Abstract:
Embodiments of the invention generally provide a method for detecting air contamination within a fluid dynamic bearing used with a disc drive. In one embodiment, the invention provides a method to determine the amount of air contamination with hydrodynamic fluid by comparing the differential displacement of the fluid dynamic bearing between vacuum and non-vacuum conditions. In another aspect, the invention provides an air-contamination detecting apparatus adapted to detect air contamination within the fluid of a fluid dynamic bearing. In another aspect, the invention provides a method to determine air contamination within fluid dynamic bearings using the change in fly height of one or more probes disposed above the surface of a rotating surface of the disc drive.

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This invention is based on U.S. Provisional Patent Application Ser. No. 60/342,605 filed Dec. 20, 2001, entitled “Air Detection Method In Fluid Dynamic Bearings Via Fly Height” filed in the name of Anthony Joseph Aiello and Klaus Dieter Kloeppel. The priority of this provisional application is hereby claimed. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates generally to the field of disc drives, and more particularly to an apparatus and method for detecting air contamination of fluid dynamic bearings within a disc drive. 
     2. Description of the Related Art 
     Disc drives are capable of storing large amounts of digital data in a relatively small area. Disc drives store information on one or more recording media. The recording media conventionally takes the form of a circular storage disc, e.g., media, having a plurality of concentric circular recording tracks. A typical disc drive has one or more discs for storing information. This information is written to and read from the discs using read/write heads mounted on actuator arms that are moved from track to track across surfaces of the discs by an actuator mechanism. 
     Generally, the discs are mounted on a hub that is turned by a spindle motor to pass the surfaces of the discs under the read/write heads. The spindle motor generally includes a shaft supported from a base plate of the housing. Permanent magnets attached to the hub interact with a stator winding to rotate the hub. One or more bearings usually support the hub for rotation. 
     Over time, disc drive storage density has tended to increase and the size of the storage system has tended to decrease. This trend has lead to greater precision and lower tolerance in the manufacturing and operating of magnetic storage discs. For example, to achieve increased storage densities the read/write heads must be placed increasingly close to the surface of the storage disc. 
     From the foregoing discussion, it can be seen that the bearing assembly which supports the storage disc is of critical importance. One typical bearing assembly comprises ball bearings supported between a pair of races which allow a hub of a storage disc to rotate relative to a fixed member. However, ball bearing assemblies have many mechanical problems such as wear, run-out and manufacturing difficulties. Moreover, resistance to operating shock and vibration is poor because of low damping. 
     One alternative bearing design is a fluid dynamic bearing. In a fluid dynamic bearing, a lubricating fluid such as air or liquid provides a bearing surface between a fixed member of the housing (i.e., shaft) and a rotating member of the disc hub. In addition to air, typical lubricants include oil or ferromagnetic fluids (i.e., hydrodynamic fluid). Fluid dynamic bearings spread the bearing interface over a large surface area in comparison with a ball bearing assembly, which comprises a series of point interfaces. This is desirable because the increased bearing surface reduces wobble or run-out between the rotating and fixed members. Further, the use of fluid in the interface area imparts damping effects to the bearing which helps to reduce non-repeat run-out. 
     Generally, during the manufacturing of the fluid dynamic bearings using oil or ferromagnetic fluids, the fluid dynamic bearing undergoes a lubricating fluid fill process. During the lubricating fluid fill process, air is inadvertently introduced into the lubricating fluid in the form of bubbles. Unfortunately, the bubbles may cause fluid pressure inconsistencies within the fluid dynamic bearing. Further, during operation, the air bubbles may expand increasing non-repeatable run-out between the rotating and fixed members. 
     Generally, for non-fluid dynamic bearings, such as stationary shaft and two piece hub-shaft motors, the meniscus of the fluid is checked under a vacuum for changes due to air contamination. That is, a change typically indicates the presence of a bubble, which expands. For example, a microscope may be used to visually check the fluid meniscus change in dimension when a vacuum is applied. If air is present in the non-fluid dynamic bearings, the meniscus width, height, etc., within the capillary may vary as a function of the amount of air present. 
     Generally, differential weight changes before and after the fill process are used to inspect the air contamination of fluid within a fluid dynamic bearing. Unfortunately, this methodology is time consuming and prone to measurement variation as the amount of air within the hydrodynamic fluid may be very small. Accordingly, the measurements may lead to an increase in disc drive manufacturing time, premature disc drive failure due to inaccurate measurements, and ultimately an increase in the cost of the disc drive. 
     Therefore, a need exists for a method and apparatus to provide a reliable and repeatable fluid dynamic bearing air-bubble contamination test. 
     SUMMARY OF THE INVENTION 
     The invention generally provides a method for detecting air-bubble contamination of fluid dynamic bearings used with a disc drive. In one embodiment, the invention generally provides a method of measuring air-bubble contamination within a fluid dynamic bearing of a disc drive. The method comprises rotating a disc drive motor, then determining at least one first value responsive to air-contamination within the fluid dynamic bearing. At least one second value is then compared to the at least one first value and if the at least one second value exceeds the at least one first value then the method determines if the at least one second value is an unacceptable disc drive operational condition. 
     In another embodiment, the invention provides a disc drive fluid dynamic bearing air contamination testing apparatus, comprising a vacuum chamber adapted to hold at least one disc drive. The testing apparatus also includes an apparatus adapted to detect change within the hydrodynamic bearing when the pressure within the vacuum chamber is changed from a first pressure value to a second pressure value while the motor is activated within the vacuum chamber. 
     In another embodiment, the invention provides an apparatus for measuring air contamination within a disc drive spindle motor fluid dynamic bearing. The apparatus comprising a means for rotating a disc drive motor and a means for detecting, during the rotation of the motor, at least one disc drive change as a function of air-contamination within the fluid dynamic bearing between at least a first and second atmospheric pressure on the fluid dynamic bearing by detecting change in fly height of a probe over a rotating surface within the disc drive. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       So that the manner in which the above recited embodiments of the invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
         FIG. 1  depicts a plan view of one embodiment of a disc drive for use with aspects of the invention. 
         FIG. 2  is a sectional side view depicting one embodiment of a spindle motor for use with aspects of the invention. 
         FIGS. 3A and 3B  depict a plan view and top view of one embodiment of a testing apparatus for use with aspects of the invention. 
         FIG. 4  depicts one embodiment of a method for determining air contamination for use with aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  depicts a plan view of one embodiment of a disc drive  10  for use with embodiments of the invention. Referring to  FIG. 1 , the disc drive  10  includes a housing base  12  and a top cover  14 . The housing base  12  is combined with top cover  14  to form a sealed environment to protect the internal components from contamination by elements from outside the sealed environment. The base and top cover arrangement shown in  FIG. 1  is well known in the industry. However, other arrangements of the housing components have been frequently used, and aspects of the invention are not limited to the configuration of the disc drive housing. For example, disc drives have been manufactured using a vertical split between two housing members. In such drives, that portion of the housing half which connects to the lower end of the spindle motor is analogous to base  12 , while the opposite side of the same housing member, which is connected to or adjacent the top of the spindle motor, is functionally the same as the top cover  14 . The disc drive further includes a disc pack  16  which is mounted for rotation on a spindle motor (not shown) by a disc clamp  18 . Disc pack  16  includes one or a plurality of individual discs that are mounted for co-rotation about a central axis. Each disc surface has an associated read/write head  20  which is mounted to disc drive  10  for communicating with the disc surface. In the example shown in  FIG. 1 , read/write heads  20  are supported by flexures  22  which are in turn attached to head mounting arms  24  of an actuator body  26 . The actuator shown in  FIG. 1  is of the type known as a rotary moving coil actuator and includes a voice coil motor (VCM), shown generally at  28 . Voice coil motor  28  rotates actuator body  26  with its attached read/write heads  20  about a pivot shaft  30  to position read/write heads  20  over a desired data track along a path  32 . While a rotary actuator is shown in  FIG. 1 , the invention may be used with other disc drives having other types of actuators, such as linear actuators. 
       FIG. 2  is a sectional view of a fluid dynamic bearing spindle motor  32  in accordance with the invention. Spindle motor  32  includes a stationary member  34 , a hub  36 , and a stator  38 . In the embodiment shown in  FIG. 2 , the stationary member is a shaft that is fixed and attached to base  12  through a nut  40  and a washer  42 . Hub  36  is interconnected with shaft  34  through a fluid dynamic bearing  37  for rotation about shaft  34 . Fluid dynamic bearing  37  includes a radial working surface  46  (e.g., journal surface) and axial working surfaces  48  and  50  (e.g., thrust surface). Shaft  34  includes fluid ports  54 ,  56 , and  58  which supply hydrodynamic fluid  60  and assist in circulating the fluid along the working surfaces of the fluid dynamic bearing  37 . The fluid dynamic bearing  37  may include a series of hydrodynamic grooves  35  positioned thereon. The hydrodynamic grooves  35  may be disposed upon the shaft  34 , and/or the hub  36  to facilitate the supply and distribution of the hydrodynamic fluid  60  to the radial and axial working surfaces  46 - 50 , of the fluid dynamic bearing  37 . The hydrodynamic grooves  35  may be configured any number of ways depending on the fluid dynamic bearing load requirements. For example, the hydrodynamic grooves  35  may include sinusoidal grooves, herringbone grooves, helix grooves, and other similar grooves. The spacing between the hydrodynamic grooves  35  is defined as the “land”  39  which may vary between the hydrodynamic grooves  35  to accommodate various fluid flow requirements. Hydrodynamic fluid  60  is supplied to shaft  34  by a fluid source (not shown), which is coupled to the interior of shaft  34  in a known manner. Spindle motor  32  further includes a thrust bearing  45 , which forms the axial working surfaces  48  and  50  of fluid dynamic bearing  37 . A counterplate  62  bears against working surface  48  to provide axial stability for the fluid dynamic bearing  37  and to position the hub  36  within spindle motor  32 . An O-ring  64  is provided between counterplate  62  and hub  36  to seal the fluid dynamic bearing  37 . The seal prevents hydrodynamic fluid  60  from escaping between counterplate  62  and hub  36 . Hub  36  includes a central core  65  and a disc carrier member  66  which supports disc pack  16  (shown in  FIG. 1 ) for rotation about shaft  34 . Disc pack  16  is held on disc carrier member  66  by disc clamp  18  (also shown in FIG.  1 ). A permanent magnet  70  is attached to the outer diameter of hub  36 , which acts as a rotor for a spindle motor  32 . Core  65  is formed of a magnetic material and acts as a back-iron for magnet  70 . Rotor magnet  70  can be formed as a unitary, annular ring or can be formed of a plurality of individual magnets which are spaced about the periphery of hub  36 . Rotor magnet  70  is magnetized to form one or more magnetic poles. Stator  38  is attached to base  12  and includes a magnetic field focusing member or lamination stack  72  and a stator winding  74 . Stator winding  74  is attached to back-iron  72  between back-iron  72  and rotor magnet  70 . Stator winding  74  is spaced radially from rotor magnet  70  to allow rotor magnet  70  and hub  36  to rotate about a central axis  80 . Stator  38  is attached to base  12  through a known method such as one or more C-clamps  76  which are secured to the base through bolts  78 . Commutation pulses applied to stator winding  74  generate a rotating magnetic field that communicates with rotor magnet  70  and causes hub  36  to rotate about central axis  80  on bearing  37 . In the embodiment shown in  FIG. 2A , spindle motor  32  is a “below-hub” type motor in which stator  38  is positioned below hub  36 . Stator  38  also has a radial position that is external to hub  36 , such that stator winding  74  is secured to an inner diameter surface  82  of lamination stack  72 . 
       FIGS. 3A and 3B  depict a simplified plan view and top view for one embodiment of an apparatus to detect air contamination in a fluid dynamic bearing  37  of a disc drive  10 .  FIGS. 1-2  are referenced as needed in the discussion of  FIGS. 3A and 3B . 
       FIGS. 3A and 3B  illustrate an air-detection apparatus  300  adapted to detect air-contamination in fluid dynamic bearings  37 . The air-detection apparatus  300  includes a back-end system  301  configured to supply power and control signals to activate and/or control at least some of the operations of the disc drive  10 . For example, the back-end system  301  may be adapted to provide power to the spindle motor  32 . Further, while in one aspect the back-end system  301  includes a vacuum pump (not shown) to draw a vacuum within a vacuum chamber  302  disposed thereon, it also contemplated that the vacuum chamber  302  may be coupled to an external vacuum source to provide the vacuum within the vacuum chamber  302 . The vacuum chamber  302  is configured to hold one or more assemblies such as the disc drive  10 , spindle motor  32 , and the like, therein to draw a vacuum therefrom. 
     In one aspect, the air-detection apparatus  300  includes an air-detection apparatus  304  disposed within the vacuum chamber  302 . The air-detection apparatus  304  includes a moveable detection arm  306  having a detection probe  310  thereon. The detection probe  310  includes a surface detection tool  312  such as a capacitance probe, laser depth tool, ultrasonic depth finder, and other devices adapted to measure distances between the surface detection tool  312  and one or more surfaces of disc drive  10 . In one aspect, the testing probe  310  detects the change in distance between the testing probe  310  and the hub  36 . For example, if the distance between the surface detection tool  312  and hub  36  or standoff distance is 250 microns, the surface detection tool  312  and detects changes relative of about 50 microns. In one configuration, the air-detection apparatus  304  is coupled to a data processing system (not shown) to receive/transmit data with respect to the distance changes. 
     In one aspect of the invention, the data processing system may include a computer or other controller adapted to analyze and display distance changes between the surface detection tool  312  and one or more rotating surfaces of disc drive  10 , and may display the data on an output device such as a computer monitor screen. In general, the data processing system may include a controller, such as programmable logic controller (PLC), computer, or other microprocessor-based controller. The data processing system may include a Central Processing Unit (CPU) in electrical communication with a memory, wherein the memory may contain an air-contamination detection program that, when executed by the CPU, provides support for controlling the air-detection apparatus  300 . In another aspect of the invention, the data processing system may provide control signals to the disc drive  10  as part of the process of measuring the air-contamination of the fluid dynamic bearings  37 . The air-contamination detection program may conform to any one of a number of different programming languages. For example, the program code can be written in PLC code (e.g., ladder logic), C, C++, BASIC, Pascal, or a number of other languages. 
       FIG. 4  depicts a flow diagram of a method  400  to detect air-contamination within fluid dynamic bearings  37 .  FIGS. 1-3  are referenced as needed with the discussion of FIG.  4 . Specifically, the method  400  starts at step  402  when a measurement process is, for example, initiated by a user activating an air-detection apparatus  304  configured to detect air-contamination within a fluid dynamic bearing  37 . At step  404  the testing probe  310  is positioned a desired distance with respect to a disc drive  10  disposed within the vacuum chamber  302  and a rotating surface such as the spindle motor  32 . In one configuration the spindle motor  32  is activated to rotate at a desired RPM between about 4200 and 10000, depending primarily on the working speed at which the disc operates. 
     At step  406 , the detection probe  310  is configured to establish the threshold levels for detecting air-contamination using parameters such as sample rate, sensitivity, and other factors required to operate the detection probe  310 . For example, a critical proximity change value between a spinning rotor  36  or a disc surface and a reference distance may be set as a threshold to flag the process controller of air-contamination within the fluid dynamic bearing  37 . At step  408 , the method  400  sets at least one reference distance between the surface being detected, and the detection probe  310 , under given atmospheric pressure levels within the vacuum chamber  302 , such as ambient air pressure. It is contemplated, the reference distance may be established any number of ways including, an RMS value of a plurality of detected distance values, a least squares regression model, and the like. Once the reference distance is stored, a vacuum of between about 1-500 Torr is pulled within the vacuum chamber  301  at step  410 . In one aspect, the method  400  operates the back-end  310  to establish a vacuum within the vacuum chamber  301 . 
     At step  412 , the method measures the change in one or more probe measurement values. In one aspect, the change in distance between the detection probe  310  and a rotating hub  36  is measured. As air-contamination within the hydrodynamic fluid  60  increases, the greater the deflection between the detection probe  310  and the rotating hub  36  as the volume of air expands within the hydrodynamic fluid  60 . At step  414 , the method  400  compares the amount of measured deflection to the reference value and to acceptable threshold values. If the changes in hub deflection do not exceed the reference and/or threshold values, the method  400  proceeds to step  418  described below. If the changes in hub deflection exceed the reference and/or threshold values, the method  400  proceeds to step  416  to issue a message, such as a failure message to the process controller, for example. At step  418 , the method  400  determines if the detection process is complete. If the detection process is complete, the method  400  proceeds to step  420  and exits. However, if the detection process is not complete, the method returns to step  404 . 
     While the foregoing is directed to embodiments of the invention, other and further embodiments of the invention may be devised without departing from the scope thereof, and the scope thereof is determined by the claims that follow. For example, the first could be made at a higher than atmospheric pressure, and the second at atmospheric pressure. Alternatively, the two measurements could be made at two artificial pressures. 
     Other variations may be adopted by those of skill in the art.