Abstract:
A method for improving servo seek settling performance by selectively activating a gain enhancement control signal during settling. As an actuator approaches a target location, absolute position and velocity of the read/write transducer head are monitored. If the absolute position changes absolute values (positive to negative or vice-versa) or the velocity approaches zero, the gain enhancement control signal is activated and combined with a nominal actuator control signal to further urge the actuator towards the target location. The gain enhancement control signal decays exponentially so as not to excite mechanical resonance of the actuator arms.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates generally to methods for use in disk drives for computer systems. More particularly, the present invention relates to methods for improving servo seek settling performance by selectively boosting the servo loop bandwidth during settling. 
     BACKGROUND OF THE INVENTION 
     FIG. 1 is a diagrammatic representation of a conventional disk drive for use in a host computer system. Disk drive  10  includes at least one rotating rigid disk  14  mounted on a rotating hub  16 . The hub  16  is typically secured to a spindle shaft (not shown) that is rotatably mounted to a bearing assembly and driven by a spindle motor. Disk drive  10  also includes an actuator assembly  20  mounted to the baseplate  11 . Included in the actuator assembly  20  is a motor assembly  25  (e.g. voice coil motor) that produces a current profile induced torque for pivotably actuating a head actuator  21  about pivot  24  to position a read/write transducer head  19  back and forth between specific tracks  31 ,  32  concentrically defined on the disk  14 . Although not shown it should be understood that actuator assembly  20  may include a plurality of integrated head actuators and read/write transducers to access a plurality of disks. 
     Each disk  14  includes a plurality of concentric tracks defined thereon for storing data. In addition, each disk  14  may also contain multiple spokes containing servo information that is useful with a head positioner servo system. Head positioner servo systems for disk drives are well known. Disk drive head positioner servos provide essentially two functions: track seeking or accessing, and track following. Ideally, a track seeking servo configuration moves the read/write head between two tracks in the minimum possible time, while track following servo configuration maintains the head at the centerline of a track being followed. These two quite different functions require different control circuit configuration for practical implementation, even though the same actuator structure, actuator driver circuit and mechanical system components are used for both tasks. 
     A third transitional mode, known as track settle, covers the situation where the track seeking servo has completed its task, and the servo circuitry has switched to the track following configuration, but the head transducer has yet stabilized at the target track location. Since read and write operations cannot be accomplished until the head has stabilized at the target track location, it is most desirable to minimize settle time. 
     With advancements in disk drive performance, seek times have decreased to approximately 7 msecs. As a result, higher torque levels are also required to effectively move the head positioners to meet the faster access time. A typical current profile  260  for creating such a torque, illustrated in FIG. 2, has a substantially exponential shape, after an initial sudden change of slope. The ruggedness of beginning section  264  signifies an abrupt change in the motor input current which commonly initiates vibration of the disk drive in addition to the desired actuator movement. A “polarity reversal” section  268  of input current profile  260  signifies a transition from acceleration to deceleration. An ending section  272  signifies a final deceleration which causes the actuator to decelerate and eventually return to rest. The deceleration during ending section  272  represents the settling of the read/write head as it approaches the target location. The settling profile on approaching the target location typically entails some sort of ringing induced by an interaction between the drive and the environment. Depending on the drive mounting scheme, the disk drive  10  may also be susceptible to recoil disturbances as a result of the high torque levels generated by the actuator motor. Such recoil forces tend to prevent the read/write head from initially reaching the target location, thereby adversely lengthening the settle time, as illustrated in the FIG. 3 graphical representation. As shown, curve  120  represents the settling time T 1 , when disk drive is not subjected to recoil. When subjected to recoil, curve  110  never reaches the target location A, at time T 1 . In fact the recoil will tend to “push” the actuator away from the target location A before finally settling at time T 2 . Traditionally, the solution to compensate for this recoil was to increase the bandwidth for both the servo settling controller as well as the track following controller. However, mechanical resonance of the head positioners (actuator arms) limit the flexibility to increase servo bandwidth. Another alternative to reducing recoil disturbance was to slow the seek operation. However, this may not be a viable alternative in high TPI and high seek performance disk drive systems where an aggressive seek profile is necessary. 
     Therefore, there exists a need for a method of improving the settling time to overcome the adverse effects of disk drive recoil during seek operation. 
     SUMMARY OF THE INVENTION 
     The present invention satisfies this need. 
     A general object of the present invention is to improve seek settling performance by selectively activating a momentary gain to servo loop bandwidth during settling. 
     More specifically, a gain enhancement control signal is selectively activated and combined with a closed loop control signal to provide a momentary boost to servo loop bandwidth when two conditions are satisfied during seek settling. 
     Another object of the present invention is to provide a disk drive having a controller that urges a read read/write transducer head from an initial location on a disk to a target location and selectively activates an enhanced control signal to further urge the read/write transducer head towards the target location. 
     In one aspect of the present invention, an absolute position value and a velocity of the read/write head are monitored as the read/write transducer head approaches the target location during a seek. When either the velocity approaches zero or the absolute position value changes signs, a gain enhanced control signal is activated to be combined with the closed loop control signal, and sent to an actuator amplifier to further urge the read/write transducer head towards the target location. 
     In another aspect of the present invention, the momentary gain activated to produce the gain enhanced control signal decays exponentially so as not to excite mechanical resonance within the drive. 
     These and other advantages of the present invention will become apparent upon reading the following detailed descriptions and studying the various figures of the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention may best be understood by reference to the following description taken in conjunction with the accompanying drawings in which: 
     FIG. 1 is a diagrammatic representation of a disk drive assembly. 
     FIG. 2 is a graph representing a current profile for creating torque to accelerate the actuator of FIG.  1 . 
     FIG. 3 compares, in graphical form, the effect to settle time when the disk drive is subjected to recoil disturbance induced by acceleration of the actuator. 
     FIG. 4 is a flow diagram representation of a typical closed-loop seek in a disk drive assembly. 
     FIG. 5 a  is a flow diagram representation of an improved closed-loop seek in accordance with principles of the present invention. 
     FIG. 5 b  is a schematic representation of the improved closed-loop seek in accordance with principles of the present invention. 
     FIG. 6 a  represents a graph of absolute position of a read/write transducer head over time, during a seek, illustrating one condition when a gain enhancement control signal is activated. 
     FIG. 6 b  is a graph of absolute position of the read/write transducer head over time, during a seek, illustrating another condition when a gain enhancement control signal is activated. 
     FIG. 6 c  represent the graph of FIG. 6 a  when the gain enhancement control signal is applied, in accordance with principles of the present invention. 
     FIG. 7 a  is a graph of position error of the read/write transducer head over time during a seek, illustrating one condition when a gain enhancement control signal is activated. 
     FIG. 7 b  is a graph of position error of the read/write transducer head over time during a seek, illustrating another condition when a gain enhancement control signal is activated. 
     FIG. 7 c  represent the graph of FIG. 7 a  when the gain enhancement control signal is applied, in accordance with principles of the present invention. 
     FIG. 8 is a representation of the exponential decay of the gain enhancement control signal in accordance with principles of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     The present invention will now be described in detail with reference to a few preferred embodiments thereof as illustrated in the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art, that the present invention may be practiced without some or all of these specific details. In other instances, well known structures and process steps have not been described in detail in order not to unnecessarily obscure the present invention. 
     The present invention involves the selective increase in bandwidth of a servo seek settling control signal. To begin discussion of the present invention, we first review a typical servo seek scheme, as represented in the flow diagram of FIG.  4 . As shown, a seek typically begins at step  41  with the servo system receiving a seek request from the disk drive controller. At step  42 , the servo system generates a position error signal. The position error signal is then enhanced at step  43 , typically with a signal compensator, producing a closed-loop control signal. The closed-loop control signal is then transmitted to an actuator amplifier to drive the read/write head towards the target location at step  44 . At step  45 , the servo system determines whether the read/write head has reached the target location. If the read/write head has not reached the target location, a position error signal is again generated, the position error signal is again compensated by the signal compensator, thereby further urging the read/write head towards the target location. This settling loop continues until the target location has been reached, at which point the system enters a track following mode at step  46 . As explained herein above, when a disk drive assembly is subjected to recoil disturbance, seek settling time increases due to the additional signal enhancement required to enable the read/write head to reach the target location. 
     Now refer to FIG. 5 a,  which shows a flow diagram of a servo seek settling scheme in accordance with the present invention. Steps  41 - 46  are identical to the steps shown in FIG.  4 . However, as a position error signal is generated, after the actuator is accelerated towards the target location, the servo system monitors the velocity and absolute position of the re/write head. Absolute position is defined as the relative and directional distance between the readwrite head and the target location as the read/write head approaches the target location. Directional distance may be better understood by referring to FIGS. 6 a  ,  6   b  ,  7   a  , and  7   b  , which will be discussed herein below. Thus, absolute position is defined by both a relative direction and a relative distance. When either tho velocity approaches zero, in step  51  or the absolute position changes signs (either negative to positive, or positive to negative), the position error signal is further enhanced by a gain to produce an enhanced control signal as in step  53 . The enhanced control signal is then combined with the closed loop control signal in step  54 , as the actuator is again urged towards the target location, in step  55 . The servo system then determines whether the read/write head has stablized at its target location, i.e. has a velocity that is approximately zero at the target location. If the read/write head has not stabilized at its target location, the system loops back to step  42  to generate a position error signal until the read/write head has stabilized at the target location. if neither of the conditions in steps  51  and  52  is satisfied, the actuator is urged towards the target location with only the closed loop control signal from step  43 , as the enhanced control signal in step  53  is not activated. When the read/write head has stabilized at the target location, the system enters the track following mode in step  46 . 
     To provide further understanding of the present invention, FIG. 5 b  provides a schematic representation of FIG. 5 a  . As shown in FIG. 5 b,  position error signal  151  is provided by servo system  143  and transmitted to both a signal compensator  141  and a decision-making element  144 . As position error signal  151  is enhanced by compensator  141 , decision making element  144  determines whether either of the two conditions are satisfied i.e. 1) when the absolute position of read/write head relative to the target location changes signs or 2) when read/write head velocity approaches zero. These two conditions are illustrated in FIGS. 6 a,    6   b,    7   a  and  7   b  . FIGS. 6 a  and  6   b  illustrate the first condition, when, for example, a recoil of the disk drive forces the read/write head to initially “overshoot” its target location. In FIG. 6 a,  read/write head starts out from initial location A and accelerates toward target location B in a positive to negative direction. When the read/write head overshoots the target location B, the absolute position changes from a positive value to negative value. Referring back to FIG. 5 b,  this situation satisfies the first condition, which prompts switch  156  to close, enabling the position error signal  151  to reach gain multiplier  142 . Gain multiplier  142  enhances the position error signal  151  to produce enhanced control signal  152 , which is then combined with closed loop control signal  155  at adding junction  139 . 
     FIG. 6 b  illustrates another way of satisfying the first condition. In this situation, the read/write head is accelerated from an initial location A to target location B, in a negative to positive direction. In this scenario, the absolute position changes from a negative to a positive value from the target location. It should be understood that FIGS. 6 a  and  6   b  represent the performance of the read/write head when the present invention is not included into the servo system. That is, the read/write head eventually stabilizes on the target location B, but with an extended settling time. As shown in FIG. 6 c,  which represents FIG. 6 a  when the present invention is implemented into the servo system, the additional gain reduces settling time by enabling the target location to be stabilized upon at time T. 
     FIGS. 7 a  and  7   b  illustrate the second aforementioned condition, wherein the read/write head is accelerated towards target location B but does not reach the target location initially, because of, for example, recoil to the disk drive. In FIG. 7 a,  the read/write head is accelerated towards target location B in a positive to negative direction. At time T 1 , the read/write head does not reach the target location B as the read/write head changes direction and moves away from target location B. FIG. 7 b  shows seek acceleration in the opposite direction, i.e. negative to positive wherein the read/write head experiences a similar recoil at time T 1  and is initially driven away from the target location B. In both instances, the target location B is reached at time T 2 . 
     FIG. 7 c  illustrates the improved settle time when the present invention is implemented. As recoil similarly is shown to appear at time T 1 , velocity of the read/write head approaches zero (as the read/write head starts to change direction). A gain is applied to the position error signal to produce an enhanced control signal  152  (see FIG. 5 b  ). The summation of enhanced control signal  152  and closed loop control signal  155  produces combined signal  160  and provides increased bandwidth to further urge the read/write head to reach the target location B at time T 3 . 
     In accordance with one aspect of the present invention, the gain that is applied to the position error signal, when either of the two aforementioned conditions is satisfied, is one that is not constant in magnitude but rather one that decays exponentially over each digital servo sample. FIG. 8 illustrates this decaying nature, which is necessary in order to prevent possible mechanical resonance in the drive. The exponential decay of gain G may be represented as a function of digital servo sample by the following relationship 
     
       
           G ( k )= G   0   *d   k   
       
     
     G=gain 
     G 0 =initial gain 
     k=digital servo sample 
     d=positive constant 
     where d is less than 1 so that the gain, G, decreases exponentially with each digital servo sample. 
     Although the present invention has been described to be effective in overcoming the adverse effects of recoil disturbance, those skilled in the art will understand that the present invention is also effective in overcoming adverse effects of other types of disturbances and forces that impede the seek settling time. 
     Having thus described an embodiment of the invention, it will now be appreciated that the objects of the invention have been fully achieved, and it will be understood by those skilled in the art that many changes in construction and widely differing embodiments and applications of the invention will suggest themselves without departing from the spirit and scope of the invention. The disclosure and the description herein are purely illustrative and are not intended to be in any sense limiting.