Abstract:
A HDD comprising a temperature sensor disposed inside the HDD configured to periodically measure temperature inside of said hard disk drive; a magnetic disk; a read head; a write head; memory for storing RWO data. The RWO data is a function of a distance between the read head and the write head. The HDD also includes a RWO data adjustor configured to adjust the RWO data in response to a change in temperature of the HDD to compensate for a change in the distance between the read head and the write head based on the change in temperature.

Description:
BACKGROUND 
     Read elements and write elements are located separately in a magnetic heads (e.g., magnetoresistive (MR) heads). Therefore, write head position during data writing requires a different offset than read head position during data reading, which is referred to as read/write offset (RWO) or micro-jog. In particular, RWO changes at different cylinder locations because the actuator motion is rotary and the skew angle of the head against the track direction is different at different tracks. For example, RWO is different at the inner diameter (ID) than the outside diameter (OD) of the magnetic disk. Moreover, temperature changes within a hard disk drive (HDD) can change RWO requirements due to, but not limited to, thermal expansion/contraction of track pitch and the magnetic head 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of a HDD, in accordance with an embodiment of the present invention. 
         FIG. 2  illustrates an example of a read/write head, in accordance with an embodiment of the present invention. 
         FIG. 3  illustrates an example of a flow chart of a method for thermal read/write offset compensation, in accordance with an embodiment of the present invention. 
     
    
    
     The drawings referred to in this description should be understood as not being drawn to scale except if specifically noted. 
     DESCRIPTION OF EMBODIMENTS 
     Reference will now be made in detail to embodiments of the present technology, examples of which are illustrated in the accompanying drawings. While the technology will be described in conjunction with various embodiment(s), it will be understood that they are not intended to limit the present technology to these embodiments. On the contrary, the present technology is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the various embodiments as defined by the appended claims. 
     Furthermore, in the following description of embodiments, numerous specific details are set forth in order to provide a thorough understanding of the present technology. However, the present technology may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present embodiments. 
     With reference now to  FIG. 1 , a schematic drawing of one embodiment of an information storage system including a magnetic hard disk file or HDD  110  for a computer system is shown, although only one head and one disk surface combination are shown. What is described herein for one head-disk combination is also applicable to multiple head-disk combinations. In other words, the present technology is independent of the number of head-disk combinations. 
     In general, HDD  110  has an outer sealed housing  113  usually including a base portion (shown) and a top or cover (not shown). In one embodiment, housing  113  contains a disk pack having at least one media or magnetic disk  138 . The disk pack (as represented by disk  138 ) defines an axis of rotation and a radial direction relative to the axis in which the disk pack is rotatable. 
     A spindle motor assembly having a central drive hub  130  operates as the axis and rotates the disk  138  or disks of the disk pack in the radial direction relative to housing  113 . An actuator assembly  115  includes one or more actuator arms  116 . When a number of actuator arms  116  are present, they are usually represented in the form of a comb that is movably or pivotally mounted to base/housing  113 . A controller  150  is mounted on a printed circuit board attached to base  113  for moving the actuator arms  116  relative to the disk  138 . Controller  150  moves all arms at the same time, but it selectively adjusts the location of each head. Actuator assembly  115  may be coupled with a connector assembly, such as a flex cable to convey data between arm electronics and a host system, such as a computer, wherein HDD  110  resides. 
     In one embodiment, each actuator arm  116  has extending from it at least one cantilevered integrated lead suspension (ILS)  120 . The ILS  120  may be any form of lead suspension that can be used in a data access storage device. The level of integration containing the slider  121 , ILS  120 , and read/write head is called the Head Gimbal Assembly (HGA). 
     The ILS  120  has a spring-like quality, which biases or presses the air-bearing surface of slider  121  against disk  138  to cause slider  121  to fly at a precise distance from disk  138 . ILS  120  has a hinge area that provides for the spring-like quality, and a flexing cable-type interconnect that supports read and write traces and electrical connections through the hinge area. A voice coil  112 , free to move within a conventional voice coil motor magnet assembly  111  is also mounted to actuator arms  116  opposite the head gimbal assemblies. Movement of the actuator assembly  115  by controller  150  causes the head gimbal assembly to move along radial arcs across tracks (e.g., tracks  140  and  145 ) on the surface of disk  138 . In particular, tracks (e.g., tracks  140  and  145 ) include servo information that is used to center the read/write head. RWO data adjustor in the controller  150  is configured to adjust RWO data in response to a change in temperature of HDD  110 , which will be described in detail later. 
     HDD  110  also includes temperature sensor  160  configured to measure temperature within HDD  110 . Temperature sensor  160  can be electrically coupled to detect temperature in read/write head of slider  121 . Temperature sensor  160  measures ambient temperature in housing  113 . 
       FIG. 2  depicts a top-down view of head  200 , in accordance to various embodiments. In one embodiment, head  200  is a MR head. Head  200  includes a read head  210  and a write head  220 . Read head  210  is offset from the write head  220 . As a result of the gap between read head  210  and write head  220 , centering read head  210  will not necessarily center write head  220  with respect to a track, and vice versa. 
     RWO is used to properly adjust head  200  during reading/writing operations. RWO is a function of the physical distance between read head  210  and write head  220  and a skew angle. In other words, RWO varies for different radial positions of head  200  with respect to disk  138 . For example, RWO  240  at one radial orientation of head  200  is different than RWO  245  at another radial orientation of head  200 . 
     RWO includes offset values for various radial disk positions. RWO data is then used to adjust the position of head  200  to center read head  210  and/or write head  220 . For example, if read head  210  is at Track X (determined by servo information), then the location of write head  220  can be interpolated via RWO (e.g., RWO data at Track X). In one embodiment, RWO data is in the form of a skew table. In another embodiment, RWO data is pre-determined and stored in memory (not shown). In one embodiment, memory is HDD firmware. It should be appreciated that if written servo tracks have a variation of track pitch from head-to-head or cylinder-to-cylinder, then a calibration to compensate for the variation is incorporated into RWO data. 
     As stated above, temperature changes (e.g., change in ambient temperature, temperature change due to write current and/or TFC) within HDD  110  can affect RWO. For example, an increase in ambient temperature of HDD  110  can thermally expand head  200  and change the gap between read head  210  and write head  220 . As a result, a location of read head  210  and/or write head  220  that is calculated via pre-determined RWO data can be incorrect. Moreover, thermal expansion can also skew track pitch. As a result, a location of read head  210  and/or write head  220  that is calculated via RWO data can be incorrect. In one embodiment, track pitch is about 100 nanometers (nm) and a 25 Celsius (C) temperature change causes a track pitch change of about 0.8 nm or an error of 1-2%. It should be appreciated that heat generated by TFC  230  and heat generated by write current can facilitate in thermal expansion which can skew RWO. 
     In reference to  FIG. 1 , RWO data adjustor  170  in controller  150  is configured to compensate for temperature changes in HDD  110 . RWO data adjustor  170  in controller  150  receives temperature readings from temperature sensor  160  and RWO data. In one embodiment, in response to a change in temperature and a predetermined temperature coefficient, RWO data adjustor  170  in controller  150  compensates for a change in distance between read head  210  and write head  220 . In other words, RWO data is updated in real-time (or time intervals) to compensate for the thermal expansion of head  200  (and resulting change in distance between read head  210  and write head  220 ). 
     In another embodiment, RWO data adjustor  170  in controller  150  compensates for a change in track pitch based on a change in temperature. As a result, RWO data is adjusted to compensate for thermal expansion/contraction of track pitch. In a further embodiment, updated RWO data is stored in memory. It should be appreciated that RWO data is applied to servo-positioning (e.g., micro-jog) during data writing based on cylinder location and/or head dependency obtained by calibration at manufacturing process. 
       FIG. 3  illustrates a flow chart of a method  300  for thermal read/write offset compensation, in accordance with an embodiment of the present invention. In one embodiment, method  300  is carried out by processors and electrical components under the control of computer readable and computer executable instructions. The computer readable and computer executable instructions reside, for example, in a data storage medium such as computer usable volatile and non-volatile memory. However, the computer readable and computer executable instructions may reside in any type of computer readable storage medium. In one embodiment, method  300  is performed at least by a system described in  FIGS. 1 and 2 . 
     At step  310 , RWO data is determined as a function of a distance between a read head and a write head. At step  320 , an ambient temperature of a hard disk drive is measured. For example, ambient temperature measured by temperature sensor  160 . At step  325 , temperature change of the hard disk drive is measured. For example, a temperature change based on write current and/or TFC is measured. 
     At step  330 , in response to the change in temperature of the hard disk drive, RWO data is adjusted to compensate for a change in the distance between the read head and the write head based on the change in temperature. Similarly, at step  340 , in response to the change in temperature of the hard disk drive, RWO data is adjusted to compensate for a change in track pitch based on the change in temperature. 
     At step  350 , a location of a head (e.g., head  200 ) is adjusted based on the adjusted RWO data based on the temperature change. 
     Various embodiments of the present invention are thus described. While the present invention has been described in particular embodiments, it should be appreciated that the present invention should not be construed as limited by such embodiments, but rather construed according to the following claims.