Patent Application
20080106818
The present invention relates to the field of air bearing sliders such as those used in hard disk drives. More specifically, the present invention relates to an air-bearing design for a slider utilizing a pitch ladder and an air-chair channel with a sub-ambient pressure cavity for controlling the pitch profile for CSS drives.
Hard disk drives are common information storage devices essentially consisting of a series of rotatable disks that are accessed by magnetic reading and writing elements. These data transferring elements, commonly known as transducers, are typically carried by and embedded in a slider body that is held in a close relative position over discrete data tracks formed on a disk to permit a read or write operation to be carried out. In order to properly position the transducer with respect to the disk surface, an air bearing surface (ABS) formed on the slider body experiences a fluid air flow that provides sufficient lift force to “fly” the slider and transducer above the disk data tracks. The high speed rotation of a magnetic disk generates a stream of air flow or wind along its surface in a direction substantially parallel to the tangential velocity of the disk. The air flow cooperates with the ABS of the slider body which enables the slider to fly above the spinning disk. In effect, the suspended slider is physically separated from the disk surface through this self-actuating air bearing. The ABS of a slider is generally configured on the slider surface facing the rotating disk, and greatly influences its ability to fly over the disk under various conditions. For a typical disk drive (−160 GB/platter) on the market, the distance between a magnetic head and the media is less than 10 nm. In order to correctly read and write data, it is essential that the sliders fly stably over the magnetic recording media during reading and writing.
There are currently two types of drive designs on the market. The first is a Load/Unload (LUL) design, where sliders stay on a ramp that is outside the perimeter of the magnetic disk when no reading or writing is performed. The second is a Contact Start Stop (CSS) design, where the sliders park on the magnetic media at the innermost diameter (also referred to as the Landing Zone) of the magnetic disk when no reading or writing is performed. Once the disk stops rotating, the slider comes to rest on the surface of the disk. When the rotation of the disk begins again, the air-bearing is formed once again and the slider separates from the disk. In order to reduce friction between the slider and the disk, a very thin layer of lubricant on the order of a few nanometers, is applied to the surface of the disk. A common problem with the operations of CSS drives is the starting friction (“stiction”). Stiction is caused by viscous lubricant between the slider and the disk. In some cases, especially in a humid environment, the slider is held down strong enough that the disk fails to rotate. To correct this problem, pads are introduced onto the surface of the slider to reduce the area of contact between the slider and the disk. The pads are extremely tall to minimize the stiction problem described above.
The stiction consideration for CSS drives with padded sliders requires that the angle between the disk and backplane of the slider (commonly referred to as the “pitch angle” or “pitch”) be above a certain value such that there is no pad contact. As a result of this need, it is not uncommon for a CSS drive to have a high flying pitch angle of greater than 150 microradians.
It is further realized that the flying pitch of a conventional ABS design for a CSS drive is not uniform across the diameter of a magnetic disk. Due to the greater tangential velocity, and hence greater air flow, at the outer diameter of the disk (OD), the pitch angle at the OD of the disk is usually much higher than that in the start stop zone.
Another important reliability issue is the uneven flying heights (FH) at high altitudes due to uneven flying pitch. The lift force experienced by the ABS is a product of both air density and the amount of air flow. Therefore, a conventional ABS design will have a lower flying height in the less-dense air of high altitudes than it has at sea level. This can be especially problematic at the inner diameter (ID) where there is less air flow to lift the ABS. The FH at the inner diameter is usually low with a low flying pitch, which can result in head-disk contact or failure at high altitude. Reducing altitude FH sensitivity is one way to deal with this difficulty. However, this approach does not solve the issues associated with pitch drop at certain tracks. For example, the FH at the sensor area may change only slightly, but the flying pitch could still drop significantly, moving the minimum FH location to the leading edge of the ABS (such as anti-stiction pads), causing altitude failure at the inner radius of the disk. Therefore, a high enough pitch angle should be maintained at the ID.
In view of these reliability issues associated with current CSS drives with high pitch ABS designs, the present invention proposes using a pitch ladder and an air channel for controlling the pitch profile of CSS drives.
a-b show the number of particles passing through the magnetic sensor at the outer diameter for an exemplary design with a pitch ladder and for an exemplary design without a pitch ladder.
The embodiment shown uses three-step stairs 260 to increase the effective air squeezing cross-area for the air bearing pitch angle. The effective air squeezing cross-area is defined as the summation of the product of the length and depth of each step for all steps in the pitch ladder. The dynamic pitch angle of an air bearing surface is nonlinearly proportional to the effective air squeezing cross-area for a given amount of incoming air flow. The air bearing dynamic pitch saturates when the air flow squeeze cross-area is large enough. Due to the minimum incoming air flow difference from the inner diameter to the outer diameter and increased effective air squeezing cross-area, the air bearing dynamic profile can be controlled uniformly from the ID to OD since the air bearing dynamic pitch angle has reached a saturation value at all diameters.
The squeezing air will produce a high pressure on the leading pad and generate a sub-ambient pressure region in the cavity. When the pressure on the leading pad saturates with the pitch ladder, the pitch angle has reached a steady value for different disk tracks.
The embodiment of
For the traditional ABS without a pitch ladder feature, the dynamic pitch at the ID drops significantly because the incoming air flow at the ID is much smaller than at the OD, and also because the air squeeze cross-area is not big enough to reach a saturation level, causing the air bearing pitch angle to vary substantially with the amount of incoming airflow. Alternatively, for the ABS with the pitch ladder feature, the dynamic pitch varies only minimally from the ID to the OD. As evidenced by
An illustrative embodiment of the role of the pitch ladder is shown in
Although embodiments are specifically illustrated and described herein, it will be appreciated that modifications and variations of the present invention are covered by the above teachings and are within the purview of the appended claims without departing from the spirit and intended scope of the invention.
