Patent Publication Number: US-2019186993-A1

Title: Voice coil motor, and movable mirror unit and interference spectrophotometer equipped with same

Description:
TECHNICAL FIELD 
     The present invention relates to a voice coil motor (hereinafter, abbreviated as “VCM”) and an interference spectrophotometer such as a Fourier transform infrared spectrophotometer (hereinafter, abbreviated as “FTIR”). 
     BACKGROUND ART 
     Michelson 2-beam interferometers that are used for an FTIR have such a configuration that infrared rays emitted from an infrared ray source are divided into two directions, one directed towards a fixed mirror and the other directed towards a movable mirror, by a beam splitter, and light that has returned by being reflected from the fixed mirror and light that has returned by being reflected from the movable mirror are combined by the beam splitter so as to be sent to one light path. At this time, the movable mirror can be moved to the front or rear along the direction of the axis of the entering light (forward and backward direction) in order to change the difference in the light path between the two divided beams, and therefore, the resulting light provides an interferometer where the light intensity changes in accordance with the location of the movable mirror. 
       FIG. 6  is a diagram showing the configuration of the essential part of a conventional FTIR, and  FIG. 7  is a horizontal cross-sectional diagram showing the movable mirror unit in  FIG. 6 . Here, one direction that is horizontal relative to the ground is the X direction, the direction that is horizontal relative to the ground and perpendicular to the X direction is the Y direction, and the direction that is perpendicular to the X direction and the Y direction is the Z direction. 
     An FTIR  101  is provided with a main interferometer essential part  140 , a light source  10  for emitting infrared rays, a light detection unit  20  for detecting an interferometer, and a computer (control unit)  130 . 
     The light source  10  is provided with an infrared ray source for emitting infrared rays, a converging mirror and a collimator. As a result, the infrared rays emitted from the infrared ray source pass through the converging mirror and the collimator before entering into the beam splitter  42  in the main interferometer essential part  140 . 
     The light detection unit  20  is provided with an ellipsoidal mirror and a light detector for detecting an interferogram. As a result, the light with which a sample S is irradiated transmits through (or is reflected from) the sample S and is collected by the ellipsoidal mirror so as to be emitted to the light detector. 
     The main interferometer essential part  140  is provided with a housing  41 , a beam splitter  42 , a movable mirror unit  150  equipped with a movable mirror  53 , a fixed mirror unit  60  equipped with a fixed mirror  61  and an alignment mechanism  62 . 
     The movable mirror unit  150  is provided with a hollow pipe  51  in cylindrical shape having a central axis in the forward and backward direction (X direction) and a piston  52  in cylindrical shape that is disposed within the hollow pipe  51  such that reciprocal movement is possible in the forward and backward direction, a movable mirror  53  fixed to the front portion of the piston  52  and a VCM  170  (see Patent Literature 1). 
     The VCM  170  is provided with a static unit  171  and a movable unit  172 . 
     The static unit  171  is provided with a cylindrical part  173   a  in cylindrical shape having a central axis in the forward and backward direction, a yoke  173  made of iron (a magnetic material) having a rear sidewall  73   b  in disc form, two magnets  74  ( 74   a,    74   b ) in columnar form having a central axis in the forward and backward direction, and a pole piece  75  in columnar form having a central axis in the forward and backward direction. The first magnet  74   a,  the pole piece  75  and the second magnet  74   b  are fixed to the center portion of the front surface of the rear sidewall  73   b  of the yoke  173  in this order, and thus are disposed within the cylindrical portion  173   a  of the yoke  173 . In addition, the front portion of the cylindrical part  173   a  is attached to the rear portion of the housing  41  (hollow pipe  51 ). Furthermore, a slit (oblong hole)  173  that extends in the forward and backward direction is created in the right sidewall of the cylindrical part  173   a  of the yoke  173 . 
     The movable unit  172  is provided with a bobbin  72   a  in cylindrical shape having a central axis in the forward and backward direction, and a circular coil  72   b  wound around the outer peripheral surface of the rear portion of the bobbin  72   a.  In addition, the front portion of the bobbin  72   a  is attached to the rear portion of the piston  52 . Furthermore, the coil  72   b  is disposed between the cylindrical part  173   a  of the yoke  173  and the pole piece  75  so as to be electrically connected to the power supply (not shown) via a power supply terminal (power supply line)  72   c  disposed so as to penetrate through the slit  173   c  in the upward and downward directions (Y direction). 
     When a current is made to flow through the coil  72   b  via the power supply terminal  72   c,  the coil  72   b  receives an electromagnetic force (Lorentz force) due to the magnetic field generated between the yoke  173  and the pole piece  75  and moves in the forward and backward direction, and as a result, the movable mirror  53  that is fixed to the piston  52  also moves in the forward and backward direction. 
     In the thus-formed FTIR  101 , it is necessary to suppress the angular deviation between the movable mirror  53  and the fixed mirror  61  to one second or less in order to measure the sample S having a high S/N ratio while securing a sufficient throughput. In order to do so, the computer  130  controls in real time the angle of the fixed mirror  61  in response to the angular deviation of the movable mirror  53  by using an alignment mechanism  62 . 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Japanese Translation of International Patent Publication H6 (1994)-505804 
     SUMMARY OF THE INVENTION 
     1. Technical Problem 
     Depending on the measurement mode (measurement type of sample S) in an interference spectrophotometer such as an FTIR, a short stroke high-speed drive or a long stroke low-speed drive of the movable mirror  53  have been required. 
     In the FTIR  101  as described above, however, a large acceleration is applied at the time of return during the short stroke high-speed drive, and therefore, such a problem arises that the angular control of the fixed mirror  61  by the computer  130  cannot follow the return movement (see  FIG. 5( b ) ). 
     2. Solution to Problem 
     In order to solve the above-described problem, the present inventors examined a method for suppressing the angular deviation between the movable mirror  53  and the fixed mirror  61  to one second or less, even at the time of high-speed drive.  FIG. 4  is a table showing the simulation results of the difference in the magnetic flux density between the right sidewall (with a slit  173   c ) of the cylindrical part  173   a  of the yoke  173  and the left sidewall (with no slit) of the cylindrical part  173   a,  and thus, it can be seen that the magnetic flux is weaker on the side with a slit. The magnetic flux density is proportional to the impellent (Lorentz force), and therefore, a difference is created in the impellent in the same manner as the difference in the magnetic flux density. 
     When a current flows through the coil  72   b  wound around the bobbin  72   a,  the piston  52  fixed to the bobbin  72   a  receives a force in the forward direction (−X direction) or the rear direction (X direction) so as to slide. At this time, a weak impellent is generated on the side with the slit  173   c,  and therefore, it was found that the surface of the movable mirror  53  faces such a direction that the mirror surface is turned away even in the state where the piston  52  is held within the hollow pipe  51 . Typically, such a momentum that accompanies the rotation to the right with the Z direction being the rotational axis is applied to the movable unit (the piston  52  and the like) when the coil  72   b  moves in the forward direction, and such a momentum that accompanies the rotation to the left is applied to the movable unit when the coil  72   b  moves in the rear direction. 
     Thus, it was found that another slit can be provided on the side of the cylindrical part of the yoke in such a manner that the slits are symmetrical relative to the cylindrical part in order to cancel the difference in the impellent. Here,  FIG. 5( b )  shows the results of evaluation of the angular deviation (3.4 seconds pp) in the case where a slit is provided only on one side of the cylindrical part of the yoke, and  FIG. 5( a )  shows the results of evaluation of the angular deviation (0.6 seconds pp) in the case where a slit is provided on two sides of the cylindrical part of the yoke. In the case where a slit is provided on two sides, the angular deviation can be reduced to approximately ⅕ (0.6 seconds pp) as compared to the case where a slit is provided only on one side. 
     Namely, the voice coil motor according to the present invention is provided with: a static unit including a yoke having a cylindrical part fixed to the static unit and a magnet disposed in the cylindrical part fixed to the static unit; a movable unit including a circular coil fixed thereto, the circular coil disposed between the cylindrical part of the yoke and the magnet; and a power supply line for connecting the coil to a power supply, wherein the cylindrical part of the yoke has a slit through which the power supply line is to pass is created, the movable unit is configured to reciprocally move relative to the static unit in response to an electromagnetic force generated by the magnet in conjunction with the activated coil, and another slit is created in the cylindrical part of the yoke in such a manner that the slits are symmetrical with respect to a central axis of the cylindrical part. 
     3. Advantageous Effects of the Invention 
     As described above, the voice coil motor according to the present invention is provided with another slit in such a manner that the slits are symmetrical relative to the central axis of the yoke in order to cancel the difference in the impellent, and thus, such a momentum that might cause a rotational movement in the movable unit can be prevented from being generated. 
     4. Other Solutions to Technical Problem and Advantageous Effects Thereof 
     In the invention, a first slit that is parallel to the above-described central axis and a second slit having the same shape as the first slit may be created in the cylindrical part of the yoke. (In the invention, the slits(oblong hole) extend in parallel with the central axis.) 
     In addition, the movable mirror unit in the present invention may be provided with a voice coil motor as described above, a hollow pipe having a cylindrical shape, and a piston disposed in the hollow pipe, the piston being reciprocally movable in the hollow pipe, wherein the piston includes a movable mirror fixed thereto and the movable unit are fixed to the piston. 
     In the movable mirror unit in the present invention, the difference in the impellent in the voice coil motor is cancelled so that such a momentum that might cause rotational movement in the movable mirror can be prevented from being generated. 
     Furthermore, the interference spectrophotometer according to the present invention may be provided with: a movable mirror unit as described above; a light source for emitting light; a fixed mirror; a beam splitter configured for the processes of splitting light received from the light source into two beams, directing one beam towards the fixed mirror and the other beam towards the movable mirror, receiving first returning light reflected from the fixed mirror and second returning light reflected from the movable mirror, combining the first and second reflecting beams of light into interference light; a light detection unit on which a sample is arranged, the detector configured to detect the interference light that has transmitted through or has been reflected from the sample; and a control unit for controlling the speed of the movable unit or the moving distance of the movable unit with the coil activated via the power supply line. 
     In the interference spectrophotometer according to the present invention, the difference in the impellent in the voice coil motor is cancelled, and such a momentum that might cause rotational movement is prevented from being generated, and thus, the angular deviation between the movable mirror and the fixed mirror when driven at a high speed can be made small. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a diagram showing the configuration of the essential part of the FTIR according to the present invention; 
         FIG. 2  is a horizontal cross-sectional diagram showing the movable mirror unit in  FIG. 1 ; 
         FIGS. 3( a ) and 3( b )  are cross-sectional diagrams showing the VCM in  FIG. 2 ; 
         FIG. 4  is a graph showing the simulation results of the difference in the magnetic flux density depending on the existence of a slit; 
         FIGS. 5( a ) and 5( b )  are graphs showing the evaluation results of the angular deviation; 
         FIG. 6  is a diagram showing the configuration of the essential part of a conventional FTIR; and 
         FIG. 7  is a horizontal cross-sectional diagram showing the movable mirror unit in  FIG. 6 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     In the following, the embodiments of the present invention are described in reference to the drawings. Here, the present invention is not limited to the below-described embodiments, and various types of modifications are included as long as the gist of the present invention is not deviated from. 
     An FTIR is cited as an example of the interference spectrophotometer according to the present invention, and  FIG. 1  shows the configuration of the essential part thereof.  FIG. 2  is a horizontal cross-sectional diagram showing the movable mirror unit  50  in  FIG. 1 . In addition,  FIGS. 3( a ) and 3( b )  are cross-sectional diagrams showing the VCM  70  in  FIG. 2 , where  FIG. 3( a )  is a longitudinal cross-sectional diagram and  FIG. 3( b )  is a horizontal cross-sectional diagram. Here, the same symbols are attached to the same components as in the above-described FTIR  101 , and thus, the descriptions thereof are not repeated. 
     An FTIR  1  is provided with a main interferometer essential part  40 , a light source  10  for emitting infrared rays, a light detection unit  20  for detecting an interferogram, and a computer (control unit)  30 . 
     The main interferometer essential part  40  is provided with a housing  41 , a beam splitter  42 , a movable mirror unit  50  having a movable mirror  53 , and a fixed mirror unit  60  having a fixed mirror  61  and an alignment mechanism  62 . 
     The movable mirror unit  50  is provided with a hollow pipe in cylindrical shape having the center in the forward and backward direction (X direction), a piston  52  in columnar form that is disposed within the hollow pipe  51  so that reciprocal movement is possible in the forward and backward direction, a movable mirror  53  fixed to the front portion of the piston  52 , and a VCM  70 . 
     The VCM  70  is provided with a static unit  71  and a movable unit  72 . 
     The static unit  71  is provided with: a yoke  73  made of iron (magnetic material) having a cylindrical part  73   a  with the central axis being in the forward and backward direction and a rear sidewall in disc form; two magnets  74  ( 74   a,    74   b ) in columnar form having a central axis in the forward and backward direction; and a pole piece  75  in columnar form having a central axis in the forward and backward direction. The first magnet  74   a,  the pole piece  75  and the second magnet  74   b  are fixed to the center portion on the front surface of the rear sidewall  73   b  of the yoke  73  in this order, and thus are disposed within the cylindrical part  73   a  of the yoke  73 . In addition, the front portion of the cylindrical part  73   a  is attached to the rear portion of the housing  41  (hollow pipe  51 ). 
     A first slit  73   c  is created in the right sidewall of the cylindrical part  73   a  of the yoke  73  so as to extend in the forward and backward direction, and at the same time, a second slit  73   d  is created in the left sidewall of the cylindrical part  73   a  of the yoke  73  so as to extend in the forward and backward direction. That is to say, the first slit  73   c  and the second slit  73   d  are created in such locations that the slits are point symmetrical relative to the central axis (X direction) of the cylindrical part  73   a  of the yoke  73 . 
     The movable unit  72  is provided with a bobbin  72   a  in cylindrical shape having a central axis in the forward and backward direction, and a circular coil  72   b  wound around the outer peripheral surface of the rear portion of the bobbin  72   a.  In addition, the front portion of the bobbin  72   a  is attached to the rear portion of the piston  52 . Furthermore, the coil  72   b  is disposed between the cylindrical part  73   a  of the yoke  73  and the pole piece  75 , and is electrically connected to the power supply (not shown) via a power supply terminal (power supply line)  72   c  that is disposed so as to pass through the first slit  73   c  in the upward and downward directions (Y direction). Moreover, a dummy power supply terminal  72   d  having the same shape as the power supply terminal  72   c  that is disposed so as to pass through the second slit  73   d  in the Y direction is formed in the movable unit  72 . That is to say, the power supply terminal  72   c  and the dummy power supply terminal  72   d  are formed in such locations that the terminals are point symmetrical relative to the central axis (X direction) of the cylindrical part  73   a  of the yoke  73 . 
     As a result, the coil  72   b  receives an electromagnetic force (Lorentz force) due to the magnetic field generated between the yoke  73  and the pole piece  75  so as to move in the forward and backward direction when a current is made to flow through the coil  72   b  via the power supply terminal  72   c,  and thus, the moving mirror  53  that is fixed to the piston  52  also moves in the forward and backward direction. At this time, a magnetic flux density as on the “there is a slit” side in  FIG. 4  is generated on both sides, left and right sidewall sides, of the cylindrical part  73   a  of the yoke  73 . 
     The computer  30  is provided with a CPU  31  and an input device  32 . The CPU  31  can be divided into the following parts using the functions processed by them. The CPU  31  has: a light intensity information acquisition part  31   a  for acquiring an interferogram from the light detection unit  20 ; a sample measurement part  31   b  for calculating the absorbance spectrum and the like of the sample S; a movable mirror control part  31   c  for controlling the speed or the moved distance of the movable mirror in the movable mirror unit  50  on the basis of the input information that has been inputted through the input device  32 ; and a fixed mirror control part  31   b  for controlling the alignment mechanism  62  in the fixed mirror unit  60 . 
     As described above, in the FTIR  1  according to the present invention, the first slit  73   c  and the second slit  73   d  are provided in such locations that the slits are symmetrical relative to the central axis of the yoke  73 , and therefore, the difference in the impellent is cancelled so as to prevent a momentum that might cause a rotary motion from being generated. Thus, the angular deviation between the movable mirror  53  and the fixed mirror  61  when driven at a high speed can be suppressed to one second or less (see  FIG. 5( a ) ). 
     Other Embodiments 
     (1) Though the above-described FTIR  1  has a configuration that is provided with a dummy power supply terminal  72   d,  such a configuration is also possible where a coil is electrically connected to the power supply via the power supply terminal, and at the same time is electrically connected to the power supply via a dummy power supply terminal. 
     (2) Though the above-described FTIR  1  has a configuration that is provided with a dummy power supply terminal  72   d,  such a configuration is also possible where no dummy power supply terminal is provided. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be preferably applied to interference spectrophotometers such as a Fourier transform infrared spectrophotometer. 
     REFERENCE SIGNS LIST 
       1  FTIR (interference spectrophotometer) 
       70  VCM (voice coil motor) 
       71  static unit 
       72  movable unit 
       72   b  coil 
       72   c  power supply terminal (power supply line) 
       73  yoke 
       73   a  cylindrical part 
       73   c,    73   d  slit 
       74  magnet