Abstract:
A servo control system for a voice coil motor including a servo controller using position feedback to provide an acceleration current to the voice coil motor and voice coil brake providing a deceleration current based upon velocity feedback from the voice coil motor. In one embodiment, the velocity of the voice coil motor is derived from current feedback. A method for braking an actuator driven by a voice coil motor including the steps or receiving and processing current feedback from the voice coil motor and supplying a deceleration current to the voice coil motor derived from the current feedback from the voice coil motor.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims priority to Provisional Application Ser. No. 60/100,139 filed Sep. 14, 1998 and entitled “IMPROVED DISK DRIVE VOICE COIL MOTOR BRAKE”. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a data storage device. In particular, the present invention relates to a brake assembly for a voice coil motor for positioning heads for read and write operations. 
     BACKGROUND OF THE INVENTION 
     Disc drives are used to store digitally encoded information. Digital information is stored on rigid discs supported for rotation for read and write operations. Heads including transducer elements are supported relative to the disc surface to read data from and write data to the disc surface. An E-block movably supports heads relative to the disc surface to read data from concentric data tracks on the disc. A voice coil motor moves E-block to position heads relative to selected data tracks on the disc surface. 
     The movement of the head to a desired data track is referred to as seeking. Maintaining the head over the center of the desired data track during read and write operations is referred to as track “following”. Operation of the E-block (and heads) is controlled by a servo control system using prerecorded servo information or position feedback either on a dedicated servo disc or on sectors interspersed among the data on a data disc. Loss of position feedback interferes with servo control and can cause the E-block and head to contact or slam into an end stop. Rapid movement or acceleration of the E-block into the end stop can damage heads and degrade air bearing stiffness. 
     In prior servo control systems, the E-block is slowed when there is a loss of position feedback to reduce the force at which the E-block slams into the end stop. To slow the E-block, the voltage potential across the voice coil motor is electrically shorted and the back Electro-motive force (Emf) is used to supply a brake current to slow the E-block. Although, the back Emf slows the E-block to reduce the force at which the E-block contacts the end stop, the lag time for the back Emf to stop the E-block can be relatively long in comparison to the operating stroke of the E-block. A relatively long lag time increases the stopping time and force at which the E-block contacts the end stop. The present invention addresses these and other problems, and offers other advantages over the prior art. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a servo control system including servo position feedback to energize the voice coil motor and a voice coil brake providing a deceleration current. The deceleration current i b  is proportional to the velocity of the voice coil motor. In one embodiment, the velocity of the voice coil motor is derived from current feedback. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a data storage device. 
     FIG. 2 is a perspective view of an actuator or E-block and voice coil of a data storage device. 
     FIG. 3 illustrates operation of a voice coil motor for moving E-block for head placement for read and write operations. 
     FIG. 4 is a schematic illustration of a servo control system and a prior art voice coil motor brake. 
     FIG. 5 graphically illustrates a voice coil motor brake of the prior art. 
     FIG. 6 is a graphical illustration of the braking velocity of the voice coil motor brake of FIG.  5 . 
     FIG. 7 is a schematic illustration of a servo control system including an embodiment of a voice coil motor brake of the present invention. 
     FIG. 8 graphically illustrates a voice coil motor brake of the present invention supplying a deceleration current to brake the voice coil motor. 
     FIG. 9 is a graphical illustration of the velocity of the voice coil motor brake of FIG.  7 . 
     FIG. 10 is a flow chart illustrating operation of an embodiment of a servo control system and voice coil motor brake of the present invention. 
     FIG. 11 is a schematic illustration of an alternate embodiment of a voice coil motor brake of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a voice coil motor brake for braking an actuator or E-block of a data storage system supporting data heads for read-write operations. The present invention has application for a rotary-type magnetic disc drive, as illustrated in FIG.  1 . As illustrated, disc drive  50  includes deck  52 , discs  54 , and E-block  56 . Discs  54  are rotationally coupled to deck  52  via a spindle motor (not shown) for rotation, as illustrated by arrow  58 . E-block  56  is rotationally coupled to deck  52  to supports heads  60  for reading and writing data to and from selected data tracks of discs  54 . Heads  60  are supported by a plurality of actuator arms  62  on E-block  56  (only one shown in FIG. 1) and the E-block  56  rotates as illustrated by arrow  64  to move heads  60  along an arcuate path for seek operations via operation of a voice coil motor (“VCM”)  66 . 
     As shown in FIG. 2, E-block  56  includes a wound coil  68  which is supported in a permanent magnetic field  70  formed in a gap between upper and lower magnets  72 ,  74  supported by upper and lower backirons  76 ,  78  separated by spaces  80 ,  82  as illustrated in FIG.  3 . Current is supplied through coil  68  in one direction to rotate E-block in a first direction and is supplied in a second direction to rotate E-block  56  in the opposite direction. Although a particular magnet and backiron assembly is shown, the invention is not limited to the particular embodiment shown and other constructions may be used as is known in the art. 
     A permanent magnetic field is produced by the magnets  72 ,  74 . Current is supplied to coil  68  aligned in the flux path of the magnetic field to generate a force based upon: 
     
       
           F=iln×B   Eq. 1 
       
     
     where: 
     F—is the force generated; 
     i—is the current through the coil; 
     l—is the length of the active coil legs; 
     B—is the magnetic field; and 
     n—is the number of turns in the coil. 
     Thus current is supplied through coil  68  to generate a force to accelerate the E-block  56  for seek operations. The velocity of E-block  56  is proportional to the current supplied to the coil  68  as follows: 
     
       
           V   m   =K∫idt   Eq. 2 
       
     
     V m —is the velocity of the E-block; 
     i—is the current supplied to the coil; and 
     K—is a constant proportional to the length of the active coil legs, the magnetic field, and the number of turns in the coil. 
     A servo control system  100  (shown schematically in FIG. 4) operates VCM  66  to move heads  60  from one track to another based upon servo information and maintain the heads  60  in radial alignment with a selected track. For seek and follow operations, a voice coil motor driver  102  supplies current to the voice coil  68  based upon a position signal  103  generated by the servo controller  104 . The position signal  103  is generated based upon a seek position  106  and servo feedback  108 . In the embodiment shown servo feedback  108  provides position feedback from servo information  110  interspersed in a data disc as illustrated in FIG.  4 . Alteratively, position feedback can be provided from a dedicated servo disc. 
     As illustrated in FIGS. 4-6, voice coil motor drive  102  supplies an acceleration current  114 , the magnitude and direction of which is based upon the seek position  106  and servo feedback  108 . The acceleration current  114  operates the VCM  66  to move the E-block  56 . As shown in FIG. 6, the velocity of E-block is proportional to the integral of the acceleration current  114  (or ∫i a ) as shown by line  116 . During a seek operation, servo feedback can be interrupted or lost (due to a bad disc sector or head instability) as illustrated schematically by line  118  in FIGS. 5-6. The loss of servo feedback data  108  can result in loss of control of the E-block  56  resulting in the E-block contacting or slamming into an end stop  119  (as illustrated in FIG. 1) which is referred to as a crash stop. A crash stop can damage the head and can degrade the air bearing stiffness of a slider (not separately shown) of the head. The loss servo data  110  can result in the loss of velocity as well as position feedback  108  for E-block  56  control. 
     In prior devices upon the loss of servo feedback  108 , the servo controller  104  supplies a brake signal  120  to operate a voice coil motor brake. To brake the voice coil motor, current in the coil  68  was shorted and the back Emf  122  (shown schematically in FIG. 4) of the system was used to drive a brake current  124  to slow the E-block  56  to reduce impact at the end stop  119 , as illustrated in FIGS. 4-5. Back Emf  122  is proportional to the velocity of the E-block so the higher the velocity of the E-block  56 , the higher the back Emf  122  and brake current  124 . The brake current  124  slowed the velocity of the E-block  56  as illustrated by line  126  in FIG.  6 . 
     The present invention relates to a voice coil motor brake which provides improved dynamic response to reduce crash impact of the E-block  56  upon servo feedback loss. An embodiment of the voice coil motor brake is shown in FIGS. 7-9 where like numbers are used to refer to like elements in the previous figures. As shown in FIGS. 7-9, the servo control system  100 - 1  includes velocity feedback V m    130  for the moving coil  68 . The servo controller  104  processes the velocity feedback V m    130  to supply an opposing force proportional to the velocity of the moving coil  68  of the E-block  56  to stop the VCM  66  to reduce the impact of a crash stop. 
     As shown in FIGS. 7-9, the servo controller  104  processes the velocity feedback  130  to provide a brake current signal  132  to the VCM driver  102  to supply a deceleration or current i b    134  to coil  68  in the opposite direction of the acceleration current  114  so that the velocity  136  of the E-block  56  is slowed to zero to limit contact force between the E-block  56  and the end stop  119 . The magnitude and duration of the deceleration current i b    134  is derived from 
     
       
           V   m1   −V   m2   =K∫i   b   dt   Eq. 3 
       
     
     where: 
     V m1 —is the voice coil motor velocity at servo feedback loss  118   
     V m2 —is the braked velocity of the voice coil motor (e.g. near zero velocity) 
     i b —is the brake or deceleration current  134   
     In the embodiment shown in FIGS. 7-9, the velocity V m  feedback  130  from the VCM  66  is provided by a current sensor  138  to measure feedback current i m  from the voice coil  68 . The feedback current i m  is proportional to the velocity of the voice coil motor as follow: 
     
       
         
           V 
           m 
           =K∫i 
           m 
           dt 
         
       
     
     The feedback current i m  from the voice coil motor is used to derive a deceleration current i b  having a sufficient magnitude and duration to slow the velocity feedback V m    130  from the voice coil motor to zero. The duration of the deceleration current i b  is determined based upon the magnitude of the deceleration current i b  and the monitored integral of current i m  of the motor. When the integral of current i m  of the motor drops to or near zero, the brake or deceleration current i b  drops to zero and the VCM  66  and E-block  56  should be close to zero velocity. 
     Thus, as shown in FIG. 10, heads  60  read servo feedback  108  from discs as illustrated by block  140 . Servo feedback is processed as illustrated by block  142  to supply a position control signal  103  for head placement as illustrated by block  144 . Servo control continues as illustrated by line  146  until operation is complete as illustrated by block  148 . If servo data is interrupted or lost, as illustrated by line  150 , velocity feedback v m    130  from the voice coil motor is processed, as illustrated by block  152 , to supply a deceleration or brake current  134  as illustrated by block  154  to brake the E-block  56  to limit impact of a crash stop. Velocity feedback v m  is derived from a current sensor  130  coupled to the VCM  66 . Alternatively, velocity feedback v m  can be derived from an accelerometer  160  coupled to the E-block  56  as shown in FIG.  11  and application of the invention is not limited to the specific embodiments shown. 
     Thus, as described, the present invention relates to a servo control system  100 - 1  for a voice coil motor  66  including a brake. The servo controller  104  supply a brake or deceleration current signal  132  to the VCM driver  102  based upon velocity feedback V m  of the VCM  66  to slow the VCM  66  upon the loss of servo feedback  108 . The magnitude and duration of the brake deceleration current  134  i b  is based upon v m =K∫i b dt 
     It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this disclosure is illustrative only, and changes may be made in detail, especially in matters of structure and arrangement of parts within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed. For example, the particular elements may vary depending on the particular application while maintaining substantially the same functionality without departing from the scope and spirit of the present invention. In addition, although the preferred embodiment described herein is directed to a magnetic disc drive system, it will be appreciated by those skilled in the art that the teachings of the present invention can be applied to other systems, like an optical disc drive system, without departing from the scope and spirit of the present invention.