Method and apparatus for feedforward repeatable runout compensation in a selected frequency range

#This application relates generally to data storage devices and more particularly to servo positioning in such devices.

#Disc drives are data storage devices that store digital data on a rotating disc. Modern disc drives, for example, comprise one or more rigid information storage discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers mounted to a radial actuator for movement of the heads relative to the discs. During a data write operation sequential data is written onto the disc track, and during a read operation the head senses the data previously written onto the disc track and transfers the information to an external environment. Important to both of these operations is the accurate and efficient positioning of the head relative to the center of the desired track on the disc. Head positioning within a desired track is dependent on head-positioning servo patterns, i.e., a pattern of data bits recorded on the disc surface and used to maintain optimum track spacing and sector timing. Servo patterns or information can be located between the data sectors on each track of a disc (“embedded servo”), or on only one surface of one of the discs within the disc drive (“dedicated servo”). Regardless of whether a manufacturer uses “embedded” or “dedicated” servos, the servo patterns are typically recorded on a desired disc during the manufacturing process of the disc drive.

#Recent efforts within the disc drive industry have focused on developing cost-effective disc drives capable of storing more data onto existing or smaller-sized discs. One potential way of increasing data storage on a disc surface is to increase the recording density of the magnetizable medium by increasing the track density (i.e., the number of tracks per inch). Increased track density requires more closely-spaced, narrow tracks and therefore enhanced accuracy in the recording of servo-patterns onto the desired disc surface. This increased accuracy requires that servo-track recording be accomplished within the increased tolerances, while remaining cost effective.

#FIG. 1 shows a data storage device constructed in accordance with a preferred embodiment of the present invention.

#FIG. 2 shows a flowchart for a “PES-based” compensation method of the present invention.

#FIG. 3 is a block diagram of a servo system of the present invention configured to use repeatable runout (RRO) compensation that is RRO-based.

#Although the examples below show more than enough detail to allow those skilled in the art to practice the present invention, subject matter regarded as the invention is broader than any single example below. The scope of the present invention is distinctly defined, however, in the claims at the end of this document.

#Numerous aspects of data storage device technology that are not a part of the present invention (or are well known in the art) are omitted for brevity, avoiding needless distractions from the essence of the present invention. For example, this document does not include specifics of how PES is manipulated in a conventional servo loop to control actuators that position transducer heads. Neither does it include specific methods for receiving, queuing, and executing user commands. Specific techniques for implementing transfer functions specified by formulae or Bode plots in circuitry or software are likewise omitted, typically being a matter of design choice to those of ordinary skill in servo controller design.

#Definitions and clarifications of certain terms are provided in conjunction with the descriptions below, all consistent with common usage in the art but some described with greater specificity. A typical disc-based data storage system has a data storage disc that rotates at a controlled frequency during normal read/write operations. Energy in position error signal that is concentrated at harmonics (integer multiples) of this controlled frequency is called repeatable runout (“RRO”). Coherent RRO (“CRRO”) is that portion of the RRO that is common for groupings of many adjacent tracks (e.g. in an annular zone).