Abstract:
A disk drive and methods for preventing a coil in a voice coil motor from overheating due to the application of excess current. The disk drive comprises a servo control system. The servo control system is adapted for applying a current to a coil of the voice coil motor thereby causing the voice coil motor to move the head according to a seek distance. The servo control system generates a plurality of seek profiles for each of a plurality of seek distances and a plurality of current limits for the plurality of seek profiles. Each of the plurality of seek profiles defines a plan for controlling the current to be applied to the coil while the voice coil motor is operated over the seek distance. The plurality of current limits each define a maximum current allowed while controlling the current to be applied to the coil.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to enhancing the performance of disk drives by controlling thermal rise and, more particularly, to improved systems and methods for preventing a coil of a voice coil motor (VCM) from overheating by preventing excessive current from being applied to the coil. 
     2. Background of the Invention 
     A typical hard disk drive includes a head disk assembly (HDA) and a printed circuit board assembly (PCBA). The HDA includes at least one magnetic disk (disk), a spindle motor for rotating the disk, and a head stack assembly (HSA) that includes a read/write head with at least one transducer for reading and/or writing data. The HSA is controllably positioned by a servo system to read or write information from or to particular tracks on the disk. The typical HSA has three primary portions: (1) an actuator assembly that moves in response to the servo control system; (2) a head gimbal assembly (HGA) that extends from the actuator assembly and biases the head toward the disk; and (3) a flex cable assembly that provides an electrical interconnect with minimal constraint on movement. 
     A “rotary” or “swing-type” actuator assembly comprises a body portion that rotates on a pivot bearing cartridge between limited positions, a coil portion that extends from one side of the body portion to interact with one or more permanent magnets to form a VCM, and an actuator arm that extends from an opposite side of the body portion to support the HGA. 
     Within the HDA, the spindle motor rotates the disk or disks, that are the media to and from which the data signals are transmitted via the read write/head(s) on the gimbal attached to the load beam. The performance of the disk drive is largely dominated by its mechanical latencies. One such mechanical latency is the rotational latency of the drive, which is a function of rotational speed of the disk and hence of the spindle motor. Another such mechanical latency is the seek latency of the drive, which is a function of the speed at which the actuator radially moves across the disk. 
     Competitive pressures in the disk drive market have compelled disk drive designers and manufacturers to simultaneously boost performance and reduce cost. Historically, higher performance has been achieved by, for example, increasing the rotational speed of the spindle motor and/or performing faster seek operations. Faster seek operations, in turn, can be achieved by increasing the control current flowing through the VCM, thereby increasing the actuator&#39;s acceleration and deceleration as it moves across the disk. Excessive VCM control currents or control current profiles having a high average value, however, can cause the VCM assembly (typically overmolded with a plastic material) to overheat, causing damage to the coil and the drive. For example, when subjected to an instantaneous or average current that is beyond the VCM&#39;s design limitations, the coil can generate excessive heat with consequences such as delamination of the coil overmold material, or loss of rigidity, thus drooping and contacting adjacent magnets; and/or outgassing particulates into the disk drive enclosure, with deleterious results. Such outgassing from the coil overmold, coil insulators, and/or from other materials applied to the coil wires (such as wire insulators, for example) can occur even at relatively low temperatures (85° C., for example). A need, therefore, exists to monitor the temperature of the VCM coil and to prevent damage thereto. 
     One possible solution that addresses the need to prevent excessive VCM temperatures is to limit the VCM control current so that the heat generated therein remains at all times within conservative limits, independent of present actuator current usage patterns. This solution, while effectively preventing the VCM from overheating and obviating the need to monitor the temperature thereof, also results in unacceptably slow drive performance. Another solution is proposed in the U.S. Pat. No. 5,594,603, issued to Mori et al. In the Mori patent, the current applied to the VCM is used to calculate an approximation of the VCM temperature. This method attempts to mathematically model the thermal behavior of the VCM by devising a number of coefficients and by quantifying and inter-relating the VCM control current, the heat naturally radiated by the VCM, the ambient temperature, the thermal capacity of the VCM, and the ambient temperature thereof, among other factors. However, such a mathematical model, although providing an indication of the present VCM temperature, may not accurately provide a calculated temperature value that accords with the present and actual temperature of the VCM. Indeed, a number of factors can skew the results obtained from such mathematical models. For example, the present temperature of the drive or the resistance of the VCM coil may not remain constant and result in changing VCM control current magnitudes. As the VCM control current is used as the basis for the temperature calculations, the VCM is not driven (i.e., supplied with VCM control current) in an optimal manner and the actuator may not sweep as rapidly across the disk as it might otherwise have, thereby needlessly limiting the overall performance of the drive. Alternatively, should the mathematical model prove to be an inaccurate predictor of actual VCM temperature in certain situations, excessive VCM control currents can be generated, potentially causing damage to the VCM and to the drive. Over many iterations, recursively-applied mathematical models can cause a relatively small error in each calculation to grow to such a degree that the model no longer accurately reflects present operating conditions. Reliance upon such an inexact mathematical model in modulating the VCM control current can understandably result in less than optimal drive performance characteristics. 
     Another proposed solution is proposed in the U.S. Pat. No. 5,128,813, issued to Lee. In this patent, a discrete temperature-sensing element is used to dynamically sense the VCM temperature during the operation of the drive. The output of the temperature-sensing element (e.g., thermistor) is quantized and used to calculate a multiplication factor. The multiplication factor, in turn, is multiplied by a reference velocity command during a seek operation to produce a velocity command that then is compared with a feedback velocity value to generate an error signal that modulates the operation of the actuator (e.g., the VCM control current) during seek operations. This patent discloses that the thermistor is mounted for thermal conduction directly to the head and disk assembly. While the temperature sensing element can, in fact, provide a direct measurement of the temperature of the VCM (in contrast to the Mori patent above, for example), this method requires mounting a high precision thermistor to the HDA and requires that appropriate signal conditioning means be provided to measure, quantize and interpret the resistance thereof. In many aspects, however, disk drive designers and manufacturers operate in an environment that has acquired many of the characteristics of a commodity market. In such a market, the addition of even a single, inexpensive part can directly and adversely affect competitiveness. In this case, therefore, the addition of the thermistor and associated signal conditioning means discussed in the Lee patent would be of little practical value. 
     Other proposed solutions to prevent a coil of a VCM from overheating have included the addition of a dwell time between successive seek operations. By adding the dwell time, no current is applied to the coil for some period after each seek operation. As a result, the disk drive permits the coil to cool during the dwell time; however, no further seek operations can be commenced until the dwell time ends. Thereby, although the coil is provided with an opportunity to cool, the performance of the drive is adversely affected by increasing the average seek time. 
     Similarly, it has been proposed that the temperature of the coil can be controlled by selecting a fixed maximum current for all seek distances exceeding a certain seek distance and then adjusting the acceleration and deceleration intervals during which the fixed maximum current is applied to the coil. The fixed maximum current is applied to all seek distances over the certain seek distance without regard to the existence of a coast interval. The coast interval is a time period that occurs between the acceleration and deceleration intervals. At the end of the acceleration interval, the head has reached a maximum velocity, and the fixed maximum current is removed from the coil. The head then effectively “coasts” until the beginning of the deceleration interval when the fixed maximum current again is applied to the coil, but in an opposite direction, to decelerate the head. Since no current is applied to the coil as the head coasts, the coil is permitted to cool during the coast interval. As the coast interval increases with longer seek distances, the time during which the coil cools also increases. The proposed solution that uses fixed maximum current however does not take advantage of the increased cooling provided by the coast interval. Due to the increased cooling for the longer seek distances, the current applied to the coil for the longer seek distances can exceed the fixed maximum current, increasing the performance of the disk drive without causing the coil to overheat. 
     What are needed, therefore, are methods for preventing the application of excessive VCM control currents to a disk drive voice coil motor that are accurate, reliable and inexpensive in their implementation. More specifically, without relying upon complex and error prone mathematical modeling schemes or upon costly temperature sensing circuitry, methods for optimizing a maximum VCM control current to be applied to the voice coil motor for preselected seek distances are needed. Further, methods are needed for allowing the VCM control current to be modulated in an optimal manner to optimize seek operations. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a disk drive that provides the capability to prevent a coil of a voice coil motor from overheating due to the application of excessive current while moving a head over a recording surface of a disk. 
     A disk drive in accordance with an embodiment of the present invention comprises a disk with a recording surface, a head for reading and/or writing data on the recording surface, a voice coil motor for moving the head over the recording surface, and a servo control system. The servo control system applies a current to a coil of the voice coil motor, causing the head to move a seek distance over the recording surface of the disk. The servo control system generates a plurality of seek profiles for each of a plurality of seek distances and a plurality of current limits for the plurality of seek profiles. Each of the plurality of seek profiles defines a plan for controlling the current to be applied to the coil while the voice coil motor is operated over the seek distance. The plurality of current limits each define a maximum current allowed while controlling the current to be applied to the coil. 
     Each of the plurality of current limits is determined by examining a seek distance that represents one seek distance or a range of seek distances. For the seek distance, an appropriate seek profile and a nominal maximum current level are selected. The seek profile provides, among other things, time intervals during which the current is applied to the coil for the seek distance. The time intervals of the seek profile include an acceleration interval and a deceleration interval. If the seek distance exceeds a certain threshold length, typically thirty-five percent of full stroke, the seek profile also includes a coast interval, during which no current is applied to the coil. The nominal maximum current level comprises a starting point for determining the calculating the current limit for the seek distance and includes a nominal maximum acceleration current level and a nominal maximum deceleration current level. 
     A maximum stabilized RMS power for the seek distance then is calculated for the coil based upon several factors, including the acceleration interval, the deceleration interval, the nominal maximum acceleration current level, and the nominal maximum deceleration current level. If the maximum stabilized RMS power falls outside a preselected range of a maximum RMS power level for the coil, the nominal maximum acceleration current level and the nominal maximum deceleration current level each are adjusted, and the maximum stabilized RMS power is re-calculated. When the maximum stabilized RMS power is within the preselected range, the nominal maximum acceleration current level and the nominal maximum deceleration current level each, as adjusted, are stored as the current limit for the seek distance and, if desired, a next seek distance is examined. Once the current limit has been calculated for each of the plurality of seek distances, a current limit function, comprising the current limit for each seek distance, is generated. The current limit function may be generated in the form of a table, an equation, an algorithm, and/or any other form of generalized function, and, upon receiving a seek distance, produces a relevant current limit for the seek distance. 
     In operation, the servo control system receives a seek distance for moving the head over the recording surface. Upon receiving the seek distance, a relevant current limit, comprising a maximum acceleration current level and a maximum deceleration current level, is determined via the current limit function. The seek distance and the relevant current limit then are provided to a seek profile generator. In the seek profile generator, the relevant current limit is combined with a relevant seek profile, a seek profile from the plurality of seek profiles that is relevant to the seek distance. The relevant seek profile includes an acceleration interval, a deceleration interval, and, depending on the length of the seek distance, a coast interval. A current then is generated having a maximum amplitude substantially equal to the maximum acceleration current level during the acceleration interval and the maximum deceleration current level during the deceleration interval. No current is applied to the coil during the coast interval, if applicable. Once generated, the servo control system applies the current to the coil to move the head by the seek distance, maintaining the performance objectives of the disk drive but without exceeding the power handling capabilities of the coil. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram illustrating a disk drive in accordance with the present invention. 
     FIGS. 2A-D each are exemplary graphs illustrating the application of a current to a coil of a voice coil motor for various preselected seek distances. 
     FIGS. 3,  4 ,  5 , and  6  each are flow diagrams illustrating the steps in a method performed by the disk drive of FIG.  1 . 
     FIG. 7 is a graph of coil current limits as a function of seek distance in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Since a disk drive&#39;s performance can be adversely affected by reducing an amount of current applied to a coil of a voice coil motor, the use of a plurality of seek profiles in conjunction with a plurality of current limits can prove to be an advantageous method for preventing the coil from overheating. By adjusting both an amplitude and an application time of the current for each of a plurality of seek distances, the coil of the voice coil motor is prevented from overheating while maintaining performance objectives for seek operations. This result is achieved, according to one embodiment of the present invention, by employing a disk drive  100  as shown in FIG.  1 . 
     The disk drive  100  comprises a disk  105 , a head  115 , a voice coil motor  120 , and a servo control system  130 . The disk  105  includes a recording surface  110  and is rotatably coupled with the disk drive  100 . The head  115  extends over the recording surface  110  and may be employed to read data and/or to write data on the recording surface  110  of the disk  105 . The head  115  also is movably coupled to the disk drive  100  via the voice coil motor  120 . The voice coil motor  120  includes a coil  125 , which is electrically coupled with the servo system  130 , and moves the head  115  over the recording surface  110  of the disk  105  in accordance with a current  135  generated by the servo system  130 . 
     The servo control system  130  includes a current limit function  150  for determining a relevant current limit for a seek distance  140 . The current limit function  150  includes a plurality of current limits  155 , each comprising a maximum acceleration current level and a maximum deceleration current level for each of a plurality of seek distances. Each maximum acceleration current level is a maximum current that should be applied to the coil  125  to accelerate the head  115  from a starting track  170  toward a destination track  180 , located the seek distance  140  from the starting track  170 ; likewise, each maximum deceleration current level is the maximum current that can be applied to the coil  125  to decelerate the head  115  as the head  115  approaches the destination track  180  from the starting track  170 . Whereas the current  135  travels through the coil  125  in a first direction to accelerate the head  115 , the current  135  travels through the coil  125  in a second direction, opposite to the first direction, to decelerate the head  115 . The current limit function  150  may be generated in the form of a table, an equation, an algorithm, and/or any other form of generalized function, and produces a relevant current limit upon receiving the seek distance  140 . Further, each of the plurality of current limits  155  may be relevant to one seek distance and/or to a plurality, such as a range, of seek distances. 
     Similarly, the servo control system  130  also includes a seek profile generator  160  for selecting a relevant seek profile for a seek distance  140 . The seek profile generator  160  generates a plurality of seek profiles  165  for determining a rate and one or more time intervals for applying the current  135  to the coil  125 . The plurality of seek profiles  165  each may be applicable to one seek distance and/or to a plurality, such as a range, of seek distances. As shown in FIG. 2C, the plurality of seek profiles  165  each includes an acceleration interval from T 1  to T 2  and a deceleration interval from T 3  to T 4 . The current  135 , termed an acceleration current when applied to the coil  125  during the acceleration interval, and a termed a deceleration current when applied to the coil  125  during the deceleration interval. For each preselected seek distance  140  that exceeds a certain threshold distance, typically based on a velocity limit which occurs at about thirty-five percent of full stroke, the plurality of seek profiles  165  also includes a coast interval from T 2  to T 3 , during which no current  135  is applied to the coil  125 . The seek profile generator  160  also shapes the current  135  during the acceleration interval and the deceleration interval, to minimize the effect of resonances. 
     Returning to FIG. 1, the seek profile generator  160  can determine the plurality of seek profiles  165  in real-time via, for example, an algorithm performed by a processing system as the disk drive  100  operates. Alternatively, the plurality of seek profiles  165  may be determined in advance according to the algorithm and stored in a table, for example, in a memory system, preferably comprising non-volatile memory, for subsequent retrieval during operation of the disk drive  100 . The application of the plurality of current limits  155  and the plurality of seek profiles  165  prevents the coil  125  from overheating during the operation of the disk drive  100  while maintaining performance objectives for seek operations. 
     In operation, a servo control system  130  can control a current  135  to be applied to a coil  125  in a voice coil motor  120  of a disk drive  100  as the voice coil motor  120  moves a head  115  over a recording surface  110  of a disk  105  as shown in FIG.  3 . The servo control system  130  generates a plurality of seek profiles  165  in step  310  and calculates a plurality of current limits  155  for the plurality of seek profiles  165  in step  320 . The servo control system  130  then performs a series of seek operations of varying seek distance  140  using the plurality of seek profiles  165  and the plurality of current limits  155  in step  330 . 
     As described above, when defining the plurality of seek profiles  165  in step  310 , the servo control system  130  determines a rate and at least one time interval for applying the current  135  to the coil  125 . The plurality of seek profiles  165  each includes an acceleration interval and a deceleration interval. Also, each of the plurality of seek profiles  165 , corresponding to a preselected seek distance  140  that reaches a velocity limit, has a coast interval during which the coil  125  is permitted to cool because little or no current  135  is being applied to the coil  125 . The velocity limit is typically reached at approximately thirty-five percent of full stroke. At lower velocities, there is no coast interval. The application of current  135  to coil  125  further is shaped during the acceleration interval and the deceleration interval to minimize resonances. 
     The plurality of current limits  155  for the plurality of seek profiles  165  then are calculated by the method illustrated in FIG.  4 . The servo control system  130  establishes a maximum RMS power level for the coil  125  from a temperature handling capability of the coil  125  and a thermal rise of the coil  125  in step  410 . The maximum RMS power level for the coil  125  is determined by a series of simulation runs performed to characterize the power-handing capacity for the coil  125 . Since temperature is directly related to power, the simulation runs begin by applying a preselected current  135  to the coil  125  at an ambient temperature. The ambient temperature typically comprises a maximum temperature for the environment in which coil  125  is specified to operate, for example, 55° C. When the preselected current  135  is applied, the coil  125  will experience a thermal rise, and a coil winding resistance of the coil  125  will increase. After determining the thermal rise in the coil  125 , a resultant temperature of the coil  125  is calculated by adding the thermal rise to the ambient temperature. The current  135  then is increased and again applied to the coil  125  with the increased coil winding resistance and at the resultant temperature, resulting in an additional thermal rise and an additional increase in the coil winding resistance of the coil  125 . As the current  135  continues to be incrementally increased, the incremental thermal rise in the coil  125  and the incremental increase in the coil winding resistance both decrease for each successive increase in current  135 . When the incremental thermal rise falls below a preselected limit, the temperature and coil winding resistance of the coil  125  both have substantially stabilized, and the simulation test is ended. The current  135  and the coil winding resistance of the coil  125  at the end of the simulation test each are recorded as a final current and a final coil winding resistance, respectively. The maximum RMS power level for the coil  125  is substantially equal to the product of a square of the final current and the final coil winding resistance. 
     Based upon the maximum RMS power level for the coil  125 , a current limit for each of the plurality of seek distances is determined in step  420 . As shown in FIG. 5, each current limit is determined by first selecting an initial seek distance  140  to analyze in step  510 . An appropriate seek profile then is selected from the plurality of seek profiles  165  for the initial seek distance  140  in step  520 . The appropriate seek profile provides an acceleration interval, a deceleration interval, and/or a coast interval for the initial seek distance  140 . After a nominal maximum current level, comprising a nominal maximum acceleration current level and a nominal maximum deceleration current level, has been provided in step  530 , a maximum stabilized RMS power for the nominal maximum current level is calculated in step  540 . The maximum stabilized RMS If power preferably is calculated in accordance with the equation:                P     R                 MS       =           ∫     BEGIN                 ACCEL       END                 ACCEL              i   a   2               tR   w          T       +       ∫     BEGIN                 DECEL       END                 DECEL              i   d   2               tR   w          T             T   ACCEL     +     T   COAST     +     T   DECEL     +     T   LATENCY     +     T   DWELL                 Equation                 1                                
     where i a  is the nominal maximum acceleration current level, i d  is the nominal deceleration current level, T is the time current is applied and R w  is the coil winding resistance. T ACCEL , T COAST , T DECEL , T LATENCY  and T DWELL  are the lengths of the acceleration interval, the coast interval, the deceleration interval, a rotational latency time, and a dwell time, respectively. 
     The nominal maximum acceleration current level is applied to the coil  125  during the acceleration interval, and the nominal maximum deceleration current level is applied to the coil  125  during the deceleration interval. No current  135  is applied to the coil  125  during the coast interval, permitting the coil  125  to cool. As noted above, the coil winding resistance of the coil  125  exhibits a positive temperature coefficient, causing the coil winding resistance of the coil  125  to increase as the coil temperature rises. Preferably, the coil winding resistance, as used in Equation 1, reflects a stabilized coil winding resistance and substantially comprises the final coil winding resistance described above. 
     The rotational latency time, T LATENCY , is the time that the head  115  must wait while the disk  105  rotates such that a preselected sector is located substantially below the head  115 . During the rotational latency time, the coil  125  cools because only small amounts of current  135  is applied to the coil  125  during track following. The average latency time is one-half of a time required for the disk  105  to complete a full revolution. The dwell time, T DWELL , comprises an idle period of time between successive seek operations. As shown in FIG. 2A, the rotation latency time and the dwell time both occur during the time interval from T 4  and T 5 . Since no current  135  is applied to the coil  125 , the disk drive  100  permits the coil  125  to cool during the dwell time. Through the use of the concepts of the present invention, dwell times do not need to be inserted between successive seek operations to permit additional time for the coil  125  to cool. Thus, for purposes of step  540 , the dwell time is assigned a zero value. 
     Once calculated for the nominal maximum current level in step  540 , the maximum stabilized RMS power of the coil  125  is compared in step  550  with the maximum RMS power level that was established in step  410 . If the maximum stabilized RMS power of the coil  125  does not fall within a preselected range below the maximum RMS power level, the nominal maximum current level is adjusted in step  560 . The nominal maximum acceleration current level and/or the nominal maximum deceleration current level may be adjusted in step  560 . The preselected range is substantially between a preselected percentage of the maximum RMS power level and the maximum RMS power level. The preselected range should not include any RMS power levels exceeding the maximum RMS power level. The preselected percentage of the maximum RMS power level is less than the maximum RMS power level and preferably is substantially equal to ninety-seven percent of the maximum RMS power level. 
     For example, if the maximum stabilized RMS power of the coil  125  exceeds the maximum RMS power level in step  550 , the nominal maximum current level is decreased in step  560 , and the maximum stabilized RMS power applied to the coil  125  in step  540  is recalculated based upon the nominal maximum current level, as decreased. These steps are repeated until the maximum stabilized RMS power of the coil  125  is substantially within the preselected range. Conversely, if the maximum RMS power level exceeds the stabilized RMS power of the coil  125  in step  550 , the nominal maximum current level is increased in step  560 . The maximum stabilized RMS power applied to the coil  125  in step  540  is recalculated based upon the nominal maximum current level, as increased, and these steps are repeated until the maximum stabilized RMS power of the coil  125  is substantially within the preselected range. 
     Once the maximum stabilized RMS power of the coil  125  based upon the nominal maximum current level, as adjusted in step  560 , is substantially within the preselected range, the nominal maximum current level is stored as a current limit in the plurality of current limits  155  and associated with the initial seek distance  140  in step  570 . The plurality of current limits  155  may be stored and retained in any format. Upon storing and retaining the current limit for the initial seek distance  140 , the plurality of current limits  155  are examined for completeness in step  580 , and, if desired, a next preselected seek distance  140  can be selected to be analyzed in step  590 . The current limit may be calculated for all possible seek distance  140  for the disk drive  100  or for selected seek distances  140 . Further, a specific current limit may be defined to apply to a single seek distance or to a plurality, such as a range, of seek distances. 
     A graph of the plurality of current limits  155  for the coil  125  as a function of seek distance  140  is shown in FIG.  7 . As shown by the graph, the current limits for very short seek distances, such as track-to-track seeks, are relatively high because the current  135  is applied to the coil  125  during acceleration intervals and deceleration intervals that are relatively short. For seek lengths which are below the velocity limit point at which a coast interval is applied, the current limits are relatively low in order to maintain coil temperature below a critical point. Beyond the velocity limit point, approximately 35% of full stroke, the coast interval applies, permitting the coil  125  to cool during the coast interval. As the seek distance  140  continues to increase to full stroke, the coast interval and the current limits both also increase. As the coil  125  is permitted to cool for increasingly longer periods of time, higher current limits can be permitted during the acceleration interval and the deceleration interval as shown in FIG.  7 . Preferably, a table  156  is used to store the current limits, indexed by seek length. 
     FIGS. 2A-2D show the current cycle for a range of seek distances  140  from a track-to-track seek, as in FIG. 2A, to a full stroke seek, as in FIG.  2 D. In FIG. 2A, since the current  135  is applied for a short period of time, current limits A 1  and A 2  of the current  135  comprise a current limit imposed by the servo control system  130 . The current  135  is applied to the coil  125  for a short period of time and generates a RMS power PRMS that is substantially equal to a constant K 1 . In contrast, FIG. 2B shows a current cycle for a longer seek distance  140 . As the seek distance  140  approaches approximately thirty-five percent of the full stroke for the disk  105 , current limits B 1  and B 2  decrease from the current limit A 1  and A 2 , respectively, because the current  135  is applied to the coil  125  for a longer period of time. As result, the RMS power PRMS being generated in the coil  125  remains at a constant K 1 . Further, the coil  125  is not subject to a coast interval, preventing the coil  125  from cooling during the seek operation. The current limits B 1  and B 2  represent the minimum current limits in the plurality of current limits  155  as shown in FIG.  7 . 
     IF The coil  125  is subject to a coast interval beginning in FIG. 4C because the seek length  140  is sufficient to reach a velocity limit. The voice coil motor  120  moves the head  115  across the recording surface  110  of the disk  105  by applying the current  135  with current limits C 1  and C 2  to the coil  125 . Since the coil  125  is permitted to cool during the coast interval, the current limits C 1  and C 2  of the current  135  begin to increase from the current limits B 1  and B 2 , respectively, as the seek distance  140  increases. The maximum seek distance  140  occurs when the seek distance  140  is substantially equal to the full stroke for the disk  105  as shown in FIG.  2 D. At full stroke, current limits D 1  and D 2  for the current  135  increase from the current limit C 1  and C 2 , respectively, because the coast time during which the coil  125  is permitted to cool also increases. Since the coil  125  has a longer period of time to cool, the current limits D 1  and D 2  of the current  135  increase during the acceleration and deceleration periods, respectively. These changes in the respective current limits with respect to the seek distance  140  preferably are reflected in the plurality of current limits  155  in the current limit function  150 . 
     Returning to FIG. 4, once the plurality of seek profiles  165  have been defined as in step  310  and the plurality of current limits  155  have been calculated as in step  320 , a current limit function  150  then is generated in step  430 . The current limit function  150  comprises the current limit for each of the plurality of seek distances, selected seek distances, and/or at least one range of seek distances. The current limit function  150  may be generated in the form of a table, an equation, an algorithm, and/or any other form of generalized function, and, upon receiving a preselected seek distance  140 , produces a relevant current limit for the preselected seek distance  140 . Upon receiving the preselected seek distance  140 , the current limit function  150  responds with a relevant current limit. 
     The current limit function  150  may determine the relevant current limit for the preselected seek distance  140  in real-time via, for example, an algorithm performed by a processing system. Alternatively, the current limit function  150  may comprise a table of pre-calculated current limits. If the current limit function  150  comprises the table, each of the plurality of current limits  155  is discretely stored and associated with the appropriate seek distance  140  or the appropriate range of seek distances. The current limit function  150  can interpolate between the current limits of two seek distances if no current limit has been associated with the preselected seek distance  140 . The servo control system  130  may include a memory system for storing the table. The memory system preferably comprises non-volatile memory. 
     The steps for performing the series of seek operations of varying seek distance  140  as described in step  330  is shown in FIG.  6 . Upon receiving a seek distance  140  in step  610 , the servo control system  130  generates a relevant seek profile for the seek distance  140  in step  620 . The relevant seek profile is relevant to the seek distance  140  and includes a relevant current limit. To generated the relevant seek profile, the servo control system  130  provides the seek distance  140  to the current limit function  150 . The current limit function  150  then determines a relevant current limit for the seek distance  140  in step  640 . The relevant current limit includes a relevant acceleration current limit and a relevant deceleration current limit and may be determined in real-time via, for example, an algorithm as the disk drive  100  operates or via a table that was compiled prior to the operation of the disk drive  100 . 
     Once the relevant current limit has been determined, the servo control system  130  provides the seek distance  140  and the relevant current limit to a seek profile generator  160  to generate a relevant seek profile in step  650 . The seek profile generator  160  selects a relevant seek profile from the plurality of seek profiles  165  and generates a current  135  for the coil  125 . The relevant seek profile includes an acceleration interval, a deceleration interval, and, if appropriate, a coast interval. The current  135  comprises an acceleration current and a deceleration current. The acceleration current is applied during the acceleration interval of the relevant seek profile and has a maximum amplitude substantially equal to the relevant acceleration current limit. Similarly, the deceleration current is applied during the deceleration interval of the relevant seek profile and has a maximum amplitude substantially equal to the relevant deceleration current limit. The relevant seek profile may be generated in real-time during the operation of the disk drive  100  or may be pre-calculated and, for example, stored in a table. The seek profile generator  160  also shapes the current  135  during the acceleration interval and the deceleration interval, ramping the amplitude of the current  135  up toward the relevant current limit and back down to prevent an occurrence of undesired consequences, such as resonances. Little or no current  135  is applied to the coil  125  during the coast interval. After the current  135  has been generated, the servo control system  130  applies the current  135  to the coil  125  in step  630 .