System for correcting transducer tangential skew in a disk drive system having two actuators
This application is a continuation of Ser. No. 08/007,585, filed Jan. 22, 1993, now abandoned. 
    
    
     A portion of the disclosure of this patent document contains material which is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by any one of the patent documents or the patent disclosure as it appears in the United States Patent &amp; Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 
     CROSS-REFERENCE TO RELATED APPLICATIONS 
     DISK DRIVE SYSTEM USING MULTIPLE EMBEDDED QUADRATURE SERVO FIELDS, Ser. No. 386,504, filed Jul. 27, 1989, assigned to the assignee of the present application; 
     ADAPTIVE READ EQUALIZER WITH SHUTOFF MODE FOR DISK DRIVES, Ser. No. 559,899, filed Jul. 30, 1990, assigned to the assignee of the present application; 
     DISK DRIVE SYSTEM EMPLOYING ADAPTIVE READ/WRITE CHANNEL CONTROLS AND METHOD OF USING SAME, Ser. No. 420,371, filed Oct. 12, 1989, assigned to the assignee of the present application; 
     HIGH PERFORMANCE DISK DRIVE ARCHITECTURE, Ser. No. 07,612,427, filed Nov. 9, 1990, assigned to the assignee of the present application; and MULTIPLE ACTUATOR DISK DRIVE, Ser. No. 07,614,853, filed Nov. 9, 1990, assigned to the assignee of the present application. 
     Each of these Related Applications is hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally related to hard disk drives and disk drive electronic control systems. In particular, the present invention relates to a system for tangential skew correction of the transducers for reading and writing data from the disk in a disk drive system having two actuators. 
     2. Description of the Related Art 
     A disk drive system having two actuators is shown in FIG. 1 and is comprised of disk assembly 7 and actuators 8 and 9. Transducer 14 on actuator 8 and transducer 13 on actuator 9 read and write information to and from the disk on disk assembly 7. Further it is desirable for each of the transducers 13 and 14 to read data regardless which of the transducers recorded the data. 
     FIG. 2 shows in more detail the disk drive system having two actuators of FIG. 1. Actuator 8 has a head arm assembly 15 where each arm of the head arm assembly 15 carries a transducer to communicate with one of the surfaces of a disk in disk assembly 7. The head arm assembly 15 is connected to a motor 11 for placing a given transducer over a desired track on one of the disk surfaces of disk assembly 7. Disk assembly 7 is comprised of a plurality of disks 10, 16 and 18, shaft 19 and a motor 20. Actuator 8 employs transducers 14 and 22 to interact with top and bottom surfaces of disk 10 respectively, transducers 24 and 26 to interact with the top and bottom surfaces of disk 16 respectively and transducers 28 and 30 to interact with the top and bottom of disk 18 respectively. Actuator 9 is comprised of a motor 12, a head arm assembly 17 and transducers 13, 23, 25, 27, 29 and 31. 
     In a disk drive system having two actuators, one of the actuators is used as a packwriter to record on each disk surface system operational information such as servo data, sector marks, address marks and like information. Actuator 8 is used as the packwriter to record the system information on the surfaces of disks 10, 16 and 18. Thus there is no radial or tangential skew problem associated with transducers 14, 22, 24, 26, 28 and 30 because their physical location with respect to each other and with respect to the disk surface remains a constant. 
     The spatial relationship between transducers 14, 22, 24, 26, 28 and 30 of actuator 8 will be different than the spatial relationships between actuators 13, 23, 25, 27, 29 and 31 of actuator 9. There is no physical means to align the transducers on the head arm assembly of actuator 9 to accurately duplicate the spatial relationship of the transducers on actuator 8. The radial and tangential skew between the transducers on the head arm assemble of the nonpackwriter actuator does cause a delay in obtaining proper transducer placement over a desired track during a transducer switching operation between transducers associated with the nonpackwriter actuator. 
     Assuming that transducer 13 is used as the reference transducer, it can be seen from FIG. 3 that transducers 23, 25, 27, 29 and 31 can be radially offset either to the right or left of reference transducer 13. Such an offset to the right or left of transducers 23, 25, 27, 29 and 31 may cause these transducers to be located over different tracks than the reference transducer 13. Of course, a transducer may in fact be in alignment with the reference transducer 13 as shown with respect to transducer 31. Therefore, transducer 31 will be over the same track as transducer 13 which still may not be the correct track because the spatial relationship between transducers 13 and 31 may be different than the spatial relationship between packwriter transducers 14 and 30. Thus a track correction may have to be made for transducers 23, 25, 27, 29 and 31 to place the transducer over the correct track before data may be read or written properly. 
     FIG. 4 uses transducer 13 as a reference and shows that transducers 23, 25, 27, 29 and 31 may be tangentially offset to the front or to the rear of reference transducer 13. Where the transducer is offset to the front of reference transducer 13, the address mark will appear earlier than the address mark at the reference transducer 13. Where the transducer is set to the rear of reference transducer 13, the address mark will occur later than the address mark at the reference transducer 13. 
     In the disk drive system using a sectored embedded servo, control circuitry is synchronized to data read from the desired track on the disk surface. The system expects the occurrence of servo data, sector marks and address marks to occur at specific times and sets timers to trigger circuitry to search for the occurrence of servo data, sector marks and address marks. 
     If reference transducer 13 is being used and the system switches to transducer 23, the system would still expect the address mark to occur at the time that the address mark would have occurred for transducer 13. Therefore the start of the address mark search by the system can begin after the address mark has passed beneath transducer 23 since transducer 23 is physical ahead of transducer 13. The address mark will therefore be missed and the system would have to go through the entire synchronization procedure before the system could ascertain the track location of transducer 23. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to establish a system to compensate for the tangential offset of the transducers on the nonpackwriting actuator such that the next address mark appearing on the data track after a transducer switching operation will be properly read by the newly selected transducer, thereby increasing the overall efficiency of the disk drive system. 
     The present invention develops a reference table of the time offset between designated transducer and all of the other transducers of the nonpackwriting actuator in the drive disk drive system having two actuators. The table is stored in memory and is used in each transducer switching operation of the nonpackwriting actuator. During a transducer switching operation, means calculate the correction factor for the newly selected transducer from the table stored in memory and adjust the time when the start of the next index mark search should occur, as established by the prior transducer, such that the time for the start of the address mark search will be that time for properly starting the address mark search for the newly selected transducer. This system will cause the newly selected transducer to sense the first address mark to occur after the transducer switching operation. As a result of obtaining the address mark information, the transducer can then be placed over the correct track such that a data transfer operation may proceed. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with respect to the particular embodiments thereof and reference will be made to the drawings, in which: 
     FIG. 1 is a high level diagram of a disk drive system having two actuators; 
     FIG. 2 is a diagram showing the relationships of the various transducers associated with each of the actuators with a disk surfaces of the disk drive system; 
     FIG. 3 is a diagram of the skew, along the X axis, to the left or right of a reference transducer of the other transducers associated with the nonpackwriting actuator in a disk drive system having two actuators; 
     FIG. 4 is a diagram showing the skew, along the Z axis, ahead or to the rear of a reference transducer of the other transducers associated with the nonpackwriting actuator of a disk drive system having two actuators; 
     FIG. 5 is a logic diagram for tangential skew correction during a transducer switching operation for the nonpackwriting actuator in a disk drive system having two actuators; and 
     FIG. 6 is a table describing the tangential skew correction factors stored in memory to be used in conjunction with the logic of FIG. 5. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 5, the logic associated with establishing the timing for sensing an address mark in a disk drive system having two actuators is shown. 
     A disk drive system normally has associated with it a central processing unit (CPU) 35 to control the operation of the disk drive system. CPU 35 will have a processor 40 for controlling such operations as the track seeking, transducer switching, track following, data reading and writing, and transferring data to and from the disk drive system to another processor or memory. CPU 35 has an associated memory unit 42 to store the various control program to be used by processor 40 in controlling the operation of the disk drive system. 
     The system employs self-synchronizing techniques and uses encoded data in specific areas on the disk to obtain data relevant to system control functions and procedures. Once such type of data is an address mark which is used to determine the identity of the data being read by a transducer in the system. To locate the address mark, processor 40 will establish the time for the start of the search for an address mark in terms of a count value of counter 46. Counter 46 is stepped by clock pluses appearing on line 43. Comparator 48 compares the count value of counter 46 on line 47 and the count value for the for initiating an address mark search appearing on line 44. When a comparison occurs, an address mark search signal is generated on line 51 to signal the start of an address mark search by the system. The system (by logic not herein shown) will open a window during which time the data being read by the selected transducer is expected to be an address mark. 
     This invention incorporates into the foregoing, subtractor 52 to subtract the tangential skew correction factor for the presently selected transducer on line 38 from the tangential skew correction factor for the transducer to be selected on line 39. The results of the subtraction is provided on line 45 as the transducer switching correction factor to be used in the transducer switching procedure. Adder 49 adds the transducer switching correction factor on line 45 to the anticipated start address mark search count on line 44 to generate a new start address mark search count on line 50 to be used by comparator 48. It should be noted that the transducer switching correction factor is only used to search for the first address mark after a transducer switching operation. After the first address mark has been sensed, processor 40 will thereinafter generate the proper next start address mark search count in accordance with the normal address mark search procedure. 
     More specifically, referring to FIG. 6, a table is stored within memory of the tangential skew correction factors for transducer switching operations between reference transducer 13 and transducers 23, 25, 27, 29 and 31 of actuator 9 The usable recording surface of the disk is divided into four zones and a correction factor is established for each of the zones. It has been found that the arc path transversed by the transducers on the packwriting actuator is different from the arc path transversed by transducers of the nonpackwriting actuator. As a result of the differences in the arc path followed by the transducer associated with both actuators for interacting with the same disc surface, the tangential skew correction factor is not a constant. To mitigate this problem, the tracks have been divided into four zones and the tangential skew correction factor is established in each zone with the reference transducer located over the center track of the zone. During a transducer switching operation, the processor 40 will read the correction factor for the present transducer selected and the correction factor for the transducer to be selected. The correction factor of the present selected transducer is subtracted from the value of the newly selected transducer and this value becomes the value of the correction factor which appears on line 45 of FIG. 5 and sent to adder 49. This procedure allows the correction factor between any two transducers in any zone to be determined while only storing the correction factor for switching between the reference transducer and each of the other transducers for each of the zones. The amount of data to be stored in support of this invention is thereby minimized. 
     For example, if reference transducer 13 is selected and it is desired to switch to transducer 27 and the reference transducer 13 is presently within zone 2, then correction factor C23 will be used. Since the reference transducer is the transducer to be switched from, the correction value for that transducer is deemed to be zero and, therefore, the effective correction factor for that transducer switching operation is the value C23 as shown in FIG. 6. 
     Next assume that transducer 31 is selected and it is desired perform a transducer switching operation to select transducer 23. Further assume that transducer 31 is presently within zone 3. The processor 40 recovers from memory 42 the value of C35 for transducer 31 and C32 for transducer 23. The transducer switching correction factor for this transducer switching operation will be C35 minus C31. It should further be realized that the correction factors can be of a positive or negative number as they are adjusting the count in a forward or backward direction. By viewing FIG. 4, transducer 31, being to the rear of the reference transducer 13, would have a positive correction factor which would cause the start address mark search signal on line 51 to occur at a later time than had reference actuator 13 been used. In similar fashion, actuator 23 is ahead of reference actuator 13 and its correction factor would be a negative correction factor which will cause the start address mark search signal on line 51 to occur at an earlier time than the address mark signal which would have been generated for reference transducer 13. The correction factor for the transducer switching operation from transducer 31 to transducer 23 would mean that transducer 23 is in front of transducer 31 and, therefore, a negative value should result to cause the start of an address mark search to occur at an earlier time than would have been predicted for the start of an address mark search when using transducer 31. Therefore the subtracting from the newly assigned transducer 23, the value of the present selected transducer 31 will yield a negative number which will cause the start of the address mark search for transducer 23 to occur at an earlier time than the start of the address mark search that would have occurred for transducer 31. 
     Once the address mark has been properly read by transducer 23 then a correction can be made to bring transducer 23 to the desired track. Since the address mark identifies the track presently being read, the system can adjust transducer 23 to the correct track such that a data operations may be initiated. 
     The correction factors as shown in FIG. 6 are obtained by means of the circuitry shown in FIG. 5. During the manufacture of the disk drive system having two actuators, a microprogram is run to determine the values of the correction factors. Since there is no simple way, without additional hardware, of measuring the time difference directly within the system, a trial and error system is employed. The trial and error system will be described for a transducer switching operation from transducer 13 to transducer 23 in zone 1 for exemplary purposes. The same procedure is performed for the transducer switching operation for each transduce with regard to the reference transducer for each zone on the disk. The procedure calls for a transducer switching operation to be performed between transducer 13 and 23 and the procedure notes if an address mark was sensed by transducer 23 at the expected time. If no address mark was sensed during the address window, then a correction factor is added in a forward direction and the switch operation is again attempted. The method will alternate by adding a forward or backward correction factor to the initial setting, increasing the value of the correction factor each time used, until an address mark is first obtained. The system then incrementally increases the correction factor in the direction in which the address mark was first sensed until the address mark is lost. This indicates a maximum correction factor. The correction factor is then incrementally decreased during transducer switching operations until the address mark is again lost and this determines the minimum correction factor for the transducer switching operation. The maximum value and minimum value are averaged together such that the correction factor would be in the middle of the maximum and minimum correction values. This value is then stored as the tangential skew correction factor C11 in the table in memory. It should be noted that the process of determining the minimum and maximum values are repeated a number of times to ensure the correctness of the minimum and maximum values. 
     In the preferred embodiment, the method for generating the tangential skew correction factors is performed by a microprogram stored within memory 42. Further, the method for generating the new address mark search count is performed by a microprogram stored within memory 42 to be used by processor 40 during a transducer switching operation. The invention as described heretofore uses known digital software implementation and the microprogram was assembled using Motorola 68C11 Assembler, series 5.0. The microprogram for carrying out the method for establishing the tangential skew correction factors as shown in FIG. 6 is provided in Appendix A. The microcode for establishing the new corrected address mark search count appearing on line 50 of comparator 48 is provided in Appendix B. The programs of Appendix A and Appendix B carry out the functions as described within the specification. It should be understood that the method can be embodied in other microprograms using other programming languages and assemblers. The microprogram may be permanently stored in a read only memory such that the microprograms become a permanent part of the overall operating system. It is also possible for the transducer switching system to be embodied in hardware logic using the correction factor stored in memory and a state machine to provide the necessary timing. 
     While the invention has been particularly shown and described with reference to the preferred embodiments therefore, it will be understood by those skilled in the art that changes in form and detail may be made therein without departing from the spirit and scope of the invention. Given the above disclosure of general concepts and specific embodiments, the scope of the protection sought is defined by the following. ##SPC1##