Adaptive skew setting for a disk drive

Details Adaptive skew setting for a disk drive Adaptive skew setting for a disk drive

2001-03-30

2002-12-10

Sniezek, Andrew L. (Department: 2651)

Dynamic magnetic information storage or retrieval

Automatic control of a recorder mechanism

Controlling the head

C360S063000, C360S078040

Reexamination Certificate

active

06493171

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates generally to methods for preparing and operating disk drives for computer systems. More particularly, the present invention relates to methods for improving disk drive performance by determining and implementing skew settings for each data transducer of each disk drive.
BACKGROUND
Data storage devices are widely used in computers and data processing systems for storing information in digital form. These devices include disk drives, which commonly use one or more rotating magnetic storage disks to store data. Each storage disk typically includes a data storage surface on each side of the storage disk. These storage surfaces are divided into a plurality of narrow, annular, concentric regions of different radii, commonly referred to as “tracks”. Additionally, each storage surface typically includes a plurality of wedge-shaped “sectors”. Typically, a read/write head is utilized to transfer data to and from the tracks of the data storage surfaces. As used herein, the read/write head is also sometimes referred to as a “head” or a “data transducer”.
Within data storage devices, e.g. disk drives, if a sequential data transfer crosses a boundary from one head to another, or crosses a cylinder boundary, i.e., when data associated with the last sector of a track and the first sector of the next sequential track are included in the transfer, there is a negative impact on data throughput since the transfer of data must be interrupted while a head switch or single-cylinder seek procedure is completed. The term “seek time” describes the latency that occurs from the initiation of a head switch or single-cylinder seek operation until the data transfer can be continued, i.e. restarted.
A “seek” process encompasses activities associated with positioning the heads to a predetermined location on the disk drive medium (sometimes referred to herein as a “target track”) to initiate data recording and data retrieval sequences. The seek process will typically involve one or both of the following steps:
1. A “single-cylinder seek”, also known as “cylinder switch”, involves operating the actuator to move the heads across the disk surfaces. This motion permits moving the heads from cylinder to cylinder, as required. Moving the heads from a cylinder to a next adjacent cylinder, e.g. moving from cylinder to cylinder, is known as a single-cylinder seek.
2. “Head selection”, also known as “head switch”, involves activating different heads which are associated with different storage surfaces of the storage disks to be used during a required data transfer. As illustrated in
FIG. 2
, a head switch occurs when a data transfer first requires head
12
a,
then head
12
b
to be sequentially activated, for example
A “skew” of a track is defined as the physical offset of logical sector zero of the target track from logical sector zero of the preceding track. Stated another way, because the storage disks continue to rotate during the latency period caused by a single-cylinder seek or a head switch, a particular measurable offset occurs, called a “skew”. By configuring the disk drive to account for these known “delays” or skews, the disk drive is able to operate with greater efficiency, with a decrease in the interruption time while reading and/or writing data to a storage disk. The purpose of the skew is to improve disk drive performance by compensating for the rotation of the medium that occurs during the time it takes to complete a corresponding seek operation. The size of the physical offset, or the “skew value”, is determined by the seek time associated with the required operation.
Disk drive units typically use two different track skew values: “head skew”, which is associated with a head switch operation, i.e. switching from one head to another, and “cylinder skew”, which involves a single-cylinder seek operation, i.e. movement of the head from one cylinder to another. Since the time required to complete a head switch operation will usually differ from that required for a single-cylinder seek operation, different skews are associated with each operation. For example, during sequential data transfer operations, in a disk drive unit having n heads, track zero of each cylinder is always assumed to be the target track during a single-cylinder seek operation and is given a cylinder skew consistent with that operation. During a head switch operation, each of the tracks 1 through n-1 associated with each cylinder is assumed to be a target track and receives a head skew consistent with that operation.
In the past, skew values associated with a particular disk drive product were determined during the product development cycle. Commonly, all disk drive units in a product line carried the same skew values. Skew values have typically been determined by several factors, among them head mass, disk rotation speed (RPM), flexibility of the flex cable, etc. The prevailing approach was to use greater skew values that would avoid “slipped revolutions”, i.e. when a data transfer is delayed a full disk revolution because of an intervening head switch or single-cylinder seek operation. The prevailing thought has been that the skew values should be higher than measured in order to provide a “margin of safety” in order to prevent slipped revolutions. Unfortunately, the problem with this approach has been that by using somewhat higher skew values to avoid slipped revolutions, a decrease in efficiency was realized over time. Higher skew values result in longer access times, which is not conducive to high performance. In other words, excessively increasing the skew value in an attempt to avoid a slipped revolution results in a greater loss of time than more precisely setting the skew value and suffering an occasional slipped revolution.
In addition, slight changes in shape occur to the heads of a disk drive during operation. These changes occur as a result of heat, aerodynamic forces, etc. Oftentimes, one head can become misaligned relative to the other heads following usage of the disk drive. Head misalignment can impact the skew value of each individual head. This effect on the skew values can adversely affect the speed and efficiency of the disk drive.
Therefore, there exists a need for a method of determining skew values for each individual disk drive and implementing the values into each drive in a manner that is conducive to high performance disk drives. Further, the need exists to provide a method for determining skew values for each individual head of every disk drive, and implementing the skew values into each drive. Still another need exists to provide a disk drive consistent with this methodology which is relatively easy and inexpensive to manufacture.
SUMMARY
The present invention satisfies these needs. A general object of the present invention is to determine and implement skew values for each disk drive to improve performance of the disk drive. Preferably, head skew and cylinder skew values are calculated and implemented for each of the heads of the disk drive during a production test phase of the disk drive manufacturing process. Uniquely, data is compiled during the production test phase of the disk drive manufacturing process and is used to calculate and implement skew values for each individual head of each disk drive.
Preferably, the method of manufacturing a disk drive includes initiating a head switch test series for a first head, measuring a head switch time for the first head, computing an average head switch time for the first head, computing a standard deviation of the head switch times for the first head, and computing a head skew based on the average and the standard deviation of the head switch times for the first head. This process is typically repeated for each head in the disk drive.
The present invention is also directed toward a disk drive that includes a base, at least one storage disk mounted to the base, and at least one actuator arm for positioning a head near the storage disk. Importantly, the disk drive also includes skew settings for each of the

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