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Why HALT/HASS vibration should be simultaneous multiaxis random.

In his March column in this magazine, Peter Grundy mentions "HALT and HASS employ both thermal and vibration analysis together so the product can be exercised in controlled environments that it would not see regularly in normal life."¹ Grundy notes that the applied mechanical vibration must be random, but fails to point out that the random vibration should simultaneously be applied in all axes. This column, emphasizes these two points.

Random vibration needed. Willoughby² in 1979 decried the time wasted on sinusoidal, one-frequency-at-a-time vibration for ESS (environmental stress screening). He stated that the applied vibration should be all-frequencies-simultaneously random. What is random vibration? When the array of tuned reeds in Figure 1 is touched to the table of a classical one-frequency-at-a-time sinusoidally-vibrating shaker, one reed will respond, as shown. But if it is touched to an all-frequencies-simultaneously vibrating shaker table, all reeds will respond simultaneously.

The former represents, in the test lab, exciting one resonance at a time within a product. The classical sweeping of shaker frequency eventually excites all the resonances, but only one at a time. Random vibration testing or screening excites those resonances simultaneously, and is far more effective in finding weaknesses in a product.

Simultaneous multiaxis shaking is needed. But in 1979, state-of-the-art products underwent three random vibration tests or screens on the electrodynamic shakers of that era: with first the product's x, then its y and finally with its z axis aligned with the shaker axis of motion. This procedure was necessary with mechanical shakers and had been accepted with electrodynamic shakers such as the one being assembled in Figure 2.

Since 1979 a number of laboratory investigations of field failures have found them impossible to duplicate on standard single-axis-at-a-time electrodynamic shakers. But they are possible to duplicate on, for example, three shakers simultaneously driving a load in its x, y and z axes (Figure 3).

Multiple electrodynamic shakers are expensive. Worldwide, only a few labs have capabilities comparable to Figure 3: three or more shakers, three or more power amplifiers, three or more control channels. They are too costly for the many facilities that need to perform ESS, HALT and HASS.

Pneumatic vibrators can do the job. Imagine yourself on the lab floor, looking upward at the horizontal vibrating platform of a modern HALT/HASS chamber. You see something like Figure 4. Operating off compressed air and lacking much if any periodicity, a number of tilted RS or repetitive-shock devices (bangers) attach (on different compass headings) to the bottom of this softly-sprung horizontal platform. Because the RS units are not synchronized, the platform simultaneously translates north-south, east-west and vertically. It also rolls, pitches and yaws. That platform is the bottom of a fast-ramping thermal chamber. Devices you wish to test or screen are attached topside (Figure 5), where they receive thermal and simultaneous multiaxis vibration stimuli. Such combined stimuli are being used in labs worldwide to detect hardware (especially electronics) weaknesses.

References

1. Peter Grundy, "Will Your Product Survive in the Field?" Circuits Assembly, March 2006.

2. Willis J. Willoughby, Jr., "Navy Manufacturing Screening Program - Decrease Corporate Costs - Increase Fleet Readiness," NavMat P9492, May 1979.

Bibliography

• Equipment-reliability.com, "Random Vibration & Shock Testing, Measurement, Analysis & Calibration - as applied to HALT, ESS, HASS & COTS," ISBN 0-9741466-0-9.

Wayne Tustin is an author and founder of Equipment Reliability Institute (equipment-reliability.com), a specialized engineering school; tustin@equipment-reliability.com. His next short course is Aug. 22-24 in Santa Barbara, CA.

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