Hard Disk Drive Spindle Motor 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.

Reduction in Adsorption of Bearing Oil From Fluid Dynamic Bearings of Hard Disk Drive Spindle Motor by Diamagnetism

In this study, we focused on the diamagnetism of ester lubricant oil for fluid dynamic bearings (FDBs) and investigated the effect of a magnetic field on oil vapors. We vaporized the lubricant oil and compared the amount of oil adsorbed on disk surfaces through two slits-one with magnets and the other without a magnet-in a preliminary experiment. As a result, we confirmed that it is possible to reduce the amount of oil adsorbed on a disk surface with the aid of a magnetic field. Based on the preliminary experiment, the outflow of the FDB oil from the hard disk drive (HDD) spindle was compared with regard to a narrow clearance with and without the magnetic field gradient of permanent magnets. The amount of outflow was decreased by the magnetic field gradient, and the reduction ratio depended on the environmental temperature. Finally, our results suggest the possibility of a gas seal generated by the magnetic field gradient.

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.

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.

Effects of Tolerance Design on Suppression of Electromagnetic-Induced Acoustic Noises and Vibration Transmission in Hard Disk Drive Spindle Motors

In this paper, a mechanism of acoustic noise generation in hard disk drive (HDD) spindle motors is presented. Three parts are reviewed and analyzed here: (1) the characteristics of electromagnetic (EM) excitations, which are the major noise source in the motors; (2) the transmission of EM-induced vibrations to the motor exterior; and (3) the propagating vibration energy capable of converting to acoustic noise. Lastly, the approach to reduce the transmitted vibration energy by optimizing the interference fit between the stator and base assembly of the motor is experimentally investigated.

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.

Vibration of 1.8-in Hard Disk Drive Spindle Motors at Various Ambient Temperatures

In this paper, vibration of 1.8-in hard disk drive (HDD) spindle motors is studied experimentally and theoretically. In the experimental study, vibration of 1.8-in HDD motors is measured at 5degC, 25degC, and 70degC in a thermal chamber. Experimental results reveal two important phenomena. First, vibration of 1.8-in HDD spindle motors is dominated by a (0,0) unbalanced mode that has a significant bending deformation of the base plate. Second, the natural frequency of the (0,0) unbalanced mode and the static gain of the frequency response function change significantly as the ambient temperature varies. To explain these experimental observations, we apply a mathematical model developed at the University of Washington to analyze the vibration of the 1.8-in HDD spindle motors. The analysis shows that (0,0) unbalanced mode shifts its natural frequency because membrane stresses are developed in the base plate during temperature changes as a result of a significant mismatch of coefficients of thermal expansion between the base plate and the fixture. Moreover, the change of the static gain results from reduction of fluid-dynamic bearing coefficients as the temperature varies.

Design of Hydrodynamic Bearing for Miniature Hard Disk Drives

This paper has two specific goals to achieve. The first goal is to address major challenges encountered in design and development of miniature hydrodynamic bearings for small form factor hard disk drive spindle motors. These challenges include extremely small thickness of the spindle motors, mass productivity of tiny bearing parts, and bearing reliability and performance in relation to changes in atmospheric pressure. To overcome these challenges, we have developed a prototype hydrodynamic bearing that is resistant to environmental changes, free from oil film disruption, and tolerant of variations in processing precision. The prototype hydrodynamic bearing has two special features: an oil circulation function and a gas/liquid separation function. The oil circulation function is achieved by using asymmetric hydrodynamic grooves together with circulation passages. The gas/liquid separation function is achieved by using tapered capillary seals in the circulation passages. As a result of these two special features, the prototype hydrodynamic bearing can successfully expel air bubbles out of the bearing, thus enhancing the performance and reliability of the hydrodynamic bearing. The second goal is to evaluate the prototype hydrodynamic bearing in a miniature spindle motor for small form factor hard disk drives. Two specific bearing designs are evaluated. The first bearing design is a pushing type that generates no low-pressure area. However, the flow rate increases at high temperature. The second design is just the opposite. It is a pulling type design that generates a low-pressure area. But the flow rate is constant and is independent of temperature changes. To accommodate manufacturing tolerances, a parametric study is conducted to identify the effects of manufacturing tolerance on bearing performance.

Development of High Performance Multi-Lobe Fluid Dynamic Bearings Using a Sintered Material

Recently, in spindle motors for hard disk drive (HDD) devices, fluid dynamic bearings (FDB) with herringbone grooves have come to be used instead of ball bearings due to the demand for high density recording of the devices, improvement in the speed of data transfer, and the quietness of the motor. In this study, a 5-lobe bearing with high bearing stiffness using a sintered material, as a new trial, was developed, and the bearing performance was examined by simulated calculations and experiments. As a result, it was clarified that the 5-lobe bearing had the required performance for practical use in the spindle motor for HDD by means of optimizing the bearing’s dimensions. In addition, bearing loss of the 5-lobe bearing is lower than that of the herringbone bearing, and the 5-lobe bearing showed clearly that it is effective in being used in the miniaturization of HDD spindle motors.

Optimization of 3.5-in. HDD spindle motors for OP-vibration performance: theoretical prediction and experimental verification

The purpose of this paper is to optimize OP-vibration performance of 3.5-in. hard disk drive (HDD) spindle motors through theoretical prediction and experimental verification. OP-vibration performance of HDD is closely related to the first rocking vibration of spindle motors because excited frequencies of 3.5-in. HDD from the environment are mostly below 500 Hz and the first rocking vibration is the only resonance in the corresponding frequencies. Therefore, minimizing first rocking vibration leads to improve OP-vibration performance of the spindle motors. In order to minimize the first rocking vibration key parameters of FDB spindle motors were selected from a previous work done by Heo and Shen (Microsyst Technol 11:1204–1213, 2005). Then, the selected parameters have been optimized to minimize the first rocking vibration through a theoretical model developed at University of Washington. Then, experiments with ten prototype FDB spindle motors have been conducted to verify the theoretical results. Each prototype motor has different spindle parameter configurations including bearing coefficients, bearing locations, and center of gravity location, etc. Also, this paper demonstrated that radial measurements of spindle rocking vibration have better correlation with OP-vibration performance than axial measurements through PES measurements. Finally, the optimized design has been manufactured by a motor maker and has also successfully verified the theoretical prediction experimentally.