Spacecraft Dynamics Block not matching intuition

I am using the spacecraft dynamics block in Simulink.

I am giving two nonzero moment inputs as shown below in one of the attached screenshots. It is just a PRBS input for both.

However, my angular velocity output looks like the next attached photo.

How is it that my angular velocities are all going negative and becoming larger in magnitude negative as time goes on when the input is swinging negative and positive? You would think that the input given would cause the spacecraft to essentially wobble back and forth, but instead we see it abruptly start to spin one direction and spin faster and faster in that same direction as time goes on. This does not match my intuition and makes me think something is wrong. I would really appreciate if someone could clarify.

Answer 1

[Balavignesh]

Hi Tristan,

It sounds like you're encountering an unexpected behavior in your Simulink model related to the spacecraft dynamics under a pseudo-random binary sequence (PRBS) input. While I would need the model and the respective signals to reproduce the issue on my side, I can suggest some potential reasons and considerations for this behavior:

• PRBS inputs are designed to switch between values in a pseudo-random manner. Ensure that the amplitude and frequency of the PRBS input are appropriate for your system. If the PRBS input is too aggressive, it might be causing large torques that lead to rapid changes in angular velocity.
• Check the solver settings in Simulink (Ctrl + E). Using an inappropriate solver or step size can lead to numerical instability. Try using a different solver or adjusting the step size to see if the behavior changes.
• If your model includes feedback control, ensure that the control logic is correctly implemented and does not introduce instability.
• Monitor the angular momentum of the spacecraft. An unexpected increase in angular momentum might indicate an issue with the applied torques or the integration of the equations of motion.

The behavior you're observing could be due to several factors, including incorrect initial conditions, inappropriate PRBS input characteristics, numerical instability, or issues with the equations of motion. By simplifying the input, monitoring intermediate states, and reviewing the model parameters, you can diagnose and address the issue.

Kindly have a look at the following documentation link to have more information:

• Spacecraft Dynamics Block: https://www.mathworks.com/help/aeroblks/getting-started-with-the-spacecraft-dynamics-block.html

Hope that helps!

Balavignesh