Abstract:
A magnetic head and disk tester includes a magnetic head which is selectively positioned relative to a rotatably mounted magnetic disk to accomplish testing of either of the head or disk or both. The magnetic head is secured to a carriage which is under electro-mechanical control, wherein displacement of the carriage causes a corresponding displacement of the head relative to the disk and, to some degree, may cause yaw in the carriage. Measurements of the linear position of a left and a right side of the carriage are detected, measured, and fed back to a control system. The control system responds to a difference between a desired command position and the actual position of each side of the carriage and causes actuators to adjust the position of each side of the carriage until the difference, and thereby the yaw, is substantially eliminated. Additionally, during a positioning operation, vibrations in the yaw direction are decreased, which decreases a corresponding settling time of the magnetic head.

Description:
REFERENCE TO RELATED APPLICATION 
     This application is related to U.S. Application Ser. No. 09/241,512, entitled “Magnetic Head and Disk Tester with Pitch Correction” filed on even date herewith and assigned to the assignee of the present invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to magnetic head and disk testers, and more particularly to testers with improved accuracy in positioning a magnetic head with respect to a disk. 
     A magnetic head and disk tester is an instrument that is used for testing the characteristics of magnetic heads and disks such as signal-to-noise ratio, pulse width and so on. Each tester includes two main assemblies, i.e., an electro-mechanical assembly that performs movements of the head with respect to the disk, and an electronic assembly that is responsible for measurements, calculations, and analysis of the measured data. The electro-mechanical assembly of the tester is known as the spinstand. The spinstand generally simulates the motions of the head with respect to the disk that occur in, for example, a hard disk drive. Whatever the accuracy of the electronic measurement portion of the tester, the results of measurements will also depend upon the positioning accuracy provided by the spinstand. 
     An exemplary spinstand  5  of a prior-art head and disk tester is shown schematically from a top view in FIG.  1 A. The spinstand  5  includes a stationary base element  30  that supports the positioning system and the head  12  and disk  10  to be tested. The disk  10  is supported in a preferably horizontal plane in a manner allowing rotary motion of the disk  10  about a spindle axis perpendicular to that horizontal plane. The spinstand  5  includes a coarse positioning system and a fine positioning system arranged in series to effect controlled movement of head  12  with respect to disk  10 . The coarse positioning system positions the magnetic head  12  close to its desired position relative to a magnetic disk  10 . In the illustrated form, the coarse positioning system includes a stepper motor  28  affixed to base  30 . The stepper motor  28  rotationally drives a lead screw  32  that rotates within bearings  24  and engages a nut  34 . Nut  34  is rigidly fixed to a slide  18  so that rotary motion of lead screw  32  effects linear motion of slide  18  along guides (not shown) with respect to base element  30 , along a translation axis X, or X-axis. 
     The fine positioning system of spinstand  5  resides on slide  18  and effects relatively minor positional changes to the position of head  12  illustrated by the slide  18 . In the illustrated form, the fine positioning system includes a piezo actuator  26  that is disposed between a stop  36  that is rigidly mounted on slide  18  and a deformable (in the direction of x-axis) body  16  also mounted on slide  18 . Two bolts  22   a  and  22   b  are screwed into deformable body  16  through openings in the stop  36 . Piezo actuator  26  is preloaded by springs  20   a ,  20   b  that are compressed between the heads of the bolts  22   a ,  22   b  and the stop  36 . The deformable body  16  at its base is rigidly coupled to slide  18 . The top of body  16  is moveable, in response to the piezo actuator  26 , supports arm  14 , which in turn supports head  12 . Arm  14  is coupled to link  16   a  by a shaft  25 . Body  16  functions as a parallel-link mechanism that is sensitive to the expansions and contractions of piezo actuator  26  to small linear displacements (e.g., 0.001 in) for head  12 , (relative to disk  10 , as supported on base  30 ) in addition to the major displacements effected by the coarse positioning system. 
     FIG. 1B shows side view of an exemplary form of deformable body  16  in the system of FIG.  1 A. In this form, the deformable body  16  is a parallelogram-structured deformable body comprised of a top and a bottom rigid links  16   a  and  16   b , disposed in parallel, coupled by two side rigid links  16   c  and  16   d , wherein flexures are at the junction of link pairs to allow for angular displacement of the elements while substantially maintaining the parallelogram integrity of the structure. With this structure the piezo element  26  drives the uppermost, as shown, or the top link  16   a  of deformable body  16  in the x direction relative to slide  18  (and base  30 ), whereby the magnetic head  12  to be tested remains substantially at the same height throughout the range of its displacement. 
     Movements of the link  16   a  of deformable body  16  are measured by an optical linear encoder  38   a ,  38   b , as shown in FIG.  1 A. The optical linear encoder  38  consists of a moveable portion  38   a  (i.e., a glass scale) that is rigidly attached to the top link  16   a  of deformable body  16  and a stationary portion  38   b  (i.e., an optical detector) fixed to base  30 . A signal generated by optical detector  38   b  corresponds to movements of top link  16   a  of deformable body  16  relative to base  30 . That signal corresponds to a sum of the linear displacement established by the steppers motor  28  and by the piezo actuator  34  (together with deformable body  16 ). 
     Thus, to achieve high accuracy in linear positioning of head  12  over magnetic disk  10 , the positioning process is split into steps of coarse and fine positioning. The coarse positioning is provided, in part, by the rotation of lead screw  32  by stepper motor  28 . Rotational movement of lead screw  32  is translated into a linear movement of slide  18  by nut  34 . Upon completion of coarse positioning, fine positioning is activated by applying a voltage to piezo actuator  26  from an external power supply (not shown). In a manner known in the art, under the effect of the voltage, actuator  26  changes its linear dimension in proportion to the level of the applied voltage. As a result, the top link  16   a  together with arm  14  and a magnetic head  12  is shifted with respect to magnetic disk  10  in the X direction. The displacement of magnetic head  12  is measured by optical linear encoder  38  and sent to a feedback circuit (not shown) to control the amount of displacement of the deformable body  16 , in a manner known in the art. 
     During the testing, when the top link  16   a  of deformable body  16  moves arm  14  with magnetic head  12  mounted thereto, an optical linear encoder  38  is used to determine the position of magnetic head  12 . In the prior art, the displacement measured by optical linear encoder  38  is considered to be substantially the same as the displacement of the magnetic head  12 . However, in practice, the top link  16   a  of the deformable body  16  may experience yaw (i.e. rotational displacement about an axis perpendicular to the nominal (horizontal) plane of allowed movement) during the movement. Yaw can occur due to different (asymmetrical) stiffness of the weakened portions (i.e. the flexures) of the deformable body  16 , or due to different stiffnesses of the springs  20   a  and  20   b . FIG. 2 shows the effect of the parallelogram-structured deformable body  16  rotating about a point O in the direction indicated by arrow A. As shown, the head  12  moves from an original point P to a point Q. This movement corresponds to a shift X 1  in the X direction, and to a shift Y 1 , in the Y direction. Optical linear encoder  38   a ,  38   b  can only detect movements in the X direction; in this particular case, it detects, a movement of X 2 , which is not equal to X 1 . The difference X 1 -X 2  and the shift Y 1 , cannot be compensated by the normal, prior art feedback circuit, since the yaw component is undetectable. Therefore, the prior art spinstand  5  shown in FIGS. 1A and 1B cannot achieve very high positioning accuracy. 
     This problem of accuracy is solved to some degree in a prior art disk and head tester designated as Model 1701, developed and manufactured by Guzik Technical Enterprises, San Jose, Calif. This tester uses a high-precision micropositioning mechanism that performs fine movements. Although this mechanism operates very efficiently and is advantageous for some applications, it is expensive to manufacture because it requires the use of many interacting parts, relative to, for example, the tester of FIGS. 1A,  1 B, and  2 . 
     Another disadvantage of the prior art spinstand shown in FIGS. 1A,  1 B, and  2  is that the parallelogram-structured deformable body  16 , the arm  14 , and the head  12  tend to oscillate in the direction indicated by arrow A when the piezo actuator  26  changes its length. The reason for this is that the center of mass of the combination consisting of deformable body  16 , arm  14 , and magnetic head  12  is not on the longitudinal axis of piezo actuator  26 . As a result, this configuration increases the settling time of magnetic head  12  (the time that is necessary to move magnetic head  12  from one point to another). 
     It is, accordingly, an object of the present invention to provide a magnetic head and disk tester, with relatively few parts, that ensures high accuracy of positioning of a magnetic head over a magnetic disk by compensating for yaw. It is yet another object of the present invention is to decrease the settling time of a head and disk tester. 
     SUMMARY OF THE INVENTION 
     According to the present invention, a magnetic head and disk tester comprises an assembly for rotationally supporting a magnetic disk (or disk) in a first plane and a dual-stage positioning system that moves in the direction of an X-axis, and a magnetic head with respect to the magnetic disk substantially within a second (or translation) plane which is parallel to the disk plane. The first stage of the positioning system is a coarse positioner that positions the magnetic head in the vicinity of a predetermined command position relative to a magnetic disk. In a preferred embodiment, the coarse positioner includes a lead screw rotated along a translation X-axis by a stepper motor which causes linear movement of a carriage which contains the second stage of the positioning system. The second stage of the positioning system is a fine positioner that comprises a deformable body in the carriage, preferably in the form of a parallelogram-structured deformable body having four flexure coupled planar links. In that preferred form, the deformable body has a top link that supports a magnetic head, and a bottom link that is rigidly connected to the slide. The top link is moveable in a plane substantially parallel to the translation plane. A pair of piezo actuators, spaced apart in the translation plane, and transverse to the x axis, are coupled between the top link and the carriage to provide controlled adjustment of the orientation of that link about axes perpendicular to the nominal translation plane. Preferably, the top link includes a left and a right side that can be independently moved in the direction of the x axis and with respect to the carriage by the two piezo actuators located behind the front link and proximate to the top link. Positions of both left and right sides of the top link of the deformable body are measured by separated optical linear encoders, one on each side. The results of these measurements are input to a closedloop positioning system with two separate feedback loops, each controlling one of the piezo actuators. By using two piezo actuators, any yaw of the deformable body and the magnetic head is eliminated, which significantly increases positioning accuracy of the magnetic head respect to the magnetic disk. Furthermore, vibrations in the yaw direction during a positioning operation are damped by the fine positioner, which decreases the settling time (the time that is necessary to move the magnetic head from one point to another) of the tester of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The foregoing and other objects of this invention, the various features thereof, as well as the invention itself, may be more fully understood from the following description, when read together with the accompanying drawings as described below. 
     FIG. 1A is a schematic representation of a prior art spinstand used in a magnetic head and disk tester. 
     FIG. 1B is a schematic side view of the fine positioning system of a prior art spinstand of FIG.  1 A. 
     FIG. 2 is a schematic representation of the consequences of yaw in the prior art magnetic head and disk tester of FIGS. 1A and 1B. 
     FIG. 3A is a schematic representation of the spinstand of the magnetic head and disk tester of a preferred embodiment of the present invention. 
     FIG. 3B is a schematic side view of the preferred embodiment of the fine positioning system of the magnetic head and disk tester of FIG.  3 A. 
     FIG. 4 is a functional block diagram of the closed-loop control system used in the magnetic head and disk tester of FIGS.  3 Aa and  3 B. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A spinstand  100  of the preferred embodiment of a magnetic head and disk tester in accordance with the present invention is schematically shown from a top view in FIG.  3 A. The spinstand  100  is similar to the spinstand  5  illustrated in FIG. 1A, except that spinstand  100  includes the yaw correction assembly of the present invention. In FIG. 3A elements which corresponds to elements in FIG. 1A, are identified with the same reference designations. A magnetic disk  10  is rotationally (about a vertical axis) supported in a first (or disk) plane on a base  30  of spinstand  100 . A magnetic head  12  is positioned relative to disk  10  to permit testing. Similar to the prior art tester of FIG. 1A, the spinstand  100  includes a dual-stage positioning system having a coarse positioner and a fine positioner for selectively positioning head  12  along a transition or X-axis. The coarse positioner includes base  30  that supports a stepper motor  28  which rotationally drives a lead screw  32 . Stepper motor  28  is a standard commercially available module, for example a stepper motor of model ESCAP P850-508-C produced by Portescap U.S., Inc., Hauppauge, N.Y., USA. Lead screw  32  rotates within bearings  24  and engages a nut  34  that is rigidly fixed to a carriage  18 , thereby being adapted for translating the rotational motion of the lead screw  32  into linear motion of the carriage  18  in the X direction. The carriage  18  moves along guides (not shown) with respect to base  30 , and along the X-axis. 
     The fine positioner includes a four link parallelogram deformable body  16  mounted to the carriage  18 , as in the embodiment of FIG.  1 A. Also, as in that embodiment the magnetic head  12  is mounted to horizontally oriented arm  14 , which is attached to the top link of deformable body  16  by a vertical coupler, such as shaft  25 . The coarse positioner moves the fine positioner and, thereby, positions the magnetic head  12  near a predetermined command position relative to the magnetic disk  10 , and the fine positioner effects minor change to that position established by the coarse positioner. 
     The fine positioner includes a correction mechanism for correcting yaw experienced by the top link of deformable body  16  (and the magnetic head  12 ). 
     The fine positioner of the preferred embodiment of the invention includes two piezo actuators  56   a  and  56   b  that are positioned between a stop  36 , which is rigidly mounted to carriage  18 , and the top link  16   a  of deformable body  16 . The piezo actuators  56   a  and  56   b  are laterally offset from each other (about the x axis) so that each can effect a displacement of a different side of link  16   a , and offset yaw. Deformable body  16  is sensitive to the expansions and contractions of piezo actuators  56   a  and  56   b  and thereby achieves fine positioning at the magnetic head  12 , while eliminating yaw. 
     As shown, piezo actuators  56   a  and  56   b  are placed approximate to two ends (i.e., left and right) of a top link  16   a  of deformable body  16 , and along the X-axis, such that the direction of force applied by actuators  56   a  and  56   b  is parallel to the X-axis. A bolt  52  is screwed into the center of deformable body  16  through an opening in stop  36 . Piezo actuators  56   a  and  56   b  are preloaded by a spring  50  that is compressed between the head of bolt  52  and the stop  36 , the stop being rigidly attached to carriage  18 . In the preferred embodiment, piezo actuators  56   a  and  56   b  are standard piezoelectric devices that change their linear dimensions in response to voltage applied to their respective inputs. An example of commercially available piezo actuator is a device P-844.20 produced by Physik Instrumente (PI) GMbH, Waldbronn, Germany. 
     As shown in FIG. 3B, in the preferred form of deformable body  16  is a four link ( 16   a ,  16   b ,  16   c  and  16   d ) parallelogram-structured deformable body  16  having a top link  16   a  that supports arm  14 , and a bottom link  16   b  that is secured to carriage  18 . The flexures at the vertices of the parallelogram-structure allow angular displacement of adjacent links while maintaining the parallelogram integrity of the structure. The top element  16   a  can be moved linearly in the x direction, and also can be selectively rotated about axes perpendicular to the translation plane using piezo actuators  56   a  and  56   b  which expand and contract independently. This selective rotation of link  16   a  provides a mechanism by which yaw can be corrected. Deformable body  16  also includes a bottom element  16   b  which rigidly attaches deformable body  16  to carriage  18 . Because of the fine degree of movement caused by piezo actuators  56   a  and  56   b  and the parallelogram-structure of deformable body  16 , the top link  16   a  remains substantially in a plane parallel to the translation plane throughout its range of displacement. Accordingly, Magnetic head  12  substantially remains in its plane throughout its range of displacement. 
     Referring again to FIG. 3A, linear movements of deformable body  16  along the X-axis are measured by two optical linear encoders  68   a ,  68   b ,  70   a  and  70   b , in the preferred embodiment. These encoders consist of moveable portions  68   a  and  70   a  that are secured to the left side  72  and the right side  74 , respectively, of link  16   a  of deformable body  16  and stationary portions  68   b  and  70   b  that are secured to base  30 . The moveable portions  68   a  and,  70   a  are glass scales and the stationary portions  68   b  and  70   b  are optical detectors. Signals generated by optical detector  68   b  and  70   b  correspond to linear positions of the left side  72  and the right side  74  of the top link  16   a  of deformable body  16 , respectively. Thus, independent signals representative of the linear displacements of the left and right sides of top link  16   a  are generated. Optical linear encoders  68   a ,  68   b  and  70   a ,  70   b  are standard devices, such as the encoder LIP401R produced by Heidenhain Corporation, Schaumburg, Ill. Those skilled in the art will appreciate that alternative embodiments of the present invention could use other types of devices to measure the linear displacements of the right and left sides of top link  16   a , such as magnetic, electrical, or mechanical encoders or similar devices, or some combination thereof. 
     The positioning process is split into steps of coarse and fine positioning in order to achieve high accuracy in linear positioning of magnetic head  12  with respect to magnetic disk  10 . The coarse position is effected by stepper motor  28  as it rotates lead screw  32  and generally places the magnetic head  12  in the vicinity of magnetic disk  10 . Rotational movement of lead screw  32  within nut  34  is translated into a linear movement of carriage  18  to accomplish this coarse positioning. Upon completion of coarse positioning, the fine positioner is activated by applying control voltages to piezo actuators  56   a  and  56   b . These control voltages are produced by a feed back system  110 , that includes a closed-loop control system  80 , as shown in FIG.  4 . The closed-loop control system  80  includes two identical portions A and B that are controlled by a controller  92 . Each portion, A and B, contains two adders  82   a ,  88   a  or  82   b ,  88   b , an integrator  84   a  or  84   b , a differentiator  86   a  or  86   b , a filter  90   a  or  90   b , and an amplifier  91   a  or  91   b . Controller  92  prescribes a predetermined command position (e.g., a position X 0 ) to both parts A and B of the closed-loop control system  80 . Once the left and right sides of top link  16   a  are positioned in accordance with the command position, any previously experienced yaw in the top element is substantially eliminated. 
     Part A of the closed-loop control system  80  receives a signal representative of the linear displacement or position of the left side of top element  16   a  from the left optical linear encoder  68   a ,  68   b . The position of the left optical detector  68   b  with respect to left glass scale  68   a  is compared with the command position X 0  in adder  82   a . The adder  82   a  calculates the difference between the command position and the detected position, referred to as the “left positioning error P L ”. This left positioning error is integrated by integrator  84   a , and is differentiated by differentiator  86   a . Adder  88   a  calculates the weighted sum of the positioning error, its integral I L , and its derivative D L . The output signal of adder  88   a  is filtered by filter  90   a  (e.g., a low-pass filter) and amplified by amplifier  91   a , which drives left piezo actuator  56   a . As a result, piezo actuator  56   a  moves the left side  72  of the top link  16   a  of deformable body  16  in the direction opposite to the positioning error, i.e., to the command position X 0 . 
     Part B of the closed-loop control system  80  gets positioning information from the right optical linear encoder  70   a ,  70   b . In a manner similar to that described with respect to part A, part B generates a control signal for the right piezo actuator  56   b  using two adders  82   b  and  88   b , integrator  84   b , differentiator  86   b , filter  90   b , and amplifier  91   a . Accordingly, a right positioning error P R , integral I R , and derivative D R  are determined to produce the control signal. This control signal is a function of the difference between command position X 0  and detected position of the right optical detector  70   b  with respect to right glass scale  70   a . As a result, piezo actuator  58   a  moves the right side  74  of the top link  16   a  of deformable body  16  in the direction opposite to the positioning error P R  (for right side  74 ), i.e., to the command position X 0 . 
     As a result of the above-described closed-loop positioning, the positions of both the left side  72  and the right side  74  of link  16   a  of deformable body  16  are substantially the same (i.e., X 0 ) along the X-axis at the end of any movement. Accordingly, the angle αbetween a horizontal axis which passes between the axis of shaft  25  and magnetic head  12  and the X-axis of linear horizontal movement (FIG. 3A) remains the same in the end of any movement (when head testing will be performed). Therefore, positioning errors associated with yaw are eliminated. 
     Another advantage of the tester of the present invention is that the fine positioner actively damps oscillations of deformable body  16 , arm  14 , and magnetic head  12  in the yaw direction during a positioning process, wherein the actuators  56   a  and  56   b  iteratively adjust for yaw and thereby control oscillations. This improvement in the dynamic characteristics of the tester results in shorter settling times, which corresponds to more efficient and effective testing. 
     In another embodiment, feedback from both the left and the right optical linear encoders  68   a ,  68   b  and  70   a ,  70   b  can be used to control each of two piezo actuators  56   a  and  56   b  instead of feedback from only the linear encoder on the same side as the piezo actuator. For example, weighted sums a 11 X L +a 12 X R  and a 21 X 1 +a 22 X R  can be used as inputs of adders  82   a  and  82   b , respectively, where X L  and X R  are detected positions of the left side  72  and the right side  74  of the top link  16   a  of deformable body  16 , a 11 , a 12 , a 21  and a 22  are weighting coefficients related to alignment of the encoders with the piezo actuators. The preferred embodiment described above is a special case of this equation, where a 11 =a 22 =1 and a 12 =a 21 =0. Non-zero values for weighting coefficients a 12  and a 21  can improve dynamic characteristics of the tester in the case where the axes of piezo actuators  56   a  and  56   b  do not coincide with the axes of optical linear encoders  68   a ,  68   b  and  70   a ,  70   b , for example actuators could be placed on the right or left side of link  16   a  of deformable body  16 , or some combination thereof. Also, the present invention could be used with other types of actuators. 
     The invention may be embodied in other specific forms without departing from the spirit or central characteristics thereof. For example, a head stack with many magnetic heads and a disk pack can be used instead of the single magnetic head and the single magnetic disk. Additionally, rather than a dual-stage positioning system, a positioning system having only one or more than two positioning stages could also be used, so long as at least one stage is capable of performing fine positioning. As an example, the coarse positioner could take a variety of forms, such as a belt driven (rather than screw driven) positioner, and the fine positioner need not be displaced by a piezoelectric device. The carriage could alternatively be placed on one or more rollers or bearings, instead of rails and displaced by gears, pistons, belts or similar devices. Also, the bottom link of the deformable body could be integral with the carriage and the deformable body could take a form other than a parallelogram-structured deformable body, so long as the fine positioner maintains movement of the magnetic head substantially in the second plane. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by appending claims rather than by the foregoing description, and all changes that come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.