Abstract:
A servo accelerometer has a pair of housings having a tubular part, one end opened and the other end closed with a closing part. A frame that supports a pendulum is held between the housings. A permanent magnet is attached to each of the closing parts with a bottom pole piece interposed therebetween. Coils arranged in annular magnetic gaps are attached to the pendulum. The closing part has a recess, and the bottom pole piece is disposed in the recess. The outer circumference of the bottom pole piece faces the inner circumference of the recess with a predetermined gap interposed therebetween.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to a servo accelerometer that has a pendulum provided with a coil interlinked with a magnetic field produced by a permanent magnet and is configured to apply a current determined by the amount of swing of the pendulum to the coil to balance the pendulum in the vicinity of the zero point. 
         [0003]    2. Description of Related Art 
         [0004]      FIGS. 1A and 1B  shows a configuration of a conventional servo accelerometer of this type. 
         [0005]    A pendulum  12  disposed inside a circular frame  11  has the shape of a circular plate having a part of the circumference cut by a chord. The pendulum  12  is supported by the frame  11  by the cut part  12   a  of the pendulum  12  being coupled to the frame  11  by a pair of hinges  13 . The frame  11 , the pendulum  12  and the pair of hinges  13  are integrally formed of quartz glass. The pair of hinges  13  is thin in order to allow a required swing of the pendulum  12 . 
         [0006]    The opposite surfaces of the frame  11  abut against a first housing  14  and a second housing  15 , respectively, and the frame  11  is held between the pair of the first and second housings  14  and  15 . Both the first and second housings  14  and  15  substantially have the shape of a cylinder one end of which is open and the other end of which is closed, and the open ends abut against the frame  11 . Annular protrusions  14   g  and  15   g  are formed along the inner circumference of the open ends of the first and second housings  14  and  15 , which abut against the frame  11 . The first and second housings  14  and  15  serve also as a magnetic yoke and are made of a magnetic material. The magnetic material may be Invar, which is magnetic and has a low thermal expansion coefficient. 
         [0007]    At the center of the inside of the first and second housings  14  and  15 , a first permanent magnet  16  and a second permanent magnet  17 , both of which are cylindrical samarium-cobalt magnets, are disposed. In this example, the first and second permanent magnets  16  and  17  are installed on the inner surface of closing parts  14   a  and  15   a  of the first and second housings  14  and  15  with disk-shaped bottom pole pieces  18  and  19  interposed therebetween, respectively, and disk-shaped pole pieces  21  and  22  having an increased thickness along the circumference are disposed on the upper surface of the first and second permanent magnets  16  and  17 , respectively. 
         [0008]    The first and second permanent magnets  16  and  17 , the bottom pole pieces  18  and  19 , and the pole pieces  21  and  22  are assembled by adhesion, for example. The bottom pole pieces  18  and  19  and the pole pieces  21  and  22  are made of an electromagnetic soft iron (compliant with Japanese Industrial Standards C 2503), and the bottom pole pieces  18  and  19  are fixed to the closing parts  14   a  and  15   a  of the first and second housings  14  and  15  by adhesion or laser welding, for example. The bottom pole pieces  18  and  19  serve to accommodate the difference in thermal expansion between the first and second housings  14  and  15  and the first and second permanent magnets  16  and  17 . 
         [0009]    For example, the first permanent magnet  16  is arranged so that the N pole abuts against the pole piece  21 , and the S pole abuts against the bottom pole piece  18 . The first permanent magnet  16  and the first housing  14  form a primary magnetic circuit, and a first magnetic gap  23  is formed between the inner circumference of the protrusion  14   g  at the open end of the first housing  14  and the first permanent magnet  16  or, more specifically, the outer circumference of the pole piece  21  in this example. In the second housing  15 , a similar second magnetic gap  24  is formed. 
         [0010]    In the cylindrical first and second magnetic gaps  23  and  24 , a first coil  27  wound around a bobbin  25  and a second coil  28  wound around a bobbin  26  are disposed, respectively. The first and second coils  27  and  28  are coaxial with the first and second permanent magnets  16  and  17  and attached to the opposite surfaces of the pendulum  12 . The ends of the bobbins  25  and  26  on the side of the pendulum  12  are closed by attachment plates  25   a  and  26   a , and the first and second coils  27  and  28  are attached to the pendulum  12  by fixing the attachment plates  25   a  and  26   a  to the pendulum  12  by adhesion. 
         [0011]    In this example, capacitance-type displacement detecting means detects displacement (swing) of the pendulum  12 . On the opposite surfaces of the pendulum  12 , arc-shaped electrodes  29   a  and  29   b  surrounding the first and second coils  27  and  28  are formed by gold plating or the like. The first and second housings  14  and  15  constitute electrodes opposed to the electrodes  29   a  and  29   b . Within the angular range of the electrodes  29   a  and  29   b  formed on the pendulum  12 , the open end of the first and second housings  14  and  15  has frame abutting surfaces  14   b  and  15   b , recesses  14   c  and  15   c , and electrode surfaces  14   d  and  15   d  arranged in this order from the outside, as shown in FIG.  1 A. The electrode surfaces  14   d  and  15   d  are spaced apart from the electrodes  29   a  and  29   b  on the pendulum  12  by a predetermined distance. 
         [0012]    The servo accelerometer configured as described above detects the swing of the pendulum  12  caused by an input acceleration in the X direction as a variation in capacitance between the electrodes  29   a  and  14   d  and between the electrodes  29   b  and  15   d . The electrode surfaces  14   d  and  15   d  are at a common potential, the detection signals on the electrodes  29   a  and  29   b  are differentially amplified by a predetermined electric circuit (not shown), and a current determined by the capacitance difference detected on the electrodes  29   a  and  29   b  flows through the first and second coils  27  and  28 . The current flowing through the first and second coils  27  and  28  and the magnetic field produced by the first and second permanent magnets  16  and  17  interact to restore the pendulum  12  to the original position, and the pendulum  12  is balanced in the vicinity of the zero point. The current flowing in this case is proportional to the acceleration applied to the pendulum  12 , so that the input acceleration can be determined from the current. 
         [0013]    As described above, for the conventional servo accelerometer, the frame  11 , the pendulum  12  and the hinges  13  are made of quartz glass, and the first and second housings  14  and  15  are made of Invar. By using those materials having low thermal expansion coefficients in this way, dimensional changes or displacements caused by temperature changes or strains induced by stresses are minimized. 
         [0014]    However, Invar, which is used for the first and second housings  14  and  15  to make the housings participate in the magnetic circuit, has a saturation flux density of about 1.2 T at room temperature (25 degrees C.), and the saturation flux density of Invar highly depends on the temperature. In particular, in a high temperature environment, the magnetic circuit is easily saturated. Thus, the operating temperature range and the measurement range are limited, and the magnetic circuit can be downsized only to a limited extent. 
         [0015]      FIGS. 2A and 2B  shows a configuration of a servo accelerometer described in Japanese Patent Application Laid-Open No. H8-292208 (issued Nov. 5, 1996, referred to as Patent literature 1 hereinafter) that solves the problem of the saturation of the magnetic circuit described above. In this example, magnetic reinforcing plates  31  and  32  are attached to the outer surface of the closing parts  14   a  and  15   a  of the first and second housings  14  and  15 , respectively. 
         [0016]    The magnetic reinforcing plates  31  and  32  are made of a material having a higher saturation flux density than the first and second housings  14  and  15 . The material may be an electromagnetic soft iron, for example. 
         [0017]    The magnetic reinforcing plates  31  and  32  disposed on the outer surface of the closing parts  14   a  and  15   a  of the first and second housings  14  and  15  serve to eliminate the saturation of the magnetic circuit at the closing parts  14   a  and  15   a.    
         [0018]    However, the servo accelerometer having the magnetic reinforcing plates  31  and  32  shown in  FIGS. 2A and 2B  cannot be easily downsized because of the magnetic reinforcing plates  31  and  32  attached to the outer surface of the first and second housings  14  and  15 . The servo accelerometer further has a problem that the cost also increases because of the increased number of components. 
       SUMMARY OF THE INVENTION 
       [0019]    An object of the present invention is to provide a compact servo accelerometer that can eliminate saturation of a magnetic circuit and can be manufactured inexpensively. 
         [0020]    According to a first aspect of the present invention, there is provided a servo accelerometer having a pair of housings having a tubular part, made of a magnetic material, and having one end opened and the other end closed with a closing part, in which a frame that supports a pendulum inside thereof via hinges is held between said one ends of the pair of housings, a permanent magnet is attached to an inner surface of each of the closing parts of the pair of housings with a bottom pole piece made of a magnetic material interposed therebetween, an annular magnetic gap is formed between each permanent magnet and the inner circumference of said one end of the corresponding one of the pair of housings, and a pair of coils arranged in the annular magnetic gaps to be coaxial with each other is attached to the opposite surfaces of the pendulum, wherein each of the closing parts has a recess formed in the inner surface thereof, and the bottom pole piece is disposed in the recess, and the outer circumference of the bottom pole piece faces the inner circumference of the recess with a predetermined gap interposed therebetween. 
         [0021]    According to a second aspect of the present invention, there is provided a servo accelerometer having a pair of housings having a tubular part, made of a magnetic material, and having one end opened and the other end closed with a closing part, in which a frame that supports a pendulum inside thereof via hinges is held between said one ends of the pair of housings, a permanent magnet is attached to an inner surface of each of the closing parts of the pair of housings with a bottom pole piece made of a magnetic material interposed therebetween, an annular magnetic gap is formed between each permanent magnet and the inner circumference of said one end of the corresponding one of the pair of housings, and a pair of coils arranged in the annular magnetic gaps to be coaxial with each other is attached to the opposite surfaces of the pendulum, wherein the bottom pole pieces are sized to cover substantially the entire inner surface of the closing part, and the outer circumference of the bottom pole piece faces the inner circumference of the tubular part with a predetermined gap interposed therebetween. 
         [0022]    According to the present invention, the first and second housings are each arranged in the vicinity of the bottom pole piece to face the outer circumference of the bottom pole piece with a gap interposed therebetween, and a second magnetic path is formed across the gaps when magnetic saturation occurs at the center part of the closing parts of the first and second housings where the magnetic flux is particularly concentrated. Thus, saturation of the magnetic circuit can be avoided. 
         [0023]    Therefore, the servo accelerometer according to the present invention is compact and manufactured inexpensively, compared with the conventional servo accelerometer that has magnetic reinforcing plates attached to the first and second housings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1A  is a cross-sectional view of a conventional servo accelerometer; 
           [0025]      FIG. 1B  is a cross-sectional view taken along the line C-C in  FIG. 1A ; 
           [0026]      FIG. 2A  is a cross-sectional view of a conventional servo accelerometer having magnetic reinforcing plates; 
           [0027]      FIG. 2B  is a side view of the conventional servo accelerometer having magnetic reinforcing plates; 
           [0028]      FIG. 3A  is a cross-sectional view of a servo accelerometer according to a first embodiment of the present invention; 
           [0029]      FIG. 3B  is a cross-sectional view taken along the line C-C in  FIG. 3A ; 
           [0030]      FIG. 4A  is a cross-sectional view of a servo accelerometer according to a second embodiment of the present invention; and 
           [0031]      FIG. 4B  is a cross-sectional view taken along the line C-C in  FIG. 4A . 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    In the following, embodiments of the present invention will be described. 
         [0033]      FIGS. 3A and 3B  show a configuration of a servo accelerometer according to a first embodiment of the present invention. Parts corresponding to those shown in  FIGS. 1A and 1B  are denoted by the same reference numerals, and detailed illustration thereof will be omitted. 
         [0034]    As with the first and second housings  14  and  15  shown in  FIG. 1A , first and second housings  14 ′ and  15 ′ have tubular parts  14   e  and  15   e  one ends of which are open, and closing parts  14   a  and  15   a  that close the other end of the tubular parts  14   e  and  15   e , respectively, and annular protrusions  14   g  and  15   g  are formed along the inner circumference of the open ends of the tubular parts  14   e  and  15   e.    
         [0035]    In this embodiment, the closing parts  14   a  and  15   a  have circular recesses  14   f  and  15   f  formed in the inner surface, and disk-shaped bottom pole pieces  18  and  19  are disposed in the recesses  14   f  and  15   f , respectively. Predetermined gaps  41  and  42  are formed between the outer circumference of the bottom pole pieces  18  and  19  and the inner circumference of the recesses  14   f  and  15   f , and the outer circumference of the bottom pole pieces  18  and  19  face the inner circumference of the recesses  14   f  and  15   f  with the gaps  41  and  42  interposed therebetween. 
         [0036]    Comparing the shape of the closing parts  14   a  and  15   a  of the first and second housings  14 ′ and  15 ′ having the recesses  14   f  and  15   f  with the shape of the closing parts  14   a  and  15   a  of the first and second housings  14  and  15  shown in  FIG. 1A , the thickness of the part on which the bottom pole piece  18 ,  19  is disposed is not changed, and the thickness of the part surrounding the bottom pole piece  18 ,  19  is increased (that is, the part surrounding the bottom pole piece is raised). 
         [0037]    In the configuration as described above, the first and second housings  14 ′ and  15 ′ are made of Invar, and the bottom pole pieces  18  and  19  are made of an electromagnetic soft iron (compliant with JIS C 2503), as with the conventional servo accelerometer. The electromagnetic soft iron has a saturation flux density of about 2.2 T at room temperature (25 degrees C.). In addition, the saturation flux density of the electromagnetic soft iron is higher than and less temperature dependent than the saturation flux density of Invar. Thus, even at higher temperatures, the decrease of the saturation flux density is low. 
         [0038]    In this embodiment configured as described above, a magnetic path described below is formed. 
         [0039]    (1) When the servo accelerometer is at room temperature, magnetic saturation does not occur even at the center part of the closing parts  14   a  and  15   a  of the first and second housings  14 ′ and  15 ′ where the magnetic flux particularly tends to be concentrated. Thus, the center part of the closing parts  14   a  and  15   a  (the part where the recesses  14   f  and  15   f  are formed) serves as a magnetic path, and the magnetic flux flows through the bottom pole pieces  18  and  19  in the thickness direction thereof. 
         [0040]    (2) When the temperature of the servo accelerometer rises to higher temperature, the saturation flux density of the first and second housings  14 ′ and  15 ′ made of Invar decreases, and magnetic saturation occurs from the part where the cross section of the magnetic path is small, or more specifically, the center part of the closing parts  14   a  and  15   a.    
         [0041]    The occurrence of the magnetic saturation causes a magnetic flux flowing across the gaps  41  and  42  between the outer circumference of the bottom pole pieces  18  and  19  and the inner circumference of the recesses  14   f  and  15   f  of the closing parts  14   a  and  15   a . That is, in addition to the first magnetic path in which a magnetic flux flows in the thickness direction of the bottom pole pieces  18  and  19 , a second magnetic path is formed in which a magnetic flux flows between the bottom pole pieces  18  and  19  and the circumference of the closing parts  14   a  and  15   a  (the part surrounding the recesses  14   f  and  15   f ) across the gaps  41  and  42 . 
         [0042]    In this embodiment, as described above, the second magnetic path is formed across the gaps  41  and  42  in a high temperature environment. Therefore, saturation of the magnetic circuit can be avoided without the magnetic reinforcing plates  31  and  32 , which are required in the example of prior art shown in  FIG. 2A , for example. 
         [0043]    The size of the gaps  41  and  42  depends on the difference in thermal expansion between the first and second housings  14 ′ and  15 ′ and the bottom pole pieces  18  and  19 . That is, the size of the gaps  41  and  42  is determined so that the gaps  41  and  42  do not disappear even when the bottom pole pieces  18  and  19  are more significantly thermally expanded than the first and second housings  14 ′ and  15 ′ as the temperature rises. The gaps  41  and  42  prevent the radial thermal stress in the bottom pole pieces  18  and  19  from directly acting on the first and second housings  14 ′ and  15 ′ and thereby causing a stress-induced strain in the pendulum  12 , for example. 
         [0044]    Next, a second embodiment of the present invention will be described with reference to  FIGS. 4A and 4B . 
         [0045]    In this embodiment, the inner surface of the closing parts  14   a  and  15   a  of first and second housings  14 ″ and  15 ″ has no recess formed therein but is a flat surface. Bottom pole pieces  18 ′ and  19 ′ have a larger diameter than the bottom pole pieces  18  and  19  shown in  FIG. 3A  and substantially cover the entire inner surface of the closing parts  14   a  and  15   a  as shown in  FIG. 4A . Predetermined gaps  43  and  44  are formed between the outer circumference of the bottom pole pieces  18 ′ and  19 ′ and the inner circumference of the tubular part  14   e  and  15   e  of the first and second housings  14 ″ and  15 ″, and the outer circumference of the bottom pole pieces  18 ′ and  19 ′ face the inner circumference of the tubular parts  14   e  and  15   e  with the gaps  43  and  44  interposed therebetween, respectively. 
         [0046]    In this embodiment, as in the first embodiment shown in  FIGS. 3A and 3B , when the temperature of the servo accelerometer rises to higher temperature, and magnetic saturation occurs at the center part of the closing parts  14   a  and  15   a  of the first and second housings  14 ″ and  15 ″, a magnetic flux starts flowing between the outer circumference of the bottom pole pieces  18 ′ and  19 ′ and the inner circumference of the tubular parts  14   e  and  15   e  across the gaps  43  and  44 , thereby forming a second magnetic path. Thus, saturation of the magnetic circuit can be avoided. 
         [0047]    In this embodiment, to facilitate attachment of the bottom pole pieces  18 ′ and  19 ′ having a larger diameter to the inside of the first and second housings  14 ″ and  15 ″, the first and second housings  14 ″ and  15 ″ are divided into the tubular parts  14   e  and  15   e  and the closing parts  14   a  and  15   a , respectively. The tubular parts  14   e  and  15   e  and the closing parts  14   a  and  15   a  can be integrated by adhesion or welding, for example. As in the first embodiment, the size of the gaps  43  and  44  is determined so that the gaps  43  and  44  do not disappear even when the bottom pole pieces  18 ′ and  19 ′ are more significantly thermally expanded than the first and second housings  14 ″ and  15 ″ as the temperature rises. 
         [0048]    As described above, according to the present invention, the first and second housings  14 ′ and  15 ′ ( 14 ″ and  15 ″) are arranged in the vicinity of the bottom pole pieces  18  and  19  ( 18 ′ and  19 ′) to face the outer circumference of the bottom pole pieces  18  and  19  ( 18 ′ and  19 ′) with the gaps  41  and  42  ( 43  and  44 ) interposed therebetween, and a second magnetic path is formed across the gaps  41  and  42  ( 43  and  44 ) when magnetic saturation occurs at the center part of the closing parts  14   a  and  15   a  of the first and second housings  14 ′ and  15 ′ ( 14 ″ and  15 ″) where the magnetic flux is particularly concentrated. Thus, the servo accelerometer can be prevented from being affected by the magnetic saturation in the closing parts  14   a  and  15   a.    
         [0049]    In general, to reduce the size of the servo accelerometer, the closing parts  14   a  and  15   a  of the first and second housings  14 ′ and  15 ′ ( 14 ″ and  15 ″) have a smaller magnetic path cross section than the tubular parts  14   e  and  15   e . However, the present invention can solve the problem of the magnetic saturation in the closing parts  14   a  and  15   a.