Hard Disk Drive Spindle Research Articles

Modified structure for reduction of cavitation at fluid dynamic bearing of HDD spindle motor under non-operating axial vibration

A hard disk drive (HDD) is subjected to the axial vibration while it is transported long distance or stored in vibrating environments. The cavitation may occur in the lubricant film of the spindle thrust bearing due to the axial vibration. The cavitation bubble may push the lubricating oil out of the bearing clearance so that it may be resulted in system failure of the HDD. The formation of cavitation bubble is studied for a specially designed fluid dynamic thrust bearing (FDTB) for the HDD. A novel design method for the FDTB is proposed, which suppresses cavitation effectively.

Robust hub design to prevent mechanical contact between the hub and sleeve of FDBs in an HDD due to disc clamping force

We investigated the rotational harmonic source of the runout in a 2.5? hard disk drive (HDD) with a height of 5 mm, especially at low rotating speeds. After disassembling the HDD, circumferential scratches between the upper thrust surface of the hub and sleeve were observed. Through a finite element static analysis of the hub, the deflection resulting from disc clamping force was identified as a source of mechanical contact between the rotating hub and the stationary sleeve. This was determined to generate large-amplitude harmonics in the runout and flying height lower than design values. To prevent mechanical contact between these components, we propose a robust hub design in which the hub thickness radially decreases between the inner and outer thrusts. And HDD spindle motor was prototyped with the proposed hub. Experiments showed that the flying height is increased almost same as design value and the harmonics of the runout were significantly reduced, even at low rotating speeds. After disassembling the prototyped HDD spindle motor, we confirmed that there were no scratches on the surface of the thrust bearing.

Effect of additional harmonics of driving current on torque ripple and unbalanced magnetic force in HDD spindle motors with stator and rotor eccentricity

This paper mathematically investigates the characteristics of torque ripple and unbalanced magnetic force (UMF) caused by additional harmonics of the driving current in hard disk drive (HDD) spindle motors with stator and rotor eccentricities. It shows that additional harmonics of the driving current, stator eccentricity, and rotor eccentricity independently affect torque ripple. It also shows that UMF is not generated in an ideal HDD spindle motor even when the driving current has additional harmonics. However, additional harmonics of the driving current generate additional harmonics of UMF in an HDD spindle motor with stator or rotor eccentricities. The characteristics of torque ripple and UMF caused by additional harmonics of the driving current were indirectly verified by measuring the acoustic noise and vibration of an HDD spindle motor with stator and rotor eccentricities.

Robust optimal design of a magnetizer to reduce the harmonic components of cogging torque in a HDD spindle motor

This research proposes a robust optimal design methodology to reduce the cogging torque of a hard disk drive (HDD) spindle motor due to the coil-positioning error of the magnetizer. The design optimization problem of the magnetizer is formulated with an objective function of the cogging torque and the constraints of the torque constant. The coil-positioning errors measured by computerized tomography are considered as the random variables of the robust optimal design problem. Additional design variables of the magnetizer are chosen in the optimization problem, such as back-yoke thickness, notch depth, etc. Magnetic finite element analysis of the HDD spindle motor is also performed to calculate the cogging torque and torque constant. The cogging torque and torque constant of the optimal design are compared with those of the conventional design, demonstrating that the proposed method effectively reduces the cogging toque of the HDD spindle motor.

Noise and vibration due to rotor eccentricity in a HDD spindle system

This paper numerically and experimentally investigates the characteristics of torque ripple and unbalanced magnetic force (UMF) due to rotor eccentricity and their effects on noise and vibration in a hard disk drive (HDD) spindle motor with 12 poles and 9 slots. The major excitation frequencies of a non-operating HDD spindle system with rotor eccentricity are the least common multiples (LCM) of pole and slot numbers of the cogging torque and the harmonics of slot number ±1 of the UMF. An experimental setup is developed to measure the UMF generated by rotor eccentricity and to verify the simulated UMF. In the operating HDD spindle motor, the harmonics of the commutation frequency of torque ripple (multiplication of pole and phase) are increased by the interaction of the driving current and rotor eccentricity, and they are the same as the LCM of pole and slot numbers for a HDD spindle motor with 12 poles and 9 slots. The major excitation frequencies of the UMF while operating condition are also the harmonics of slot number ±1 and the harmonics of commutation frequency ±1. We verify that the source of the harmonics of slot number ±1 and the harmonics of commutation frequency ±1 in acoustic noise and vibration is rotor eccentricity of the UMF through experiments.

Development of a new spindle motor for a hard disk drive

A new spindle motor is developed with a sloped permanent magnet (PM) for a hard disk drive (HDD). In a conventional spindle motor, a pulling plate is installed at the stationary part under the rotating PM to pull down rotating bodies. This axial force is required for stable operation of the spindle motor using a hydrodynamic bearing. However, the pulling plate has considerable iron loss and a negative torque opposing the direction of rotation due to the induced eddy currents. Our proposed model has a sloped PM surface to generate the required axial force as well as torque without the pulling plate. Optimal design is carried out by a response surface methodology, and the new spindle motors are prototyped. The resulting electrical and mechanical performance of the prototyped motors is compared with that of conventional models, showing the possibility of adapting the proposed model for an HDD spindle motor.

Vibration and Noise in a HDD Spindle Motor Arising from the Axial UMF Ripple

We investigated numerically the characteristics of axial unbalanced magnetic force (UMF) due to axial magnetic design in the spindle motor of a hard disk drive (HDD). The HDD spindle motor has a magnet-overhang and a pulling plate to generate the axial magnetic force, which is applied to the fluid dynamic bearings (FDBs) as a preload in order to increase the axial stiffness of the HDD spindle system. However, the axial UMF ripple with the least common multiple (LCM) harmonics of pole and slot is generated by the pulling plate and magnet-overhang in the HDD spindle motor. We also investigated the characteristics of the axial magnetic forces generated in the pulling plate and the stator core, separately. We found that the axial magnetic forces generated in the pulling plate and the stator core have opposite phases. Furthermore, we proposed an optimal position of permanent magnet with respect to the stator core. We experimentally verified that the LCM harmonics of pole and slot in the vibration and acoustic noise mostly originate from axial UMF ripple and that the proposed design can effectively minimize the LCM harmonics in the vibration and acoustic noise of HDD systems.

Frequency Characteristics of BEMF, Cogging Torque and UMF in a HDD Spindle Motor due to Unevenly Magnetized PM

This research mathematically derives the frequency equations of back electromotive force (BEMF), cogging torque and unbalanced magnetic force (UMF) of a brushless DC (BLDC) motor due to unevenly magnetized permanent magnet (PM). The proposed mathematical equations are experimentally validated by comparing with the measured BEMF, cogging torque and UMF of a hard disk drive (HDD) spindle motor with 12 poles and 9 slots (12P9S). Only 3-multiple harmonics originated from uneven magnetization of the PM in the spindle motor with 12P9S introduce same frequency components to BEMF and slot-multiple harmonics to cogging torque, respectively. However, other harmonics of the uneven magnetization of PM except the 3-multiple harmonics introduce slot-multiple ±1 harmonics to UMF.

J162015 小型HDD用流体軸受スピンドルの揺動応答特性([J162]メカニカルデバイス・システムの知能化:機械の知能化を学際的に考える)

This paper describes the experimental study on swaying impact response of fluid film bearing (FDB) spindle for Hard Disk Drives (HDDs). Recently, the opportunity for the HDD spindle motors exposed to external vibration has been increasing because the HDDs are used for recording device such as slimline laptop, video cameras, car navigation systems and so on. Hence, the rotating shaft contact the bearing from impact, there is a possibility of data loss. In order to avoid the problems, it is extremely important to know how the spindle moves against the large impact on HDDs experimentally. However, as far as the authors know, there are few experimental studies treating the swaying impact response. In this paper, firstly, we propose a new test rig and experimental method for impact response of FDB spindle. Then we conduct the swaying impact test on FDB spindle for 2.5" HDD and show the experimental results of swaying impact response waveform and maximum displacement of disk.

Robust Optimum Design of Fluid Dynamic Bearing for Hard Disk Drive Spindle Motors

This paper describes the robust optimum design considering dimensional tolerances for fluid dynamic bearings (FDBs) of 2.5 in. hard disk drives (HDDs). Recently, 2.5 in. HDDs are widely used for mobile devices such as laptops, video cameras, and car navigation systems. Therefore, in mobile devices, high durability toward external vibrations is essential for high HDD performance. On the other hand, FDBs for HDD spindle motors are generally manufactured by mass production processes which will eventually require reduction of production costs. Consequently, the FDBs are demanded to be easily manufactured and expected to have an insensitive design with low variability of bearing characteristics due to manufacturing errors. In this paper, first, the vibration model of the spindle motor is constructed, and then the vibration experiment was carried out in order to verify the appropriateness of the vibration model. Second, the bearing characteristics are calculated considering dimensional tolerance using optimum design combined with the statistical method in which the dimensional tolerance is assumed to distribute according to the Gaussian distribution. The bearing characteristics are estimated by expectation and standard deviation. Finally, the results of this robust optimum design compared with ones of optimum design neglecting tolerance, and the validity of this technique, were clarified. It was found from the results that the tolerances of radial clearance and groove depth are important factors to be considered to reduce the variability of the amplitude and friction torque. In addition, the variability of the amplitude strongly depends on the groove depth.

Analysis of PDT noise originating from driving method of a HDD spindle motor

This paper investigates the prominence discrete tone (PDT) noise originating from the driving method of a hard disk drive (HDD) spindle motor with 12 poles and 9 slots. Torque ripple of a HDD is reconstructed by the multiplication of measured back electromotive force (BEMF) and measured switching current. It shows that the frequency components of PDT noise match with those of torque ripple. It also investigates the frequency change of PDT noise due to two driving methods which have the different switching-off periods to detect the zero-crossing of the BEMF. The BEMF has odd harmonics of the number of pole pair, but the current has even harmonics of the number of pole pair due to the switching-off period as well as those odd harmonics of BEMF. We theoretically derive the torque equation in terms of the frequency components of BEMF and switching current. We also verify that the even harmonics of the driving current due to driving method generate the 54th harmonic of torque ripple, and show that the pure sinusoidal BEMF with the 6th harmonic can decrease the 36th harmonic of torque ripple as well as PDT noise in the HDD spindle systems.

Development and optimal design of a HDD spindle motor with pulling magnet to reduce electrical loss

A conventional hard disk drive (HDD) spindle motor has a pulling plate to generate the axial magnetic force. However, the pulling plate consumes significant amount of iron loss due to the alternating magnetic field on the pulling plate. We propose the new design of a HDD spindle motor with pulling magnet to generate the pre-load as well as to eliminate the iron loss of the pulling plate. We also develop an optimal design methodology to minimize iron and copper losses from the spindle motor of a computer HDD while maintaining the same level of torque ripple and pulling force. The new design is optimized by the developed optimal design methodology. A metamodel is constructed from the three-dimensional finite element analysis of the magnetic field and the meta-modeling techniques, and the accuracies of the metamodels are discussed. The proposed optimal design problem is solved by the progressive quadratic approximation method. The proposed design reduces the electrical loss of the HDD spindle motor by 30.42 % while maintaining the same level of torque ripple and pulling force.

Robust Optimum Design of Thrust Hydrodynamic Bearings for Hard Disk Drives

This paper describes the robust optimum design which combines the geometrical optimization method proposed by Hashimoto and statistical method. Recently, 2.5″ hard disk drives (HDDs) are widely used for mobile devices such as laptops, video cameras and car navigation systems. In mobile applications, high durability towards external vibrations and shocks are essentials to the bearings of HDD spindle motor. In addition, the bearing characteristics are influenced by manufacturing error because of small size of the bearings of HDD. In this paper, the geometrical optimization is carried out to maximize the bearing stiffness using sequential quadratic programming to improve vibration characteristics. Additionally, the bearing stiffness is analyzed considering dimensional tolerance of the bearing using statistical method. The dimensional tolerance is assumed to distribute according to the Gaussian distribution, and then the bearing stiffness is estimated by combining the expectation and standard deviation. As a result, in the robust optimum design, new groove geometry of bearing can be obtained in which the bearing stiffness is four times higher than the stiffness of conventional spiral groove bearing. Moreover, the bearing has lower variability compared with the result of optimum design neglecting dimensional tolerance.

Torque Performance Analysis of HDD Spindle Motors Based on Geometrical Interpretation

By visualizing the torque as an area enclosed by locus of magnet operating points in B-H plane, several surface mounted permanent magnet (SPM) motors with different pole/slot combinations for hard disk drive (HDD) spindle motors are analyzed to clarify their geometrical characteristics of the maximum torque area. Then, a simple and fast calculation algorithm of predicting the maximum torque is proposed based on the torque geometrical interpretation and its validity is verified through comparison with FEA results. In addition, optimum pole/slot combinations for modern HDD spindle motors are recommended from the view points of reducing vibration resource caused by cogging torque and increasing speed control accuracy.

Source and Reduction of Uneven Magnetization of the Permanent Magnet of a HDD Spindle Motor

Uneven magnetization of a permanent magnet (PM) generates cogging torque with slot harmonics and an unbalanced magnetic force, which are the major sources of vibration and noise in a hard disk drive (HDD) spindle motor. We investigated several potential sources of uneven magnetization of a ring-shaped PM magnetized in a capacitor-discharge impulse magnetizer by transient finite element analysis. We verified our simulation model by comparing the simulated surface magnetic flux density along the ring-shaped PM with the measured flux density. Our research findings demonstrate that positioning error of the coil of the magnetizing yoke is the dominant source of uneven magnetization. High magnetic field intensity in the magnetization moment due to high magnetizing voltage or the presence of a back yoke decreases the uneven magnetization of the PM by compensating for the coil-positioning error.

Optimum Design of Thrust Air Bearing for Hard Disk Drive Spindle Motor

This paper describes the application of geometry optimization method proposed by Hashimoto to the design of air lubricated thrust bearings used for hard disk drive (HDD) spindle motors. The optimization is carried out to maximize the dynamic stiffness of air films because the low stiffness is a serious problem for thrust air bearings in the actual application to HDD spindle motor. The optimized dynamic stiffnesses are obtained by changing the allowable film thickness, which is corresponding to the tolerance of bearings. The results obtained show that the optimized thrust air bearings have the comparable stiffness to the oil lubricated thrust bearings and it is verified theoretically that this type of thrust air bearing can be used for HDD spindle motors.

Robust design of a HDD spindle system supported by fluid dynamic bearings utilizing the stability analysis of five degrees of freedom of a general rotor-bearing system

This paper proposes a method to improve the robustness of a hard disk drive (HDD) spindle supported by fluid dynamic bearings (FDBs) by utilizing the stability analysis of the five degrees of freedom of a general rotor-bearing system. The Reynolds equations and the perturbed equations of the coupled journal and thrust bearings were solved by FEM to calculate the dynamic coefficients. The paper introduces the radius of gyration to the equations of motion in order to consistently define the stability problem with respect to a single variable, i.e., the mass. The critical mass, which is the threshold between the stability and instability of the HDD spindle, is determined by solving the linear equations of motion. The proposed method was applied to improve the robustness of a HDD spindle supported by FDBs by varying the groove parameters. It shows that the optimized groove design obtained using the proposed method increases both the stability and the modal damping ratio of the half-speed whirl mode. This research also determines the motions of the rotating disk-spindle system by solving its nonlinear equations of motion with the Runge–Kutta method. It shows that the groove design optimized using the proposed method has a small whirl radius in the steady state. It also shows that it has very little displacement due to the shock excitation, and that it quickly recovers to the equilibrium state.

Thermal analysis of helium-filled enterprise disk drive

Enterprise hard disk drives (HDDs) are widely used in high-end storage systems for data center. One of key performance requirements for enterprise HDDs is data access rate, which demands very high rotational speed (e.g. 15 k rpm or more) to permit fast access time. To reach such high speed, the disk spindle motor draws more power to spin and hence the temperature of HDD enclosure increases due to large windage loss. It has been known, temperature rise is one of the most fundamental factors that affect the reliability of the disk drive. In order to develop high reliable enterprise HDDs, thermal management of enterprise HDDs needs to be optimized to improve heat dissipation. One possible approach is to fill disk drive with helium because of its lower density and higher thermal conductivity. This paper investigates thermal performances of helium-filled enterprise disk drives through FEM simulations with experimental validations. Windage loss and heat convection of the HDD filled with helium and air are analysed. The simulated and measured temperature distributions of one commercial enterprise HDD with helium-filled and helium-air mixture are compared with those of an air-filled one. The results show 41% reduction of temperature rise of HDD enclosure can be achieved by filling with helium in comparison with that of air-filled HDD. It is also projected that in terms of equivalent cooling capability like air-filled HDD at 15 k rpm, helium-filled HDD spindle can spin up to 19 k rpm, which will greatly increase data access rate by 25% for future enterprise applications.

Dynamics of a Hard Disk Drive Spindle System Due to Its Structural Design Variables and the Design Variables of Fluid Dynamic Bearings

This research investigates the effect of the groove depth, a design variable of fluid dynamic bearings (FDBs), and the shaft diameter, a structural design variable of a hard disk drive (HDD) on the dynamics of a HDD spindle system. The flying height of the HDD spindle system is determined by using the static analysis of the FDBs. The stiffness and damping coefficients are calculated by using the dynamic analysis of the FDBs. The free vibration characteristics and shock response of the HDD spindle system are analyzed by using the finite element method and the mode superposition method. An experimental modal test is also performed to verify the accuracy of the proposed method. This research shows that the shaft diameter changes the rocking frequencies in wider range than the groove depth of FDBs and that the shock response of a HDD spindle system is affected by the groove depth of FDBs. It also shows that the stiffness coefficients of journal bearing affect the rocking frequencies because their magnitudes are within the range of the stiffness of the supporting structure and that the damping coefficients of thrust bearing affect the axial frequency because the stiffness of thrust bearing is much smaller than that of the supporting structure.

An Integrated System Modeling and Analysis of Hard Disk Drive Spindle Motors

Continuously increasing a real recording density requires systematic optimal analysis of hard disk drive (HDD) spindle motors (SPMs) down to submicro/nanometer-scale level. Conventional SPM investigations have usually consisted of discrete analyses under separate disciplines, and thus are unable to achieve such systematical optimum among the motor's principal components. In order to bridge the relational gap existing between the principal components, an integrated system modeling and analysis that can quantitatively analyze the influences of individual components such as the control algorithm, drive circuit, motor, rotor, and bearing on overall characteristics is developed for HDD spindle motors. The influences of pulse-width modulation modes and driving circuit parameters on startup characteristics are investigated to demonstrate the importance of the proposed method.