Abstract:
A system, method, and device for moving a plurality of heads in a disk drive is disclosed. The system comprises hard disks, read/write heads for reading from or writing to the hard disks, a voice coil motor for moving the plurality of heads, a voice coil motor driver for energizing the voice coil motor, a voltage clamp device for regulating a voltage across the voice coil motor, and a park voltage source. The voltage clamp device comprises a transistor, a switch, and a resistive divider network. During head parking, a park voltage source activates the voltage clamp device, which clamps the voltage across the voice coil motor to a fixed value, resulting in movement of the voice coil and heads towards a park location with fixed velocity.

Description:
RELATED APPLICATIONS 
     Embodiments of this invention relate to Provisional Application Ser. No. 60/056,023, filed Sep. 2, 1997. The contents of that application are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of this invention relate generally to disk drives of the type generally used for storing digital data, and in particular to methods and devices for parking the heads used in a disk drive at a constant velocity, and disk drive systems incorporating the same. 
     2. Description of Related Art 
     Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that store and retrieve data from hard disks have thus become popular components of computer systems. 
     FIG. 1 illustrates a conventional disk drive system that could be used to implement embodiments of the present invention. FIG. 1 shows a disk drive system  2  comprising a disk drive microprocessor  4 , control logic  6 , voice coil motor driver  8 , voice coil motor  10 , head assembly  12 , read/write heads  14 , hard disks  16 , spindle motor  18 , and spindle motor drivers  20 . In operation, a computer  22  communicates through controller  24  with the disk drive microprocessor  4 . In response to commands from the controller  24 , the disk drive microprocessor  4 , by means of control logic  6 , activates voice coil motor driver  8 . The voice coil motor driver  8  energizes the voice coil motor  10  to position the head assembly  12  and read/write heads  14  over specific track locations on the hard disks  16 , which are rotating at a substantially constant velocity under the impetus of the spindle motor  18  and spindle motor drivers  20 . Once the read/write heads  14  have stabilized over the appropriate tracks, the read/write heads  14  can read data from, or write data to, the hard disks  16 . 
     Those skilled in the art will recognize that the disk drive system  2  of FIG. 1 is not intended to limit embodiments of the present invention. Indeed, those skilled in the art will recognize that alternative hardware configurations may be used without departing from the scope of the present invention. 
     In typical disk drive systems, the hard disks rotate at high velocities and read/write heads are positioned over the hard disks with very little air gap separation. In this configuration, read/write head contact with the hard disks (a head crash) can be catastrophic. Data can be permanently lost, or the read/write heads or hard disks can be damaged such that the entire disk drive system no longer functions. Therefore, modern disk drive systems avoid head contact with the hard disks as much as possible. To minimize read/write head contact with the hard disks, many disk drives park their read/write heads when the disk drive system is powered down so that the read/write heads rest over a parking zone (an area on the hard disks where no data is stored, typically the innermost central region of the disks) instead of an area used for storing data. The use of a parking zone minimizes wear on the recording area of the disks and thus increases the reliability of the disk drive system and the integrity of the stored data. Head parking circuitry activates when the disk drive system is being powered down or when the hard disks temporarily stop spinning. Such circuitry energizes the voice coil motor and moves the read/write heads to the parked position on the hard disks. 
     FIG. 2 illustrates a conventional head positioning system that includes a head parking system. Under normal operating conditions where data is being written to or read from the hard disks, a voice coil motor driver  8 , consisting of a first driver  8   a  and a second driver  8   b , produces a current flow through a voice coil motor  10 . This current flow magnetizes a voice coil  10   a , and causes the voice coil  10   a  to push or pull on a fixed permanent magnet  10   b  surrounding the voice coil  10   a . These forces of repulsion or attraction cause the voice coil  10   a  to move in relation to the fixed permanent magnet  10   b . Because the voice coil  10   a  is fixedly attached to the read/write heads  14  through the head assembly  12 , movement of the voice coil  10   a  results in movement of the read/write heads  14  in relation to the hard disks  16 . 
     Activation of the parking circuitry is triggered by the application of a park voltage source  26  to the voice coil motor  10  through a park voltage resistive network  28 . The park voltage source  26  is typically generated by stored energy in the spindle motor  18  or a storage capacitor  18   a  When the park voltage source  26  is applied, a constant current is sourced through the voice coil motor  10 , which magnetizes the voice coil  10   a  and results in movement of the voice coil  10   a  and fixedly attached read/write heads  14  towards the park position. 
     However, the constant current provided by the park voltage source  26  causes the read/write heads  14  to accelerate towards the park position, creating high gravitational forces and mechanical stress on the head assembly  12 . Acceleration of the head assembly  12  and gravitational forces can be minimized if the read/write heads  14  are parked at a constant velocity. Constant velocity parking will drastically reduce the chance of head slap (heads slapping on the disc) and resultant media defects. 
     One proposed way of parking read/write heads at a constant velocity is disclosed in the Carobolante patent (U.S. Pat. No. 5,566,369), incorporated herein by reference, which uses a feedback loop comprised of an active component (op amp) and resistors to maintain a constant voltage across the voice coil. In Carobolante, knowledge of the resistive component of the voice coil is necessary to select proper resistor values. 
     SUMMARY OF THE DISCLOSURE 
     Therefore, it is an object of embodiments of the invention to provide a system, method, or device for parking the read/write heads in a disk drive system at a constant velocity. 
     It is a further object of preferred embodiments of the invention to provide a system, method, or device for parking the read/write heads in a disk drive system at a constant velocity, wherein the system, method, or device is adjustable so that the optimum park velocity can be selected. 
     It is a further object of preferred embodiments of the invention to provide a system, method, or device for parking the read/write heads in a disk drive system at a constant velocity which avoids the use of active feedback and the necessity of knowing the resistivity of the voice coil. 
     These and other objects are accomplished according to a system for moving a plurality of heads in a disk drive, wherein the system is comprised of (1) hard disks, (2) a plurality of heads for reading from or writing to the hard disks, (3) a voice coil motor for moving the plurality of heads, (4) a voice coil motor driver for energizing the voice coil motor and moving the heads during normal read/write operations, (5) a voltage clamp device for regulating the voltage across the voice coil motor and moving the heads with constant velocity during head parking, and (6) a park voltage source for energizing the voltage clamp device through a park voltage resistive network during head parking. The voltage clamp device is further comprised of (1) a first transistor for providing a constant voltage at the output of the voltage clamp device, (2) a switch for providing current to the first transistor, and (3) a voltage divider for dividing down the input voltage to the voltage clamp device and turning on the first transistor. 
     These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional disk drive system. 
     FIG. 2 is a symbolic diagram of a conventional head positioning system that includes a head parking system. 
     FIG. 3 is a symbolic diagram of a system for parking the heads of a disk drive with constant velocity. 
     FIG. 4 is a schematic diagram of an embodiment of a voltage clamp device for parking the heads of a disk drive with constant velocity. 
     FIG. 5 is a schematic drawing of a preferred embodiment of a voltage clamp device using matched transistors for parking the heads of a disk drive with constant velocity. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description of preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the preferred embodiments of the present invention. 
     Modern computers require a media in which digital data can be quickly stored and retrieved. Magnetizable (hard) disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that store and retrieve data from hard disks have thus become popular components of computer systems. 
     To minimize the dangers of read/write head contact with hard disks, many disk drives park their read/write heads when the disk drive system is powered down. However, when a park voltage from a head parking circuit is applied, the constant current sourced through the voice coil motor results in acceleration of the read/write heads and high gravitational forces, causing mechanical stress on the head assembly. Acceleration of the head assembly and resultant gravitational forces can be minimized if the read/write heads are parked at a constant velocity. Thus, preferred embodiments of the present invention relate to a system, method, and device for parking the read/write heads in a disk drive at a constant velocity. 
     A system for parking the heads of a disk drive with constant velocity according to an embodiment of the invention is shown in FIG.  3 . Referring to FIG. 3, the constant velocity head parking system comprises hard disks  16 , read/write heads  14 , head assembly  12 , a voice coil motor  10 , a voice coil motor driver  8 , a voltage clamp device  30 , a park voltage resistive network  28 , and a park voltage source  26 . 
     The voltage clamp device  30  comprises an input terminal  44 , an output terminal  39 , a first transistor  32 , a switch  34 , and a voltage divider  37 . The first transistor  32  comprises a clamp terminal coupled to the voltage clamp device output terminal  39 , a reference terminal coupled to a voltage reference  40  (for example, ground), and a control terminal. The switch  34  is coupled between the voltage clamp device input terminal  44  and the voltage clamp device output terminal  39 . The voltage divider  37  has an input terminal coupled to the voltage clamp device input terminal  44  and an output terminal coupled to the control terminal of the first transistor  32 . 
     The voice coil motor  10  contains a voice coil  10   a  nested within a fixed permanent magnet  10   b . Under normal operating conditions where information is being written to or read from the hard disks  16 , the voice coil motor  10  is energized by a voice coil motor driver  8  that contains, in preferred embodiments, a first driver  8   a  and a second driver  8   b . During these normal read/write operations, the first driver  8   a  and second driver  8   b  are configured by control logic (not shown) to produce a current flow through the voice coil motor  10 . This current flow magnetizes the voice coil  10   a  within the voice coil motor  10  and causes the voice coil  10   a  to push or pull on the fixed permanent magnet  10   b . These forces of repulsion or attraction cause the voice coil  10   a  to move in relation to the permanent fixed magnet  10   b . Because the voice coil  10   a  is fixedly attached to the read/write heads  14 , movement of the voice coil  10   a  results in movement of the heads  14  in relation to the hard disks  16 . By properly sequencing the configuration of first driver  8   a  and second driver  8   b , the read/write heads  14  can be moved from one track to another on the hard disks  16 . 
     Under power-down conditions when the read/write heads  14  are to be parked, a park voltage source  26  typically generated by stored energy in the spindle motor  18  or a storage capacitor  18   a  is applied by the closing of a switch (not shown in FIG. 3) to the input terminal  44  of the voltage clamp device  30  through the park voltage resistive network  28 . The voltage at the voltage clamp device input terminal  44  closes the switch  34  and allows current to flow through the closed switch  34  to the clamp terminal of the first transistor  32 . The voltage at the voltage clamp device input terminal  44  is also divided down with the voltage divider  37 . The divided-down voltage at the voltage divider output terminal activates the control terminal of the first transistor  32  and allows a regulated amount of current to flow from the clamp terminal to the reference terminal of the first transistor  32 . Because of this current, a voltage develops across the clamp and reference terminals of the first transistor  32  and also at the voltage clamp device output terminal  39 . This voltage remains generally constant even though the voltage at the voltage clamp device input terminal  44  varies, because the voltage divider  37  compensates for voltage changes at the voltage clamp device input terminal  44  by changing the amount of current flowing from the clamp terminal to the reference terminal of the first transistor  32 . This constant voltage is then applied to the voice coil motor  10 . 
     The constant voltage across the voice coil motor  10  produces a current flow through the voice coil  10   a  that magnetizes the voice coil  10   a  and causes the voice coil  10   a  to push or pull on the fixed permanent magnet  10   b . These forces of repulsion or attraction cause the voice coil  10   a  to move with constant velocity in relation to the permanent fixed magnet  10   b , and also cause the read/write heads  14  to move with constant velocity in relation to the hard disks  16  toward the park position. 
     In an embodiment of a voltage clamp device  30  for parking the read/write heads  14  of a disk drive with constant velocity shown in FIG. 4, the switch  34  and voltage divider  37  of FIG. 3 have been replaced by a diode  34 , first resistor  36 , and a second resistor  38  connected at one end to a voltage reference  40 . The clamp, control, and reference terminals of the transistor  32  of FIG. 3 have also been replaced by collector, base, and emitter terminals, respectively. When a voltage V 44  is applied to the input terminal  44  of the voltage clamp device  30 , the voltage V 44  forward-biases the diode  34 . The first resistor  36  and second resistor  38  act as a voltage divider such that a voltage V 46  appears at node  46  and is approximately equal to V 44 (R 38 /(R 36 +R 38 )), where R 36  is the resistance of the first resistor  36  and R 38  is the resistance of the second resistor  38 . As FIG. 4 illustrates, V 46  is also equal to V BE1 , the voltage across the base and emitter terminals of the first transistor  32 . A sufficiently large V BE1  will forward-bias the base-emitter junction  42  of the first transistor  32 , turning on first transistor  32  and allowing increased current to flow from the collector terminal to the emitter terminal of first transistor  32 . 
     The voltage (identified as V CE1 ) that develops across the collector and emitter terminals of first transistor  32  is equal to V 44 −V D , where V D  is the voltage drop across the diode  34 . Thus, V 44 =V CE1 +V D . Because V BE1 =V 44 (R 38 /(R 36 +R 38 )) as noted in the paragraph above, by manipulating the equation it is also true that 
     
       
         V BE1 =(V CE1 +V D )(R 38 /(R 36 +R 38 )) 
       
     
     and 
     
       
         V CE1 =V BE1 ((R 36 +R 38 )/R 38 )−V D . 
       
     
     In preferred embodiments, the diode  34  and first transistor  32  are selected such that V D  is approximately equal to V BE1 , and thus V CE1 =V BE1 (R 36 /R 38 ). Because V BE1  is constant at approximately 0.6V when the base-emitter junction  42  is forward-biased during parking, V CE1  is also constant during parking and is dependent only on the ratio of R 36  to R 38 . 
     When the voltage clamp device  30  is part of the constant velocity head parking system of FIG. 3, during parking V CE1  appears at the voltage clamp device output terminal  39  and across the voice coil  10   a , and acts as a clamp voltage for the voice coil  10   a . This constant voltage across the voice coil  10   a  moves both the voice coil  10   a  and the read/write heads  14  with constant velocity, minimal acceleration, and low gravitational forces, minimizing the mechanical stress on the heads  14  during parking. 
     The clamp voltage (V CE1 ) is directly proportional to the velocity at which the voice coil  10   a  moves such that V CE1 =k*Vel VC , where k is a constant and Vel VC  is the velocity of the voice coil  10   a . Therefore, the clamp voltage (V CE1 ) can be adjusted to achieve a target park velocity by choosing R 36  and R 38  according to the formula V CE1 =V BE1 (R 36 /R 38 ). 
     The above description assumes that in preferred embodiments, V D =V BE1 . Differences in the value of V D  and V BE1  due to design or manufacturing differences between the diode  34  and first transistor  32  used in such preferred embodiments will result in a deviation between the target park velocity and the actual park velocity. An alternative preferred embodiment of the present invention, illustrated in FIG. 5, minimizes these differences and hence the error between the target park velocity and the actual park velocity. 
     The preferred embodiment of a voltage clamp device  30  for parking the read/write heads  14  of a disk drive with constant velocity shown in FIG. 5 is identical to that of FIG. 4, except that the diode  34  of FIG. 4 has been replaced by a second transistor  48  and third resistor  50  in FIG.  4 . The second transistor  48  has an collector terminal coupled to the first end of the first resistor  36  and a first end of the third resistor  50 , a base terminal coupled to a second end of the third resistor  50 , and an emitter terminal coupled to the collector terminal of the first transistor  32 . 
     In the preferred embodiment of FIG. 5, the second transistor  48  is chosen to have a design and fabrication process similar to the first transistor  32 . When a voltage V 44  is applied to the input terminal  44  of the voltage clamp device  30 , the voltage V 44  forward-biases a base-emitter junction  52  of the second transistor  48 . The third resistor  50  limits the current flowing into the base terminal of the second transistor  48  to a safe value. The voltage (identified as V CE2 ) across the collector and emitter terminals of second transistor  48  is approximately equal to the voltage (identified as V BE2 ) across the base-emitter junction  52  of the second transistor  48 . This approximation (V CE2 =V BE2 ) can be made due to the small amount of current flowing through the third resistor  50  and into the base terminal of the second transistor  48 . Because V BE2 =V BE1  due to the closely matched properties of the first transistor  32  and second transistor  48 , it is also true that V CE2 =V BE1 . By substituting V CE2  for V D  and making the assumption that V CE2 =V BE1  in the equations associated with FIG. 4, the equation V CE1= V BE1 (R 36 /R 38 ) is again derived for FIG.  5 . 
     Therefore, as in FIG. 4, the clamp voltage (V CE1 ) in FIG. 5 can be adjusted to achieve a target park velocity by choosing R 36  and R 38  according to the formula V CE1 =V BE1 (R 36 /R 38 ). However, the preferred embodiment of FIG. 5 will generally be able to produce a more accurate park velocity than the embodiment of FIG. 4 because of the matched properties of the first transistor  32  and second transistor  48 . 
     The foregoing description of preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.