There are two ways to specify an objective in OptiStruct. Either a single response can be minimized or maximized or you can choose to minimize the maximum value, or maximize the minimum value, of a number of normalized responses.
In the first instance, where a single response is defined as the objective, a DESOBJ card must be included in the Subcase Information Section of the input file. The DESOBJ card references a response, (DRESP1 or DRESP2), which is defined in the Bulk Data Section of the input file. If the response, to which the DESOBJ card refers, is associated with a single subcase, the DESOBJ card must be placed within that subcase definition. If the response is associated with more than one subcase, the DESOBJ card must appear before the first SUBCASE statement.
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Example: |
Objective is to minimize the value of the response with ID 1. |
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DESOBJ(MIN) = 1 |
The second instance, where the objective references multiple responses, requires DOBJREF bulk data entries and MINMAX or MAXMIN subcase information entries. The DOBJREF cards reference responses (DRESP1 or DRESP2) and provide positive and negative reference values for these responses. Multiple DOBJREF cards may occur in the input file and they may or may not use the same Design Objective IDs (DOIDs). The reference values allow for normalization of different responses. The value of the response is divided by the appropriate reference value. When the value of the response is positive, the positive reference value is used. When the value of the response is negative, the negative reference value is used.
The MINMAX or MAXMIN cards reference the DOBJREF cards. If all DOBJREF cards use the same DOID, only one occurrence of MAXMIN or MINMAX is required. If different DOIDs are used on the DOBJREF cards, multiple occurrences of MINMAX and MAXMIN cards may be required, but a MINMAX statement cannot appear in the same input file as a MAXMIN statement. MINMAX or MAXMIN statements must appear before the first SUBCASE statement.
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Example: |
Objective is to minimize the maximum of all DOBJREF's with DOID 1 and DOID 2. |
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MINMAX = 1 |
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MINMAX = 2 |
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Example: |
Design objective for MINMAX (MAXMIN) problems - DOID 1 - references design response 10 in subcase 2 - negative reference value = -1.0, positive reference value = 1.0. |
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(1) |
(2) |
(3) |
(4) |
(5) |
(6) |
(7) |
(8) |
(9) |
(10) |
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DOBJREF |
1 |
10 |
2 |
-1.0 |
1.0 |
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