# Patent

## Title
System, method, and apparatus for breaking up large-scale eddies and straightening air flow inside rotary disk storage devices

## Abstract

A system, method, and apparatus for solving flow-induced track misregistration (TMR) problems in hard disk drives (HDD) are directed to breaking up large-scale eddies, and straightening air flows with honeycomb structures, woven wire screens, and guide vanes or holes. In addition, boundary layer manipulation techniques are applied to the airflow in the HDD, such as boundary layer suction with slots or holes, and wall damping techniques, such as an open honeycomb seal and Helmholtz resonators. These flow-conditioning solutions reduce the turbulence intensity throughout the HDD to reduce TMR. These solutions achieve these goals while minimizing increases in the running torque needed to overcome their inherent rotational drag.

## Background
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates in general to an improved architecture for conditioning air flow inside data storage devices and, in particular, to an improved system, method, and apparatus for breaking up large-scale eddies and straightening air flow inside rotary disk storage devices.
2. Description of the Related Art
Data access and storage systems generally comprise one or more storage devices that store data on magnetic or optical storage media. For example, a magnetic storage device is known as a direct access storage device (DASD) or a hard disk drive (HDD) and includes one or more disks and a disk controller to manage local operations concerning the disks. The hard disks themselves are usually made of aluminum alloy or a mixture of glass and ceramic, and are covered with a magnetic coating. Typically, one to five disks are stacked vertically on a common spindle that is turned by a disk drive motor at several thousand revolutions per minute (rpm). Hard disk drives have several different typical standard sizes or formats, including server, desktop, mobile (2.5 and 1.8 inches) and micro drive.
A typical HDD also uses an actuator assembly to move magnetic read/write heads to the desired location on the rotating disk so as to write information to or read data from that location. Within most HDDs, the magnetic read/write head is mounted on a slider. A slider generally serves to mechanically support the head and any electrical connections between the head and the rest of the disk drive system. The slider is aerodynamically shaped to glide over moving air in order to maintain a uniform distance from the surface of the rotating disk, thereby preventing the head from undesirably contacting the disk.
The head and arm assembly is linearly or pivotally moved utilizing a magnet/coil structure that is often called a voice coil motor (VCM). The stator of a VCM is mounted to a base plate or casting on which the spindle is also mounted. The base casting with its spindle, actuator VCM, and internal filtration system is then enclosed with a cover and seal assembly to ensure that no contaminants can enter and adversely affect the reliability of the slider flying over the disk. When current is fed to the motor, the VCM develops force or torque that is substantially proportional to the applied current. The arm acceleration is therefore substantially proportional to the magnitude of the current. As the read/write head approaches a desired track, a reverse polarity signal is applied to the actuator, causing the signal to act as a brake, and ideally causing the read/write head to stop and settle directly over the desired track.
One of the major hurdles in hard disk drive (HDD) development is track misregistration (TMR). TMR is the term used for measuring data errors while a HDD writes data to and reads data from the disks. One of the major contributors to TMR is flow-induced vibration. Flow-induced vibration is caused by turbulent flow within the HDD. The nature of the flow inside a HDD is characterized by the Reynolds number, which is defined as the product of a characteristic speed in the drive (such as the speed at the outer diameter of the disk), and a characteristic dimension (such as the disk diameter or, for some purposes, disk spacing). In general, the higher the Reynolds number, the greater the propensity of the flow to be turbulent.
Due to the high rotational speed of the disks and the complex geometries of the HDD components, the flow pattern inside a HDD is inherently unstable and non-uniform in space and time. The combination of flow fluctuations and component vibrations are commonly referred to as “flutter” in the HDD literature. The more precise terms “disk flutter” and “arm flutter” refer to buffeting of the disk and arm, respectively, by the air flow. Unlike true flutter, the effect of the vibrations in HDDs on the flow field is usually negligible. Even small arm and disk vibrations (at sufficiently large frequencies, e.g., 10 kHz and higher), challenge the ability of the HDD servo system to precisely follow a track on the disk.
Since the forcing function of vibrations is directly related to flow fluctuations, it is highly desirable to reduce any fluctuating variation in the flow structures of air between both co-rotating disks and single rotating disks. Thus, a system, method, and apparatus for improving the architecture for conditioning air flow inside data storage devices is needed.
SUMMARY OF THE INVENTION
One embodiment of a system, method, and apparatus of the present invention attempts to apply several techniques to solve track misregistration (TMR) problems in hard disk drives (HDD). Some of the solutions presented herein are related to straightening airflows in wind tunnel applications. Several key components for breaking up large-scale eddies or straightening HDD air flows are honeycomb structures, woven wire screens, and guide vanes or holes. The flow-conditioning solutions presented in the present application reduce the turbulence intensity throughout the HDD to reduce TMR. These solutions achieve these goals while minimizing increases in the running torque needed to overcome their inherent rotational drag.
Three of the solutions may be categorized as large-eddy break-up (LEBU) devices. By installing these devices inside an HDD, the turbulent energy generated by the devices is confined to a range of smaller eddies that are more easily dissipated. The LEBU devices are positioned in the flow stream between the disks. The passages in the devices are aligned tangentially along a framing structure. The framing structure extends radially between the disks such that the various sets of passages are interleaved relative to the disk stack. Moreover, the LEBU devices can be placed as a single unit or multiple units in series, depending upon the application.
Another solution affects the stability of the HDD flow and enables the flow to follow complex geometries and regions with adverse pressure gradients (i.e., increasing pressure in the direction of flow) without flow separation. Separated regions are a major source of flow fluctuations when the Reynolds number is sufficiently large. The latter is true for typical prior art HDD configurations. Suction inhibits turbulent mixing between the Ekman layers spun off the disk and their return flow. Reduced mixing leads to a reduction in the aerodynamic torque needed to spin the disk pack.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the present invention, taken in conjunction with the appended claims and the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
So that the manner in which the features and advantages of the invention, as well as others which will become apparent are attained and can be understood in more detail, more particular description of the invention briefly summarized above may be had by reference to the embodiment thereof which is illustrated in the appended drawings, which drawings form a part of this specification. It is to be noted, however, that the drawings illustrate only an embodiment of the invention and therefore are not to be considered limiting of its scope as the invention may admit to other equally effective embodiments.
FIG. 1 is a top plan view of one embodiment of a disk drive constructed in accordance with the present invention.
FIG. 2 is a top plan view of another embodiment of a disk drive constructed in accordance with the present invention.
FIG. 3 is a sectional side view of one embodiment of a interleaf structure taken along the line 3-3 of FIG. 2.
FIG. 4 is a sectional side view of an alternate embodiment of the interleaf structure of FIG. 3.
FIG. 5 is a sectional side view of another alternate embodiment of the interleaf structure of FIG. 3.
FIG. 6 is a sectional side view of yet another alternate embodiment of the interleaf structure of FIG. 3.
FIG. 7 is a sectional side view of still another alternate embodiment of the interleaf structure of FIG. 3.
FIG. 8 is a partial isometric view of one embodiment of a boundary layer device for a disk drive enclosure.
FIG. 9 is a partial isometric view of an alternate embodiment of the boundary layer device of FIG. 8.



DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, one embodiment of a system, method, and apparatus for reducing track misregistration in disk drives is shown. This embodiment employs an information storage system comprising a magnetic hard disk file or drive 111 for a computer system. Drive 111 has an outer housing or base 113 (e.g., an enclosure) containing at least one magnetic disk 115. Disk 115 is rotated by a spindle motor assembly having a central drive hub 117. An actuator 121 comprises a plurality of parallel actuator arms 125 (one shown) in the form of a comb that is pivotally mounted to base 113 about a pivot assembly 123. A controller 119 is also mounted to base 113 for selectively moving the comb of arms 125 relative to disk 115.
In the embodiment shown, each arm 125 has extending from it at least one cantilevered load beam and suspension 127. A magnetic read/write transducer or head is mounted on a slider 129 and secured to a flexure that is flexibly mounted to each suspension 127. The read/write heads magnetically read data from and/or magnetically write data to disk 115. The level of integration called the head gimbal assembly is head and the slider 129, which are mounted on suspension 127. The slider 129 is usually bonded to the end of suspension 127.
Suspensions 127 have a spring-like quality, which biases or urges the air bearing surface of the slider 129 against the disk 115 to enable the creation of the air bearing film between the slider 129 and disk surface. A voice coil 133 housed within a conventional voice coil motor magnet assembly is also mounted to arms 125 opposite the head gimbal assemblies. Movement of the actuator 121 (indicated by arrow 135) by controller 119 moves the head gimbal assemblies radially across tracks on the disk 115 until the heads settle on their respective target tracks. The head gimbal assemblies operate in a conventional manner and always move in unison with one another, unless drive 111 uses multiple independent actuators (not shown) wherein the arms can move independently of one another.
Referring now to FIGS. 1 and 2, drive 111 further comprises a flow-conditioning device 141 that is mounted to the enclosure 113 adjacent to the disk 115. The flow-conditioning device 141 comprises one or more “flow straighteners” that may be either symmetrically arrayed or asymmetrically about the disk 115, depending upon the application. FIG. 2 illustrates the symmetrical arrangement. Each flow-conditioning device 141 comprises a foundation or support post 145 that is mounted to the enclosure 113. As shown in FIG. 3, one embodiment of support post 145 is mounted to and extends between both portions of the enclosure 113: base plate 113a and top cover 113b. 
The flow-conditioning device 141 includes at least one projection or finger 143 (e.g., five shown for four disks 115 in FIG. 3) having passages 147. The fingers 143 extend radially with respect to the disks 115 and their axis 116, and parallel to the surface 118 of the disks 115. When two or more fingers 143 are used, the adjacent fingers 143 define a slot 144 that closely receives the two parallel surfaces of the disk 115. The fingers 143 originate at the support post 145 and preferably extend to or near the disk hub 117. However, there is no contact between any portion of the flow-conditioning device 141 and the disks 115.
Each finger 143 comprises a small, generally rectangular frame having a plurality of the passages 147 that permit air flow to move all the way through the finger 143. The passages 147 are formed in the finger 143 in directions that are axially and radially transverse (e.g., perpendicular) with respect to the disks 115. Each of the fingers 143 has the passages 147 to reduce air flow turbulence intensity and track misregistration.
The fingers 143 are positioned in the air flow stream generated by the disks 115 so that, as the disks 115 rotate, the passages 147 are aligned with the air flow stream and reduce an air flow turbulence intensity and track misregistration between the heads on the sliders 129 and the read/write tracks on the disks 115. The turbulent energy generated by the flow-conditioning device(s) 141 is confined to a range of smaller eddies that are more easily dissipated within the disk drive 111 than prior art large eddies. Each finger 143 has an angular or arcuate width in a range of approximately 5 degrees or less. Each finger 143 also can be configured to have a constant width along the radial direction of the disk 115.
As shown in FIGS. 3-7, the passages 147 may comprise many different configurations or combinations thereof. For example, in FIG. 3, the passages 147 are configured in a honeycomb structure 151 having a tight array of one or more hexagonal feature(s) that extend across the entire face of the fingers 143. In one embodiment, the passages 147 (honeycomb cell size) are on the order of five times smaller than the axial disk spacing. In addition, the fingers 143 may have an arcuate width that is approximately equal to said axial distance minus a mechanical clearance on the order of 0.5 mm.
In another embodiment (FIG. 4), the passages are formed from wire screen walls 153, which may be woven, mounted on a framing structure. The wire screen dimensions are dictated by the size of the disk diameter and spacing. Typically, the wire screen walls 153 comprise at least two or three passages across the vertical direction. In one version, the wire screen walls 153 have a thickness on the order of 0.1 mm.
In FIG. 5, the passages comprise sets of guide vanes 155 that extend axially with respect to the disk 115. In the embodiment shown, the guide vanes 155 are grouped in small sets of three that radially offset from each other. The individual vanes in guide vanes 155 are parallel to each other. Other guide vane configurations are possible, including those of the slanted type. The guide vanes have a thickness that is sufficient to ensure mechanical stability and ruggedness, which may be on the order of 0.3 mm.
Alternatively, the passages may be formed by cylindrical tubes 157, as shown in FIG. 6. The cylindrical tubes 157 may comprise many different configurations, such as side-by-side in a flat array having a single row of the axially parallel cylindrical tubes 157. In another embodiment (FIG. 7), the cylindrical tubes 159 form a plurality (two shown) of parallel rows that are configured in an alternating pattern of upper and lower positions.
Referring now to FIG. 8, an inner wall of the enclosure 113 may be configured with a boundary layer device 161. The boundary layer device 161 is designed to manipulate the air flow inside the disk drive 111 to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations.
The boundary layer device may comprise many different forms. For example, in FIG. 8, a suction plenum 163 having an array of suction apertures 165 is shown. The apertures 165 may comprise slots, holes, and/or combinations thereof, and are used to evacuate air flow from the interior of the disk drive 111 into the suction plenum 163. The air flow (see arrows 167) is then reintroduced into the interior of the disk drive 111 at a suitable location, such as at perforations 169 in hub 117 (for clarity, disks 115 are not shown). Moreover, the suction air also may be passed through an air filter 171 before being reintroduced into the pack of disks 115.
An alternative embodiment of the boundary layer device is shown in FIG. 9 as a lining of cavities 173 on the inner wall of enclosure 113. In one version, the cavities 173 comprise a honeycomb of hexagonal walls 175, each of which is perforated by a small orifice 177. Collectively, these cavities 173 form a close-packed array of Helmholtz resonators. Because these resonators can be tuned, they are particularly effective in suppressing narrow-band turbulence fluctuations. In particular, the Helmholtz resonators may be tuned to act as acoustic notch filters or certain prominent frequencies in the file. For example, one such frequency is the vortex shedding frequency associated with the actuator arm. In addition, the cavities may comprise closed and open cell acoustic foam.
The present invention also comprises a method of reducing track misregistration in a disk drive. In one embodiment (FIG. 1), the method comprises providing a disk drive 111 having an enclosure 113, a disk 115 having a surface 118 with tracks, and an actuator 121 having a head for reading from and writing to the tracks. The method further comprises positioning a flow-conditioning device 141 (FIG. 3) adjacent to the surface 118 of the disk 115, rotating the disk 115 to generate an air flow, flowing the air flow through passages 147 in the flow-conditioning device 141, and thereby reducing air flow turbulence intensity and track misregistration between the head and the tracks on the disk 115.
The method also may comprise positioning the disk 115 in an elongated slot 144 in the flow-conditioning device 141. The method may further comprise orienting the passages 147 at axially and radially transverse positions with respect to the disk 115, and forming the passages in a configuration selected from the group consisting of: a honeycomb structure (FIG. 3), wire screen walls (FIG. 4), guide vanes (FIG. 5), and cylindrical tubes (FIGS. 6 and 7). In another embodiment, the method may further comprise forming a symmetrical array (FIG. 2) of the flow-conditioning devices 141 about the disk 115.
Alternatively, or in combination with any of the foregoing steps of the method, the method may further comprise forming a boundary layer device 161 (FIGS. 8 and 9) on an inner surface of the enclosure 113, and manipulating the air flow inside the disk drive 111 with the boundary layer device 161 to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations. The method may comprise evacuating air flow from an interior of the disk drive 111 into a suction plenum 163 (FIG. 8), and reintroducing the air flow into the disk drive 111. In addition, the method may comprise configuring the boundary layer device as a lining of walled cavities 173 (FIG. 9), each having a small orifice 177 in communication with the interior of the disk drive 111.
The present invention has several advantages, including the ability to reduce TMR problems in hard disk drives HDDs. These solutions break up large-scale eddies, straighten air flows, and manipulate the boundary layers. As a result, the turbulence intensity is reduced throughout the HDD to reduce TMR while minimizing increases in the running torque needed to overcome rotational drag. The turbulent energy generated by the devices is confined to a range of smaller eddies that are more easily dissipated. The LEBU devices can be used individually or as multiple units in series.
The present invention also enables the flow to follow complex geometries without flow separation. Suction inhibits turbulent mixing between the Ekman layers spun off the disk and their return flow. Reduced mixing leads to a reduction in the aerodynamic torque needed to spin the disk pack. In addition, turbulent fluctuations are dampened via the dissipation generated by the special linings, some of which can be tuned to suppress narrow-band fluctuations. The application of these special linings along the interior walls of the HDD provide aerodynamic and acoustic damping. In particular, the Helmholtz resonators may be tuned to act as acoustic notch filters or certain prominent frequencies in the file.
While the invention has been shown or described in only some of its forms, it should be apparent to those skilled in the art that it is not so limited, but is susceptible to various changes without departing from the scope of the invention.

## Claims

1. A system for reducing track misregistration in a disk drive, comprising:
an enclosure;
a disk mounted to the enclosure for rotation relative thereto, the disk having a surface with tracks;
an actuator having a head and mounted to the enclosure for moving the head relative to the disk such that the head reads information from and writes information to the tracks on the disk;
a flow-conditioning device mounted to the enclosure adjacent to the disk, the flow-conditioning device having a pair of fingers extending radially with respect to the disk and parallel to the surface of the disk, the pair of fingers forming an elongated slot in which the disk is located;
a plurality of passages formed in each of the fingers, the passages being axially and radially transverse with respect to the disk, and the fingers is positioned in an air flow stream generated by the disk so that, as the disk rotates, the passages align with the air flow stream and reduce an air flow turbulence intensity and track misregistration between the head and the tracks on the disk; wherein
the flow-conditioning device comprises a plurality of the flow-conditioning devices, and the plurality of flow-conditioning devices are mounted to the enclosure in a symmetrical array about the disk;
the passages comprise a pair of parallel rows of the cylindrical tubes that are configured in an alternating pattern along a radial length of each of the fingers, such that a centerline of each of the cylindrical tubes is radially spaced apart from centerlines of other ones of the cylindrical tubes in respective ones of the fingers;
a boundary layer device mounted directly to a side wall of the enclosure to form a lining along the side wall, the boundary layer device also extending radially from a side wall of the enclosure but radially spaced apart from the disk, and the boundary layer device being separate and spaced apart from the flow-conditioning device to further manipulate the air flow inside the disk drive to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations to further reduce the air flow turbulence intensity and track misregistration between the head and the tracks on the disk; and
the lining comprises cavities on the inner wall of enclosure forming a honeycomb of hexagonal walls, each of which is perforated by a small orifice, to form a close-packed array of Helmholtz resonators.

2. A disk drive, comprising:
an enclosure;
a disk mounted to the enclosure for rotation relative thereto, the disk having a surface with tracks;
an actuator having a head and mounted to the enclosure for moving the head relative to the disk such that the head reads information from and writes information to the tracks on the disk;
a flow-conditioning device mounted to the enclosure adjacent to the disk, the flow-conditioning device having a finger extending radially w surface of the disk, and a plurality of passages formed in the finger that are axially and radially transverse with respect to the disk;
the finger is positioned in an air flow stream generated by the disk so that, as the disk rotates, the passages align with the air flow stream and reduce an air flow turbulence intensity and track misregistration between the head and the tracks on the disk;
the passages are formed by sets of guide vanes that extend axially with respect to the disk, and the guide vanes are grouped in sets of three along a radial dimension of the finger, and each of the guide vanes have a thickness on the order of 0.3 mm;
a boundary layer device mounted directly to a side wall of the enclosure to form a lining along the side wall, the boundary layer device also extending radially from a side wall of the enclosure but radially spaced apart from the disk, and the boundary layer device being separate and spaced apart from the flow-conditioning device to further manipulate the air flow inside the disk drive to provide aerodynamic and acoustic damping that promote viscous dissipation of turbulent fluctuations to further reduce the air flow turbulence intensity and track misregistration between the head and the tracks on the disk; and
the lining comprises cavities on the inner wall of enclosure forming a honeycomb of hexagonal walls, each of which is perforated by a small orifice, to form a close-packed array of Helmholtz resonators that are tuned to suppress narrow-band turbulence fluctuations and act as acoustic notch filters for a vortex shedding frequency associated with the actuator.