Abstract:
A magnetic recording hard disk drive (HDD) uses adaptive braking of the voice coil motor (VCM) actuator upon unexpected loss of power during a track seek. An adaptive braking controller applies a preselected value of brake voltage to the VCM to reverse the motion of the freely-moving actuator. The value of the selected brake voltage is determined from the actuator velocity. A set of brake voltage values is stored in memory in the HDD, and each brake voltage value corresponds to a band of track seek lengths, with each band representing a range of actuator velocities. For each seek, the value of brake voltage corresponding to the band in which the seek length falls is stored in a register. If emergency power-off (EPO) occurs during the seek, the value of brake voltage is recalled from the register and applied to the VCM to brake the VCM. After the VCM has been adaptively braked in this manner, actuator retract occurs to unload the heads.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to disk drives, and more particularly to a magnetic recording load/unload type of disk drive that unloads the recording heads when disk drive power is removed. 
     2. Description of the Related Art 
     Magnetic recording hard disk drives (HDDs) are information storage devices that use rotatable disks with concentric data tracks containing the information, a head or transducer for reading and/or writing data onto the various tracks of each disk surface, and an actuator for moving the heads. Each head is located on a head carrier and each carrier is connected to the actuator by a suspension. The actuator is a voice coil motor (VCM) comprising a coil movable through a magnetic field generated by a fixed permanent magnet assembly. The HDD has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track (track “following”) and move them to the desired track (track “seeking”) for reading and writing of data. The disks are stacked on a hub that is rotated by a disk drive motor, also called a spindle motor. A housing supports the spindle motor and actuator, and surrounds the heads and disks to provide a substantially sealed environment for the head-disk interfaces. 
     The head carrier is typically an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained next to the disk surface by a suspension that connects the slider to the actuator. The slider is either biased toward the disk surface by a small spring force from the suspension, or is “self-loaded” to the disk surface by means of a “negative-pressure” air-bearing surface on the slider. 
     In a “load/unload” type of HDD, the sliders are mechanically unloaded from the disks when power is turned off, and then loaded back to the disks when the disks have reached a speed sufficient to generate the air bearing. The loading and unloading is typically done by means of ramps that contact the suspensions when the actuator is moved away from the data regions of the disks. Each slider is thus “parked” off its disk surface with its suspension, or a tap extending from the suspension, supported in a recess of the ramp. Load/unload HDDs provide a benefit in laptop computers because the parking of the sliders on the ramps away from the disk surfaces also provides some resistance to external shocks caused by moving or dropping the computer. 
     The parking of the sliders on the load/unload ramps when HDD power is removed is typically accomplished by use of the back electromotive force (EMF) generated by the freely rotating spindle motor. When the HDD is powered down, or in the event of unexpected loss of power (an emergency power-off or EPO event), actuator retract circuitry disconnects the VCM from its driver circuitry and connects it to a rectifier circuit that is coupled to the spindle motor. The output of the freely-rotating spindle motor is converted by the rectifier circuit to a DC current supplied to the coil of the VCM. This causes the VCM to retract to move the sliders to the ramps. A significant amount of torque is needed to ensure that the sliders are fully parked on the ramp, regardless of the VCM position or velocity at power down or EPO. The actuator velocity during retract needs to be controlled to avoid the sliders hitting the ramps at high speed. Excessive slider motion can cause the sliders to contact the ramp structure, or perhaps other sliders. Such contact can result in slider damage or transfer of contamination to the air-bearing surface, which can lead to head-disk interface failures. 
     In normal power down, the movement of the actuator can be controlled by software that controls the retract circuitry. However, during EPO, in which power to the HDD is lost unexpectedly, the software control is not active. The current supplied to the VCM during EPO retract must be large enough to unload the sliders to the ramps but small enough to prevent the suspensions and sliders from impacting the ramps at excessive speed. The EPO retract is acceptable if EPO occurs when the actuator is track following and thus has no initial velocity. However, safe retract becomes problematic if EPO occurs during track seeking, when the actuator is moving. The actuator seek velocity depends on the seek length, and can be considerably high, so that the actuator retract velocity can be significantly increased due to the initial velocity of the actuator. To address this problem, if EPO occurs during a track seek HDDs use dynamic braking of the VCM to release residual energy inside the VCM to ground before initiating actuator retract. The VCM dynamic braking is designed to brake the actuator for short track seeks, when the actuator velocity is low. For medium and long track seeks from outside diameter (OD) to inside diameter (ID) direction, the dynamic braking may be too weak so that the actuator impact speed on the ID crash stop is still high and later causes the sliders to stall at the ramps or rebound back from the ramps towards the disks. The stalling and rebounding of the sliders can cause damage to the heads and disks. For medium and long track seeks from ID to OD direction, the dynamic braking is not strong enough to avoid high speed impact during unloading. High speed impact between the sliders and the ramps also can cause damage to the heads and disks. 
     What is needed is an HDD with a reliable method for braking the actuator if EPO occurs during a track seek. 
     SUMMARY OF THE INVENTION 
     The invention is a HDD with adaptive braking of the VCM upon unexpected loss of power during a track seek. An adaptive braking controller applies a preselected value of brake voltage to the VCM to reverse the motion of the freely-moving actuator. The value of the selected brake voltage is determined from the actuator velocity. In one embodiment, a set of brake voltage values is stored in memory in the HDD, and each brake voltage value corresponds to a band of track seek lengths. Because the actuator velocity is directly related to the length of the seek being performed, the track seek lengths can be grouped into bands, with each band representing a range of actuator velocities. For each seek, the value of brake voltage corresponding to the band in which the seek length falls is stored in a register. If EPO occurs during the seek, the value of brake voltage is recalled from the register and applied to the VCM to brake the VCM. After the VCM has been adaptively braked in this manner, actuator retract occurs to unload the heads. In one embodiment the adaptive braking only occurs if the actuator velocity at EPO is above a predetermined threshold value. The actuator velocity at EPO can be determined from the VCM back EMF voltage at EPO. The VCM back EMF voltage is compared to a reference voltage and if it is below the reference voltage, conventional VCM dynamic braking occurs by shorting the VCM to ground. 
     For a fuller understanding of the nature and advantages of the present invention, reference should be made to the following detailed description taken together with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a sectional schematic of a conventional HDD. 
         FIG. 2  is an open top view of the HDD depicted schematically in  FIG. 1  and shows the head load/unload ramp. 
         FIG. 3  is a block diagram of a HDD with VCM dynamic braking and actuator retract. 
         FIG. 4  is a block diagram of the VCM dynamic braking circuit for the HDD depicted in  FIG. 3 . 
         FIG. 5  is a block diagram of a HDD according to the invention with adaptive VCM braking and actuator retract. 
         FIG. 6  is a block diagram of the VCM brake control logic for adaptive VCM braking for the HDD depicted in  FIG. 5 . 
         FIG. 7  is a flow chart illustrating the adaptive VCM braking method of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to  FIG. 1 , there is illustrated in sectional view a schematic of a conventional HDD. For ease of illustration and explanation, the HDD depicted in  FIGS. 1 and 2  is shown as having a single recording head and associated disk surface, although conventional HDDs typically have multiple heads and disks. The HDD comprises a base  10  to which are secured a spindle motor  12  and an actuator  14 , and a cover  11 . The base  10  and cover  11  provide a substantially sealed housing for the HDD. Typically, there is a gasket  13  located between base  10  and cover  11 . A magnetic recording disk  16  is connected to spindle motor  12  by means of a spindle or hub  18 . A read/write head or transducer  25  is formed on the trailing end of an air-bearing slider  20 . Transducer  25  may be an inductive read and write transducer or an inductive write transducer with a magnetoresistive (MR) read transducer formed by thin-film deposition techniques as is known in the art. The slider  20  is connected to the actuator  14  by means of a rigid arm  22  and a suspension  24 , the suspension  24  providing a biasing force that urges the slider  20  onto the surface of the recording disk  16 . The arm  22 , suspension  24 , and slider  20  with transducer  25  are referred to as the head-arm assembly. During operation of the HDD, the spindle motor  12  rotates the disk  16  at a constant speed, and the actuator  14  pivots on shaft  19  to move the slider  20  generally radially across the surface of the disk  16  so that the read/write transducer  25  may access different data tracks on disk  16 . The actuator  14  is a rotary voice coil motor (VCM) having a coil  21  that moves through the fixed magnetic field of magnet assembly  23  when current is applied to the coil. 
       FIG. 2  is a top view of the interior of the HDD with the cover  11  removed, and illustrates in better detail the suspension  24  that provides a force to the slider  20  to urge it toward the disk  16 . The suspension  24  provides a gimbaled attachment of the slider  20  which allows the slider  20  to pitch and roll as it rides on the air bearing. The data detected from disk  16  by the transducer  25  is processed into a data readback signal by signal amplification and processing circuitry in the integrated circuit chip  15  located on arm  22 . The signals from transducer  25  travel via cable  17  to chip  15 , which sends its output signals via cable  27 . 
     A load/unload ramp  30  is mounted to the base  10 . Ramp  30  contacts suspension  24  and lifts the slider  20  away from the disk  16  when the actuator  14  rotates the slider  20  toward the disk outside diameter when the HDD is powered down. An actuator crash stop  34  is mounted to the base  10  to prevent excessive movement of the actuator arm  22  and to assure that the suspension  24  does not move too far up ramp  30 . The parking location for the actuator  14  when the HDD is stopped will be with the slider  20  unloaded off the disk  16  by the ramp  30  and the actuator up against crash stop  34 . 
     Referring now to  FIG. 3 , the HDD includes a microprocessor  100  that controls the VCM  14  and the spindle motor  12 . During read and write operations, the microprocessor  100  receives head position servo information from disk  16  and generates digital output to a digital-to-analog converter (DAC)  104 . DAC  104  is coupled to a VCM driver  106  that sends current pulses to the coil of VCM  14  to move the head  25  ( FIG. 2 ) on slider  20  to the appropriate data track on disk  16  during track seeking, and to maintain the head  25  on track during track following. Microprocessor  100  also controls and is connected to the spindle motor  12  via spindle driver  108 . The spindle driver  108  controls the current from the power supply (e.g., a 12V power source as shown in  FIG. 3 ) to the windings of spindle motor  12  to rotate the motor at a constant rotational speed during HDD operation. In addition, the spindle driver  108  provides a status signal to microprocessor  100  indicating whether or not the spindle motor  12  is rotating at its operating speed. The spindle motor  12  may be a “delta” or “Y” (as shown in  FIG. 3 ) type brushless, three-phase motor with fixed windings  110 ,  112 ,  114  as field coils and a permanent magnet rotor. 
       FIG. 3  also illustrates the spindle motor power stage/rectifier circuit  70  for driving the spindle motor  12  and for energizing the VCM  14  at HDD power down with the back EMF from spindle motor  12  windings  110 ,  112 ,  114 . The spindle motor power stage/rectifier circuit  70  includes the isolation MOSFET  1  and six other MOSFETS  2 ,  3 ,  4 ,  5 ,  6 ,  7 , which drive the three-phase spindle motor  12 . The HDD can be powered by single +5V or +12V or both +5V and +12V power supply. In  FIG. 3 , +12V power is used. In normal HDD operation, the “power on reset” or POR signal is high, which makes the switches  58 ,  59  open. As a result, the DC/DC converter  72  and the VCM dynamic braking/actuator retract circuit  90  are disconnected from the system. The VCM  14  is also connected directly to rectifier circuit  70  via voltage line V MAN  which is used to retract the actuator at normal or manual power down (other than EPO) when dynamic braking is not required because retract is under software control and HDD power. 
     The design of the VCM driver  106  and spindle driver  108  is such that when power is removed (POR signal goes low), their respective output lines to the VCM  14  and spindle motor  12  behave as open circuits (disconnected). When power is removed, MOSFET  1  is off, which is an open circuit to isolate the spindle power stage  70  from the 12V power supply. At the moment power is removed, switches  58  and  59  close. Switches  58 ,  59  may be relays, solid state switches such as field-effect transistors (FETs), or other switching devices. The VCM  14  and spindle motor  12  are then effectively disconnected from the VCM driver  106  and spindle driver  108 , and connected to rectifier circuit  70 . The rectifier circuit  70  includes semiconductor directional current control devices in the form of MOSFETS  2 - 7  which form a conventional three-phase, full-wave rectifier. When power is removed, the spindle motor  12  (and the disk stack mounted on it) continue spinning due to rotational inertia. Back EMF in the spindle motor  12  results in the generation of AC currents in the motor windings  110 ,  112 ,  114 . The spindle motor  12  essentially behaves as a three-phase AC generator, and the resulting output current is rectified by the rectifier circuit  70 . The output of the rectifier circuit  70  is a DC current that flows through the now closed switches  58  and  59  to DC/DC converter  72 . The input of DC/DC converter is the spindle motor back EMF voltage. For example, if the spindle motor back EMF is about 9 volts, the output of DC/DC converter  72  would be about 3 volts and the DC/DC converter would continue to operate until the input decayed to 3 volts. The DC/DC converter  72  output on line  73  is connected to VCM dynamic braking/actuator retract circuit  90  which is coupled to the VCM  14 . When circuit  90  has completed dynamic braking of the VCM, actuator retract is enabled and the output of DC/DC converter  72  is input to VCM  14 , causing the VCM  14  to move slider  20  ( FIG. 2 ) to the ramp  30 . Various modifications of this actuator retract technique are known; for example U.S. Pat. No. 6,025,968 describes a multistage retract technique that applies first a low-level retract followed by a high-level retract. 
       FIG. 4  shows the details of the VCM dynamic braking/actuator retract circuitry  90 . VCM dynamic braking is active only if the actuator has an initial velocity less than some threshold or reference velocity V ref  when EPO occurs. Circuit  90  includes a NOR gate  91  that receives as input the POR and a signal on line  233  that is low if the velocity is less than V ref . NOR gate  91  drives MOSFET driver  92 . If at the time of EPO (POR line low) the heads are track following or are performing short track seeks, then the VCM has essentially low actuator velocity so the output of line  233  will be low. When both POR line and line  233  are low, the output of NOR gate  91  will be high. When the control line (output of NOR gate  91 ) to MOSFET driver  92  is high, VCM dynamic braking is initiated. The MOSFET driver  92  puts both upper MOSFETs  93 ,  94  in tri-state mode (not driving the MOSFETs) and turns on both lower MOSFETs  95 ,  96  for a fixed period of time. This dynamic brake action causes residual energy inside the VCM motor to be released through MOSFETs  95 ,  96  to ground so the actuator can be easily retracted later. After the fixed time period, MOSFET driver  92  opens MOSFETs  93 ,  96  and closes MOSFETs  94 ,  95  so power on line  73  from DC/DC converter  72  will be directed to VCM  14  and actuator retract will occur. 
     However, if a long track seek is being performed at the time of EPO (POR line low), the actuator will have an initial velocity greater than V ref  and line  233  will be high. The output of NOR gate  91  will be low and VCM dynamic braking will not occur. 
     In the present invention VCM braking is tuned depending on the velocity of the actuator when EPO occurs. By properly tuning of the VCM brake voltage (or current) and the brake time, the actuator speed can be controlled to a safe value. In one embodiment there are multiple brake voltages and brake times corresponding to multiple bands of seek lengths. The actuator velocity depends on the seek length. For long seeks, the actuator speed is high, hence hard VCM braking is required, while for short seeks less VCM braking is required. The invention uses a register in the VCM driver to store the VCM brake parameters (voltage and time). If seek length changes, this register is updated. When EPO occurs, the VCM brake is initiated based on the VCM brake parameters stored in the register. For example, in one particular HDD with three-band VCM braking, a brake voltage (3 volts) and brake time (2 msec) is applied if the seek length is more than 9,000 tracks, a brake voltage (3 volts) and brake time (1 msec) is applied if the seek length is more than 5,000 tracks but less than or equal to 9,000 tracks, and a brake voltage (2 volts) and brake time (1 msec) is applied if the seek length is less than or equal to 5,000 tracks. 
       FIG. 5  is a block diagram of the HDD implementing the invention. The adaptive VCM braking circuit  200  includes oscillator  202 , VCM brake delay control  204 , VCM brake control logic  206 , VCM driver register  208  and comparator  210 . The VCM brake voltage and brake time are programmed into the VCM driver register  208  by microprocessor  100  which merely writes the proper control bits to the VCM driver register. For each seek, the servo control system knows whether a seek is a forward seek from outside diameter (OD) to inside diameter (ID), or a reverse seek from ID to OD, so the microprocessor  100  also writes the proper polarity of the brake voltage to VCM driver register  208 . Before each seek, the servo control system will check the target seek length. If the target seek length is not in the same band as the previous seek, then the VCM driver register is updated with the new brake voltage (BV) and new brake time (BT). If the target seek length is in the same band as the previous seek, then the VCM driver register is not updated. When EPO occurs during a seek, the VCM brake control logic  206  will apply the BV and BT stored in the VCM driver register. The VCM driver register  208  feeds BT to VCM brake delay control  204  via line  231  and feeds the reverse BV to VCM brake control logic  206  via line  232 . The reverse BV is the voltage that generates the current to oppose the seek current to slow down the actuator velocity. When EPO occurs, the POR signal goes low, which closes the switches  58 ,  59 . The POR signal also enables the VCM brake delay control  204 , the VCM brake control logic  206 , and the VCM dynamic braking/actuator retract circuit  90 . 
     In one example of a HDD with the invention, the output V out  of DC/DC converter  72  is 3 volts. The DC/DC converter  72  continues to operate until the input decays to 3 volts. Because the spindle motor back EMF depends on motor design and its rotational speed, care should be taken when choosing the output of the DC/DC converter  72  to assure there is enough back EMF voltage and time to maintain V out , for the adaptive VCM brake operation and actuator retract. V out  drives the oscillator  202 , the VCM brake delay control  204 , the VCM brake control logic  206  and comparator  210 . The oscillator  202  provides timing to VCM brake delay control  204  and VCM brake control logic  206 . 
     The inverting terminal of the comparator  210  is connected to a capacitor, which is charged to 1.0 volt before power is removed. This voltage is the reference voltage, V ref , for this comparator and represents a reference actuator velocity. The reference actuator velocity is not limited to the use of V ref ; it can be represented by threshold or reference value stored in a memory cell instead of a capacitor. Immediately upon the occurrence of EPO, the VCM driver  106  is tri-state (not driven), causing VCM coasting for a period of time, e.g., 486 microseconds in one particular HDD, during which actuator velocity is determined. Since the VCM is coasting, the VCM back EMF voltage, which is a function of actuator velocity, can be measured across the VCM coil terminals, VCMN and VCMP, and this voltage is input to comparator  210 . In the example of  FIG. 5 , V ref  of 1.0 volt corresponds to a velocity of 440 mm/s. The actuator velocity is compared with V ref  at comparator  210 . If the velocity is greater than 440 mm/s, the output of comparator  210  on line  233  will be high. 
       FIG. 6  is a block diagram of VCM brake control logic  206 . The comparator output on line  233  and the inverse of the POR line are input to AND gate  221 . When the output of AND gate  221  is high, it will initiate the VCM brake control circuit to slow down the seek velocity, applying the previously loaded BT and reverse BV to VCM  14  via lines  223 ,  225 . After adaptive VCM braking, the logic will initiate actuator retract via line  215  to unload the sliders to the ramps. If the actuator velocity is less than 440 mm/s, the output of comparator  210  on line  233  will be low, the output of AND gate  221  will be low, and adaptive VCM braking will not occur. However, the comparator output on line  233  and the POR line are input to NOR gate  91  ( FIG. 4 ) in VCM dynamic braking circuitry  90 . So as described previously, when both POR and comparator  210  output on line  233  are low, the output of NOR gate  91  will be high and VCM dynamic braking will occur. 
       FIG. 7  is a block diagram explaining operation of the HDD with the invention. In block  310 , for each seek the target seek length is checked. In blocks  321 , 322  and  323  the seek length is divided into three bands; however the invention is not limited to three bands. Block  321  tests if the seek length equal to or less than 5,000 tracks (short seek). Block  322  tests if the seek length is greater than 5,000 tracks, but less than 9,000 tracks (medium seek). Block  323  tests if the seek length is equal to or greater than 9,000 tracks (long seeks). 
     At block  324  the microprocessor checks the target seek length and if the target seek length is not in the same seek band (short seek band, medium seek band, or long seek band) as the previous seek, then at block  326  the VCM driver register is updated with the VCM brake control parameters: the brake time (BT) and the reverse brake voltage (BV). If the target seek length is in the same seek band as the previous seek, then the VCM driver register does not need to be updated (block  328 ) because the content of the VCM driver register is already set for this seek length. Skipping this unnecessary register update saves time and operation. 
     Next at block  350  the seek operation is executed. At block  352 , EPO occurs during the seek. At EPO, the VCM is in coasting mode for time T (in one example T is 486 microseconds) during which time actuator velocity is measured (block  354 ). If at block  358  the velocity is less than the reference velocity (in the example described above this would represent the output of comparator  210  being low) then conventional VCM dynamic braking is initiated (block  360 ). Thus MOSFETs  95 ,  96  ( FIG. 4 ) are turned on for a fixed time, e.g., 2.5 ms in one example, to release the stored energy in the VCM coil to ground to slow down the VCM. 
     If at block  358  the velocity is greater than the reference velocity (in the example described above this would represent the output of comparator  210  being high) then adaptive VCM braking according to this invention is initiated (block  365 ). The appropriate VCM brake control parameters BT and BV, which have been stored in the VCM driver register before EPO occurred, are applied to the VCM to slow down the actuator. The reverse voltage is the voltage that will generate a current which opposes the seek current. In one example of a HDD a short brake time (1 msec) and low brake voltage (2 volts) is applied across the VCM coil in the short seek band, a short brake time (1 msec) and high brake voltage (3 volts) is applied in the medium seek band, and a longer brake time (2 msec) and higher brake voltage (3 volts) is applied in the long seek band. 
     After conventional dynamic braking (block  360 ) or adaptive VCM braking (block  365 ), actuator retract occurs. The VCM braking reduces the seek velocity to near zero, but does not unload the sliders to the ramps. Therefore, actuator retract is required. In one embodiment of a multistage retract, a low level retract applies 1V across the VCM coil for 194 ms (block  371 ), and a high level retract applies 3V for 200 ms (block  372 ) to move the sliders to the ramps. 
     At block  380 , after actuator retract when the sliders have been unloaded to the ramps, all of the low sides of the MOSFETs in the three phase spindle motor are turned on to dump the stored energy in the spindle motor to ground to stop the spindle motor. 
     While the present invention has been particularly shown and described with reference to the preferred embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the spirit and scope of the invention. Accordingly, the disclosed invention is to be considered merely as illustrative and limited in scope only as specified in the appended claims.