Abstract:
A system for positioning a transducing head in a disc drive device over a selected track of a rotatable disc includes an actuator arm which is rotatable about an axis, a head suspension mechanism connected to the actuator arm, and a slider carrying a transducing head and supported by the head suspension mechanism. A low resolution motor moves the actuator arm about the axis to effect coarse movement of the head between tracks of the disc. A piezoelectric element is embedded in the actuator arm to distort the arm to effect fine positioning of the head. Control circuitry distributes electrical signals to the low resolution motor and the piezoelectric element to selectively control movement thereof. The piezoelectric element is embedded in the actuator arm by removing a predetermined amount of material from the actuator arm and bonding the piezoelectric element in the resulting space in the actuator arm.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     This application is a divisional of U.S. application Ser. No. 08/836,292 filed May 12, 1997 now U.S. Pat. No. 6,052,251, for “Actuator Arm Integrated Piezoelectric Microactuator” by K. Mohajerani, J. Sampietro, A. Fard, J. Barina and M. Hawwa, which is the national phase of PCT International Application PCT/US97/07892 filed May 12, 1997, and which in turn claims priority from Provisional Application No. 60/030,406 filed Nov. 1, 1996 for “Eblock Integrated Piezo Electric Actuator” by K. Mohajerani, J. Sampietro, A. Fard, J. Barina and M. Hawwa. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to a mechanism for positioning a transducing head in a disc drive system, and more particularly relates to a piezoelectric microactuator integrated into an actuator arm of a disc drive system to provide high resolution head positioning over a selected track of a rotatable disc. 
     Concentric data tracks of information are being recorded on discs with increasing track densities, which reduces the margin for error in positioning a transducing head over a selected track due to the reduced radial distance between tracks and the narrow radial width of the tracks themselves. Typical actuator motors lack sufficient resolution to accurately position a head in a system implementing a disc with a high track recording density. 
     Various proposals have been made to provide a second, high resolution motor, or microactuator, to finely position a head at a radial position over a track, in addition to the low resolution actuator motor. These “dual-stage actuation” systems have taken a variety of forms. Some of the proposed designs would install a microactuator in the head slider itself. These designs require significant changes in the manufacturing of head sliders. A solution that allows existing mass manufacturing techniques for sliders to be used would be more desirable. Other proposed designs would replace a conventional gimbal with a specially designed silicon gimbal having a microactuator formed directly on the gimbal itself. Again, these designs require new, complex gimbal manufacturing techniques, which are less efficient than a solution that utilizes existing disc drive components. Still other proposed designs would mount a microactuator motor where the actuator arm meets the head suspension. While these designs often require only minimal changes in the actuator arm head suspension designs, the connection between the actuator arm and the head suspension must be carefully designed to include the microactuator motor. In addition, none of the proposed designs includes a microactuator having the capability of sensing a position of the head slider based on a state of the microactuator. A solution with this capability, that requires minimal additional design steps to conventional actuator assembly design, would be a significant improvement over the presently proposed dual-stage actuation systems. 
     The present invention is directed to a piezoelectric microactuator embedded in the actuator arm of a disc drive system. U.S. Pat. No. 4,814,908 to Schmitz discloses a system for radially positioning a transducing head over the center of a track on a rotatable disc by placing a thermal element on one side of the actuator arm. The arm is made of a material which expands upon heating and contracts upon cooling, so that the arm can be expanded or contracted (thereby radially moving the transducing head carried by the arm) in response to controlled heating or cooling of the thermal element. However, the thermal element has a relatively slow response time, making it inadequate for some high performance disc drive systems. Also, expansion of the thermal element in response to a given input stimulus is not sufficiently precise and predictable to serve as an effective high resolution positioning mechanism. The introduction of heat into the actuator arm affects the environmental conditions of the disc drive, which can have significant effects on the operation of the positioning system. Finally, the current state of a thermal element cannot be readily detected, making it difficult to determine the appropriate input stimulus to effect incremental transformation of the microactuator to precisely position the head over a selected track. 
     SUMMARY OF THE INVENTION 
     A system is provided by the present invention to position a transducing head in a disc drive device over a selected track of a rotatable disc having a plurality of concentric tracks. The disc drive device includes an actuator arm rotatable about an axis to move a slider carrying a transducing head mounted by a head suspension mechanism to the actuator arm. A low resolution motor moves the actuator arm about the axis to effect coarse movement of the head on the slider between tracks of the rotatable disc. A piezoelectric element is embedded in the actuator arm to distort the arm to effect fine positioning of the head on the slider. Control circuitry distributes electrical signals to selectively operate the low resolution motor and the piezoelectric element. 
     According to an optional feature of the present invention, the control circuitry includes an input circuit providing a track number corresponding to the selected track, and a feedback loop including a summing circuit comparing the selected track number and a current track number to determine the desired movement of the head, a piezoelectric element controller for operating the piezoelectric element to effect fine movement of the head and distributing the control signals representative of a number of tracks remaining to be traversed, and a low resolution motor controller receiving the control signals from the piezoelectric element controller and operating the low resolution motor in response to the control signals to effect coarse movement of the head. 
     One aspect of the present invention encompasses a method of manufacturing an actuator arm. A predetermined amount of material is removed from a side portion of the actuator arm to create a space in the actuator arm. A piezoelectric element is bonded into the space in the actuator arm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a top view of a dual-stage actuation system utilizing a piezoelectric element embedded in the actuator arm according to the present invention. 
     FIG. 2 is a side view of the dual-stage actuation system of FIG.  1 . 
     FIG. 3 is a top view of a dual-stage actuation system utilizing two piezoelectric elements embedded in opposite sides of the actuator arm in accordance with the present invention. 
     FIG. 4 is a side view of the dual-stage actuation system of FIG.  3 . 
     FIG. 5 is a flow diagram illustrating the process of embedding a piezoelectric element in the actuator arm according to the present invention. 
     FIG. 6 is a block diagram illustrating the functional elements of a feedback servo controller circuit usable with the dual-stage actuation system of the present invention. 
     FIG. 7 is a block diagram illustrating the functional element of an alternative feedback servo controller circuit usable with the dual-stage actuation system of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 is a top view, and FIG. 2 is a side view, of a dual-stage actuation system  10  according to the present invention. Actuation system  10  includes a voice coil motor  12  operable to rotate actuator arms  16  of an E-block about axis  14  of shaft  17 . Screw  15  fastens the top of actuator shaft  17  to a top cover (not shown). Head suspension  18  is connected to a distal end of actuator arm  16  by head suspension mounting block  20 . Gimbal  22  is attached to a distal end of head suspension  18 . Slider  24  is mounted to gimbal  22  in a manner known in the art. Voice coil motor  12  is a low resolution motor for coarse positioning of actuator arms  16  of the E-block. Voice coil motor  12  is operatively attached to actuator arm  16 . Actuator arm  16  is rotatable around axis  14  in response to operation of voice coil motor  12 , and has a longitudinal axis  25  normal to axis  14 . Actuator arm  16  includes a space  19  forming arm side portions  21   a  and  21   b  on each side of longitudinal axis  25 . Voice coil motor  12 , actuator arm  16 , head suspension  18 , head suspension mounting block  20 , gimbal  22 , and slider  24  are all standard disc drive system components, manufactured in a manner known in the art. 
     Piezoelectric element  26  is embedded in side portion  21   b  of actuator arm  16 , and expands and contracts in response to a voltage applied to its terminals  27   a  and  27   b.  The size of piezoelectric element  26  is varied in proportion to the voltage across its terminals  27   a  and  27   b.  Relief  28  is provided in side portion  21   a  of actuator arm  16 , to reduce the force required to distort actuator arm  16  by selective expansion and contraction of piezoelectric element  26 . 
     In operation, voice coil motor  12  is operated to rotate actuator arm  16  around axis  14  to effect coarse positioning of slider  24  over a selected region of a rotatable disc  30 . Disc  30  rotates around disc axis  32 , and includes a plurality of concentric tracks  34  radially positioned around disc axis  32 . Once coarse positioning has been achieved, a voltage is applied to piezoelectric element  26  to cause selective expansion or contraction of the piezoelectric element, thereby causing distortion of actuator arm  16  to effect fine positioning of slider  24  over a selected track of rotatable disc  30 . 
     Piezoelectric element  26  is preferably positioned as near to rotational axis  14  of actuator arm  16  as possible, and as near to longitudinal axis  25  of actuator arm  16  as possible, so that the arc of fine positioning of slider  24  by expansion and contraction of piezoelectric element  26  approximates the designed head positioning arc as nearly as possibly, thereby minimizing head skew and maximizing the displacement of slider  24  for a corresponding expansion or contraction of piezoelectric element  26 . Although many locations of piezoelectric element  26  along the length of actuator arm  16  are effective, piezoelectric element  26  is located within 20% of the length of actuator arm  16  from axis  14  (“near” axis  14 ) in a preferred embodiment of the invention, to achieve maximum amplification of expansion and contraction of piezoelectric element  26 , minimize head skew, and minimally affect the balance and inertia of actuator arm  16 . To assure distortion close to axis  14 , relief  28  is formed in side portion  21   a  as near as possible to axis  14  as well. 
     Because the voltage across the piezoelectric element  26  is directly proportional to the size of the element, a current state of piezoelectric element  26  is readily ascertainable. This enables the actuation system to easily determine the incremental displacement (and voltage) required to adjust the piezoelectric element to position the head over the selected track of the disc. More efficient fine positioning of the head can thereby be achieved. 
     FIG. 3 is a top view, and FIG. 4 is a side view, showing an alternative embodiment of the dual-stage actuation system  10  of the present invention. Actuation system  10  includes a voice coil motor  12  operable to rotate actuator arms  16  of an E-block about axis  14  of shaft  17 . Screw  15  fastens the top of actuator shaft  17  to a top cover (not shown). Head suspension  18  is connected to a distal end of actuator arm  16  by head suspension mounting block  20 . Gimbal  22  is attached to a distal end of head suspension  18 . Slider  24  is mounted to gimbal  22  in a manner known in the art. Voice coil motor  12  is a low resolution motor for coarse positioning of actuator arms  16  of the E-block. Voice coil motor  12  is operatively attached to actuator arm  16 . Actuator arm  16  is rotatable around axis  14  in response to operation of voice coil motor  12 , and has a longitudinal axis  25  normal to axis  14 . Actuator arm  16  includes a space  19  forming arm side portions  21   a  and  21   b  on each side of longitudinal axis  25 . Voice coil motor  12 , actuator arm  16 , head suspension  18 , head suspension mounting block  20 , gimbal  22 , and slider  24  are all standard disc drive system components, manufactured in a manner known in the art. 
     Piezoelectric elements  26  are embedded in side portions  21   a  and  21   b  actuator arm  16 , and are preferably implemented with opposite polarities, so that a voltage introduced across terminals  27   a  and  27   b  of both piezoelectric elements induces expansion of one side portion of actuator arm  16  and contraction of the other side portion of actuator arm  16 . This complementary arrangement of piezoelectric elements  26  allows a greater distortion of actuator arm  16  to be achieved, thereby enabling greater displacement of slider  24 . Piezoelectric elements  26  are preferably positioned as near to rotational axis  14  of actuator arm  16  as possible, and as near to longitudinal axis  25  of actuator arm  16  as possible, so that the arc of fine positioning of slider  24  by expansion and contraction of piezoelectric elements  26  approximates the designed head positioning arc as nearly as possibly, thereby minimizing head skew and maximizing the displacement of slider  24  for a corresponding expansion or contraction of piezoelectric elements  26 . While many locations of piezoelectric elements  26  are effective, piezoelectric elements  26  are located within 20% of the length of the actuator arm from axis  14  (“near” axis  14 ) in a preferred embodiment of the invention, to maximize amplification of expansion and contraction of piezoelectric elements  26 , minimize head skew, and minimally affect the balance and inertia of actuator arm  16 . 
     FIG. 5 is a flow diagram illustrating the process steps for embedding a piezoelectric element into the actuator arm according to the present invention. First, at step  40 , the actuator arm is formed such that space  19  creates arm side portions  21   a  and  21   b,  space  19  extending as close as possible to axis  14 . At step  42 , the actuator arm is placed in a fixture and aligned to known reference points. A predetermined section of material is then removed at step  44 , from one or both of side portions  21   a  and  21   b  of the actuator arm at the end of space  19  closest to axis  14 . Finally, at step  46 , an insulated and terminated piezoelectric element is bonded in the section in the arm portion where material was removed. If only one side portion  21   a A or  21   b  is fitted with a piezoelectric element, it is preferred that step  44  additionally includes machining relief  28  (FIG. 1) into the other size portions. 
     By embedding the piezoelectric element in a conventional actuator arm, the present invention provides a microactuator without requiring additional design of the actuator arm, head suspension, head suspension mounting block, gimbal, or slider. These components are manufactured according to existing processes known in the art. 
     FIG. 6 is a logical block diagram of the functional elements of a dual-stage actuation control system of the present invention. The actuation control system includes a step input circuit  50 , summing circuit  52 , piezoelectric element controller  54 , piezoelectric element  56 , VCM controller  58 , VCM  60 , summing block  62 , and head  64 . 
     Step input  50  provides an electrical signal representative of the number of the destination track to which the head is to be moved. Summing circuit  52  subtracts the track number over which the head is currently positioned, as interpreted from the servo information read by head  64  from the disc, from the destination track number provided by step input  50 . Thus, summing circuit  52  provides a signal indicative of the number of tracks that the head must traverse, and the direction in which the head must move. Piezoelectric element controller  54  analyzes the number of tracks which the head must traverse, and distributes the required movement among piezoelectric element  56  and VCM  60 . Piezoelectric element controller  54  provides the necessary signals to control the movement of piezoelectric element  56  (that is, provides a voltage across the terminals of piezoelectric element  56 ), and VCM controller  58  provides the signals necessary to control the movement of VCM  60 . Summing block  62  represents the total movement effected by VCM  60  and piezoelectric element  56 , so that the output of summing block  62  represents the total physical movement of the head. Head  64  reads servo information from the disc, which is interpreted to determine the track over which the head is currently positioned. The current track number is subtracted by summing circuit  52  from the destination track number provided by step input circuit  50 , and the functional loop is iterated again. 
     The dual-stage actuation control system of the present invention may be operated with a disc having a track recording density that is so high that VCM  60  only has sufficient resolution to move the head in increments of five tracks. For example, step input  50  may provide a signal indicating that the head is to move from track  100  to track number  208 . Summing circuit  52  subtracts the current track number ( 100 ) from the desired track number ( 208 ) to determine that the head must move 108 tracks in the positive displacement direction. This information is provided to piezoelectric element controller  54 . Piezoelectric element controller  54  may, for example, be configured with the capability of operating piezoelectric element  56  to move the head up to five tracks. Thus, when piezoelectric element controller  54  analyzes the desired movement of 108 tracks, it sends a signal to piezoelectric element  56  that causes piezoelectric element  56  to move the head its maximum radial displacement, five tracks. This movement is not enough to obtain the desired head movement (108 tracks), so piezoelectric element controller  54  distributes the remainder of the head movement to VCM  60 . In this example, VCM controller  58  receives a signal from piezoelectric element controller  54  that indicates there are 103 tracks left to traverse. VCM controller  58  then operates VCM  60  to move the head 100 tracks. The total movement by VCM  60  and piezoelectric element  56 , symbolized as being summed in block  62 , is 105 tracks. Thus, the track number over which head  64  is currently positioned is  205 . 
     This current track number ( 205 ) is subtracted from the destination track number ( 208 ) by summing circuit  52 , yielding a desired track movement of three tracks in the positive displacement direction. However, piezoelectric element controller  54  has already operated piezoelectric element  56  to its maximum extent. Therefore, piezoelectric element controller  54  distributes the desired three-track movement by sending a signal to VCM controller  58  to operate VCM  60  to move the head one more increment (5 tracks), and operates piezoelectric element  56  to displace the head two tracks less than its maximum (3 tracks). Thus, the movement of head  64  effected by VCM  60  is 105 tracks, and the movement of head  64  effected by piezoelectric element  56  is three tracks. These movements are symbolically added in block  62 , to yield a total movement of 108 tracks, and the head is positioned over track number  208 , as determined from the servo information read by head  64 . This current track number ( 208 ) is subtracted from the destination track number ( 208 ) at summing circuit  52 , yielding a desired track movement of zero tracks. The logical loop continues in this steady state until a new desired track number is input by step input circuit  50 . 
     The actuation system is preferably also designed to compensate small off-track errors, such as one-quarter or other fractional track errors, for example. Thus, when head  64  detects an off-center condition, a correction signal is passed through summing circuit  52  to controller  54  to operate piezoelectric element  56 . Piezoelectric element  56  has sufficient resolution to correct these off track errors, to center the head over the desired track. When these small adjustments need to be made, piezoelectric controller  54  serves to distribute the head centering movement to piezoelectric element  56 , so that VCM  60  is not operated for such minuscule movements. 
     FIG. 7 is a logical block diagram of the functional elements of an alternative dual-stage actuation control system of the present invention, including a step input circuit  70 , summing circuit  72 , piezoelectric element controller  74 , unity gain inverter  76 , summing circuit  78 , piezoelectric element  80 , VCM controller  82 , VCM  84 , summing block  86 , and head  88 . 
     Step input  70  provides an electrical signal representative of the number of the destination tracks to which the head is to be moved. Summing circuit  72  subtracts the track number over which the head is currently positioned, as interpreted from the servo information read by head  88  from the disc, from the destination track number provided by step input  70 . Thus, summing circuit  72  provides a signal indicative of the number oftracks that the head must traverse, and the direction in which the head must move. Piezoelectric element controller  74  analyzes the number of tracks which the head must traverse, and provides a signal to control the movement of piezoelectric element  80  (that is, provides a voltage across the terminals of piezoelectric element  80 ) based on the required track movement received from summing circuit  72 . The signal provided from piezoelectric element controller  74  is inverted by inverter  76 , and summing circuit  78  adds the required track movement from summing circuit  72  and the inverted movement achieved by piezoelectric element  80  under the control of piezoelectric element controller  74 , yielding a signal representing the required track movement remaining. VCM controller  82  analyzes the number of tracks left for the head to traverse, and provides signals to control the movement of VCM  84  to achieve that motion. Summing block  86  represents the total movement effected by VCM  84  and piezoelectric element  80 , so that the output of summing block  86  represents the total physical movement of the head. Head  88  reads servo information from the disc, which is interpreted to determine the track over which the head is currently positioned. The current track number is subtracted by summing circuit  72  from the destination track number provided by step input circuit  70 , and the functional loop is iterated again. 
     The dual-stage actuation control system shown in FIG. 7 operates in a manner substantially similar to the actuation control system shown in FIG.  6  and described previously. The control system shown in FIG. 7 contains slightly more components than the system shown in FIG. 6, but also requires a less complex piezoelectric element controller. It will be apparent to one skilled in the art that the control systems shown in FIGS. 6 and 7 effectively operate a low resolution motor to effect coarse positioning of a head, and also operate a high-resolution piezoelectric microactuator to effect fine positioning of the head, while preventing application of a voltage to the high resolution piezoelectric microactuator that exceeds the range of allowable voltages, which would saturate the microactuator and inhibit the performance of the system. 
     The dual-stage actuation system of the present invention efficiently controls the positioning of a head over a selected track of a rotatable disc. A piezoelectric microactuator is integrated into the actuator arm of the system, providing high resolution head positioning without requiring additional design of existing disc drive components. The piezoelectric microactuator is located in the actuator arm to maximize the attainable range of head movement and to minimize head skew. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.