Abstract:
A system for low acoustic noise spindle motor commutation is disclosed. The system contains a control device that generates commutation control signals which cause spindle motor drivers to source or sink current through windings of a spindle motor. The windings create electromagnetic fields which induce rotational movement in a spindle motor rotor. Low acoustic noise snubber devices are coupled to each winding and are dynamically configurable by the control device to provide low spindle motor driver charging current upon initial application of power to the spindle motor, and reduced acoustic noise and back EMF-generated current and voltage surges at the spindle motor driver during spindle motor commutation.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Embodiments of this invention relate to Provisional Application Ser. No. 60/056,029, filed Sep. 2, 1997. The contents of that application are incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Embodiments of this invention relate generally to spindle motors of disk drives of the type generally used for storing digital data, and in particular embodiments to methods for generating low acoustic noise spindle motor commutation, and disk drive systems incorporating the same. 
     2. Description of Related Art 
     Modem computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. To access memory locations on a hard disk, a read/write head is positioned over the hard disk, and the hard disk rotates at an essentially constant velocity. By moving the read/write head radially over the rotating hard disk, all memory locations on the hard disk can be accessed. 
     A spindle motor coupled to a spindle and the hard disk is often used to rotate the hard disk at an essentially constant velocity. Spindle motors are often brushless DC motors, which develop torque by the interaction of radial magnetic fields produced by permanent magnets on the rotor and rotating radial magnetic fields produced by sequencing alternating currents in the multi-phase windings of the stator. Rotation of the rotor occurs as the rotor&#39;s magnetic fields, and hence its permanent magnets, “follow” the rotating magnetic fields of the stator. 
     A plurality of spindle motor drivers repetitively source and sink current through the stator windings to produce the alternating currents in the windings, with each winding&#39;s alternating current maintaining a fixed phase relationship with respect to the alternating currents in the other windings. This phasing of alternating current in the windings of a motor is known as commutation. Because each winding acts essentially as an inductor, as the current in each winding changes direction at the switching frequency of its corresponding spindle motor driver, the reduction of current flowing through the winding causes its magnetic field to collapse, producing a back electromotive force (EMF) across the winding. The back EMF causes a surge voltage and a corresponding surge current to appear at the spindle motor driver at the switching frequency of the spindle motor driver. The repetitive switching of current in the winding results in a fluctuating magnetic field, creating a voice-coil effect with small forces of attraction and repulsion between the winding and the adjacent housing causing slight vibrations and acoustic noise at the switching frequency. 
     Snubber circuits are often coupled between each winding and ground to reduce the surge voltages and currents caused by the back EMF. An example of a snubber circuit is disclosed in U.S. Pat. No. 4,334,254, incorporated herein by reference. Capacitive snubber circuits suppress voltage transients by supplying a discharge path between the inductive winding and ground, but require high charging currents when the spindle motor drivers are sourcing current into the windings. High charging current is undesirable because it increases the power dissipation of transistors in the spindle motor drivers, requiring more expensive larger-geometry transistors. 
     To minimize these charging currents, a resistance is often included in series with the snubber capacitance. The resistance provides a resistive charging path for the capacitor, slowing down the charging rate of the capacitor and decreasing the instantaneous current sourcing requirements of the spindle motor drivers. The reduced charging current rate is also beneficial because many spindle motor driver circuits rely on a specific current ramp profile during the initial application of power to the spindle motor to determine the start phase of commutation, and without the resistance the high charging currents would distort the current ramp profile and introduce commutation start-up errors. 
     However, the resistance also creates drawbacks. The resistance impedes the flow of current through the capacitance to ground during spindle motor commutation, reducing the ability of the capacitance to suppress voltage transients, surge currents, and acoustic noise. 
     SUMMARY OF THE DISCLOSURE 
     Therefore, it is an object of embodiments of the invention to provide a system and method for reducing the voltage transients, surge currents, and acoustic noise generated during spindle motor commutation by decreasing the resistance and increasing the capacitance of discharge paths between the windings and a reference voltage. 
     It is a further object of preferred embodiments of the invention to provide a system and method for reducing the acoustic noise generated during spindle motor commutation by decreasing the resistance and increasing the capacitance of discharge paths between the windings and a reference voltage, while causing minimal disturbance to the start-up current ramp profile and ensuring the proper start-up of commutation by increasing the resistance and decreasing the capacitance of the discharge paths during the initial application of power to the spindle motor. 
     These and other objects are accomplished according to a system for low acoustic noise spindle motor commutation. The system contains a control device that generates commutation control signals which cause spindle motor drivers to source or sink current through windings of a spindle motor. The windings create electromagnetic fields which induce rotational movement in a spindle motor rotor. Low acoustic noise snubber devices are coupled to each winding and are dynamically configurable by the control device to provide low spindle motor driver charging current upon initial application of power to the spindle motor, and reduced acoustic noise and back EMF-generated current and voltage surges at the spindle motor driver during spindle motor commutation. 
     These and other objects, features, and advantages of embodiments of the invention will be apparent to those skilled in the art from the following detailed description of embodiments of the invention, when read with the drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a spindle motor commutation system. 
     FIG. 2 is a schematic diagram of a spindle motor commutation system according to an embodiment of the present invention. 
     FIG. 3 is a schematic diagram of a low acoustic noise snubber device according to an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the following description of preferred embodiments, reference is made to the accompanying drawings which form a part hereof, and in which is shown by way of illustration specific embodiments in which the invention may be practiced. It is to be understood that other embodiments may be utilized and structural changes may be made without departing from the scope of the preferred embodiments of the present invention. For example, although the drawings reference a three-phase brushless DC motor with windings located on the stator, it is understood that other multi-phase implementations fall within the scope of preferred embodiments of the present invention, including excited rotor, hysteresis, and reluctance-type synchronous motors. In addition, other types of motors utilizing commutation, including induction motors, fall within the scope of preferred embodiments of the present invention, and that in alternate embodiments the windings may also be located on the rotor. Furthermore, although the description and drawings reference a disk drive spindle motor commutation system, snubber devices according to embodiments of the present invention may be used with any system where acoustic noise is generated by repeatedly switching the direction of current flow through inductive loads. 
     Modern computers require media in which digital data can be quickly stored and retrieved. Magnetizable (hard) layers on disks have proven to be a reliable media for fast and accurate data storage and retrieval. Disk drives that read data from and write data to hard disks have thus become popular components of computer systems. To access memory locations on a hard disk, a read/write head is positioned over the hard disk, and the hard disk rotates at an essentially constant velocity. By moving the read/write head radially over the rotating hard disk, all memory locations on the hard disk can be accessed. A spindle motor coupled to a spindle and the hard disk is often used to rotate the hard disk at an essentially constant velocity. 
     FIG. 1 illustrates a prior art example of a spindle motor commutation system  2  for use in a disk drive system. The spindle motor commutation system  2  comprises a control device  34 , spindle motor driver circuit  4  containing a plurality of spindle motor drivers  6 , a plurality of snubber devices  8 , and a spindle motor  60  containing a spindle motor rotor  58 , a spindle motor stator  14 , and a plurality of stator windings  16 . Each stator winding  16  is coupled between an output node  20  of a corresponding spindle motor driver  6  and a star node  18 . Each snubber device  8  comprises a snubber resistive device  10  and a first snubber capacitive device  12  having a first end  22  and a second end  24 . Each snubber resistive device  10  is coupled between the output node  20  of a corresponding spindle motor driver  6  and the first end  22  of a corresponding first snubber capacitive device  12 . Each second end  24  of the first snubber capacitive devices  12  is coupled to a reference voltage  26  (for example, ground). The control device  34  includes a plurality of commutation control terminals  54 , each coupled to an input node  56  of a single spindle motor driver  6 . 
     The operation of one stator winding  16  and associated spindle motor driver  6  and snubber device  8  will now be described. It should be noted that this description applies similarly to the other stator windings  16 , or windings located on the spindle motor rotor  58 . In operation, a current  28  flows through the stator winding  16 , either in the direction of the arrow (see FIG. 1) or opposing it. When current  28  flows in the direction of the arrow (“positive” current flow), the spindle motor driver  6  sources the current  28  and the output node  20  associated with the spindle motor driver  6  is at a voltage state greater than the reference voltage  26  (a “high” state). When current  28  flows in a direction opposing the arrow (“negative” current flow), the spindle motor driver  6  sinks the current  28  and the output node  20  associated with the spindle motor driver  6  is at a voltage state less than or equal to the reference voltage  26  (a “low” state). 
     When the direction of the current  28  is to be changed from negative to positive, the control device  34  configures the input node  56  of the corresponding spindle motor driver  6  to transition the output node  20  from a low state to a high state. Once the output node  20  is at a high state, current  28  begins to flow in the positive direction, and an additional charging current  30  flows through the corresponding snubber resistive device  10  to charge up the corresponding first snubber capacitive device  12 . The snubber resistive device  10  impedes and limits the flow of charging current  30 , minimizing the instantaneous output current and power dissipation requirements of the spindle motor driver  6 . 
     When the direction of the current  28  is to be changed from positive to negative, the control device  34  configures the input node  56  of the corresponding spindle motor driver  6  to transition the output node  20  from a high state to a low state. Once the output node  20  is at a low state, current  28  begins to flow in the negative direction. As the current  28  in the stator winding  16  changes direction, the temporary reduction of current  28  flowing through the stator winding  16  causes its magnetic field to collapse, producing a back electromotive force (EMF) across the stator winding  16 . The back EMF causes a surge voltage and a corresponding surge current to appear at the output node  20  of the spindle motor driver  6 . The snubber device  8  reduces these surge voltages and currents by providing a capacitance in the first snubber capacitive device  12  which resists sudden changes in voltage and by supplying a path for discharge current  32  to flow to the reference voltage  26 . However, the snubber resistive device  10  and any other resistance in the discharge path impedes the path of the discharge current  32  and limits the ability of the snubber device  8  to reduce the surge voltages and currents. 
     The current  28  through the stator winding  16  repetitively changes direction at a periodic rate, as controlled by the control device  34  and spindle motor driver  6 . The remaining two stator windings in the three-phase spindle motor stator  14  of FIG. 1 also repetitively change direction at the same frequency. However, the current  28  in each stator winding  16  changes direction at a time 120 degrees out of phase with respect to the current changes in each of the other stator windings  16 . This phased switching of current  28  in the stator windings  16  results in the generation of magnetic fields that appear to rotate about the spindle motor stator  14 , and is known as spindle motor commutation. Rotation of the spindle motor rotor  58  is produced by electromagnetic forces upon the spindle motor rotor  58  created by rotating electromagnetic fields generated by the spindle motor commutation. Spindle motors are often brushless DC motors, which develop torque by the interaction of radial magnetic fields produced by permanent magnets  62  on the spindle motor rotor  58  and the rotating radial magnetic fields of the spindle motor stator  14 . Rotation of the spindle motor rotor  58  occurs as the rotor&#39;s magnetic fields, and hence its permanent magnets  62 , “follow” the rotating magnetic fields of the spindle motor stator  14 . 
     FIG. 2 illustrates an example of a spindle motor commutation system  36  for use in a disk drive system according to an embodiment of the present invention. The description of FIG. 1 is generally applicable to FIG. 2, except for the elements and operation of the snubber device  8  in FIG.  1 . In FIG. 2, each snubber device  8  of FIG. 1 is replaced by a respective low acoustic noise snubber device  38 . Each low acoustic noise snubber device  38  comprises a snubber resistive device  10 , a first snubber capacitive device  12  having a first end  22  and a second end  24 , a second snubber resistive device  40 , and a switch  42  having an input terminal  44 , and output terminal  46 , and a control terminal  48 . In preferred embodiments of a low acoustic noise snubber device  38  shown in FIG. 3, the switch  42  is a low drain-source on-resistance metal oxide semiconductor field-effect transistor (MOSFET) switch of less than one ohm. Referring again to FIG. 2, each snubber resistive device  10  is coupled between the output node  20  of a corresponding spindle motor driver  6  and the first end  22  of a corresponding first snubber capacitive device  12 . Each second end of the first snubber capacitive devices  12  is coupled to a reference voltage  26 . Each second snubber capacitive device  40  is coupled between the first end  22  of the corresponding first snubber capacitive device  12  and the input terminal  44  of the corresponding switch  42 , while the output terminal  46  of the switch  42  is coupled to the reference voltage  26 . The control terminal  48  of each switch  42  is coupled to the start-up control terminal  50  of the control device  34 . In addition, a pull-down resistor  52  is coupled between the start-up control terminal  50  and the reference voltage  26 , to pull down the control terminals  48  of the switches  42  towards the reference voltage  26  when the start-up control terminal  50  is in its low state. 
     The second snubber capacitive device  40  is normally coupled in parallel with the first capacitive device  12  by having the control device  34  close the switch  42 . In doing so, the parasitic resistances of the first and second snubber capacitive devices  12  and  40  are paralleled, decreasing the equivalent resistance of the low acoustic noise snubber device  38  and further reducing current surges caused by the back EMF by providing a less resistive discharge path to the reference voltage  26 . In addition, the equivalent capacitance of the low acoustic noise snubber device  38  is increased, further reducing voltage surges caused by the back EMF. 
     Upon the initial application of power to the spindle motor, many spindle motor driver circuits trigger the start of spindle motor commutation by sensing a particular output current level being sourced by the spindle motor drivers. The current ramp profile must therefore be well-defined to ensure proper start up of commutation. However, low impedance in associated snubber devices allows the snubber capacitor to charge rapidly, which can produce an unacceptable current spike in the current ramp profile. In embodiments of the present invention, the addition of the second snubber capacitive device  40  lowers the overall resistance of the low acoustic noise snubber device  38  and may result in a sharp and unpredictable current ramp profile, creating problems in the start-up of commutation. 
     To avoid this problem, in an embodiment of the present invention the control device  34  generates a high voltage at a start-up control terminal  50  only during the initial application of power to the spindle motor. This high voltage at the start-up control terminal  50  configures the switches  42  to be open, effectively removing the second snubber capacitive device  40  from the low acoustic noise snubber device  38 . Without the second snubber capacitive device  40 , the overall resistance of the low acoustic noise snubber device  38  is increased while its capacitance is decreased, reducing the charging current  30  and producing a smoother current ramp profile for the spindle motor drivers  6 . The smooth current ramp allows proper commutation start-up for those spindle motor driver chips that rely on a consistent current ramp profile. 
     Once the spindle motor has attained a preset speed, the control device  34  generates a low voltage at the start-up control terminal  50 . A pull-down resistor  52  pulls down the low voltage at the start-up control terminal  50  towards the voltage reference  26 . This low voltage at the start-up control terminal  50  configures the switches  42  to be closed, effectively adding the second snubber capacitance  40  back into the low acoustic noise snubber device  38 . With the second snubber capacitance  40  in the circuit, the overall resistance of the low acoustic noise snubber device  38  is decreased while its capacitance is increased, increasing the discharge current  32  and reducing the acoustic noise and current and voltage surges seen by the spindle motor drivers  6 . 
     It should be understood that although the preceding discussion of an embodiment of the invention described a high voltage at the start-up control signal  50  to open the switch  42  upon the initial application of power, and a low voltage at the startup control signal  50  to close the switch  42  once a preset spindle motor speed is attained, in other embodiments of the invention the switch  42  may close when a high voltage appears at the start-up control signal  50 , and may open when a low voltage appears. In such embodiments, the start-up control signal  50  will be at a low voltage upon the initial application of power, and will be at a high voltage once a preset spindle motor speed is attained. 
     Therefore, according to the foregoing description, preferred embodiments of the present invention will result in proper start-up of spindle motor commutation upon the initial application of power to the spindle motor, and reduced current and voltage surges and acoustic noise during repetitive spindle motor commutation. 
     The foregoing description of preferred embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Many modifications and variations are possible in light of the above teaching. It is intended that the scope of the invention be limited not by this detailed description, but rather by the claims appended hereto.