Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1998-12-11
2003-09-02
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06614617
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to the field of mass storage devices. More particularly, this invention relates to a method for performing a track to track seek in a disc drive.
BACKGROUND OF THE INVENTION
One of the key components of any computer system is a place to store data. Computer systems have many different places where data can be stored. One common place for storing massive amounts of data in a computer system is on a disc drive. The most basic parts of a disc drive are a disc that is rotated, an actuator that moves a transducer to various locations over the disk, and electrical circuitry that is used to write and read data to and from the disk. The disc drive also includes circuitry for encoding data so that it can be successfully retrieved and written to the disc surface. A microprocessor controls most of the operations of the disc drive as well as passing the data back to the requesting computer and taking data from a requesting computer for storing to the disk.
The transducer is typically housed within the slider. The slider is a small ceramic block which is passed over the disc in a transducing relationship with the disk. The small ceramic block, also referred to as a slider, is usually aerodynamically designed so that it flies over the disk. Most sliders have an air bearing surface (“ABS”) which includes rails and a cavity between the rails. When the disc rotates, air is dragged between the rails and the disc surface causing pressure, which forces the head away from the disk. At the same time, the air rushing past the depression in the air bearing surface produces a negative pressure area. The negative pressure or suction counteracts the pressure produced at the rails. The slider is also attached to a load spring which produces a force on the slider directed toward the disc surface. The various forces equilibrate so the slider flies over the surface of the disc at a particular fly height. The fly height is the thickness of the air lubrication film or the distance between the disc surface and the transducing head. This film eliminates the friction and resulting wear that would occur if the transducing head and disc were in mechanical contact during disc rotation. In some disc drives, the slider passes through a layer of lubricant rather than flying over the surface of the disk.
Information representative of data is stored on the surface of the memory disk. Disc drive systems read and write information stored on tracks on memory disks. Transducers, in the form of read/write heads attached to the sliders, located on both sides of the memory disk, read and write information on the memory disks when the transducers are accurately positioned over one of the designated tracks on the surface of the memory disk. The transducer is also said to be moved to a target track. As the memory disc spins and the read/write head is accurately positioned above a target track, the read/write head can store data onto a track by writing information representative of data onto the memory disk. Similarly, reading data on a memory disc is accomplished by positioning the read/write head above a target track and reading the stored material on the memory disk. To write on or read from different tracks, the read/write head is moved radially across the tracks to a selected target track. The data is divided or grouped together on the tracks. In some disc drives, the tracks are a multiplicity of concentric circular tracks. In other disc drives, a continuous spiral is one track on one side of a disc drive. Servo feedback information is used to accurately locate the transducer. The actuator assembly is moved to the required position and held very accurately during a read or write operation using the servo information.
The actuator assembly is moved by a voice coil motor (“VCM”). Attached to the actuator arm of the actuator assembly is a coil, most commonly known as a voice coil. The voice coil is one of the major portions of the VCM. Magnets attached to the base of the disc drive form the other major portion of the VCM. By controlling the amount and direction of the current passing through the voice coil, the direction and the speed at which the actuator arm can be moved is regulated. Of course, the voice coil does have physical constraints one of which is a maximum current which can be passed through the voice coil for a given supply voltage. When maximum current is passed through the voice coil this is called operating in saturation mode. The amount of current applied to the voice coil during saturation mode is referred to as the saturation current.
When the actuator assembly moves the read/write head from a beginning track radially across the other tracks to a selected target track this is referred to as a seek. Many times seeks are to adjacent tracks. Other times the seeks can span up to 2000-3000 tracks or even more depending on the track density of the drive. Actuator assemblies within disc drives generally accelerate and decelerate during long seeks to keep access times to a minimum. Generally the actuator assemblies follow what is known as a velocity profile during a seek to move the read/write head from a start track to a target track. A velocity profile is a preprogrammed equation or table which lists a desired velocity verses the stopping distance remaining until reaching the target track. The profile velocity value is the highest possible value of velocity the actuator can have at a particular distance so that the actuator can still be decelerated to a stop upon reaching the target track. The amount of deceleration that can be applied to the actuator is a function of many variables including voice coil resistance, file torque constant and power supply voltage. These variables are generally not known for each specific disc drive and as a result, the velocity profile is designed using worst case values to assure that there will always be adequate deceleration capability to stop the read/write head actuator upon reaching the target track.
A typical seek is accomplished by calculating distance left to go to the target track, selecting the velocity from the velocity profile which corresponds to the calculated distance to go, determining the actual actuator velocity and subtracting the actual actuator velocity from the selected velocity obtained from the velocity profile. This value is then multiplied by a gain to give a control current output to the voice coil. The acceleration produced is a function of many factors. Amongst the major factors are the magnetic flux density of the VCM and the environmental operating conditions, such as temperature, of the drive. These parameters vary from disc drive to disc drive.
During the design process of velocity or seek mode, a fixed acceleration constant is used as a reference point to determine optimized seek trajectories resulting in characterized move times and settle times to the target track center for a given seek length. If the acceleration constant deviates away from the reference point chosen during the design phase, seek and settle time characteristics will change. This results in unpredictable move times and will cause either overshooting or undershooting on the position error signal during the settle phase of the seek. At worst case, the seek may not complete successfully at the target track resulting in a seek error. A force constant is a multiplier associated with the gain for the VCM drive current that equalizes the results amongst the disc drives.
A standard technique used throughout the industry is to maintain a fixed acceleration/deceleration for a fixed demand. The way this is achieved is to scale the output current by multiplying the demand by a term called the force constant. For example, if the acceleration constant for a drive is 10% less than the design reference point at 1.0 amps flowing through the VCM, the force constant parameter will increase the VCM current by 10% to 1.1 amps. This will ensure that the accelerations required at the design stage will be met under all conditions. The force constant ei
Berger Derek J.
Hudspeth David
Seagate Technology LLC
Wong K.
LandOfFree
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