Consider any electronic or mechanical piece of equipment intended for military use. Stating that the equipment is “rugged” is not sufficient. There’s no neat category that defines what rugged means. It has to be quantified in terms of the laboratory test intensities at which the equipment has continued to operate satisfactorily. This means the whole gamut of climatic environments including high temperature, low temperature, humidity, altitude and depth. Also important are the dynamic, man-made environments of vibration, shock and sound pressure.

All those potentially damaging climatic environments “come at” hardware simultaneously from all directions. Take temperature, for example. In the “real world” and therefore in the lab, most if not all surfaces are warmed simultaneously or cooled simultaneously. Take altitude for another example. In the real world as in the lab, most if not all surfaces are exposed to partial vacuum simultaneously.

Last-Century Methods: Sequential Axis Shaking

The problem today is that while climatic test is by nature simultaneous, testing of dynamic vibrations and shocks often isn’t. Why should procurement agencies continue to condone last-century testing? Why should laboratories continue to waste time and money shaking test hardware first in its X axis, then its Y, then finally in its Z axis? That common sequential-axis testing, familiar to generations of test engineers and technicians, is time consuming and labor intensive. It’s detrimental on a number of levels. It means paying for three tests and paying for three fixtures. Further, it necessitates much potentially harmful handling. Worse, sequential-axis testing is not as effective nor as quick at finding product weaknesses as simultaneous multi-axis testing.

Two words—“simultaneous” and “multi-axis”—are very important when specifying a dynamic (vibration and shock) test for equipment intended for use aboard military and commercial land, sea and air vehicles. The new Test Method 527 (multi-exciter testing) in the late-2008 “G” revision to the venerable MIL-STD-810 was overdue, but certainly is welcome.

Land Vehicle Simultaneous Multi-Axis Shaking

Only single-axis-at-a-time shaking was possible with mechanical shakers prior to 1950, limited typically to 10 to 55 Hz. Wider frequency range (typically 10 to 200 or to 500 Hz) EH or electrohydraulic (servohydraulic) shakers also are single axis. However, automotive test engineers long ago combined three or more EH shakers for multi-axis shaking, replicating, for example, damaging railroad transport inputs. So did seismic test engineers, replicating multi-axis earthquake inputs to buildings.

In Figure 1 are multiple EH shakers creating realistic simultaneous multi-axis railroad transport vibrations on a laboratory vibrating platform. Nearly every automobile manufacturer has such a vibrating platform. Why? Because new automobiles suffered railcar-induced damage (surprises) en route to dealer showrooms. It is far cheaper to find railcar-induced weaknesses before a new model automobile goes into production.

Nearly every automobile manufacturer also has a setup in which multiple EH shakers drive the four wheels in a manner that represents various road inputs that relate to various road and off-road conditions and various vehicle maneuvers at various speeds. It is far better to find roadway-induced weaknesses before a new model automobile goes into production. Reductions in warranty expense far more than pay for testing.

Higher Frequency Simultaneous Multi-Axis Shaking

Testing to 2,000 Hz is desired for aircraft and missile hardware, also for engine-mounted hardware. This necessitated the development of ED or electrodynamic shakers. In operating principle these resemble electrodynamic loudspeakers. ED shakers are driven by power amplifiers under specialized computer control.

At relatively few U.S. military establishments, three or more ED shakers have been on-site combined for simultaneous multi-axis shaking. Figure 2 was taken at the Army Research Lab, Adelphi, Maryland, after two ED shakers were added. Earlier, with just one shaker, some field failures could not be replicated in the lab. After adding two more shakers, those field failures were replicated. More recent multi-exciter ED shaker systems include White Sands Proving Ground in New Mexico, Hill Air Force Base in Utah (Figure 3) and Keyport Naval Undersea Warfare Center in Washington State. Experience at those facilities led to the new Test Method 527 (Multi-Exciter Testing) mentioned earlier.