamics

The temperature evolution follows the heat balance equation:
C × dT/dt = Pin - Pout
where:
Pin = I² × R(T) (Joule heating)
Pout = (T - Tambient) / Rth (Heat dissipation)

• C: Thermal capacitance (J/K)
• Rth: Thermal resistance (K/W)
• Tambient: Ambient temperature (K)

The temperature is updated using Euler integration:
T(t + Δt) = T(t) + (dT/dt) × Δt

Implementation Details

Files Added:

• plugin/thermal/thermal.h: Plugin header with configuration structure
• plugin/thermal/thermal.cc: Core implementation of thermal model
• plugin/thermal/register.cc: Plugin registration
• plugin/thermal/CMakeLists.txt: Build configuration
• test/plugin/thermal/thermal_test.cc: Unit tests
• test/plugin/thermal/testdata/thermal_test.xml: Test configuration
• test/plugin/thermal/CMakeLists.txt: Test build configuration

Key Features:

• Per-actuator temperature state tracking
• Configurable thermal parameters via XML attributes
• Sensor output for temperature monitoring
• Reset functionality to ambient temperature
• Comprehensive input validation

Configuration Parameters

Parameter	Symbol	Unit	Description
C	C	J/K	Thermal capacitance
Rth	Rth	K/W	Thermal resistance
RNorm	R₀	Ω	Nominal electrical resistance at 25°C
TempCoeff	α	1/K	Temperature coefficient of resistance
Kt25	Kt₂₅	Nm/A	Torque constant at 25°C
Kt130	Kt₁₃₀	Nm/A	Torque constant at 130°C
G	G	-	Gear ratio

Usage Example

<mujoco>
  <extension>
    <plugin plugin="mujoco.sensor.thermal">
      <instance name="motor_thermal">
        <config key="C" value="42"/>
        <config key="Rth" value="3.4"/>
        <config key="TempCoeff" value="0.039"/>
        <config key="RNorm" value="0.46"/>
        <config key="Kt25" value="0.068"/>
        <config key="Kt130" value="0.061"/>
        <config key="G" value="3141.59"/>
      </instance>
    </plugin>
  </extension>

  <custom>
    <numeric name="ambient_temperature" data="298.15"/>
  </custom>

  <sensor>
    <plugin name="thermal_sensor" instance="motor_thermal"
            objtype="actuator" objname="motor"/>
  </sensor>
</mujoco>

Testing

- Unit tests verify correct temperature evolution and parameter validation
- Test ensures plugin properly initializes, computes, and resets
- Validates that temperature state is properly tracked per actuator
- Confirms ambient temperature initialization and proper sensor data output

[vidurvij-apptronik]

@quagla @yuvaltassa It needed the number of plugins to be updated in the test. Just did it!

[yuvaltassa]

2025

[yuvaltassa]

Vidur Vij -> DeepMind Technologies Limited

[yuvaltassa]

Same

[yuvaltassa]

Add terminating blank line

[yuvaltassa]

2025

[yuvaltassa]

2025