Abstract:
An electromagnetic induction type absolute position measuring encoder having two or more rows of scale coils, each of the rows having a scale pitch different from that of another row; a transmitter coil and a receiver coil arranged on a grid that is movable relative to the scale in the measuring direction so as to face the scale coils; and the track is constituted by the scale coils, the transmitter coil and the receiver coil. The encoder is capable of measuring an absolute position of the grid with respect to the scale from a flux change detected at the receiver coil via the scale coils when the transmitter coil is excited, in which at least one loop-shaped additional scale coil is added between the scale coils in at least one of the tracks.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The disclosure of Japanese Patent Application No. 2011-170510 filed on Aug. 3, 2011 including specifications, drawings and claims are incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an electromagnetic induction type absolute position measuring encoder, and, in particular, to an electromagnetic induction type absolute position measuring encoder suitable for use in a electronic digital caliper, an indicator, a linear scale, a micrometer, or the like, and capable of achieving a high precision in measurement due to an improvement in S/N ratio and/or capable of reducing a scale width and therefore an encoder width. 
       BACKGROUND ART 
       [0003]    As described in Japanese Patent Application Laid-Open No. Hei. 10-318781 (hereinafter referred to as Patent Literature 1) or in Japanese Patent Application Laid-Open No. 2003-121206 (hereinafter referred to as Patent Literature 2), there is known an electromagnetic induction type encoder as in  FIG. 1  showing an example of Patent Literature 2. The electromagnetic induction type encoder includes: a large number of scale coils  14  and  16  arranged on a scale  10  along a measuring direction thereof; and transmitter coils  24  and  26  and receiver coils  20  and  22  arranged on a grid (also referred to as a slider)  12  capable of moving relative to the scale  10  in the measuring direction. The electromagnetic induction type encoder detects an amount of relative movement between the scale  10  and the grid  12  from a flux change detected at the receiver coil via the scale coil when the transmitter coil is excited. In this figure, reference numeral  28  denotes a transmission controller, and  30  denotes a receiving controller. 
         [0004]    As shown in  FIG. 2 , in order to reduce an offset, which is an excess signal, in such an electromagnetic induction type encoder, an offset has been reduced by placing the receiver coil  20  at a portion where magnetic fields generated by the transmitter coils  24  are cancelled out to be net zero (a central portion between the transmitter coils on the both sides thereof in the example of  FIG. 2 ). Note that in Patent Literature 2, the second receiver coils  22  are also arranged on the both sides of the second transmitter coil  26  as shown in  FIG. 3  in addition to the configuration formed by the first transmitter coils  24  and the first receiver coil  20  shown in  FIG. 2 . 
         [0005]    However, this configuration requires three rows of scale coils, and the line length of the scale coils therefore becomes long. Thus, there is the problem that the generated induced current is attenuated due to an impedance of the scale coil itself and it is therefore difficult to obtain a strong signal. 
         [0006]    In order to solve such a problem, the present applicant has suggested in Japanese Patent Application Laid-Open No. 2009-186200 (hereinafter referred to as Patent Literature 3) that a plurality of sets of transmitter coils  24 A and  24 B, receiver coils  20 A and  20 B, and scale coils  14 A and  14 B are arranged symmetrically with respect to the center of the scale  10 , and one of the scale coils symmetrically positioned with respect to the scale center ( 14 A, for example) has a positional relationship shifted from the other one of the scale coils ( 14 B, for example) by a ½ phase of a scale pitch λ as shown in  FIG. 4  of the present application corresponding to  FIG. 6  in Patent Literature 3. 
         [0007]    Furthermore, as shown in  FIG. 5 , it is conceivable to enable absolute position measurement by placing two sets of a track including scale coils, a transmitter coil, and a receiver coil in a scale width direction (grid width direction) with different scale pitches of λ 1  and λ 2 . The two sets includes a set formed by a transmitter coil  24 - 1  on the lower side of the figure, and scale coils  14 - 1   a  and a receiver coil  20 - 1  at the scale pitch λ 1  on the upper side of the figure and a set formed by a transmitter coil  24 - 2  on the upper side of the figure, and scale coils  14 - 2   a  and a receiver coil  20 - 2  at the scale pitch λ 2  on the lower side of the figure. In this figure, reference numeral  14 - 3  denotes a coil for connecting the scale coil  14 - 1   a  with the scale coil  14 - 2   a  (referred to as a connecting coil). 
         [0008]    With the configuration of  FIG. 5 , however, the receiver coil  20 - 1  ( 20 - 2 ) and the transmitter coil  24 - 1  ( 24 - 2 ) need to be placed at positions spaced apart from each other in order to reduce a direct crosstalk amount from the transmitter coil  24 - 1  or  24 - 2  to the receiver coil  20 - 2  or  20 - 1  on the grid  12 . Thus, a length of the scale coils on the scale  10  (a length of the scale coil  14 - 1   a +a length of the scale coil  14 - 2   a +a length of the connecting coil  14 - 3 ) becomes long, resulting in attenuation of generated induced current Ia due to the impedance of the scale coil itself. Thus, there is the problem that a strong signal is difficult to be obtained. 
         [0009]    Moreover, if it is attempted with the configuration of  FIG. 5  to reduce the encoder width, for example, by reducing a space between the scale coils  14 - 1   a  and  14 - 2   a , a magnetic field generated by the driving of the transmitter coil also directly affects the scale coil directly facing the receiver coil. As a result, there is generated an induced current component in a direction opposite to that of the normal induced current flowing through the scale coil, thereby reducing an induced current generated in the scale coil. This leads to the problem of a decrease in signals detected at the receiver coil. 
         [0010]    A description thereof will be given below. 
         [0011]      FIG. 6  shows an operation of detecting the scale coils  14 - 1   a  at the scale pitch λ 1  on the upper side of  FIG. 5  by the receiver coil  20 - 1 . As shown in the figure, the principle of the detection is essentially such that a magnetic field generated by the driving of the transmitter coil  24 - 1  with a driving current I D  leads to the generation of the induced current Ia at the scale coil  14 - 2   a  and a magnetic field generated by the induced current Ia flowing through the scale coil  14 - 1   a  via the connecting coil  14 - 3  is then detected by the receiver coil  20 - 1 . As the scale coil  14 - 1   a  approaches to the transmitter coil  24 - 1 , however, an induced current component Id in a direction opposite to that of the induced current Ia is generated at the scale coil  14 - 1   a  due to the magnetic field generated by the driving of the transmitter coil  24 - 1 . As a result, the total induced current at the scale coil  14 - 1   a  becomes (Ia−Id), i.e., a reduction by an amount of Id. In other words, the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the scale coil  14 - 2   a  and the induced current component Id (in the direction opposite to that of Ia) generated by the magnetic field directly entered into the scale coil  14 - 1   a  from the transmitter coil  24 - 1  are generated at the scale coil  14 - 1   a.    
         [0012]    On the other hand,  FIG. 7  shows an operation of detecting the scale coil  14 - 2   a  on the lower side of  FIG. 5  at the scale pitch λ 2  by the receiver coil  20 - 2 . As shown in this figure, the principle of the detection is essentially such that a magnetic field generated by the driving of the transmitter coil  24 - 2  with the driving current I D  leads to the generation of the induced current Ia at the scale coil  14 - 1   a  and a magnetic field generated by the induced current Ia flowing through the scale coil  14 - 2   a  via the connecting coil  14 - 3  is then detected by the receiver coil  20 - 2 . As the scale coil  14 - 2   a  approaches to the transmitter coil  24 - 2 , however, the induced current component Id in the direction opposite to that of the induced current Ia is generated at the scale coil  14 - 2   a  due to the magnetic field generated by the driving of the transmitter coil  24 - 2 . As a result, the total induced current at the scale coil  14 - 2   a  becomes (Ia−Id), i.e., a reduction by an amount of Id. In other words, the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the scale coil  14 - 1   a  and the induced current component Id (in the direction opposite to that of Ia) generated by the magnetic field directly entered into the scale coil  14 - 2   a  from the transmitter coil  24 - 2  are generated at the scale coil  14 - 2   a.    
         [0013]    Further, if it is attempted with the configuration of  FIG. 5  to reduce the encoder width, for example, by reducing the space between the scale coils  14 - 1   a  and  14 - 2   a , the following disadvantage also occurs. 
         [0014]    In other words, as shown in  FIG. 8 , a magnetic field generated by the induced current (Ia−Id) flowing through the scale coil  14 - 2   a  directly affects the receiver coil  20 - 1 , and a crosstalk current component Ic (Ic 1  at the left end side of the scale on the left side of  FIG. 8 ) therefore flows through the receiver coil  20 - 1 . 
         [0015]    In a case of the scale having the two-track configuration with different scale pitches as shown in  FIG. 5 , the crosstalk current component generated at the receiver coil  20 - 1  varies depending on a position of the scale (Ic 2  at the right end side of the scale on the right side of  FIG. 8 ). Thus, the crosstalk current component, which varies depending on a position in the measuring direction of the scale, is superimposed on a true position detection signal, thereby causing a problem of affecting a measurement accuracy (in particular, a wide range accuracy) over the entire length of the scale. 
         [0016]    In the scale of  FIG. 5  having tracks with the scale pitch λ 1  and the scale pitch λ 2  (λ 1 &lt;λ 2  in this example) respectively, if the scale at the scale pitch λ 1  is detected (measured) by the receiver coil  20 - 1  as in the example of  FIG. 8 , the wide range accuracy thereof has errors in a plus direction as shown in  FIG. 9  due to the effect of the scale at the scale pitch λ 2 . If the scale at the scale pitch λ 2  is detected (measured) by the receiver coil  20 - 2 , on the other hand, the wide range accuracy thereof has errors in a minus direction as shown in  FIG. 9  due to a cause similar to that described above, i.e., the effect of the scale at the scale pitch λ 1 . 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0017]    The present invention has been made in order to solve the above-described problems in the conventional technique, and an object thereof is to improve a detection signal strength at a receiver coil by increasing an induced current in a scale coil when a transmitter coil is excited and thereby achieve a high measurement precision due to an improvement in S/N ratio and/or to downsize an encoder by reducing a scale width thereof and therefore an encoder width thereof, as well as to improve a wide range accuracy by reducing an induced current due to a crosstalk magnetic field. 
       Solution to Problem 
       [0018]    The present invention solves the above-described problems by an electromagnetic induction type absolute position measuring encoder having two or more tracks, the encoder including: two or more rows of scale coils, each of the rows including a large number of scale coils arranged on a scale along a measuring direction so as to have a scale pitch different from that of another row; a transmitter coil and a receiver coil arranged on a grid movable relative to the scale in the measuring direction so as to face the scale coils; and the track constituted by the scale coils, the transmitter coil and the receiver coil, and capable of measuring an absolute position of the grid with respect to the scale from a flux change detected by the receiver coil via the scale coils when the transmitter coil is excited, wherein at least one loop-shaped additional scale coil is added between the scale coils in at least one of the tracks. 
         [0019]    Here, the scale coil (hereinafter, sometimes referred to as the “original scale coil”) may have a coil length twice or more as long as a coil length of the additional scale coil. 
         [0020]    Furthermore, the receiver coil may have a shape covering both of the original scale coil and the additional scale coil. 
         [0021]    Furthermore, the shape of the receiver coil may be a figure of eight with the top and bottom direction of the figure of eight coinciding with the measuring direction. 
         [0022]    Furthermore, a signal strength detected at the receiver coil may be increased to (Ia+Id)/(Ia−Id) times more than or equal to a signal strength obtained when no additional scale coil is added, where Id is an induced current component generated by a magnetic field directly entered into the original scale coil on the row to be measured from the transmitter coil, and Ia is an induced current component via the original scale coil on the other row. 
         [0023]    Furthermore, a line width of the additional scale coil may be adjusted so as to cancel out induced currents due to crosstalk magnetic fields from the scale coils to the receiver coil. 
         [0024]    Furthermore, a line width of the scale coil may be changed in accordance with a position in the measuring direction of the scale so as to cancel out induced currents due to crosstalk magnetic fields from the scale coils to the receiver coil. 
         [0025]    Furthermore, three or more rows of the scale coils may be provided. 
         [0026]    Furthermore, the scale coils may include first scale coils arranged at a scale pitch λ 1 , and second scale coils arranged at a scale pitch λ 2  different from the scale pitch λ 1  disposed on either side of the first scale coils, and further include an additional scale coil added only between the second scale coils disposed on either side of the first scale coils while no additional scale coil is added in the first scale coils at the center. 
         [0027]    Furthermore, the shape of the scale coil may be a rectangular frame shape. 
       Advantageous Effects of Invention 
       [0028]    According to the present invention, it is possible to increase the signal detection strength at the receiver coil when the transmitter coil is excited due to the induced current of the additional scale coil added between the original scale coils and to reduce the induced current due to the crosstalk magnetic field as compared to the configuration of  FIG. 5 . Therefore, there are provided advantageous effects such as (1) it becomes possible to reduce a space between two or more tracks and thus to downsize an encoder by reducing a scale width and accordingly an encoder width, (2) a measurement accuracy can be improved by an improvement in S/N ratio, (3) a gap between a grid and a scale can be widened, (4) it becomes possible to reduce a width of a scale coil and thus to downsize an encoder by reducing a scale width and accordingly an encoder width, and (5) a wide range accuracy can be improved by reducing an effect of a crosstalk magnetic field. 
         [0029]    These and other novel features and advantages of the present invention will become apparent from the following detailed description of preferred embodiments. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0030]    The preferred embodiments will be described with reference to the drawings, wherein like elements have been denoted throughout the figures with like reference numerals, and wherein: 
           [0031]      FIG. 1  is a perspective view showing the entire configuration of a conventional electromagnetic induction type encoder described in Patent Literature 2; 
           [0032]      FIG. 2  is a plan view similarly showing an arrangement and a first function of coils on a grid; 
           [0033]      FIG. 3  is a plan view similarly showing an arrangement and a second function of coils on a grid; 
           [0034]      FIG. 4  is a plan view showing a grid and a scale of a conventional electromagnetic induction type encoder described in Patent Literature 3; 
           [0035]      FIG.5  is a plan view of a grid and a scale showing a basic configuration of an electromagnetic induction type absolute position measuring encoder under consideration by the present inventor; 
           [0036]      FIG. 6  is a plan view of a main section illustrating how a transmitter coil on a lower side of  FIG. 5  is driven to perform detection at a receiver coil on an upper side thereof; 
           [0037]      FIG. 7  is a plan view of a main section illustrating how a transmitter coil on the upper side of  FIG. 5  is driven to perform detection at a receiver coil on the lower side thereof; 
           [0038]      FIG. 8  is a plan view of a main section illustrating occurrences of induced currents due to crosstalk magnetic fields in a conventional example; 
           [0039]      FIG. 9  is a graph similarly showing an example of wide range errors; 
           [0040]      FIG. 10  is a plan view of a scale according to a first embodiment of the present invention; 
           [0041]      FIG. 11  is a plan view of a main section illustrating how a transmitter coil on a lower side of  FIG. 10  is driven to perform detection at a receiver coil on an upper side thereof; 
           [0042]      FIG. 12  is a plan view of a main section illustrating how a transmitter coil on the upper side of  FIG. 10  is driven to perform detection at a receiver coil on the lower side thereof; 
           [0043]      FIG. 13  is a plan view of a main section illustrating occurrences of induced currents due to crosstalk magnetic fields in the first embodiment; 
           [0044]      FIG. 14  is a graph similarly showing an example of wide range errors; 
           [0045]      FIG. 15  is a plan view of a scale similarly showing a modification; 
           [0046]      FIG. 16  is a plan view of a scale similarly showing another modification; and 
           [0047]      FIG. 17  is a plan view of a scale according to a second embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0048]    Embodiments of the present invention will be described below in detail with reference to the drawings. 
         [0049]    As shown in  FIG. 10 , according to the first embodiment of the present invention, scale coils  14 - 1  are configured by adding scale coils (referred to also as additional scale coils)  14 - 1   b  shown by broken lines in the figure between the originally-provided scale coil main bodies (referred to also as original scale coils or main body scale coils)  14 - 1   a  similar to those shown in  FIG. 5 . Similarly, scale coils  14 - 2  are configured by adding scale coils (referred to also as additional scale coils)  14 - 2   b  shown by broken lines in the figure between the originally-provided scale coil main bodies  14 - 2   a  (referred to also as original scale coils or main body scale coils) similar to those shown in  FIG. 5 . 
         [0050]    A reason why a signal strength detected at the receiver coil is increased with the above-described configuration even when an encoder width is reduced, for example, by reducing a space between the scale coils  14 - 1   a  and  14 - 2   a  will now be described below. 
         [0051]    First, as shown in  FIG. 11 , when the scale coils  14 - 1  at the scale pitch λ 1  positioned on the upper side of  FIG. 10  are detected by the receiver coil  20 - 1 , due to a magnetic field generated by the driving of the transmitter coil  24 - 1  with the driving current I D , the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the main body scale coil  14 - 2   a  and the induced current component Id (in a direction opposite to that of Ia) generated by the magnetic field directly entered into the main body scale coil  14 - 1   a  from the transmitter coil  24 - 1  is generated at the main body scale coil  14 - 1   a . Also, at the additional coil  14 - 1   b , there is generated an induced current Ib (in a direction opposite to that of the induced current (Ia−Id) at the main body scale coil  14 - 1   a ) by a magnetic field directly entered thereto from the transmitter coil  24 - 1 . When the magnitudes of the induced current Id and the induced current Ib are compared to each other, (the induced current Id)&lt;(the induced current Ib/2) holds true since the coil length of the scale coils ( 14 - 1   a + 14 - 2   a + 14 - 3 ) is more than or equal to double the coil length of the additional scale coil  14 - 1   b.    
         [0052]    The receiver coil  20 - 1  has a coil shape in the figure of eight covering both of the main body coil  14 - 1   a  and the additional coil  14 - 1   b  with the top and bottom direction of the figure of eight coinciding with the measuring direction. Thus, a detection signal current at the receiver coil  20 - 1  is equal to that obtained by adding an induced signal current at the receiver coil  20 - 1  due to the current Ib of the additional scale coil  14 - 1   b  to an induced signal current at the receiver coil  20 - 1  due to the current (Ia−Id) of the main body scale coil  14 - 1   a.    
         [0053]    Thus, as shown in the following expressions, a signal strength A detected at the receiver coil  20 - 1  is increased to (Ia+Id)/(Ia−Id) times more than or equal to a signal strength B obtained when no additional scale coil  14 - 1   b  is added. 
         [0000]        A ∞( Ia−Id+Ib )≅( Ia−Id+ 2 Id )=( Ia+Id )
 
         [0000]      B∞(Ia−Id)
 
         [0000]      ∴A/B∞(Ia+Id)/(Ia−Id)
 
         [0054]    As shown in  FIG. 12 , when the scale coils  14 - 2  at the scale pitch λ 2  positioned on the lower side of  FIG. 10  are detected by the receiver coil  20 - 2 , on the other hand, due to a magnetic field generated by the driving of the transmitter coil  24 - 2  with the driving current I D , the induced current (Ia−Id) corresponding to a difference between the induced current component Ia via the main body scale coil  14 - 1   a  and the induced current component Id (in a direction opposite to that of Ia) generated by the magnetic field directly entered into the main body scale coil  14 - 2   a  from the transmitter coil  24 - 2  is generated at the main body scale coil  14 - 2   a . Also, at the additional scale coil  14 - 2   b , there is generated the induced current Ib (in the direction opposite to that of the induced current (Ia−Id) at the main body scale coil  14 - 2   a ) by a magnetic field directly entered thereto from the transmitter coil  24 - 2 . When the magnitudes of the induced current Id and the induced current Ib are compared to each other, (the induced current Id)&lt;(the induced current Ib/2) holds true since the coil length of the scale coils ( 14 - 2   a  + 14 - 1   a + 14 - 3 ) is more than or equal to double the coil length of the additional scale coil  14 - 2   b.    
         [0055]    The receiver coil  20 - 2  has a coil shape in the figure of eight covering both of the main body scale coil  14 - 2   a  and the additional scale coil  14 - 2   b  with the top and bottom direction of the figure of eight coinciding with the measuring direction. Thus, a detection signal current at the receiver coil  20 - 2  is equal to that obtained by adding an induced signal current at the receiver coil  20 - 2  due to the current Ib of the additional scale coil  14 - 2   b  to an induced signal current at the receiver coil  20 - 2  due to the current (Ia−Id) of the main body scale coil  14 - 2   a.    
         [0056]    Thus, in a similar manner as that described above, a signal strength C detected at the receiver coil  20 - 2  is increased to (Ia+Id)/(Ia−Id) times more than or equal to the signal strength B obtained when no additional scale coil  14 - 2   b  is added as shown in the following expressions. 
         [0000]        C ∞( Ia−Id+Ib )≅( Ia−Id+ 2 Id )=( Ia+Id )
 
         [0000]      B∞(Ia−Id)
 
         [0000]      ∴C/B≅(Ia+Id)/(Ia−Id)
 
         [0057]    Further, with the addition of the loop-shaped scale coils ( 14 - 1   b  and  14 - 2   b ), as shown in  FIG. 13 , when the scale coils  14 - 1  at the scale pitch λ 1  on the upper side of  FIG. 10  are detected by the receiver coil  20 - 1 , crosstalk magnetic fields from the scale coils  14 - 2  to the receiver coil  20 - 1  become substantially uniform without depending on a position of the scale in the measuring direction. As a result, a difference between the induced currents due to the crosstalk magnetic fields becomes small (Ic 1 ≈Ic 2 ), thereby improving the wide range accuracy as shown in  FIG. 14 . Also when the scale coils  14 - 2  at the scale pitch λ 2  on the lower side of  FIG. 10  are detected by the receiver coil  20 - 2 , the wide range accuracy can be improved due to the similar action as shown in  FIG. 14 . In particular, when the receiver coil has a shape in the figure of eight with the top and bottom direction of the figure of eight coinciding with the measuring direction, the induced currents due to the crosstalk magnetic fields are cancelled out each other. As a result, the induced current generated at the receiver coil  20 - 1  is substantially reduced, thereby further improving the wide range accuracy. 
         [0058]    Further, as illustrated in  FIG. 15  by way of example, the above-described induced currents can be completely cancelled out each other by adjusting a line width of the added scale coils ( 14 - 1   b  and  14 - 2   b ). Alternatively, as illustrated in  FIG. 16  by way of example, a line width of the scale coil can be changed in accordance with the position of the scale in the measuring direction so as to satisfy Ic 1 =Ic 2 , thereby achieving a higher degree of accuracy. 
         [0059]    Further, the application of the present invention is not limited to those including two rows of tracks as shown in  FIG. 10 . As in the second embodiment shown in  FIG. 17 , the present invention can also be applied, for example, to those including three rows of tracks in which the scale coils  14 - 2  at the scale pitch λ 2  are arranged on the upper side and lower side of the scale coils  14 - 1  at the scale pitch λ 1 . 
         [0060]    In the second embodiment, the additional scale coils  14 - 2   b  are added only between the main body scale coils  14 - 2   a  in the scale coils  14 - 2  on the upper and lower sides, and no additional scale coils are added to the scale coils  14 - 1  arranged at the center. It is also possible to partially omit the addition of the scale coils in such a manner. 
         [0061]    Although the shape of the scale coil is described as a rectangular frame shape in any of the above-described embodiments, the shape of the scale coil is not limited thereto. For example, it may be a plate shape having an electrode inside the rectangle or alternatively a plate shape obtained by removing the rectangular portion. 
         [0062]    It should be apparent to those skilled in the art that the above-described embodiments are merely illustrative which represent the application of the principles of the present invention. Numerous and varied other arrangements can be readily devised by those skilled in the art without departing from the spirit and the scope of the invention.