Airborne equipment experiences random vibrations that change at every measurement interval due to various conditions during flight operation — such as motors, noise, turbulence, and projectile launches — and structural safety and fatigue durability evaluations are required for these conditions. In this example, we will explore methods for verifying random vibration analysis of a condition monitoring system.
The topic we will be covering today is "Random Vibration Evaluation" of airborne equipment.
Helicopter Example
The most commonly seen airborne equipment around us are airplanes and helicopters.
"It's amazing how well it flies with just small propellers...“ "I want to travel abroad by plane..."
"What would happen if something that large suddenly crashed??"
Airborne equipment, which represents a concentration of engineering technology, is composed of countless interlocking components. Because this can be a cause of very large casualties, it is important to define and verify the loads generated by random occurrences during flight.
Example of internal component configuration of an airborne vechicle
The automated transfer equipment that is the subject of this topic refers to equipment that automatically transports parts or products to locations where repeated transfer is required in order to improve production efficiency.
In semiconductor manufacturing processes, OHT (Overhead Hoist Transport) equipment — which transports wafer FOUPs (Front Opening Unified Pods) — is commonly used, and the transfer equipment is operated automatically according to the applied logic.
Considering the process of an OHT transfer system, it will repeatedly cycle through the following sequence: (stop – product loading – acceleration – constant-speed travel – deceleration – stop – product unloading). Since loaded products may be damaged during transfer, it is important to predict how the OHT transfer equipment and the loaded products (wafer FOUPs) behave during the transfer process and to identify potential issues.
Since airborne equipment is also subjected to loads that vary at every moment and location, the concept of probability distribution is applied to quantify and evaluate them. By measuring the loads that vary at every moment and location over a long period of time and performing statistical analysis, a single normalized probability distribution curve can be obtained. The results are classified by probability of occurrence into 1-Sigma (68.27%), 2-Sigma (95.45%), and 3-Sigma (99.70%) levels, and these are used to perform safety evaluations against random loads.
Random loads are defined not only for airborne equipment but also for ground-based vehicles such as automobiles and trains. Military standard mil-std-810 and international test standard IEC 60068-2-64, among others, define random loads for operating equipment and enable safety evaluations to be performed. These are generally provided as PSD (Power Spectral Density) or ASD (Acceleration Spectral Density) functions; you must first confirm which unit — (g²/Hz) or (m²/sec³) — is applied to the y-axis density amplitude, and then carefully apply the correct unit load value when performing frequency response analysis under unit loading.