An Experimental and Simulation Study of Touchdown Dynamics
Dynamic flying height technology has been widely employed for reducing the mechanical spacing between the magnetic heads and the disk. As the recording density of hard disk drives approach 1 Tbit/in2, the spacing is decreased to sub 1 nm. At such low spacing, the touchdown (TD) dynamics becomes extremely critical. First, it affects the accuracy of the spacing setting, or TD detection. Second, it affects the hard disk drive reliability, such as writing modulation, wear, instability, etc. It decides how low the slider can fly stably and reliably. In this paper, we tried to have a better understanding of the TD dynamics with several designs of experiments first. We found that there were two stages in the whole TD process. In the first TD stage, a low frequency (30-150 kHz) vibration appeared. It was a suspension mode that was excited by the lubricant on the disk. In the second stage, a high frequency vibration (200-400 kHz) appeared. It was the second pitch mode of the slider air bearing excited by the contact between the slider and the disk. Based on these experimental observations, we propose modeling and simulation procedures with a combination of a full suspension model and a simplified air bearing model. Simulation can predict these two TD frequencies very well. Therefore, it could be applied to design head/disk interface to help achieve preferred TD behaviors.
Available from: Xiaohang Jin
- "During flying, slider may encounter the soft contact, or hard contact; the contact force may be in out-of-plane ( -direction) or in-plane (off-track and down-track) . In this paper, the out-of-plane force is denoted as , and the in-plane force is denoted as . "
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ABSTRACT: To achieve the areal density of 1 TB/in<sup>2</sup> in hard disk drives (HDDs), the magnetic spacing between the slider and disk needs to reduce to 1 nm or smaller. At such small clearance, the contact between the slider and disk is inevitable. Slider-disk contact may induce a drastic slider response, which has a significant negative impact on the reading/writing performance. In this paper, the effect of a low-frequency vibration in the Z-direction (out-of-plane) brought by the tip of gimbal (TG) on slider dynamics is studied. The impacts of three different excitations on the TG resonance are explored. Under such excitations, the bending mode of TG can be easily excited, which consequently induces the vigorous slider vibration. The influences of the air-bearing stiffness on slider dynamics subject to TG mode are also studied. A slider designed with weaker air-bearing stiffness will suffer a stronger TG resonance than that of stiffer air bearing. These experimental and simulation results show that better TG design and more strict environmental or excitations control are required to achieve a robust head disk interface (HDI).
Available from: Yu Wang
- "The end results may be incapable of providing sufficient information to identify the HDI degradation mechanisms that are crucial in guiding the enhancements of the slider/disk designs as well as in facilitating the extension of HDD life. HDI degradation can be reflected by more than just mechanical damage (wear) to the interface; the vigorous slider vibration that can be used to characterize the instability of HDI system is also a significant sign of HDI degradation –. Fig. 2. Diagram of experimental setup. "
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ABSTRACT: To further increase the areal storage density of hard disk drive (HDD), one solution is to reduce the spacing between the magnetic head and disk to sub-1-nanometer regime. Such ultra-low spacing introduces great challenges to tribological reliability in the head-disk interface (HDI). Therefore, it is necessary to understand the characteristics and mechanisms of HDI degradation in the tribological condition. This paper investigates the degradation behaviors of HDI based on an accelerated wear experiment. Acoustic emission sensor and thermal asperity sensor were used simultaneously to monitor the touch down (TD) dynamics before and after each cycle of overdrive test in order to characterize the HDI degradation behaviors. Through analyzing the evolutions of HDI dynamic characteristics, the degradation mechanisms, including wear mechanism shift and HDI instability propagation, were figured out.
Available from: Aravind N. Murthy
- "At 5400 RPM, the interaction is a combination of the above two modes: as the TFC powers increase, a gradual rise in the 148 kHz component occurs first, but a strong vibration in the pitch-2 mode (321 kHz) eventually marks touchdown. These results are in agreement with recent studies that show that at close spacing and at the onset of lubricantcontact , the in-plane shear forces and friction can destabilize the slider for certain ABS designs resulting in vibrations dominantly occurring at suspension and lower air bearing frequencies (60–200 kHz in our case), while stronger contact with the disk causes slider vibrations with higher frequency content (above 200 kHz)  "
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ABSTRACT: Accurate touchdown power detection is a prerequisite for read-write head-to-disk spacing calibration and control in current hard disk drives, which use the thermal fly-height control slider
technology. The slider air bearing surface and head gimbal assembly design have a significant influence on the touchdown behavior, and this paper reports experimental findings to help understand the touchdown process. The dominant modes/frequencies of excitation at touchdown can be significantly different leading to very different touchdown signatures. The pressure under the slider at touchdown and hence the thermal fly-height control efficiency as well as the propensity for lubricant pickup show correlation with touchdown behavior which may be used as metrics for designing sliders with good touchdown behavior. Experiments are devised to measure friction at
the head-disk interface of a thermal fly-height control slider actuated into contact. Parametric investigations on the effect of disk roughness, disk lubricant parameters, and air bearing surface design on the friction at the head-disk interface and slider burnishing/wear are conducted and reported.
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