Table of Contents

Editor: Sun Safe Pointing Chain

Description

This guide showcases how to create a sun-safe pointing chain on a spacecraft. This chain is designed to direct a solar panel’s normal direction towards the sun vector; a direction between the spacecraft and the sun.

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This example scenario can be found in the Power/Demo_SunPointing level. It uses reaction wheels to orient the spacecraft's attitude to face the sun and optimize power budgets.

Creating the Spacecraft

Start by configuring a level with a spacecraft spawned in orbit. The spacecraft must have the following components for this chain to work:

  • Reaction Wheels
  • Computer
  • Solar Panel

The spacecraft can have any other number of components and can have multiple solar panels. However, for this software chain, the first added solar panel (or the top in the hierarchy of the spacecraft editor) will be the primary solar panel that will be used as the direction vector. The spacecraft shown is used for this guide.

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Adding the Navigator

Next, after spawning the spacecraft in the level blueprint, flight software can be added to the spacecraft. This is usually done in a blueprint function but can be done directly on the Event Graph. The first step is to add the Navigator component. This is required for all flight software chains, and the sun-pointing chain is no exception. The Navigator produces the estimated position and rotation of the spacecraft, with some noise values applied. It is a source of truth and does not simulate the data from a sensor. This component needs to be added to the Computer on the spacecraft, which can be done by first fetching the computer and constructing the Navigator.

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The Navigator takes three input flags, which define how the errors are added to the module:

  • Cross Translation: A flag that defines a position error depending on its velocity
  • Cross Attitude: A flag that defines an attitude error depending on its attitude rate
  • Use Eclipse: A flag that defines whether the sun pointing vector on the navigation attitude message depends on whether the spacecraft is in an eclipse.

Adding the Sun Pointing Software

The next software to add is the Sun Safe Pointing Software, which will define the guidance error between the current spacecraft state (with the errors defined by the navigator) and the sun direction vector. This module takes in two input messages, which are both connected to the same output message. The navigation attitude message defines the current orientation of the spacecraft and the sun direction vector. Although both messages appear to be added to the same value, each input message uses a different set of properties within the message to determine the current orientations. If the Use Eclipse flag on the Navigator is enabled, then the sun direction vector will be zero when the spacecraft is in Eclipse.

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The sun safe pointing software takes in a few parameters that can define how the software handles small angles and when in Eclipse.

  • Sun Body Vector: This is the unit vector in the direction that the software should orient the alignment vector towards. Conventionally, this is the normal vector of the primary solar panel.
  • Omega RN_B: This is the desired body rate vector if no sun direction is available, in the case when the spacecraft is in Eclipse.
  • Min Unit Mag: The minimally acceptable normal of sun body vector.
  • Small Angle: The smallest possible angle between the normal vector and the sun vector before the flight software stops orienting the spacecraft closer to the alignment.
  • Sun Axis Spin Rate: The desired constant spin rate about the sun heading vector.

Adding the MRP Feedback Software

A standard software module that is used for all pointing chains is the MRP Feedback Software. This software module takes in a desired guidance direction and compares the direction with the current orientation. It then uses a PID controller to create a torque message that will define how much torque is given to the effectors (such as a reaction wheel or external force module) to start orienting the spacecraft in the desired vector. The guidance message is a requirement as the input for the module, however, the reaction wheel messages (speed, availability and config) are optional and can be used when a reaction wheel exists. These messages provide information to the controller on the configuration and speed of each reaction wheel and can optimize the controller for specific wheels.

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The MRP controller is configurable and the PID constants can be adjusted. These parameters are dependent on the state of the system and the friction models on the reaction wheels. The default constants are suggestive and work in a majority of use cases.

  • K: The proportionality-gain constant (P) to the MRP errors.
  • P: The rate-error constant (D) for the feedback gain.
  • Ki: The integration feedback error constant (I) on rate error.
  • Integral Limit: The integration limiting torque to avoid wind-up issues.
  • Known Torque Point B_B: The defined additional external torque, defined in body frame coordinates, that is applied to the controller in the \(B\) frame.

Adding the Reaction Wheel Motor Torque Software

The MRP controller will return an expected torque for some effector to apply on the spacecraft in the body frame. If a reaction wheel module is used, then this torque needs to be converted to a reaction wheel command, that can be sent to the reaction wheel array on the spacecraft. This is the Reaction Wheel Motor Torque Software and it takes in the state of the reaction wheels (as a configuration message) and the required torque command and produces an array motor torque message, containing a list of torques that are to be applied to a reaction wheel. By default, if no availability message is defined, the module will assume all reaction wheels that are configured are available and operable in the system.

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The reaction wheel motor torque software has one additional parameter, defined as the control axes. This parameter defaults to the identity matrix but can be configured to define a non-symmetrical axis of control, in the case that a reaction wheel or software produces the torques in a rotation that is not equivalent to the reaction wheel direction vectors.

Connecting the Reaction Wheel Message

The reaction wheel motor torque software produces an array message that the reaction wheel uses to control the torques of each wheel. The final task of the flight software chain is to connect the message to the reaction wheel. This can be done using the Set In Messages method, which is the standard syntax for updating the input messages of a component.

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These are all the required software modules for the sun-pointing chain to be implemented on a spacecraft. Additional software modules can be added for specific use cases, such as a power voltage conversion.