Analysis and optimization of dynamic response of air bearing sliders to disk waviness

Flying stability has been becoming more critical for air bearing sliders with extremely low flying height (FH). Therefore, the effects of disk waviness on flying height modulation (FHM) cannot be neglected. This paper presents an analytical study on the mechanism of FHM of air bearing sliders due to disk waviness, and a design optimization for increasing waviness following ability of sliders. An analytical three-degree-of-freedom (3-DOF) model is developed, where the air bearings are modeled as six lumped linear springs and dampers. The purpose of this model is to develop a quantitative understanding of how air bearing sliders respond to disk waviness. The dynamic characteristics of the slider-air bearing system are then analyzed, and the closed-form frequency resonance function (FRF) of FHM to disk waviness is derived. The impact of disk surface features and the positions of the trailing pad, the side pads, the leading pads and the negative pressure center on FHM are also investigated using parametric analysis. The analysis results show that the improvement of the roll-off characteristics of the disk surface waviness can also decrease the FHM. In addition, shortening the distance between the trailing pad pressure center and the head position, moving backward the side pads and leading pads and forward the negative pressure center can increase waviness following ability of the slider. Finally, an air bearing slider is designed according to the proposed design strategies for reducing the FHM due to disk waviness.     

Topological design sensitivity on the air bearing surface of head slider

In this study, a topological design sensitivity of the air bearing surface (ABS) is suggested by using an adjoint variable method. The discrete form of the generalized lubrication equation based on a control volume formulation is used as a compatible condition. A residual function of the slider is considered as an equality constraint function, which represents the slider in equilibrium. The slider thickness parameters at all grid cells are chosen as design variables since they are the topological parameters determining the ABS shape. Then, a complicated adjoint variable equation is formulated to directly handle the highly nonlinear and asymmetric coefficient matrix and vector in the discrete system equation of air-lubricated slider bearings. An alternating direction implicit (ADI) scheme is utilized for the numerical calculation. This is an efficient iterative solver to solve large-scale problem in special band storage. Then, a computer program is developed and applied to a slider model of a sophisticated shape. The simulation results of design sensitivity analysis (DSA) are directly compared with those of FDM at the randomly selected grid cells to show the effectiveness of the proposed approach. The overall distribution of DSA results are reported, clearly showing the region on the ABS where special attention should be given during the manufacturing process.     

Mechanism studies of the multiple flying states of the air bearing slider

The multiple flying states of a negative pressure air bearing slider is demonstrated and verified by simulation. It is observed that a slider may have one to three balanced flying states under certain conditions. One of these states is the intended flying state, while the other two are much higher and could make the read/write operations unreliable. It is observed that there is no suction force when the slider is at the highest flying state, but there is a small or no suction force in the median flying state. It is noticed that these three states could interchange with each other under certain circumstances. Some aspects of their physical meanings and characteristics are discussed. Simulations were done for loading forces of 2.5 and 1.5 g, respectively. Negative air bearing stiffness is found in certain regions, and the regions could affect the load/unload performance of the slider.     

Simulation of a head slider considering a discrete track recording technology

Discrete track recording (DTR) is a new application technology that utilizes a separate physical storage disk with grooves and ridges comprised of radial and circumferential direction in order to achieve higher data transfer rates and storage densities on a hard disk drive (HDD). However, the grooves and ridges on the DTR media cause flying stability issues. Therefore, we analyze and compare the dynamic performance of a head slider with three types of DTR media in which different DTR parameters are defined, such as groove width and pitch. Prior to simulations, we propose the DTR flying height (FH) loss equation which estimates a loss of FH on the DTR media using the defined DTR parameters. The accuracy and creditability of this equation is then predicted by comparing the results of the equation to those of the simulation. Consequently, we propose a method for designing the air bearing surface (ABS) on the DTR media using the static performance of the ABS on a continuous track recording (CTR) along with the proposed DTR FH loss equation.     

Optimum design of externally pressurized air bearing using Cluster OpenMP

This study presents a performance evaluation of a new portable parallel programming paradigm, the Cluster OpenMP (CLOMP) for distributed computing, in conducting an optimum design of air bearings. The multi-objective optimization was carried out by using a genetic algorithm (GA) incorporating Pareto optimality criterion. Since the GA is natural parallel evolution algorithm, the computation of the search was carried out in parallel by using the CLOMP. In this study, the performance of a CLOMP cluster of four dual-core computers for the air bearing optimization was compared with a shared-memory processing (SMP) computer equipped with two quad-core processors. To examine the parallel efficiency of the CLOMP in the GA optimization, several multithread applications of various task sizes were tested. It is shown that the air bearing optimization can be effectively dealt with by the CLOMP (parallel efficiency of 96.2-98.8%) as well as the SMP computing (93.1-99.4%) in the studied cases. The CLOMP retains the characteristics of directive-based OpenMP, such as incremental programming and serial-coding compatibility. The verified high parallel efficiency of the CLOMP cluster demonstrates its potential applications of the scalable computing in many tribological optimizations.     

The behavior of the liquid-like Langmuir's adsorbed film on air bearing surfaces

The contamination of slider air bearing surfaces may lead to a critical failure of hard disk drive systems. The contamination may be classified into two types: the liquid-like contamination and the particle contamination. In this study, the liquid-like contamination is concerned. It is assumed that the liquid-like contamination is caused by the adsorption of contaminant molecules in air. A mathematic model has been developed to predict the accumulating process of the liquid-like contaminant on air bearing surfaces. Numerical analysis of the behavior of the liquid-like adsorbed film for two different types of air bearing surfaces was conducted by using the proposed model based on the Langmuir adsorption. The results show that the contaminant tends to accumulate on the recession area of the air bearing surface when the viscosity of the liquid-like adsorbed film is low. However it may also accumulate in the front area of the air bearing surface when the viscosity is as high as 20 Pa s.     

Effect of environment humidity and temperature on stationary and transient flying responses of air bearing slider

It is well known that the environment humidity and temperature have a significant influence on the flying height of an air bearing slider. However, not many research papers address this topic, especially when the transient flying response is considered. This paper studies the influences of the environment humidity and temperature on both the stationary and transient flying responses of slider by simulation. A slider design for the thermal protrusion application is addressed. The reason for causing the drop of the air bearing pressure is discussed, and the methods for decreasing the drop are proposed. It is observed that the environment humidity and temperature may determine whether the slider is in full flying state or in partial flying/partial dragging state, when the slider is released from a certain height. The reason may be due to the high humidity and temperature which weakens the air bearing. As a result, the air bearing becomes not strong enough to support well the full flying of slider when the influence of the intermolecular force is significant. Slider vibrations for the full flying case and the partial flying/partial dragging case are analyzed in frequency domain, and the slider vibration frequencies are discussed. It shows that the environment temperature and humidity have significant effects on both the stationary and transient flying responses of the slider.     

Air Bearing Design to Prevent Reverse Flow from the Trailing Edge of the Slider

A simulation approach that relies on an analysis of the flow patterns closest to an air bearing surface (ABS) was used to predict the lubricant accumulation on the ABS of a head slider. The lubricant accumulation patterns obtained through the simulation were in good agreement with experimental results and with our experimental apparatus. We used this method to study and analyze flow pattern droplets close to the trailing edge of a number of sliders and found that there was a reverse flow from the slider’s trailing edge on both sides of the trailing pad and behind the read/write element, which could result in a lubricant accumulation on the slider surface close to the trailing edge of a slider and thus lead a transient slider vibration and magnetic-signal loss in a hard disk drive. Further simulations and analyses revealed that the reverse flow is dependent on the depth of slider surface on adjacent to the trailing edge of the slider, and that if the depth is less than a critical depth, which is dependent on the velocity of the disk, the reverse flow could be eliminated. On the basis of these findings, we propose a new ABS design concept for effectively suppressing the reverse flow of lubricants from the trailing edge of the slider. In this concept, the slider has a “smooth flow pad” and the depths of outlet recesses are specified as being smaller than the critical depth. It was confirmed by both simulation and experiment that lube accumulation on the slider surface is obviously decreased and the reliability of a hard disk drive with this air bearing design is consequently improved.     

Effects of temperature dependent air properties on the performances of a thermal actuated slider

Thermal actuated sliders have been widely used in today's hard disk drive industry for its advantages of easier control of flying height (FH) and less risk of contacts with the disk over the conventional slider. In this paper, we used a coupled-field analysis method, which includes an air bearing model, a heat transfer model and a thermal-structural finite element (FE) model to investigate the flying and thermal performances of a thermal actuated slider at various environmental temperatures. We also proposed a generalized mean free path model to incorporate various molecular dynamics models and consider temperature effects of the mean free path. Some temperature dependent air properties, such as the viscosity and the thermal conductivity are also considered in the simulation. It is found that the mean free path is a crucial parameter in determine air bearing and heat transfer across the head-disk interface (HDI). Our simulation results also show that the temperature effects of the viscosity and the thermal conductivity are contrary to that of the mean free path, which limit the variations of air bearing and heat transfer as the environmental temperature increases. However, their temperature effects still need to be considered for an accurate simulation, especially when the disk drives operate in a wide temperature range.     

An Air Bearing Slider for Small Diameter Disk Storage Devices

The authors propose a novel slider design. The tri-pad slider has three separate air bearing surfaces, one in the front of the slider and the other two in the back of the slider. This design permits the optimization of the air bearing stiffness and flying attitude without changing the minimum flying height. It can minimize the effect of skew angle on the flying height, and offers ample room for a larger size thin film head element. Samples were fabricated and flying characteristics at several velocities, as well as the dynamic flying stability, were investigated.     

Reliability-based design optimization of slider air bearings

This paper presents a design methodology for determining configurations of slider air bearings considering the randomness of the air-bearing surface (ABS) geometry by using the iSIGHT. A reliability-based design optimization (RBDO) problem is formulated to minimize the variations in the mean values of the flying heights from a target value while satisfying the desired probabilistic constraints keeping the pitch and roll angles within a suitable range. The reliability analysis is employed to estimate how the fabrication tolerances of individual slider parameters affect the final flying attitude tolerances. The proposed approach first solves the deterministic optimization problem. Then, beginning with this solution, the RBDO is continued with the reliability constraints affected by the random variables. Reliability constraints overriding the constraints of the deterministic optimization attempt to drive the design to a reliability solution with minimum increase in the objective. The simulation results of the RBDO are listed in comparison with the values of the initial design and the results of the deterministic optimization, respectively. To show the effectiveness of the proposed approach, the reliability analyses are simply carried out by using the mean value first-order second-moment (MVFO) method. The Monte Carlo simulation of the RBDO’s results is also performed to estimate the efficiency of the proposed approach. Those results are demonstrated to satisfy all the desired probabilistic constraints, where the target reliability level for constraints is defined as 0.8.     

Magnetic-fluid-based porous inclined slider bearing

A porous inclined slider bearing, lubricated with a magnetic fluid, in the presence of an externally applied magnetic field which is oblique to the lower surface of the bearing is discussed. The load capacity of the magnetic-fluid-based porous inclined slider bearing is found to be greater than that of a viscous porous inclined slider bearing. It is shown that the magnetic-fluid-based porous inclined slider bearing has a performance superior to that of the viscous porous inclined slider bearing.     

Static and Dynamic Slider Air-Bearing Behavior in Heat-Assisted Magnetic Recording Under Thermal Flying Height Control and Laser System-Induced Protrusion

The air bearing’s response to regions of elevated temperature on its bounding surfaces (the slider and disk) may be an important consideration in the head–disk interface design of heat-assisted magnetic recording (HAMR) systems. We implement the general non-isothermal molecular gas lubrication equation into an iterative static solver and dynamic air-bearing solver to evaluate the effect of localized heating of the air-bearing surface (ABS) due to the near-field transducer (NFT). The heat-dissipating components in our simplified HAMR design are the NFT, laser diode, and thermal flying height control (TFC) heater. We investigate the effect of each HAMR slider component on ABS temperature and thermal deformation and the slider’s flying height. The NFT induces a localized thermal spot and protrusion on the larger TFC bulge, and it is the location of maximum temperature. This ABS temperature profile alters the air-bearing pressure distribution, increasing the pressure at the hot NFT location compared with predictions of an isothermal air-bearing solver, so that the center of the pressure acting on the ABS is slightly closer to the trailing edge, thereby decreasing the pitch angle and increasing the minimum flying height. Other researchers have shown that the NFT’s thermal response time may be much faster than its protrusion response time (Xu et al. in IEEE Trans Magn 48:3280–3283, 2012). The slider’s dynamic response to a time-varying NFT thermal spot on the ABS while the combined TFC and NFT induced thermal protrusion remains constant is investigated with our dynamic air-bearing solver. We simulate the slider’s step response to a suddenly applied ABS temperature profile and a pulsed temperature profile that represents laser-on over data zones and laser-off over servo zones. The sudden (step) or rapid (pulse) increase in ABS temperature induces a sudden or rapid increase in pressure at the NFT location, thereby exciting the air bearing’s first pitch mode. For the slider design and simulation conditions used here, the result of the pitch mode excitation is to alter the position of the center of pressure in the slider’s length direction, thereby changing the pitch moment. In response, the pitch angle and minimum flying height change. The step response decays after approximately 0.15 ms. Because the laser duty cycle is much shorter than this response time, a periodic disturbance is predicted for the center of pressure coordinate, pitch angle, and minimum flying height. The peak-to-peak minimum flying height modulations are relatively small (only up to 0.126 nm); more significantly, the time-averaged minimum flying height increases 0.5 nm for the NFT that reached 208 °C compared to simulations of the isothermal ABS at ambient temperature.     

Lubricant Flow and Accumulation on the Slider’s Air-Bearing Surface in a Hard Disk Drive

Lubricant accumulation on the slider’s surface of a hard disk drive (HDD) has a detrimental effect on its read/write performance. Air flow through the slider-disk clearance moves some of the lubricant from the air-bearing surface (ABS) toward the slider’s lateral walls where it accumulates. In this article, we show by numerical simulations that the lubricant accumulation characteristics are strongly dependent on the slider’s flying height, skew angle and ABS design. The lubricant flow on the slider’s surface is quantified numerically. Air shear stress, air pressure and disjoining pressure are used as driving forces in the simulations. The lubricant thickness profile and volume evolution are calculated for two states of the HDD: operating and at rest. In the first state, lubricant is driven by air shear stress toward the trailing edge of the slider where it accumulates on the deposit end. In the second state, lubricant from the deposit end flows back into the ABS driven by the action of disjoining pressure. Lubricant accumulation on the four lateral walls of the slider is taken into account. The lateral walls are unfolded to study the flow using a two-dimensional lubrication model. The effects of flying height, skew angle and slider design on the accumulation removal of lubricant from the ABS are determined for the two states of the drive.     

Head and media design considerations for reducing thermal asperity

Recent use of magnetoresistive (MR)/giant-magnetoresistive (GMR) heads in disk drives requires tighter control on particle contamination that may generate thermal asperities at the head/disk interface. In this study, the effects of slider air bearing surface (ABS) design and media on TA reduction capability are investigated. The motion of particles at the head/media interface is simulated numerically. Drive level TA tests are performed using a particle injection chamber. It is observed that a new ABS design, that has an aerodynamic U-shaped rail and a central airflow passage, is beneficial in reducing the particle contamination on the slider. Scratch-resistance of the media surface is correlated to TA reduction capability of the media; more scratch-resistant media produced less TA. On the other hand, the adhesion properties of the media have minor effects on TA reduction, as the particles that usually generate TA in a drive are too small to spin off the media.     

Head slider designs using approximation methods

This paper presents an approach to optimally design the air bearing surface (ABS) of the head slider by using the approximation methods. The reduced basis concept is used to reduce the number of design variables. In the numerical calculation, the progressive quadratic response surface modeling (PQRSM) is used to handle the non-smooth and discontinuous cost function. A multi-criteria optimization problem is formulated to enhance the flying performances over the entire recording band during the steady state and track seek operations. The optimal solutions of the sliders, whose target flying heights are 12 nm and 9 nm, are automatically obtained. The flying heights during the steady state operation become closer to the target values and the flying height variations during the track seek operation are smaller than those for the initial one. The pitch and roll angles are also kept within suitable ranges over the recording band.     

The optimum profile for a porous slider bearing

The load capacity of a porous slider bearing was investigated. The optimum profile was found to be a step form with the riser location and step height ratio depending on the porosity. Porosity decreases the load capacity of a slider bearing. A porous slider bearing of optimum profile supports more load than a porous inclined slider bearing     

Optimization of slider air bearing contours using the combined genetic algorithm-subregion approach

A hybrid optimization approach, the combined genetic algorithm-subregion method, which combines the advantage of the genetic algorithm and the subregion approach, is presented. Using a binary string to represent a selected design space, the combined genetic algorithm-subregion method adopts the genetic algorithm to perform the optimization process. Starting from a pico slider design originally flying at 14 nm, optimized designs were obtained for sliders with target flying heights of 7, 5 and 3.5 nm, respectively. The results show that the combined genetic algorithm-subregion method has good convergence with a substantial reduction of computation time.     

Contact-induced off-track vibrations of air bearing-slider-suspension system in hard disk drives

Contact-induced vibration of air bearing-slider-suspension system is a crucial issue for slider flying stability and head positioning precision of 1 Tbit/in² hard disk drives. In this paper, the contact-induced off-track vibrations of air bearing-slider-suspension system are investigated by simulation. A dynamic simulator is developed to calculate the interactions between the air bearing dynamics and vibrations of slider-suspension assembly. The simulation model consists of a finite element model of suspension assembly, an air bearing model based on the generalized lubrication equation, and a slider–disk contact model based on the probability distributions of surface roughness. A sequential method is used to couple all these models and analyses. The time history of the slider and suspension motions, together with the time-varying forces including air bearing force, air shear forces, contact force and friction force can be obtained. The effects of different contact conditions, such as the contact intensity, friction coefficient, and disk surface waviness on off-track vibrations are investigated numerically in details. The results reveal some mechanisms on how these factors contribute to the off-track vibrations of suspension assembly.     

The effects of slider design on thermal asperity rejection capability

Particle contamination has been an ongoing problem affecting the reliability of the magnetic hard disk drives. Especially the recent use of MR head requires much tighter control of particle contamination due to thermal asperity (TA) phenomenon. In this study, the effects of slider air bearing surface design on TA reduction capability were investigated by manufacturing two types of sliders. Numerical methods were used to simulate the motion of particles in the head/media interface. Experiments were conducted to verify the results predicted by the numerical simulation. Drives were built and exposed to particle contamination using a particle injection chamber, which turned out to be a very simple and reliable particle generation method over conventional aerosol technique. Then the number of TA generated in the drives was recorded and compared. Also the contacts between slider and particles were investigated by acoustic emission study. It was found that a new ABS design, which has aerodynamic U-shaped rail and central flow passage, was beneficial in reducing the particle contamination on the slider.