Importance of Input Vibration Spectrum and Mechanical Response of Test Article

Repetitive Shock (RS) and Electrodynamic (ED) vibration systems produce substantially different vibration conditions at the input point to the test article. These differences are most evident in terms of peak G level and spectrum content.

The "RS" System produces vibration in short bursts which contain extremely high G amplitudes at the leading edge of each air hammer impact. The frequency content of the "RS" spectrum is non-uniform and exhibits many "holes" in the test spectrum.

The "ED" System produces a continuous vibration time history that contains peak G amplitudes that vary within a moderate, programmable range. The distribution of vibration energy over the test spectrum is uniform and easily programmed using accelerometer feedback (closed-loop) control.

The successful implementation of Accelerated Reliability Stress Testing requires a thorough understanding of a product's failure modes as well as the proper selection of the screening test system and its capabilities (ref 2). Without prior knowledge of product responses and the actual dynamic inputs produced by the test system, effective stress test results may be unachievable.

It has been shown that screening test results may be significantly different when the same product is tested on vibration systems with different operating characteristics and performance limitations (ref 1,3). It follows, therefore, that the product's failure modes and the test system's operating characteristics must be understood before selecting the proper stress function, stress magnitude and type of test equipment (ref 2).

Case Study

Two types of vibration screening systems are presently installed in the HALT/HASS lab at a prominent disk drive manufacturer in the San Francisco bay area. One test system is a Repetitive Shock machine, which produces vibration using an array of pneumatically powered impactors, or air hammers. In this discussion, the Repetitive Shock system will be referred to as the "RS" System.

The second vibration screening system in the HALT/HASS lab is an Electrodynamic table, which generates vibration using an electrodynamic (voice coil) shaker powered by an electronic amplifier with closed-loop spectrum control. In this discussion, the Electrodynamic system will be referred to as the "ED" System.

A variety of 3.5 inch disk drive products were tested to establish the response characteristics of these drives using input vibration from the "RS" System and from the "ED" System. The results produced by these two Systems were remarkably different. For example, drives screened on the "RS" System failed almost immediately (within seconds) from initial vibration turn-on, even with the "RS" System controller set for a vibration level of 1.0 G rms. By comparison, drives screened with the "ED" System did not fail abruptly upon turn-on for the same 1 G rms control level. As a rule, failures during "ED" System operation increased generally in proportion to increasing G rms test levels.

Why were the "RS" and "ED" drive failures so dramatically different at presumably low vibration inputs from both machines? To clear the air, a comparative technical evaluation of the "RS" and "ED" systems was undertaken.

The following material describes some technical characteristics of the "RS" and "ED" Systems and offers commentary on the pros & cons of each System with respect to the testing of disk drive assemblies. The application results noted herein may vary for other products as a function of product size, mass and dynamic responses associated with specific failure modes.

The test parameters of primary technical interest for this study were the actual vibration spectrum generated by each System and the maximum vibration amplitude (g pk) contained within the spectrum produced by each System.