scenarioAsteroidLanding

It’s recommended to review the following scenario(s) first (and any recommended scenario(s) that they may have):

1. examples/mujoco/scenarioArmWithThrusters.py

This scripts shows how one could simulate a spacecraft landing on an asteroid. It illustrates force-vector actuators, loading meshes for bodies, and collisions. This is done using the MuJoCo-based DynamicObject MJScene.

The multi-body system is defined in the XML file sat_ast_landing.xml. This XML illustrates various interesting features. First, is the use of the <default> element to avoid data repetition in the XML file. Second, it’s the use of multiple free-floating bodies, in this case the ‘hub’ of the spacecraft and the ‘asteroid’ body. Multiple independent spacecraft are also possible (see examples/mujoco/scenarioSimpleDocking.py). Finally, this showcases how to load a mesh (for the geometry of the asteroid). The actual file path must then be provided on construction of MJScene.

Unless specifically included, gravity is not modeled in :ref:`MJScene<MJScene> simulations. However, in this scenario we want to simulate a spacecraft landing on the asteroid, thus we need to model gravity. This is done by creating a custom ConstantGravity model that applies a constant force in the inertial frame. This is obviously a simplification, akin to considering a constant downward gravity near the surface of the Earth.

Previously seen scenarios have always used actuators defined in the XML file, and of the MJSingularActuator type. MJSingularActuator are characterized by being controlled by a single scalar value. For the case of MJSingularActuator tied to a site, these generate a force/torque in the local frame of their site. The resulting force and/or torque vectors are the product of the scalar input of the MJSingularActuator times the constant direction vectors in the site reference frame defined by the gear attribute of this actuator.

MJSingularActuator are ideal to model forces/torques that always applied along a fixed direction in the site frame. However, in this scenario, we need to apply a force in the inertial frame. To do so, we use a MJForceActuator, which allows us to define a force vector with arbitrary direction and magnitude (thus 3 scalar values are used to control it). Still, the desired force vector given to this actuator is still in the site reference frame. To produce a force fixed in the inertial frame, we need to use the attitude of the site frame to continuously rotate the force in the site-fixed frame such that it’s always acting in the same inertial direction.

This is implemented through two mechanisms. First, we need to add a new actuator. Previous scripts retrieved actuators defined in the XML through getSingleActuator. In this case, we create a new actuator of the MJForceActuator type with addForceActuator. Second, we use the ConstantGravity model, which accounts for the rotation of the site frame to produce a force vector that produces a gravity force that is constant in the inertial frame.

It is possible to add MJSingleActuator, MJForceActuator, MJTorqueActuator, and MJForceTorqueActuator through Python or C++ function calls (addSingleActuator, addForceActuator, etc.). These can later be retrieved with getSingleActuator, getForceActuator, etc.

class scenarioAsteroidLanding.ConstantGravity(*a, **kw)

Bases: SysModel

A class to model constant gravity force in a simulation.

force_N

The constant gravitational force vector in the inertial frame (N frame).

Type:

Sequence[float]

frameInMsg

Reader for spacecraft state messages.

Type:

messaging.SCStatesMsgReader

forceOutMsg

Message to output the computed force in the body-fixed frame (B frame).

Type:

messaging.ForceAtSiteMsg

scenarioAsteroidLanding.run(showPlots: bool = False, visualize: bool = False)

Main function, see scenario description.

Parameters:

• showPlots (bool, optional) – If True, simulation results are plotted and show. Defaults to False.

• visualize (bool, optional) – If True, the MJScene visualization tool is run on the simulation results. Defaults to False.