## 2 Mechanical Vibration Tests
### Vibration Tests Equipment  
Mechanical vibration tests are performed using the following essential components [26; 27; 28]:  
1. Electrodynamic vibration exciters (shakers)--devices producing mechanical vibration based on an electrical current provided from a power amplifier; when the armature coils are supplied with an alternating current, they produce an alternating magnetic field which will causes the armature to move;
2. Slip plate (table)--the slip table is placed on an extremely smooth stone surface and a hydraulic pump delivers oil through holes in this stone, reducing friction between the slip table and the surface on which it rests, meaning that the table is excited by horizontally oriented shakers;
3. Power amplifiers--supplies the vibrator with the electric power necessary to generate vibration motion according to a control signal from closed-loop control system;
4. Accelerometers--measures the vibration motion; during vibration tests they allow researchers to verify the vibratory levels reached by chosen DUT regions;  
The main reason to fabricate a vibration test fixture is that it is not possible to mount the DUT directly to the vibration table or armature due to the following issues:  
* The DUT mounting hole pattern not matching the vibration table hole pattern;
* The DUT size being much bigger than the size of the armature;
* The necessity for the DUT to be mounted in a certain orientation (for example: car audio systems, aircraft avionics, unmanned aerial vehicle cameras, etc.). Figure 5 shows the standard fixture shapes, like L, T, plates, cubes, or a tri-axis, which are used for DUTs with flat mounting surfaces. For DUTs with complex shapes or mounting points, custom fixtures are created specifically for a given object [30; 31]. There are many requirements for the vibration test fixtures, such as the following:  
1. The shape of the fixture--the fixture for attaching the DUT to the vibration equipment must match the shape of the exciter table and fit the test object. It utilizes specific points on the DUT, such as feet or screw holes, to replicate real-world force application. The DUT is mounted as it would be in actual use, but the fixture is designed to be stiffer to endure the intense vibrations of accelerated testing;
2. Transmissibility (most important)--defined as the ratio of output to input vibration acceleration amplitudes at each test frequency, crucial for vibration testing, it requires the fixture to evenly transfer vibratory energy to the DUT across all frequencies. Ideally, it should maintain unitary transmissibility within the test range, with minimal distortion. However, practical limitations, like weight and cost, lead to imperfections, causing resonance or attenuation in vibration transfer;  
Figure 4: Examples of a car’s battery pack fastened to the fixture on a slip table [29]. Figure 5: Examples of standard shape vibration test fixtures: (**a**) L type, (**b**) T type, (**c**) plate, and (**d**) cube. Special test fixtures: (**e**) custom fixture and (**f**) tri-axis fixture (on the base [30; 31]). 3. The Natural vibration frequency of a fixture should be lower than the required frequency in the tests to avoid resonance phenomena. An ideal fixture couples the motion from the vibrator to the DUT with high fidelity, with zero distortion at all amplitudes and frequencies used in the test. Practical limitations, like weight and cost, lead to imperfections, causing resonance or attenuation in vibration transfer;
4. Mass--the fixture should be lightweight to maximize the acceleration achievable by the vibration exciter, allowing for simultaneous testing of multiple samples and improved cost efficiency. Fixtures should be low profile and keep the sample as close to the system input as possible;
5. Stiffness--to achieve reliable results with good transmission and minimal resonance, the system must be stiff and rigid. High stiffness in the vibration test fixture is necessary for efficient force transfer from the table to the test object with minimal loss and distortion.