Abstract:
Provided is a linear motor comprising a magnet including a first region polarized to have a magnetic pattern for driving and a second region polarized to have a magnetic pattern for position detection, the first and second regions being arranged linearly in a direction of the driving; a drive coil that is provided opposite the first region and generates a drive force exerted on the magnet in the direction of the driving; a magnetic sensor arranged opposite the second region; and a base member that supports the magnet, the drive coil, and the magnetic sensor such that the magnet can be moved relative to the drive coil and the magnetic sensor in the driving direction.

Description:
The contents of the following Japanese patent application are incorporated herein by reference: No. 2009-209736 filed on Sep. 10, 2009. 
     The contents of the following International patent application are incorporated herein by reference: No. PCT/JP2010/005544 filed on Sep. 10, 2010. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a linear motor and a lens unit. 
     2. Related Art 
     A configuration using a wire and a rotary encoder is conventionally used as a position detecting mechanism for moving bodies in a linear motor. This mechanism detects position by using the rotary encoder to detect the amount of relative change with respect to a wire pulled between a scanning table, which is a moving component, and a fixed point on a rail, as shown in Patent Document 1, for example. 
     Patent Document 1: Japanese Patent Application Publication No. H09-222318 
     With this mechanism, however, a wire coiling mechanism is used in addition to the rotary encoder to apply a prescribed tensile force to the wire. Therefore, the size of the apparatus is increased. 
     SUMMARY 
     To solve the above problem, according to a first aspect related to the innovations herein, provided is a linear motor comprising a magnet including a first region polarized to have a magnetic pattern for driving and a second region polarized to have a magnetic pattern for position detection, the first and second regions being arranged linearly in a direction of the driving; a drive coil that is provided opposite the first region and generates a drive force exerted on the magnet in the direction of the driving; a magnetic sensor arranged opposite the second region; and a base member that supports the magnet, the drive coil, and the magnetic sensor such that the magnet can be moved relative to the drive coil and the magnetic sensor in the driving direction. 
     According to a second aspect related to the innovations herein, the linear motor is provided in a lens unit. 
     The summary clause does not necessarily describe all necessary features of the embodiments of the present invention. The present invention may also be a sub-combination of the features described above. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view schematically showing the overall configuration of a linear motor according to an embodiment of the present invention. 
         FIG. 2  is a cross-sectional view schematically showing the configuration of the linear motor. 
         FIG. 3  schematically shows the fixing portion of the intermediate area in the moving member. 
         FIG. 4  is a cross-sectional view schematically showing a moving-coil linear motor as another embodiment of the present invention. 
         FIG. 5  is a schematic cross-sectional view of an overall configuration of an image capturing apparatus. 
         FIG. 6  is a schematic view of the configuration of the lens unit. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, some embodiments of the present invention will be described. The embodiments do not limit the invention according to the claims, and all the combinations of the features described in the embodiments are not necessarily essential to means provided by aspects of the invention. 
       FIG. 1  is a perspective view schematically showing the overall configuration of a linear motor  100  according to an embodiment of the present invention. The linear motor  100  includes a moving member  110 , a stator  120 , hall elements  131  and  132 , a base member  140 , and a calculating section  150 . The moving member  110  is a cylindrical magnet, and the stator  120  includes a hollow cylindrical drive coil. 
     The moving member  110  is supported by linear movement bearings  144  provided on supports  142  of the base member  140 . The stator  120  is supported on the base member  140 , has an inner diameter that is greater than the outer diameter of the moving member  110 , and the inner surface of the stator  120  is distanced from the moving member  110 . With this configuration, the moving member  110  can move smoothly along the stator  120  in the longitudinal direction thereof. In the following description, this longitudinal direction is referred to as the X-axis direction, a direction orthogonal to the mounting surface of the linear motor  100  is referred to as the Z-axis direction, and a direction orthogonal to the X-axis and the Z-axis is referred to as the Y-axis direction. 
       FIG. 2  is a cross-sectional view schematically showing the configuration of the linear motor  100 . The moving member  110  has three regions, which are a drive region  112  that is polarized to have a magnetic pattern for driving, a position detection region  114  that is polarized to have a magnetic pattern for position detection, and an intermediate area  116 . 
     The intermediate area  116  includes a fixing portion that can fix the member being driven when the moving member  110  moves relative to the base member  140 . Since the intermediate area  116  is not polarized, the intermediate area  116  can decrease the effect that the magnetic fields of the drive region  112  and the position detection region  114  have on each other. This effect can be achieved simply by not polarizing the intermediate region, and it is not necessary to include the fixing portion of the driven member. Furthermore, the intermediate area  116  need not be provided, and the drive region  112  and the position detection region  114  may be adjacent to each other. 
     The following describes the configuration of the position detecting section and the drive section of the linear motor  100 . As shown in  FIG. 2 , the drive region  112  of the moving member  110  is polarized to achieve a polarization pattern in which the poles are arranged in the X-axis direction. On the other hand, the position detection region  114  is polarized to achieve a polarization pattern in which the poles in a direction orthogonal to the X-axis direction, which is a suitable orientation for position detection. 
     The drive coil portion  122  of the stator  120  is a series of coils having a U phase, a V phase, and a W phase, which are oriented in the X-axis direction. Each coil has the same width in the X-axis direction, and this width is ⅓ of the width of one pole of the drive region  112  of the moving member  110 . 
     The drive coil portion  122  is supplied with a three-phase current corresponding to positions that have a phase difference of 120 degrees when the pitch of the poles (between N and S) of the drive region  112  has a phase of 180 degrees. As a result, thrust is generated in the X-direction due to the relationship with the magnetic pattern of the drive region  112 . By controlling the current supplied to the coils according to the position of the moving member  110 , the movement of the moving member  110  in the X-axis direction can be controlled. 
     The position of the moving member  110  is detected using the position detection region  114  and the hall elements  131  and  132  arranged at positions opposite the position detection region  114 . The hall elements  131  and  132  each output a voltage corresponding to the polarity and strength of the magnetic field at the position thereof, and this output voltage changes when the moving member  110  moves. The calculating section  150  performs a calculation using this output voltage, and outputs position information of the moving member  110  as a result. 
     For example, it is assumed that the surface facing the hall element  131  on one end of the position detection region  114  is an S pole, and the initial position of the moving member  110  is such that one end of the position detection region  114  is positioned opposite the position of the hall element  131 . When the moving member  110  moves such that the position opposite the hall element  131  moves across the width of two poles, first an N pole and then an S pole of the position detection region  114 , and then stops, the resulting state is such that an S pole is positioned opposite the hall element  131 . At this time, the output voltage of the hall element  131  is a sinusoidal signal (signal A) resulting from the sequential detection of the magnetic fields of an S pole, an N pole, and an S pole. The hall element  132  outputs a sinusoidal signal (signal B) in which the phase of the magnetic field is shifted by 90 degrees, such that the magnetic field output is 0 at the boundary from N to S poles, at which time the hall element  131  corresponds to an S pole. 
     The sinusoidal signals having a phase difference of 90 degrees therebetween can be frequency-divided by the calculating section  150  to generate pulses at positions having a prescribed resolution. Any position can be detected based on a count value, by resetting the count value of the pulses at an original sensor position or when the moving member  110  is at a prescribed mechanical stopper position. As a result, the position of the moving member  110  can be more accurately detected. 
       FIG. 3  schematically shows the fixing portion of the intermediate area  116  in the moving member  110 . The driven body can be fixed by connecting a restraint in a central aperture. Since the intermediate area  116  is a non-magnetic region, the effect of the magnetic fields of the drive region  112  and the position detection region  114  on each other can be decreased, and the non-magnetic intermediate area  116  can also function as the fixing portion of the driven member, thereby effectively using the regions of the moving member  110 . 
     As described in the above embodiment, the position detection can be realized using a configuration in which a single component is polarized to form the drive region  112  and the position detection region  114  and a magnetic sensor is provided. In other words, there is no need to provide an additional structure for the position detection, and therefore the linear motor can be made smaller. Furthermore, the drive region  112  and the position detection region  114  can be formed by one performance of a polarization process, and therefore the number of steps used in the manufacturing is decreased. Accordingly, the manufacturing cost can be decreased. 
     As shown in  FIGS. 2 and 3 , the pitch of the magnetic pattern of the drive region  112  in the above embodiment is greater than the pitch of the magnetic pattern of the position detection region  114 . With this configuration, the position can be controlled more precisely than allowed by the magnetic pattern of the drive region  112 . 
     For example, when there are three phases of coils and the width of one coil is ⅓ the width of one pole of the drive region  112 , as in the above embodiment, setting the coils of the position detection region  114  to have the same pitch as the coils enables the polarity of the drive region  112  at positions opposite each of the coils to be known. Furthermore, even more precise positional control can be realized by using a lower pitch to control the position using one or two phase modes, for example. The present invention is not limited to three phases of coils, and may instead include two phases or four or more phases of coils, or may use a single phase. 
     The above embodiment describes an example in which the moving member  110  has a cylindrical shape. Compared to forming the moving member  110  as a square pillar, for example, forming the moving member  110  as a cylinder achieves the effects of effectively using space and being suitable for complementing the linear movement bearing, for example. On the other hand, the moving member  110  may be formed as a square pillar or a flat shape, depending on the intended use and environment. 
     In the above embodiment, a hall element is used as the magnetic sensor, but the present invention is not limited to this. Instead, a magnet diode or magnetoresistance effect element, for example, may be used. 
     In the above embodiment, the drive coil portion  122  of the stator  120  is arranged on the outside of the moving member  110 , but the present invention is not limited to this. As another example, a central axis may be fixed to the base member as the stator, and an annular magnet may be arranged around the central axis as the moving member. In this case, the fixing portion of the intermediate area  116  is preferably arranged to avoid the central axis, and the intermediate area  116  may be removed such that the drive region  112  and the position detection region  114  are adjacent. The hall elements  131  and  132  may be arranged on the stator, instead of on the base member. 
     The above embodiment describes a moving-magnet linear motor in which the moving member  110  is a magnet, but the present invention is not limited to this. Instead, a moving-coil linear motor may be used, in which the coils operate as the moving member. 
       FIG. 4  is a cross-sectional view schematically showing a moving-coil linear motor as another embodiment of the present invention. The linear motor  200  includes a stator  210 , a moving member  220 , hall elements  224  and  225  arranged on the moving member  220 , a base member  230 , and a calculating section  240 . 
     The stator  210  is supported by the base member  230 , and is a hollow cylinder with an inner diameter that is greater than the outer diameter of the cylindrical moving member  220 . The inner surface of the stator  210  is distanced from the moving member  220 . The moving member  220  is supported via a linear movement bearing provided on the support of the base member  230 . With this configuration, the moving member  220  moves smoothly in the X-axis direction along the stator  210 . 
     The stator  210  includes a magnet  212 , and the magnet  212  has three regions, which are a drive region  214 , a position detection region  216 , and an intermediate area  218 . The drive region  214  is polarized to have a magnetic pattern for driving, in which the poles are arranged in the X-axis direction. The position detection region  216  is polarized to have a magnetic pattern for position detection, in which the poles are arranged in a direction orthogonal to the X-axis direction. The intermediate area  218  is not polarized, in order to decrease the effect of the magnetic fields of the drive region  214  and the position detection region  216  on each other. 
     The moving member  220  includes a central axle and a drive coil portion  222 , which has a plurality of coils disposed around the central axle. The drive coil portion  222  may include coils having a U phase, a V phase, and a W phase, which are oriented in the longitudinal direction of the moving member  220 . Each coil has the same width in the longitudinal direction, and this width is ⅓ of the width of one pole of the drive region  214 . 
     The drive coil portion  222  is supplied with a three-phase current corresponding to positions that have a phase difference of 120 degrees when the pitch of the poles (between N and S) of the drive region  214  is a phase of 180 degrees. As a result, thrust is generated in the X-direction due to the relationship with the magnetic pattern of the drive region  214 . By controlling the current supplied to the coils according to the position of the moving member  220 , the movement of the moving member  220  in the X-axis direction can be controlled. 
     The position of the moving member  220  is detected using the position detection region  216  and the hall elements  224  and  225  arranged at positions opposite the position detection region  216 . The hall elements  224  and  225  each output a voltage corresponding to the polarity and strength of the magnetic field at the position thereof, and therefore the hall elements  224  and  225  output sinusoidal voltages that are phase-shifted from each other by 90 degrees according to movement of the moving member  220 . The calculating section  240  outputs position information concerning the moving member  220  by performing a counting operation and a computation using the output voltages. 
     As described above, the position detection can be realized by polarizing the magnet  212  of the stator  210  to create the drive region  214  and the position detection region  216  and providing a magnetic sensor. In other words, there is no need to provide an additional structure for the position detection, and therefore the linear motor can be made smaller. 
       FIG. 5  is a schematic cross-sectional view of an overall configuration of an image capturing apparatus  500  including a lens unit  300 , which is an exemplary application of the linear motor according to the above embodiment. The image capturing apparatus  500  is formed by combining the lens unit  300  and an image capturing section  400 . 
     The lens unit  300  includes a lens barrel  310 , a stator  320 , a moving member  330 , hall elements  341  and  342 , a lens holding frame  350 , a guide axle  360 , a sliding part  370 , and a lens group  380 . The lens group  380  includes focusing lenses  382  and  384 , which are arranged on a common optical axis C. The lens unit  300  is formed integrally with the image capturing section  400  by being connected to a mount section  460  of the image capturing section  400 , which is described further below. 
     The moving member  320  is a cylindrical magnet, and is polarized to have the magnetic pattern for driving. The stator  330  is fixed to the barrel  310 , and includes a hollow cylindrical drive coil. The stator  330  has an inner diameter that is greater than the outer diameter of the moving member  320 , and the inner surface of the stator  330  is distanced from the moving member  320 . 
     The hall elements  341  and  342  are fixed to the barrel  310 , and detect a change in the magnetic field caused by movement of the moving member  320 . The change in the magnetic field detected by the hall elements  341  and  342  is used to detect the position of the moving member  320 , and a detailed explanation of this process is provided further below. The lens holding frame  350  is connected to the moving member  320  and moves together with the moving member  320 . The sliding part  370  has a hollow cylindrical portion with an inner diameter that is slightly larger than the outer diameter of the guide axle  360 , and the inner surface thereof freely fits with the surface of the guide axle  360 . With this configuration, current is supplied to the drive coil of the stator  330  to move the moving member  320 , thereby driving the focusing lens  382  held by the lens holding frame  350 . 
     The image capturing section  400  includes an optical system having a main mirror  440 , a sub mirror  442 , a pentaprism  470 , and a finder optical system  490 , and a control system having a focal point detecting section  430 , a main control section  450 , a calculating section  452 , and a photometric unit  480 . When the main mirror  440  is in an image capture position the sub mirror  442  is raised out of the optical path of the incident light. 
     When in the standby position, the main mirror  440  is inclined relative to the incident light and guides a majority of the incident light to a focusing screen  472  arranged thereabove. The focusing screen  472  is arranged at the focal position of the optical system, and displays the image formed by the optical system. The image formed by the focusing screen  472  can be seen from the finder optical system  490  via the pentaprism  470 . The incident light passed by the main mirror  440  is reflected by the sub mirror  442  and guided to the focal point detecting section  430 . The focal point detecting section  430  detects the focal state of a subject from the lens unit  300 . 
     The image capturing section  400  includes a shutter  420 , an optical filter  412 , and an image capturing element  410  that are arranged in the stated order on the optical axis C behind the main mirror  440  in a direction of the incident light from the lens unit  300 . When the release switch of the image capturing section  400  is pressed, the main mirror  440  moves to the image capture position and removed from the optical path of the incident light. Therefore, the incident light is directed to be incident to the shutter  420 . When the shutter  420  is opened, the incident light progresses to be incident to the image capturing element  410 , and the subject image is formed on the light receiving surface of the image capturing element  410 . As a result, the image formed on the light receiving surface is converted into an electrical signal by the image capturing element  410 . 
     When the image capturing section  400  performs an autofocus operation, the control section  450  drives the focusing lens  382  by applying a drive pattern current to the drive coil of the lens unit  300 . The present embodiment adopts a phase difference AF method. With this method, a separating lens is used to generate two images from incident light from the lens unit  300 , and a line sensor is used to measure the interval between the images and detect the blur amount. The focal position of the focusing lens  382  is then determined, and the drive pattern current is applied to the drive coil to drive the focusing lens  382  to the focal position. Although this embodiment uses the phase difference AF method, the contrast AF method may be used instead, particularly if the main objective is capturing moving images. 
       FIG. 6  is a schematic view of the configuration of the lens unit  300 . The moving member  320  is a cylindrical magnet, and includes a drive region  322  that is polarized to have a magnetic pattern for driving, a position detection region  324  that is polarized to have a magnetic pattern for position detection, and a fixing portion  326 . The stator  330  includes a cylindrical drive coil  332  and a linear movement bearing  334 . 
     The drive region  322  of the moving member  320  is polarized to achieve a polarization pattern in which the poles are arranged in the X-axis direction. On the other hand, the position detection region  324  is polarized to achieve a polarization pattern in which the poles are arranged in a direction orthogonal to the X-axis direction. The fixing portion  326  is not polarized, and is connected to the lens holding frame  350 . 
     The moving member  320  is supported via the linear movement bearing  334 . Linear movement bearings  372  are arranged within the inner surface of the sliding part  370 , at both ends thereof, and the sliding part  370  is supported from the guide axle  360  via the linear movement bearings  372 . With this configuration, the focusing lens  382  can move smoothly relative to the direction of the optical axis. 
     In the manner described above, the sliding part  370  freely fits with the guide axle  360 . In other words, the sliding part  370  and the guide axle  360  engage with each other while keeping space therebetween. The fit tolerance of the linear movement bearing  334  is set to be less than the fit tolerance of the guide axle  360 . In other words, the moving member  320  has less freedom than the guide axle  360 , and is highly restrained. 
     As a result, compared to a configuration in which the moving member  320  is less restrained than the guide axle  360 , the above configuration can drive the moving member  320  with more stability. When the linear motor and the guide axles are used in this way, the linear motor can be driven with more stability by using the guide axle having the highest restraint among the plurality of guide axles as the moving member. 
     The drive coil  332  of the stator  330  is a series of coils having a U phase, a V phase, and a W phase, which are oriented in the X-axis direction. Each coil has the same width in the X-axis direction, and this width is ⅓ of the width of one pole of the drive region  322  of the moving member  320 . 
     The drive coil  332  is supplied with a three-phase current corresponding to positions that have a phase difference of 120 degrees when the pitch of the poles (between N and S) of the drive region  322  has a phase of 180 degrees. As a result, thrust is generated in the X-direction due to the relationship with the magnetic pattern of the drive region  322 . By controlling the current supplied to the coils according to the position of the moving member  320 , the movement of the moving member  320  in the X-axis direction can be controlled. 
     The position of the moving member  320  is detected using the position detection region  324  and the ball elements  341  and  342  arranged at positions opposite the position detection region  324 . The hall elements  341  and  342  each output a voltage corresponding to the polarity and strength of the magnetic field at the position thereof, and therefore the hall elements  341  and  342  output sinusoidal voltages that are phase-shifted from each other by 90 degrees according to movement of the moving member  320 . The calculating section  452  outputs position information concerning the moving member  320  by performing a counting operation and a computation using the output voltages. 
     With the above configuration, the focusing lens  382  connected to the moving member  320  can move in the direction of the optical axis, which is parallel to the driving direction of the moving member. In the above embodiment, the image capturing section  400  includes the calculating section  452 , but the lens unit  300  may include the calculating section  452  instead. In this case, the positional information concerning the moving member  320  calculated by the calculating section  452  is transmitted to the control section  450  of the image capturing section  400  via the mount section  460 . 
     While the embodiments of the present invention have been described, the technical scope of the invention is not limited to the above described embodiments. It is apparent to persons skilled in the art that various alterations and improvements can be added to the above-described embodiments. It is also apparent from the scope of the claims that the embodiments added with such alterations or improvements can be included in the technical scope of the invention.