It is often helpful in the test planning and evaluation process to compare two similar structures. A common application is the examination of the interaction of a test fixture with the structure. In this case one structure consists of the test article fixed to ground, while the other structure includes a test fixture that is designed to act as ground. A well designed test fixture will result in very little change in the target modes. In this case the first structure is a subset of the second. Another application is the case where a portion of the test article will be replaced by a “mass simulator.” A good mass simulator will result in very little change to the target modes. TAMKIT provides the compare_modes alter that will use cross-orthogonality and MAC measures to compare the modes of two different structures that include a common subset of nodes. The comparison is applied only to the common subset.
There are two cases in which the compare_modes alter can be applied. In the first case model A is a subset of model B. In this case the FEM mode shapes and mass matrix are written using the write_modes alter, and the comparison is made on model B. In the second case, only a subset of model A is common with model B. In this case the subset of nodes is identified on USET,U2 cards when writing out the modes from model A, and the comparison is again made on model B. In both cases a negative value of the OMODES parameter must be selected since the mass matrix is only written using the OUTPUT2 file option.
An example of a deck that compares a model with a test fixture to a model without is illustrated in Figure 3-24. In this case the modes of the structure without the test fixture were written to the OUTPUT2 file gpsc_fem.op2 using the write_modes alter with PARAM,OMODES,-13. The fixture is a 12” thick steel plate with airbags at its corners, weighing about 6 times more than the test article.
gpsc_compare.dat
ASSIGN INPUTT2='gpsc_fem.op2', UNIT=13, FOMRATTED
ASSIGN MASTER ='gpsc_fixture.MASTER', DELETE
ASSIGN DBALL ='gpsc_fixture.DBALL', DELETE
$
DIAG 8 $
SOL 103 $ Normal modes
$
$ Write modal data
$
INCLUDE 'compare_modes.v2001'
$
CEND
TITLE =GENERAL PURPOSE SPACECRAFT (GPSC)
SUBTITLE =COMPARE MODES WITH FIXTURE
$
METHOD = 70 $ Modes to 70 Hz
$
DISP(PUNCH) = ALL $ Recover but do not print mode shapes
$
BEGIN BULK
$
PARAM WTMASS .00259
PARAM POST -2
PARAM MAC YES
PARAM OMODES 13
$
EIGRL 70 70.0
$
$ Spacecraft bulk data
$
INCLUDE 'gpsc.blk'
INCLUDE 'gpsc.prp'
$
$ Fixture bulk data
$
INCLUDE 'fixture.blk'
INCLUDE 'fixture.prp'
$
ENDDATA
Figure 3-24. Sample input file for model comparison.
The results from the compare_modes run can be read into Matlab using the read_tam function, and reported using the rpt_tam function. The results for this example are summarized in Table 3-16. The FEM modes are the modes with the fixture and the TAM modes are the modes without. The 1st six modes with the fixture are rigid body modes of the fixture on the airbags, all below 1 Hz. The 7th – 23rd modes with the fixture align fairly well with the 1st 17 fixed base modes, though there are some significant frequency shifts, implying that the test fixture will need to be accounted for when comparing test and analysis results.
Table 3-16. Effect of Fixture on Fixed Base Modes
Fixture |
Grounded |
|
|
|
||
Mode |
Freq (Hz) |
Mode |
Freq (Hz) |
Difference |
XORTHO |
Off-Diagonal |
7 |
13.75 |
1 |
13.59 |
-1% |
100% |
3% |
8 |
16.04 |
2 |
16.04 |
0% |
100% |
1% |
9 |
17.88 |
3 |
17.65 |
-1% |
100% |
5% |
10 |
20.58 |
4 |
20.26 |
-2% |
99% |
6% |
11 |
25.71 |
5 |
25.25 |
-2% |
99% |
12% |
12 |
25.71 |
6 |
25.27 |
-2% |
99% |
11% |
13 |
36.31 |
7 |
36.26 |
0% |
100% |
1% |
14 |
39.80 |
8 |
37.35 |
-6% |
92% |
34% |
15 |
43.19 |
9 |
40.06 |
-7% |
94% |
31% |
16 |
43.65 |
10 |
42.70 |
-2% |
96% |
25% |
17 |
47.53 |
11 |
46.34 |
-2% |
97% |
23% |
18 |
50.20 |
12 |
49.10 |
-2% |
99% |
10% |
19 |
54.64 |
13 |
54.54 |
0% |
100% |
3% |
20 |
57.27 |
14 |
57.25 |
0% |
100% |
2% |
21 |
60.90 |
16 |
60.90 |
0% |
100% |
0% |
22 |
62.60 |
15 |
60.23 |
-4% |
100% |
3% |
23 |
65.28 |
17 |
69.47 |
6% |
99% |
0% |