Disk drives are well known in the computer art for providing secondary mass storage with random access. A disk drive comprises one or more magnetic data storage disks rotated on a spindle by a spindle motor within an enclosed housing. A magnetic transducer head is placed on an actuator arm and positioned very closely to a corresponding disk surface by a slider suspended upon an air bearing. Servo information is typically written in servo sectors which are interleaved between data sectors or blocks. Servo information provides a servo controller with head position information to enable a head positioner, such as a rotary voice coil motor (VCM), to move the actuator arm and therefore the head from track-to-track during random access track seeking operations, and to maintain the head in proper alignment with a track centerline during track following operations when user data is written to or read from the available data sectors of the disk surface. As such, the servo controller controls head positioning as the head is moved transversely across the tracks by the actuator arm, and maintains the head over a particular track as the disk spins. The servo controller also controls the acceleration of the head which results from a force supplied by the VCM to the actuator arm.
The servo controller receives head position readings from the head. The head position is determined from the servo information written directly onto the disk by e.g. a servo writer as part of the manufacturing process. The servo information may include the track number and indicate how far the head is from the track centerline. That is, certain information on each track is reserved for indicating head position. As the head passes over the servo information, the track identification and position indicators are read by the head and supplied to the servo controller. The position indicators are at regularly spaced locations. Thus, the servo controller input from the head is not continuous but is sampled.
The servo writer is typically stabilized on a large granite base to minimize unwanted vibration and employs an encoder (e.g. laser interferometry) for position measurements. The servo writer supplies power to the spindle motor for rotating the disk. The servo writer may include a fixed head for writing a clock track onto one disk surface. The servo writer may also include a positioning system for moving a push-pin which extends through an opening in the disk drive housing and mechanically contacts the actuator arm. The positioning system uses the push-pin to move the actuator arm and the head radially across the disk, and the head writes the track address and the servo information at several specified locations called servo wedges that extend radially across the tracks. The servo wedges provide servo sectors for each track on the disk.
The servo writer attempts to write the servo information in circular tracks that are evenly spaced across the disk surface. However, because of mechanical and electrical limitations, it not possible to obtain even track spacing. This track mispositioning is referred to as “squeeze” which limits the off-track read capability (OTRC) of the disk drive and can cause encroachment (overwrite) leading to data loss. Existing methods of correcting squeeze and encroachment are not effective when two adjacent tracks are radially positioned too close to or too far from one another.
This problem is increasingly significant as track densities (measured in tracks-per-inch (TPI)) are increased. Increased TPI makes track spacing errors and the resulting squeeze more significant to data integrity. Existing methods of detecting squeeze include detecting data corruption due to encroachment. For example, during disk drive manufacturing, a flaw scan test identifies the disk locations which do not provide reliable read and write operations. These disk locations are logged within the disk drive and mapped out of the available customer data area. However, such techniques are time consuming and expensive.
There is, therefore, a need to efficiently detect squeeze during disk drive manufacturing. There is also a need for correcting squeeze in order to prevent encroachment and degradation in the OTRC of the disk drive.