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Vibration monitoring of computer hard disk drives
ARTICLE IN PRESS Mechanical Systems and Signal Processing Mechanical Systems and Signal Processing 20 (2006) 1008–1013 www.elsevier.com/locate/jnlabr/ymssp
Letter to the Editor Vibration monitoring of computer hard disk drives 1. Introduction Computer hard disk drives (HDDs) are one of the most important components within the personal computer (PC). They are used to store and retrieve huge amounts of data. Over the last few decades their reliability, capacity and speed have improved. It is hard to really understand the factors that affect performance, reliability and interfacing without knowing how the drive works internally. Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate—the platter’s base material—must be non-magnetic and capable of being machined to a smooth finish. Both sides of each platter are coated with a magnetic medium to allow data storage. The data is stored in magnetic patterns. Typically, one or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute (Fig. 1). Special electromagnetic read/write devices called heads are mounted onto sliders and used to either record information onto the disk or read information from it. The sliders are mounted onto arms, all of which are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator which contains a voice coil—an electromagnetic coil that can move a magnet very rapidly. A logic board controls the activity of the other components and communicates with the rest of the PC. Each surface of each platter on the disk can hold tens of billions of individual bits of data. These are organised into larger ‘‘chunks’’ for convenience, and to allow for easier and faster access to information. Each platter has two heads, one on the top of the platter and one on the bottom, so a hard disk with three platters has six surfaces and six total heads. Each platter has its information recorded in concentric circles called tracks. Each track is further broken down into smaller pieces called sectors, each of which holds 512 bytes of information. There’s a gap between the platters, making room for magnetic read/write head, mounted on the end of the actuator arm. The head is so close to the surface of the platters that it ‘‘flies’’ on an air bearing a fraction of a millimeter above the disk. On early HDDs this distance was around 0.2 mm. In modern-day drives this has been reduced to 0.07 mm or less. The reduction of the flying height is important for increasing the areal recording density in HDDs. Typical flying heights in recent disk drives are less than 10 nm, resulting in a recording density of 78 Mbit/mm2 [1]. A small particle of dirt could cause a head to ‘‘crash’’, touching the disk and scraping off the magnetic coating. There’s a read/write head for each side of each platter, mounted on arms, which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice coil—an electromagnetic coil that can move a magnet very rapidly. 2. Hard disk drive failures The increasing capacity of current HDDs places a great importance on reliability, as tens of gigabytes of data can be lost in an instant due to a single tribological failure event. To improve the reliability of the head–disk interface there is a need to predict, measure and monitor any interactions. Smoother disk surfaces and lower flying heads increasingly affect tribological reliability whereas, disk drive reliability requirements remain unchanged or increase with time. The industry standard reliability demonstration test requires no more than about 1% of disk drives failures (unrecoverable errors resulting in data loss) over 1000 power-on hours for a population of 1000 drives reading and writing constantly [2]. If there is no head–disk contact then there is 0888-3270/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ymssp.2005.08.012
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1009
Fig. 1. Hard disk drive.
almost no danger of tribological problems and associated errors. However, occasional head–disk contact do occur. These can be caused due to contaminant or wear particle getting stuck between the head and the disk, disk lubricant, disk corrosion, worn bearings or due to direct slider/head–disk interaction [2]. Continuous hard contacts/rubbing between head and disk can result in disk crash. Leo and Sinclair [3] developed a model, which appears to provide improved estimates of representative forces, pressures, etc.; during a head/disk collision. This model takes account of asperity deformation and the statistical nature of asperity interaction. Khurshudov and Ivett [2] have discussed some techniques that can be used for head–disk contact detection. These techniques range from acoustic emission method to some drive specific tests, such as thermal asperity (TA) detection and variable gain amplifier (VGA) signal measurements. 3. Vibrations in HDDs For the past 20 years, the disk drive industry has continued to increase the track density (measured by tracks per inch (TPI)) and the rotational speed (measured by revolutions per minute (RPM)) in order to increase storage capacity and to reduce data access time. For high-TPI, high-RPM drives, disk and spindle vibration becomes a critical issue and a major concern. Some studies on HDD vibration generation and prediction have been reported [4,5]. If the vibration exceeds an allowable limit called the track misregistration budget, read/write errors may occur. Computer HDDs are subjected to environmental and operational vibrations that hinder performance. The application of physical vibration or shock to the drive chassis or internally generated vibration or shock to the head disk assembly itself tends to cause positioning errors. These disturbances may be introduced by spindle imbalance forces, external shock and vibration, self-induced
ARTICLE IN PRESS 1010
Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
Fig. 2. Sketch of open HDD showing accelerometer and weight positions.
resonance excitation, as well as head disk contacts. Gola et al. [6] have presented a device that allows compensation for the head mis positioning caused by the mechanical vibrations of HDD itself. It is a capacitive rotational accelerometer sensor, which uses micro-electro-mechanical system (MEMS). The vibrations of the disk drive cause tiny rotational displacements of the moving part of the MEMS structure. These movements are measured by sensing the small variations in capacitance that they cause. The minute rotational vibrations are detected and measured in order to generate a feedforward correction signal for the voice coil drive circuit to keep the head in the correct position. Vibrations are most common in the platters, actuator shaft/arm, the suspension or load beam and the spindle motor shaft. Severe vibration in these components deteriorates drive performance, leading to increase seek time, track errors and misregistration. These vibrations may also cause a drive to crash resulting in total data loss. An MEMS accelerometer-based motion sensor system that detects acceleration, such as in a fall, in notebook computers has been reported [7]. The system responds by stopping the hard disk drive and parking the head. Vibration monitoring is a useful technique for the detection of machinery problems [8]. The HDD crash can be minimised or eliminated by detecting when the vibration velocity of head is going to increase suddenly and taking appropriate action. This change can be able to sense by deploying the MEMS/miniature accelerometers over the read/write head arm or over the actuator. Placing the accelerometer over head is not easy, because it moves dynamically over the platter. In the present work, head–disk clearance was intentionally reduced gradually to promote contact by increasing the amount of tiny weights put on the actuator arm (Fig. 2). These weights cause gradual reduction in the head–disk gap and introduce, first, light rubbing and then stronger rubbing of head against platters resulting in permanent damage or crash. This gradual increase in pressure between head and platters is expected to result in increased vibrations. The vibration velocity of HDD’s head was measured with the help of miniature accelerometer fixed at the rotational axis of the actuator arm with an objective to find out the point where the abrupt change in velocity of head takes place. The vibration of head was measured for 3 different HDDs having different speeds. HDD rotation cut-off circuit can be designed to operate when unacceptable vibration velocity levels are reached. 4. Experiment, results and discussion Three new HDD of 4500, 5400 and 7200 rpm having a capacity of 1.6, 40 and 40 GB, respectively, were used in the study. All the HDDs had 3.5 in platters. Their casing was opened and the vibration level of actuator arm was measured experimentally by increasing the head and platter contact. The head and platter gap was gradually decreased by placing a tiny weight on the actuator arm and increasing its amount. A miniature accelerometer was fixed at the end of this arm to measure overall vibration levels and to obtain vibration spectrum on an FFT analyser. The Accelerometer position was chosen as the screw, which assembles the
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1011
Vibration of actuator arm, mm/s
actuator arm to the base. The location was chosen from practical mounting point of view. Initially, vibration level and spectrum were measured without any weight on the arm i.e. HDD’s normal condition, later weights were added on the actuator arm from 0.25 to 4 g in steps. Placing the mass on the actuator arm (close to head) gradually increases the surface contact between disk and the head and increase in the vibration velocity is expected. The measurement results show that the vibration velocity increased slightly at initial stage but as mass was increased more than 2 g the actuator arm vibration velocity level increased rapidly—much more than about 2 mm/s (Figs. 3–5). When the mass was around 3.5 g, permanent scratch started to form on the platter and this became very clear circular marks at 4 g mass indicating strong continuous head–disk contact. The important observation is that the vibration levels have started increasing much before total damage to the disk drive. This means that one gets enough lead-time to take action to avoid permanent damage. In general, as expected, the vibration levels are more in the case of higher RPM HDDs. This means that the safe limits for these drives have to be different. Small MEMS accelerometers, which are cheap if bought in bulk, can be fixed permanently in HDDs and an HDD rotation cut off and braking circuit can be designed to operate when unacceptable vibration levels are reached. The device can also send a signal to the voice coil drive circuit to bring the head immediately to parking position in addition to stopping the rotation of the platters. Vibration velocity spectra of 7200 RPM HDD with no weight, 2 and 4 g weight on the actuator arm are shown in Figs. 6–8, respectively. When there is no weight on the arm, main peaks in the vibration spectrum at the line frequency (50 Hz) and at rotational speed (120 Hz) are observed. The peaks at 50 Hz appear to be the electrically induced vibrations due to power supply/transformer circuits. The level of these peaks increases when the head–disk contact takes place and the levels at higher frequencies also rise. External harmless 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm
Vibration of actuator arm, mm/s
Fig. 3. Vibration velocity of actuator arm with increasing weight on it (4500-RPM HDD).
6 5 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm Fig. 4. Vibration velocity of actuator arm with increasing weight on it (5400-RPM HDD).
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
Vibration of actuator arm, mm/s
1012
8 7 6 5 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm Fig. 5. Vibration velocity of actuator arm with increasing weight on it (7200-RPM HDD).
-30 -40 Velocity, dB
-50 -60 -70 -80 -90 -100 -110 -120 0
100
200
300
400
500
600
700
800
900
1000
Frequency, Hz Fig. 6. Actuator arm vibration velocity spectrum of 7200 RPM HDD with no mass.
0
Velocity, dB
-20 -40 -60 -80
-100 -120 0
100
200
300
400
500
600
700
800
900
1000
Frequency, Hz Fig. 7. Actuator arm vibration velocity spectrum of 7200 RPM HDD with 2 g weight.
vibrations such as the ones due to movement of vehicles are expected to be of very low frequency, whereas head–disk contact vibration is of high frequency also as seen in these spectra. So, the two can be differentiated and separate limits can be set for the two frequency bands.
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1013
0
Velocity, dB
-20 -40 -60 -80
-100 -120 0
100
200
300
400 500 600 Frequency, Hz
700
800
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1000
Fig. 8. Actuator arm vibration velocity spectrum of 7200 RPM HDD with 4 g weight.
5. Conclusions HDD’s head–disk contact vibrations were studied. The vibration velocity of actuator arm was measured and analysed after gradually increasing the amount of weight put on the actuator arm. The overall vibration levels increased slowly till the weight was 2 g, but increased drastically when the weight was further increased. Permanent circular scratch marks appeared on the disk at 4 g weight. The results showed that the vibration levels increased before total disk crash i.e. before scratch marks appeared on the disk. So vibration monitoring can be used to take preventive action. Vibration velocity spectra showed that the levels at main peaks (50 & 120 Hz) and at high frequencies increased substantially when the head–disk contact takes place. HDD rotation cut off and braking circuit can be designed to operate when unacceptable vibration levels are reached. The device can also send a signal to the voice coil drive circuit to bring the head immediately to parking position in addition to stopping the rotation of the platters. References [1] S. Yonemura, S. Weissner, L. Zhou, F.E. Talke, Investigation of disk damage caused during load/unload using a surface reflectance analyzer, Tribology International 38 (2005) 81–87. [2] A. Khurshudov, P. Ivett, Head–disk contact detection in the hard disk drives, Wear 255 (2003) 1314–1322. [3] H.L. Leo, G.B. Sinclair, So how hard does a head hit a disk?, IEEE Transactions on Magnetics 27 (6) (1991) 5154–5156. [4] I.Y. Shen, Recent vibration issues in computer hard disk drives, Journal of Magnetism and Magnetic Materials 209 (2000) 6–9. [5] J.P. Yang, S.X. Chen, Vibration predictions and verifications of disk drive spindle system with ball bearings, Computers and Structures 80 (2002) 1409–1418. [6] A. Gola, N. Bagnalasta, P. Bendiscioli, E. Chiesa, S. Delbo’, E. Lasalandra, F. Pasolini, M. Tronconi, T. Ungaretti, A MEMS-based rotational accelerometer for HDD applications with 2.5 rad/s2 resolution and digital output, ESSCIRC Proceedings, 2001. [7] J. Karoub, New IBM Thinkpad retains data even if owner is thoughtless, http://www.smalltimes.com/document_display.cfm? document_id=7025, 2003. [8] N. Tandon, A. Choudhury, A review of vibration and acoustic measurement methods for the detection of defects in rolling element bearings, Tribology International 32 (1999) 469–480.
N. Tandon, V.P. Agrawal, V.V.P. Rao, ITMME Centre, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India E-mail address: [email protected] Corresponding author. Tel: +91 11 26591276; fax: +91 11 26596222.
Letter to the Editor Vibration monitoring of computer hard disk drives 1. Introduction Computer hard disk drives (HDDs) are one of the most important components within the personal computer (PC). They are used to store and retrieve huge amounts of data. Over the last few decades their reliability, capacity and speed have improved. It is hard to really understand the factors that affect performance, reliability and interfacing without knowing how the drive works internally. Hard disks are rigid platters, composed of a substrate and a magnetic medium. The substrate—the platter’s base material—must be non-magnetic and capable of being machined to a smooth finish. Both sides of each platter are coated with a magnetic medium to allow data storage. The data is stored in magnetic patterns. Typically, one or more platters are stacked on top of each other with a common spindle that turns the whole assembly at several thousand revolutions per minute (Fig. 1). Special electromagnetic read/write devices called heads are mounted onto sliders and used to either record information onto the disk or read information from it. The sliders are mounted onto arms, all of which are mechanically connected into a single assembly and positioned over the surface of the disk by a device called an actuator which contains a voice coil—an electromagnetic coil that can move a magnet very rapidly. A logic board controls the activity of the other components and communicates with the rest of the PC. Each surface of each platter on the disk can hold tens of billions of individual bits of data. These are organised into larger ‘‘chunks’’ for convenience, and to allow for easier and faster access to information. Each platter has two heads, one on the top of the platter and one on the bottom, so a hard disk with three platters has six surfaces and six total heads. Each platter has its information recorded in concentric circles called tracks. Each track is further broken down into smaller pieces called sectors, each of which holds 512 bytes of information. There’s a gap between the platters, making room for magnetic read/write head, mounted on the end of the actuator arm. The head is so close to the surface of the platters that it ‘‘flies’’ on an air bearing a fraction of a millimeter above the disk. On early HDDs this distance was around 0.2 mm. In modern-day drives this has been reduced to 0.07 mm or less. The reduction of the flying height is important for increasing the areal recording density in HDDs. Typical flying heights in recent disk drives are less than 10 nm, resulting in a recording density of 78 Mbit/mm2 [1]. A small particle of dirt could cause a head to ‘‘crash’’, touching the disk and scraping off the magnetic coating. There’s a read/write head for each side of each platter, mounted on arms, which can move them towards the central spindle or towards the edge. The arms are moved by the head actuator, which contains a voice coil—an electromagnetic coil that can move a magnet very rapidly. 2. Hard disk drive failures The increasing capacity of current HDDs places a great importance on reliability, as tens of gigabytes of data can be lost in an instant due to a single tribological failure event. To improve the reliability of the head–disk interface there is a need to predict, measure and monitor any interactions. Smoother disk surfaces and lower flying heads increasingly affect tribological reliability whereas, disk drive reliability requirements remain unchanged or increase with time. The industry standard reliability demonstration test requires no more than about 1% of disk drives failures (unrecoverable errors resulting in data loss) over 1000 power-on hours for a population of 1000 drives reading and writing constantly [2]. If there is no head–disk contact then there is 0888-3270/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.ymssp.2005.08.012
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1009
Fig. 1. Hard disk drive.
almost no danger of tribological problems and associated errors. However, occasional head–disk contact do occur. These can be caused due to contaminant or wear particle getting stuck between the head and the disk, disk lubricant, disk corrosion, worn bearings or due to direct slider/head–disk interaction [2]. Continuous hard contacts/rubbing between head and disk can result in disk crash. Leo and Sinclair [3] developed a model, which appears to provide improved estimates of representative forces, pressures, etc.; during a head/disk collision. This model takes account of asperity deformation and the statistical nature of asperity interaction. Khurshudov and Ivett [2] have discussed some techniques that can be used for head–disk contact detection. These techniques range from acoustic emission method to some drive specific tests, such as thermal asperity (TA) detection and variable gain amplifier (VGA) signal measurements. 3. Vibrations in HDDs For the past 20 years, the disk drive industry has continued to increase the track density (measured by tracks per inch (TPI)) and the rotational speed (measured by revolutions per minute (RPM)) in order to increase storage capacity and to reduce data access time. For high-TPI, high-RPM drives, disk and spindle vibration becomes a critical issue and a major concern. Some studies on HDD vibration generation and prediction have been reported [4,5]. If the vibration exceeds an allowable limit called the track misregistration budget, read/write errors may occur. Computer HDDs are subjected to environmental and operational vibrations that hinder performance. The application of physical vibration or shock to the drive chassis or internally generated vibration or shock to the head disk assembly itself tends to cause positioning errors. These disturbances may be introduced by spindle imbalance forces, external shock and vibration, self-induced
ARTICLE IN PRESS 1010
Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
Fig. 2. Sketch of open HDD showing accelerometer and weight positions.
resonance excitation, as well as head disk contacts. Gola et al. [6] have presented a device that allows compensation for the head mis positioning caused by the mechanical vibrations of HDD itself. It is a capacitive rotational accelerometer sensor, which uses micro-electro-mechanical system (MEMS). The vibrations of the disk drive cause tiny rotational displacements of the moving part of the MEMS structure. These movements are measured by sensing the small variations in capacitance that they cause. The minute rotational vibrations are detected and measured in order to generate a feedforward correction signal for the voice coil drive circuit to keep the head in the correct position. Vibrations are most common in the platters, actuator shaft/arm, the suspension or load beam and the spindle motor shaft. Severe vibration in these components deteriorates drive performance, leading to increase seek time, track errors and misregistration. These vibrations may also cause a drive to crash resulting in total data loss. An MEMS accelerometer-based motion sensor system that detects acceleration, such as in a fall, in notebook computers has been reported [7]. The system responds by stopping the hard disk drive and parking the head. Vibration monitoring is a useful technique for the detection of machinery problems [8]. The HDD crash can be minimised or eliminated by detecting when the vibration velocity of head is going to increase suddenly and taking appropriate action. This change can be able to sense by deploying the MEMS/miniature accelerometers over the read/write head arm or over the actuator. Placing the accelerometer over head is not easy, because it moves dynamically over the platter. In the present work, head–disk clearance was intentionally reduced gradually to promote contact by increasing the amount of tiny weights put on the actuator arm (Fig. 2). These weights cause gradual reduction in the head–disk gap and introduce, first, light rubbing and then stronger rubbing of head against platters resulting in permanent damage or crash. This gradual increase in pressure between head and platters is expected to result in increased vibrations. The vibration velocity of HDD’s head was measured with the help of miniature accelerometer fixed at the rotational axis of the actuator arm with an objective to find out the point where the abrupt change in velocity of head takes place. The vibration of head was measured for 3 different HDDs having different speeds. HDD rotation cut-off circuit can be designed to operate when unacceptable vibration velocity levels are reached. 4. Experiment, results and discussion Three new HDD of 4500, 5400 and 7200 rpm having a capacity of 1.6, 40 and 40 GB, respectively, were used in the study. All the HDDs had 3.5 in platters. Their casing was opened and the vibration level of actuator arm was measured experimentally by increasing the head and platter contact. The head and platter gap was gradually decreased by placing a tiny weight on the actuator arm and increasing its amount. A miniature accelerometer was fixed at the end of this arm to measure overall vibration levels and to obtain vibration spectrum on an FFT analyser. The Accelerometer position was chosen as the screw, which assembles the
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1011
Vibration of actuator arm, mm/s
actuator arm to the base. The location was chosen from practical mounting point of view. Initially, vibration level and spectrum were measured without any weight on the arm i.e. HDD’s normal condition, later weights were added on the actuator arm from 0.25 to 4 g in steps. Placing the mass on the actuator arm (close to head) gradually increases the surface contact between disk and the head and increase in the vibration velocity is expected. The measurement results show that the vibration velocity increased slightly at initial stage but as mass was increased more than 2 g the actuator arm vibration velocity level increased rapidly—much more than about 2 mm/s (Figs. 3–5). When the mass was around 3.5 g, permanent scratch started to form on the platter and this became very clear circular marks at 4 g mass indicating strong continuous head–disk contact. The important observation is that the vibration levels have started increasing much before total damage to the disk drive. This means that one gets enough lead-time to take action to avoid permanent damage. In general, as expected, the vibration levels are more in the case of higher RPM HDDs. This means that the safe limits for these drives have to be different. Small MEMS accelerometers, which are cheap if bought in bulk, can be fixed permanently in HDDs and an HDD rotation cut off and braking circuit can be designed to operate when unacceptable vibration levels are reached. The device can also send a signal to the voice coil drive circuit to bring the head immediately to parking position in addition to stopping the rotation of the platters. Vibration velocity spectra of 7200 RPM HDD with no weight, 2 and 4 g weight on the actuator arm are shown in Figs. 6–8, respectively. When there is no weight on the arm, main peaks in the vibration spectrum at the line frequency (50 Hz) and at rotational speed (120 Hz) are observed. The peaks at 50 Hz appear to be the electrically induced vibrations due to power supply/transformer circuits. The level of these peaks increases when the head–disk contact takes place and the levels at higher frequencies also rise. External harmless 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm
Vibration of actuator arm, mm/s
Fig. 3. Vibration velocity of actuator arm with increasing weight on it (4500-RPM HDD).
6 5 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm Fig. 4. Vibration velocity of actuator arm with increasing weight on it (5400-RPM HDD).
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
Vibration of actuator arm, mm/s
1012
8 7 6 5 4 3 2 1 0 0
0.5
1
1.5
2
2.5
3
3.5
4
4.5
Weight, gm Fig. 5. Vibration velocity of actuator arm with increasing weight on it (7200-RPM HDD).
-30 -40 Velocity, dB
-50 -60 -70 -80 -90 -100 -110 -120 0
100
200
300
400
500
600
700
800
900
1000
Frequency, Hz Fig. 6. Actuator arm vibration velocity spectrum of 7200 RPM HDD with no mass.
0
Velocity, dB
-20 -40 -60 -80
-100 -120 0
100
200
300
400
500
600
700
800
900
1000
Frequency, Hz Fig. 7. Actuator arm vibration velocity spectrum of 7200 RPM HDD with 2 g weight.
vibrations such as the ones due to movement of vehicles are expected to be of very low frequency, whereas head–disk contact vibration is of high frequency also as seen in these spectra. So, the two can be differentiated and separate limits can be set for the two frequency bands.
ARTICLE IN PRESS Letter to the Editor / Mechanical Systems and Signal Processing 20 (2006) 1008–1013
1013
0
Velocity, dB
-20 -40 -60 -80
-100 -120 0
100
200
300
400 500 600 Frequency, Hz
700
800
900
1000
Fig. 8. Actuator arm vibration velocity spectrum of 7200 RPM HDD with 4 g weight.
5. Conclusions HDD’s head–disk contact vibrations were studied. The vibration velocity of actuator arm was measured and analysed after gradually increasing the amount of weight put on the actuator arm. The overall vibration levels increased slowly till the weight was 2 g, but increased drastically when the weight was further increased. Permanent circular scratch marks appeared on the disk at 4 g weight. The results showed that the vibration levels increased before total disk crash i.e. before scratch marks appeared on the disk. So vibration monitoring can be used to take preventive action. Vibration velocity spectra showed that the levels at main peaks (50 & 120 Hz) and at high frequencies increased substantially when the head–disk contact takes place. HDD rotation cut off and braking circuit can be designed to operate when unacceptable vibration levels are reached. The device can also send a signal to the voice coil drive circuit to bring the head immediately to parking position in addition to stopping the rotation of the platters. References [1] S. Yonemura, S. Weissner, L. Zhou, F.E. Talke, Investigation of disk damage caused during load/unload using a surface reflectance analyzer, Tribology International 38 (2005) 81–87. [2] A. Khurshudov, P. Ivett, Head–disk contact detection in the hard disk drives, Wear 255 (2003) 1314–1322. [3] H.L. Leo, G.B. Sinclair, So how hard does a head hit a disk?, IEEE Transactions on Magnetics 27 (6) (1991) 5154–5156. [4] I.Y. Shen, Recent vibration issues in computer hard disk drives, Journal of Magnetism and Magnetic Materials 209 (2000) 6–9. [5] J.P. Yang, S.X. Chen, Vibration predictions and verifications of disk drive spindle system with ball bearings, Computers and Structures 80 (2002) 1409–1418. [6] A. Gola, N. Bagnalasta, P. Bendiscioli, E. Chiesa, S. Delbo’, E. Lasalandra, F. Pasolini, M. Tronconi, T. Ungaretti, A MEMS-based rotational accelerometer for HDD applications with 2.5 rad/s2 resolution and digital output, ESSCIRC Proceedings, 2001. [7] J. Karoub, New IBM Thinkpad retains data even if owner is thoughtless, http://www.smalltimes.com/document_display.cfm? document_id=7025, 2003. [8] N. Tandon, A. Choudhury, A review of vibration and acoustic measurement methods for the detection of defects in rolling element bearings, Tribology International 32 (1999) 469–480.
N. Tandon, V.P. Agrawal, V.V.P. Rao, ITMME Centre, Indian Institute of Technology, Hauz Khas, New Delhi 110016, India E-mail address: [email protected] Corresponding author. Tel: +91 11 26591276; fax: +91 11 26596222.