Patent Publication Number: US-2005141375-A1

Title: Operating a rotatable media storage device at multiple spin-speeds

Description:
CLAIM OF PRIORITY  
      This application claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/530,504, filed Dec. 18, 2003, and entitled “Operating A Rotatable Media Storage Device At Multiple Spin-Speeds.” 
      This application also claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/530,491, filed Dec. 18, 2003, and entitled “Reducing the Time-To-Ready in a Rotatable Media Storage Device.” 
      This application is also a continuation-in-part of U.S. patent application Ser. No. 10/366,237, filed Feb. 13, 2003, and entitled “Intermediate Power Down Mode for a Rotatable Media Data Storage Device,” which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Patent Application No. 60/436,946, entitled “Intermediate Power Down Mode for a Rotatable Media Data Storage Device,” filed Dec. 30, 2002.  
      Each of the above applications is incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention relates generally to rotatable media data storage devices, as for example magnetic or optical hard disk drive technology.  
     BACKGROUND OF THE INVENTION  
      Disk drives typically include a number of rapidly rotating disks having surfaces upon which data is written to and read from. Each disk surface that is used to store data is matched with a head which is held very close to the disk surface. The head can thereby read and write data from/to the disk surface as it rotates under the head.  
      A head suspension assembly (HSA) typically includes the head attached to a slider, which is further attached to a flexible suspension member (also known simply as a suspension). The suspension is in-turn connected to a pivoting actuator arm, whose motion is typically controlled using a voice coil motor. The rotating or spinning of the disk creates air pressure beneath the slider that lifts the slider and consequently the head off of the surface of the disk, creating a micro-gap of typically less than one micro-inch between the disk and the head. The suspension is often bent or shaped to act as a spring such that a load force is applied to the surface of the disk. The air bearing or cushion created by the spinning of the disk resists the spring force applied by the suspension, and the opposition of the spring force and the air bearing to one another allows the head to trace the surface contour of the rotating disk surface, which is likely to have minute warpage, without contacting the disk surface.  
      It is preferred that the head and disk surface not come in contact while the disk is rotating, since this can result in damage to both the disk surface and the head. For example, data can be permanently destroyed if excessive contact should occur. Also, the head can be damaged by the contact. When the disk is rotating at a sufficient speed, contact between the disk surface and head is prevented by the air bearing, as just explained above.  
      It is also preferred that the head and disk surface not come in contact while the disk is not rotating (e.g., when the hard drive is not powered, or when there have been no recent read or write requests, causing the disk drive to go into a power saving mode where the disk stops spinning). This is because the head and disk surface may stick together, if the disk and the head are at rest and in contact for a period of time, resulting in damage to the disk surface and/or head when the disk starts to rotate. Also, since the disk starts spinning from rest, and a certain minimum velocity is required for the head to float over the disk surface, each startup of the hard drive can result in the head and disk surface rubbing for a distance until the disk achieves sufficient speed to form the aforementioned air cushion.  
      For the above mentioned reasons, load/unload ramp structures and techniques have been used in many hard drives to hold a head away from a disk surface while a disk is not spinning. In such systems, the head is not released from the ramp structure until the disk has achieved its normal operating spin-speed. Similarly, when the disk drive is to be stopped, the slider is unloaded to a standby position on the ramp structure before the disk rotation speed is spun-down to rest.  
      Over the past few years, portable computing devices, such as notebook computers, have become progressively thinner and lighter, and battery technology has improved significantly. However, though both thinner and lighter, portable computing devices have incorporated ever-more powerful CPUs, larger and higher resolution screens, more memory and higher capacity hard disk drives. Feature-rich models include a number of peripherals such as high-speed CD-ROM drives, DVD drives, fax/modem capability, and a multitude of different plug-in PC cards. Each of these features and improvements creates demand for power from system batteries. Many portable electronics, such as MP3 players and personal digital assistants, now use rotatable data storage devices as well, and by their nature and size place great demands for power on batteries.  
      The rotation of disks, within rotatable storage drives of portable computing devices, consumes power from the batteries. Accordingly, many manufacturers of rotatable data storage devices have reduced demand on batteries by employing power savings schemes. For example, many manufacturers perform a ramp unload, and spin down the rotating storage medium to rest, after a period of inactivity. While such schemes have been useful for extending battery life, it would be beneficial if the rotation of disks could be further optimized to save additional power.  
      As mentioned above, a head is typically not released from a ramp structure until the disk has achieved its normal operating spin-speed. This affects a drive&#39;s “time-to-ready,” meaning the time to which the head can start reading data from, or written data to, the disk. It would be beneficial if a disk drive&#39;s “time-to-ready” could be reduced. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES  
       FIG. 1  is a diagram showing components of an exemplary disk drive that can be used to implement embodiments of the present invention.  
       FIG. 2  shows additional details of the actuator assembly from  FIG. 1  and related elements.  
       FIG. 3  shows additional details of the voice coil motor (VCM) driver from  FIG. 1 , and related elements.  
       FIGS. 4A and 4B  shows additional details of the spindle motor (SM) driver from  FIG. 1 , and related elements.  
       FIG. 5  is an exemplary graph of disk spin-speed versus time, which is useful for explaining embodiments of the present invention.  
       FIGS. 6-9  are high level flow diagrams that are useful for describing various embodiments of the present invention.  
       FIG. 10  is an exemplary graph that shows the number of blocks to transfer (during a read or write operation) versus the amount of time necessary to perform the data transfer, assuming the disk is spinning up from rest.  
       FIG. 11  is a high level flow diagram that is useful for describing embodiments of the present invention in which a disk can be read from or written to at more than one spin-speed. 
    
    
     DETAILED DESCRIPTION  
      Embodiments of the present invention are useful for reducing the amount of time it takes for a head to begin reading from and/or writing to a disk, after an actuator assembly has been parked on a load/unload ramp and the disk has not been rotating. In accordance with an embodiment of the present invention, a disk spin-speed is monitored as a disk spins up from rest. The disk may have been at rest, for example, because the drive had just been powered on, or because the drive had been in a power saving (e.g., idle) mode. In accordance with an embodiment of the present invention, the drives “time-to-ready” is reduced by beginning a ramp load operation before the disk spin-speed reaches its nominal spin-speed, and in accordance with specific embodiments, even before the disk spin-speed achieves a minimum float spin-speed for causing the head to float over a surface of the disk. However, before explaining further details and embodiments of the present invention, it is first useful to describe exemplary environments in which embodiments of the present invention can be useful.  
       FIG. 1  shows an exemplary disk drive  100 , which includes at least one rotatable storage medium  102  (i.e., disk) capable of storing information on at least one of its surfaces. In a magnetic disk drive as described below, the storage medium  102  is a magnetic disk. The numbers of disks and surfaces may vary from disk drive to disk drive. A closed loop servo system, including an actuator assembly  106 , can be used to position a head  104  over selected tracks of the disk  102  for reading or writing, or to move the head  104  to a selected track during a seek operation. In one embodiment, the head  104  is a magnetic transducer adapted to read data from and write data to the disk  102 . In another embodiment, the head  104  includes separate read and write elements. For example, the separate read element can be a magnetoresistive head, also known as an MR head. It will be understood that various head configurations may be used with embodiments of the present invention.  
      A servo system can include a voice coil motor driver  108  to drive a voice coil motor (VCM)  130  for rotation of the actuator assembly  106 , a spindle motor driver  112  to drive a spindle motor  132  for rotation of the disk  102 , a microprocessor  120  to control the VCM driver  108  and the spindle motor driver  112 , and a disk controller  128  to accept information from a host  122  and to control many disk functions. The host  122  can be any device, apparatus, or system capable of utilizing the disk drive  100 , such as a personal computer or Web server. The disk controller  128  can include an interface controller in some embodiments for communicating with the host  122 , and in other embodiments a separate interface controller can be used. Servo fields on the disk  102  are used for servo control to keep the head  104  on track and to assist with identifying proper locations on the disk  102  where data is written to or read from. When reading servo fields, the head  104  acts as a sensor that detects position information to provide feedback for proper positioning of the head  104 .  
      The microprocessor  120  can also include a servo system controller, which can exist as circuitry within the drive or as an algorithm resident in the microprocessor  120 , or as a combination thereof. In other embodiments, an independent servo controller can be used. Additionally, the microprocessor  120  may include some amount of memory such as SRAM, or an external memory such as SRAM  110  can be coupled with the microprocessor  120 . The disk controller  128  can also provide user data to a read/write channel  114 , which can send signals to a current amplifier or preamp  116  to be written to the disk  102 , and can send servo signals to the microprocessor  120 . The disk controller  128  can also include a memory controller to interface with memory  118 . Memory  118  can be DRAM, which in some embodiments, can be used as a buffer memory.  
      Although shown as separate components, the VCM driver  108  and spindle motor driver  112  can be combined into a single “hard disk power-chip.” It is also possible to include the spindle speed control circuitry in that chip. The microprocessor  120  is shown as a single unit directly communicating with the VCM driver  108 , although a separate VCM controller processor (not shown) may be used in conjunction with processor  120  to control the VCM driver  108 . Further, the processor  120  can directly control the spindle motor driver  112 , as shown. Alternatively, a separate spindle motor controller processor (not shown) can be used in conjunction with microprocessor  120 .  
       FIG. 2  shows some additional details of the actuator assembly  106 . As shown in  FIG. 2 , the actuator assembly  106  includes an actuator arm  204  that is positioned proximate the disk  102 , and pivots about a pivot point  206  (e.g., which may be an actuator shaft). Attached to the actuator arm  204  is the read/write head  104 , which can include one or more transducers for reading data from and writing data to a magnetic medium, an optical head for exchanging data with an optical medium, or another suitable read/write device. Also, attached to the actuator arm  206  is an actuator coil  210 , which is also known as a voice coil or a voice actuator coil.  
      The voice coil  210  moves relative to one or more magnets  212  when current flows through the voice coil  210 . The magnets  212  and the actuator coil  210  are parts of the voice coil motor (VCM)  130 , which applies a force to the actuator arm  204  to rotate it about the pivot point  206 . The actuator arm  204  includes a flexible suspension member  226  (also known simply as a suspension). At the end of the suspension  226  is a mounted slider (not specifically shown) with the read/write head  104 .  
      The VCM driver  108 , under the control of the microprocessor  120  (or a dedicated VCM controller, not shown) guides the actuator arm  204  to position the read/write head  104  over a desired track, and moves the actuator arm  204  up and down a load/unload ramp  224 . The ramp  224  will typically include a latch (not shown) to hold the actuator arm  204  when in the parked position. The drive  100  also includes crash stops  220  and  222 . Additional components, such as a disk drive housing, bearings, etc. which have not been shown for ease of illustration, can be provided by commercially available components, or components whose construction would be apparent to one of ordinary skill in the art reading this disclosure.  
      Disk(s)  102  generally rotate at a constant set rate ranging from 3,600 to 15,000 RPM, with speeds of 4,200 and 5,400 RPM being common for hard disk drives designed for mobile environments, such as laptops. The actuator assembly sweeps an arc between the inner and outer diameters of the disk  102 , that combined with the rotation of the disk  102  allows a read/write head  104  to access approximately an entire surface of the disk  102 . The head  104  reads and/or writes data to the disks  102 , and thus, can be said to be in communication with a disk  102  when reading or writing to the disk  102 . Each side of each disk  102  can have an associated head  104 , and the heads  104  are collectively arranged within the actuator assembly such that the heads  104  pivot in unison. As mentioned above, the spinning of the disk  102  creates air pressure beneath the slider to form a micro-gap of typically less than one micro-inch between the disk  102  and the head  104 .  
       FIG. 3  shows exemplary details of the VCM driver  108  of  FIG. 1  as connected to the VCM  130 . As shown, the exemplary VCM driver  108  includes a VCM current application circuit  350 , which applies current to the coil  210  of the VCM  130  with a duration and magnitude controlled based on a signal received from the microprocessor  120  (or separate VCM controller). The coil  210  is modeled in  FIG. 3  to include a coil inductance L coil , a coil resistance R coil  and a back emf voltage generator  370 . Current provided through the coil  210  controls movement of a rotor  350 , and likewise movement of the rotor  350  generates a back emf voltage in the back emf voltage generator  370 .  
      The VCM driver  108  further includes a back emf detection circuit  352  for sensing the velocity of the actuator arm  204  based on an estimate of the open-circuit voltage of the VCM  130 . The open-circuit voltage of the VCM  130  is estimated by observation of the actual VCM voltage and the VCM current (either the commanded current or the sensed current, sensed using a series sense resistor R sense ), and multiplication of the current by an estimated VCM coil resistance (R coil ) and subtraction of that amount from the measured coil voltage. Referring briefly back to  FIG. 2 , during shut down, the actuator arm  204  is positioned on the ramp  224  situated off to the side of the disk  102  to prevent contact between the head  104  and the disk  102 . During startup, actuator velocity down the ramp  224  is controlled using measurements from the VCM back emf detection circuit  352  to ensure that the head  104  “flies” or “floats” when it gets to the bottom of the ramp  224  and does not contact the disk  102 .  
       FIG. 4A  shows exemplary details of the spindle motor  132  supporting a rotor shaft  470 , and the spindle motor driver circuit  112 . The exemplary spindle motor  132  includes a coil  460  with three windings  462 ,  464  and  466  electrically arranged in a Y configuration. A rotor  468  of the spindle motor  132  has magnets that provide a permanent magnetic field. The spindle motor driver circuit  112  supplies current to windings  462 - 466  to cause the rotor  468  to rotate at a desired operating spin-rate. The spindle motor driver  112  includes a commutation and current application circuit  450  to apply different commutation state currents across windings  462 - 466  at different times. The commutation and current application circuit  450  applies the commutation state currents based on signals received from the microprocessor  120 . The microprocessor  120  monitors the time period between back emf zero crossings using a spindle motor back emf detector  452  and uses this time period information to determine the speed of spindle motor  132 . The speed indication can then used by the microprocessor  120  (or separate SM controller) to control the commutation voltages applied across windings  462 - 466  to accomplish a desired speed. In accordance with embodiments of the present invention described below, the speed indication can also be used by the microprocessor  120  for triggering certain events, such as initiation of a ramp load operation.  
       FIG. 4B  shows an alternative configuration of the spindle motor driver circuit  112 . As shown, the commutation and current application circuit  450  receives the back emf zero crossing signals from the spindle motor back emf detector  452 . In this embodiment, the commutation circuit  450  includes circuitry to calculate the current application states needed to obtain a desired speed based on a spindle motor speed indication determined from the spindle motor back emf detector  452  (during steady-state operation; during open-loop startup, commutation states are determined internally or provided from the microprocessor  120 ). In the embodiment of  FIG. 4B , some (or all) of the processing that was performed by the microprocessor  120  in the configuration of  FIG. 4A , is included in the commutation circuit  450 . Thus, in the configuration of  FIG. 4B , the microprocessor  120  may only provide clocking or desired spindle motor speeds to the commutation circuit  450 .  
      In the configurations of both  FIGS. 4A and 4B , measurements of spindle motor speed, and thus of disk spin-speed, can be made using the spindle motor back emf detector  452 . The spindle motor back emf information is provided to the microprocessor  120 . After the head  104  is over the disk  102 , the processor  120  can also determine spin-speed using the servo data from disk  102 .  
      Reducing Time-to-Ready  
      Referring back to  FIG. 2 , the load/unload ramp  224  is used to hold the head  104  away from a disk surface while the disk  102  is not spinning. As mentioned above, conventionally the head  104  is not released from the ramp (i.e., does not begin moving down the ramp  224 ) until the disk  102  has achieved its normal operating spin-speed. This effects a drive&#39;s “time-to-ready,” meaning the time to which data can start being read from, or written to, a disk. Embodiments of the present invention, discussed with reference to  FIGS. 5-7 , are used to reduce a disk drive&#39;s “time-to-ready.” 
       FIG. 5  is an exemplary graph of disk spin-speed versus time. Three different spin-speeds are labeled on the vertical axis, including nominal spin-speed  506 , minimum “float” spin-speed  504 , and a spin-speed  502  when a ramp load operation begins, in accordance with an embodiment of the present invention. The nominal spin-speed  502  is the spin-speed at which the drive is designed to normally read and write data. The nominal spin-speed  502  is shown as occurring at a time  516 , labeled t nom , after the disk begins to spin-up from rest. The minimum “float” spin-speed  504 , is the minimum spin-speed at which a sufficient air-gap is formed between the slider and the disk&#39;s surface to prevent the head  104  from contacting the disk  102  following a ramp load operation (i.e., when the head  104  leaves the ramp  224  and is positioned over an outer diameter of the surface of disk  102 ). The minimum float spin-speed is shown as occurring at a time  514 , labeled t min . The minimum float spin-speed  504 , which is typically be about two-thirds of the nominal spin-speed  502 , can be determined through simple experiments. As will be discussed in more detail below, in accordance with some embodiments of the present invention, a drive is designed to read and write at more than one spin-speed (e.g., at a reduced spin-speed and a nominal spin-speed). In such embodiments, it is assumed that the nominal spin-speed is higher than the lowest spin-speed at which the drive can read and write.  
      A ramp load operation is the operation in which the head  104  is moved from a parked position on the ramp  224 , down the ramp  224  toward the disk  102 , and eventually over the surface of the disk  102 . The terms “on the ramp” and “down the ramp” are not meant to convey that the head itself contacts the ramp  224 . Rather, it is more likely that a lift tab (not shown) or similar lifting feature associated with the head suspension assembly, engages the ramp  224 . A ramp load operation begins at the time when the head  104  begins to moved from the parked position toward the disk  102 .  
      Conventionally, a ramp load operation is not began until the disk  102  has achieved its nominal spin-speed  502 . In accordance with an embodiment of the present invention, discussed below with reference to the high level flow diagram of  FIG. 6 , the ramp load operation begins at a spin-speed  506  that is less than the minimum float speed  504 . The time at which the load operation begins is shown as occurring at a time  512 , labeled toad.  
      Referring to  FIG. 6 , the spin-speed of the disk (i.e., the disk spin-speed) is monitored as the disk spins-up from rest, as specified at a step  602 . The disk may have been at rest because the disk-drive was just powered up, or because the disk drive was in a power saving mode in which the disk was spun down to rest. As mentioned above, the head suspension assembly is parked on the ramp while the disk is at rest.  
      At a next step  604 , a ramp load operation is began before said disk spin-speed reaches a minimum float spin-speed for causing the head to float over a surface of the disk. This includes beginning to move the head from a parked position on the ramp toward the disk. The initiating of the ramp load operation can be triggered when the disk spin-speed reaches a threshold spin-speed, that is less than said minimum float spin-speed, yet fast enough that the spin-speed will reach the minimum float speed by the time the head reaches the outer diameter of the disk.  
      In accordance with an embodiment of the present invention, tests are performed to determined the amount of time it takes for the disk to spin from rest up to the minimum float spin-speed. Tests are also performed to determine the amount of time it takes for the head to move from the parked position to the outer diameter of the disk, once a ramp load operation is initiated (i.e., begins). Using the results of these tests, a graph of time versus spin-speed, resembling the graph in  FIG. 5 , can be produced. A determination can be made of the earliest time (and corresponding earliest spin-speed) at which the ramp load operation can begin, such that the minimum float speed will be achieved just prior to the head reaching the outer diameter of the disk.  
      For example, starting from rest, assume that it takes 2.000 seconds for the disk to reach its nominal spin-speed, yet only 1.500 seconds for the disk to reach its minimum float spin-speed. Also assume that it takes 200 msec. (i.e., 0.200 seconds) for the head to move from the parked position to the outer diameter of the disk during a ramp load operation. Thus, if a ramp load operation begins at, or slightly later than, 1.300 seconds after the disk begins to spin-up from rest, then the disk should achieve the minimum float speed by the time the head moves from the parked position to the outer diameter of the disk. Also assume that the nominal spin-speed of the disk is 5,400 rpm, and that the minimum float speed is 4,000 rpm. Further, assume that at 1.300 seconds after the disk begins to spin-up from rest that disk achieves a spin-speed of 3,200 rpm. Using these exemplary assumptions, the threshold spin-speed could be set at (or slightly above) 3,200 rpm. In other words, if the ramp-load operation is initiated when the disk spin-speed reaches 3,200 rpm, then the disk will have just achieved the minimum float spin-speed of 4,000 rpm by the time the head reaches the outer diameter of the disk. Then, as soon as the disk reaches its nominal spin-speed, the head should ready for servoing and reading and/or writing. Or, if reading and writing can occur at less than the nominal spin-speed, e.g., at a reduce spin-speed, as explained below, then the head will be ready to read and write once the reduced spin-speed is reached.  
      It is also possible, in accordance with an embodiment of the present invention, that the drive can use measures of back EMF to get the heads close to a desired track on the disk (for reading or writing), even before the disk reaches a spin-speed at which the drive can begin servoing (e.g., the nominal spin-speed). For example, if the drive knows that the first track to read from or write to is near the inner diameter of the disk, then the drive can use measures of back EMF to guide the head toward a location near the inner diameter of the disk such that once the disk reaches a desired spin-speed (e.g., the nominal spin-speed) at which it can begin servoing, there is less distance that the actuator arm will need to travel in order to place head over the desired track.  
      In the above manners, the time-to-ready can reduced. It is noted that the above mentioned spin-speeds and times are just exemplary values, which are not meant to be limiting.  
      To the knowledge of the inventor, a disk drive&#39;s nominal spin-speed has always been higher than the minimum float spin-speed. Also, to the knowledge of the inventor, reading and/or writing have not been performed at less than a disk drive&#39;s nominal spin-speed. In accordance with embodiments of the present invention, a disk drive is adapted to perform reading and/or writing at the minimum float spin-speed, and/or at some other spin-speed that is less than the nominal spin-speed. Such embodiments are discussed below in the “Multiple Spin-speed” section. As will be appreciated from the discussion of the multiple spin-speed embodiments, it can be beneficial to combine the above discussed embodiments (relating to early initiation of a ramp load operation) with the embodiments relating to multiple spin-speeds.  
      As explained above with reference to  FIGS. 4A and 4B , the disk spin-speed can be monitored by monitoring the spin-speed of the spindle motor  132  that rotates the disk  102 . This can be accomplished, for example, by using measurements of back EMF determined by the spindle motor back emf detection circuit  452 . Alternative schemes for monitoring disk spin-speed, while the head  104  is not over the disk  102 , can also be used.  
      In accordance with a further embodiment of the present invention discussed with reference to the flow diagram of  FIG. 7 , the spin-up time since the disk began to spins-up from rest is monitored, at a step  702 . In this manner, at a step  704  a ramp load operation is initiated before the spin-up time reaches a predicted time at which the disk achieves a minimum float spin-speed for causing the head to float over a surface of the disk. The threshold spin-up time should be selected such that the disk will achieve the minimum float spin-speed prior to the head reaching an outer diameter of the disk. For example, a ramp load operation can be initiated when time since spin-up began reaches a threshold spin-up time, which that is less than the predicted time at which the disk will achieve the minimum float spin-speed. Using the exemplary values just discussed above, the threshold spin-up time can be 1.300 seconds, for example.  
      In the above described embodiments, the head  104  should reach the outer diameter of the disk  102  after the disk spin-speed reaches the minimum float spin-speed, but before the disk reaches the nominal spin-speed. In accordance with alternative embodiments of the present invention, the ramp load operation can be initiated prior to the disk spin-speed reaching nominal spin-speed, but such that by the time the head  104  reaches the outer diameter of the disk  102 , the disk spin-speed will have reached the nominal spin-speed (which is always at least as great as the minimum float spin-speed). In these embodiments, summarized in the flow diagrams of  FIGS. 8 and 9 , it is possible that the ramp load operation may not even begin until after the disk spin-speed reaches the minimum float spin-speed. Nevertheless, the time-to-ready will still be less than if the ramp load operation did not begin until after the disk spin-speed reached the nominal spin-speed. In such embodiments, the time since spin-up from rest and/or the spin-speed can be monitored, as discussed above. Similarly, a spin-up speed threshold or a time threshold (since spin-up began) can be used as the triggering threshold to begin a ramp load operation. In another embodiment, where reading and writing can occur at a reduced spin-speed (which is less than the nominal spin-speed), the ramp load operation can be initiated prior to the disk spin-speed reaching the reduced spin-speed, but such that by the time the head  104  reaches the outer diameter of the disk  102 , the disk spin-speed will have reached the reduced spin-speed.  
      For all of the embodiments discussed above, it would be useful to have a safety feature that causes the head  104  to retract back up the ramp  224  if the head is about to reach the outer diameter of the disk, but the disk has not yet reached the minimum float spin-speed (or some other predetermined spin-speed). Generally, the exact location of the head  104  is difficult to determine while the head suspension assembly moving along the ramp  224 . However, there are certain ways that a location of the head can be approximated. For example, the location of the head  104  can be estimated using angular velocity measurements that are made, e.g., using the VCM back emf detection circuit  352 , discussed with reference to  FIG. 3 . Such measurements of angular velocity can be used to estimate where the head  104  is along the ramp  224 . Most ramps  224  have a flat portion followed by a declined portion adjacent the outer diameter of the disk  102 . When the head  104  begins to move down the declined portion of the ramp  224 , the velocity of the head  104  will typically increase, as compared to when the head is moving along the flat portion of the ramp  224 . This increase in acceleration can be detected by the VCM back emf detection circuit  352 . In these manners, the location of the head  104  relative to ramp  224  can be estimated. In accordance with an embodiment of the present invention, if the disk spin-speed has not reached the minimum float spin-speed (or some other predetermined threshold) by the time the head  104  reaches a predefined location with respect to the ramp  224 , then the VCM driver  108  is instructed to abort the ramp load operation for the time being, and to move the head back up the ramp  224  (e.g., to the parked position).  
      Other schemes for determining the position of the head  104  relative to the ramp  224  can alternatively or additionally be used, in order to determine whether to abort the ramp load operation. For example, it would be possible to use the teachings in commonly assigned U.S. patent application Ser. No. 10/349,798, entitled “Ramp Arrangement and Method for Measuring the Position of an Actuator Arm in a Rotating Media Storage Device,” which is incorporated herein by reference. As taught in the just mentioned application, the ramp  224  can be electrically connected with the actuator assembly (e.g., with the lift tab or suspension), such that a closed circuit is formed when a portion of the actuator assembly contacts a portion of the ramp  224 . The closed circuit has a resistance that varies when the actuator assembly moves along the ramp, as would the resistance in a potentiometer including an adjustable wiper. By measuring the resistance, or changes in resistance, the position of the head  104  relative to the ramp  224  can be determined.  
      Multiple Spin-Speeds  
      As mentioned above, the minimum float spin-speed for a disk drive is typically about two-thirds of the disk drive&#39;s nominal spin-speed (i.e., the spin-speed at which the drive is designed to read and write data). Typically, the faster the nominal spin-speed, the faster data can be read from and written to a disk. However, the faster the nominal spin-speed, the more power necessary to spin the disk. Accordingly, the nominal spin-speed selected for use in a portable computing device (and more specifically, in a rotatable data storage device of the portable computing device) is typically selected with both performance and power consumption in mind. In other words, the nominal spin-speed of the disk drive can be selected to provide an acceptable trade-off between performance and power.  
      Embodiments of the present invention, which shall now be explained, also relate to methods and systems in which more than one spin-speed is used for reading from and writing to a disk. More specifically, in accordance with an embodiment of the present invention, a reduced spin-speed is used in certain situations, wherein the reduced spin-speed is greater than or equal to the minimum float spin-speed, yet less than the nominal spin-speed. It should be understood that for each of the embodiments involving multiple spin-speeds, the read/write channel (e.g.,  114 ) of the rotatable media storage device implementing the inventions should be designed to operate at multiple frequencies (i.e., one for each spin-speed).  
      There are many instances where a host may need read a small amount of data from, or write a small amount of data to, a disk which has been spun down to rest. In such instances, it may often be faster and less power consuming for the host to perform the read or write operation while the disk rotates at a spin-speed that as less than its nominal spin-speed. This is because it takes less time to achieve the reduced spin-speed, thereby reducing the time-to ready, and enabling the host to begin reading and/or writing at an earlier point in time. Additionally, less power is consumed at the reduced spin-speed than would be consumed a higher nominal spin-speed.  
       FIG. 10  is an exemplary graph that shows the number of blocks to transfer (during a read or write operation) versus the amount of time necessary to perform the data transfer, assuming the disk is spinning up from rest. As can be seen from the graph, if the amount of data is less than X blocks, then it will take less time to transfer the data at the reduced speed. This is because the transfer can begin sooner (i.e., at time t 1 , as opposed to at time t 2 ) if the reduced spin-speed is used.  
      However, as can also be seen from the graph, if the amount of data is greater than X blocks, then it would be faster to wait until the disk reached the nominal spin-speed (at time t 2 ) to begin the data transfer. Nevertheless, even though an entire data transfer may occur faster if the transfer does not begin until the disk reaches the nominal spin-speed, there are other advantages to beginning a data transfer at the reduced spin-speed. For example, if a user makes a request for information stored as data on a disk that is at rest (i.e., not spinning), the response time (i.e., the time it takes to display at least a portion of the information to the user) may be quicker if the data is read from the disk at the reduced spin-speed. For a more specific example, a user of a laptop computer may click on a HELP button in an attempt to learn about certain features of a software program. Assume that the information necessary to display an initial HELP screen is stored on a disk that is at rest, and that the sooner the HELP screen can be displayed the better (from the standpoint of the user). If the information (relating to the initial HELP screen) is read from the disk as soon as the disk reaches the reduced spin-speed, then the user will experience a faster response time. Then, while the user provides more details about the help that they desire (e.g., as prompted by the initial HELP screen), the disk can be spun-up to the nominal spin-speed (although this is not necessary).  
      In accordance with an embodiment of the present invention, if less than a specific amount of work is to be performed (e.g., a transfer of less than X blocks of data) following a disk spinning up from rest, then the disk is spun up to its reduced spin-speed and the work is began at the reduced spin-speed.  
      Further embodiments of the present invention, which will now be described, provide systems and methods for controlling a spin-speed at which an initial data transfer occurs after a disk begins to spin-up from rest. The disk may have been at rest because the storage device was powered down (i.e., off), or because the disk was purposefully spun-down to conserve power (i.e., as part of a power saving scheme). In either situation, it is likely that the head is parked on a ramp when the disk is at rest, to prevent the head from contacting the disk, as was explained above.  
      As will be described with reference to  FIG. 11 , in accordance with embodiments of the present invention, whether the initial data transfer occurs at a reduced spin-speed or a nominal spin-speed can be based on whether an amount of work to be performed is less than a threshold. This determination, which is performed at a step  1102 , can be performed, for example, by a disk controller, by a microprocessor within a storage device, or by a host that is interfacing with the storage device, or any combination thereof. In accordance with an embodiment of the present invention, the amount of work can be defined at least in part by an amount of data to be transferred (i.e., read from or written to the disk). In accordance with another embodiment of the present invention, the amount of work can be defined at least in part by an estimated amount of time necessary to complete the work. A disk controller, microprocessor and/or host may perform such an estimate, e.g., using a lookup table, model, etc. These are just examples, which are not meant to limit the present invention.  
      The threshold, which can be predefined or selectable (e.g., by the host) should be consistent with how the amount of work is defined. For example, if the amount of work is defined by the amount of data blocks to be transferred between a disk and a host, then the threshold should specify a threshold number of data blocks. For example, referring back to  FIG. 10 , the threshold can be set as X data blocks, which is the point at which it would be faster to wait until the disk spins up to the nominal spin-speed to begin the data transfer, rather than begin the data transfer at the reduced speed.  
      As specified at step  1104 , if it is determined that the amount of work is less than the threshold, then the disk is spun-up to the reduced spin-speed, at which point the initial data transfer can begin. If, however, it is determined that the amount of work is greater than the threshold, then the initial data transfer does not begin until the disk is spun-up to the nominal spin-speed, as specified at step  1106 .  
      As was explained above with reference to  FIGS. 4A and 4B , the spindle motor back EMF detection circuit  452  can be used to monitor the spin-speed of the disk. Referring back to  FIG. 1 , the disk controller  128  can be used to be used to control the reading from and writing to the disk  102 . The microprocessor  120  and/or the VCM driver  108  can control the spin-speed of the VCM  130 , and thereby the spin-speed of the disk  102 .  
      While an initial data transfer is being performed at a reduce spin-speed (or after the transfer is complete), the storage device may receive instructions to perform additional work (e.g., transfer more data). Referring again to  FIG. 11 , in accordance with an embodiment of the present invention, if the addition work is requested, then the disk is spun-up from the reduced spin-speed to the nominal spin-speed, and the additional work (e.g., data transfers) are performed at the nominal spin-speed, as specified at step  1112 . For example, this can occur regardless of the extent of the additional work (as shown by the dashed line), or alternatively, only if the additional work is above the a threshold, as expressed by step  1108 . This threshold can be the same threshold used in step  1102 , or it may be that the additional work requested must be above a further threshold in order to cause the disk to be spun-up to the nominal spin-speed. In still another embodiment, the disk is spun up to the nominal spin-speed if the additional work causes a total amount of work to exceed a threshold (e.g., the threshold used in step  1102 , or some other threshold), or if a total amount of remaining work exceeds a threshold. If the additional work does not exceed a threshold, in accordance with how the threshold is defined, the spin-speed can be kept at the reduced spin-speed to save power, as specified at steps  1108  and  1110 . Further, a ramp unload operation can be performed and the disk can be spun back down to rest, if a defined period of inactivity is exceeded, e.g., following step  1104 ,  1106 ,  1110  or  1112 .  
      In accordance with another embodiment, an initial data transfer, after the disk has been at rest, is performed at the reduced spin-speed regardless of the amount of data to be transferred. Then the disk is spun up to the nominal spin-speed so that additional data transfers (if necessary) can be performed at the nominal spin-speed. Such an embodiment is especially useful in situations where a user makes a request for information that requires data to be read from a disk that is at rest at the time of the request, because at least a portion of the information can be presented to the user more quickly (than if the disk had to spin-up to the nominal spin-speed before performing a read). This was explained above in some more detail.  
      In accordance with some embodiments of the present invention, it is the host that decides the appropriate spin-speed at which to read and write to a disk. In such embodiments, the host instructs the storage device to achieve a specific spin-speed at which to perform data transfers (initial or otherwise). More generally, the drive will perform data transfers at instructed spin-speeds that are specified by the host.  
      The embodiments just described above, which relate to performing reads and writes at more than one speed, can be combined with the other embodiments discussed above relating to beginning ramp loads operations at lower spin-speeds and/or earlier times than is conventional. Such combinations can be used to further reduce a time-to-ready in a rotatable media storage device. For example, assume a storage device can read or write to a disk at both a reduced spin-speed and a nominal spin-speed, and that the reduced spin-speed is used for initial data transfers (e.g., if an initial data transfer does not exceed a threshold). In accordance with an embodiment of the present invention, a ramp load operation is initiated before the disk spin-speed reaches the reduced spin-speed, such that the disk will achieve the reduced spin-speed just prior to a head reaching an outer diameter of the disk.  
      It is also possible that more that two spin-speeds can be used. For example, there can be a low spin-speed, a medium spin-speed and a high spin-speed, or even more than three different spin-speeds. Thresholds, similar to those described above, can be used to select which of the spin-speeds should be used to perform initial data transfers and/or further data transfers following the initial data transfers. Unless otherwise specified, it should be assumed that the lowest spin-speed (i.e., a reduced spin-speed) is not the nominal spin-speed, and that any spin-speed greater than the lowest spin-speed may be defined as the nominal spin-speed.  
      While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention.  
      The present invention has been described above with the aid of functional building blocks illustrating the performance of specified functions and relationships thereof. The boundaries of these functional building blocks have often been arbitrarily defined herein for the convenience of the description. Alternate boundaries can be defined so long as the specified functions and relationships thereof are appropriately performed. Any such alternate boundaries are thus within the scope and spirit of the claimed invention.  
      The breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.