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Aerospace Modeling Tutorial Lecture 1 – Rigid Body Dynamics

Learn about modeling rotations in aerospace engineering, from 1-dimensional to 3-dimensional using Euler angles, quaternions, and rotation matrices. Understand the challenges and solutions in expressing and using rotations in rigid body dynamics simulations.

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Aerospace Modeling Tutorial Lecture 1 – Rigid Body Dynamics

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  1. Aerospace Modeling TutorialLecture 1 – Rigid Body Dynamics Greg and Mario January 21, 2015

  2. Reference frames North-East-Down: Body: Relative rotation:

  3. Translational Dynamics

  4. Rotational Dynamics (1 dimensional) (3 dimensional) ]

  5. Rotational sim

  6. Rotational sim

  7. We know the angular velocity, but not the angle super easy 1 dimension

  8. How to model rotations – 1 dimension θ

  9. How to model rotations – 3 dimensional First Idea: Euler angles (yaw, pitch, roll)

  10. Euler angles – how do we express the rotation?

  11. How do we use this with our rigid body equations? ] ?

  12. How do we use this with our rigid body equations? (Sorry for different time scales)

  13. How do we use this with our rigid body equations? (Sorry for different time scales)

  14. Euler angles – why don’t we use them? These are very nonlinear Major problem: Harry Ball Theorem (Every cow must have at least one cowlick) (You can’t comb the hair on a coconut)

  15. A: Quaternions or Rotation Matrices! Q: What do we use instead of Euler Angles?

  16. Quaternions in 15 seconds Very compact and elegant representation of attitude …which we will not discuss today

  17. Rotation Matrices a.k.a. Direction Cosine Matrices How to rotate vectors from one frame to another? Word to the wise: Everyone uses different conventions. Stick to one. Always check other people’s convention. Convention:

  18. Rotation Matrices a.k.a. Direction Cosine Matrices Derivative with respect to ω

  19. DCM sim

  20. DCM sim

  21. What makes DCMs hard? • Hard to visualize • Solution: convert to Euler angles before plotting - • Must be initialized right-handed and orthonormal • Easiest solution: initialize to identity • Easy solution: initialize as Euler angles then convert to DCM. • If initial angle is free/unknown, use hard solution: enforce orthonormality as a constraint Only enforce this at one time point! Doing this more than once destroys LICQ! Only enforce these three • Matching conditions have 9 equations and 3 degrees of freedom • Solution: Enforce small relative rotation = 0 IMPORTANT: Also enforce diagonal elements positive

  22. Homework 1: Reproduce these plots (Sorry for different time scales)

  23. Homework 2:Match twosimulationswith differentstates

  24. Homework 3:minimum torque satellite de-tumbleMultiple shooting with 200 timesteps, rk4 integrator Objective=

  25. Homework 4: OPTIONAL BONUS QUESTIONsame problem as homework 3, but in minimal time Objective is now end time Add bounds on the control:

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