Abstract:
A servo system positions a transducer over a disc surface in a disc drive system. Disc surfaces in the drive each have a plurality of spaced servo samples recorded thereon. The servo samples on at least two of the disc surfaces are recorded in skewed relation to one another. A plurality of transducers are provided and one transducer is associated with each one of the plurality of disc surfaces. An actuator arm assembly is coupled to the transducers to move the transducers relative to the disc surfaces. A servo control system is coupled to the actuator arm assembly to control position of the actuator arm assembly. The servo control system includes a reader configured to read servo samples from at least two disc surfaces such that at least two servo samples are read within one servo time period.

Description:
REFERENCE TO RELATED APPLICATION 
     The present application is based on a provisional application Ser. No. 60/018,302 filed on May 24, 1996. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to disc drives. More particularly, the present invention relates to a servo positioning system in a disc drive. 
     A typical disc drive includes one or more magnetic discs mounted for rotation on a hub or spindle. A typical disc drive also includes a transducer supported by a hydrodynamic air bearing which flies above each magnetic disc. The transducer and the hydrodynamic air bearing are collectively referred to as a data head. A drive controller is conventionally used for controlling the disc drive based on commands received from a host system. The drive controller controls the disc drive to retrieve information from the magnetic discs and to store information on the magnetic discs. 
     An electromechanical actuator operates within a negative feedback, closed-loop servo system. The actuator moves the data head radially over the disc surface for track seek operations and holds the transducer directly over a track on the disc surface for track following operations. 
     Information is typically stored in concentric tracks on the surface of the magnetic discs by providing a write signal to the data head to encode flux reversals on the surface of the magnetic disc representing the data to be stored. In retrieving data from the disc, the drive controller controls the electromechanical actuator so that the data head flies above the magnetic disc, sensing the flux reversals on the magnetic disc, and generating a read signal based on those flux reversals. The read signal is typically conditioned and then decoded by the drive controller to recover data represented by flux reversals stored on the magnetic disc, and consequently represented in the read signal provided by the data head. 
     In an embedded servo-type system, servo information is recorded on tracks which also contain data stored on the disc drive. The servo data (or servo bursts) are written on the data tracks and are commonly evenly temporally spaced about the circumference of each track. Data to be stored on the disc drive is written between the servo bursts. 
     As a transducer reads the servo information, the transducer provides a position signal which is decoded by a position demodulator and presented in digital form to a servo control processor. The servo control processor essentially compares actual radial position of the transducer over the disc (as indicated by the embedded servo burst) with desired position and commands the actuator to move in order to minimize position error. 
     In the past, dedicated servo-type systems were used. In a dedicated servo system, an entire disc surface in a disc drive was dedicated to servo information. Thus, high sample rates from the servo information could be maintained. However, in order to increase disc storage capacity, the above-described embedded (or sectored) servo systems are used. One disadvantage of this type of system is that, since data is also stored on the tracks containing servo information, the sample rate obtainable for servo information is lower than with a dedicated servo system. As the sample rate of the servo position information decreases, certain performance limitations increase. 
     SUMMARY OF THE INVENTION 
     A servo system positions a transducer over a disc surface in a disc drive system. Disc surfaces in the drive each have a plurality of spaced servo samples recorded thereon. The servo samples on at least two of the disc surfaces are recorded in skewed relation to one another. A plurality of transducers are provided and one transducer is associated with each one of the plurality of disc surfaces. An actuator arm assembly is coupled to the transducers to move the transducers relative to the disc surfaces. A servo control system is coupled to the actuator arm assembly to control position of the actuator arm assembly. The servo control system includes a reader configured to read servo samples from at least two disc surfaces such that at least two servo samples are read within one servo time period. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a portion of a disc drive according to the present invention. 
     FIGS. 2A and 2B are timing diagrams illustrating operation of the present invention. 
     FIG. 3 is a block diagram illustrating another embodiment of a portion of a disc drive according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a block diagram of a portion of a positioning system  10  in a disc drive according to the present invention. Positioning system  10  includes disc stack  12 , electromechanical actuator  18 , preamplifier  22 , read/write channel  24 , servo demodulator  26 , servo processor  28 , and power amplifier  30 . 
     The stack  12  of magnetically encodable discs  14  is mounted for rotation about a spindle  16 . Electromechanical actuator  18  is used to position a number of data transducers  20  radially with respect to discs  14 . Preferably, one transducer  20  is associated with each surface of each disc  14 . Transducers  20  read data, including servo bursts, from the surfaces of their associated magnetic discs  14 . The transducers  20  read the data by providing a read signal which represents the flux reversals encoded on the associated surfaces of magnetic discs  14 . 
     When it is desired to read from one of the transducers  20 , the specific transducer  20  is selected using appropriate multiplexing circuitry (not shown) and its read signal is provided to a preamplifier  22 . Preamplifier  22  amplifies the read signal from the selected transducer  20  and provides an amplified signal to read/write channel  24 . Read/write channel  24  recovers information from the read signal provided by preamplifier  22 . The information recovered by read/write channel  24  includes data stored on disc stack  12 , as well as servo information written on the disc surfaces of disc stack  12 . The data is provided to a disc drive controller or host system (not shown). 
     The servo information which is recovered from read/write channel  24  is provided to servo demodulator  26 . Servo demodulator  26  decodes the servo burst to extract position information and presents that information, in digital form, to servo control processor  28 . The position information represents the actual position of the selected transducer  20  over its associated disc surface. Servo control processor  28  compares the decoded position signal received from servo position demodulator  26  with a desired position signal to determine a transducer position error. The transducer position error represents the difference between the actual position of the selected transducer  20 , indicated by the decoded position signal, and the desired position indicated by the desired position signal. 
     Servo control processor  28  then generates a position correction signal which is converted to an analog signal by a digital-to-analog (D/A) converter (not shown) and applied to actuator  18  through power amplifier  30 . The position correction signal causes actuator  18  to move transducers  20  radially with respect to the surfaces of discs  14  in order to minimize the transducer position error. 
     FIG. 2A is a timing diagram which illustrates a preferred encoding of servo bursts on the surfaces of discs  14 . In the description relative to FIG. 2A, discs  14  are assumed to have two opposite surfaces on which data is recorded. Those surfaces are referred to as surface  0  and surface  1 . In the preferred embodiment, the servo bursts on surface  0  are recorded in skewed relation to the servo bursts on surface  1 . 
     For example, FIG. 2A illustrates that servo bursts  32  are recorded each servo time period, t 1  on disc surface  0 . Data to be stored on surface  0  is written between servo bursts  32 . In addition, in the preferred embodiment, servo information stored on surface  1  (the surface opposite surface  0  on the same disc  14 ) is recorded once each servo time period t 3 , where t 1  is equal to t 3 . However, the servo bursts  34  stored on surface  1  are skewed in time relative to servo bursts  32  on surface  0  such that servo bursts  34  occur a time t 2  after servo burst  32 . In the preferred embodiment, time t 2  is equal to one-half of time periods t 1  and t 3  such that servo bursts  34  occur half-way between servo bursts  32 . 
     During a track seek operation (and referring again to FIG. 1) servo control processor  28  is provided with a destination track signal from a host or other controller. The destination track signal represents a track over which servo processor  28  is to position a desired transducer  20 . Servo control processor  28  then provides a position signal through power amplifier  30  to actuator  18 . Actuator  18 , in turn, seeks to the desired track (i.e., moves transducers  20  radially relative to associated surfaces of disc  14  so that transducers  20  are positioned over the desired track). 
     During such a track seek operation, the disc drive is not reading back data from disc stack  12 , but is only reading position information so that servo control processor  28  can determine when actuator  18  has moved transducers  20  to the correct track. In prior systems, servo processor  28  would simply choose a desired transducer  20  and read the servo bursts (such as servo burst  32  from surface  0 ) once every servo time period t 1 . 
     However, in one preferred embodiment of the present invention, servo control processor  28  alternately reads servo bursts  32  and  34  from surfaces  0  and  1  during the track seek operation. In other words, servo control processor  28  first selects a transducer  20  corresponding to surface  0  and reads a first servo burst  32 . Then, after reading the first servo burst  32 , servo control processor  28  selects a second transducer  20  corresponding to surface  1  and reads a servo burst  34  from surface  1 . Servo control processor  28  continues this alternate reading of servo bursts from surfaces  0  and  1  until the track seek operation is complete. This allows servo control processor  28  to receive a servo burst from the selected disc  14  every time period t 2  which is approximately one-half of the normal servo time period t 1 . This effectively doubles the sample rate for servo system  10  thereby significantly enhancing drive performance without adding any additional hardware to the disc drive. Also, since no data is being read during a track seek operation, the present invention can be accomplished without any deterioration in data access times or throughput. 
     FIG. 2B is a timing diagram of another preferred embodiment of the present invention. In FIG. 2B, servo bursts are recorded on three surfaces in disc stack  12  (surface  0 , surface  1  and surface  2 ). In this embodiment, the servo bursts  32  and  34  (on surfaces  0  and  1 , respectively) are not offset from one another by one-half of servo time period t 1 . Rather, servo bursts  32  and  34  are offset from one another by a time period t 3  which is approximately one-third of servo time period t 1 . In addition, servo bursts  36 , which are recorded on surface  2  in disc stack  12 , are separated from servo bursts  34  by another time period t 3  which is one-third of servo time period t 1 . Therefore, from the initiation of the first servo burst  32 , servo burst  34  is offset by a time period t 3  which is one-third of servo time period t 1  and servo burst  36  is offset by a time period t 2  which is two-thirds of servo time period t 1 . 
     With this recording scheme, feedback system  10  switches between three transducers  20  associated with surfaces  0 ,  1  and  2  during a track seek operation. This allows servo control processor  28  to obtain three servo bursts during a single servo time period t 1 , rather than only the single servo burst which was obtained in prior drives. This effectively triples the sample rate for servo control processor  28  without adding additional hardware to feedback system  10 . 
     Of course, the technique of skewing servo bursts on the plurality of surfaces can be carried out for any suitable number of surfaces. However, in the preferred embodiment, the servo samples should be equally spaced from one another among the several disc surfaces which are used. Also, it has been found that a preferred embodiment of the present invention is to use the opposite surfaces of a single disc  14 . Discs  14  have been found to have certain eccentricities. However, those eccentricities typically affect both the top and bottom surfaces of the disc  14  approximately equally. Thus, the eccentricities have little affect on accuracy. However, working across multiple discs  14  makes the correlation of the servo samples among the surfaces in disc stack  12  a bit more cumbersome and possibly less accurate. 
     FIG. 3 is another embodiment according to the present invention in which the sample rate of the servo bursts is increased even during a track following operation. In a track following operation, actuator  18  holds a selected transducer  20  in a single radial position over a desired track so that data can be recovered from that track. The embodiment shown in FIG. 3 provides a small quantity of additional hardware on the drive so that multiple servo bursts can be read even during a track following operation. 
     The positioning circuit  38  shown in FIG. 3 is similar to positioning circuit  10  shown in FIG. 1, and similar items are similarly numbered. However, positioning system  38  differs from positioning system  10  in that a plurality of preamplifiers  22   0  through  22   n  are provided in the circuit, along with additional filtering circuit  40  and a multiplexer  42 . In addition, the output of read/write channel  24  is no longer provided to servo position demodulator  26 , but instead simply goes to the data output. 
     With feedback system  38 , and during a track following operation, the selected transducer  20  provides its read signal to preamplifier  22   0  which, in turn, provides an amplified signal to read/write channel  24 . As with the system shown in FIG. 1, read/write channel  24  recovers data from the amplified read signal and provides that data to the host or other similar controller. 
     However, the output of preamplifier  22   0  is also provided to filter circuit  40 . In addition, a suitable number of other transducers  20  also provide read signals to the remainder of preamplifiers  22 . The output of those preamplifiers provide amplified read signals to filtering circuit  40  which, in turn, provides a filtered output signal to multiplexer  42 . 
     Servo control processor  28  provides a select signal to multiplexer  42  to choose one of the transducers  20  from which to read a servo burst. That servo burst is provided from multiplexer  42  to servo demodulator  26  which demodulates the servo position information and provides it, in digital form, to servo control processor  28 . Servo control processor  28  controls multiplexer  42  to switch through the various transducers  20  in order to read multiple servo bursts during each servo time period t 1 . 
     Of course, servo control processor  28  can control multiplexer  42  to read from any suitable number of transducers  20  during a track following operation such that the servo sampling rate is increased to a desired level. It should also be noted that feedback system  38  shown in FIG. 3 is suitable for obtaining the increased sampling rate during a track seek operation as described with respect to FIG.  1 . However, with the simple addition of a small amount of preamplifying circuitry  22   0  through  22   n  as well as filtering circuitry  40  and multiplexing circuitry  42 , feedback system  38  provides the ability to increase the servo sampling rate even while data is being read during a track following operation. 
     Therefore, it can be seen that the present invention increases servo sampling rates by alternately switching between two transducers on opposite sides of a disc to double the sample rate. The present invention can also optionally switch between three or more transducers  20  to read from the surfaces of multiple discs to drastically increase the servo sampling rate. 
     It should also be noted that, while the present invention has been discussed with respect to reading a servo burst from a single transducer and then switching among a plurality of transducers, the transducers can also be read in parallel with the duplication of additional circuitry (such as servo demodulator circuitry  26 ) in the positioning system. 
     It should be further noted that, while the present invention has been discussed with respect to an embedded servo-type system, it could also be used in a hybrid servo system. Hybrid systems typically include both embedded servo information and a dedicated servo surface. The present invention would be used in reading information from the embedded servo portion of the drive. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.