1. Field of the Invention
This invention is concerned with writing embedded servo disks for fixed media hard disk drives while said disks are off the drive spindle and with the techniques required for seeking and following a xe2x80x9cvirtual trackxe2x80x9d derived from the pre-written tracks but concentric with the center of rotation of the disks once they are mounted in the drive.
2. Description of the Related Art
Disk drives are magnetic data storage devices on which user created data can be written and read. Typically such drives contain one or more flat disks which rotate at high angular velocity (3600 rpm being typical) about a central spindle and on which data is encoded by magnetic read/write heads that move above the surface of the disk and are radially positioned by actuator arms. Under ideal circumstances the data written by the read/write head forms narrow circular tracks, as thin as 0.8 microns, that are concentric with the rotation axis of the spindle. During the operation of the disk drive mechanism, however, thermal and mechanical shocks and stresses can warp individual disks and also cause a disk to shift from the true center of the rotating spindle. If this occurs, an attempt by the actuator mechanism to fix the read/write head over a track that is still assumed to be circular and concentric with the spindle will not succeed and data will be read or written inaccurately. In the case where the disk slips relative to its concentric position with the spindle, a new, non-concentric circular path will form, which will overlap several of the previously written concentric paths. This displacement of concentricity is called xe2x80x9crunoutxe2x80x9d and, when it occurs, new writes by the read/write head will overwrite previously written data. In short, accurate positioning of the read/write head becomes nearly impossible without additional information being present on the disk to guide it to the correct track positions and, when possible, to correct for runout. This information, which actuates locating and position-correcting servomechanisms within the drive unit, is called servo information and it is pre-written on the, hard disks after the drive unit is assembled, but before customer data is Written on them.
Winchester technology introduced fixed disk drives in about 1970. In the construction of these drives with embedded servos, which are essentially the drives used today, disks are mounted on the spindle, heads are mounted in the drive, the entire drive is incorporated within a device called a servo track writer. (STW) and the required servo information is then written onto the disk by the drive""s own read/write heads. The degree of precision required is such that the write head must be positioned by laser interferometry or some other technique of equal precision. In this regard, Tsai (U.S. Pat. No. 5,315,372) teaches a method of constructing a servo track writing apparatus with a laser interferometer for positioning a remote servo track positioning arm. In addition, the accuracy with which the write head of the disk drive must be advanced to write each successive piece of servo data often requires the use of a mechanical xe2x80x9cpusher block assemblyxe2x80x9d to achieve the required definition. In this regard, Fioravanti (U.S.
Patent No. 5,774,294) teaches a method for incorporating a pusher block with a tuned damper that reduces the effects of errors in the written servo data that result from resonances in the pusher block assembly.
Typically, the servo data for an embedded servo comprises a data field that identifies the track, a servo xe2x80x9cburstxe2x80x9d that is used to properly align the head with the track and other fields for read/write and system operation. The modern read/write head, wherein the read function and write function are performed by separate elements (eg. a magnetoresistive read head, an induction write head) that are offset from one another, requires different alignment for reading and writing. Codilian et al (U.S. Pat. No. 5,946,157) teaches a method of recording circumferentially successive servo bursts that overlap each other and allows the magnetoresistive read head to always be positioned within its linear width on one of the bursts.
The embedded servo data field and user data field of adjacent concentric tracks are radially aligned to form wedge shaped sectors, each containing a servo wedge and a data wedge. The accurate writing of the servo data requires that a clock track be written first, usually along the outside circumference (OD) of the disk. The magnetic transitions written into the clock track divide the disk into a predetermined number of angular positions. The servo information is then written synchronously with the clock track information and the servo wedges are determined by the angular partitioning produced by the clock track. In this regard, Stein et al (U.S. Pat. No. 5,796,541) teach a method of writing a clock track that is independent of variations in the spindle rotational speed.
The role of the servo track remains the same today as it was in 1970. The servomechanism attempts to follow the written servo track as closely as possible. Normal servos can eliminate approximately 90% of the tracking error usually encountered. If the runout is excessive as would be the case if the drive had been subjected to mechanical or thermal shocks, then xe2x80x9crunout compensatorsxe2x80x9d can be applied to eliminate an additional 90% of the remaining error. Andrews, Jr. et al (U.S. Pat. No. 5,539,714) teach a method for designing an xe2x80x9cadaptive runout compensator,xe2x80x9d which provides xe2x80x9con-line, real-time compensation for disk runout.xe2x80x9d According to this method, the regular rotation of the off-center track (the track with runout) produces a periodic signal whose harmonics are an indication of the eccentricity of the track. The compensator compiles a given number of the harmonic coefficients (ie. does a limited spectral analysis by a method of a discrete Fourier analysis) of the signal during a predetermined number of disk rotations. These coefficients or Fourier components are then used to provide a compensating signal to the read/write head actuator for each data sector defined on the disk. In U.S. Pat. No. 5,930,067, Andrews et al teach a method of designing a multi-harmonic compensator for track following which is similar in principle to the design in U.S. Pat. No. 5,539,714, but which calculates more harmonic components to provide a more accurate degree of compensation.
Whether or not compensators are incorporated into a disk drive design, the writing of the servo information is a costly and time-consuming process. With track densities approaching 20,000 tracks per inch (TPI) and the data capacities of 2.5 inch disks approaching 10 gigabytes (GB), the time required to write the servo information is approaching 1 hour. As one might suspect from the complex designs and technologies taught in the patents cited above, the cost of the servo track writer (STW), which writes the servo information on the disks, is quite high, between $50,000 and $250,000. In all, it is a very high cost, very low throughput operation which requires a great deal of space in a clean-room, is difficult to automate and requires a great deal of technical personnel support.
This invention teaches a method of writing servo information on loose disks rather than on disks which have already been mounted in a disk head assembly. Only after the disks have been servo- written would they be mounted on the drive spindle and only then would the read/write head and actuator mechanism be introduced. This approach greatly relieves the assembly operation and reduces the burden on the clean room. In addition, the loose-disk servo track writer (STW),would be a simpler and less costly machine to manufacture then the servo track writer presently used to write disks that are already mounted in a head assembly. It would be an easier device to maintain and would lend itself to the automated processing of disks. It is conceivable that the servo writing process could run, unattended, on a 24 hour basis. The disks could be handled in caddies, which are more space-efficient than assembled drives. A well designed loose-disk STW could simultaneously write a stack of disks and do so at high speeds with a single clock disk and dedicated write heads. It is therefore a first object of this invention to simplify, make more efficient and reduce the cost of the writing of servo data without sacrificing the capability of that data to enable the accurate location of data tracks during the normal operation of the drive. It is a second object of this invention to provide a method by which servo data can be used for the accurate reading and writing of data tracks in a way that allows the normal operating configuration of the head actuator to be one of quiescence rather than one of constantly following rotationally off-center tracks with runout.
Both objects of this invention will be achieved by the introduction and use of a novel concept, the xe2x80x9cvirtual track,xe2x80x9d which is a track followed by the read/write head that is not actually a physical locus of magnetically stored data on the disk. The virtual track is an imaginary circle centered on the actual axis of rotation of the disk and defined xe2x80x9cvirtuallyxe2x80x9d by an array of stored addresses which are the intersection points of the track and the pre-written servo information patterns. Since the rotationally concentric virtual tracks are an invariant property of the mounted disk, it is their use that permits the servo data to be written in another servo track writer (STW) unit. Although the servo data is used initially to define the virtual track, the quiescent mode of the head actuator assembly ultimately verifies that definition.
The addresses used to define the virtual track can be conceived as consisting of a sequence XXX.yyy, where XXX represents the pre-written servo identification code of the particular track and yyy is the fractional track position of the intersection, which can be derived from the information stored in the servo bursts. With the virtual track as the guide for the read/write head, that head is no longer required to constantly chase after the runout tracks. The ordinary status of the head actuator is quiescence. The virtual track is followed by loading up the next track address in the stored array and finding it. In fact, the quiescence of the actuator is a measure of goodness of the virtual track as defined by its array.
Initially, a virtual track is defined at three radial positions on the disk, the inside diameter (ID), the middle diameter (MD) and the outside diameter (OD). This is done by locking on to a pre-written track and using a runout compensator to determine the harmonic components of its runout motion. Then the intermediate values are generated by interpolation.
The advantages of this invention extend to both the process of servo writing the disks and to the operation of the drive assembly once the disks are mounted within it. As already noted, the servo writing process is simplified, it can be automated and it requires a servo track writer (STW) that is much less expensive (on a per disk basis) than that presently employed. During the normal operation of the drive, the actuator is now configured to strive for minimum activity, rather than to constantly follow the runout of physical tracks. The state of normal quiescence implies that the actuator is subjected to minimal forces and accelerations, which, in turn, minimizes tracking errors associated with the inertia of the mechanism. In short, the drive is defining its own xe2x80x9cperfectxe2x80x9d track, then using it. Minimization of tracking errors due to the constant following of tracks with runout effectively increases acceptable offtrack tolerances, leaving more margin for error within the system.