Patent Publication Number: US-2011075291-A1

Title: Disk drive controlled to detect head-disk interference

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority under 35 U.S.C. §119 to Korean Patent Application No. 10-2009-0091146 filed on Sep. 25, 2009, the disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND 
     Embodiments of the inventive concept relate generally to disk drives. More particularly, embodiments of the inventive concept relate to disk drives that operate to detect head-disk interference by comparing one or more parameters measured during a write operation with one or more reference parameters measured while head-disk interference is generated under reference conditions. 
     A hard disk drive uses a magnetic head to perform read and write operations on a disk. During the read and write operations, the head is maintained at a distance from the disk while the disk spins. Where the head is not properly aligned with data storage tracks of the disk, or where the head is not located at a proper distance from the disk, head-disk interference can occur. 
     Head-disk interference can cause various defects in the disk and can prevent writing operations from being performed properly. Accordingly, it is desirable to both detect and prevent head-disk interference. 
     SUMMARY 
     Embodiments of the inventive concept provide disk drives and methods of controlling the disk drives. In certain embodiments, the disk drives are controlled to determine head-disk interference. 
     According to one embodiment of the inventive concept, a method of controlling a disk drive comprises determining a reference frequency and a reference amplitude from a first position error signal of a head of the disk drive, determining a frequency and an amplitude from a second position error signal generated during a writing operation of the disk drive, comparing the determined frequency with the reference frequency, and comparing the determined amplitude with the reference amplitude, and determining the occurrence of head-disk interference in the disk drive based on the comparisons. 
     In certain embodiments, determining the reference frequency and the reference amplitude comprises measuring the first position error signal while generating head-disk interference on a normal track of the disk drive, and performing a fast Fourier transform on the first position error signal and selecting the reference frequency and the reference amplitude from the fast Fourier transform. 
     In certain embodiments, measuring the first position error signal comprises iteratively writing predetermined data in the normal track and then decreasing a flying height of the head until HDI occurs, and measuring the first position error signal after the head-disk interference occurs. 
     In certain embodiments, determining the reference frequency and the reference amplitude comprises determining reference frequencies and reference amplitudes for a plurality of heads of the disk drive as references for determining the occurrence of HDI between each of the plurality of heads and a corresponding disk. 
     In certain embodiments, determining the frequency and the amplitude comprises counting a number of times a write retry operation is performed during the writing operation, comparing the number of write retry operations with a predetermined critical value, and where the counted number of write retry operations is greater than the predetermined critical value, performing a fast Fourier transform on the second PES, and selecting the frequency and the amplitude from the fast Fourier transform. 
     In certain embodiments, determining the occurrence of head-disk interference comprises determining a difference between the frequency and the reference frequency, determining a difference between the amplitude and the reference amplitude, and determining that head-disk interference occurs where the difference between the frequency and the reference frequency is less than or equal to a predetermined value and the difference between the amplitude and the reference amplitude is greater than the reference amplitude. 
     In certain embodiments, the method further comprises upon determining that HDI occurs, increasing a flying height of the head and then performing another writing operation, and determining whether data is successfully written to the disk drive in the another writing operation. 
     In certain embodiments, the method further comprises determining a reference value of a predetermined parameter as a reference for determining the occurrence of head-disk interference, measuring a value of the parameter during a writing operation, and comparing the measured value of the parameter with the reference value. 
     In certain embodiments, the parameter comprises at least one of a servo automatic gain control, a jitter representing a difference between a current velocity and a target velocity of a spindle motor of the disk drive, and a bias applied to a voice coil motor of the disk drive. 
     In certain embodiments, the occurrence of head-disk interference is determined based on at least one of a comparison result relating to the second position error signal, a comparison result relating to the servo automatic gain control, a comparison result relating to the jitter, and a comparison result relating to the bias. 
     According to another embodiment of the inventive concept, a method of controlling a disk drive comprises determining a reference value of a predetermined parameter of the disk drive, measuring a value of the parameter during a writing operation of the disk drive, comparing the measured value of the parameter with the reference value, and determining the occurrence of head-disk interference based on a result of the comparison. 
     In certain embodiments, the parameter comprises at least one of a servo automatic gain control, a jitter representing a difference between a current velocity and a target velocity of a spindle motor of the disk drive, and a bias applied to a voice coil motor of the disk drive. 
     In certain embodiments, the occurrence of head-disk interference is determined based on at least one of a comparison result related to a position error signal of the disk drive, a comparison result related to the servo automatic gain control, a comparison result relating to the jitter, and a comparison result related to the bias. 
     In certain embodiments, determining the reference value comprises iteratively writing predetermined data in a track of the disk drive and then decreasing a flying height of a head of the disk drive until head-disk interference occurs, and determining the reference value by measuring a value of the parameter during occurrence of the head-disk interference. 
     According to another embodiment of the inventive concept, a disk drive comprises a disk, a head for writing data to the disk, and a controller for controlling the head. The controller comprises a determination unit that determines a reference frequency and a reference amplitude from a first position error signal and determines a frequency and an amplitude from a second position error signal generated during a writing operation, a comparison unit that compares the frequency with the reference frequency and outputs a comparison signal indicating a result of the comparison, and a control signal generating unit that generates a control signal based on the comparison signal, and the control signal is used to control the head. 
     In certain embodiments, the first position error signal is measured while head-disk interference is generated on a normal track of the disk drive. Additionally, the determination unit determines the reference frequency and the reference amplitude by performing a fast Fourier transform on the first position error signal, and the determination unit determines the frequency and the amplitude by performing a fast Fourier transform on the second position error signal. 
     In certain embodiments, the control signal generating unit generates the control signal to increase a flying height of the head upon determining that a difference between the frequency and the reference frequency is less than or equal to a predetermined value and the amplitude is greater than the reference amplitude. 
     In certain embodiments, the determination unit determines a reference value related to a predetermined parameter of the disk drive, and measures a value of the predetermined parameter during a writing operation, and the comparison unit compares the measured value of the parameter with the reference value and outputs a comparison signal based on the comparison. 
     In certain embodiments, the disk drive further comprises a spindle motor for rotating the disk at a predetermined speed, and a voice coil motor for driving the head. The parameter comprises at least one of a servo automatic gain control, a jitter representing a difference between a current velocity and a target velocity of the spindle motor, and a bias applied to the voice coil motor. 
     In certain embodiments, the control signal generating unit generates the control signal using at least one of a comparison result related to the second PES, a comparison result related to the servo automatic gain control, a comparison result related to the jitter, and a comparison result related to the bias. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The attached drawings illustrate various embodiments of the inventive concept. In the drawings like reference numerals denote like features. 
         FIG. 1  is a schematic diagram of a head disk assembly of a disk drive according to an embodiment of the inventive concept. 
         FIG. 2  is a block diagram illustrating electrical connections of the disk drive according to an embodiment of the inventive concept. 
         FIG. 3A  is a flowchart illustrating a method of controlling a disk drive according to an embodiment of the inventive concept. 
         FIG. 3B  is a flowchart illustrating a method of determining a reference frequency and amplitude in the method of  FIG. 3A  according to an embodiment of the inventive concept. 
         FIG. 4A  is a graph illustrating a waveform of a position error signal where head-disk interference occurs. 
         FIG. 4B  is a graph illustrating a waveform obtained by transforming the position error signal of  FIG. 4A  using a fast Fourier transform. 
         FIG. 5A  is a flowchart illustrating a method of controlling a disk drive according to another embodiment of the inventive concept. 
         FIG. 5B  is a flowchart illustrating a method of determining a reference value for a predetermined parameter in the method of  FIG. 5A  according to an embodiment of the inventive concept. 
         FIG. 6  is a flowchart illustrating an example of the method of  FIG. 5  where the predetermined parameter is a servo automatic gain control. 
         FIG. 7A  is a graph illustrating a servo automatic gain control value and a reference value. 
         FIG. 7B  is a graph illustrating a servo automatic gain control value measured in the method of  FIG. 6 . 
         FIG. 8  is a flowchart illustrating an example of the method of  FIG. 5  where the predetermined parameter is a jitter. 
         FIG. 9A  is a graph illustrating a jitter value of a disk drive in a normal state. 
         FIG. 9B  is a graph illustrating a jitter value and a reference value of a disk drive having head-disk interference. 
         FIG. 9C  is a graph illustrating a jitter value measured in the method of  FIG. 8 . 
         FIG. 10  is a flowchart illustrating an example of the method of  FIG. 5  where the parameter is a bias. 
         FIG. 11A  is a graph illustrating a bias value of a disk drive in a normal state. 
         FIG. 11B  is a graph illustrating a bias value and a reference value of a disk drive having head disk interference. 
         FIG. 11C  is a graph illustrating a bias value measured in the method of  FIG. 10 . 
         FIG. 12  is a flowchart illustrating a method of controlling a disk drive according to another embodiment of the inventive concept. 
         FIG. 13  is a block diagram illustrating a controller of  FIG. 2  according to an embodiment of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Selected embodiments of the inventive concept are described below with reference to the accompanying drawings. These embodiments are presented as teaching examples and should not be interpreted to limit the scope of the inventive concept. 
     In general, the described embodiments relate to a disk drive comprising a head disk assembly (HDA). 
       FIG. 1  is a schematic diagram of a HDA  100  of a disk drive according to one embodiment of the inventive concept. 
     Referring to  FIG. 1 , HDA  100  comprises at least one magnetic disk  12  that is rotated by a spindle motor  14 . HDA  100  further comprises a head  16  located adjacent to a surface of disk  12 . 
     Head  16  comprises one or more transducers that sense a magnetic field of disk  12  or magnetize disk  12  to read or write information from or to disk  12  as it is rotated. For instance, head  16  can comprise a write transducer for magnetizing disk  12  and a read transducer for magnetizing disk  12 . The read transducer typically comprises a magneto-resistive (MR) device. 
     Head  16  is integrated with a slider  20 , and slider  20  is coupled to a head stack assembly  22 . Head stack assembly  22  is attached to an actuator arm  24  having a voice coil  26 . Voice coil  26  is disposed adjacent to a magnetic assembly  28  to form a voice coil motor (VCM)  30 . A current supplied to voice coil  26  generates torque for rotating actuator arm  24  with respect to bearing assembly  32 . Due to the rotation of actuator arm  24 , transducer  16  is moved across the surface of disk  12 . 
     Data is stored in a plurality of ring-shaped tracks  34  of disk  12 . Where multiple disks are stacked on top of each other, cylinders are formed by corresponding tracks on different disks. Each of tracks  34  typically comprises a plurality of sectors. Each sector comprises a data field and a servo field. The servo field stores a preamble, a servo address/index mark (SAM/SIM), a gray code, and a burst signal. The transducer moves across the surface of disk  12  to read or write information on different tracks. 
     Head  16  comprises a structure for forming an air bearing between the surface of disk  12  and the transducers, and a heater (not shown) for heating the structure that forms the air bearing. 
     In some embodiments, HDA  100  comprises a plurality of disks  12  and a plurality of heads  16  corresponding to the surfaces of the plurality of disks  12 . For example, where HDA  100  comprises two disks  12 , HSA  22  can comprise four heads  16  and corresponding heaters. 
       FIG. 2  is a diagram illustrating electrical connections of the disk drive of  FIG. 1 , according to an embodiment of the inventive concept. 
     Referring to  FIG. 2 , the disk drive comprises disk  12 , head  16 , a pre-amplifier  210 , a read/write channel  220 , a host interface  230 , a controller  240 , a read only memory (ROM)  250 A, a random access memory (RAM)  250 B, a VCM driving unit  260 , and a heater current supply circuit  270 . 
     ROM  250 A stores firmware and control information for controlling the disk drive. During initialization of the disk drive, information for controlling the disk drive is read from ROM  250 A or disk  12  and stored in RAM  250 B. 
     Controller  240  analyzes instructions received from a host device (not shown) through host interface  230  and performs control operations based on the analysis. Controller  240  sends control signals to VCM driving unit  260  and heater current supply circuit  270  to control the movement of head  16 . 
     In a read operation of the disk drive, pre-amplifier  210  amplifies an electrical signal detected from disk  12  by the read transducer. Next, the amplified signal is gain-controlled by a gain control circuit (not shown) of read/write channel  220  to amplify the signal to a predetermined level. Then, the signal amplified by the gain control circuit is encoded into a digital signal readable by the host device, and the digital signal is converted into stream data and sent to the host device through host interface  230 . 
     In a write operation of the disk drive, data received from the host device through host interface  230  is converted into a binary data stream suitable for a write channel by read/write channel  220 . Then, pre-amplifier  210  amplifies a write current and the write transducer of head  16  writes the binary data stream on disk  12  using the amplified write current. In the write operation, read/write channel  220  provides information to controller  240  for controlling track-seek and track-following operations while reproducing the preamble, SAM/SIM, gray code, and burst signals. 
     In a servo copy process, read/write channel  220  provides information to controller  240  for controlling track-seek and track-following operations while reproducing a reference servo pattern recorded on a surface of disk  12  or a plurality of disks  12  using a reference head. 
     Controller  240  generally receives at least one parameter value to generate a control signal CON for controlling head  16 . Control signal CON comprises a flying on demand (FOD) signal for determining a current to be applied to the heater of head  16 . Heater current supply circuit  270  determines the current to be applied to the heater of head  16  in response to control signal CON. Thermal expansion of a pole tip of head  16  varies according to the magnitude of the current applied to the heater of head  16 , and therefore controller  240  controls the flying height of head  16  by controlling the current supplied to the heater of head  16 . The parameter value received by controller  240  typically comprises at least one of a servo automatic gain control value, a jitter value representing a difference between the current velocity of spindle motor  14  and a target velocity, and a bias value applied to voice coil motor VCM  30 . 
     The operation of controller  240  is described in further detail with reference to  FIGS. 3A through 12 , and the structure of controller  240  is described in further detail with reference to  FIG. 13 . 
       FIG. 3A  is a flowchart illustrating a method of controlling a disk drive according to an embodiment of the inventive concept. In the description that follows, example method steps or operations are denoted by parentheses (SXXX). 
     Referring to  FIGS. 1 through 3A , controller  240  determines a reference frequency and a reference amplitude that are related to a position error signal (PES), as reference values for detecting head-disk interference (HDI) (S 310 ). In one example, controller  240  determines the reference frequency and amplitude by detecting a PES generated by head  16  when induced to generate HDI on a normal track under controlled conditions, such as those described below with reference to  FIG. 3B . The generated PES is then transformed into the frequency domain using a fast Fourier transform (FFT), and a maximum amplitude of the FFT is used as the reference amplitude while the frequency corresponding to the maximum amplitude is used as the reference frequency. In addition, controller  240  determines reference frequencies and reference amplitudes for all heads  16  and stores them in a table. A further explanation of operation  310  is provided below with reference to  FIGS. 3B and 4B . 
     After the reference frequency and the reference amplitude are determined, head  16  attempts to write data to disk  12  (S 320 ). Where a write error occurs in operation S 320 , writing is retried in a write retry operation. Controller  240  counts the number of times the write retry operation is performed and compares the number of write retry operations with a first critical value (S 330 ). The first critical value can be determined according to various properties of the disk drive. Where the number of write retry operations is less than or equal to the first critical value, the method returns to operation S 320  so that head  16  attempts another writing operation. Otherwise, where the number of write retry operations is greater than the first critical value, controller  240  measures a PES from the head  16  that is performing a current writing operation and determines a frequency and an amplitude of the PES (S 340 ). For example, in operation S 340 , controller  240  can measure a PES from head  16  that is currently performing a writing operation, and determine a frequency and an amplitude using values obtained by transforming the PES using the FFT. In one example, the amplitude is the maximum of the values obtained by the FFT, and the frequency is a frequency corresponding to the maximum. 
     Controller  240  compares the frequency determined in operation S 340  with the reference frequency determined in operation S 310 , and the amplitude determined in operation S 340  with the reference amplitude determined in operation S 310  (S 350 ). For example, where there are 1st through n-th heads (n is a natural number) and a frequency and an amplitude are determined for a k-th head in operation S 340  (1≦k≦n), controller  240  compares the frequency and amplitude of the k-th head, which are determined in operation S 340 , with a reference frequency and a reference amplitude of the k-th head, which are determined in operation S 310 . 
     Where the difference between the frequency determined in operation S 340  and the reference frequency determined in operation S 310  is greater than a predetermined value but the amplitude determined in operation S 340  is less than or equal to the reference amplitude determined in operation S 310  (S 350 =No), the method determines that there is no HDI, and writing is retried until the data is written without an error (S 355 ). On the other hand, where the difference between the frequency determined in operation S 340  and the reference frequency determined in operation S 310  is less than or equal to the predetermined value but the amplitude determined in operation S 340  is greater than the reference amplitude determined in operation S 310  (S 350 =Yes), it is determined that there is HDI (S 360 ). For example, suppose the reference frequency is 300 Hz, the reference amplitude is 10 dB, and the predetermined value is 50 Hz. Where the frequency and amplitude determined in operation S 340  respectively are 100 Hz and 8 dB, it is determined that there is no HDI, and operation S 355  is performed. As an alternative example, where the frequency and amplitude determined in operation S 340  are respectively 330 Hz and 12 dB, it is determined that there is HDI in operation S 360 . 
     Where it is determined that there is HDI in operation S 360 , controller  240  increases the flying height of the head  16  that is performing a current writing operation (S 370 ). For example, controller  240  can output a control signal CON to heater current supply circuit  270  as an FOD signal, and heater current supply circuit  270  can adjust a current supplied to head  16  in response to control signal CON to control the flying height of head  16 . 
     After the flying height of head  16  is controlled, another writing operation is performed, and then it is determined whether the writing operation has been completed (S 380 ). Where the writing operation has not been completed (S 380 =No), the method returns to operation S 370  to increase the flying height of head  16  and retry the writing operation. In other words, until the writing operation is completed, the flying height of head  16  is increased and the writing operation is repeated. On the other hand, where it is determined that the writing operation has been completed (S 380 =Yes), the method checks whether the data is normally written (S 390 ). In other words, operation S 390  checks for errors such as those caused by HDI or by an increase of the flying height of head  16 . 
       FIG. 3B  is a flowchart illustrating an embodiment of operation S 310  of  FIG. 3A . 
     Referring to  FIGS. 1 through 3B , controller  240  controls head  16  to write data to a track having a good PES, i.e., a normal track (S 311 ). Where an error occurs in operation S 311 , the writing operation is performed again in a write retry operation. Controller  240  counts the number of times the write retry operation is performed and compares the number of write retry operations with a second critical value (S 312 ). The second critical value can be determined based on various properties of the disk drive. 
     Where the number of write retry operations is not greater than the second critical value (S 312 =No), the flying height of head  16  is decreased (S 313 ), and operation S 311  is repeated. In other words, operations S 312  and S 313  are performed to generate HDI intentionally at the normal track. Where the flying height of head  16  is sufficiently decreased and the number of write retry operations becomes greater than the second critical value (S 312 =Yes), controller  240  determines that there is HDI and measures a PES. Then, based on the measured PES, controller  240  determines a reference frequency and a reference amplitude (S 314 ). For example, controller  240  can determine the reference frequency and the reference amplitude using values obtained from an FFT of the PES. The determination of the reference frequency and the reference amplitude from the FFT will be described in further detail with reference to  FIGS. 4A and 4B . 
     After the reference frequency and the reference amplitude are determined in operation S 314 , the method determines whether reference frequencies and reference amplitudes have been determined for all heads (S 315 ). Where there is a head for which a reference frequency and a reference amplitude are not determined (S 315 =No), operations  311  through  314  are performed for the head. For example, where there are 1st through n-th heads, reference frequencies and reference amplitudes are determined for the 1st through n-th heads. The reference frequencies and amplitudes can then be stored in a table. 
       FIG. 4A  is a graph illustrating a waveform of a PES when HDI occurs in the disk drive. 
     Referring to  FIG. 4A , the PES fluctuates between positive (+) and negative (−) values about a zero line “0”. As the PES approaches the zero line, a head gets closer to the center of a track, and as the PES gets farther from the zero line, the head distances from the center of the track. Referring to  FIG. 4A , due to HDI, the PES is often removed from the zero line. 
       FIG. 4B  is a graph illustrating a waveform obtained by transforming the PES of  FIG. 4A  using a FFT. 
     Referring to  FIGS. 4A and 4B , the PES of  FIG. 4A  is transformed by the FFT and shown in the frequency domain in  FIG. 4B . The maximum amplitude of the transformed PES is 10 dB, which occurs at a frequency of 600 Hz. 
     In the following description, it is assumed that the PES of  FIG. 4A  is measured upon determining in operation S 312  that the number of write retry operations is greater than the second critical value. A reference frequency and a reference amplitude are determined in operation S 314  using the graph of  FIG. 4B . For example, in  FIG. 4B , the maximum amplitude of the PES transformed by the FFT is 10 dB, and a frequency corresponding to the maximum is 600 Hz. In this case, 600 Hz is determined as the reference frequency, and 10 dB is determined as the reference amplitude. Alternatively, a value smaller than the maximum amplitude can be determined as the reference amplitude. 
       FIG. 5A  is a flowchart illustrating a method of controlling a disk drive according to another embodiment of the inventive concept. 
     Referring to  FIGS. 1 ,  2 , and  5 A, controller  240  determines a reference value related to a predetermined parameter, as a reference for determining the occurrence of HDI (S 510 ). The parameter typically comprises at least one of a servo automatic gain control, a jitter, and, and a bias. The servo automatic gain control is a parameter used to control a signal amplified by pre-amplifier  210  at a predetermined level. The jitter represents a difference between the current velocity of spindle motor  14  and a target velocity, and the bias is a voltage applied to voice coil motor VCM  30 . Controller  240  determines reference values for all heads  16  and stores the reference values in a predetermined table according to the different heads  16 . Operation S 510  for determining the reference value is described in further detail below with reference to  FIG. 5B . 
     After the reference value is determined, head  16  attempts to write data to disk  12  (S 520 ). Where a write error occurs in operation S 520 , writing is retried in a write retry operation. Controller  240  counts the number of times the write retry operation is performed and compares the number of write retry operations with a predetermined critical value (S 530 ). The critical value can vary according to the type of the parameter. Where the number of write retry operations is not greater than the critical value (S 530 =No), the method returns to operation S 520  so that head  16  attempts a writing operation again. Otherwise, where the number of write retry operations is greater than the critical value (S 530 =Yes), controller  240  measures a parameter relating to the current writing operation (S 540 ). 
     Controller  240  compares the parameter value measured in operation S 540  with the reference value determined in operation S 510  (S 550 ). For example, where there are 1st through n-th heads and a parameter of a k-th head (1≦k≦n) is measured in operation S 540 , controller  240  compares the parameter value of the k-th head, which is measured in operation S 540 , with a reference value of the k-th head, which is determined in operation S 510 . A method of determining the occurrence of HDI according to the relationship between the parameter value measured in operation S 540  and the reference value determined in operation S 510  will be explained for various parameters with reference to  FIGS. 6 through 11 . 
     The method next determines whether HDI has occurred based on the comparison result obtained in operation S 550  (S 560 ). Where it is determined that there is no HDI (S 560 =No), writing is retried until the data is written without an error (S 565 ). Otherwise, where it is determined that there is HDI (S 560 =Yes), controller  240  increases the flying height of the head  16  that is performing a current writing operation (S 570 ). For example, controller  240  can output a control signal CON to heater current supply circuit  270  as an FOD signal, and heater current supply circuit  270  can adjust a current supplied to head  16  in response to control signal CON to control the flying height of head  16 . 
     After the flying height of head  16  is controlled, a writing operation is repeated, and the method determines whether the writing operation has been completed (S 580 ). Where the writing operation has not been completed (S 580 =No), the method returns to operation S 570  to increase the flying height of head  16  and retry the writing operation. In other words, until the writing operation has been completed, the flying height of head  16  is incrementally increased, and the writing operation is repeated. Otherwise, where it is determined that the writing operation has been completed (S 580 =Yes), the method checks whether the data is normally written (S 590 ). In other words, operation S 590  can be optionally performed to check an error such as an error caused by HDI or an error caused by an increase of the flying height of head  16 . 
       FIG. 5B  is a flowchart illustrating an example of operation S 510  of  FIG. 5A . 
     Referring to  FIGS. 1 ,  2 ,  5 A, and  5 B, controller  240  finds a normal track and controls head  16  so that head  16  writes data on the normal track (S 511 ). Where an error occurs in the writing operation of operation S 511 , the writing operation is repeated in a write retry operation. Controller  240  counts the number of times the write retry operation is performed and compares the number of write retry operations with a predetermined critical value (S 512 ). The critical value can vary according to the type of the parameter. Moreover, the critical value can be equal to or different from the critical value of operation S 530 . 
     Where the number of write retry operations is not greater than the critical value (S 512 =No), the flying height of head  16  is decreased (S 513 ), and operation S 511  is repeated. That is, operations S 512  and S 513  are performed to generate HDI intentionally at the normal track. Where the flying height of head  16  is sufficiently decreased and the number of write retry operations becomes greater than the critical value (S 512 =Yes), controller  240  determines that there is HDI and measures a parameter. Then, by using the measured parameter, controller  240  determines a reference value (S 514 ). Methods of determining reference values according to parameters will be described in further detail with reference to  FIGS. 6 through 11 . 
     After the reference value is determined in operation S 514 , the method determines whether reference values have been determined for all heads (S 515 ). Where there is a head for which a reference value has not determined, operations  511  through  514  are performed for the head. For example, where there are 1st through n-th heads, reference values are determined for the 1st through n-th heads, and the reference values are stored in a table. 
       FIG. 6  is a flowchart illustrating an example of the method of  FIG. 5  where the predetermined parameter is servo automatic gain control.  FIG. 7A  is a graph illustrating a servo automatic gain control value and a reference value, and  FIG. 7B  is a graph illustrating a servo automatic gain control value measured in operation S 640  of  FIG. 6 . 
     Referring to  FIGS. 1 ,  2 , and  5 A through  7 B, controller  240  determines a reference value relating to servo automatic gain control as a reference for determining the occurrence of HDI (S 610 ). Operation S 610  comprises the operations described with reference to  FIG. 5B . A servo automatic gain control value decreases as the distance between head  16  and disk  12  decreases and increases as the distance between head  16  and disk  12  increases. Therefore, in a normal state, a servo automatic gain control value corresponding to a curve “x” is detected. Where HDI occurs, a relatively smaller servo automatic gain control value corresponding to a curve “y” is detected. In operation S 514  of  FIG. 5B , a value greater than or equal to the average amplitude of a servo automatic gain control signal corresponding to curve “y” of  FIG. 7A  is determined as a reference value relating to servo automatic gain control. 
     After the reference value is determined in operation S 610 , the same operations S 520  and S 530  explained with reference to  FIG. 5A  are performed. Because operations S 520  and  530  are described above, further descriptions of these operations will be omitted to avoid redundancy. In operation S 530 , where the number of write retry operations is greater than a critical value, controller  240  measures a servo automatic gain control value relating to the current writing operation (S 640 ). 
     Next, controller  240  compares the servo automatic gain control value measured in operation S 640  with the reference value determined in operation S 610  (S 650 ). Where the servo automatic gain control value measured in operation S 640  is greater than or equal to the reference value determined in operation S 610 , as shown by a curve “m” in  FIG. 7B  (S 650 =No), operation S 565  is performed. Otherwise where the servo automatic gain control value measured in operation S 640  is smaller than the reference value determined in operation S 610  (S 650 =Yes), as shown by a curve “n” in  FIG. 7B , it is determined that there is HDI (S 660 ), and operations S 570  through S 590  explained with reference to  FIG. 5A  are performed. Since operations  565  through  590  have been described in detail with reference to  FIG. 5A , further descriptions of these operations will be omitted to avoid redundancy. 
       FIG. 8  is a flowchart illustrating an example of the method of  FIG. 5  where the predetermined parameter is jitter.  FIG. 9A  is a graph illustrating a jitter value of a disk drive in a normal state.  FIG. 9B  is a graph illustrating a jitter value and a reference value of a disk drive having HDI.  FIG. 9C  is a graph illustrating a jitter value measured in operation S 840  of  FIG. 8 . 
     Referring to  FIGS. 1 ,  2 ,  5 A,  5 B, and  8  through  9 C, controller  240  determines a reference value relating to jitter as a reference for determining the occurrence of HDI (S 810 ). Operation S 810  comprises the operations described with reference to  FIG. 5B . A jitter value is relatively low where there is no HDI but relatively high where there is HDI. For instance, the jitter value is relatively low in a normal state shown in  FIG. 9A  but relatively high in an example having HDI as shown in  FIG. 9B . In operation S 514 , a value less than or equal to the average of the jitter value shown in  FIG. 9B  is determined as a reference value relating to jitter. 
     After the reference value is determined in operation S 810 , operations S 520  and S 530  explained with reference to  FIG. 5A  are performed. Because these operations are described above, further descriptions thereof will be omitted to avoid redundancy. In operation S 530 , where the number of write retry operations is greater than a critical value, controller  240  measures a jitter value relating to the current writing operation (S 840 ). Controller  240  compares the jitter value measured in operation S 840  with the reference value determined in operation S 810  (S 850 ). 
     For explanation purposes, it will be assumed that the jitter value measured in operation S 840  has a jitter range value JITTER 1  shown in  FIG. 9C . Where jitter range value JITTER 1  is less than or equal to the reference value determined in operation S 810  (S 850 =No), operation S 565  is performed. Otherwise, where the jitter range value JITTER 1  measured in operation S 840  is greater than the reference value determined in operation S 810  (S 850 =Yes), it is determined that there is HDI (S 860 ), and the same operations S 570  through S 590  explained with reference to  FIG. 5A  are performed. Since operations  565  through  590  have been described in detail with reference to  FIG. 5A , further descriptions will be omitted to avoid redundancy. 
       FIG. 10  is a flowchart illustrating an example of the method of  FIG. 5  where the predetermined parameter is bias.  FIG. 11A  is a graph illustrating a bias value of a disk drive in a normal state,  FIG. 11B  is a graph illustrating a bias value and a reference value in a disk drive having HDI, and  FIG. 11C  is a graph illustrating a bias value measured in operation S 840  of  FIG. 10 . 
     Referring to  FIGS. 1 ,  2 ,  5 A,  5 B, and  10  through  11 C, controller  240  determines a reference value relating to a bias, as a reference for determining the occurrence of HDI (S 1010 ). Operation S 1010  comprises the operations described with reference to  FIG. 5B . A bias value is relatively low where there is no HDI but relatively high where there is HDI. That is, the bias value is relatively low in a normal state as shown in  FIG. 11A  but relatively high where HDI is present, as in the example of  FIG. 11B . In operation S 514 , a value less than or equal to an average of the bias value shown in  FIG. 11B  is determined as a reference value relating to a bias. 
     After the reference value is determined in operation S 1010 , operations S 520  and S 530  are performed as explained with reference to  FIG. 5A , and thus detailed descriptions of these operations will not be repeated. Where operation S 530  determines that the number of write retry operations is greater than a critical value (S 530 =Yes), controller  240  measures a bias value relating to the current writing operation (S 1040 ). Controller  240  compares the bias value measured in operation S 1040  with the reference value determined in operation S 1010  (S 1050 ). For explanation purposes, it will be assumed that the bias value measured in operation S 1040  has a bias range value BIAS 1  as shown in  FIG. 11C . Where bias range value BIAS 1  measured in operation S 1040  is less than or equal to the reference value determined in operation S 1010  (S 1050 =No), operation S 565  is performed. On the other hand, where bias range value BIAS 1  measured in operation S 1040  is greater than the reference value determined in operation S 1010  (S 1050 =Yes), it is determined that there is HDI (S 860 ), and operations S 570  through S 590  explained with reference to  FIG. 5A  are performed. Since operations  565  to  590  have been described in detail with reference to  FIG. 5A , a further description of these operations will be omitted to avoid redundancy. 
       FIG. 12  is a flowchart illustrating a method of controlling a disk drive according to another embodiment of the inventive concept. The method of  FIG. 12  combines aspects of the methods described with reference to  FIGS. 3A ,  5 A,  6 ,  8 , and  10 . In particular, in the method of  FIG. 12 , the occurrence of HDI may be determined using at least one of a comparison result relating to a PES, a comparison result relating to a servo automatic gain control, a comparison result relating to a jitter, and a comparison result relating to a bias. 
     Referring to  FIG. 12 , controller  240  determines a reference frequency and a reference amplitude that relate to a PES (S 1211 ), and a first reference value relating to servo automatic gain control (S 1212 ). In addition, controller  240  determines a second reference value relating to a jitter (S 1213 ), and a third reference value relating to bias (S 1214 ). Operations  1211  through  1214  can be selectively performed where necessary. Operation S 1212  is similar to operation S 310  of  FIG. 3A , operation S 1212  is similar to operation S 610  of  FIG. 6 , operation S 1213  is similar to operation S 810  of  FIG. 8 , and operation  1214  is similar to operation S 1010 . Accordingly, additional detailed descriptions of these operations will be omitted to avoid redundancy. 
     After operations S 1211  through S 1214  are performed, head  16  writes data to disk  12  (S 1220 ) and counts the number of times a write retry operation is performed. Controller  240  compares the number of write retry operations with a predetermined critical value (S 1230 ), and repeats operation S 1220  upon determining that the number of write retry operations is not greater than the critical value (S 1220 =No). Operation S 1220  is similar to operation S 320  of  FIG. 3A  and operation S 520  of  FIG. 5A , and operation S 1230  is similar to operation S 330  of  FIG. 3A  and operation S 530  of  FIG. 5A . Accordingly, further description thereof will be omitted to avoid redundancy. 
     Where the number of write retry operations is greater than the critical value (S 1220 =Yes), controller  240  measures the frequency and amplitude of a PES (S 1241 ), a servo automatic gain control value (S 1242 ), a jitter value (S 1243 ), and a bias value (S 1244 ) from the head  16  that is performing a current writing operation. Like operations S 1211  through S 1214 , operations S 1241  to S 1244  can be selectively performed where necessary. 
     Controller  240  compares the frequency measured in operation S 1241  with the reference frequency measured in operation S 1211 , and the amplitude determined in operation S 1241  with the reference amplitude determined in operation S 1211  (S 1251 ). In addition, controller  240  compares the servo automatic gain control value measured in operation S 1242  with the first reference value determined in operation S 1212  (S 1252 ), the jitter value measured in operation S 1243  with the second reference value determined in operation S 1213  (S 1253 ), and the bias value measured in operation S 1244  with the third reference value determined in operation S 1214  (S 1254 ). Like operations S 1211  through S 1214 , operations S 1251  through S 1254  can be selectively performed where necessary. 
     Operations S 1255 , S 1260 , S 1270 , S 1280 , and  1290  are performed according to the results of operations S 1251  through S 1254 . Operations S 1255 , S 1260 , S 1270 , S 1280 , and  1290  are similar to operations S 355 , S 360 , S 370 , S 380 , and S 390  of  FIG. 3A , respectively, and therefore further detailed descriptions of these operations will be omitted to avoid redundancy. 
     In various alternative embodiments, the method of  FIG. 12  is performed using only a subset of the illustrated operations. For instance, where occurrence of HDI is determined using only a PES and a jitter, operations S 1211  and S 1213  are performed but operations S 1212  and S 1214  are not performed; operations S 1241  and S 1243  are performed but operations S 1242  and S 1244  are not performed; and operations S 1251  and S 1253  are performed but operations S 1252  and S 1254  are not performed. Furthermore, in the case where occurrence of HDI is determined using a PES and a jitter, operation S 1260  can be performed where both or at least one of the conditions of operations S 1251  and S 1253  is satisfied. Where operation S 1260  is performed upon satisfaction of at least one of the conditions of operations S 1251  and S 1253 , operation S 1255  is not performed. 
       FIG. 13  is a block diagram illustrating controller  240  of  FIG. 2  according to an embodiment of the inventive concept. 
     Referring to  FIGS. 1 through 13 , controller  240  comprises a determination unit  1310 , a comparison unit  1320 , and a control signal generating unit  1340 . According to the various embodiments described with reference to  FIGS. 1 through 12 , determination unit  1310  can determine a reference frequency and a reference amplitude that relate to a PES as reference values REF for determining the occurrence of HDI, and can determine a frequency and an amplitude DET of a PES generated during a writing operation. Determination unit  1310  can also determine a reference value REF relating to a servo automatic gain control SAGC for determining the occurrence of HDI and may measure a servo automatic gain control value DET during a writing operation. Determination unit  1310  can also determine a reference value REF relating to jitter for determining the occurrence of HDI and can measure a jitter value DET during a writing operation. Determination unit  1310  can also determine a reference value REF relating to a bias for determining the occurrence of HDI and may measure a bias value DET during a writing operation. In other words, determination unit  1310  can perform various operations such as operations S 310 , S 340 , S 510 , and S 540 . 
     Comparison unit  1320  can generate a comparison signal COMP by comparing the frequency and amplitude with the reference frequency and amplitude, respectively. In addition, comparison unit  1320  can generate a comparison signal COMP by comparing each of the measured servo automatic gain control value, the jitter value, and the bias value with a corresponding reference value. That is, comparison unit  1320  can perform operations such as operations S 350  and S 550 . 
     Control signal generating unit  1340  outputs a control signal CON in response to comparison signals COMP to control head  16 . In other words, to control head  16 , control signal generating unit  1340  generates a control signal CON using at least one of a comparison signal COMP relating to the PES, a comparison signal COMP relating to servo auto gain control SAGC, a comparison signal COMP relating to a jitter, and a comparison signal COMP relating to a bias. As described above, heater current supply circuit  270  applies a predetermined current to head  16  in response to control signal CON to adjust the flying height of head  16 . 
     The foregoing is illustrative of embodiments and is not to be construed as limiting thereof. Although a few embodiments have been described, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of the inventive concept. Accordingly, all such modifications are intended to be included within the scope of the inventive concept as defined in the claims.