Abstract:
In an electromagnetic linear actuator including a fixed portion with a predetermined axis and a movable portion holding an object, a single guide shaft is coupled to the fixed portion and the movable portion and guides the movable portion to be movable with respect to the fixed portion only along the predetermined axis. A position sensor is coupled to the fixed portion and the movable portion and detects a current position of the movable portion with respect to the fixed portion. The position sensor is cooperated with the single guide shaft to inhibit rotation of the movable portion around the predetermined axis.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to a tape drive for use in a linear tape storage system represented by DLT (Digital Linear Tape) or LTO (Linear Tape Open) and, in particular, to a linear actuator which can be used as a head feed mechanism for moving a magnetic head used in the tape drive.  
           [0002]    A linear tape storage system of the type has been developed as a backup for a computer system. A variety of linear tape storage systems have heretofore been proposed. For example, a digital linear tape drive as the DLT is disclosed in U.S. Pat. No. 5,862,014.  
           [0003]    The digital linear tape drive is adapted to receive a tape cartridge having a single supply reel. The digital linear tape drive includes a take-up reel in the interior thereof. When the tape cartridge is received in the tape drive, a magnetic tape is pulled out from the tape cartridge to be wound up around the take-up reel through a head guide assembly. The head guide assembly serves to guide to a magnetic head the magnetic tape pulled out from the tape cartridge. The magnetic head exchanges information to and from the tape.  
           [0004]    As disclosed in U.S. Pat. No. 5,793,574 for example, a tape drive typically comprises a generally rectangular housing having a common base. The base has two spindle motors. The first spindle motor has a permanently mounted spool permanently mounted to the base to serve as a take-up reel. The spool is dimensioned to accept a magnetic tape streaming at a relatively high speed. The second spindle motor is adapted to receive a removable tape cartridge.  
           [0005]    The removable tape cartridge is manually or automatically inserted into the drive via a slot formed on a housing of the drive. When the tape cartridge is inserted into the slot, the cartridge is engaged with the second spindle motor. Prior to rotation of the first and the second spindle motors, the tape cartridge is connected to the permanently mounted spool by means of a mechanical buckling mechanism. A number of guide rollers are positioned between the tape cartridge and the permanently mounted spool and guide the magnetic tape as it streams at a relatively high speed back and forth between the tape cartridge and the permanently mounted spool.  
           [0006]    The tape drive further comprises a head assembly. The head assembly is positioned between the spool and the tape cartridge along a tape path defined by a plurality of rollers. During operation of the tape drive, the magnetic tape streams between the spool and the tape cartridge along the tape path, coming into close proximity to the head assembly during streaming. An example of the head assembly is disclosed in U.S. Pat. No. 5,793,574 mentioned above. On the other hand, Japanese Unexamined Patent Publication No. 2000-149491 (JP 2000-149491 A) discloses an example of the tape cartridge to be received in the digital linear tape drive.  
           [0007]    In order to move the head assembly up and down in a widthwise direction of the magnetic tape, the tape drive further includes a head feed mechanism. For example, the head feed mechanism comprises a threaded shaft, i.e., a lead screw as disclosed in U.S. Pat. No. 5,793,574 mentioned above. By rotating the lead screw, the head assembly is linearly moved up and down. The head feed mechanism of the type is herein referred to as a “mechanical linear actuator”.  
           [0008]    In the mechanical linear actuator, position control of the head assembly is carried out by so-called open-loop control. Therefore, it is difficult to controllably bring the head assembly to a desired position with high accuracy.  
           [0009]    In view of the above, the present inventors have proposed a head feed mechanism for moving a head assembly up and down by electromagnetic force. The head feed mechanism of the type will herein be referred to as an “electromagnetic linear actuator”.  
           [0010]    In the electromagnetic linear actuator, position control of the head assembly can easily be carried out by closed-loop (feedback) control. It is therefore possible to controllably bring the head assembly to a desired position with high accuracy.  
           [0011]    However, the electromagnetic linear actuator requires a plurality of guide shafts for guiding the movement of the head assembly and a position sensor for detecting a current position of the head assembly. It is therefore difficult to reduce the number of components and the number of assembling steps and to save a mounting space occupied by the electromagnetic linear actuator.  
         SUMMARY OF THE INVENTION  
         [0012]    It is therefore an object of this invention to provide an electromagnetic linear actuator which is reduced in number of components and in number of assembling steps and which requires a small mounting space.  
           [0013]    It is another object of this invention to provide a tape drive comprising the above-mentioned electromagnetic linear actuator.  
           [0014]    Other objects of the present invention will become clear as the description proceeds.  
           [0015]    According to an aspect of the present invention, there is provided an electromagnetic linear actuator which comprises a fixed portion having a predetermined axis, a movable portion holding an object, a single guide shaft coupled to the fixed portion and the movable portion for guiding the movable portion to be movable with respect to the fixed portion only along the predetermined axis, and a position sensor coupled to the fixed portion and the movable portion for detecting a current position of the movable portion with respect to the fixed portion. The position sensor is cooperated with the single guide shaft to inhibit rotation of the movable portion around the predetermined axis.  
           [0016]    According to another aspect of the present invention, there is provided a tape drive which comprises the above-mentioned electromagnetic linear actuator and a head assembly as the object held by the movable portion. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0017]    [0017]FIG. 1 is a partially cut-away perspective view of an electromagnetic linear actuator obtained in the course of the work leading to this invention as seen from a rear side;  
         [0018]    [0018]FIG. 2 is a perspective view of the electromagnetic linear actuator illustrated in FIG. 1 as seen from a front side;  
         [0019]    [0019]FIG. 3 is a plan view of a tape drive comprising an electromagnetic linear actuator according to a first embodiment of this invention;  
         [0020]    [0020]FIG. 4 is a perspective view showing the electromagnetic linear actuator in the tape drive illustrated in FIG. 3;  
         [0021]    [0021]FIG. 5 is an exploded perspective view of the electromagnetic linear actuator illustrated in FIG. 4;  
         [0022]    [0022]FIG. 6 is a sectional view of the electromagnetic linear actuator illustrated in FIG. 4;  
         [0023]    [0023]FIG. 7 is a perspective view of an electromagnetic linear actuator according to a second embodiment of this invention;  
         [0024]    [0024]FIG. 8 is an exploded perspective view of an electromagnetic linear actuator according to a third embodiment of this invention; and  
         [0025]    [0025]FIG. 9 is a sectional view of the electromagnetic linear actuator illustrated in FIG. 8.  
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0026]    At first referring to FIGS. 1 and 2, description will be made of an electromagnetic linear actuator obtained in the course of the work leading to this invention.  
         [0027]    The electromagnetic linear actuator illustrated in the figure comprises a fixed portion  20 ′, a movable portion  30 ′ holding a head assembly (not shown) so that the head assembly is movable up and down with respect to the fixed portion  20 ′, a guide  40 ′ for constraining the movement of the movable portion  30 ′ except up-and-down movement, and a base  11 ′ for mounting the guide  40 ′, and a position sensor  50 ′ for detecting a current position of the movable portion  30 ′ with respect to the fixed portion  20 ′. The fixed portion  20 ′ includes a peripheral yoke  21 ′ fixedly mounted to the base  11 ′ by screws and a center yoke  22 ′ standing at the center of the peripheral yoke  21 ′. The movable portion  30 ′ has a box-like bracket  32 ′ extending in upward and downward directions A and B to cover the peripheral yoke  21 ′.  
         [0028]    The bracket  32 ′ has a front surface provided with a pair of upper and lower holding members  33 ′ for holding the above-mentioned head assembly. The bracket  32 ′ has a rear surface provided with a protruding potion  325 ′ protruding from its upper end to fix the position sensor  50 ′ (which will later be described).  
         [0029]    The guide  40 ′ comprises first and second guide shafts  41 ′ and  46 ′ disposed on both sides of the bracket  32 ′ and extending from the base  11 ′ in the upward direction A. The base  11 ′ is provided with first and second stoppers  16 ′ and  17 ′ extending in the upward direction A. The first stopper  16 ′ is arranged adjacent to one side surface of the bracket  32 ′ at a position spaced from the first guide shaft  41 ′ while the second stopper  17 ′ is arranged adjacent to the other side surface of the bracket  32 ′ at a position spaced from the second guide shaft  46 ′. The first guide shaft  41 ′ has a lower end attached to the base  11 ′ and an upper end attached to an end portion of the first stopper  16 ′. On the other hand, the second guide shaft  46 ′ has a lower end attached to the base  11 ′ and an upper end attached to an end portion of the second stopper  17 ′.  
         [0030]    The first guide shaft  41 ′ is coupled to a pair of ring bearings  324 ′ protruding from the one side surface of the bracket  32 ′ so that the ring bearings  324 ′ are slidable in the upward and the downward directions A and B. On the other hand, the second guide shaft  46 ′ is coupled to a U-shaped protrusion  326 ′ protruding from the other side surface of the bracket  32 ′ so that the protrusion  326 ′ is slidable in the upward and the downward directions A and B. The first guide shaft  41 ′ is used exclusively for up and down moving of the movable portion  30 ′ while the second guide shaft  46 ′ is exclusively used as a rotation stopper for inhibiting the rotation of the movable portion  30 ′.  
         [0031]    The first guide shaft  41 ′ is provided with first and second ring dampers  43 ′ and  44 ′ fitted thereto at positions adjacent to the upper and the lower ends thereof, respectively. The first and the second ring dampers  43 ′ and  44 ′ are called upper and lower dampers, respectively. The upper damper  43 ′ is fixed to the end portion of the first stopper  16 ′ by adhesion while the lower damper  44 ′ is fixed to an upper surface (principal surface) of the base  11 ′ by adhesion. The ring bearings  324 ′ of the movable portion  30 ′ come into contact with the upper and the lower dampers  43 ′ and  44 ′ so that a stroke of the movable portion  30 ′ is limited and mechanical shock is absorbed.  
         [0032]    The position sensor  50 ′ for detecting the current position of the movable portion  30 ′ in the upward and the downward directions A and B is disposed on the rear side of the bracket  32 ′ and extends from the base  11 ′ in the upward direction A.  
         [0033]    More specifically, the position sensor  50 ′ comprises a hollow sensor bobbin  52 ′ with a sensor coil  51 ′ wound therearound, a screw  53 ′ engaged with the sensor bobbin  52 ′ to fix the sensor bobbin  52 ′ to the protruding portion  325 ′ of the bracket  32 ′, and a sensor shaft  54 ′ extending in the upward and the downward directions A and B and having one end fixed to the base  11 ′ and the other end inserted into the sensor bobbin  52 ′. The sensor shaft  54 ′ has an outer diameter substantially equal to an inner diameter of the sensor bobbin  52 ′. The position sensor  50 ′ detects the amount or length of insertion of the sensor shaft  54 ′ into the sensor bobbin  52 ′ as the change in inductance value so that the current position of the movable portion  30 ′ with respect to the base  11 ′ (fixed portion  20 ′) is detected as a detected position.  
         [0034]    The first and the second stoppers  16 ′ and  17 ′ are bridged at their lower ends by a beam  18 ′ fixed onto the base  11 ′ by screws  19 ′ on the rear side of the bracket  32 ′. One end of the sensor shaft  54 ′ is fixed to an approximate center of the beam  18 ′.  
         [0035]    The above-mentioned electromagnetic linear actuator uses the two guide shafts  41 ′ and  46 ′ as the guide  40 ′. In addition to a space for mounting the guide shafts  41 ′ and  46 ′, another space for mounting the position sensor  50 ′ is also required. Such increase in space has an adverse influence upon a mounting space of other components. Therefore, it is desired to save the space required to mount the electromagnetic linear actuator.  
         [0036]    Referring to FIG. 3, description will be made of a tape drive comprising an electromagnetic linear actuator according to a first embodiment of this invention.  
         [0037]    The tape drive is depicted by a reference numeral  1  and includes a chassis  2  having an upper surface  3  and a lower surface opposite to the upper surface  3 . The chassis  2  comprises a sheet metal chassis made of an iron-based magnetic material. The upper surface  3  is divided into first and second upper regions  3   a  and  3   b.  The tape drive further comprises a take-up reel  4  rotatably mounted to the chassis  2  on the first upper region  3   a.  The take-up reel  4  is driven and rotated by a take-up motor (not shown) attached to the chassis  2  on the lower surface.  
         [0038]    A slot portion  5  is formed on the second upper region  3   b  of the upper surface  3  of the chassis  2 . The slot portion  5  is adapted to receive a tape cartridge (not shown) comprising a rotatable supply reel (not shown). In the slot portion  5 , a supply rotor  6  to be engaged with the supply reel of the tape cartridge is exposed. The supply rotor  6  is driven and rotated by a supply motor (not shown) attached to the chassis  2  on the lower surface.  
         [0039]    The tape drive illustrated in the figure further includes a head assembly  7  having a magnetic head. The head assembly  7  is positioned on a tape path defined by a plurality of rollers. During operation of the tape drive, a magnetic tape streams between the take-up reel  4  and the tape cartridge along the tape path, coming into close proximity to the head assembly  7  during streaming. Thus, information exchange between the magnetic head and the magnetic tape can be carried out.  
         [0040]    In order to move the head assembly  7  in a widthwise direction of the magnetic tape, i.e., in a direction perpendicular to a sheet plane of the drawing, the tape drive further includes a head feed mechanism  8 . The head assembly  7  corresponds to an object to be moved up and down.  
         [0041]    Referring to FIGS. 4 through 6 in addition to FIG. 3, description will be made of the head feed mechanism  8 .  
         [0042]    The head feed mechanism  8  comprises an electromagnetic linear actuator of a “movable magnet type”. The electromagnetic linear actuator comprises a fixed portion  20 , a movable portion  30  holding the head assembly  7  and movable in upward and downward directions A and B with respect to the fixed portion  20 , a guide  40  for guiding the movable portion  30  with inhibiting the movement thereof except up-and-down movement, and a position sensor  50 .  
         [0043]    The fixed portion  20  has a yoke base  10  of an integral formed product which comprises a plate-like base  11  and a cylindrical yoke  12  integrally formed. The yoke base  10  is formed by drawing a plate as the base  11  to form a portion having a cylindrical shape.  
         [0044]    The base  11  is provided with a guide attaching hole  11   b  and a sensor attaching hole  11   c.  The guide  40  is attached to the guide attaching hole  11   b  of the base  11 . The position sensor  50  is attached to the sensor attaching hole  11   c.  The base  11  is further provided with four through holes  11   d  at four corner portions thereof. The base  11  is attached to the chassis  2  of the tape drive  1  by inserting four screws (not shown) into the through holes  11   d.    
         [0045]    The yoke  12  has a cylindrical portion  121  and an annular upper surface portion  122  arranged at an upper end of the cylindrical portion  121  and having a circular opening  122   a  formed at its center. The yoke  12  contains a hollow bobbin  22  with a coil  21  wound therearound.  
         [0046]    On the other hand, the movable portion  30  comprises a cylindrical magnet  31  extending in the upward and the downward directions A and B, a bracket  32  mounted on the magnet  31 , and a center yoke  33  inserted into the magnet  31  to connect the magnet  31  and the bracket  32 . The magnet  31  is received in the bobbin  22  to be slidable in the upward and the downward directions A and B. The magnet  31  has an outer diameter substantially equal to the inner diameter of the bobbin  22 . A combination of the yoke  12 , the coil  21 , the magnet,  31 , and the center yoke  33  is referred to as an electromagnetic device which generates electromagnetic power acting between the coil  21  and the magnet  31  when the coil  21  is supplied with an electric current in the manner known in the art.  
         [0047]    The bracket  32  has a cylindrical portion  321  standing on the magnet  31  and extending in the upward direction A, a sector portion  322  disposed at an upper end of the cylindrical portion  321  and extending outward in a radial direction perpendicular to the upward and the downward directions A and B, and a curved portion  323  disposed at an outer peripheral edge of the sector portion  322  and extending in the downward direction B in parallel to the cylindrical portion  321 . The head assembly  7  is attached to the curved portion  323 .  
         [0048]    The guide  40  comprises a guide shaft  41  attached to the base  11  and extending on a guide attaching hole  11   b  in the upward direction A, and a cylindrical bushing  42  disposed at a lower end of the guide shaft  41  and fitted to the guide attaching hole  11   b  to thereby attach the guide shaft  41  to the base  11  on the guide attaching hole  11   b.  A cylindrical bearing  324  protrudes from one lateral end of the curved portion  323  of the bracket  32  outward in the radial direction and is attached to the guide shaft  41  to be slidable in the upward and the downward directions A and B.  
         [0049]    The guide shaft  41  has a top end  41   a  in the upward direction A, a first ring groove  41   b  formed adjacent to the top end  41   a , and a second ring groove  41   c  formed adjacent to the bushing  42  in the downward direction B on an upper side thereof. A first ring damper  43  is fitted to the first ring groove  41   b  while a second ring damper  44  is fitted to the second ring groove  41   c.  Therefore, the bearing  324  is slidable between the first and the second ring dampers  43  and  44  along the guide shaft  41  in the upward and the downward directions A and B.  
         [0050]    On the other hand, the position sensor  50  is attached to the base  11  and extends on the sensor attaching hole  11   c  in the upward direction A.  
         [0051]    More specifically, the position sensor  50  comprises a hollow sensor bobbin  52  with a sensor coil  51  wound therearound, a screw  53  engaged with the sensor bobbin  52  through the sensor attaching hole  11   c  so that the sensor bobbin  52  is attached to the base  11  to stand on the sensor attaching hole  11  in the upward direction A, and a sensor shaft  54  extending in the upward and the downward directions A and B. The sensor shaft  54  has one end  54   a  fixed to the sector portion  322  at an upper position corresponding to the sensor attaching hole  11   c  and the other end  54   b  inserted into the sensor bobbin  52 . The sensor shaft  54  has an outer diameter substantially equal to an inner diameter of the sensor bobbin  52 . The position sensor  50  detects the amount or length of insertion of the sensor shaft  54  into the sensor bobbin  52  as the change in inductance value so that the current position of the movable portion  30  with respect to the base  11  (fixed portion  20 ) is detected as a detected position.  
         [0052]    In the electromagnetic linear actuator used as the head feed mechanism  8  in FIGS. 4 through 6, the position sensor  50  comprises a mechanism in which the sensor shaft  54  moves in the sensor bobbin  52 . The sensor bobbin  52  of the position sensor  50  is made of a material excellent in slidability to thereby achieve smooth movement of the sensor shaft  54 . With this structure, the position sensor  50  can be used also as a rotation stopper for inhibiting the rotation of the movable portion  30 . Thus, the second guide shaft  46 ′ as the rotation stopper and the position sensor  50 ′ in the electromagnetic linear actuator illustrated in FIGS. 1 and 2 can be replaced collectively by the position sensor  50  as a single common component. Therefore, it is possible to reduce the components involved in the guide shaft  46 ′ used as the rotation stopper in the electromagnetic linear actuator in FIGS. 1 and 2. As a consequence, the cost is lowered and the space is saved in correspondence to the above-mentioned reduction of the components.  
         [0053]    The position sensor  50  produces a position detection signal representative of the current position, i.e., the detected position of the movable portion  30  and transmits the position detection signal to a controller (not shown). The controller compares the detected position represented by the position detection signal and a target position of the movable portion  30  and controls the electric current supplied to the coil  21  wound around the bobbin  22  so that the detected position is coincident with the target position. Thus, by the use of the electromagnetic linear actuator, position control of the head assembly (object to be moved up and down)  7  can be carried out by closed loop (feedback) control.  
         [0054]    Referring to FIG. 7, description will be made of an electromagnetic linear actuator according to a second embodiment of this invention. The same parts as those of the electromagnetic linear actuator illustrated in FIGS. 4 through 6 are designated by the same reference numerals and description thereof will be omitted. Similar parts are designated by the same reference numerals with a suffix “A” added thereto.  
         [0055]    The position sensor  50 A comprises a hollow sensor bobbin  52 A with a sensor coil  51 A wound therearound, a screw  53 A engaged with the sensor bobbin  52 A so that the sensor bobbin  52 A is attached to a lower surface of the sector portion  322  of the bracket  32  at an outer peripheral end thereof to extend in the downward direction B, and a sensor shaft  54 A extending in the upward and the downward directions A and B. The sensor shaft  54 A has one end fixed to the base  11  at a lower position corresponding to the sensor bobbin  52 A and the other end inserted into the sensor bobbin  52 A. The sensor shaft  54 A has an outer diameter substantially equal to an inner diameter of the sensor bobbin  52 A. The position sensor  50 A detects the amount or length of insertion of the sensor shaft  54 A into the sensor bobbin  52 A as the change in inductance value so that the current position of the movable portion  30  with respect to the base  11  (fixed portion  20 ) is detected as the detected position.  
         [0056]    In the electromagnetic linear actuator illustrated in FIGS. 4 through 6, the sensor bobbin  52  is attached to the base  11  while the sensor shaft  54  is fixed to the movable portion  30 . On the other hand, in the electromagnetic linear actuator illustrated in FIG. 7, the sensor shaft  54 A stands on the base  11  while the sensor bobbin  52 A is attached to the movable portion  30 .  
         [0057]    In the electromagnetic linear actuator illustrated in FIG. 7 also, the position sensor  50 A comprises a mechanism in which the sensor shaft  54 A moves in the sensor bobbin  52 A. The sensor bobbin  52 A of the position sensor  50 A is made of a material excellent in slidability to thereby achieve smooth movement of the sensor shaft  54 A. With this structure, the position sensor  50 A can be used also as a rotation stopper for inhibiting the rotation of the movable portion  30 . Thus, the second guide shaft  46 ′ as the rotation stopper and the position sensor  50 ′ in the electromagnetic linear actuator illustrated in FIGS. 1 and 2 can be replaced collectively by the position sensor  50 A as a single common component. Therefore, it is possible to reduce the components involved in the guide shaft  46 ′ used as the rotation stopper in the electromagnetic linear actuator in FIGS. 1 and 2. As a consequence, the cost is lowered and the space is saved in correspondence to the above-mentioned reduction of the components.  
         [0058]    In the electromagnetic linear actuator illustrated in FIG. 7, the head assembly  7  is attached to an outer surface of the curved portion  323  of the bracket  30 . The base  11  is fixedly mounted onto the chassis of the tape drive by the use of the screws (not shown) inserted into the four through holes  11   d.    
         [0059]    Referring to FIGS. 8 and 9, description will be made of an electromagnetic linear actuator according to a third embodiment of this invention. The same parts as those of the electromagnetic linear actuator illustrated in FIGS. 4 through 6 are designated by the same reference numerals and description thereof will be omitted. Similar parts are designated by the same reference numerals with a suffix “A” added thereto.  
         [0060]    The electromagnetic linear actuator illustrated in FIGS. 4 through 6 is of a “movable magnet type” in which the magnet is movable. On the other hand, the electromagnetic linear actuator illustrated in FIGS. 8 and 9 is of a “movable coil type” in which the coil is movable.  
         [0061]    The fixed portion  20 A comprises a cylindrical magnet  31 A received in the yoke  12 , and a center yoke  33 A fixed at a lower end of the yoke  12  and extending in the upward direction A. The movable portion  30 A includes a hollow bobbin  22 A disposed between the magnet  31 A and the center yoke  33 A and having a coil  21 A wound therearound. The bobbin  22 A is slidable in the upward and the downward directions A and B with respect to the center yoke  33 A. The bobbin  22 A has an inner diameter substantially equal to an outer diameter of the center yoke  33 A.  
         [0062]    Although this invention has thus far been described in conjunction with the preferred embodiments, it will readily be understood that this invention is not restricted thereto. For example, a bearing, such as a linear bearing, an oilless metal, and a resin bearing, may be attached to the sensor bobbin ( 52 ,  52 A) at a portion brought into contact with the sensor shaft ( 54 ,  54 A). In the foregoing embodiment, description is directed to the case where the object to be moved up and down is the head assembly. However, the object is not restricted thereto at all.