Abstract:
An inductive rotational accelerometer for assisting in servo control of a hard disc drive using a torsional mass-spring system in combination with an inductance sensitive circuit to detect and measure the rotational vibrations imposed on a hard disc drive. The inductive rotational accelerometer includes a frame member coupled to the disc drive, the frame member supporting a pin on which is disposed a rotational mass. The rotational mass supports conductive blocks which with the rotational mass and the frame member provide a path for inductance.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of provisional application Ser. No. 60/126,397 entitled “Rotational Inductive Accelerometer For Measuring And Canceling Rotational Vibration In Disc Drives” filed Mar. 26, 1999. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to a disc drive system. More particularly, the present invention relates to an apparatus for detecting and measuring rotational vibrations impacting a disc drive in order to improve track following. 
     BACKGROUND OF THE INVENTION 
     In a contemporary disc drive, a transducer records information onto a magnetic disc in concentric tracks. Each piece of data that is recorded on the magnetic disc is assigned a location. When the information is needed, the transducer must return to the exact location and track where the piece of data has been stored. 
     As track densities have increased, disc drives have become more sensitive to vibrations which deflect the transducer from the track it follows or which cause the magnetic disc to vibrate beneath the transducer. In effect, vibrations within the disc drive cause the disc to move or slip underneath the transducer. Motion of the magnetic disc relative to the transducer can cause the transducer to slip further along the track producing read/write errors. Furthermore, a contemporary disc drive needs to meet exacting standards with respect to the speed with which data can be accessed and recorded. Movement of the magnetic disc relative to the transducer slows down both information retrieval times and data recording times for the system. There exists a need to detect and compensate for these vibrations before they cause slipping of the magnetic disc. 
     Rotational accelerations as low as 21 radians/second 2  can cause track slipping. One source of rotational vibration involves disc drives stacked in close proximity to each other. An actuator arm controls the movement of the transducer relative to the magnetic disc for each disc drive. During a seeking mode, the actuator arm of a disc drive will move the transducer rapidly over the surface of the magnetic disc. The rapid movement of the actuator arms in such close proximity to other disc drives can cause rotational vibrations which affect the track following performance of nearby disc drives. When dozens of disc drives are stacked together, the effect can be significant. 
     Several solutions to this problem have been suggested. Dedicated servo surface systems attempt to maintain constant information regarding the transducer&#39;s position relative to the magnetic disc by dedicating a portion of the magnetic disc space to storing this information. This information is then used by a servo control system to compensate for track skipping during use. This solution suffers from the obvious disadvantage of consuming disk space which would otherwise be available for other data. 
     Embedded servo surface systems embed periodic reference points on the surface of the magnetic disk to provide the system with position information. This system requires less disc surface space than the dedicated servo surface systems, but they do not provide constant position information. Embedded reference points only provide position information periodically as the transducer passes over a reference point. Therefore, embedded servo surface systems do not provide instantaneous and constant position information. 
     SUMMARY OF THE INVENTION 
     The use of accelerometers to detect and measure rotational vibrations offers the advantage of requiring little magnetic disc space while at the same time providing constant information to the servo control system enabling the servo control to compensate for rotational vibrations. 
     The present invention relates to an inductive accelerometer for detecting and measuring rotational vibrations in a disc drive. In accordance with one embodiment of the present invention there is provided a rotational mass disposed on a pin having two ends. The pin is held at its ends by a top frame member and a bottom frame member. Both frame members are secured to the hard disc drive. The pin and rotational mass act as a torsional mass-spring system. Disposed on the rotational mass are ferro-magnetic blocks. The ferro-magnetic blocks overlap the bottom frame member. Together the rotational mass, the ferro-magnetic blocks, and the bottom frame member make a path for magnetic flux. A wire coil is disposed around the bottom frame member. Rotational accelerations cause the rotational mass to twist the pin and rotate momentarily relative to the bottom frame member. This rotation causes a portion of the ferro-magnetic blocks not to overlap the bottom frame member. A change in the magnetic flux will result which induces a change in the inductance of the wire coil. The change in inductance of the wire coil is proportional to the rotational acceleration applied to the hard disc drive. By measuring the change in inductance of the wire coil, the system can supply a servo control system with information necessary to compensate for the rotational acceleration so detected. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a top view of a disc drive. 
     FIG. 2 shows a perspective view of one embodiment of the present invention. 
     FIG. 3 shows a front plan view of one embodiment of the present invention. 
     FIG. 4 shows a side plan view of one embodiment of the present invention. 
     FIG. 5 shows a bottom view of one embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 1 is a top view of a disc drive  100 . Disc drive  100  includes a magnetic disc  102  mounted for rotational movement about an axis defined by spindle  104  within housing  106 . Disc drive  100  also includes a stacked actuator system  108  mounted to a base plate  110  of the housing  106  and pivotally movable relative to disc  102  about axis  112 . The actuator system  108  supports a transducer assembly  126  for reading information on the disc and for encoding information on the disc. The information on the disc  102  is stored in concentric tracks. A cover  114  covers a portion of stacked actuator system  108 . Drive controller  116  is coupled to stacked actuator system  108 . Drive controller  116  is either mountable within disc drive  100  or is located outside of disc drive  100  with suitable connection to stacked actuator system  108 . 
     FIG. 2 is a perspective view of a rotational inductive accelerometer  200  configured according to the present invention. The accelerometer  200  comprises a top frame member  203 , a ferro-magnetic bottom frame member  207 , a pin  215 , a ferro-magnetic rotational mass  217 , first and second ferro-magnetic blocks  219  and  221 , and first and second wire coils  225  and  227 . 
     The top frame member  203  and the bottom frame member  207  are both secured to the housing  106  of the disc drive  100  in FIG.  1 . The top frame member  203  includes a top bar  205  which is parallel to the surface of the disc drive housing  106 . The bottom frame member  207  includes a bottom bar  213  which is parallel to the surface of the disc drive housing  106 . The top frame member  203  is configured to straddle the bottom frame member  207  so that the bottom bar  213  lies between the top bar  205  and the surface of the disc drive housing  106  and so that the top bar  205  and bottom bar  213  are oriented perpendicular to each other. The top frame member  203  may be comprised of steel or an alternative metal, ceramic, plastic or composite. The bottom frame member  207  which includes two side posts  209  and  211 , the blocks  219  and  221 , and the rotational mass  217  is preferably formed of a ferro-magnetic material. 
     The pin  215  includes a top end  214  and a bottom end  216  (shown in FIG.  5 ). The pin  215 , which preferably is formed of steel, is mechanically pressed through the rotational mass  217 . Similarly, the top end  214  is pressed though the top bar  205 , and the bottom end  216  is pressed through the bottom bar  213 . The pin  215  is disposed between the top bar  215  and the bottom bar  213  so that the pin  215  is substantially perpendicular to the surface of the disc drive housing  106 . The pin  215  is capable of twisting along an axis defined by its length. 
     The rotational mass  217  is characterized by a rotational moment of inertia. That is, the rotational mass  217  may be configured to resist rotational acceleration in a plane parallel to a plane defined by the magnetic disc  102  of the disc drive  100 . FIG. 2 shows one preferred embodiment of the system wherein the rotational mass is configured in the shape of a disk. 
     Together the rotational mass  217  and the pin  215  act as a torsional mass-spring system. The top bar  205  of the top frame member  203  and the bottom bar  213  of the bottom frame member  207  define fixed boundary positions for each end of the pin  215 . When not subject to rotational acceleration, the rotational mass  217  maintains a fixed initial position relative to the bottom frame member  207 . When rotational acceleration is applied to the disc drive housing  106  and thereby to the top and bottom frame members  203  and  207 , which are secured to the disc drive housing  106 , each end of the pin  215  experiences rotational acceleration. The rotational mass  217 , however, resists the rotational acceleration due to its moment of inertia. This resistance causes the pin  215  to twist momentarily, and the rotational mass  217  is temporarily rotated away from its initial position relative to the bottom frame member. The extent of displacement of the rotational mass  217  away from its initial position is proportional to the magnitude of the rotational acceleration applied to the disc drive housing  106 . 
     The bottom frame member  207  may include two side posts, a first side post  209  and a second side post  211 . The side posts  209  and  211  are configured to be perpendicular to the disc drive housing  106 . The side posts  209  and  211  may be evenly spaced so that each post is equidistant from the pin  215 . The side posts  209  and  211  have uppermost ends  210  and  212 . 
     Two ferro-magnetic blocks  219  and  221  are coupled to a surface of the rotational mass facing the bottom frame member  207 . The ferro-magnetic blocks  219  and  221  may be configured to be equidistant from the pin  215  so that each ferro-magnetic block is suspended above one of the side posts  209  and  211  of the bottom frame member  207 . As shown in FIGS. 3 and 4, the ferro-magnetic blocks  219  and  221  and the uppermost ends  210  and  212  of the side posts over which they are suspended define an air gap. The ferro-magnetic blocks  219  and  221  may be configured to have a cross section, defined by a plane parallel to the disc drive housing  106 , substantially identical in size and shape of a similarly defined cross section of the uppermost ends  210  and  212  of the side posts  209  and  211 . 
     A portion of the cross section of each ferro-magnetic block overlaps the cross section of the uppermost ends of the side posts. The ferro-magnetic blocks  219  and  221  are configured so that when the accelerometer is at rest only a portion of the cross section of each block is suspended above the uppermost ends of the side posts. As shown in FIGS. 2 and 3 ferro-magnetic blocks  219  and  221  are offset so that not all of the cross section of the blocks is suspended above the side posts  209  and  211 . By configuring the ferro-magnetic blocks in this manner, the direction of the acceleration can be identified by the change in inductance of the system being either positive or negative. As is discussed below, depending on the direction of the angular acceleration, the portion of the ferro-magnetic blocks that overlaps the side posts will either increase or decrease. The ferro-magnetic blocks  219  and  221  may be soft iron, stainless steal, magnets or any other ferromagnetic material. 
     Around at least one of the side posts is wound a first wire coil  225 . Preferably, a second wire coil  227  identical to the first wire coil  225  is wound around the second side post  211  as shown in FIG.  5 . Each wire coil  225  and  227  is characterized by an inductance. The wire coils  225  and  227  are coupled to an inductance meter  231  or any other inductance sensing circuit. 
     The bottom cross bar  213 , the side posts  209  and  211 , the ferro-magnetic blocks  219  and  221 , and the rotational mass  217  together define a path for magnetic flux. The magnetic flux is dependent in part on the size of the portion of the cross section of each ferro-magnetic block  219  and  221  that overlaps the cross section of the uppermost end  210  and  212  of the side posts  209  and  211 . When the overlapping cross section decreases in size due to rotation of the rotational mass  217  in a first direction relative to the bottom frame member  207 , the magnetic flux passing through the path decreases. Alternatively, when the overlapping cross section increases in size due to rotation of the rotational mass  217  in an direction opposite to the first direction relative to the bottom frame member  207 , the magnetic flux passing through the path increases. Changing the magnetic flux passing through a wire coil will produce a proportional change in the inductance of the wire coil. The wire coils  225  and  227  are configured so that a change in the magnetic flux of the system will produce a change in the inductance of the wire coils. 
     The inductive rotational accelerometer  200  measures the magnitude of a rotational vibration by measuring the change in the inductance of the wire coils  225  and  227  caused by changing the magnetic flux passing through the side posts around which they are coiled. The change in magnetic flux through the side posts  209  and  211  is caused by rotation of the rotational mass  217  and ferro-magnetic blocks  219  and  221  relative to the side posts  209  and  211 . The inductance meter used to detect and measure the change in inductance of the wire coils may be electrically coupled to a voltage circuit which would produce a voltage signal proportional to the change in inductance of the wire coils. The change in the inductance of the wire coils is proportional to the magnitude of rotational acceleration applied to the accelerometer. 
     The voltage signal produced by the present invention may be used by a servo control device of the drive controller  116  to sense and dynamically cancel rotational disturbances to a disc drive. Rotational accelerations as small as 21 rad/second 2  may cause the transducer of the transducer assembly  126  to lose its track placement. Therefore in a preferred embodiment the rotational accelerometer is configured to detect rotational accelerations as low as 21 rad/second 2 . 
     The sensitivity of the accelerometer can be modified by varying the dimensions of the rotational mass  217  and the pin  215 . The dimensions of the pin  215  such as length and diameter define a spring constant value for the pin. The change in inductance of the system produced by any angular acceleration is inversely proportional to the spring constant of the pin  215 . For example, lengthening the pin  215  or decreasing its diameter will decrease the spring constant value for the pin  215  resulting in an increase in the change of inductance produced by angular acceleration. Similarly, the dimensions of the rotational mass  217  may be varied in order to provide a voltage signal response for rotational accelerations within a measurement bandwidth of interest. For example, increasing the rotational inertia of the rotational mass  217  will increase the displacement of the mass when the system is subject to rotational acceleration thereby increasing the sensitivity of the accelerometer. In addition, the design of the accelerometer is such that the symmetry of the accelerometer cancels out the effects of translational accelerations thereby minimizing the effect of translational acceleration on the accelerometer. 
     The dimensions of the rotational mass  217  are preferably configured so that the system is characterized by a natural frequency which is several times the value of a maximum frequency of angular accelerations to be measured by the accelerometer. The natural frequency of the system is proportional to the square root of the quantity, the spring constant of the pin  215  divided by the moment of inertia of the rotational mass  217 . This relationship is expressed in the formula: ω∝(K/J), where ω is the natural frequency of the system, K is the spring constant of the pin  215 , and J is the inertia of the rotational mass  217 . For a maximum frequency of angular accelerations of 800 Hz, the system is preferably configured with a natural frequency between 3000 and 5000 Hz. 
     In summary, the present invention is directed to an apparatus for detecting and measuring rotational vibration of a disc drive  100  having a top frame member  203 , a bottom frame member  207 , a pin  215 , a rotational mass  217  attached to the pin  215 , a first and second ferro-magnetic blocks  219  and  221  and a wire coil  225 . The top frame member  203  has a cross bar and is coupled to the disc drive. The bottom frame member  207  has two side posts  209  and  211  and a bottom cross bar  213 . The bottom cross bar  213  is coupled to the disc drive  100  and couples the two lower side posts  209  and  211 . The pin  215  has a top end  214  and a bottom end  216  and the top end  214  is coupled to the cross bar  205  of the top frame member  203  and the bottom end  216  is coupled to the cross bar  213  of the bottom frame member  207 . The rotational mass  217  is disposed on the pin  215  so as to surround the pin  215 . The rotational mass  217  has a moment of inertia. The first and second ferro-magnetic blocks  219  and  221  are disposed on a surface of the rotational mass  217  facing the bottom frame member  207 . Each of the ferro-magnetic blocks  219  and  221  is suspended above one of the uppermost ends of the two side posts  209  and  211  of the bottom frame member  207  to define an air gap therebetween. The ferro-magnetic blocks  219  and  221  have a surface area that overlaps the side posts  209  and  211 . The wire coil  225  is disposed around one of the side posts of the bottom frame member  207  and is electrically coupled to an inductance meter  231 . When an angular accelerating force is applied to the disc drive  100 , the top and bottom frame members  203  and  207  rotate relative to the rotational mass  217  due to the inertia of the rotational mass  217  thereby causing the cross sections of the conducting blocks  219  and  221  that overlap the side-posts  209  and  211  to change in area resulting in a change in the inductance of the wire coil  225 . 
     In addition there is provided an apparatus for detecting and measuring rotational vibration and acceleration of a disc drive  100  to assist in servo control. The apparatus includes a ferro-magnetic rotational mass  217  connected to a frame member that is mounted on the disc drive  100  by a mounting means  215 . The mounting means is capable of torsional movement where the rotational mass  217  is characterized by a moment of inertia. A ferro-magnetic block  219  is disposed on the rotational mass  217  and acts with the rotational mass  217  and the frame member to define a path for magnetic flux. The path for magnetic flux is characterized at at least one point by a cross sectional area defined by an area that the ferro-magnetic block  219  overlaps the frame member. Also included is a wire coil  225  around the frame member that is electrically connected to a means for measuring  231  the inductance of the coil. When the disc drive  100  experiences angular acceleration, the frame member connected to the disc drive  100  moves rotationally relative to the rotational mass  217  to cause a change in the cross-sectional area of the path for magnetic flux thereby producing a change in the inductance of the wire coil  225  as measured by the measuring means  231 . The change in inductance of the wire coil  225  is proportional to the angular acceleration of the disc drive  100 . The apparatus detects and measures the angular acceleration of the disc drive  100 . 
     The above specification, examples and data provide a complete description of the manufacture and use of the composition of the invention. Since many embodiments of the invention can be made without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.