Abstract:
The present invention relates to an apparatus for an axial control and outside impact resistance of a hard disk drive (HDD), and more particularly to an impact resistant apparatus and its method by way of an axial control of a hard disk drive continuously maintaining a head at a constant flying height under an unsteady state by way of the control of an active element after equipping a head suspension with the active element for the stabilization of signals read/written on a disk. In the method to control the location of the constant flying height of the head by way of the control of suspension of the hard disk drive, this invention for the performance of the purposes is comprised of the steps of sensing the tensile or the compressive state due to the bend of suspension following the increase of the flying height of the head by the active damper, providing the value for the increase of a reverse tensile strength for the active damper in order to return to the normal flying height when the head is higher than the normal flying height after the suspension becomes tensed in the sensing step, and providing the value for the increase of the reverse compressive strength for the active damper in order to return to the normal flying height when the head is lower than the normal flying height after the suspension is compressed due to the flying height decrease of the head in the sensing step.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application makes reference to, incorporates herein and claims all benefits accruing from our application earlier filed in the Korean Industrial Property Office Dec. 31, 1993 of our application entitled OUTSIDE IMPACT RESISTANT APPARATUS AND METHOD BY AXIAL CONTROL OF HARD DISK DRIVE, which application was assigned Ser. No. 31792/1993. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus providing axial control and outside impact resistance for a hard disk drive (HDD), and more particularly to an impact resistant apparatus and a method of axial control of a hard disk drive for continuously maintaining a magnetic head at a constant flying height (CFH) over a disk in an unsteady state. The method controls an active element equipped on a head suspension to achieve stabilization of signals read and written on the disk. 
     Recent trends in hard disk drive technology have created incentives to produce disk drive devices that are more light-weight and have a higher storage capacity. Accordingly, the track pitch and bit cell of contemporary hard disks have become minute. As a result, technologies have been devised wherein servo-mechanisms are carefully controlled to enable high precision read and write operations. These technologies basically require that the magnetic head of an actuator be maintained at a constant flying height above a top side of the disk (typically, less than 0.8 micrometers). 
     It is essential that the head be maintained at this requisite flying height above the disk in order to stabilize the electrical signals generated during reading and writing operations. The stabilization of these signals during the reading and writing operations is attained when the servo-mechanism is controlled with high precision. 
     A conventional device for maintaining a constant flying height of the head is disclosed in U.S. Pat. No. 5,012,369 entitled Head Suspension Mechanism Of A Recording Apparatus With A Constant Flying Height issued to Owe et al. on Apr. 30, 1991. In this invention, a load is beam is engaged by a pressing member to control the flying height of the head. The force of the pressing member is adjusted by a screw to maintain a constant flying height. While this invention purports to control flying height of the head, it contains no provision for the problem associated with the head contacting the surface of the disk. 
     In U.S. Pat. No. 5,115,664 entitled Tunable Feedback Transducer For Transient Friction Measurement issued to Hegde et al. on May 26, 1992, another conventional system for maintaining a fixed distance between system members is disclosed. In Hegde et al. &#39;664, an output is taken from a servomechanism to provide a signal proportional to a force exerted on a movable system member due to friction. While this invention may possess merit in its own right, it suffers from a problem in that the magnetic head is not prevented from contacting the disk. Accordingly, this system risks data destruction due to this contact and we therefore believe that an improved device can be constructed. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an improved hard disk drive and control method therefor. 
     It is another object to provide a method and apparatus which can ensure the stabilization of electrical signals during reading and writing operations by continuously maintaining a desired constant flying height of a head. 
     It is still another object to provide a method and apparatus which can ensure the stabilization of electrical signals during reading and writing operations in both steady and unsteady operational states of a hard disk drive. 
     It is yet another object to provide a method and apparatus which can ensure the stabilization of electrical signals during reading and writing operations by installing an active element at a suspension of the hard disk drive to control axial bead displacement of a spindle motor. 
     It is still yet another object to provide an apparatus for improving access time of a hard disk drive by stabilizing an initial settlement of a head by using an active damper. 
     It is also an object to provide an apparatus which moves a head to a safety area of a disk during a reading and writing operation when an outside impact exceeds a specified margin, and then returns the head to its prior track position. 
     These and other objects can be accomplished according to the principles of the present invention by: sensing tensive and compressive states of a suspension attributable to increases and decreases in flying heights of a head, providing to an active damper a value indicative of a reverse tensive force necessary to return the head to a normal flying height when the head is determined to be higher than the normal flying height in the sensing step, and providing to the active damper a value indicative of a reverse compressive force necessary to return the head to the normal flying height when the head is determined to be lower than the normal flying height in the sensing step. 
     Accordingly, in the sensing step, the tensive or compressive state due to a bend in the suspension following the flying height increase or decrease is sensed by the active damper. 
     The values indicative of the reverse tensive or compressive forces are provided to the active damper as a control signal to enable the active damper to adjust to the tensive or compressive forces in order to maintain constant flying height. 
     Moreover, the hard disk drive of the present invention resists impacts by: sensing an outside impact of the hard disk, determining whether a value indicative of the sensed impact is less than a standard marginal impact value, transferring the head to a safety area on the disk where no data exists when the sensed impact value is greater than the standard marginal impact value, confirming a constant flying height of the head, and returning the head to the track position occupied prior to transfer after a constant flying height of the head is confirmed. Accordingly, the head can be replaced to the track position previously occupied after the flying height is confirmed as steady and constant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of this invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein: 
     FIG. 1 is a schematic perspective view showing a configuration of head suspension ( 105 ) with swing arm ( 103 ) and active element ( 165 ). 
     FIG. 2 is a schematic plan view of the configuration of FIG. 1 wherein two heads are employed. 
     FIG. 3 is a schematic plan view illustrating the constant flying height (CFH) of head ( 161 ) above disk ( 106 ) of FIG.  2 . 
     FIG. 4 is a schematic perspective view of head ( 161 ) of FIG.  3 . 
     FIG. 5 is a block diagram illustrating the control of head suspension ( 105 ) according to the principles of the present invention. 
     FIG. 6 is a curve diagram illustrating flying height changes when using a rotary-type voice coil motor (VCM). 
     FIG. 7 is a schematic plan view illustrating a return to reference null position (RNP) in accordance with the tension/compression of suspension ( 105 ). 
     FIG. 8 is a circuit diagram illustrating suspension control circuit ( 500 ) of FIG. 5 
     FIG. 9 is a circuit illustrating another embodiment of suspension control circuit ( 500 ) of FIG.  5 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIGS. 1 through 4, a head  161 , such as the one illustrated in FIG. 4, is attached to gimbals  162  at the end of a suspension  105  of a swing arm  103  as illustrated in FIG.  1 . Swing arm  103  rotates about a pivot bearing axis  102  to access disk  106  by controlling electrical current provided to a voice coil motor (VCM). Once rotated to a desired position on disk  106 , head  161  is able to read or write data, as illustrated in FIG.  3 . For stabilization of the reading and writing operations, head  161  should maintain a constant flying height  163  above the surface of disk  106 , as illustrated in FIG.  3 . 
     Head  161 , to which transducers  601  and  602  are attached and used to read and write data on disk  106 , as illustrated in FIG. 4, and the main body to which head  161  is attached can also be referred to as a head slider. Accordingly, head  161  can be referred to as a head slider depending upon the case. 
     As shown in FIG. 2, head  161  is attached to one end of a plate spring (hereinafter referred to as “suspension”) composed of a stainless steel material by gimbals  162 . Suspension  105  is attached to swing arm  103 . 
     In FIG. 2, head  161  is attached to swing arm  103  by gimbals  162  through suspension  105  on both the upper and lower sides of disk  106 . When the hard disk drive is not operating, head  161  remains in a safety area on the surface of disk  106  where no data exists. When the hard disk drive is operating, head  161  moves to the constant flying height  163  above disk  106  by an air current created between head  161  and disk  106  when disk  106  is rotated by a spindle motor. The combined configuration of head  161  and suspension  105  can be referred to as the head gimbals assembly (HGA). 
     Each head  161 , as illustrated in the FIG. 2, is attached to suspension  105  through gimbals  162  which are composed of ultra-thin plate springs. The position of head  161  attached to gimbals  162  is maintained according to a predetermined slope (Θ), as illustrated in FIG.  3 . 
     Transducer  601  of FIGS. 3 and 4, which is located at a trailing edge of head  161 , reads and writes electrical signals on the surface of disk  106 . The flying height  163  of head  161  refers to the gap between the trailing edge of head  161  and the surface of disk  106 , as illustrated in FIG.  3 . 
     The flying of head  161  above disk  106  is enabled by air pressure formed between the surface of disk  106  and the surface of head  161  when disk  106  rotates. 
     The surface of head  161  proximal to disk  106  is referred to as an air bearing surface (ABS). A taper  404  formed on the air bearing surface (ABS) at a portion where air flow enters (see arrows in FIG. 3) enables head  161  to receive a lifting force which maintains its position over disk  106 . 
     FIG. 4 shows the air bearing surface (ABS) of a flat taper type of head  161  and a thin film inductive type of transducer  601 . One of the two transducers  601  and  602  is not used. 
     It is possible to maintain the desired constant flying height during steady state motion of the hard disk drive due to the change in shape of head  161 , but it becomes impossible to maintain the desired constant flying height during an unsteady state of motion. 
     The reason it is impossible to maintain the desired constant flying height under the unsteady state is because there is no way to compensate for problems such as: the change in flying height resulting from twisting transformations in the resonant frequency of suspension  105 , the change in flying height resulting from an axial non-repeatable runout (NRRO) of the spindle motor (i.e. a non-periodical displacement phenomenon that occurs when a disk rotates about its axis), and collisions between surfaces of disk  106  and head  161  resulting from the impact of the hard disk drive. Compensation for the dynamic axial disturbance of the spindle motor (i.e. the axial displacement of the spindle motor) requires an effective way to control the flying height of head  161 . 
     FIG. 5 is a block diagram illustrating the control of head suspension  105  according to the principles of the present invention. 
     Referring to FIG. 5, displacements (i.e. tension and compression) of suspension  105  corresponding to flying height changes of head  161  are detected and compensated for by attaching a first actuator  701  composed of a piezoelectric ceramic and a second actuator composed of a piezoelectric polymer to an upper part of suspension  105 . Second actuator  702  detects changes in the tension and compression of suspension  105  caused by changes in the flying height of head  161 , and provides an output representative of such tension and compression changes to a suspension control circuit  500 . Suspension control circuit  500  generates a compensation value and applies this value to first actuator  701 , so that appropriate adjustments can be made to maintain a desired constant flying height. 
     Second actuator  702 , which acts as a “strain gauge”, detects the state of suspension  105  by detecting changes in the flow of electrical current generated by a voltage Vcc provided to second actuator  702 . That is, changes in the tension and compression of suspension  105  produces corresponding changes in the electrical resistance of second actuator  702 . Accordingly, changes in the flow of electrical current in second actuator  702  reflect the tension and compression of suspension  105 . 
     In order to maintain the desired constant flying height, first and second actuators  701  and  702 , which are active elements for sensing and controlling axial head displacement of the spindle motor, should have the following characteristics: 
     First, they should be able to respond rapidly to dynamic disturbances. Secondly, the maximum control displacement should be 1 micrometer (axially) or less. Thirdly, the minimum control displacement should be at least 0.1 micrometers (axially). And fourthly, they should be made of a light-weight material to avoid adversely influencing voice coil motor (VCM) inertia. 
     First and second actuators  701  and  702  should be installed on the upper end of suspension  105 . Also, they should be very flexible in the axial direction of the spindle motor and located as close to head  161  as possible. 
     The configuration of first and second actuators  701  and  702  attached to suspension  105  serves as a sensing device to detect minute tension and compression changes in suspension  105  indicative of changes in the flying height of head  161 . 
     FIG. 6 is a curve diagram illustrating flying height changes in head  161  when using a rotary-type voice coil motor (VCM). The horizontal axis represents the radius of the disk and the vertical axis represents flying height of head  161 . Point A represents an initial state of head  161 ; that is, head  161  is at a flying height indicated by reference numeral  691  when at radial position T 1  of disk  106 . Point B represents the maximum flying height of head  161  caused by air pressure formed between the surface of disk  106  and the surface of head  161  when disk  106  rotates. This maximum flying height, which is indicated by reference numeral  692 , occurs when head  161  is at radial position T 2  of disk  106 . Point C represents flying height of head  161  when at radial position T 3  (i.e. the outermost track) of disk  106 . 
     FIG. 7 illustrates the state of suspension  105  and flying height changes corresponding to the tension and compression of first actuator  701  attached to the upper end of suspension  105 . 
     FIG. 8 is a detailed circuit of suspension control circuit  500  of suspension  105  constructed in accordance with the principles of the present invention. Suspension control circuit  500  is comprised of an amplification and signal processing circuit  662  for outputting a signal Y(S) representative of a change detected in the tension or compression of suspension  105  by second actuator  702 , an error value extraction device  664  for extracting an error value E(S) after comparing a current flying height offset value R(S) input to a flying height FH offset terminal  668  with a value represented by the signal Y(S) output from amplification and signal processing circuit  662 , a signal truth-delay compensator  663  for compensating for the phase delay of the signal E(S) output from error value extraction device  664 , and an amplifier  661  for amplifying a signal U(S) output from signal truth-delay compensator  663  to provide an amplified output to first actuator  701 . 
     FIG. 9 is a diagram illustrating a second embodiment of suspension control circuit  500  constructed according to the principles of the present invention. While the circuit of FIG. 8 utilizes an analog method, the circuit of FIG. 9 uses a digital method. Suspension control circuit  500  of FIG. 9 is comprised of an analog-to-digital (A/D) converter  667  for analog-to-digitally converting the output of amplification and signal processing circuit  662 , error value extraction device  664  for extracting error value E(Z) after comparing the flying height offset value R(Z) input to flying height FH offset terminal  668  with the value represented by signal Y(Z) output from A/D converter  667 , a digital proportional-integral-derivative (PID) type controller  669  for receiving and digitally controlling the signal E(Z) output from error value extraction device  664  and generating an adjustment signal U(Z) after summing the received signal according to a constant ratio following the differentiation and integration, and a digital-to-analog (D/A) converter  668  for converting the output of digital PID controller  669  to provide a converted analog signal to amplifier  661 . 
     The piezoelectric ceramic and piezoelectric polymer used as first and second actuators  701  and  702 , respectively, are extended due to piezoelectric characteristics when an electric field is applied. This principle is described on page 196 of “ Examples and Basis of Application Laser Guide ” published by the domestic Electric and Electronic Research Institute on Sep. 5, 1985. 
     Referring now to FIGS. 1 through 9, the preferred embodiments of the present invention will hereinafter be described. 
     In FIG. 7, no displacement change is detected in accordance with the sensing value detected by first and second actuators  701  and  702  of suspension  105  when normal flying height  892  of head  161  is maintained. When the flying height of head  161  increases by an amount equal to +ΔZ as depicted by reference numeral  893 , however, the resulting tension is detected at a lower portion of suspension  105 . That is, second actuator  702  detects the tension due to the distortion (i.e. bending) of suspension  105  since the piezoelectric polymer acts as a “strain gauge”, as described earlier. A tension detection value is input to suspension control circuit  500  of FIG. 8 or  9  which performs an adjustment operation. That is, a reverse tensive force corresponding to the tensive force detected at suspension  105  can be generated from the flying height offset value input to flying height FH offset terminal  668 . 
     Control of the reverse tensive force of suspension  105  is performed by suspension control circuit  500  as follows. First, a signal representative of the tensive force detected at second actuator  702  is amplified through amplification and signal processing circuit  662 . Output signal Y(S) from amplification and signal processing circuit  662  is then combined with flying height offset value R(S) from flying height FH offset terminal  668  at error value extraction device  664  and error value E(S) is determined by a subtraction operation. Compensation for the phase delay of E(S) is provided for by signal delay-truth compensator  663 . The output U(S) of signal delay-truth compensator  663  is amplified by amplifier  661  and then provided to first actuator  701  composed of the piezoelectric ceramic. In response to receipt of the amplified signal, first actuator  701  generates the reverse tensive force necessary to return suspension  105  (and head  161 ) to the reference null position (RNP), as illustrated in FIG.  7 . Similarly, when the flying height of head  161  decreases by an amount equal to −ΔZ, as depicted by reference numeral  891  in FIG. 7, the sensing part of second actuator  702  generates a value representative of detected compressive force. If this value is input to suspension control circuit  500 , the reverse compressive force is generated and applied to first actuator  701 , according to the process described above, in order to return suspension  105  to reference null position (RNP)  892 . 
     FH changes attributable to the tension and compression of suspension  105  in cases where a rotary-type voice coil motor (VCM) is used are generally the same as the flying height changes in cases where a flat taper-type of head  161 , as illustrated in FIG. 4, is used. 
     Referring to FIG. 6, flying height changes increase at the outer radial portions of disk  106  due to relative differences in the linear velocity of disk  106  on inner and outer tracks. These differences in disk velocity produce varying levels of air pressure between disk  106  and head  161 , which in turn produce different degrees of force upon head  161 . With a rotary-type of voice coil motor (VCM), a flying height change curve as shown in FIG. 6 results because a large skew angle attributable to changes in head angle exists between the center-line of head  161  and the normal line of the track. Accordingly, a decrease in flying height occurs on the outermost track of disk  106 . 
     Magnitudes of read/write signals of transducer  601  in FIG. 4 are dependent upon changes in flying height. For example, when constant density reading is desired for design purposes, it is essential to maintain the constant flying height of head  161  over all data tracks. Maintaining head  161  at the constant flying height above the surface of disk  106  in this manner requires a series of developments changing the air bearing surface of head  161 , i.e. TPC (Transverse Pressure Contours), TAB (Tri-rail Air Bearing) and NPAB (Negative Pressure Air Bearing). 
     Accordingly, it is possible to use the flat taper-type of head  161  for normal operation in the present invention, even without using a head slider which takes advantage of the specific air bearing surface and maintains the constant flying height over the entire data track section, which is a requirement of the constant density reading design. 
     Continuous maintenance of a desired flying height becomes enabled by using suspension control circuit  500  to actively compensate for dynamic disturbances inside and outside the hard disk drive. Suspension control circuit  500  detects and compensates for the tension or compression of suspension  105  by using first and second actuators  701  and  702  installed at suspension  105 , even during abnormal operating states of the hard disk drive, such as the following situations A-C: 
     A. Flying height changes due to resonance in the self-resonant frequency of the HGA suspension. 
     B. Flying height changes due to the axial NRRO of the spindle motor. 
     C. Flying height changes due to outside impact of the hard disk drive. 
     The piezoelectric polymer sensor of second actuator  702  also plays a role in data protection by preventing the collision of head  161  and disk  106  due to an outside impact. This helps strengthen the impact resistance of the hard disk drive which is especially important in view of the trend of hard disk drive miniaturization. Head  161  moves to a safety area on disk  106  where no data exists when an impact beyond the marginal impact occurs. This is enabled by installing the sensor for detecting outside acceleration on the PCB to protect the data from impacts, for instance like, “Safe-Rite HDD” from the Seagate Co. of U.S.A. The piezoelectric polymer sensor of second actuator  702  can be used for the same purpose as mentioned above. 
     That is, when a flying height change of head  161  occurs due to an outside impact beyond the constant margin, the servo control of the present invention first moves head  161  performing the reading or writing into the safety area on the disk, and then when flying height is normal, head  161  returns to the track location it previously occupied. 
     Control of the voice coil motor (VCM) is divided into a seek mode and a track following mode, as is well-known in hard disk drive design. The seek mode represents a voice coil motor (VCM) control mode for transferring head  161  from a currently occupied track to a track at a target location. The track following mode represents a voice coil motor (VCM) control mode where head  161  performs the read/write function at the center of the track at the target location. There is also a settling characteristic associated with head  161 , which serves as an important control function while the seek mode changes to the track following mode. The sooner head  161  settles, the faster reading and writing can be performed. Radial settling can be improved with voice coil motor (VCM) control, but axial settling can be improved only marginally with a passive damper attached to the upper portion of suspension  105 . Access time of the hard disk drive, however, can be improved by stabilizing the initial settling of the track following mode as rapidly as possible with the active damper using the piezoelectric ceramic/polymer of first and second actuators  701  and  702 . 
     Although not every practical illustration of the present invention has been described, it will be clear to those of ordinary skill in the art that equivalents can be substituted in the present invention without departing from the central scope thereof. Another great advantage of the present invention achieved by using the piezoelectric element on the suspension or flexible arm is to solve problems such as increases in frictional force and the wear which can occur due to the contact of head  161  and disk  106  while starting and stopping the hard disk drive by separating head  161  and disk  106  after applying the necessary power source to the actuator. 
     Moreover, the head of the hard disk drive is set to have a flying height that is constant and as low as possible depending upon upgrade capabilities and capacity. It is known in the present invention, that there are various methods to adjust the pressure distribution in accordance with the air currents between the air bearing surface (ABS) of head  161  and disk  106 . There is, however, a problem in that the starting load of the spindle motor increases or data is lost as a result of an increase in friction due to wear because head  161  and disk  106  contact each other when the spindle motor fails to rotate at the designated constant velocity, for instance during the starting or stopping of the spindle motor. 
     The present invention, however, can solve the defects due to contact of head  161  and disk  106  by utilizing an actuator having a piezoelectric element. Head  161  and disk  106  are placed in close proximity from the beginning of disk rotation until its rotation at a constant velocity and from the time of rotation at the constant velocity until the time immediately after rotation is stopped. Head  161  and disk  106  are separated from each other immediately after application of the power source so that the compressive strength can be generated and applied to the actuator attached to the upper portion of suspension  105 .