scenarioUnbalancedThrusters

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

  1. examples/mujoco/scenarioReactionWheel.py

This script shows how to simulate a CubeSat with 4 thrusters attached each to a different fuel tank. All thrusters produce the same thrust, but one of the consumes fuel at twice the rate of the others (which simulates a defect). This is done using the MuJoCo-based DynamicObject MJScene.

The multi-body system is defined in the XML file sat_w_thrusters.xml. This XML file defines a ‘hub’ body with 4 tank bodies as their sub-bodies (‘tank_1’, ‘tank_2’, etc.). The ‘hub’ body is a cube, while the ‘tank’ bodies are ‘pills’ and are contained within the ‘hub’ body. Each ‘tank’ body has a site attached to it (‘tank_X_thrust_point’), which defines the point at which the thrust vector is applied. The thrust of each thruster is modeled as a single actuator (‘tank_X_thrust’) that applies a force at the corresponding site. The attribute gear of the actuators is set to gear="0 0 -1 0 0 0", which means ‘apply a force along the negative z-axis’.

For more information on how to define a multi-body system in MuJoCo, see https://mujoco.readthedocs.io/en/stable/XMLreference.html

To model a thruster, two things must be achieved:

  1. The thrust force must be applied.

  2. The fuel consumption must be modeled.

To accomplish the first, we tie a standalone SingleActuatorMsg to the control input of each of the thrusters. The payload of this message is kept constant at 5 N.

To model the fuel consumption, we need to tell the system that the mass of the tank bodies is decreasing. The mass of each of the bodies in the system is a state, and thus should be updated by defining its time derivative. This is done by tying a SCMassPropsMsg to the derivativeMassPropertiesInMsg of the tank bodies. In this script, the payload of this message is kept constant at -1 kg/s for the first 3 tanks, and -2 kg/s for the last tank.

The simulation is run for 2.5 minutes, and the mass of each of the tanks and the state of the ‘hub’ body are recorded. The mass of the tanks should decrease linearly with time, with the last tank decreasing at twice the rate of the others. Because of the initial symmetry of the configuration, the ‘hub’ body should appear to accelerate in a straight line. However, because the last tank is consuming fuel at twice the rate of the others, the inertial properties of the system become unbalanced, and the ‘hub’ body should start to rotate.

scenarioUnbalancedThrusters.run(showPlots: bool = False, visualize: bool = False)[source]

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.