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Structural Model Reduction Tool

Aerospace Control Systems

Spacecraft GNC Systems Engineering

 
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The Structural Model Reduction Tool is used to reduce the order of Nastran structural models while retaining essential characteristics of the full model. The full Nastran model can have hundreds or thousands of modes and nodes, whereas a practical model for control system design and analysis has a much smaller number structural modes, inputs, and outputs.

The Structural Model Reduction tool has several essential features:

  • The tool reads a binary Nastran op2 file.
  • Various order-reduction criteria are provided, including modal gain, and modal dominance.
  • The user can specify particular modes to retain or omit.
  • The user can select various inputs and outputs based on numbers (Grid IDs) assigned to the nodes in the Nastran model.
  • An orthogonal transformation is used to separate the rigid-body rotational and translational modes, which usually appear in linear combinations in a Nastran model. If only rotational dynamics are to be controlled, the translational modes are unobservable and uncontrollable, and they have to be removed from the model.
  • A gain-phase tracking algorithm is used to produce accurate frequency and phase response plots of the structural model so that modal peaks and phase changes are not missed

The order-reduction criterion selected depends on the objective of the reduced-order model. For control system stability analysis, one would choose the modal dominance criterion, where the frequency response decreases as the square of the frequency. The modal gain criterion would be chosen to obtain a reduced-order model suitable for analyzing the response to wheel imbalance. Other order-reduction criterion and methods such as peak amplitude, Gregory's method, modal cost, and frequency-weighted balanced truncation are being implemented.