Abstract:
A position sensor includes a stationary platform and a moveable platform. The position sensor further includes at least one beam coupling the moveable platform to the stationary platform. The at least one beam includes piezoresistive material that is positioned tolprovide an indication of a movement of the moveable platform relative to the stationary platform.

Description:
TECHNICAL FIELD OF THE INVENTION 
     This invention relates generally to position sensors and more particularly to a position sensor for a microactuator in a mass storage device. 
     BACKGROUND OF THE INVENTION 
     Mass storage devices such as hard disk drive systems generally include a magnetic storage media for storing information, a spindle motor for rotating the storage media, a magnetic read/write head for reading information from or writing information to the magnetic storage media, and an actuator for positioning the read/write head over the storage media. A control system associated with the actuator controls the movement of the actuator. 
     The read/write head is often positioned on an arm. This arm is positioned using the actuator mentioned above. However, such a system only provides coarse positioning of the read/write head. In order to further increase the data storage capacity of hard disk drives, the size of each data bit is constantly being reduced. The reduced data bit size requires increased accuracy in the positioning of the read/write head over the storage media. In order to provide finer adjustment of the position of the read/write head, microactuators are used that are positioned on an extreme end of the arm, where the read/write head is positioned. 
     The positioning force created by the microactuator can take many forms. For example, past actuator designs have used electrostatic, ferromagnetic, and piezoresistive actuation. An example system is described in “Design and Feedback Control of Electrostatic Actuators for Magnetic Disk Drives” by David A. Horsley, et. al. This paper was published at the Solid-State Sensor and Actuator Workshop, Jun. 8-11, 1998. 
     A problem that arises from the use of such a microactuators is determining the precise location of the read/write head in response to positioning by the microactuator. Position sensors or “pick-offs” are used for this purpose. 
     In addition to hard disk drives, there are numerous other applications that require precise positioning information. 
     SUMMARY OF THE INVENTION 
     Accordingly, a need has arisen for an apparatus for determining the precise position of a device, and more specifically, a read/write head in a hard disk drive. The present invention provides such a system and method for determining the position of a device. 
     According to one embodiment of the invention, a position sensor includes a stationary platform and a moveable platform. The position sensor further includes at least one beam coupling the moveable platform to the stationary platform. The at least one beam includes piezoresistive material that is positioned to provide an indication of a movement of the moveable platform relative to the stationary platform. 
     According to another embodiment of the invention, a hard disk drive system includes a disk storage media for storing information. The hard disk drive system also includes an arm operable to move over the disk storage media and an actuation system for positioning the arm relative to the disk storage media. Furthermore, the hard disk drive system includes a stationary platform coupled to an end of the arm and a head coupled to the stationary platform by a pair of beams. The head is used for recording and reproducing data in the disk storage media. A microactuator is used for positioning the head relative to the stationary platform, and piezoresistive material is positioned on the pair of beams to provide an indication of this movement of the head. 
     Embodiments of the present provide numerous technical advantages. For example, in one embodiment of the invention, a position sensor incorporates piezoresistive material to determine the precise location of a read/write head in a hard disk drive. Such precise location information allows for very fine adjustments in the position of the read/write head. In turn, this allows for storage of data on a hard disk drive at greater densities. 
    
    
     Other technical advantages are readily apparent to one skilled in the art from the following figures, descriptions, and claims. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a more complete understanding of the present invention and the advantages thereof, reference is now made to the following descriptions taken in connection with the accompanying drawings in which: 
     FIG. 1 is a block diagram illustrating a portion of a hard disk drive system, including a disk drive apparatus; 
     FIG. 2 is a schematic diagram illustrating a partial view of the disk drive apparatus of FIG. 1, showing the control of the movement of a read/write head by a control system; 
     FIG. 3 is an enlarged drawing of the hard disk drive of FIG. 2 showing in greater detail an arm and the read/write head positioned on the arm of the hard disk drive; 
     FIG. 4A is a top view of the read/write head illustrated in FIG. 3, and shown on a stationary platform positioned on the arm illustrated in FIG. 2 in an undeflected,position; 
     FIG. 4B is an isometric view of a beam illustrated in FIG. 4A; 
     FIG. 5A is a top view of the read/write head illustrated in FIG. 3 shown on a stationary platform positioned on the arm illustrated in FIG. 2 in an deflected position; and 
     FIG. 5B shows an enlarged view of a beam illustrated in FIG. 5A, showing areas of tension and compression in the beam. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Embodiments of the present invention and its advantages are best understood by referring to FIGS. 1 through 5B of the drawings, like numerals being used for like and corresponding parts of the various drawings. 
     FIG. 1 is a block diagram of a hard disk drive system  12  used for retrieving data during read operations and for storing data during write operations. Hard disk drive system  12  interfaces and exchanges data with a host  14  during read and write operations. Hard disk drive system  12  includes a disk drive apparatus  16 , a read channel  18 , a write channel  20 , and a hard disk control system  22 . 
     Disk drive apparatus  16  is used to magnetically store data. Disk drive apparatus  16  is shown in greater detail in FIG.  2 . Read channel  18 , write channel  20 , and hard disk control system  22  are used to process data that are read from and written to disk drive apparatus  16 . Hard disk control system  22  also controls various operations of hard disk drive system  12 . Write channel  20  is coupled to disk drive apparatus  16  through a write data path  24 . Read channel  18  is coupled to disk drive apparatus  16  through a read data path  23 . Read channel  18  is coupled to hard disk control system  22  through a read data path  27 . Write channel  20  is coupled to hard disk control system  22  through a write data path  26 . Host  14  exchanges data with hard disk control system  22  through data bus  28 . 
     During read operations, read channel  18  receives an analog data signal from disk drive apparatus  16  through data path  23 . Read channel  18  conditions, decodes, and formats the analog data signal and provides a digital data signal to hard disk control system  22  through data path  27 . Read channel  18  may include any of a variety of circuit modules such as an automatic gain control circuit, a low pass filter, a variable frequency oscillator, a sampler, and a synchronization field detection circuit. 
     During write operations, write channel  20  receives a digital data signal from hard disk control system  22  through data path  26 . Write channel  20  reformats and codes the digital data signal for storage and provides an analog data signal to disk drive apparatus  16  through data path  24 . Write channel  20  may include any of a variety of circuit modules such as a register, a scrambler, a phase locked loop, an encoder, a serializer, and a write precompensation circuit. 
     Hard disk control system  22  is used to control various operations of hard disk drive system  12  and to exchange digital data with host  14 , including disk drive apparatus  16 . Hard disk control system  22  generates a control signal that is received by disk drive apparatus  16 . Hard disk control system  22  may include a microprocessor, a random access memory, a read-only memory, and a disk controller (not explicitly shown). The microprocessor, random access memory, read-only memory, and disk controller together provide control and logic functions to read channel  18 , write channel  20 , and disk drive apparatus  16  so that data can be received from host  14 , stored, and later retrieved and provided back to host  14 . 
     FIG. 2 is a schematic diagram of disk drive apparatus  16  shown in FIG.  1 . Disk drive apparatus  16  is used to magnetically store and retrieve data. Disk drive apparatus  16  includes a storage media  50 , a disk drive mechanism  52 , a read/write head  54 , and an actuation system  56 . Actuation system  56  includes a read/write head arm  53  and actuation circuitry  57  for positioning read/write head arm  53 . Actuation system  56  further includes a microactuator (not explicitly shown) located on the tip of read/write head arm  53 . This microactuator is used for fine positioning of read/write head  54 , and is described in greater detail in conjunction with FIG.  3 . 
     In one embodiment, storage media  50  is a rotating magnetic disk or platter that stores data represented as magnetic transitions on a surface of the magnetic platter. Although storage media  50  is illustrated in FIG. 2 as a single magnetic platter, disk drive apparatus  16  can include multiple magnetic disks or platters. Storage media  50  illustrated in FIG. 2 has a center  55  and an outer edge  58 . 
     Disk drive mechanism  52  rotates storage media  50  at a desired rate. An example disk drive apparatus  16  uses disk drive mechanism  52  that rotates storage media  50  at a rate of approximately 10,000 revolutions per minute. Disk drive mechanism  52  may be any of a number of available mechanisms operable to rotate storage media  50  such as a spindle motor. 
     Read/write head  54  stores and retrieves data from a single surface of storage media  50 . Although only one read/write head  54  is illustrated in FIG. 2, a second read/write head can be provided to store data to and retrieve data from the opposite side of the magnetic platter illustrated in FIG.  2 . Also, if multiple magnetic platters are used for storage media  50 , read/write heads can be provided for each surface of each magnetic platter. Read/write head  54  may be any of a number of available read/write heads such as magneto-resistive heads. 
     During read operations, read/write head  54  deciphers the magnetic transitions stored on storage media  50 . Read/write head  54  then sends an analog data signal to read channel  18  through read data path  23  (not explicitly shown in FIG.  2 ). During write operations, read/write head  54  receives an analog data signal from write channel  20  through write data path  24  (not explicitly shown in FIG.  2 ). Read/write head  54  then records the analog data signal as magnetic transitions on storage media  50 . 
     Read or write operations cannot occur until read/write head  54  is in an appropriate location over the surface of storage media  50 . Actuation circuitry  57  coarsely positions read/write head  54  by moving read/write head arm  53  to the appropriate location in response to receiving control signal  25  from hard disk control system  22 . In addition, the microactuator can be used to finely position read/write head relative to read/write head arm  53 . In a series of read and write operations, actuation circuitry  56  moves read/write head  54  back and forth over the surface of storage media  50 . 
     According to the teachings of the present invention, a position sensor is described below that allows one to determine the exact position of read/write head  54  in response to positioning by the microactuator. The use of the microactuator and the position sensor allows the precise positioning of read/write head  54  over storage media  50 . This precise positioning, in turn, allows for storage of data on storage media  50  at greater densities. 
     FIG. 3 is an enlarged drawing of the hard disk drive of FIG. 2 showing in greater detail read/write head arm  53  and read/write head  54  positioned on arm  53 . In general, a movable platform is coupled to a stationary platform  60  using a set of beams  62 , which are illustrated in greater detail in FIG. 4A through 5B. In the illustrated embodiment, the movable platform is specifically read/write head  54 . Stationary platform  60  is coupled to read/write head arm  53 . Stationary platform  60  is preferably constructed of polysilicon; however, other types of material may be used. Through the use of beams  62 , read/write head  54  is restricted from moving in all directions except along the axis generally perpendicular to the longitudinal axis  59  of read/write head arm  53 . A microactuator (not explicitly shown) applies a force to read/write head  54  in order to move it along this axis of movement. 
     FIG. 4A is a plan view of the read/write head and stationary platform illustrated in FIG. 3 shown in an undeflected position. As described above, microactuator  66  is operable to move read/write head  54  along axis  64 . Microactuator  66  is depicted in FIG. 4A in two alternative positions. Microactuator  66  can be located in any position which allows it to exert an actuating force on read/write head  54 . Various types of microactuators may be used to apply such an actuating force to read/write head  54 . Such microactuators can employ electrostatic, ferromagnetic, piezoresistive or other various types of actuating forces. No matter what type of microactuator is used, the operation of microactuator  66  can be generalized as the application of forces  68   a  and  68   b  to read/write head  54 . Forces  68   a  and  68   b  operate to move read/write head  54  in either direction along axis  64 . Forces  68   a  and  68   b  can be applied at any point on read/write head  54 , and can also be distributed over an area of read/write head  54 . 
     As described above, read/write head  54  is coupled to stationary platform  60  by beams  62 . Beams  62   a  and  62   b  are preferably made from polysilicon; however, other materials can be used. A thin layer  63  of polysilicon doped with boron is applied to certain areas of the top surface of beams  62   a  and  62   b . More specifically, in one embodiment of the present invention, polysilicon is doped with 10 19  particles/cm +3  of boron. However, it should be understood that other materials having piezoresistive properties can also be used. The positioning of layers  63  is depicted in FIGS.  4 A and FIG.  4 B. If the top of beams  62   a  and  62   b  are divided into imaginary quadrants, layers  63  occupy two of the four quadrants. The placement of layers  63  on beam  62   a  is generally opposite of the placement on beam  62   b.    
     Layers  63  on beams  62   a  and  62   b  are placed such that when a force  68   a  or  68   b  is applied to read/write head  54  in the direction of axis  64 , layers  63  are subjected to either substantially compressive or substantially tensile forces. Since layers  63  are placed in generally opposite positions on beams  62   a  and  62   b , if layers  63  on beam  62   a  are experiencing tension, layers  63  on beam  62   b  will experience compression. This difference in applied force in beams  62   a  and  62   b  allows the direction of movement of read/write head  54  along axis  64  to be ascertained. This will be described in more detail in conjunction with FIGS. 5A and 5B. 
     It should be noted that although FIG. 4A shows the use of two beams  62   a  and  62   b , a single beam (not explicitly shown) may be used in an alternate embodiment. In such an embodiment, the piezoresistive layers may cover an entire surface of the single beam. For example, two piezoresistive layers may be placed in a similar configuration as layers  63  are positioned on beam  62   a  in FIG. 4A, and two other layers may be placed on the same beam in a similar configuration as layers  63  are positioned on beam  62   b . In so positioning the layers, the upper surface of the single beam is substantially covered. 
     As can be seen from FIG. 4B, the thickness of layers  63  in this embodiment is very small (more than an order of magnitude smaller) in comparison to the thickness of beams  62   a  and  62   b . It should be understood that layers could also be positioned on any other surface of beams  62   a  and  62   b . For example, layers  63  could be placed along the bottom or along either side of beams  62   a  and  62   b.    
     Movement of read/write head  54  along axis  64  allows for precise positioning of read/write head  54  over storage media  50 . When force  68   a  or  68   b  is applied to read/write head  54 , it is important to know the exact position of read/write head  54  along axis  64  in relation to stationary platform  60 . In order to determine the precise position of read/write head  54 , a positioning system  70  is employed. In the illustrated embodiment, positioning system  70  includes a bridge circuit  72 . Bridge circuit  72  is a Wheatstone bridge circuit comprising a set of resistors  74 . In particular, layers  63  on beams  62   a  and  62   b  serve as resistors  74  in bridge circuit  72 . The doped polysilicon that comprises layers  63  has piezoresistive properties which cause the resistance of layers  63  to change when stress is applied to layers  63 . As will be described in more detail below, bridge circuit  72  is used to measure this change in resistance of layers  63  to determine the location of read/write head  54 . It should be understood, however, that any suitable method of determining the change in resistance of layers  63  can be implemented. 
     FIG. 5A is a plan view of the read/write head and stationary platform illustrated in FIG. 3, shown in a deflected position. Read/write head  54  has been deflected along axis  64  by force  68   b . As mentioned above, it is important to be able to know exactly how far and in which direction read/write head  54  has moved along axis  64 . The precise location of read/write head  54  is measured using the piezoresistive characteristics of doped polysilicon layers  63  on beams  62   a  and  62   b . As read/write head  54  moves along axis  64 , beams  62  are deformed. Such deformation of beam  62   a  is illustrated in FIG.  5 B. 
     As read/write head  54  moves towards side  80  of stationary platform  60  along axis  64 , the quadrants of beam  62   b  having doped polysilicon layers  63   b  are subject to a compressive force. Since layers  63   b  are attached to beam  62   b , this compressive force is also applied to layers  63   b . It should be noted that since layers  63  are much thinner than beam  62   b , layers  63   b  do not significantly alter the deformation of beam  62   b . When the compressive force is applied to layers  63   b  of beam  62   b , layers  63   b  are compressed and shortened. This compression and shortening causes the resistance of layers  63   b  to decrease. Bridge circuit  72  runs through layers  63   b  and is used to measure this change in resistance. 
     Similarly, when read/write head  54  moves towards side  80  of stationary platform  60  along axis  64 , the quadrants of beam  62   a  having doped polysilicon layers  63   a  are subject to a tensile force. This tensile force elongates layers  63   a . This tension and elongation increases the resistance of layers  63   a , through which bridge circuit  72  runs. Bridge circuit  72  is used to measure this change in resistance of layers  63   a . By determining the change in resistance of layers  63   a  and  63   b , as described above, the precise position of read/write head  54  can be determined. It should be noted that through the use of the Wheatstone bridge, any changes in resistance of layers  63  due to temperature changes are canceled out. Therefore, the measured change in resistance in only a function of the force applied to layers  63 . 
     Alternatively, if read/write head  54  is moved towards side  82  of stationary platform  60  by microactuator  66 , the same process as described above is used to determine the precise position of read/write head  54 . In this case, the only change from the above description is that layers  63   a  of beam  62   a  undergo compression, whereas layers  63   b  of beam  62   b  undergo tension. This is the opposite of the situation described above. It is this difference in the type of stress applied to layers  63   a  and  63   b  that indicates the direction read/write head  54  has moved along axis  64 . Thus, in the illustrated embodiment, if the resistance of layers  63   a  of beam  62   a  increases, thus indicating tension, and the resistance of layers  63   b  of beam  62   b  decreases, thus indicating compression, then it can be ascertained that the read/write head has moved towards side  80  of stationary platform  60 . The opposite change in resistance of layers  63   a  and  63   b  would indicate that read/write head has moved towards side  82  of stationary platform  60 . For these reasons, layers  63  are not applied to the entire surface of beams  62 . If layers  63  were so applied, the tensile and compressive forces on beams  62  would effectively cancel each other out, and the direction of movement of read/write head  54  along axis  64  could not be ascertained. 
     In addition to the direction of movement, the magnitude of the movement of read/write head  54  is also required. The piezoresistive characteristics of the doped polysilicon are well documented. These characteristics are used to determine the magnitude of deformation of layers  63  that corresponds to the measured change in resistance of layers  63 . This determination can be made either through theoretical analysis, empirical testing and calibration, or a combination of the two. Knowing the magnitude and direction of deformation of layers  63  enables one to determine the magnitude and direction of movement of the end of beams  62  that are coupled to read/write head  54 . This information can then be used to determine the magnitude and direction of movement of read/write head  54  relative to stationary platform  60 , since read/write head  54  is attached to beams  62 . 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made therein without departing from the spirit and scope of the present invention as defined by the appended claims. For example, it should be understood that the position sensor described above can be implemented in numerous other applications besides its use in a mass storage device. Any application that requires precise positioning of an object could conceivably implement the present invention. In general, the present invention can be used to measure the movement of a movable platform coupled to a stationary platform by one or more beams comprising piezoresistive material.