Numerical and experimental study on dynamic analysis of dimple-flexure interactions in hard disk drives

다국어 초록 (Multilingual Abstract)

Recently, hard disk drive (HDD) market steadily grows up. In particular, 2.5 inch mobile HDD has more market share than 3.5 inch HDD with 2010. In 2013, market of mobile HDD occupies by 70% in total HDD market. This means that growth of mobile HDD is much higher than desktop HDD. Therefore, technologies such as slim form factor platform, high speed interface, low power mechanism, high shock and temperature reliability are required. Load/unload (L/UL) technology has advantages proper to mobile HDD such as high areal density, high shock reliability and low power consumption. Currently, L/UL technology has been used in almost all HDD. Generally, mobile HDD is often faced with inevitably mechanical problems because of portable aspect. Therefore, much faster emergency parking is absolutely necessary to protect the system from a sudden power-off or an external shock. However, very fast emergency parking causes a large ramp contact, and then it is possible to create the dimple-flexure interaction such as dimple-flexure slip, head-gimbal assembly (HGA) vibration, unexpected slider motion, and so far. We expect that these dimple-flexure interactions largely affect slider dynamics and unloading performance. This means that the dimple-flexure interactions become one of the most important factors. In spite of this fact, studies for the dimple-flexure interaction are still insufficient. Recently, some laboratories and companies give thought to the dimple-flexure interaction such as the dimple-flexure slip in various cases which are flying, seeking, and L/UL.
Therefore, in this thesis, we proved existence and characteristics of the dimple-flexure interaction, analyzed the dimple-flexure interaction using experiments and finite element modeling, and investigated that the dimple-flexure interaction affects the slider dynamics and unloading performance.
At first, we evaluated feasibility of dimple-flexure slip through repeated L/UL test. From experiments, we verified the main cause of dimple-flexure slip in order to analyze the dimple-flexure interaction, and then we investigated relation between the dimple-flexure slip and a ramp contact during an emergency parking which is much faster than a controlled normal unload. We also analyzed characteristics of the dimple-flexure slip for various emergency parking velocities and ramp contact events.
Next, as a result of brief modeling, we showed that many parameters that affect the dimple-flexure slip. The dimple-flexure slip increases according to increase of unloading velocity regardless of coefficient of friction. We also measured the ramp contact force and the duration time to analyze dimple-flexure interaction. Using these results, we designed the FE ramp contact simulation based on FE HGA model, disk and air-bearing stiffness model. When the ramp contacts the suspension lift-tab, transient analysis was performed considering contact motion. As a result of analysis of simulation results, amount of dimple-flexure slip increases according to emergency parking velocity.
At last, we analyzed slider dynamics and unload performance considering the dimple-flexure interactions. When the suspension lift-tab contacts the ramp, slider dynamics instantaneously changes over disk. As the unloading velocity increases, FH loss also increases. In particular, loss of minimum FH is much larger than one of real FH at faster emergency parking velocity. In actual HDD system that needs lower flying height due to higher areal density, a FH loss is likely to cause flying instability and failure. Also, in L/UL analysis, we investigated unload performance from analysis considered the dimple-flexure interaction. The losses of minimum clearance at very fast emergency parking velocity are able to cause critical problems such as slider-disk contact and system instability in lower flying HDD system. Therefore, it is necessary that advanced unload analysis method considering the dimple-flexure interaction by the ramp contact to perform more precise unload analysis in case of fast emergency parking.