Abstract:
A disk drive that includes a base, a magnetic disk, a rotary actuator that carries a head for reading and writing data from the disk in a track-following mode under the control of a servo control system, and at least two sensors—one fixed sensor rigidly coupled to the overall disk drive and one mobile sensor mounted to the rotating actuator—for differentially detecting accelerations of the rotary actuator relative to the overall disk drive and its disk. The disk drive detects and actively compensates for accelerations imparted to a balanced actuator that has an effective imbalance. The fixed sensor is preferably mounted to a PCBA that is secured to the base. The mobile sensor is preferably mounted to an actuator arm of the rotary actuator, as far outboard as possible, and so as to align with the fixed sensor as the rotary actuator swings through its range of travel. The preferred sensors are linear accelerometers.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to magnetic disk drives and, more particularly, to active vibration cancellation within a disk drive using a multitude of accelerometers. 
     BACKGROUND OF THE RELATED ART 
     Magnetic disk drives generally read and write data on the surface of a rotating magnetic disk with a transducer or “head” that is located at the far end of a moveable actuator. A servo control system uses servo control information recorded amongst the data, or on a separate disk, to controllably move the transducer from track to track (“seeking”) and to hold the transducer at a desired position (“track following”). A detailed discussion of servo control systems is unnecessary because such systems are well known as set forth, for example, in patent application Ser. No. 09/138,841 that was filed on Aug. 24, 1998, entitled “DISK DRIVE CAPABLE OF AUTONOMOUSLY EVALUATING AND ADAPTING THE FREQUENCY RESPONSE OF ITS SERVO CONTROL SYSTEM,” and is commonly owned by the assignee of this application. 
     The industry has previously mounted various kinds of accelerometers on the disk drive in order to sense external forces. 
     One example is U.S. Pat. No. 5,426,545 entitled “Active Disturbance Compensation System for Disk Drives.” This patent discloses an angular acceleration sensor  22  that comprises an opposed pair of linear accelerometers  22   a  and  22   b . The invention is intended for use with balanced actuator assembly  26 . The overall sensor package  22  is mounted to the HDA  10  or drive housing, as shown in FIG. 1, in order to detect and correct for angular acceleration about the axis  27  of the balanced actuator assembly  26  that would otherwise produce a radial position error  30  (FIG. 2) due to the actuator&#39;s inertial tendency to remain stationary in the presence of such acceleration. 
     U.S. Pat. No. 5,521,772 entitled “Disk Drive with Acceleration Rate Sensing” discloses a variation on that theme in that it uses an “acceleration rate sensor”  50  rather than a linear acceleration sensor (conventional accelerometer) or angular acceleration sensor. The sensor  50  is mounted to the disk drive housing  9 . 
     U.S. Pat. No. 5,663,847 is yet another patent disclosing an angular accelerometer in a disk drive. It is entitled “Rejection of Disturbances on a Disk Drive by Use of an Accelerometer.” In FIG. 1, the &#39;847 patent discloses an angular accelerometer 102 that is mounted to the drive&#39;s base plate 104 in order to sense rotational motion 110. The &#39;847 patent is similar to the &#39;545 patent in that both are addressing the problem that when the disk drive containing a balanced actuator is bumped rotationally in the plane of the disk 112, a position error will arise because “the actuator 114 will retain its position in inertial space . . . ” (4:19-21). 
     PCT Application WO 97/02532 discloses another apparent use of an accelerometer that is described therein as a “shock sensor”  46  (See FIG.  3 ). This application is entitled “Disk Drive Data Protection System”. The WO 97/02532 application appears similar to the remainder of the presently known art in that it appears to disclose a single sensor that is mounted to the drive housing. According to the disclosure, the shock sensor  46  detects “physical shocks to the disk drive which may compromise data being transferred . . . ” 
     Conventional systems mount a single accelerometer to the overall disk drive and disable reading and/or writing when the output of the accelerometer surpasses a threshold. The &#39;545 patent discussed above is different in that it uses a angular acceleration sensor mounted to the overall disk drive to indicate when the drive is being shocked or vibrated about the pivot axis of a balanced actuator. However, it is only sensitive to rotational motion and it assumes that the actuator is a perfectly balanced actuator. 
     The foregoing uses of accelerometers are incapable of accurately detecting a motion of the head relative to the remainder of the disk drive and are subject, therefore, to an off-track condition due to acceleration of an imbalanced actuator. There remains a need, therefore, for a disk drive that detects the motion of the actuator relative to the disk drive and implements active vibration cancellation using a multitude of sensors. 
     SUMMARY OF THE INVENTION 
     The invention may be regarded as a disk drive comprising a head disk assembly  20  including a base  21 , a rotating disk  23 , and a rotary actuator  50  that pivots relative to the base; a first motion sensor  35  rigidly mounted relative to the base for sensing motion of the head disk assembly; and a second motion sensor  55  mounted to the rotary actuator for sensing motion of the rotary actuator relative to the motion of the head disk assembly. In a more particular embodiment, the first motion sensor is rigidly mounted relative to the base to output a first sense signal, the second motion sensor is mounted to the rotary actuator to output a second sense signal, and the disk drive further includes a means for controlling a disk function in response to a comparison of the first and second sense signals. 
     The invention may also be regarded as a method of controlling a disk drive having a head disk assembly  20  including a base  21 , a rotating disk  23 , and a rotary actuator  50  that pivots relative to the base, in order to achieve improved track following performance by reducing off-track error caused by shock and vibration, the method comprising the steps of: generating a first sense signal corresponding to a motion of the head disk assembly; generating a second sense signal corresponding to a motion of the rotary actuator relative to the motion of the head disk assembly; comparing the first and second sense signals in order to detect off-track movement of the rotary actuator while track-following; and compensating for the off-track movement. In a preferred embodiment of the method, the step of generating a first sense signal corresponding to a motion of the head disk assembly is accomplished by mounting a first motion sensor  35  rigidly relative to the base and the step of generating a second sense signal corresponding to a motion of the rotary actuator relative to the motion of the head disk assembly is accomplished by mounting a second motion sensor  55  to the rotary actuator that pivots relative to the base. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The just summarized invention can be best understood with reference to the following description taken in view of the drawings of which: 
     FIG. 1 is an exploded perspective view of a magnetic disk drive  10  according to a preferred embodiment of the invention, the disk drive having a head disk assembly  20  (“HDA”) that contains a magnetic disk  23 , a rotary actuator  50 , a first acceleration sensor  35  that moves rigidly with the HDA  20  and a second acceleration sensor  55  that rotates with the rotary actuator  50 ; 
     FIG. 2 is a simplified plan view of the disk drive  10  showing how a head  80  carried by the rotary actuator  50  moves through a first arc  58  and how the second sensor  55  carried by the rotary actuator  50  moves through a second arc  56 ; 
     FIG. 3 is a simplified plan view of the disk drive  10  showing the PCBA  30  that carries the first sensor  35  below the rotary actuator  50  that carries the second sensor  55 ; 
     FIG. 4 is a simplified plan view of the disk drive  10  showing that the second sensor  55  is preferably located over the first sensor  35  when the rotary actuator  50  is at a middle diameter of the disk  23 ; and 
     FIG. 5 is schematic diagram of a simplified system model for a microprocessor-based embodiment wherein the first and second sensors  35 ,  55  are used to compensate for motion that is otherwise undesirably imparted to the rotary actuator  50  by shock and vibration; 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1 shows a preferred embodiment of a disk drive  10  according to this invention. As shown, the disk drive  10  comprises a head disk assembly (“HDA”)  20  including a base  21 , a rotating disk  23 , and a rotary actuator  50  that pivots relative to the base  21 . In this first embodiment, the disk drive  10  further comprises a first motion sensor  35  rigidly mounted relative to the base  21  for sensing motion of the HDA  20 , and a second motion sensor  55  mounted to the rotary actuator  50  for sensing motion of the rotary actuator  50 , both with and relative to the motion of the HDA  20 . 
     There are preferably two sensors  35 ,  55 , but it is possible to use more than two sensors in a more complicated differential arrangement. The preferred sensors  35 ,  55  are linear accelerometers with a single sense axis  35   s ,  55   s  (see FIG.  2 ), but multi-axis sensors and other types of motion sensors altogether may also be used in a differential mode in accordance with this invention. 
     In the preferred embodiment, a PC Board Assembly (PCBA)  30  that contains suitable control electronics is rigidly mounted to an underside of the base  21 . The disk  23  is rotated by a spindle motor  22 . The rotary actuator  50  rotates about a pivot axis extending through a center of a pivot cartridge  51  that secures the actuator  50  to the base  21 , adjacent to the disk  23 . An actuator arm  54  extends to one side in order to carry a head  80  over the disk  23  for reading and writing data therefrom and a voice coil  52  extends from the other side for interacting with a pair of permanent magnets  60 . The voice coil  52  and magnets  60  are frequently regarded as a “voice coil motor”, or VCM  40 . A cover plate  24  encloses the foregoing components in a cavity within the base  21 . 
     The first sensor  35  is rigidly coupled to the base  21 . In the preferred embodiment, it is indirectly mounted to the base  21  by being mounted to the PCBA  30  that is itself rigidly mounted to the base  21 . It is possible, of course, to mount the first sensor  35  directly to the base  21 , or to mount it to any other structure that is, in turn, fixed to the base  21 . The second sensor  55  is mounted to the rotary actuator  50 . It is desirable to provide maximal sensitivity to rotational actuator motion. As such, the second sensor  55  is preferably positioned on the actuator arm  55 , as far as possible from the pivot cartridge  51 . The second sensor  55 , however, may be located elsewhere on the actuator  50 , such as on the voice coil  52 . Such placement however must be done while maintaining vertical registration with first sensor  35  as discussed below. 
     As shown in FIG. 5, discussed in more detail below, the first sensor  35  that is rigidly mounted relative to the base  21  outputs a first sense signal and the second sensor  55  that is mounted to the rotary actuator  50  that pivots relative to the base  21  outputs a second sense signal  56 . The preferred embodiment further comprises suitable means for controlling a motion of the actuator in response to a comparison of the first and second sense signals  36 ,  56 . The preferred means for controlling uses suitable hardware and/or firmware on the PCBA  30  to implement the control system shown in FIG. 5, but other more or less complicated control means are possible. 
     FIG. 2 is a simplified plan view of the disk drive  10  showing how the head  80  that is carried by the rotary actuator  50  moves through a first arc  58 , while the second sensor  55  moves through a second arc  56 , as the rotary actuator  50  moves from the inner diameter (ID), to the middle diameter (MD), to the outer diameter (OD), and back again. The second sensor  55  is preferably mounted on the actuator arm  50  such that its sense axis  55   s  is perpendicular to the long axis of the actuator  50 As such, the sense axis  55   s  is perpendicular to the length of the actuator  50  and tangential to the arc  56  that is traversed by the sensor  55  and the arc  58  that is traversed by the head  80 . In this manner, any component of acceleration that tends to move the head  80  off-track, is maximally imparted to the second sensor  55 . 
     The second sensor&#39;s sense axis  36  is preferably aligned with the first sensor&#39;s sense axis when the actuator is at the MD. The angular extent of the arc  58  is relatively small, but it is still necessary to consider the fact that the second sensor&#39;s sense axis  55   s  will sometimes be aligned and sometimes be skewed relative to the first sensor&#39;s sense axis  35   s  throughout the actuator&#39;s range of motion. At the ID and OD, or course, the sense axis  55   s  is slightly skewed from the ideal and the gain of the second sensor will be reduced relative to the first sensor  55 . As discussed below, however, the preferred embodiment compensates for the skew between the sense axes  35   s ,  55   s  when the actuator  50  is at the ID or the OD, and not at the MD. 
     FIG. 3 is a simplified plan view of the disk drive  10  showing the PCBA  30  that carries the first sensor  35  vertically registered with the second sensor  55  carried by the rotary actuator  50  when the rotary actuator is at the MD. In this preferred arrangement, the first sensor  35  is located below the second sensor  55 , but their respective sense axes  35   s ,  55   s  are substantially perpendicular to the long axis of the actuator  50  and aligned with one another when the actuator  50  is as the MD. 
     FIG. 4 is a simplified plan view of the preferred disk drive  10  showing that the second sensor  55  preferably moves over the first sensor  35  as the actuator  50  moves from the ID, to the MD, to the OD, and back again. The second sensor&#39;s arc of motion  56 , in other words, preferably travels over the first sensor  35 . FIG. 4 also further shows that the second sensor  55  is located directly over the first sensor  35  when the rotary actuator  50  is at the MD of the disk  23 . It is possible, however, that the sensors  35 ,  55  are located in such places that they are never in vertical alignment at any point in the actuator  50 &#39;s range of motion. In such case, however, larger gain adaptations will be required to maintain comparable signals, thereby increasing the likelihood of errors. 
     FIG. 5 is simplified diagram of a control system model that is used for controlling a disk drive  10  in order to achieve improved track following performance by reducing off-track error caused by shock and vibration. A preferred method of controlling a disk drive comprises the steps of generating a first sense signal corresponding to a motion of the head disk assembly; generating a second sense signal corresponding to a motion of the rotary actuator relative to the motion of the head disk assembly; comparing the first and second sense signals in order to detect off-track movement of the rotary actuator while track-following; and compensating for the off-track movement. 
     The preferred method may be readily understood by referring to FIG. 5 in conjunction with FIGS. 1-4. In operation, the first and second sensors  35 ,  55  are used to generate the first and second sense signals  36 ,  56  in the presence of shock and vibration, those sense signals are compared by a junction  150  to detect any resulting off-track motion, and suitable hardware and firmware is used to compensate for torque that is otherwise undesirably imparted to the rotary actuator  50  by the shock and vibration. 
     In normal operation, the control system  100  receives a digital target position  101  in accordance with a request from a host computer (not shown). An indicated position  103  is also available on a periodic basis by virtue of servo control signals that are periodically read by the head  80 , processed through a servo channel demodulator  110 , and converted to a digital value by an A/D converter  11   a.    
     A summing junction  102  subtracts the indicated position  103  from the target position  101  to produce a position error signal PES that is provided to a suitable compensator  120  to produce a nominal digital command  121  that, ordinarily, would be provided without any compensation for vibration, to a digital-to-analog converter DAC that produce an analog current “i” for accelerating the VCM  40  (see FIG. 1) in accordance with the magnitude of the PES. 
     As suggested by the gain block  131 , the drive current “i” is nominally converted to a torque T according to a torque conversion factor, K T , where T=i*K T . The applied torque, of course, accelerates the rotary actuator  50  at an angular acceleration                     2        θ              t   2                              
     that is a function of the applied torque T and the actuator&#39;s angular moment of inertia J. Over time, as suggested by the simplified        1   S                          
     system blocks  141 ,  142 , the acceleration                     2        θ              t   2                              
     results in an angular velocity             θ          t                            
     and an angular position θ. A change in the angular position Δθ causes the head  80  to move by along the arc  58  (see FIG. 2) as a function of the radial distance R h  from the pivot cartridge  51  to the head  80 . Ultimately, the head  80  is located a particular track position POS over the disk  23  and, as already discussed, that position POS is detected and returned for comparison with the target position  101 . 
     The rotary actuator  50  shown in FIGS. 1-4 is a “balanced actuator” in that the center of mass is designed to be located precisely at the pivot axis such that external accelerations will not generate a relative acceleration between the actuator  50  and the base  21 . As a practical matter, however, many rotary actuators  50  are shipped with an operational or effective imbalance even though they are nominally balanced. 
     As suggested by block  150 , an actuator  50  with an effective imbalance has a center of mass located at some distance d from the pivot axis. Such an actuator  50  is detrimentally subject to an angular acceleration whenever a linear shock or vibration imparts a force to the off-axis mass. The result is the injection of an undesired torque T vib  that tends to cause the head  80  to move off-track even while the servo control system is in a track-following mode. An inability to control the actuator  50  in the face of such undesired vibration detrimentally requires a coarser track pitch design than might otherwise be used, makes it possible that the system will have to re-read a data track, and worse yet, makes it possible that the head  80  will over-write an adjacent track when recording data. 
     In accordance with the present invention, however, two sensors  35 ,  55  may be uniquely used in order to detect and compensate for such undesired acceleration of the actuator  50 . Moreover, because of the differential approach, the system is also capable of detecting motion due to both linear and rotational shock and vibration. 
     As shown in FIG. 5, accelerations a 1 , a 2  imparted to the first and second sensors  35 ,  55  results in two corresponding sensor signals  36 ,  56  that, subject to suitable gain adjustments, are differentially compared at a junction  150 . Accordingly, if the disk drive  10  were subject to a linear shock or vibration that resulted in the head  80  moving with the disk  23  (as it would were the actuator  50  perfectly balanced), then the sensors would also move together, the sensor signals  36 ,  56  would be identical, and the output of the junction  150  would be zero, i.e. no compensation would be needed and none would take place. On the other hand, if the actuator has an effective imbalance, then a linear shock or vibration that causes the head  80  to move relative to the disk  23  would be reflected as a difference between the first and second signals  36 ,  56 . As such, the junction  150  would produce a net value and that value, after suitable treatment though an acceleration compensator  160  to produce a compensated signal  161 , would be combined (added or subtracted as appropriate) with the nominal digital command  121 , at junction  170 , to produce an adjusted digital command  171 . Preferably an adaptive gain stage G 4  is coupled between junction  150  and acceleration compensator  160  for adjusting signal gain on the basis of the formula: a 1 G 1 −(a 2 G 2 )G 3 . 
     As a result of this approach, the system  100  will actively work to cancel shock and vibration that would otherwise undesirably move the actuator  50  and the head  80  away from a desired track-following position. 
     As shown in FIG. 5, the preferred system  100  includes a gain adjust block G 3  that accounts for skewing between the sense axis of the two sensors  35 ,  55 . In particular, the gain block G 3  adaptively modifies the gain of the second sensor  55  that is mounted on the actuator  50  based on the location of the actuator  50 . When the actuator is located at the MD, the gain would be 1.0, whereas the gain at the ID or OD would increase to a larger amount (e.g. 1.2) in order to account for skew. 
     As can now be understood by reference to FIGS. 1-5 and the above description, the preferred method of generating a first sense signal  35  corresponding to a motion of the head disk assembly  20  is accomplished by mounting a first motion sensor  35  rigidly relative to the base  21  and the preferred method of generating a second sense signal  56  corresponding to a motion of the rotary actuator  50  relative to the motion of the head disk assembly  20  is accomplished by mounting a second motion sensor  55  to the rotary actuator  50  that pivots relative to the base  21 . 
     The compensating step is preferably accomplished, as shown in FIG. 5, by modifying a nominal digital command  121  in a servo control loop with a digital value  161  corresponding to the result of the comparing step. 
     As discussed above, the step of generating a first sense signal is preferably accomplished with a linear accelerometer  35  that has its sense axis  35   s  substantially, tangentially aligned with an arc  86  that is traversed by a head  80  carried by the rotary actuator  50 . In such case, the step of mounting the second linear accelerometer  55  on the rotary actuator  50  is preferably accomplished with its sense axis  55   s  substantially aligned with the sense axis  35   s  of the first linear accelerometer  35  when a read/write head  80  supported by the rotary actuator  50  is located over a middle diameter of the rotating disk  23 . 
     Finally, the preferred method includes the further step of adjusting a compensation factor G 3  to account for skew that exists between the sense axes  35   s ,  55   s  of the first and second linear accelerometers  35 ,  55  when the read/write head  80  supported by the rotary actuator  50  is located at the inside or outside diameter of the rotating disk  23 . 
     The preferred system  100  of FIG. 5 is a microprocessor implementation characterized by translation from digital-to-analog using a DAC, and back again using A/D converters  37 ,  57 , 111 . In this particular embodiment, the vibration compensation is accomplished digitally because it is most convenient. It is possible, of course, that the vibration cancellation could be implemented in a purely analog system, or in an analog portion of a hybrid system.