For submission to Automotive Test after long wait (since June 1) for Figure 1.

Perceptive visitors to vibration test labs are surprised to see automotive electronics and other assemblies receive three vibration tests, sequentially along each of the hardware’s major axes. “That’s not the way real vehicles vibrate automotive hardwar,” they remonstrate. “On real vehicles, hardware is shaken in all axes simultaneously.” The standard laboratory reply is usually “This is the way we’ve always done it.” This article announces the availability of better – simultaneous multiaxis – vibration testing

When were electrodynamic (ED) shakers first developed? In the mid 1950s. Similar in principle to electrodynamic loudspeakers, they vibrate their test loads in just one axis at a time. That shortcoming bothered few test engineers of that era. They had been using mechanical shakers with the same single-axis shortcoming. ED shakers permitted single-axis shaking to 500 Hz in the mid-1950’s and to 2,000 Hz in the early1960s.

In that same era, EH (electrohydraulic, sometimes called servohydraulic) shakers began to provide first single-axis-at-a-time sequential and then simultaneous multiaxis shaking of building components to simulate multiaxis seismic loading, multiaxis shaking of automobiles to simulate multiaxis road and off-road inputs, multiaxis shaking of railcars to simulate multiaxis track inputs, etc., all at relatively low EH shaker frequencies.

Until quite recently the ED shaker industry and users stayed with sequential single-axis-at-a-time shaking, three tests. They resisted the admittedly-difficult “ganging” of multiple ED shakers to better simulate “real world” higher frequency multiaxis excitations. Pioneering military ED simultaneous multiexciters are herein described, along with mentioning a new Test Method 527 (multiexciter testing) in this year’s “G” revision to the widely used US Military Standard 810. Finally, Japanese auto industry developments in simultaneous multiaxis shaking to 2,000 Hz (Figure 1) are described.

Three electrodynamic (ED) shakers simultaneously shake automotive headlamps in three axes. Courtesy Spectrum Technologies.

Mechanical shakers
The earliest shakers were mechanical. Smaller units were jokingly called “paint shakers.” Larger units are still used to provide 5-33 Hz sweeps for shaking shipboard hardware. Also for single-frequency shaking, predominantly vertical, possibly representing what cargo may experience in highway or rail hauling.

But mechanical shakers cannot simulate the complex vibrations from helicopter engines and rotors, from reciprocating engines in motor trucks, etc. Nor are they capable of random vibrations such as are experienced by rockets at liftoff, by jet aircraft, by land vehicles, etc.

Electrohydraulic (servohydraulic) shakers
Electrohydraulic (EH) shakers are commonly programmed to reproduce complex and random vibrations. Long strokes to 500 mm are useful for seismic simulations to perhaps 35 Hz. With lesser stroke they are highly useful to 200 Hz for automotive and other applications such as motor vehicles. Many installations require shaking of hardware first in its X axis, then in its Y axis and finally in its Z axis (three sequential tests).

Electrohydraulic multiaxis shaking
Rarely if ever, however, is “real world” vibration single axis. Far more realistically, then, three or more EH shakers have long been positioned to shake loads simultaneously in three or more axes, to perhaps 500 Hz. In Figure 2, additional EH shakers add three rotational axes: roll, pitch and yaw.

Figure 2 Simulating road-induced random vibration with multiple electrohydraulic (EH) shakers. Courtesy MTS.

Electrodynamic (ED) shaker development has lagged
ED shakers are needed for shaking relatively small electronic and other assemblies that mount on diesel and other engines to perhaps 2,000 Hz. The vast majority of ED shaker usage, unfortunately. has remained single-axis-at-a-time. ED shaker purchasers and users have until quite recently appeared satisfied with what has been available. There has been and is a lively market in used single-axis ED shakers, often powered by much newer solid-state power amplifiers.

US military multiaxis ED shaking
The US Army Research Lab at Adelphi, Maryland had a problem. A certain piece of land vehicle hardware was failing in service. But that failure could not be reproduced at any intensity nor any frequency of which their shaker was capable. Fortunately that lab was funded to add two more ED shakers, so that three-axis simultaneous excitation was possible. And the weakness was revealed. Numerous similar instances have occurred since. Figure 3 shows three shakers, oriented respectively N-S, E-W and up-down.

Three mutually perpendicular electrodynamic (ED) shakers at Army Research Lab. Courtesy US Army.

A somewhat similar lab at White Sands Proving Ground gets little use, I’m told, because clients are so familiar with sequential axis testing.

Close to Hill Air Force Base in Utah is a site-assembled eight-ED shaker 6 degree of freedom (6DoF) system long operated by Boeing to shake flight hardware. See Figure 4.

Figure 4 Eight electrodynamic (ED) shakers arranged for 6 DoF testing of Air Force loads. Courtesy USAF.

Finally, the US Navy at Keyport, Washington, more recently assembled three ED shakers into the three-axis system shown in Figure 5.

Figure 5 Three electrodynamic (ED) shakers at US Naval Undersea Warfare Center, Keyport, Washington

Factory-assembled multi-shaker systems
Commercial testing laboratories would prefer to buy complete factory-assembled systems, if available, rather than purchase three individual shakers, then design and fabricate a common foundation. Unfortunately for North America labs, few such systems have been built here. Figure 6 shows a two axis ED system from EST in Michigan.

Figure 6 Two-axis ED shaker system. Courtesy EST.

ANCO Engineering of Boulder, Colorado, with considerable seismic multiaxis EH shaker experience, has also built a few simultaneous multiaxis systems with three and with six rotational ED shakers. See Figure 7. Each shaker can be described as an electric motor that maximally rotates about 22o.

Figure 7 Three torsional shakers simultaneously shake a load N-S, E-W and up-down. Courtesy ANCO Engineering.

Within the US, only those two firms offer simultaneous multiaxis shaker systems.

Japanese auto industry testing
Meantime, IMV in Japan gained considerable seismic multiaxis shaker experience with long-stroke (about 500 mm) ED shakers. Yes, I said 500 mm ED, useful to perhaps 35 Hz. With lesser stroke of 50 mm and useable to 1,000 Hz, IMV and its Japanese competitors Shinken and EMIC have sold numerous systems to Toyota, Honda and other Japanese automobile firms.

Wishing to offer high frequency multiaxis shaking services in the Detroit region, Spectrum Technologies,, has imported two IMV systems in order to offer simultaneous multiaxis shaking to manufacturers of automotive hardware. Figure 1 shows automotive headlamps being simultaneously vibrated in three axes. A “tent” can be lowered over the vibrating table for shaking at temperature extremes.

Readers may have difficulty in seeing the three ED shakers in Figure 1. The isometric drawing of Figure 8 makes them more visible. Viewers often ask “What protects the individual shakers X, Y and Z from harming each other?” Interested readers can ask IMV for details of their ICCU unit, which accommodates and protects the three shakers.

Figure 8 Isometric view clarifies the arrangement of the three shakers of Figure 1.

Factors favoring simultaneous testing
The principal advantage of simultaneous (over sequential) multiaxis testing are
• fewer weakness “escapes”,
• time saving: 1 test instead of 3
• lesser handling, attaching of accelerometers, etc
• lesser tooling: 1 fixture instead of three.

Offsetting those advantages, of course, is the increase in investment: three shakers + three power amplifiers.