Abstract:
In one embodiment, an actuator comprises a stationary guide, a carriage movable along the guide, and a piezoelectric motor operatively coupled to the carriage and pushing on the guide such that the motor when energized moves with the carriage along the guide.

Description:
BACKGROUND 
   Tape drives store a vast amount of digital information on rolls of magnetic tape and are often used to backup information stored in computer systems. An example of a tape drive is a linear tape open (LTO) drive. 
   In a typical LTO drive, magnetic tape is stored on a supply reel contained in a removable cartridge. Data on the tape, including servo information, is arranged in 128 or more parallel tracks. During operation, the tape is passed along a series of rollers, defining the tape path, to a non-removable take up reel in the drive. The tape passes in close proximity to an assembly of read heads and write heads. The heads must be accurately positioned over the desired tracks so data can be read or written without loss and without corrupting adjacent tracks. An actuator positions the head assembly by moving it across the width of the tape. As the magnetic tape passes by the heads, the tape may have a small side-to-side motion due to the tolerances of the tape and the rollers. During coarse positioning, the actuator moves the head assembly so that a read head is close enough to a desired track to read servo information. Subsequently, during fine positioning, the servo information is read from the track and sent to servo control circuitry, which then sends a signal to the actuator to move the head directly over the desired track and to follow the small side-to-side motion of the track as it passes by the head. 
   Conventional actuators in LTO drives are often constructed as an electromagnetic actuator in which the head assembly is moved by the electromagnetic force from a strong permanent magnet and an electrically conductive coil. The electromagnetic actuator requires a coarse position sensor to determine the location of the head array during course positioning. Once the head assembly is over the desired track, a continuous electric current through the coil, called either a holding signal or holding current, is required to hold the head assembly in place. 

   
     DRAWINGS 
       FIG. 1  is a plan view of a tape drive incorporating an actuator constructed according to one embodiment of the present invention. 
       FIGS. 2 and 3  illustrate a head assembly positioned over a track on magnetic tape. In  FIG. 2 , the head assembly is positioned for a write operation. In  FIG. 3 , the head assembly is positioned for a read operation. 
       FIGS. 4-7  illustrate the operation of an exemplary piezoelectric motor. 
       FIG. 8  illustrates the path traced by the end of a piezoelectric beam in a piezoelectric motor. 
       FIG. 9  is a perspective view the actuator of  FIG. 1 . 
       FIGS. 10 and 11  are elevation views of the actuator of  FIG. 9  in different positions. 
       FIG. 12  is a plan view of the actuator of  FIG. 9 . 
       FIG. 13  is an elevation view of an actuator constructed according to another embodiment of the present invention. 
       FIG. 14  is an elevation view of an actuator constructed according to another embodiment of the present invention. 
       FIG. 15  is a flow diagram illustrating one exemplary method for coarse positioning a piezoelectric actuator. 
   

   DESCRIPTION 
   Embodiments of the present invention were developed in an effort to eliminate the coarse position sensor in a drive actuator, reduce stray magnetic fields near the surface of the tape by eliminating the permanent magnet used in conventional actuators, and reduce power consumption by eliminating the need for a holding current, all while retaining the precise positioning required for reliable operation. Embodiments of the invention will be described with reference to a head assembly actuator in a LTO drive such as the one shown in  FIG. 1 . The invention, however, is not limited to use in LTO drives. Embodiments of the invention may be implemented in other tape drives or other devices in which it may be necessary or desirable to utilize a piezoelectric actuator. 
     FIG. 1  shows a LTO drive  10 . In  FIG. 1 , magnetic tape  12  is wound on supply reel  14  inside cartridge  16 . When cartridge  16  is inserted into drive  10 , tape  12  passes around guide  18 , over head assembly  20 , around guide  22 , to take up reel  24 . Head assembly  20  contains one or more read heads, write heads, or combined read/write heads. A “head” as used in this document means a transducer that either converts an electrical signal to the form required to write the signal to a medium (a write head), or reads a signal from a medium and converts it to an electrical signal (a read head). The read and write functions can be combined in a single read/write head. Tape drives typically use a magnetic head, where an electrical signal drives a time-varying magnetic field that magnetizes spots, or domains, on the surface of the magnetic tape. A CD-ROM drive typically uses an optical head, where an electrical signal drives a laser that varies the reflectivity of an optical medium. 
   Head assembly  20  is mounted to an actuator  26  which moves head assembly  20  across the width of tape  12 . An electronic controller  28  receives read and write instructions and data from a computer or other host device  29 . Controller  28 , which may include more than one controller unit, includes the programming, processor and associated memory and electronic circuitry necessary to control actuator  26 , head assembly  20  and the other operative components of tape drive  10 . As actuator  26  carries head assembly  20  back and forth across the width of tape  12 , controller  28  selectively activates the heads to write data to tape  12  or read data from tape  12  according to the instructions received from the host device. 
   In an exemplary write operation shown in  FIG. 2 , magnetic tape  12  travels from supply reel  14  to take-up reel  24  ( FIG. 1 ) past head assembly  20 , where one or more write heads  30  write data onto the tape as one or more tracks  32 . Write data may include storage information, or servo information to assist in positioning head assembly  20 , or both storage and servo information. During a subsequent read operation shown in  FIG. 3 , data from track  32  is read by one or more read heads  34  as tape  12  travels past. Read head  34  must be aligned directly over track  32  to reliably read data. Positioning of head assembly  20  to place head  34  over track  32  occurs in two stages: coarse positioning, where read head  34  is brought close enough to track  32  to read servo information for track  32 ; and fine positioning, where the servo information is used to position read head  34  directly over track  32 . 
   A LTO drive may be configured to perform read and write operations a variety of ways. For example, write head  30  and read head  34  may be combined into a single read/write head. Head assembly  20  may deploy an array of read/write heads to read or write an array of parallel tracks simultaneously. Servo information may be read from a single track and used to position head assembly  20  to simultaneously read or write multiple tracks containing storage information. Tape drive  10  may read or write data as tape  12  moves from the supply reel  14  to take-up reel  24  or as tape  12  moves from take-up reel  24  to the supply reel  14  ( FIG. 1 ). 
     FIGS. 4-7  illustrate the operation of an exemplary piezoelectric motor. A “piezoelectric motor” as used in this document means a device that imparts stepwise motion by flexing a piezoelectric beam against an object. If the motor is stationary, then the object moves. If the object is stationary, then the motor moves. A piezoelectric motor is able to impart motion greater than the beam flexes by making a series of steps in the same direction. Piezoelectric motors are capable of linear motion or rotary motion and may include more than one piezoelectric beam. Piezoelectric motors are capable of very small, repeatable steps and smooth operation. Step sizes may be as small as a few nanometers. Piezoelectric motors that may be adapted for use in embodiments of the present invention are commercially available from various manufacturers, such as Nanomotion, Ltd. in Yokneam, Israel. 
     FIGS. 4-7  illustrate a simple piezoelectric motor  36  that includes a motor base  38  and a beam  40  of piezoelectric material attached to base  38 . In operation, sinusoidal voltages of ultrasonic frequency applied to beam  40  excite both longitudinal and bending vibrations so that the free end  42  of beam  40  traces a planar, roughly elliptical path, as shown in  FIG. 8 . Each circuit taken by end  42  generates one step of motion. A spring  44  biases beam  40  against a stationary contact surface  46  so that the motor pushes against surface  46  as beam  40  oscillates. Contact surface  46  is preferably made of ceramic material or another suitably hard substance to prevent wear. The elliptical path traced by beam end  42  is in a plane perpendicular to contact surface  46 . As beam  40  pushes against stationary surface  46 , motor  36  moves relative to surface  44 , as best seen by comparing the position of motor  36  in  FIGS. 4-7 . 
   Nanomotion&#39;s U.S. Pat. No. 5,877,579 describes the structure and operation of piezoelectric motors in more detail. The Description and Figures of U.S. Pat. No. 5,877,579 are incorporated herein by reference. With high excitation frequencies, piezoelectric motors can provide a smooth, constant driving force with the small, repeatable step size desirable for precise positioning. A piezoelectric motor may be configured to maintain its position without power upon removal of the excitation signal to eliminate the need for a holding signal, holding current, or external brake. For example, biasing spring  44  in  FIGS. 4-7  holds beam  40  against surface  46  when motor  36  is not energized. Alternatively, the size and piezoelectric characteristics of beam  40  may be selected to preload beam  40  against surface  46  without an external biasing mechanism. 
     FIG. 9  is a perspective view of actuator  26 .  FIGS. 10 and 11  are elevation views of actuator  26  in different positions.  FIG. 12  is a plan view of actuator  26 . Referring to  FIGS. 9-12 , actuator  26  includes a piezoelectric motor  48  mounted to a carriage  50 . Motor  48  includes a base  52  and a beam of piezoelectric material  54  attached to base  52 . Beam  54  of motor  48  pushes against a front guide rail  56 , which performs the function of stationary surface  46  in  FIGS. 4-7 . Carriage  50  rides along front guide rail  56  and rear guide rail  58  supported by bearings  60 . Rails  56  and  58  are secured to an actuator base  62 , which is secured to the chassis or other stable component of tape drive  10 . Motor  48  is attached to carriage  50  through pins  64  or another suitable coupling to transmit motive force from motor  48  to carriage  50 . A biasing spring  66  positioned between motor  48  and a spring base  68  on carriage  50  pushes motor  48  against front guide rail  56 . 
   Referring to  FIGS. 3 and 12 , head assembly  20  is mounted to carriage  50  so that write heads  30  and read heads  34  are adjacent to magnetic tape  12 . In operation, when piezoelectric motor  48  is energized, it moves up or down along stationary front guide rail  56  in the stepwise fashion described above for motor  36  in  FIGS. 4-7 . In this way, motor  48  carries carriage  50  up and down along guide rails  56  and  58  perpendicular to the direction of tape travel to properly position head assembly  20  for read and write operations.  FIGS. 10 and 11  show carriage  50  in different positions along guide rails  56  and  58 . 
     FIG. 13  illustrates an actuator  26  in which the piezoelectric motor  48  is placed behind rear guide rail  58 . In the embodiment of actuator  26  shown in  FIG. 13 , motor  48  pushes against rear guide rail  58 , which performs the function of stationary surface  46  in  FIGS. 4-7 . 
     FIG. 14  illustrates an actuator  26  in which piezoelectric motor  48  is not mounted to head carriage  50  and motor  48  remains stationary when energized. In the embodiment of actuator  26  shown in  FIG. 14 , motor  48  is mounted to a stationary post  70  secured to actuator base  62  adjacent to carriage  50 . Biasing spring  66  pushes motor  48  against carriage  50 . Post  70  may be located outside carriage  50 , as shown in  FIG. 14 , or inside carriage  50 , between guide rails  56  and  58  for example. In either case, when piezoelectric motor  48  is energized, it drives carriage  50  up or down along guide rails  56  and  58  in the stepwise fashion described above for motor  36  in  FIGS. 4-7 . 
     FIG. 15  illustrates one exemplary method for coarse positioning a head assembly using a piezoelectric motor. Coarse positioning starts with step  72  where the target count is loaded with the number of steps (fixed units of distance) required to place a read or write head at the expected location of a desired track on the tape. The target count may be determined prior to operation by trial and error or by calculating the number of required steps based on the step size, the physical size of the tape, the number of tracks, and the track spacing. In step  74 , the motor drives the actuator against the actuator base or another physical stop. In step  76 , the motor moves one step away from the stop. In step  78 , the step count is incremented by one step. In step  80 , the new step count is compared to the target count. Steps  76 ,  78  and  80  are repeated until the step count equals the target count. In step  82 , the motor is de-energized or otherwise signaled to lock the carriage in place. Since the stop provides a fixed reference location and the motor steps are repeatable, there is no need for a position sensor or closed loop servo control during coarse positioning. Once coarse positioning is completed, the head is in place to read servo information in step  84  to begin fine positioning. The steps shown in  FIG. 15  may be implemented as a dedicated electronic circuit or as programming executed by a processor. The processor or circuit may be located on the tape drive controller. 
   “Bearing” as used in this document means any suitable object, structure or surface that movably supports the carriage for travel along the rails. Suitable bearings may include, for example, ball bearings, roller bearings, Gothic arch bearings, journal bearings, bushings and the like. 
   The exemplary embodiments shown in the figures and described above illustrate but do not limit the invention. Other forms, details, and embodiments may be implemented. For example, the piezoelectric actuator is not limited to use in magnetic tape drives. The magnetic tape may be replaced by a rotating magnetic medium as used in a hard disk drive. The magnetic medium may be replaced by an optical medium, an optical drive for example. Guide rails need not be round, but may have any suitable cross section. Only one guide rail (or none at all) may be necessary or desirable in some applications. Guide rails may be curved to follow the surface of a curved medium. Hence, the foregoing description should not be construed to limit the spirit and scope of the invention, which is defined in the following claims.