gz_wrench_plugin

A Gazebo Harmonic (gz-sim8) C++ plugin that applies controlled forces to any model link, no flight stack needed. Currently does Z-axis altitude hold with automatic mass calculation. Runs a cascaded PD controller at 1000Hz — unlike Gazebo's built-in ApplyLinkWrench which is open-loop, this actively corrects every tick to hold altitude.

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Requirements

• Gazebo Harmonic (gz-sim8)
• ROS2 Humble

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Build

cd ~/ros_ws
colcon build --packages-select gz_wrench_plugin --cmake-args -DCMAKE_BUILD_TYPE=Release
source install/setup.bash

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Setup

export GZ_SIM_SYSTEM_PLUGIN_PATH=~/ros_ws/install/gz_wrench_plugin/lib

To persist across sessions:

echo 'export GZ_SIM_SYSTEM_PLUGIN_PATH=~/ros_ws/install/gz_wrench_plugin/lib' >> ~/.bashrc
source ~/.bashrc

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Usage

Add inside your <model> tag in your SDF:

<plugin filename="WrenchController"
        name="gz::sim::systems::WrenchController">
  <target_z>2.0</target_z>
  <link_name>base_link</link_name>
  <hover_force>18.4</hover_force>
  <kp>3.0</kp>
  <kd>8.0</kd>
  <enable_log>true</enable_log>
</plugin>

Hit play — the model lifts off and holds at target_z. Hover force is auto-calculated from model mass if omitted.

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Parameters

Parameter	Required	Default	Description
target_z	yes	—	Target altitude in metres
link_name	yes	—	Link to apply force to
hover_force	no	auto	Gravity feedforward in Newtons. Omit to auto-calculate
kp	no	3.0	Position gain
kd	no	8.0	Velocity gain
enable_log	no	true	Print status to terminal

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Example

A ready-to-run Parrot Bebop 2 example is included in examples/parrot_bebop_2/.

Run

gz sim examples/parrot_bebop_2/model.sdf

Hit play — the Bebop lifts off and holds at target_z: 2.0.

Credits

Parrot Bebop 2 model by Olivier Crave — used as an example only.

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Planning to add X/Y axis control, torque control, runtime target updates via ROS2 topic, and multi-link support.

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License

MIT