scenarioSRPInPanels

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

  1. examples/mujoco/scenarioDeployPanels.py

This scenario showcases how solar-radiation pressure (SRP) can be modeled ona spacecraft with deployable panels. This script uses the MuJoCo-based DynamicObject MJScene.

The multi-body system is defined in the XML file sat_w_deployable_panels.xml. In mujoco/scenarioDeployPanels.py we saw how to use a PD controller to compute the torque necessary to deploy the panels of this same multi-body system. In this script, we opt for constraining the panel joint angles to guarantee a smooth deployment.

The formula for the magnitude of SRP acting on a flat surface can be written as:

SRP_force_mag = C_SRP * cos(incidence_angle)**2

where C_SRP is a fixed* coefficient and incidence_angle is the angle between the sunlight vector and the normal vector to the surface. The direction of this force is along the normal to the surface, and in the direction of the sunlight ( imagine the sunrays ‘pushing’ the surface). The point of application is the centroid of the surface (akin to regular pressure).

Modeling this with the tools we have considered in previous scenarios should be straightforward! Since the thrust direction and point of application are fixed on the body-fixed reference frame, we can use an single (scalar) actuator applied on a ‘site’ in the body (recall how thrusters were modeled the same way). To compute the incidence_angle, we need the direction of sunlight in the inertial frame (we assume this is constant during the simulation) and the direction of the normal vector to the surface in the inertial frame. Fortunately, all sites (remember, these are similar to frames fixed on a body) publish their state, which includes their attitude with respect to the inertial frame. If we know the definition of the surface normal in the site reference frame (which is constant since both the surface normal and site frame are fixed in the same rigid body), then we can easily compute the direction of the surface normal in the inertial frame.

The SysModel class SRPPanel performs this calculation for a single panel. This accepts three class attributes: C_SRP, the direction of sunlight in the inertial frame, and the normal vector in the site-fixed reference frame. It also takes an input message, which defines the state of said site frame (i.e. it’s attitude, among other information). Finally, it outputs a message with the magnitude of the SRP force. This can, in turn, be tied to the control input of a (single/scalar) actuator that acts on the same site, and with the gear attribute that corresponds to the frame-fixed surface normal defined in SRPPanel.

We can create a SRPPanel for every single facet for which we want to model the SRP force. In this script, we apply it to the sunlit sides of the 6 panels. However, we could also apply the same model for the sunlit side of our CubeSat. We could even implement a system to detect when a surface is in shadow or sunlit, and then apply the SRPPanel to every surface of our spacecraft (all sides of our CubeSat, both sides of the panels, etc.).

This script is a good example of how MJScene-based simulations are flexible and require less modeling effort. The dynamic engine takes care of complicated frame transformations, so we can retrieve the state of arbitrary frames defined in the multi-body and apply forces/torques in local reference frames. It also takes care of computing inter-body forces to meet the defined joint types, joint limits, constraints, etc. Because all bodies, sites, joints, and actuators are treated equally by the engine, we obtain a very modular architecture that reduces development effort.

Note: * There are many reasons why C_SRP may not actually be constant in reality (distance from sun, eclipses, material degradation…). This is a simplification!

class scenarioSRPInPanels.SRPPanel(*a, **kw)[source]

Bases: SysModel

A simple SysModel that simulates the Solar Radiation Pressure (SRP) force that would act on a flat plate.

scenarioSRPInPanels.catchtime()[source]

Used to measure the time that the context takes to run.

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

Main function, see scenario description.

Parameters:

  • initialSpin (bool, optional) – If True, the satellite is given an initial angular velocity. Defaults to False.

  • 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.