The classic ElectroDynamic (ED) vibration system has been used for many years to accelerate vibration fatigue in Accelerated Life Testing (ALT). During the Mercury, Gemini and Apollo programs, ED shakers performing random vibration were utilized, with significant success (United States Navy, 1979). Shaped random Power Spectral Density excitation (PSDs) were specified to meet specific stress goals, and they could be generated and controlled very precisely (U.S. Department of Defense, 1996). The ‘NAVMAT’ profile became a de facto standard for vibration screening (Ciufo, 2005).

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 (Lang, 1997).

Random Shock (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 (General Motors, 2011). The generated spectral profile is referred to as ‘pseudo-random.’ Unlike the NAVMAT 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 (Qualmark Corporation, 2010). Given the significant differences in the purpose of the repetitive shock system, it must be radically different from an ED system, and arguably precipitate maximum results. This video breaks down the superior benefits of this modality in comparison to traditional ED vibe and offers some extremely telling and close up motion analysis of an actual HALT test in slow motion.

To dig deeper in to the technology and proven results of RS vibe, we encourage you to contact ESPEC (Qualmark’s parent company) with your vibration needs and inquiries today. We have a team of experts waiting to serve you. Likewise, if you'd like read the complete white paper on RS vs. ED vibration, contact us so we can share the full context with you: