Design-oriented prediction of operational shock and vibration for hard disk drives

Abstract

The hard disk drives have been increasingly used in consumer electronics and mobile computing, in which they may experience severe shock and vibration. Additionally, there is a fierce competition from the solid state drives. Hence, there is a need for rapid innovation, especially in the ever shorter drives design cycle. This in turn requires faster and accurate simulation method.

This dissertation focuses on the formulation of an efficient theoretical model for design parametric studies and optimization in modern hard disk drives. It integrates both drive structural model and air bearing model. The structural models are developed using flexible multi body dynamics formulation and state-space mode superposition theory. The air bearing model is developed using finite volume formulation and modified quasi-static concept. The coupled structural and air bearing models can simulate the shock response at drive level and also predict the operational shock tolerance effectively. Hertz elastic contact theory is incorporated to deal with structural discontinuities. A modified state-space formulation is also included to simulate the dynamic air bearing location due to disk rotation.

In the analysis of the quasi-static air bearing, it is found that air bearing equilibrium condition does not occur almost-immediately due to the squeeze term effect. Addition of optimized damping elements to quasi-static air bearing model can increase its accuracy and enable it to predict the same air bearing force as the finite volume model.

From parametric studies, it is found that lower contact stiffness between suspension (dimple) and slider (flexure) yield higher shock tolerance. A more rigid cover is also found to increase the shock tolerance.

Shock response analysis shows that the first-three most dominant actuator modes that affect the read/write head shock response are induced by the flexibility of suspension and flexure. It is also discovered that updating the position on the disk, at which the air bearing forces act, at every simulation time-step does not have significant effect on the shock response. Hence, it is not necessary to update the air bearing force location on the disk.

From HDD vibration isolation studies, it can be observed that to survive a harsh shock and random vibration defined in MIL-STD-810E, the natural frequencies of the external vibration isolation system should be between 10 to 20 Hz. The damping ratio required is relatively high (> 10%). To reduce the peak-to-peak displacement during shock event, combination of soft and stiff isolators can be used.