This tutorial introduces simulation concepts that are used in Ignition Physics.
Prerequisites
In the previous tutorial Installation, you have installed the Ignition Physics corresponding to the desired Ignition release.
Physics simulation versus animation
Dynamics simulators for robotics is more challenging than the ones used for animating virtual characters due to these aspects:
- The forces, torques, frictions, etc. are not crucial in animation since the law of physics can be violated.
- Real-time requirements and physical reality can be less constraining for purely visual display.
- Several challenges for simulating complex physical structures, such as humanoid robot:
- Numerical instability in computing inverse kinematics, fluid flows, etc. prevents correct physics simulation.
- Contact forces between bodies are hard to model due to their discontinuity characteristics, especially in the soft body case, may result in unrealistic contacts or physically unfeasible contact forces.
Ignition adopts SDFormat structure to describe not only the visual element but also the dynamic physics aspects. To get more in-depth of what you can define the environment in the SDFormat file, please refer to this SDFormat specification. For a comprehensive tutorial for constructing your robot model as SDFormat, please refer to this tutorial.
Physics concepts in Ignition Gazebo simulation
First, please see Understand the GUI tutorial for an overview of Ignition Gazebo GUI. In this tutorial, we will show how to manipulate and visualize some physics aspects using interesting models powered by Ignition Physics on Ignition Gazebo.
Differential Drive
This demo world demonstrates how we can control simulated differential drive on cars depending on physics engines and visualize the collision concept. Please download the Differential Drive simulation world demo to your home folder by:
We can now start our differential drive simulation on Ignition Gazebo by:
To see the pose changing when the simulation running, we publish a Twist message to command the vehicle_blue car to move in a circle with 2 meters radius as follow:
Then please press the Play button to start the simulation. This command tells the car to move in its coordinate frame with velocity 1.0 meter per second in the X-axis and angular velocity of 0.5 radians per second in Z-axis.
Note that the mechanism to move the car is different depending on the used physics engine. Using dartsim, the car is moved by applying force on the joints, please see DiffDrive.cc code. Using TPE, TPE directly sets model velocity in VelocityControl.cc code.
Monitoring the model and its links pose
We can monitor the model pose by selecting the moving car and then select the drop-down list Pose. Moreover, we could also read the model links' poses relative to their parent link by selecting the corresponding link on the model tree:
Note that using the model tree as shown in the above gif, we can view the parameters and properties such as visual or collision of each link or joint in the model. For more detail about the kinematics of the model tree, see this tutorial.
Visualizing collision
One of the most wonderful features of physics simulation is the capability to simulate collision. We can do a fun experiment like this: while the vehicle_blue car is moving in a circle, we move the vehicle_green to be on vehicle_blue's upcoming path. We will see the blue car will push the green car!
To see where these collision parameters are set in SDFormat and how it works, please see this tutorial.
Lift Drag
This demo world shows how joint force, torque, and pressure are supported in Ignition Physics. Please download the Lift Drag demo to your home folder by:
Like above, please start the Lift Drag demo world on Ignition Gazebo by:
To see how the rotor lifts the cube due to wind force pressure, we publish a Double message represeting the torque (Nm) applying to the rotor rod axis as follow:
Then please press Play button to start the simulation. We stops exerting torque by:
You will see the cube drops due to no lift force from support torque on the rod, and the blades will stop after some time due to friction.
The command applies a constant torque to the rotor rod, together with the mechanism to compute the upward/downward lift and drag force due to the wind pressure simulation supported by Ignition Physics, the cube is lifted. For more detail, please see the LiftDrag.cc code.
Buoyancy
This demo world shows how buoyancy is supported in Ignition Physics. This world contains the following three models:
- submarine: A simple submarine model that floats in place.
- submarine_sinking: A simple submarine model that is not buoyant and sinks.
- submarine_buoyant: A simple submarine model that is buoyant and floats.
Please download the Buoyancy demo to your home folder by:
Like above, please start the Buoyancy demo on Ignition Gazebo by:
After pressing the Play button, you will see the behaviors of the submarine as the above description.
As an overview, the buoyancy concept is realized by simulating fluid density and applying the force on the object in the fluid proportional to its volume. Hence, you can change the model buoyancy by modifying its inertia, please see link specification. For more detail on simulating buoyancy, please see the Buoyancy.cc code.
Pendulum
This demo world demonstrates how simulated inertia and gravity affect the object movement by showing free swing of the pendulum. Please download the Pendulum demo to your home folder by:
and start the Pendulum demo on Ignition Gazebo by:
After pressing the Play button, you will see that the pendulum will oscillate around its main revolute joint forever due to lack of friction (it is undeclared in the SDFormat file).
The oscillation period or the max angular speed of the joint will change if we modify the inertia of the rods. According to the demo below, you will see that gravity is defined as -9.8 m/s^2 on the Z-axis. Please refer to link specification for modifying the inertia and mass of the links.
Multicopter
This demo world shows how Ignition Physics supports gravity, actuators and inertia to control object velocity. Please download the Multicopter demo to your home folder by:
and start the Multicopter demo on Ignition Gazebo by:
To control the multicopter to ascend and hover, we send a Twist message to command the X3 multicopter ascending 0.1 m.s as follow:
then hovering:
Do the same for the X4 multicopter. After pressing the Play button, you will see both of the multicopters will ascend, this demonstrates how the physics engine utilizes model kinematics and dynamics to support simulating complex model and its controller. For more detail about the multicopter controller, please see MulticopterVelocityControl.cc.