Abstract:
A method and apparatus for controlling write operations for a data storage system during and after a shock event is disclosed. A shock sensor measures the magnitude of a shock event and compares the magnitude of the shock event to at least two predetermined thresholds. Write operations are then inhibited based upon the comparison of the magnitude of the shock event and the at least two predetermined thresholds. When the shock event meets a first upper threshold, the write is inhibited until the write is requalified. The write is executed if the measured shock event does not meet a second lower threshold and the write is paused for a predetermined time period when the measured shock event meets the second lower threshold but does not meet the first upper threshold.

Description:
This is a continuation-in-part of application Ser. No. 08/794,614, filed Feb. 3, 1997, now U.S. Pat. No. 6,115,200, which application is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates in general to data storage systems, and more particularly, to a method and apparatus for controlling write operations to a data storage medium in response to the data storage system being subjected to a shock event. 
     2. Description of Related Art 
     A typical data storage system includes a magnetic medium for storing data in magnetic form and a transducer used to read and/or write magnetic data from/to the storage medium. A disk storage device, for example, includes one or more data storage disks coaxially mounted on a hub of a spindle motor. The spindle motor rotates the disks at speeds typically on the order of several thousand revolutions-per-minute. Digital information, representing various types of data, is typically written to and read from the data storage disks by one or more transducers, or read/write heads, which are mounted to an actuator assembly and passed over the surface of the rapidly rotating disks. In a typical digital data storage system, digital data is stored in the form of magnetic transitions on a series of concentric, spaced tracks comprising the surface of the magnetizable rigid data storage disks. The tracks are generally divided into a plurality of sectors, with each sector comprising a number of information fields. One type of information field is typically designated for storing data, while other fields contain track and sector position identifications and synchronization information, for example. Data is transferred to, and retrieved from, specified track and sector locations by the transducers, which follow a given track and move from track to track, typically under the servo control of a controller. 
     Writing data to a data storage disk generally involves passing a current through the write element of the transducer assembly to produce magnetic lines of flux, which magnetize a specific location of the disk surface. Reading data from a specified disk location is typically accomplished by a read element of the transducer assembly sensing the magnetic field or flux lines emanating from the magnetized locations of the disk. As the read element passes over the rotating disk surface, the interaction between the read element and the magnetized locations on the disk surface results in the production of electrical signals in the read element. The electrical signals correspond to transitions in the magnetic field. 
     To reduce system errors, it is desirable to locate the read/write elements within the boundaries of each track during the read and write operations of the disk drive. If the read/write elements are moved toward an adjacent track by an external disturbance, the data in the adjacent track can be corrupted if a write operation is in progress. For example, if the read/write transducers move while the system is writing, the new data may write over the old data on the adjacent track, resulting in an unrecoverable loss of the old data. 
     Present data storage systems typically prevent head movement by employing a closed-loop servo control system. During normal data storage system operation, a servo transducer, generally mounted proximate the read/write transducers, or, alternatively, incorporated as the read element of the transducer, is typically employed to read information for the purpose of following a specified track (track following) and seeking specified track and data sector locations on the disk (track seeking). 
     Despite the servo system, data storage systems are susceptible to problems arising from external shock and vibrational loads. An excessive shock or vibrational load (shock event) may cause the read/write elements to move off track, for example, to an adjacent track. If this head movement occurs while the drive is writing data, the old data on the adjacent track may be lost. It is therefore desirable to have a data storage system, which prevents data from being lost when the system is subjected to a shock event. Typically servo systems are too slow to prevent at least some data from being lost, particularly if a high frequency shock event were to occur. 
     Typically systems for preventing write operations when the data storage system is subject to a shock event only inhibit write operations in the presence of the shock event. Oscillations in data storage systems caused by transient shock motion resulting from the excitation of the frequency component modes of the data storage system are not accounted for. That is, when the shock event stops, these systems allow write operations to be performed while post-shock motion or oscillations occur. 
     For example, if the initial offtrack magnitude of the read/write elements caused by a shock event is sufficiently large to be of concern, the data storage system will cause write operations to stop by setting a write inhibit flag. The write inhibit flag is then dropped when the read/write elements are positioned ontrack by the servo system. The read/write elements however are typically positioned ontrack prior to the dissipation of the energy of the shock event. In other words, the read/write elements often oscillate about the track several times before the energy of the shock dissipates. The offtrack that occurs during these oscillations is typically much larger than the initial offtrack because of the gains of the modes that are excited. If the read/write elements then move offtrack again because one or more component modes were excited by the shock, the written data may be unreadable. 
     It is also possible that data on an adjacent track can be overwritten and made unreadable. This can cause the data to be written away from track center, leading to damage to an adjacent track or a failure to overwrite old information. Both these events can cause unrecoverable corruption of data. Once way to ameliorate this problem is to have a high servo sample rate. But the size of the shock that can be tolerated is limited by the amount of real estate that can be devoted to the servo pattern, i.e., for any sample rate there is a large enough shock to cause off track writes. 
     To improve write operations during shock events, a shock sensor is often used to disable the write gate. However, this isn&#39;t a complete solution. For example, as suggested above, the worst motion caused by the shock can arise from the dynamical response that persists long after the shock itself has ended. A requalification by the servo may be forced when a shock is detected so that the write gate is re-enabled only after the requalification. Unfortunately, a requalification process is very time consuming, e.g., taking tens of milliseconds typically. Thus, invoking a requalification process may impact system throughput. 
     It can be seen then that there is a need for a method and apparatus for preventing write operations during shock events of different magnitudes while maximizing the system throughput. 
     SUMMARY OF THE INVENTION 
     To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, the present invention discloses a method and apparatus for controlling write operations for a data storage system during and after a shock event. 
     The present invention solves the above-described problems by providing a shock sensor that measures the magnitude of a shock event and compares the magnitude of the shock event to at least two predetermined thresholds. Write operations are then inhibited based upon the comparison of the magnitude of the shock event and the at least two predetermined thresholds. 
     A method and apparatus in accordance with the principles of the present invention includes detecting and measuring a shock event, determining whether the measured shock event meets a first predetermined criteria and disabling the write until the write is requalified when the measured shock event meets the first predetermined criteria. 
     Other embodiments of a method and apparatus in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the method further includes determining whether the measured shock event meets a second predetermined criteria, executing the write when the measured shock event does not meet the second predetermined criteria and pausing the write for a predetermined time period when the measured shock event meets the second predetermined criteria but does not meet the first criteria. 
     Another aspect of the present invention is that the pausing the write for a predetermined time period comprises activating an unlatched logic circuit for controlling a write gate. 
     Another aspect of the present invention is that the second predetermined criteria comprises a maximum threshold. 
     Another aspect of the present invention is that the disabling the write until the write is requalified the determining step comprises activating a latched logic circuit for controlling a write gate. 
     Another aspect of the present invention is that the first predetermined criteria comprises a minimum threshold. 
     These and various other features of novelty as well as advantages that characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects obtained by its use, reference should be made to the drawings which form a further part hereof, and to accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numbers represent corresponding parts throughout: 
     FIG. 1 is an exploded view of a data storage system according to the present invention; 
     FIG. 2 illustrates a block diagram of a data storage system in accordance with the invention; 
     FIG. 3 illustrates a block diagram of an exemplary write prevention system in accordance with the invention; and 
     FIG. 4 is a flow chart illustrating an exemplary process for controlling write operations according to the invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following description of the exemplary embodiment, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration the specific embodiment in which the invention may be practiced. It is to be understood that other embodiments may be utilized as structural changes may be made without departing from the scope of the present invention. 
     According to the present invention, a shock sensor measures the magnitude of a shock event and compares the magnitude of the shock event to at least two predetermined thresholds. Then, write operations are inhibited based upon the comparison of the magnitude of the shock event and the at least two predetermined thresholds. 
     FIG. 1 illustrates an exploded view of a disk drive system  10 . The disk drive  10  includes a housing  12  and a housing cover  14  which, after assembly, is mounted within a frame  16 . Mounted within the housing is a spindle shaft  22 . Rotatably attached to the spindle shaft  22  are a number of magnetic storage disks  24 . In FIG. 1, eight disks  24  are attached to the spindle shaft  22  in spaced apart relation. The disks  24  rotate on spindle shaft  22 , which is powered by a motor (not shown). Information is written on or read from the disks  24  by magnetoresistive (MR) heads or transducers (not shown) which are supported by sliders  26  and coupled to a channel for processing read and write information (not shown). Preferably, sliders are coupled to the suspensions or load springs  28 . The load springs  28  are attached to separate arms  30  on an E block or comb  32 . The E block or comb  32  is attached at one end of an actuator arm assembly  36 . The actuator arm assembly  36  is rotatably attached within the housing  12  on an actuator shaft  38 . The rotary actuator assembly  36  moves the integrated transducer/suspension assembly in accordance with the present invention in an arcuate path across the surface of the storage disk  24 . It should be noted that the disk drive described above is provided by way of example and not of limitation. Those skilled in the art will recognize that any data storage system, including optical, magneto-optical, and tape drives, for example, having at least one data storage medium and transducer may be subject to data corruption resulting from post-shock motion of the transducer and may benefit from the present invention. 
     FIG. 2 illustrates a block diagram of an exemplary computer disk drive system  200  suitable for practicing the invention. It shall be understood that the general read/write and servo functions of a disk drive are well known in the art, and their particular implementation is not an aspect of the present invention unless expressly noted. A host computer system  210  may be coupled to a disk drive system  200  via a buffer controller in interface block  213  and an interface processor  212 . The interface processor  212  processes commands from the host system  210  and in turn communicates with a servo controller  214  and formatter electronics  224 . The servo controller  214  includes a servo processor  216  and servo control and timing logic  218 . Data parameters may be stored in memory such as random access memory (RAM)  220  or data buffer  211 , or alternatively the data may be stored on the disk  222  itself. The servo processor  216  receives commands from the interface processor  212 . 
     Command and sequences and data to be written to the disk  222  are routed to the formatter electronics  224 . The read/write circuit  226  conditions the data and routes the data to the head  228  at the end of the actuator arm  230  for writing to the disk  222 . Data read from the disk  222  by the transducer or sensor  228  is received by the read/write circuit  226  and conditioned to provide a read pulse output. The read pulse output is then routed to the formatter electronics  224  for processing before being provided to the host  210  via the data buffer  211  and interface electronics (INFC)  213 . The interface electronics  213  is coupled to the bus from the host  210  and communicates with the interface processor  212  and data buffer  211 . 
     The servo processor  216  provides control signals to the servo control and timing logic  218 . The servo control and timing logic  218  interprets the control signals from the servo processor  216  and sends voice coil control signals to the voice coil motor  232 . The voice coil motor  232  drives the actuator arm  230  in accordance with the control signals. The actuator arm  230  supporting the head  228  is driven by the voice coil motor  232  to move the transducer  228  to a target track position on the disk  222 . The servo control and timing logic  218  outputs to the servo processor  216  position data indicative of the track position corresponding to the current position of the transducer  228  from servo data read out from the disk  222 . The position data may be used to generate servo control information, such as a position error signal (PES signal), values of which indicate the offtrack magnitude of the transducer. 
     FIG. 3 is a block diagram illustrating an exemplary write prevention system  300  for preventing a write operation to data storage medium in response to a shock event. The write prevention circuit includes a shock sensing circuit  302  for sensing and measuring shock events. The shock sensing circuit  302  is operatively coupled to an unlatched logic circuit  306  and a latched logic circuit  308 . The unlatched  306  and latched  308  logic circuits provide signals to a write gate  312  to enable and disable the write gate  312 . The write gate controls the write  322  by the data storage device. Parameters, programs and other data may be stored in the memory  320 . The servo  310  directs the read  330  and is in communication with the shock sensing device  302 , the memory  320  and the unlatched  306  and latched  308  logic circuits. 
     Though the shock sensing circuit  302  is illustrated as being separate from the servo controller  310 , it is noted that the functionality of the sensing circuit  302  may be embedded and/or programmed within the servo controller  310 . Moreover, although the various components of the servo control system and write prevention system  300  are illustrated through the use of circuits, it should be appreciated that these components may be implemented through the use of software (in addition to or in place of circuitry) without loss of functionality. 
     Generally, when a disk drive is subject to a shock event, the shock sensing circuit  302  detects the shock event and measures its magnitude. The measured magnitude is compared to a first threshold. If the measured magnitude is less than the first predetermined threshold, sensing circuit  302  does not sent a signal to either the unlatched  306  or latched  308  logic circuits to inhibit the write gate  312 . If the measured magnitude is greater than the first predetermined threshold but less than a second predetermined threshold, the sensing circuit  302  sends a signal to the unlatched logic circuit  306 . The unlatched logic circuit  306  then inhibits the write gate until the shock event passes. The unlatched logic circuit  306  may be designed so that the write is inhibited for a predetermined period of time or the unlatched logic circuit  306  may be instructed by the shock sensing device  302  to wait for a provided period of time based upon the detected magnitude of the shock. 
     If the measured magnitude is greater than the second predetermined threshold, the sensing circuit  302  sends a signal to the latched logic circuit  308 . The latched logic circuit  308  then inhibits the write gate until the latched logic circuit  308  is reset. To be reset, the latched logic circuit  308  must allow for the requalification of the write, e.g., until the servo  310  or the shock sensing device  302  determines that the write may be performed. The comparison of the measured magnitude to the first and second thresholds may be performed by the shock sensing circuit or by the servo. 
     Generally, any number of well-known circuits may be used to sense the occurrence of a shock event. Exemplary shock sensing circuits will be briefly discussed with more detailed implementation being left to those of skill in the art. For example, the shock sensing circuit  302  may receive PES values and compare these values to the first and second thresholds, and based upon the comparison of the PES values to the first and second predetermined thresholds, the shock sensing circuit  302  may provide control signals to the unlatched  306  and latched  308  logic circuits as described above. 
     In an alternate embodiment, the shock sensing circuit  302  may include a low pass filter through which a power waveform of the PES signal is passed. The filtered power signal may then be compared to the first and second predetermined threshold values to determine whether a signal should be provided to the unlatched  306  and latched  308  logic circuits as described above. 
     In yet a different embodiment, the shock sensing circuit  302  may include an external shock sensor, such as an accelerometer, which in response to a shock event generates a signal which is compared against the first and second predetermined threshold values to determine whether a signal should be provided to the unlatched  306  and latched  308  logic circuits as described above. 
     To assure post-shock movement resulting from excitation of each frequency component mode has subsided, the servo samples preferably at spans at least equal to the time required for one complete cycle of the frequency component mode in the data storage system which has the lowest frequency. 
     Turning now to FIG. 4, there is shown a flow chart  400  illustrating an exemplary process for preventing write operations in the presence of shock event of different magnitudes according to the present invention. Generally, the process involves sensing the shock event and inhibiting write operations until the energy of the shock event has dissipated a sufficient amount. 
     More specifically, a shock event is detected and measured  402 . The measured magnitude of the shock event is compared to a first threshold  404 . If the measured magnitude is less than the first predetermined threshold  406 , a signal is not sent to either the unlatched or latched logic circuits to inhibit the write gate  408 . The write is never inhibited. 
     If the measured magnitude is greater than the first predetermined threshold  410 , the measured magnitude of the shock event is compared to a second threshold  412 . If the measured magnitude of the shock event is less than a second predetermined threshold  414 , the sensing circuit sends a signal to the unlatched logic circuit and the unlatched logic circuit inhibits the write gate until the shock event passes  416 . The write is then enabled  418 . 
     If the measured magnitude is greater than the second predetermined threshold  420 , the sensing circuit sends a signal to the latched logic circuit  422 . A determination is made whether the latched logic circuit has been reset  424 . The write is inhibited until the latched logic circuit has been reset  426  by the servo requalifying the write. After the latched logic circuit is reset  428 , the write is enabled  418 . 
     The foregoing description of the exemplary embodiment of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not with this detailed description, but rather by the claims appended hereto.