The items to be tested should be held during the test in the same position and configuration of their standard installation. Vibration should be transmitted to the item in the same quantity from any fixation point of the item.
The structures that connect the item held in the proper test position and the shaker armature are called Fixtures.
When the fixture is studied for a particular item, it is called custom fixture.
Automotive and aerospace field, where the fixation of the item is critical and usually complicated, are among those fileds where a custom fixture is necessary to perform vibration tests.
Usually the custom fixture can be used along the three mutual axes by rotating it or by the use of a slip-table.
It is designed starting from item fixation points and optimized by FEM analysis to achieve the minimum weight and maximum frequency range. Before delivery, the fixture is checked on dimensions and mechanical behavior.
Any custom fixture is supplied with technical report and user manual that describes the dynamical behavior and the use of the fixture.
A customized fixture could appear as a very expensive solution, but the assurance to perform the test correctly from the fixture point of view usually could be a great saving of time, problems and money.
Also the possibility to study the fixture to mount many items at the same time will save a lot of testing time and cost.