3.2.2.      Creating a Dynamic TAM

A dynamic TAM requires the use of the dynamic_tam alter and the definition of the PARAM,DFREQ to define a central frequency in Hz. As in the case of all other TAMs the instrumented DOF are selected on ASET or ASET1 cards.

No special differences are needed for superelement or non-superelement models beyond those listed in Section 1.5.

A sample input file to create a dynamic TAM is shown in Figure 3-3. This also includes the ortho alter to calculate pseudo- and cross-orthogonality. The center frequency (PARAM,DFREQ) is chosen as 20 Hz for this example. This is approxi­mately the frequency of the fourth FEM mode and will improve the accuracy of the TAM in the range of this frequency.

gpsc_dtam.dat

 

ASSIGN INPUTT4='gpsc_fem.op4', UNIT=13

ASSIGN MASTER ='gpsc_dtam.MASTER', DELETE

ASSIGN DBALL  ='gpsc_dtam.DBALL',  DELETE

$

SOL     103     $ Normal modes

INCLUDE 'ortho.v2001'

INCLUDE 'dynamic_tam.v2001'

CEND

TITLE    =GENERAL PURPOSE SPACECRAFT (GPSC)

SUBTITLE =TAM - DYNAMIC REDUCTION AND ORTHOGONALITY

$

SPC    = 10             $ Constrain booster interface points

METHOD = 70             $ Modes to 70 Hz

$

DISP(PLOT) = ALL        $ Recover but do not print mode shapes

$

BEGIN BULK

$

$  PARAMeter cards

$  ---------------

$

PARAM   GRDPNT  0

PARAM   USETPRT 0

PARAM   WTMASS  .00259

PARAM     OMODES    13

PARAM     DFREQ     20.0

$

$  Compute eigenvalues using the Lanczos method

$  --------------------------------------------

$

EIGRL   70              70.

$

$  Spacecraft bulk data

$  --------------------

$

INCLUDE 'gpsc.blk'

INCLUDE 'gpsc.prp'

$

$  Static reduction data

$  ---------------------

$

INCLUDE 'gpsc_rke1.aset'

$

ENDDATA

Figure 3-3. An alter and one parameter are required for a Dynamic TAM.
 

For this example, the pseudo-orthogonality of the FEM mode shapes with respect to the dynamically reduced mass matrix is presented in Table 3-3. The cross-orthogonality between the TAM and FEM mode shapes is presented in Table 3-4. The dynamic TAM improves both the pseudo- and cross-orthogonality when com­pared to the static reduction, though it is not exact.
 

Table 3-3. Pseudo-orthogonality of FEM modes w.r.t. dynamically reduced mass matrix.

 

Table 3-4. Cross-orthogonality of TAM/FEM modes w.r.t. dynamically reduced mass matrix.