A METHOD OF CONTROLLING THE OPERATION OF A DISK FILE AND A DISK FILE

Abstract: The start-up of a rotating disk file is controlled by first moving the sliders (30, 32, 40, 42) supporting the read/write heads radially outwards from the inside diameter (20) of the disks (10, 12, 14) by applying a voltage from voltage source (70) to voice coil motors (34, 44), sensing the position of the sliders (30, 32, 40, 42) on the disk surfaces with a position sensor (80), initiating rotation of the disk drive motor (18) when the sliders (30, 32, 40, 42) have moved a dewetting distance, and terminating radial translation of the sliders (30, 32, 40, 42) when the disk drive motor (18) has reached operating speed and the sliders (30, 32, 40, 42) are riding on air bearings above the disks. The instrumented radial translation of the sliders (30, 32, 40, 42) overcomes the stiction forces at the slider-disk interface without damage to the read/write heads, the disks (10, 12, 14) or the slider suspensions (36, 38, 46, 48).

Description: This invention relates to a method of controlling the operation of a disk file of the type in which a transducer head is in contact with a record disk when the disk is at rest and "flies" above the disk when the disk is rotating at its operating speed, and to a disk file of that type. In many types of rotating disk files, the read/write transducer heads are supported on a slider which rides on a cushion or bearing of air above the disk when the disk is rotating at its operating speed. The slider is connected to a linear or rotary voice coil actuator by means of a relatively fragile suspension. In large capacity magnetic recording disk files there is generally a stack of rigid disks and a number of actuators with each actuator supporting a number of sliders. The actuators move the sliders radially between the disks so that each head may access the recording area of a respective disk. In these conventional disk files, the slider is biased against the disk surface by a small force from the suspension when the disk is not rotating. The slider is thus in contact with the disk from the time the disk file is turned on until the disk reaches a speed sufficient to cause the slider to ride on the air bearing. The slider is again in contact with the disk when the disk file is turned off and the rotational speed of the disk falls below that necessary to create the air bearing. In such disk files a lubricant is often maintained on the disk surface to prevent damage to the head and the disk during starting and stopping of the disk. A serious problem with such disk files is that after the slider has been in stationary contact with the disk surface for just a short period of time, the slider tends to resist translational movement or "stick" to the disk surface. This "stiction" is caused by a variety of factors, including static friction and viscous shear forces and surface tension created by the lubricant between the disk and the slider. Even in those disk files which have disks with extremely smooth unlubricated disk surfaces, striction may occur because of the strong intermolecular attraction at the interface between the smooth disk and slider surfaces. This stiction can result in damage to the head or disk when the slider suddenly breaks free from the disk surface when disk rotation is initiated. In addition, because the suspension between the actuator and the slider is relatively fragile in order to permit the slider to fly above the disk, sudden rotation of the disk can also damage the suspension. In one known technique to overcome the stiction problem, disk rotation is started very slowly so that the slider breaks free from the disk surface gradually. This approach has the disadvantage that it requires a relatively long period of time to bring the disk file up to operating speed and that it imparts tangantial forces to the suspensions, which is the direction where they are structurally weakest. EP-A-0 110 006 discloses a method of releasing a magnetic head from adherence to the surface of a record disk by imparting a small amplitude oscillatory motion to the head before rotation of the disk is commenced. The imparted oscillatory motion of the head is about its rest position on the disk. Substantially the same method is disclosed in JP-A-59-177762 concerning the problem to remove adhesion due to a surface smoothing agent. That document additionally discloses the use of detecting means to detect that the head actually oscillates in response to the application of an oscillation-including current to the head driving circuit. JP-A-57-127968 (≡ US-A-4542429) discloses a method of releasing a transducer by means of a stepper motor moving the transducer in an outward radial direction prior to start up of the disk drive motor. Translation of the transducer is halted before motion of the disk is initiated. The invention seeks to provide a solution to the stiction problem. The invention provides, according to one of its aspects a method of controlling the operation of a disk file wherein a transducer head is in contact with a rotatable data storage disk when the disk is at rest and rides on a bearing of air above the disk when the disk rotates at its operational speed, the method comprising the steps of moving the head radially in one direction and subsequently initiating rotation of the disk and continuing to move the head radially; the method being characterised by; sensing the movement of the transducer head, initiating rotation of the disk after a predetermined movement of the head corresponding to a dewetting distance, and continuing radial movement of the head until the disk is rotating at a predetermined speed. On lubricated disks the dewetting distance is the distance necessary to break loose from the lubricant in the slider-disk interface. Other aspects of the invention are defined in the claims appended hereto. When the invention is embodied in a disk file having multiple disks and actuators, means can also be provided for oscillating the sliders on one or more of the actuators radially in and out over a relatively short distance if all the actuators do not release at approximately the same time. This ensures that all of the sliders are maintained in motion over the disk surfaces until the disks have reached operating speed, at which time all of the sliders will be riding on an air bearing. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawing which is a simplified block diagram of a disk file incorporating means for implementing instrumented radial translation of the transducer heads during start up. In the disk file represented in the drawing, a plurality of rigid rotatable disks 10, 12, and 14, are supported on a spindle 16 and rotated by a disk drive motor 18, whose rotational speed is sensed by a sensor 19. The magnetic recording media on each disk is in the form of an annular pattern of concentric data tracks having an inside diameter 20 and an outside diameter 22, as indicated on disk 10. During normal data-transducing operations of the disk file, head sliders 30, 32, 40, 42 are moved radially in and out as the disks rotate so that the heads may access different portions of the disk surfaces containing the data. Each slider supports one or more read/write heads and is attached to an actuator arm by means of a suspension, as illustrated by suspensions 36, 38 connecting sliders 30, 32 to actuator arm 35. The suspensions provide a slight spring force which biases the sliders against the disk surfaces. Each actuator arm is attached to a voice coil motor (VCM) or actuator, such as arm 35 connected to VCM 34. Each VCM is a coil movable within a fixed magnetic field, the direction and velocity of the coil movement being controlled by the current supplied. The construction and operation of sliders 40, 42, suspensions 46, 48, actuator arm 45, and VCM 44 is identical to that described for sliders 30, 32 suspensions 36, 38, actuator arm 35, and VCM 34. During operation of the disk file, the rotation of the disks generates an air bearing between the sliders and the disk surfaces. This air bearing counter-balances the slight spring force of the suspensions and supports the sliders off the disk surfaces during operation. VCMs 34, 44 are controlled by means of a current source 60 and a control unit 62 which includes logic circuitry and an internal timer. The radial positions of the sliders 30, 32 and 40, 42 are provided to control unit 62 by servo signals from either a dedicated servo disk (not shown) or servo sectors on the data disks. During a "seek" operation to move the sliders from one recording track to another, the control unit 62 provides the desired current profile to current source 60 in response to the servo signals. Current source 60 in turn generates a current of the proper magnitude and duration to optimally move the sliders to the desired track. Similarly, during track following control unit 62 provides signals to current source 60 in response to the servo signals to maintain the heads over the track centre lines. The control unit 62 also receives a signal if either of the VCMs have moved the sliders to the outside diameter 22 of the disks. This is provided by crash stop sensors 64, 66 on VCMs 34, 44, respectively. In the preferred embodiment of the invention, the VCMs 34, 44 are controlled to provide the instrumented radial translation of the sliders prior to start of disk drive motor 18 by means of a separate voltage source 70 which supplies a voltage through switches 72, 74. The switches 72, 74, which are controlled by control unit 62, switch in voltage source 70 during instrumented radial translation at start-up and current source 60 during normal operation for track seeking or track following after the disks 10, 12, and 14 have reached operating speed. Alternatively, the necessary control of the VCMs for instrumented radial translation could be provided through the same current source 60 as is used to provide track seeking and track following. It is necessary to provide slider position information to control unit 62 in order to ensure that all of the sliders have moved the proper "dewetting" distance. On lubricated disks the dewetting distance is the distance necessary to break loose from the lubricant in the slider-disk interface. This position information is provided by position sensor 80 which monitors the back electromotive force (BEMF) from VCMs 34, 44 and sends slider release signals on lines 82, 83, respectively, to control unit 62. The BEMF of a VCM is that voltage generated by movement of the coil through the fixed magnetic field, regardless of whether the coil is moved manually or by means of a voltage applied to the coil from voltage source 70. The BEMF in volts is the rate of change of flux through the coil in the VCM, which is directly proportional to the velocity of the coil. Thus position sensor 80 includes a voltage sensor which monitors the radial position of the sliders by measuring the BEMF, which is proportional to slider velocity. The integration of this velocity over time produces a distance or position value. Application of the invention can be better understood by considering the function of the above-described components during start-up of the disk file. Prior to start-up, the sliders 30, 32 and 40, 42 are in contact with their respective disks 10, 12, 14 at the inside diameter 20 (i.e. the sliders are in their rest position), the sliders having been so positioned when the disk file was powered down in the previous cycle. When the disk file is turned on, the control unit 62 signals switches 72, 74 to the proper position for connecting voltage source 70 to VCMs 34, 44. Control unit 62 next signals voltage source 70 to provide a voltage profile to each of the VCMs 34, 44, which in the preferred embodiment is a fixed voltage of sufficient amplitude to cause the sliders to move radially outwardly at a constant velocity (i.e. to move in one radial direction). However, because of stiction between the sliders and their respective disk surfaces, the sliders do not necessarily release immediately upon application of the voltage to the VCMs. The sliders may release at different times because of different stiction forces. Thus actuator arms 35, 45 may begin radial translation at different times. As soon as either of the coils in VCMs 34, 44 begins to move a BEMF will be induced. The BEMF from each of the VCMs 34, 35 is input to position sensor 80 and compared with a reference voltage from control unit 62 via line 84. As soon as the BEMF from either VCM 34 or VCM 44 exceeds the predetermined reference voltage from line 84, a release signal will be sent to control unit 62 on lines 82 or 83 respectively, thus indicating that actuator arm 35 or actuator arm 45 has released and is in radial translation. Once the BEMF from a VCM exceeds the reference voltage, the coil, and thus the sliders attached to the actuator arm, are moving generally at a constant velocity because of the fixed voltage of sufficient amplitude from voltage source 70. The control unit 62 then measures the time from the receipt of the slider release signal to determine that the slider has moved the predetermined dewetting distance. In the case of lubricated disks, this dewetting distance is the distance that the slider must move to break away from the forces created by the lubricant between the slider and the disk surface. This distance can be determined experimentally for various types of disks and sliders but is within the range of approximately one to three radial contact lengths between the slider and the disk surface. When control unit 62 has received signals from position sensor 80 that both of the actuator arms 35, 45 are in radial translation, a signal is sent on output line 90 to disk drive motor 18 to begin rotation of disks 10, 12 and 14. As the disks commence rotation the sliders continue radial translation outwardly until the disks have reached at least the speed at which the sliders are flying over the disk surfaces. In the preferred embodiment, however, radial translation of the sliders is not terminated until the disks have reached operating speed. In the embodiment shown in Fig. 1, control unit 62 receives disk drive motor speed information from sensor 19. Alternatively, it is possible for control unit 62 to determine the speed of disk drive motor 18 by measuring the time from instant of turn-on. If one or both of actuator arms 35, 45 has reached its respective crash stop 64, 66 before the disks have reached operating speed, control unit 62 signals the appropriate switches 72, 74 to switch out voltage source 70 and switch in current source 60 to the appropriate VCMs 34, 44. Control unit 62 then provides an appropriate current profile to current source 60 to drive the VCMs so as to oscillate the sliders attached to those actuator arms which have "prematurely" reached their crash stops. This oscillation occurs over a relatively short radial distance near the outside diameter 22 and keeps the sliders in radial motion relative to the disks until the sliders are flying above the disks. In the preferred embodiment, this oscillation continues until the disks reach operating speed, which ensures that the sliders are flying above the disks. If the disks have reached operating speed before actuator arms 35, 45 reach their crash stops, then control unit 62, which has received disk drive motor rotational speed information from sensor 19, provides an output signal to switches 72, 74 to switch out voltage source 70 and switch in current source 60. The disk file is now in its normal mode of operation and is ready for track seeking. If either of actuator arms 35, 45 has not moved radially after a predetermined time interval from application of the fixed voltage from voltage source 70, then control unit 62 provides an emergency off signal on line 94 to voltage source 70 to immediately return the actuators which have moved to the inside diameter 20 and terminate application of voltage to the VCMs, thereby preventing any damage to the disk file. The sensing of the change of radial position of the sliders on the disk surfaces during the start-up procedure is necessary in order to ensure that the disks do not commence rotation before the stiction forces have been overcome. Otherwise suspensions 36, 38 and 46, 48 could be subjected to excessive tangential forces resulting in either structural damage to the suspensions or sudden "rebound" of the sliders, which can damage the heads and disk surfaces. The sensing of the position of the sliders by measuring velocity, through BEMF sensing, and time in motion is but one means of sensing the radial position of the sliders. For example, a photo-Darlington device can be used to sense relative displacement between the movable coil and the fixed part of each of the VCMs to indicate the distance the actuator arm has moved. When the disk file is turned off, the control unit 62 signals current source 60 to drive VCMs 34, 44 so that the sliders are at their radially innermost position when they come to rest on the disk surfaces. This ensures that the sliders are in the proper location for radial translation when the disk file is next turned on. It should be apparent that disk files may contain a large number of disks and VCMs and that each VCM may support a number of sliders. The method of instrumented radial translation at start-up of a disk file disclosed hereinbefore is fully applicable to any such disk file, provided it is of the type in which the sliders are in contact with the disk surfaces when the disks are at rest and "fly" above the disk surfaces when the disks are rotating at operating speed.

Claims: A method of controlling the operation of disk file wherein a transducer head (30, 32, 40, 42) is in contact with a rotatable data storage disk (10, 12, 14) when the disk is at rest and rides on a bearing of air above the disk when the disk rotates at its operational speed, the method comprising the steps of moving (by 62, 70, 72, 74) the head radially in one direction and subsequently initiating (by 62) rotation of the disk and continuing to move the head radially; the method being characterised by; sensing the movement of the transducer head, initiating rotation of the disk after a predetermined movement of the head corresponding to a dewetting distance, and continuing radial movement of the head until the disk is rotating at a predetermined speed. A method as claimed in claim 1, wherein the movement of the head in said one radial direction comprises moving the head radially outwards from its rest position. A method as claimed in claim 1 or claim 2, wherein the step of sensing the movement of the transducer head comprises sensing the back electromotive force generated by an actuator by which the head is moved. A method as claimed any preceding claim, wherein the step of moving the head radially comprises applying a predetermined voltage to an actuator, and removing the applied voltage if the head has not moved within a predetermined time. A method as claimed in any preceding claim, wherein the step of continuing to move the head radially further comprises oscillating the head radially in and out over a relatively short distance. A method as claimed in any preceding claim whereby the radial movement of the head is continued until the disk is rotating at its operating speed. A method as claimed in any preceding claim, wherein rotation of the disk is terminated by the steps of decreasing the speed of rotation of the disk and positioning the head to a predetermined location on the disk before the speed of rotation of the disk has dropped below the speed at which the head rides on a bearing of air above the disk. A method as claimed in claim 7, wherein the step of positioning the head to a predetermined location includes the step of moving the head to its radially innermost location on the disk surface. A method as claimed in any preceding claim, wherein the disk file has at least two rotatable data storage disks (10, 12, 14) attached to a drive motor (18) and at least two actuators (34, 44), each actuator supporting at least one transducer head (30, 32; 40, 42), and wherein rotation of the disk is initiated only when each head has moved the predetermined dewetting distance. A disk file including a rotatable data storage disk (10, 12, 14), a disk drive motor (18), and actuator (34, 44) and a transducer head (30, 32, 40, 42) attached to the actuator, wherein the head is in contact with the surface of the disk when the disk is at rest and rides on a bearing of air above the surface of the disk when the disk is rotating at its operating speed, and further including means for preventing the head from sticking to the surface of the disk during rotational start-up by first moving the head radially in one direction and subsequently initiating rotation of the disk and continuing to move the head radially , the disk file being characterised by the aforementioned means comprising : means (80, 62) for detecting a predetermined movement of the head corresponding to a dewetting distance; means (62) responsive to the detection of said predetermined movement to apply a current to the disk drive motor to cause rotation of the disk until the disk is rotating at a predetermined speed . A disk file as claimed in claim 10, wherein movement detecting means further comprises means electrically coupled to the actuator for sensing the back electromotive force generated by the actuator. A disk file as claimed in claim 11, wherein the movement detecting means further comprises timing means for timing an interval corresponding to the dewetting distance. A disk file as claimed in any of claim 10 to 12, further including means for causing the head to oscillate radially in and out over a relatively short distance when the head is at its radially outermost position and before the disk has reached its operating speed. A disk file as claimed in any of claims 10 to 13 including a plurality of rotatable data storage disks (10, 12, 14), a plurality of voice coil actuators (33, 44) and at least one transducer head (30, 32, 40, 42) attached to each actuator wherein the means (62) responsive to the detection of said predetermined movement applies a current to the disk drive motor only when each head has undergone the predetermined movement.

Inventors: BERTSCHY, JOHN RALPH, CAMERON, JAY T., DECKERT, KENNETH L., LEE, SYLVIA LUI
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1985
Main classification: G11B21
IPC codes: G11B5, G11B19, G11B21