Abstract:
A metrology system for measuring a cone angle, a cone straightness, and a cone quality of a sample and method of using the metrology system are disclosed. The system includes a rotary stage, one or more workpiece fixtures that hold the samples in the rotary stage, and a number of different sized measurement devices. The measuring devices are positioned next to the rotary stage and measure the samples using contact. The rotary stage is free to rotate when the measuring devices are in a non-measuring state. The invention also includes a processor that collects data from the measurement devices and calculates the cone angle, the cone straightness, and the cone quality of each sample based on the data.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    The current invention is in the field of metrology of objects. Particularly, the invention relates to metrology of angles, quality, and anomalies in a cone. 
         [0003]    2. Related Art 
         [0004]    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. 
         [0005]    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. 
         [0006]    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. 
         [0007]    U.S. Pat. No. 7,350,308 (“the &#39;308 patent”), herein incorporated by reference in its entirety, is an exemplary system used for measuring the angle of conical cavities. The system uses a two sphere method to determine the each cone&#39;s characteristics.  FIG. 1  illustrates aspects of the conceptual two sphere method for deriving an angle  2 θ  114  of a conical cavity  108  (shown in cross-section), that may exist for example in a conical bearing sleeve. A first sphere  112  having a known (or determinable) diameter is inserted in the conical cavity  108 . A first height  104  associated with positioning of the first sphere  112  is measured. This measurement may be with respect to reference  102 . The first sphere  112  may then be removed from conical cavity  108 . A second sphere  110  is inserted into the conical cavity  108 . A second height  106  associated with positioning of the second sphere  110  is measured; second height  106  may also be a measurement with respect to the reference  102 . After obtaining the first height  104  and the second height  106 , an angle equal to one half the angle  2 θ  114  may be calculated by application of the formula below, where R 1 , H 1 , R 2 , and H 2  respectively refer to the radius of the first sphere  112 , the first height  104 , the second sphere  110 , and the second height  106 . 
         [0000]    
       
         
           
             θ 
             = 
             
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                             R 
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                           ( 
                           
                             
                               H 
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         [0008]      FIG. 2  shows the system  200  described in the &#39;308 patent. Base  203  supports stage guide  202 . Stage guide  202  includes a first rail  282 , a second rail  284 , and a top portion  286 . The stage  204  interfaces with first rail  282  and second rail  284 , which provide guidance to stage  204  as it moves along the stage guide  202 . The stage  204  also fits closely to the top portion  286 , which is expected to aid in reducing variation of distance between a workpiece disposed in fixture  234  and gauges  214 ,  212 . By reducing variation, the stage is expected to increase accuracy and repeatability because changes in amount of extension of plungers (not shown) due to such variations would be reduced, and therefore measurement error and variations between measurements would be reduced. 
         [0009]    The stage  204  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. 
         [0010]    However, there are several drawbacks to the system disclosed in the &#39;308 patent. First of all, since only one sample can be measured at a time, it takes approximately 30 second to measure each sample. Secondly, the system is unable to measure cone straightness. Cone straightness refers to the quality of the sides of the cone.  FIG. 3  illustrates some possible undesirable defects in the sides  310  and  315  of a cone  300  that effect cone straightness. Such defects may include a bump  320  as shown in side  310  or a cavity  325  as shown in side  315 . Thirdly, the system is sensitive to the effects of particles and other system noise. 
         [0011]    Therefore, what is needed is a low-cost, accurate, and repeatable metrology system that is fast, and able to measure cone straightness and cone quality in addition to cone angle. 
       SUMMARY 
       [0012]    One aspect of the invention provides a metrology system comprising a rotary stage, at least one holding fixture, each for holding at least one conical sample. The system also comprises a plurality of differently sized measurement devices positioned adjacent to the rotary stage for interfitting, in a measuring state, with the samples. The interfitting is for obtaining data useful in determining one or more characteristics of the samples. The rotary stage is free to rotate when the measuring devices are in a non-measuring state. The system also comprises a processor in communication with the measurement devices, and operable to use the data from the measurement devices for calculating a cone angle, a cone straightness, and a cone quality of each sample. 
         [0013]    Each measurement device may include a contact element, a plunger, and a gauge to measure distance the plunger has extended when the contact element touches with the sample. The fixture may include an element to increase compliance of the fixture to misalignments between the contact element and the workpiece(s). The compliance element may include a low-friction surface on which the workpiece can move, such as a surface having sapphire. 
         [0014]    Another aspect of the invention provides a method for measuring a conical workpiece&#39;s angle of taper, straightness, quality. The method includes the steps of placing at least one sample into a sample slot in a rotary stage, extending a number of differently sized measuring devices for contacting each sample and recording an amount of extension of each measuring device, retracting the measuring devices, rotating the rotary stage to a subsequent position wherein at least one sample slot is aligned with one measuring device, repeating the above steps until each sample is measured by each measuring device, compiling a data set of recorded extensions of each measuring device for each sample, and calculating a cone angle, cone straightness, and cone quality of each sample based on the compiled set of data. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Aspect and examples will now be described in greater detail with reference to the attached drawings, in which: 
           [0016]      FIG. 1  illustrates a cone angle measurement technique of the prior art. 
           [0017]      FIG. 2  illustrates a cone angle measurement system of the prior art. 
           [0018]      FIG. 3  illustrates a cone with possible defects. 
           [0019]      FIG. 4  illustrates an embodiment of a cone measurement system; 
           [0020]      FIG. 5  is a schematic of an embodiment of a cone measurement system; 
           [0021]      FIG. 6  illustrates a holding fixture; and 
           [0022]      FIG. 7  illustrates the measurement technique. 
       
    
    
     DETAILED DESCRIPTION 
       [0023]    The following description is presented to enable a person of ordinary skill in the art to make and use various inventive aspects disclosed herein. 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 for characterizing and parameterizing any of a variety of objects. In some cases, shapes other than cones may also be characterized. The exemplary system configurations, components, exemplary tolerances, design criteria, and the like provided herein are for illustrating various aspects and are not intended to limit the configurations, components, tolerances, and/or criteria that may be accounted for in such metrology systems. 
         [0024]      FIG. 4  illustrates a sleeve cone quality measurement system  400 . System  400  may include a supportive base  405  that may be formed from granite or any other material capable of increasing isolation of the sensitive components of system  400  from ambient vibrations or other disturbances. A rotary stage guide  410  may be disposed atop supportive base  405 . Rotary stage guide  410  may provides a rotary track over which a stage  415  may rotate. Rotary stage  415  may be a rotary air-bearing stage. Rotary stage  415  may contain a plurality of workspace  425  holding fixtures  420 . Preferably, workspace  425  comprises a sleeve cone. While five holding fixtures  420  are shown, any number of holding fixtures  420  may be used. Aspects of holding fixtures  420  will be further described herein. Workspace  425  is disposed in holding fixtures  420  to provide accessibility to a cavity of the sleeve cone. An outer surface portion of the workpiece  425  may take any number of shapes, for example, the outer portion may be cylindrical, and need not be conically tapered. Holding fixtures  420  may be adapted to accommodate such variations in workspace  425 . 
         [0025]    System  400  further includes a plurality of measuring devices  430  positioned to measure the interior of the workspaces  425 . While five measuring devices  430  are shown any number of measuring devices  430  may be used. Each measuring device  430  contains a respective plunger  435  to which a respective contact element  440  is coupled. Measuring devices  430  are capable of extending plungers  435  so that contact element  440  touches workspace  425 . Once contact element  440  has reached a maximum amount of extension into workpiece  425 , measuring device  430  can measure an amount of extension. Each contact element  440  can of a different size than the other contact elements  440 , so long as two or more differently sized elements are provided. Contact elements  440  are preferably spherical, however they may be any shape capable of fitting into a sleeve cone, including but not limited to hemispherical and conical, so long as they have a circular cross section for contacting circular a cross-section of the workpiece into which such elements  440  will extend. 
         [0026]    The amount of extension may be used as indicia of the position of the contact element  440  in the workpiece  425 . These indicia of position may be viewed or otherwise interpreted into a height of the contact element  440  with respect to a reference. Each measuring device  430  may be controlled by a gauge controller that controls the amount of extension of each plunger  435  and determines when the contact element  440  touches the workpiece. 
         [0027]      FIG. 5  is a schematic of the measuring system  400  with the associated control units. Each measuring device  430  may be in communication with a gauge controller  550 , which in turn, may be in communication with a central processing unit  555 . The central processing unit  555  may interface with the gauge controller  550  to control each of the measuring devices  430  and a stage controller  560  to control rotary stage guide  410 . The central processing unit  555  may direct the stage controller  560  to rotate the rotary stage guide  410  so that at least one workspace  425  is substantially located in line with a contact element  440  of a measuring device  430 . 
         [0028]    Once the rotary stage  415  is positioned, the central processing unit  555  directs gauge controller  550  to extend the plungers  435  of each measuring device  430  so that the contact elements  440  touch the workpieces  425 . Once the amount of extension is recorded, the central processing unit  555  may then direct the gauge controller  550  to retract the plungers  435  so that the rotary stage  415  may be rotated again. The central processing unit  555  repeats the process until each measuring unit  430  has measured each workpiece  425 . 
         [0029]    In exemplary aspects, the stage controller  560  and the gauge controller  550  interface respectively with the rotary stage  416  and the measuring devices  430  at least partially pneumatically. For example, the measuring devices  430  may each include plunger controls that interface with gauge controllers  550  through pneumatic control lines. By applying air pressure through the pneumatic control lines, plunger controller may initiate extension of the plungers  435 . 
         [0030]    By applying vacuum to those pneumatic control lines, plunger controller may also slow extension of, and retract, the plungers  435 . Retraction and slowing may also be initiated by spring mechanisms associated with the plunger controller. A rate at which the plungers  435  may extend may be controlled to prevent damage to the workpieces  425 . Timing of slowing extension of the plungers  435  may be controlled to allow rapid extension, and then slowing at a time before contact with the workspace  425 . An amount of pressure (vacuum or greater than ambient) and/or volume of gas may be selectable based on the weights of the plungers  435  and the contact elements  440 . 
         [0031]      FIG. 6  is a side view of a holding fixture  420 . During rotation of rotary stage  415 , the center line  601  of workpiece  425  may not stop directly under contact element  440  (as shown in  FIG. 6 , where the center line  601  of workpiece  425  is slightly off to the side of contact element  440 ). Therefore, to correct any misalignment of the contact element  440  and the workpiece  420 , workpiece  425  may be nearly free floating so that it can self-align responsively to contact with contact element  440 . 
         [0032]    Due to the angle of workpiece  425 &#39;s walls, contact elements  440  tend to exert some force horizontally (in addition to the obvious vertical forces) when resting on workpiece  425 . Therefore, there may be a low-friction interface between surfaces of workpiece  425  and fixture  420  and/or between fixture  420  and rotary stage  415  (i.e., a coefficient of friction low enough to allow movement of workpiece  425  relative to rotary stage  415  under the horizontal force applied by contact elements  440 ). A low-friction interface between complementary surfaces of workpiece  425  and holding fixture  420  provides a desirable (e.g. lower) contact position of contact element  440  inside workpiece  425 , thereby providing increased consistency of measurement. 
         [0033]    In an exemplary embodiment of the invention, workpiece  425  comprises steel and is held by a holding fixture  420 . Workpiece  425  may sit inside a ring  660  comprised of a low friction material. Preferably, the low friction material is at least partially sapphire. A metal-sapphire surface has a low coefficient of friction of about 0.1-0.15. Ring  660  in turn sits atop a surface  665  of holding fixture  420 . Preferably, surface  665  is a polished/ground steel surface on which ring  660  can glide. 
         [0034]    During use of system  400 , gauge controller  550  may control measuring devices  430  to extend plunger  435  twice for each measurement. The first extension may properly align workpiece  425  under measuring device  430  so that contact element  440  can have a greater chance of being seated as far down as possible into workpiece  425  and the second extension may be used for measuring the amount of extension for data collection. 
         [0035]    Each measuring device  430  can measure the extension of its respective plunger  435  into a workpiece  425 , and the data can be recorded in a computer readable medium such as a RAM. The computer processor can use a Least Square model of fitting the data to determine the taper angle of the workpiece  425 ; other aspects that can be determined include cone straightness, and cone quality, as explained further below.  FIG. 7  illustrates a concept of the measurement technique using spheres as contact elements. Using a minimum of two contact elements can provide a measure of cone taper angle. Using more contact elements can allow obtaining data that can be used to obtain measures of cone straightness and cone quality, as described below. While  FIG. 7  shows two contact elements (for simplicity), any number of contact elements can be used. Preferably, five contact elements are used. The following general equations are used to determine the Least-Square fit for n contact elements: 
         [0000]    
       
         
           
             
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         [0000]    Where r is the radius of each contact element, h is the height of each contact element from a reference position (determinable based on measuring plunger extension in some examples herein), and c is a constant. F[n] and G[n] are minimization functions with respect to θ and c, respectively. By solving the equations for θ it is possible to determine the taper angle of the workpiece  425 . 
         [0036]    Once the angle is calculated, the merit function (or cone quality), which determines if any errors occurred in the measurement, and cone straightness, which determines if there are any deviations from the expected straight line of the cone wall, can be calculated. These calculations generally involve comparing data derived from individual contact elements with some averaging data for the more of the contact elements. In a particular example, the merit function is derived from the R-squared (Rsq) value and is the higher resolution of Rsq, where Rsq=0.999999896 and Merit=90. By using the following equation, Rsq can be calculated: 
         [0000]    
       
         
           
             
               R 
                
               
                   
               
                
               s 
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                
               q 
             
             = 
             
               1 
               - 
               
                 SSE 
                 SST 
               
             
           
         
       
     
       Where: 
       [0037]        SSE=Σ   i=1   n ( Rc   i   −r   i ) 2 , and 
         [0000]        SST=Σ   i=1   n ( R avg −r   i ) 2    
         [0000]    Where r is the original ball radius, Rc is the computed ball radius (computed using the equation Rc=h Sin(θ)+C Cos(θ)), h is the height, and Ravg is the average radii. Finally, the Merit can be calculated using the equation: 
         [0000]      Merit=Rsq* 10 8 −99999900 
         [0038]    The Straightness (Str) uses the location of the tangent point (using the equation L=h−r Sin(θ)) of each ball to determine the Straightness Error at each tangent point using the following equation: 
         [0000]      Str= Rc   i   −r   i    
         [0000]    From this, the overall Straightness can be determined using the following equation: 
         [0000]      Str=Max( Rc   i   −r   i )−Min( Rc   i   −r   i ) 
         [0039]    The overall Straightness and Merit can be compared with a specification, and if both are within the specification, then the workpiece can be allocated for use in a disc drive motor or some other suitable mechanism. On the other hand, if the workpiece is not within the specification, then the workpiece may be discarded, or the workpiece may be subjected to another metrology run. 
         [0040]    In certain embodiments, system  400  may include a calibration element. The calibration element may be positioned within a holding fixture  420  in place of a workpiece  425 . The central processing unit  555  may direct the gauge controller  550  to extend each plunger  435  of each measuring device  430  so that each contact element  440  touches the calibration element to determine the relative height of the gauges before measuring the workpiece  425 . Such a configuration may increase the accuracy of the measurements. 
         [0041]    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 additional metrology systems and other systems for determining suitability of a workpiece under a given specification. 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.