Method and apparatus for testing bipolar magnets having a circular curvature
BACKGROUND OF THE INVENTION 
     This invention relates generally to the testing of magnets as used in magnetic disk drives, and more particularly the invention relates to the testing of bipolar magnets. 
     Digital data is magnetically stored on the surfaces of disks in a disk drive. The disks are rotated at high speed, and pick-up heads move across the surfaces of the disks for storing and accessing data in concentric tracks on the disk surfaces. The pick-up heads are mounted on the ends of arms extending from an actuator which rotates the arms and heads to select a track. The actuator includes a voice coil which is placed in a magnetic field, and the actuator rotates in response to energization of the coil by the passage of current through the coil windings. 
     The energization of the coil in moving the pick-up heads to a desired track is controlled by a programmed computer. The program is designed to tolerate some non-uniformity in the magnets. However, it is desireable to ascertain the characteristics of the magnets prior to use in a disk drive. Moreover, by measuring the magnet characteristics, manufacturers of the magnets can modify the manufacturing processes to optimize the characteristics. 
     Heretofore, a pair of unipole (e.g. a pole on each of two major surfaces) magnets have been used in driving the actuator coil. More recently, single bipolar magnets have replaced the pair of unipolar magnets. While the characteristics of a unipole magnet can be measured through use of a Helmholz coil, the Helmholz coil cannot be used with a bipolar magnet due to the cancelling affects of the poles. Further, bipolar magnets are unique in having a transition from one pole orientation to another on each major surface. The physical location of this transition on the surface of a magnet can vary. Additionally, a neutral or null zone surrounds the transition line due to domain disorientation on each side of the transition line which results in decreased magnetic performance of that area of the magnet. 
     Characteristics of magnets are established by the magnet manufacturing processes including rare earth powder blending, pressing, sintering and heat treatment, coating, and magnetizing. By monitoring the magnetic field strength, transitional line location, and neutral zone width, the manufacturing processes can be altered to attain optimal characteristics. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the invention is apparatus for inspecting bipolar magnets. 
     Another object of the invention is a method of determining magnetic characteristics of a bipolar magnet. 
     Still another object of the invention is an improvement in manufacturing processes for bipolar magnets. 
     Yet another object of the invention is improved actuators in magnetic disc drives. 
     A feature of the invention is the use of a Hall sensor probe in measuring magnetic field strength along selected radii of a magnet. 
     Briefly, in accordance with the preferred embodiment of the invention, magnetic field strength is measured along three radii of a bipolar magnet including an inner radius, an outer radius, and a central radius. The measurements along the inner and outer radii are used to define the neutral zone and transition boundaries. The central radius is used to determine the overall magnetic field strength. 
     In carrying out the invention in accordance with a preferred embodiment, a stepper motor is used to accurately position the Hall sensor probe at a plurality of test points along each radius. All measurements are down-loaded to a computer for processing and code generation. 
     The invention and objects and features thereof will be more readily apparent from the following detailed description and appended claims when taken with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an isometric view of a disk drive illustrating the actuator therein. 
     FIG. 2 is a front elevation view of the magnetic assembly for the actuator in FIG. 1. 
     FIG. 3 is an isometric view of a bipolar magnet as used in the actuator of FIG. 1. 
     FIG. 4 is a plan view of a bipolar magnet illustrating radii thereof used in testing the magnet in accordance with the invention. 
     FIG. 5 is plot illustrating typical magnetic measurements for an acceptable bipolar magnet and for a magnet having domain disorientation. 
     FIG. 6 is a functional block diagram of apparatus for inspecting a bipolar magnet in accordance with one embodiment of the invention. 
     FIG. 7 is a flow diagram of a computer program used in controlling the apparatus of FIG. 6 in obtaining the measurements for the plots of FIG. 5. 
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     Referring now to the drawing, FIG. 1 is an isometric view of a magnetic disk drive including a base 10 which supports a spin motor (not shown) on which are mounted a plurality of disks 12. As the disks are rotated by the spin motor, data is magnetically stored on surfaces of the disks by means of magnetic pick up heads 14 mounted on the ends of arms 16 which are attached to an actuator shown generally at 18. The actuator 18 includes a rotary mounted shaft 20 on which is mounted a voice coil (not shown) that is positioned in a magnetic assembly 22. Assembly 22 includes a bipolar magnet which establishes a generally uniform magnetic field in which the voice coil is positioned. By selectively energizing the voice coil, the actuator is rotated thereby moving the pickup heads 14 on concentric tracks on the surfaces of the disks for magnetic data access and recording. 
     FIG. 2 is a front elevation view of the magnetic assembly 22 which includes a top bracket 24 and a bottom bracket 26 which together define a magnetic path for flux from the bipolar magnetic shown at 28. Flux from magnet 28 crosses a gap to the bottom plate 26 and then through bracket 24 back to magnet 28. A gap is provided between the magnet 28 and the bottom bracket 26 in which the voice coil is positioned and linearly rotated as indicated by the arrow 30. 
     FIG. 3 is an isometric view of a bipolar magnet which has a curvature defined by a center of curvature at 32. The bipolar magnet has two magnetic poles (north, south) on each major surface as shown. The line of demarkation shown generally at 34 between the north pole and south pole of the exposed surface of magnet 28 is the transition line between the poles and should be generally positioned in the center of the magnet. A neutral or null zone surrounds the transition line due to magnetic domain disorientation on each side of the transition line and results in decreased magnetic performance of that area of the magnet. The null zone can increase or decrease in severity and width from one magnet to another. 
     In accordance with the invention the characteristics of a bipolar magnet are determined from magnetic measurements taken along radii of the magnet. The magnetic characteristics can be used in determining acceptable and unacceptable magnets for use in a disk drive, and the characteristics can be used by the magnet vendors in modifying their manufacturing processes. 
     FIG. 4 is a top plan view of a magnet illustrating several radii including Rmin, R low , R nom , R hi  and R max . In a two and one half inch disk drive, the radii are as follows: 
     R min  =0.420 inch 
     R low  =0.477 inch 
     R nom  =0.610 inch 
     R hi  =0.743 inch 
     R max  =0.800 inch 
     In accordance with the invention a gaussmeter probe such as a Hall sensor is swept along a plurality of radii, and magnetic field strength is measured at equi distant points along each radius. Measurements are begun with the gaussmeter probe positioned at one edge of the magnet (position 0) and the probe is swept along the radius until the probe is positioned at the opposite edge of the radius (position 240). Two hundred and forty test points are provided between the two edges of the magnet. Thus, a total of 240 measurements are taken along each radius. From these measurements the peak value and location for each pole is determined and the overall gauss profile is determined for each radius. The transition between poles is the angular location calculated for measurements at the inner and outer radii, and the neutral zone is determined by the angular neutral zone widths of all three radii. 
     FIG. 5 is a plot of measurements along one radius for two different magnets. The measurements start at index point 0 and proceed through index point 240 as described above with reference to FIG. 4. In curve A the pole strength is relatively uniform for both the positive and negative poles of the magnet, and the transition from one pole to the other occurs near the center of the magnet with a transition being relatively uniform. However, in measurement B the transition is more inclined and off center in the magnet. Further, the strength of the magnet is non-uniform between increment positions 120 and 220, and that the slope of curve B is less than is the slope of curve A in the transition region. By knowing the overall field strength as well as the transition and neutral zone boundaries, the manufacturer of the magnets can make suitable adjustments in the manufacturing processes to achieve optimum magnet characteristics. 
     FIG. 6 is a functional block diagram of one embodiment of equipment used in carrying out the method. The gaussmeter probe 38 includes a Hall effect device 40 mounted at one end with the other end being transversely affixed to the shaft of a stepper motor 42. The stepper motor is mounted on a frame 44 with limit switches 46 provided on either side of the frame 44 for limiting the movement of the gaussmeter probe 38. The gaussmeter probe is connected to a gaussmeter 48 which converts the measurements from the Hall effect device 40 into Kilo Gauss values. The gaussmeter 48 is connected to a computer 50 which controls a printer 52 and graphics plotter 54 for readout of the measured magnetic field strength. Computer 50 is also connected to the stepper motor power supply 56 for controlling the stepper motor during a test cycle. In one embodiment a Compac desktop computer was used. 
     FIG. 7 is a flow diagram of a control program for the stepper motor. The Hall effect device is positioned on the magnet radius to be traversed at position 0, and then the probe is moved incrementally from position 0 to position 240. At each position the probe is stopped and the flux density is determined by the Hall effect device. When the count reaches 240, the measurement sequence is completed and the probe movement is stopped. Data can then be displayed and stored while the probe is moved to another radius. After the probe has traversed R min , R nom , and R max  the program is exited. 
     The method of characterizing a magnet using the measurement techniques in accordance with the invention has proved to be useful in magnet qualification and as feedback to vendors in modifying processes in fabricating the magnets. While the invention has been described with reference to a specific embodiment, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications and applications may occur to those skilled in the art without departing from the true spirit and scope of the invention as defined by the appended claims.