The classic ElectroDynamic (ED) vibration system has been used for many years to accelerate vibration fatigue in Accelerated Life Testing (ALT). Shaped random Power Spectral Density excitation (PSDs) were specified to meet specific stress goals, and they could be generated and controlled very precisely. The profile, shown in Figure 1, became a de facto standard for vibration screening.
ED shakers are familiar to virtually any engineer or technician whose work includes evaluating the ability of a product to tolerate end-use environment mechanical stresses, shipping stresses or environmental stress screening (ESS) that includes vibration stresses. The basic design and operation of ED shakers is most easily understood through the quite accurate analogy of an audio speaker and amplifier system, except on a much larger scale .
RS systems were developed more recently. Unlike ED systems, which deliver vibration with a precisely controlled spectral content along a single axis, repetitive shock systems deliver “Six Degree of Freedom” vibration, meaning the unit under test (UUT) is simultaneously stimulated in three axes as well as the rotations around these axes.
The generated spectral profile is referred to as ‘pseudo-random.’ Unlike the ( As Shown Above) profile implementation, the RS system’s spectral content is not controlled in real time, but is a characteristic of the table, fixture and product response to the impacts from the actuators beneath the table. The final vibration excitation experienced by the UUT is a combination of the responses from the fixture and table, as well as its own response to the shock inputs from the actuators. In fact, the repetitive shock system really should not be considered a vibration table. It is a repetitive shock machine.
The RS system was designed to meet a very specific purpose—to rapidly fatigue the UUT and force the weakest mechanical parts of the design to fail, allowing increased reliability and customer satisfaction. Given the significant differences in the purpose of the repetitive shock system, it must be radically different from an ED system. An intuitive understanding of the table behavior can be achieved by imagining a semi-rigid table with small jackhammers continuously beating on the bottom of it
As shown in above fig the construction of a typical RS system. The actuators are mounted at an angle to the table surface, so each impact delivers a shock in more than one axis. They are also oriented to deliver shocks in different directions on the table. The UUT is secured to the top of the vibration table by special fixturing that is very different from the fixturing used on an ED machine.
Comparison of PSDs
The random vibration generated by both of these systems is often characterized in terms of grms and the shape of the power spectral density (PSD) graph. Conclusions about the comparative effectiveness of random vibration from the two systems, particularly for inducing fatigue in a product and precipitating failures in an environmental stress screen (ESS) are often based on comparisons of these metrics. However, the vastly different design of the two systems affects this induced stress, which can lead to erroneous conclusions concerning the value of one system over another. By understanding the differences between the systems, and understanding the different tasks they were designed to do, a user can make the most effective use possible of both systems, enabling them to meet the true ultimate goal of the use of the systems—to produce the most reliable product possible, reduce warranty costs, increase product uptime and increase customer satisfaction. As stated above, the PSD of an ED system can be carefully controlled and shaped. The measured PSD on an ED system that is programmed to the fig 1 profile is shown in Figure 3.
The upper frequency limit of a larger ED system is typically near 2000 Hz, which limits its ability to excite the resonant frequencies of small electronic components. In contrast, consider the PSD from the table top of a repetitive shock system (Figure 4). The shape of the PSD is not flat, as you would expect for a random signal. Also, there is significant energy above 2000 Hz in the system, increasing its effectiveness for fatiguing smaller assemblies and components with higher resonant characteristics, such as the SMT devices that are typical in technology today.
