Abstract:
For repeatably and accurately measuring characteristics of workpieces, such angle of taper of a conical cavity defined by a workpiece, examples may include a system having a stage movable in predictable and repeatable increments, and a compliancy fixture. The compliancy feature may include a sapphire to metal interface. Such examples may also include a mechanism, such as an elongate body or an arm, for alternately disposing each of a first object of a first size and a second object of a second size in the conical cavity. Examples may use spherical objects, or portions thereof. Systems further include at least one sensor for determining positions of the first and second objects in the conical cavity and logic for determining the characteristics, such the taper angle, based on the position of the first object and the second object in the conical cavity.

Description:
BACKGROUND 
     1. Field 
     The present invention relates generally to metrology of objects, and more particularly to measuring angles and characteristics of cavities, such as cavities of conical sleeves that may be used in disc drives. 
     2. Description of Related Art 
     Magnetic disc drives are used for magnetically storing information. In a magnetic disc drive, a magnetic disc rotates at high speed and a transducing head “flies” over a surface of the disc. This transducing head records information on the disc surface by impressing a magnetic field on the disc. Information is read back using the head by detecting magnetization of the disc surface. The transducing head is moved radially across the surface of the disc so that different data tracks can be read back. 
     Over the years, storage density of media has tended to increase and the size of storage systems has tended to decrease. This trend has led to a need for greater precision, which has resulted in tighter tolerancing for components used in disc drives. In turn, achieving tighter tolerances in components requires increased precision in metrology systems for characterizing and parameterizing those components. Measuring angles of objects is one aspect of metrology, and measuring angles of conical cavities is of interest for some disc drive designs. 
     Metrology systems may include systems that use technology requiring contact with a workpiece as well as systems that obtain metrology data without contacting a workpiece. It is often the case that non-contact systems can be more precise than contact systems, but can be more expensive. Contact based systems can mar workpieces. What is needed is a low-cost, accurate, and repeatable metrology system that may be used for example in metrology of disc drive components. 
     SUMMARY 
     In an exemplary aspect, a contact-based metrology system comprises a fixture dimensioned to hold a workpiece, and a first object dimensioned to fit with at least a portion of the workpiece. The system further comprises a first structure mounting the first object to provide for movement of the first object along a path between a start position and an end position for the first object. The first object fits with at least a portion of the workpiece at the end position. The system also comprises a first sensor disposed to sense distance traveled by the first object along the path between the start position and the end position, and to produce a signal indicative of such distance. 
     In an exemplary aspect, a contact-based metrology method comprises relatively positioning a workpiece and a first object so that the first object may fit with the workpiece upon traveling a path for the first object, moving the first object along the path for the first object, and thereafter determining a position of the first object upon fitting the first object with the workpiece. The method also comprises relatively positioning the workpiece with a second object so that the second object may fit with the workpiece upon traveling a path for the second object. The method also comprises moving the second object along the path for the second object, determining a position of the second object upon fitting the second object with the workpiece. The method also comprises determining, based on the positions of the first object and the second object, a characteristic of the workpiece. 
     In another exemplary aspect, a contact-based metrology system comprises a stage for supporting a sleeve cone to provide accessibility to an inner cavity of the sleeve cone, a first object with a first size and a second object with a second size. Each of the first object and the second object are coupled to one or more supports for removably disposing each of the first object and the second object to touch a surface of the inner cavity of the sleeve cone. The system also comprises sensor equipment operatively interfacing with the one or more supports. The sensor equipment is for determining a position of the first object and a position of the second object in the inner cavity based on respective positions of the one or more supports. The system also comprises computational logic for deriving an angle of taper of the surface of the inner cavity based on the position of the first object and the position of the second object in the inner cavity. In any aspect or example discussed herein, the first and second objects may be portions of a sphere, entire spheres, or another suitable geometry. In some aspects involving metrology of sleeve cone workpieces, cone quality and cone taper angle may be measured. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a fuller understanding of aspects and examples disclosed herein, reference is made to the accompanying drawings in the following description. 
         FIG. 1  illustrates a partial section of a hydrodynamic bearing having conical sleeves; 
         FIG. 2  illustrates a concept of a two ball cone angle measurement technique; 
         FIGS. 3A-C  illustrate a diagram of an exemplary system implementing the two ball cone technique, and in succession the operation of the exemplary system from loading through measurement; 
         FIG. 4  illustrate a perspective view of the exemplary system; 
         FIG. 5  illustrates a stage component and fixture coupled with the stage component of the system; and 
         FIG. 6  illustrates exemplary method steps in the operation of the system. 
     
    
    
     DETAILED DESCRIPTION 
     The following description is presented to enable a person of ordinary skill in the art to make and use various aspects of the inventions. Descriptions of specific materials, techniques, and applications are provided only as examples. Various modifications to the examples described herein will be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the spirit and scope of the inventions. For example, aspects and examples may be employed in various metrology applications, including metrology of components of motors used in disc storage drives. Metrology equipment employing aspects disclosed herein may be designed and may operate in a number of ways. The exemplary apparatuses and systems provided herein are for illustrating various aspects and are not intended to limit the range of metrology apparatuses and systems in which in examples and aspects may be applied. 
       FIG. 1  illustrates a cross-section of a disc drive motor portion. The portion includes a hub  10  supporting discs  12 . In operation, the hub  10  rotates about a fixed shaft  14 . The fixed shaft  14  includes an upper shaft bearing cone  16  and a lower shaft bearing cone  18 . An outer surface  34  of upper shaft bearing cone  16  forms an upper hydrodynamic bearing region  20  with opposing upper conical bearing sleeve  28 . An outer surface  32  of the lower shaft bearing cone  18  forms a lower hydrodynamic bearing region  24  with opposing lower conical bearing sleeve  30 . For proper operation, there should be an engineered fit between each of the shaft bearing cones  16  and  18  and respectively opposing conical bearing sleeves  28  and  30 . 
     An aspect of this engineered fit is the angle at which the conical bearing sleeves  28  and  30  taper. To continue increasing disc drive performance, the angle at conical bearing sleeves  28  and  30  taper will likely have to be increasingly controlled, for example to within 0.01 degrees or better of an engineered specification. In turn, determining whether conical bearing sleeves  28  and  30  are within 0.01 degrees of specification requires an accurate, and repeatable metrology device and method. Since another factor considered in disc drive production is cost, the metrology device should be low cost. Cost may include such factors as whether the metrology system damages a workpiece being measured, and the speed at which a measurement may be completed.  FIGS. 2-6  illustrate systems and methods that may further these goals. 
       FIG. 2  illustrates aspects of a conceptual method for deriving an angle  2 Θ  214  of a conical cavity  208  (shown in cross-section), that may exist for example in a conical bearing sleeve. A first sphere  212  having a known (or determinable) diameter is inserted in the conical cavity  208 . A first height  204  associated with positioning of the first sphere  212  is measured. This measurement may be with respect to reference  202 . The first sphere  212  may then be removed from conical cavity  208 . A second sphere  210  is inserted into the conical cavity  208 . A second height  206  associated with positioning of the second sphere  210  is measured; second height  206  may also be a measurement with respect to the reference  202 . After obtaining the first height  204  and the second height  206 , an angle equal to one half the angle  2 Θ  214  may be calculated by application of the formula below, where R 1  and H 1  and R 2  and H 2  respectively refer to the radius of the first sphere  212  and the second sphere  210 , and the first height  204  and the second height  206 . 
     
       
         
           
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       FIGS. 3A-C  illustrate schematic aspects of an exemplary metrology system  300  implementing aspects of the above sleeve cone angle measurement strategy. In illustrating these aspects reference is made to sources of inaccuracy and non-repeatability in components of the metrology system  300 . These component sources of inaccuracy and non-repeatability combine into a complete measure of the accuracy and repeatability of the metrology system, commonly referred to as Gauge Repeatability and Reproducibility (GRR). Such references are by way of example and not limitation; other metrology systems created and operating by aspects presented herein may have fewer or additional sources of inaccuracy and non-repeatability. 
     Generally, a lower GRR signifies a more stable metrology device than a higher GRR. GRR has two primary components, repeatability and reproducibility. Repeatability is the ability of the same gauge to provide a consistent measurement during a number of uses by the same operator, and reproducibility is the ability of a gauge to give a consistent measurement regardless of the operator. GRR is typically a measurement of the variability percentage of the total available engineering tolerance for the part. For any given system, a target maximum GRR may be selected, and various parameters may be chosen or modified to avoid exceeding that GRR. 
     Metrology system  300  includes a supportive base  303  that may be formed from granite or another material that may aid in isolating sensitive components of the metrology system  300  from ambient vibrations or other disturbances. A stage guide  302  is disposed on the supportive base  303 . The stage guide  302  provides a track over which a stage  304  may move as illustrated by a movement arrow  305 . A fixture  334  is disposed on stage  304  and a workpiece  332  is disposed in the fixture  334 . Aspects of the fixture  334  will be further described herein. In exemplary aspects, the workpiece  332  is a sleeve cone. The sleeve cone is disposed in fixture  334  to provide accessibility to a cavity  338  of the sleeve cone, as illustrated in cross-section in  FIGS. 3A-C . An outer surface portion of the workpiece  332  may take any number of shapes, for example, the outer portion may be cylindrical, and need not be conically tapered. Fixture  334  may be adapted to accommodate such variations in workpiece  332 . 
     Additional components of the metrology system  300  include a first sphere  308  coupled by a plunger  328  to a first gauge  314 , which outputs information to data acquisition logic  322 . First sphere  308  is exemplary and other shapes may be used; for example, a hemisphere may also be used as further described herein. The information outputted by the first gauge  314  may include information describing an amount of extension the plunger  328 . The amount of extension may in turn be used as an indicium of a position of the first sphere  308  in workpiece  332 . This indicium of position may be viewed or otherwise interpreted into a height of the first sphere  308  with respect to a reference, in keeping with the conceptual illustration of  FIG. 2 . 
     The metrology system  300  also includes a second sphere  310  coupled by a second plunger  326  to a second gauge  312 . The second gauge  312  also outputs information that may include information describing an amount of extension of the second plunger  326 . The amount of extension may in turn be used as an indicium of a position of the second sphere  310  in workpiece  332 . This indicium of position may be viewed or otherwise interpreted into height information of the second plunger  326  with respect to the reference. The amount of extension may be transmitted to the data acquisition logic  322 . Other examples may include any amount of preprocessing of extension information. In other aspects ways other than plungers to extend and retract first sphere  308  and second sphere  310 , such as use of rotatable arms, may be employed. 
     The data acquisition logic in turn communicates with a central processing unit  318 . The central processing unit  318  interfaces with gauge controller  320  which controls aspects of both the first gauge  314  and the second gauge  312  discussed below. The central processing unit  318  also interfaces with stage controller  316 . Stage controller  316  interfaces with the stage guide  302  and controls movement and positioning of the stage  304 . Aspects of the metrology system  300  are further illustrated in  FIGS. 3B-C . 
       FIG. 3A  illustrates that stage  304  moves in an exemplary direction indicated by movement arrow  305 , and that this movement is initiated by central processing unit  318  controlling the stage through the stage controller  316 .  FIG. 3B  illustrates that stage  304 , by direction from the central processing unit  318 , positions the workpiece  332  substantially under first sphere  308 . In addition, the central processing unit  318  has directed the first gauge  314  to extend first plunger  328  for contacting first sphere  308  to workpiece  332 . Based on an amount of extension of the first plunger  328 , indicium of a position of the first sphere  308  in the workpiece are determined. Such indicium may include (or may be expressed as) a height of the first sphere  308  with respect to a reference. 
       FIG. 3C  illustrates that first sphere  308  has been retracted and that the stage  304  has moved the workpiece  332  substantially under the second sphere  310 .  FIG. 3C  also illustrates that the second gauge  312  has extended the second plunger  326  so that second sphere  310  contacts the workpiece  332 . As described above, these actions may be initiated by central processing unit  318  providing commands or other information to gauge controller  320  and to stage controller  316 . Based on an amount of extension of the second plunger  326 , indicium of a position of the second sphere  310  in the workpiece are determined. Such indicium may include (or may be expressed as) a height of the second sphere  310  with respect to a reference. 
     Gauges  314  and  312  may include sensors for determining an amount of extension of the first and second plungers  328 ,  326 . For example, such sensors may include interferometry sensors and associated supporting equipment. Exemplary gauges that may be used include Heidenhain Metro  1287  gauges. 
     In exemplary aspects, the stage controller  316  and the plunger controller  320  interface respectively with the stage  304  and the first and second gauges  314 ,  312  at least partially pneumatically. For example, the first and second gauges  314 ,  312  may each include plunger controls that interface with plunger controller  320  through pneumatic control lines. By applying air pressure through the pneumatic control lines, plunger controller  320  may initiate extension of the first and second plungers  328 ,  326 . 
     By applying vacuum to those pneumatic control lines, plunger controller  320  may also slow extension of, and retract, the first and second plungers  328 ,  326 . Retraction and slowing may also be initiated by spring mechanisms associated with the plunger controls. A rate at which the first and second plungers  328 ,  326  may extend may be controlled to prevent damage to workpiece  332 . Timing of slowing extension of the first and second plungers  328 ,  326  may be controlled to allow rapid extension, and then slowing at a time before contact with workpiece  332 . An amount of pressure (vacuum or greater than ambient) and/or volume of gas may be selectable based on the weights of the plungers  328  and  326  and first and second spheres  308 ,  310 . 
     In a general sense, aspects described in  FIGS. 3A-C  include fixture  334  dimensioned to hold a workpiece (e.g., workpiece  332 ) and a first object (e.g., first sphere  308 ) that is sized to at least fit a portion of the workpiece that is the subject of metrology. The nature of this fit may vary depending on characteristics of the portion of the workpiece subjected to metrology and characteristics of the first object, including size and shape of each. 
     Also, first plunger  328  is an example of a structural portion for mounting the first object to provide for movement of the first object along a path that results in contact with the workpiece  332 . This path, between various metrology uses, need not have precisely the same starting point or ending point, but this path would be expected to lead to contact with the workpiece  332 . This path may be predetermined based on the arrangement of the structural portion. 
     Similarly, plunger controller  320  may be generally viewed as a position controller for the structural portion for mounting the first object. As such, there may be a separate position controller for the first object structural portion and the second object structural portion. Functionality and/or functional portions of each position controller may also be distributed. For example, pneumatic valving, motors, or other actuators may be included proximate the structural portion, circuitry for controlling that valving may be at a separate location, and computation logic for controlling the circuitry may be at yet another location. 
     Upon contact, the first object fits with the workpiece at a portion of the workpiece determined by interaction between sizes and shapes of the first object and the workpiece. First gauge  314  is an example of a sensor that can be viewed as sensing a distance traveled by the first object along the path and producing a signal indicative of such distance. 
     Also, control related aspects and associated apparatuses, such as stage controller  316 , data acquisition logic  322 , plunger controller  320 , control system  250  and the like may be implemented in any of a variety of ways that provide a variety of divisions between mechanical control (e.g., valving, timing, cams, gears, and other devices useful in constructing mechanical apparatuses) and electronic control, between software control running on general purpose processors and application specific hardware implemented in ASICS, FPGAs or other suitable logic implementations. Aspects relating to second sphere  310 , second plunger  326 , second gauge  312 , and the like may similarly be generalized. 
       FIG. 4  illustrates a perspective view of an example implementation of metrology system  300 . Base  303  supports stage guide  302 . Stage guide  302  includes a first rail  402 , a second rail  404 , and a top portion  406 . The stage  304  interfaces with first rail  402  and second rail  404 , which provide guidance to stage  304  as it moves along the stage guide  302 . The stage  304  also fits closely to the top portion  406 , which is expected to aid in reducing variation of distance between a workpiece  332  disposed in fixture  334  and gauges  314 ,  312 . By reducing variation, the stage is expected to increase accuracy and repeatability because changes in amount of extension of plungers  328 ,  326  due to such variations would be reduced, and therefore measurement error and variations between measurements would be reduced. 
     The stage  304  may be an air bearing stage with a relatively small positioning error and a motion control system that can provide approximately constant velocity. Air bearing stages also help lower error because they tend to distribute load over a large surface area and often have good stiffness which is often desirable for heavy or offset loading. Also, the air bearing of an air bearing stage has an inherent averaging effect that helps in error reduction by filling small surface voids and other irregularities, which is thought to provide better pitch, roll, yaw, and straightness and flatness specifications. An exemplary air bearing stage is the ABL 1000 (FiberGlide 1000) manufactured by Aerotech. 
       FIG. 5  provides a top view of portions of metrology system  300 . Base  303  again supports stage guide  302  on which stage  304  moves. Fixture  334  is illustrated as a ring that is placed within a compliancy boundary  506 . The compliancy boundary  506  defines a surface over which fixture  334  may move under application of force on workpiece  332  (held by fixture  334 — FIGS. 3A-C ) by first sphere  308  or second sphere  310  as each contact workpiece  332 . The surface may be steel and may further be finished to be relatively smooth. In present examples, workpiece  332  includes a conical cavity. Although stage  304  may approximately align a bottom of the conical cavity under each of first sphere  308  and second sphere  310 , there may also be some misalignment, and the first sphere  308  and second sphere  310  may each initially contact workpiece  332  at a point that is not as low in the conical cavity as each sphere may reach. 
     Getting each of first sphere  308  and second sphere  310  as low as possible in the conical cavity of workpiece  332  may increase accuracy of cone angle measurement. Therefore, providing for movement of the fixture  334  allows for correction of misalignment because the first sphere  308  and second sphere  310  tend to exert some force horizontally (as well as vertically) when resting on workpiece  332 . By providing an interface (e.g., an interface between a surface of compliancy boundary  506  that faces a bottom of fixture  334  in  FIG. 5 ) having a coefficient of friction low enough to allow movement of fixture  334  under that horizontal force, the workpiece  332  may find a lower position in the conical cavity. 
     Aspects of metrology system  300  that affect an allowable approximate maximum friction coefficient include vertical force of the sphere/plunger combination on the workpiece, friction forces at the sphere/workpiece interface, and angle of contact between the sphere and the workpiece. In an example, the workpiece  332  is formed from steel and the surface defined by compliancy boundary  506  is formed from sapphire. A metal-sapphire surface has a coefficient of friction of about 0.1-0.15. In another example, the workpiece  332  is supported by a sapphire ring (e.g., fixture  334 ) having a bottom portion resting on a steel surface defined by compliancy boundary  506 . In this example, the sapphire ring glides along the steel surface. Rings of various sizes may be selected for different workpieces. Shapes other than rings may also be used; for example discs, or blocks may be used. By further example, an inner portion of a shape may be a ring, but an outer portion may be any other shape such as a square, or a hexagon. 
     Thus, compliance aspects described herein include selecting materials to provide an interface with an appropriately low coefficient of friction, where the interface is between either a workpiece and a support (e.g., the surface of compliancy boundary  506 ) or between a body (e.g., the ring example of fixture  334 ) accepting the workpiece and a support. Of course, such compliance aspects may be combined or otherwise modified. Other ways to achieve compliance are contemplated in other aspects. For example, compliance may be added to a mechanism for supporting the first and the second spheres. 
     The above examples and aspects presented used, for ease of description, spheres for objects contacting workpiece  332 . In other aspects, any of a variety of objects having other shapes may be used. For example, hemispheres and discs may be used rather than spheres. Still further aspects may use any object having a geometry from which a position of the object in a conical sleeve may be used in combination with a position of a differently sized object in the conical sleeve to calculate a characteristic, such as a taper angle of a surface in the conical sleeve. Other characteristics may include cone angle quality. 
       FIG. 6  presents an exemplary metrology method  600  (described with reference to  FIGS. 3A-C ). At  602 , workpiece  332  is loaded into fixture  334 . At  604 , stage  302  moves approximately under first sphere  308  for obtaining a first measurement. At  606 , first sphere  308  is released for contacting workpiece  332 . At  608 , a velocity of the first sphere  308  is reduced. As first sphere  308  begins contacting workpiece  332 , workpiece  332  may move by sliding on a low friction surface (e.g., the surface of compliancy boundary  506 ) or may slide with a low friction support such as fixture  334  on a surface. At  610 , an amount of extension of first plunger  328  is measured, and used as an indicium of position of first sphere  308  in the workpiece  332  (e.g., indicium may include a height of first sphere  308  with respect to a reference as discussed previously). At  612 , first sphere  308  is retracted, at  614  stage  304  is moved substantially under second sphere  310 , and at  616  second sphere  310  is released. At  618 , a velocity of the second sphere  310  is reduced. At  620 , an amount of extension of second plunger  326  is measured, and used as an indicium of position of second sphere  310  in the workpiece  332 . At  622 , second sphere  310  is retracted. At  624 , a characteristic, such as an angle of taper of the conical cavity of exemplary workpiece  332  is calculated based on the indicia of respective positions of first sphere  308  and second sphere  310  in workpiece  332 . 
     An additional aspect that may aid in improving accuracy and repeatability is to cause first and second spheres  308 ,  310  to contact the surface of workpiece  332  at various relative orientations for a number of metrology cycles. By example, first and second spheres  308 ,  310  may each be rotated through some portion of a complete revolution after completion of a cycle. Then another cycle may be commenced. Because first and second spheres  308 ,  310  have each been rotated through a portion of a revolution, first and second spheres  308 ,  310  will each contact the surface of workpiece  332  at a different relative orientation than each did previously. Where there is some deformity in one or more of first sphere  308  and second sphere  310  and the surface of workpiece  332 , rotating in this manner may help average measurements to minimize the effect of those deformities on accuracy and repeatability. Workpiece  332  may also be rotated to accomplish a similar result. 
     This description is exemplary and it will be apparent to those of ordinary skill in the art that numerous modifications and variations are possible. For example, various exemplary methods and systems described herein may be used alone or in combination with various other metrology systems, control mechanisms including program code, data collection, data visualization techniques, and the like. Various additional steps may be added to methods, including rotating of workpieces for obtaining measurements at different orientations which may improve accuracy or may be for assessing other workpiece characteristics. Additionally, particular examples have been discussed and how these examples are thought to address certain disadvantages in related art. This discussion is not meant, however, to restrict the various examples to methods and/or systems that actually address or solve the disadvantages.