Abstract:
An embodiment of a disk drive power system is described. The system is operable such that during power interruption, the system taps electric power by rectifying the back-EMF generated across each winding of a DC motor and supplying power to a voice-coil motor to park a read/write head safely away from an associated magnetic disk surface.

Description:
PRIORITY CLAIM 
     The present application claims the benefit of Singapore Patent Application Serial No. 200801720-4, filed Feb. 28, 2008, which application is incorporated herein by reference in its entirety. 
     TECHNICAL FIELD 
     An embodiment relates to a system for rectifying the back-EMF voltages of a DC motor in a magnetic disk drive during power interruption to safely park and unload a read/write head. In particular, an embodiment relates to a system for adaptive rectification of a brushless DC motor back-EMF voltages. 
     BACKGROUND 
       FIG. 1  shows a typical magnetic disk drive  10 . As shown in  FIG. 1 , the disk drive  10  includes a disk stack of one or more magnetic disks  12  rotatable about a spindle  14 . The spindle  14  is driven by a spindle motor  16 , for example, a DC brushless motor. Data is written onto a magnetic disk by one or more associated read/write head(s)  18 , and data is read back using the same read/write head(s)  18 . The read/write head(s)  18  is/are attached to a suspension arm  20 . The suspension arm  20  forms part of an actuator  22 . The actuator  22  is pivotable about a pivot  24  and is driven by a voice coil motor (VCM)  26  disposed on an arm opposite the suspension arm  20 . As can be seen from  FIG. 1 , the VCM  26  actuates the actuator  22  such that a read/write head  18  moves across a surface of an associated magnetic disk  12  in a substantially radial manner. Near the outer edge of the disk stack is a load/unload ramp  30 . At a free end of each suspension arm  20  is a tab  28  extending beyond the read/write head(s)  18 . The tab  28  is operable to engage with the load/unload ramp  30  during parking of the actuator  22 . 
     In normal use, power is supplied from the mains to the disk drive  10 , and the position and velocity of both the actuator  22  and read/write head(s)  18  are determinable from the servo information stored on the associated magnetic disk  12  surface. During a proper shut down operation, power is still available for a disk drive controller to move the actuator  22  back to its parking position. In the parked position, each tab  28  at the end of the suspension arm  20  is made to ride up an inclined surface of the associated load/unload ramp  30  such that the read/write head  18  is lifted up from the surface of the associated magnetic disk  12 ; this operation is called “unloading” of the read/write head(s)  18 . When the disk drive  10  is powered up after a shut-down operation, the actuator  22  is moved from the parked position and the read/write head  18  is “loaded” onto the associated magnetic disk  12  surface. Unloading of the read/write head(s)  18  would reduce wear and tear on both a slider carrying each read/write head  18  and the associated magnetic disk surface. 
     During normal operation, the relative speed between a read/write head  18  and the associated disk surface creates an air cushion to lift the read/write head  18  a small height from the disk surface. Thus, each read/write head  18  relatively “flies” above the associated disk surface. In the event of a power interruption, the relative speed between the read/write head and the disk surface may not be sufficient to create an air cushion to fly the read/write head above the disk surface. Immediately after such a power interruption, the spindle motor  16  continues to rotate due to the momentum stored in the rotating magnetic disk(s)  12 . During this transient period, the spindle motor  16  acts as a generator and sinusoidal back electro-motive force (BEMF) is generated across each winding in the spindle motor  16 . Therefore, it is desirable to tap the motor BEMF voltages for parking the actuator  22  and unload the read/write head(s)  18  whilst the magnetic disk(s)  12  is/are still spinning. 
     Attempts have been made to tap the power from the BEMF to bring the actuator to its parking position during this transient period.  FIG. 2  illustrates a typical disk driver system  50 . As shown in  FIG. 2 , the system  50  includes a 3-phase DC spindle motor  16 , a commutation circuit  60  consisting of six power transistors UA, UB, UC, LA, LB, LC; a bridge rectifier  80  consisting of six Schottky diodes; and a VCM  26 . Not shown in  FIG. 2  is a motor driver  70 . During normal operation, the motor driver  70  controls the commutation circuit  60  to regulate the spindle motor  16  in one direction by turning ON the relevant power transistor UA, UB . . . LB, LC coupled to windings A, B and C in a cyclical manner. After a power interruption, the momentum in the rotating disks  12  causes the spindle motor  16  to generate sinusoidal BEMF voltages across each winding A, B, C. The BEMF voltages are passively rectified to supply power to the VCM  26  for parking the actuator  22 . Capacitor  90  is used to store any excess power, which may be required by the VCM  26  to complete parking of the actuator  22 . 
     Attempts have been made to tap the power from the motor BEMF. One approach given is a driver system  100  shown in  FIG. 3 . As shown in  FIG. 3 , the system  100  includes a 3-phase motor  116 , a motor driver  110  consisting of a H-bridge of 6 power transistors UA, UB, UC, LA, LB, LC; a commutator circuit  114 , a decoder and latch circuit  118 , a VCM  126 ; a FET isolation circuit  120  and a VCM control circuit  124 . The commutator circuit  114  consists of three comparators CPA, CPB, CPC. Each of the comparator CPA, CPB, CPC is coupled across the respective motor winding A, B, C. After power interruption, the mains supply Vcc is uncoupled by the isolation circuit  120  and the output of the commutator circuit  114  is decoded and latched by the circuit  118  such that the relevant power transistor UA, UB LB, LC is sequentially turned ON to allow the BEMF across each winding A, B, C to pass through to power the VCM  126 . The VCM control circuit  124  drives the VCM  126  in one direction or another in accordance with the desired direction of the read/write head  18 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Despite development in the prior art, it can be seen that there exists a need for a system to adaptively tap the motor BEMF to park the actuator  22  during a power interruption. 
       One or more non-limiting embodiments are discussed with reference to the accompanying drawings, in which: 
         FIG. 1  illustrates a conventional magnetic disk drive system; 
         FIG. 2  illustrates a conventional motor BEMF rectification system; 
         FIG. 3  illustrates another conventional motor BEMF rectification system; 
         FIG. 4  illustrates a motor BEMF rectification system according to an embodiment; 
         FIG. 5  illustrates an embodiment of a state machine for use with the system shown in  FIG. 4 ; 
         FIG. 6  illustrates the BEMF voltages generated by the various phases of the motor windings of the system shown in  FIG. 5 ; 
         FIG. 7  illustrates components of the rectified BEMF voltage of  FIG. 6 ; and 
         FIG. 8  illustrates a schematic of the state machine shown in  FIG. 5 . 
     
    
    
     DETAILED DESCRIPTION 
     One or more specific and alternative embodiments will now be described with reference to the attached drawings. It shall be apparent to one skilled in the art, however, that the disclosed subject matter may be practiced without such specific details. Some of the details may not be described at length so as not to obscure the description. For ease of reference, common reference numerals or series of numerals will be used throughout the figures when referring to the same or similar features common to the figures. 
     One embodiment provides a system for a disk drive during power interruption. The system comprises: a plurality of power transistor arranged in a H-bridge with respect to each winding in the DC motor on an upper motor control circuit; an equal plurality of power transistors arranged in a H-bridge with respect to each winding in the DC motor on a lower motor control circuit; and a comparator operable to couple across each of the plurality of power transistors; wherein, during power interruption, a back-EMF voltage in a first motor winding is operable to increase and cross a decreasing back-EMF in second motor winding at a voltage-crossing on the upper motor control circuit, and the comparator associated with the first motor winding is operable to turn ON when the comparator input is above the supply rail voltage, thereby turning ON the transistor associated with the comparator and allowing the voltage above the voltage-crossing to pass through the transistor and, at the same time, turning OFF the comparator associated with the third motor winding; wherein the back-EMF voltage in a first motor winding is operable to decrease and cross an increasing back-EMF voltage in a third motor winding at a voltage-crossing on the lower motor control circuit, and the comparator associated with the second winding is operable to turn ON when the comparator input is below the ground rail voltage, thereby turning ON the transistor associated with the comparator and allowing the voltage below the voltage-crossing to pass through the transistor and, at the same time, turning OFF the transistor associated with the second motor winding; and the disk drive system is operable to repeat the turning ON and OFF of the comparators in a cyclical manner according to the sequence of commutating of the DC motor. 
     In another embodiment, the comparator associated with the upper motor circuit in each winding is supply-rail compliant while the comparator associated with the lower motor control circuit in each winding is ground-rail compliant. 
     In another embodiment, the disk drive system comprises a full-wave rectifier, a state machine, and a filter clamping circuit. In one embodiment of the state machine, the state machine comprises a plurality of circuits, with each circuit comprising a D flip-flop and a pre-driver. In another embodiment, the filter clamping circuit is operable to provide a 2 μs pulse for clamping the trigger of the each comparator to the output of the respective D flip-flop. 
     In another embodiment of the disk drive system, the DC motor comprises 3-phase windings. 
       FIG. 4  shows a motor BEMF power system  200  according to an embodiment. A rectified BEMF voltage Vpwr is produced at a supply rail. As shown in  FIG. 4 , the BEMF power system  200  includes a 3-phase brushless DC motor  216 ; three upper transistors UA, UB, UC and three lower transistors LA, LB, LC for commutating three windings A, B, C of the DC motor  216 ; six comparators CP 1 , CP 2  . . . CP 6 , each coupled across an associated transistor UA, UB, . . . LB, LC. The drains of the upper transistors UA, UB, UC are coupled to the supply rail at the rectified BEMF voltage Vpwr, while the sources of the lower transistors LA, LB, LC are coupled to a ground rail. In addition,  FIG. 4  also shows the supply and ground rails coupled to a VCM  226  with a capacitor  290  coupled across the VCM  226 . In one embodiment, the transistors UA, UB . . . LB, LC are lateral doped metal-oxide semiconductor (LDMOS) transistors. As shown in  FIG. 4 , the BEMF power system  200  is isolated from the mains supply Vdd by a reversed diode. 
     The voltage comparators CP 1 , CP 3 , CP 5  coupled across the upper transistors UA, UB, UC, respectively, are supply-rail compliant type, i.e., the input of each comparator can be higher than the supply voltage. The voltage comparators CP 2 , CP 4 , CP 6  coupled across the lower transistors LA, LB, LC, respectively, are ground-rail compliant, i.e., the input of each comparator can be lower than the ground voltage. 
       FIG. 5  shows a state machine  240 . The state machine  240  defines the sequential logic for turning ON and OFF of the relevant transistors UA, UB . . . LB, LC to produce the rectified BEMF supply voltage Vpwr. As shown in  FIG. 5 , the output A H  of comparator CP 1  is input to the gate of transistor UA, the output B H  of comparator CP 3  is input to the gate of transistor UB, and the output C H  of comparator CP 5  is input to the gate of transistor UC. The inverting inputs of the comparators CP 1 , CP 3 , CP 5  are coupled to the BEMF supply voltage rail (Vpwr), and H denotes the high side of the rectification control circuit. Also as shown in  FIG. 5 , the output A L  of comparator CP 2  is output to the gate of transistor LA, the output B L  of comparator CP 4  is output to the gate of transistor LB, and the output C L  of comparator CP 6  is output to the gate of transistor LC. The non-inverting inputs of the comparators CP 2 , CP 4 , CP 6  are coupled to the ground rail, and L denotes the low side of the rectification control circuit. During periods of normal operation, the state machine  240  may be disabled from generator active signal levels for any of the gate signals. 
       FIG. 6  shows sinusoidal BEMF voltages V A , V B , V C  generated across each winding A, B, C of the DC motor  216 . These BEMF voltages V A , V B , V C  are idealized for ease of illustration only. Referring to the positive side of the BEMF voltages in  FIG. 6 , when phase A voltage V A  is increasing whilst phase C voltage V C  is decreasing, they cross at point  220 . Once the phase A voltage V A  is at a threshold voltage above the supply rail voltage Vpwr, the comparator CP 1  is turned ON and the output A H  of CP 1  changes from state  0  to state  1 . The state machine  240 , in turn, turns ON the gate of transistor UA and turns OFF the transistor UC. By turning ON the gate of the transistor UA, the phase A BEMF voltage V A  is coupled to the supply rail as a phase A component voltage. 
     When V A  is decreasing and phase B voltage V B  is increasing, they cross at point  224 . Once the phase B voltage V B  is at a threshold voltage above the supply rail voltage Vpwr, the comparator CP 3  is turned ON and the output B H  of CP 3  changes from state  0  to state  1 . In response, the state machine  240  turns ON the gate of the transistor UB and turns OFF the transistor UA. By turning ON the gate of the transistor UB, the phase B BEMF voltage V B  is coupled to the supply rail as a phase B component voltage. 
     Similarly, when phase C voltage V C  is increasing while V B  is decreasing, they cross at point  228  and the comparator CP 5  is turned ON and the output C H  of CP 5  changes from state  0  to state  1 . In response, the state machine  240  turns ON the gate of the transistor UC and turns OFF the transistor UB. By turning ON the gate of transistor UC, the phase C BEMF voltage V is supplied to the supply rail as a phase C component voltage. 
     On the negative side of the BEMF voltages, as seen in  FIG. 6 , a similar sequence of switching of the relevant transistors LA, LB, LC also takes place. As shown in  FIG. 6 , as V C  is decreasing and V B  is increasing and they cross at point  230 . Once V C  is at a threshold voltage below ground, the comparator CP 6  is turned ON and the output C L  of CP 6  changes from state  0  to state  1 . In response, the state machine  240  turns ON the gate of transistor LC and turns OFF the transistor LB. With the gate of transistor LC being turned ON, the phase C voltage V C  is supplied to the ground rail. 
     When V A  is decreasing and V C  is increasing, they cross at point  234 . Once V A  is at a threshold voltage below ground, the comparator CP 2  is turned ON and the output A L  of comparator CP 2  changes from state  0  to state/and, in response, the state machine  240  turns ON the gate of transistor LA and turns OFF the transistor LC. With the gate of transistor LA being turned ON, the phase A voltage V A  is supplied to the ground rail. 
     Similarly, when V B  is decreasing and V A  is increasing, they cross at point  238 . Once V B  is at a threshold voltage below ground, the comparator CP 4  is turned ON and the output B L  of CP 4  changes from state  0  to state  1  and, in response, the state machine turns ON the gate of transistor LB and turns OFF the transistor LA. With the gate of transistor LB being turned ON, the phase B voltage V B  is supplied to the ground rail. 
     By combining the sequences of turning ON and OFF of the transistors UA, UB . . . LB, LC on a time scale and passing the components of the phase A, B and C voltages through the full-wave rectifier,  FIG. 7  shows components of a full-wave rectification of the BEMF voltage Vpwr. The components of the BEMF voltage Vpwr is a cyclic sequence of the three-phase voltages that are greater in magnitude than the BEMF crossing voltage. As can be seen from  FIG. 6 , the components of the BEMF voltage Vpwr are cyclically: Ac, cB, Ba, aC, Cb and bA where A, B, C denote the high side voltage components and a, b, c denote the low side voltage components. 
     In one BEMF power system  200  according to an embodiment, full-wave rectification of the BEMF voltage Vpwr is adaptive to the crossing voltage between two BEMF voltages across the relevant windings in the DC motor  216 , i.e., the BEMF voltages above the crossing voltage are rectified and supplied to the supply rail irrespective of time. In other words, as the DC motor  216  slows down, the BEMF voltages progressively decrease and the BEMF voltage traces become stretched out, the BEMF power system  200  produces a full-wave rectification of the BEMF voltage Vpwr as long as the BEMF voltages are higher in magnitude than Vpwr. As can be seen, the BEMF power system  200  does not require any timing information, i.e., does not rely on timing of the relevant crossing voltages. The BEMF voltages in this embodiment are full-wave rectified and are supplied to the VCM for parking the read/write head(s) safely away from the associated disk surface(s), besides powering the comparator circuit, state machine circuit, and so on. 
     In one embodiment, the state machine  240  uses logic gates and flip flops.  FIG. 8  shows a schematic of the state machine  240  coupled to the outputs of the comparators CP 1 , CP 2  . . . CP 6 . As shown in  FIG. 8 , the state machine  240  is made up of six circuits. An input of each circuit is coupled to each output of the comparators CP 1  . . . CP 6 . Each circuit has a flip flop  242 , whose output is coupled to a pre-driver  246   a ,  246   b ,  246   c , where a, b, c refers to the phases. The output of each pre-driver  246   a ,  246   b ,  246   c  produces signals to drive the gates of the relevant upper and lower transistors UA, UB, . . . LB, LC. Within each pre-driver  246   a ,  246   b ,  246   c  is a logic circuit (not shown) to produce the required outputs to turn ON and OFF the relevant transistors UA, UB . . . LB, LC. In addition, a filter circuit  248   a ,  248   b ,  248   c  provides a masking signal  260  to the pair of comparators coupled to each motor phase windings A, B, C for masking off electrical noises. As shown in  FIG. 8 , the output of flip-flop  242  associated with the positive BEMF voltage is “clamped” to the input of the filter circuit  248   a  so as not to trigger the circuits on the negative BEMF side. Similarly, the output of the flip-flop  242  associated with the negative BEMF voltage is “clamped” to the input of the filter circuit  248   a  so as not to trigger the circuits on the positive BEMF side. In this way, the filter circuit  248   a ,  248   b ,  248   c  ensures that the state machine  240  does not mis-trigger. In one embodiment, the masking signal  260  is a 2 micro-second pulse. V supply  (which may be the same as V dd  in  FIG. 4 ) may disable the state machine  240  during normal operation when V supply  is not interrupted, or, V supply  may be separate from V dd . 
     In one embodiment, the power system  200 , the state machine  240  and associated controls are implemented on a single chip or chip-set disposed on a circuit board on each disk drive. The chip or chipset, disk drive motor and circuit board may be sold separately, partially assembled or fully assembled. Furthermore, the system  200  may include a computer that includes the disk drive. 
     While specific embodiments have been described and illustrated, it is understood that many changes, modifications, variations and combinations thereof could be made without departing from the scope of the disclosure. For example, while a 3-phase DC brushless motor is used in the description, the principle is applicable to a motor with four or more phases. 
     Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many modifications and alterations. Particularly, although one or more embodiments have been described with a certain degree of particularity, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment may be incorporated in any other embodiment as a general matter of design choice.