Abstract:
An electrostatic capacitive encoder includes a scale ( 1 ) and a sensor head ( 2 ), which is arranged opposing to and relatively movable to the scale ( 1 ). A plurality of protrusions ( 24 ) for sliding, processed in a certain pattern, are located on a flat region surrounding a transmitting electrodes ( 21 ) and receiving electrodes ( 22 ) of the sensor head ( 2 ).

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a displacement measuring device such as electrostatic capacitive, photoelectric and electromagnetic inductive encoders.  
           [0003]    2. Description of the Related Art  
           [0004]    In an electrostatic capacitive encoder mounted on a small length-measuring device such a linear scale, a scale is arranged opposing to a sensor head, interposing an air gap between them. For use in specifically sustaining the air gap between the scale and the sensor head, actually available systems include a system that accommodates the sensor head within a frame having three or more protrusions for sliding in order to oppose the sensor head to the scale, and a system that employs bearings.  
           [0005]    These conventional systems are difficult to adjust the gap, however, when a more compact and smaller gap is required. Then, another system can be considered to slide the scale, directly contacting with the sensor head. This system requires a protection film for protecting electrodes on both the sensor head and scale. This protection film is required to have an excellent electrical insulation, planarity and wearability and a small coefficient of friction.  
           [0006]    An insulating film, for example, a silicon oxide film, which is deposited through chemical vapor deposition (CVD), can be considered as such the protection film. A diamond-like carbon (DLC) film, which is deposited by plasma CVD, can achieve a much further excellent wearablity and much smaller coefficient of friction compared to the CVD silicon oxide film.  
           [0007]    The following problems occur, however, when the DLC film is formed by plasma CVD as the protection film on the entire surface of the sensor head. First, there are steps of about 1 μm depth/height on the surface of the sensor head and scale, due to electrode arrangement, worsening a tight adhesion of the DLC film to the surface. As a result, a sufficient durability can not be attained. In addition, the protection film is required to have a flat surface, but the surface of the DLC film reflects the steps at the electrode portion. Thus, a planarization process is also required for planarizing the DLC film. As a planarization technology, CMP (Chemical Mechanical Polishing) is well known, for example. However, since the DLC film has a high hardness, such a planarization process can not be applied easily to the DLC film.  
         SUMMARY OF THE INVENTION  
         [0008]    The present invention has been made in consideration of the above situations and accordingly has an object to provide a displacement measuring device having an easily adjustable gap and such a sliding property that exhibits an excellent durability.  
           [0009]    The present invention is provided with a displacement measuring device, which comprises a scale having a signal transfer section arranged along a measurement axis; a sensor head arranged opposing to and relatively movable to the scale, the sensor head having a signal transmitting section and a signal receiving section, for transmitting and receiving a signal via the signal transfer section; and a plurality of protrusions, located on a flat region in at least one of opposite surfaces of the scale and sensor head, for sliding on the other surface.  
           [0010]    In the displacement measuring device according to the present invention, the protrusions are located on a plurality of positions in the flat region to avoid steps produced from arrangement of electrodes in the sensor head and scale. This allows the protrusions to tightly contact the ground material and slide with an excellent durability even though a DLC film is employed for the protrusions. In addition, it does not require any process for planarization as in the case of forming the DLC film over the entire surfaces of the sensor head and scale.  
           [0011]    Preferably, in the displacement measuring device according to the present invention, the protrusions may particularly be composed of a laminated film consisting of a buffer film and a DLC film deposited thereon. Desirably, the buffer layer has a tight adhesion to the ground material and DLC film, and a larger hardness in some extent, though it is not required as high as that of the DLC film. Specifically, the buffer film may be formed of a film of insulator such as silicon oxide and silicon nitride formed by plasma CVD or spattering. The buffer film may also comprise a film of semiconductor such as polysilicon, if the protrusions are formed at locations where no electrode is arranged.  
           [0012]    A period of time for film deposition is extremely long when a film thickness required to achieve a necessary gap is ensured only with the DLC film, though it can be reduced by use of a combination of the DLC film with another insulating film.  
           [0013]    Further, when the protrusions are formed of the buffer and DLC films, the DLC film preferably extends from the protrusions over the electrode arrangement region. This allows the DLC film to serve as a protection film for the electrode arrangement region to prevent foreign materials from mixing into the electrode arrangement region.  
           [0014]    Furthermore, when the electrode arrangement region in a sensor head substrate or scale substrate is previously formed to have a recessed region and electrodes are disposed within the recessed region, the protrusions are not required to have a much larger thickness. In this case, the protrusions can be formed of the DLC film alone.  
           [0015]    Further, the protrusions formed of the buffer and DLC films are arranged on one of the sensor head and scale, the DLC film is preferably formed on the entire surface of the other as a protection film. This can achieve protection of electrodes in the other and attain an excellent wearable sliding property with a small coefficient of friction.  
           [0016]    Other features and advantages of the invention will be apparent from the following description of the preferred embodiments thereof.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    The present invention will be more fully understood from the following detailed description with reference to the accompanying drawings in which:  
         [0018]    [0018]FIG. 1 is an exploded perspective view of an electrostatic capacitive encoder according to an embodiment of the present invention;  
         [0019]    [0019]FIG. 2 is a cross sectional view of the encoder of the same embodiment;  
         [0020]    FIGS.  3 A- 3 C show a process of forming a film for sliding in a sensor head of the same embodiment;  
         [0021]    [0021]FIG. 4 is a cross sectional view showing a sensor head according to another embodiment;  
         [0022]    [0022]FIG. 5 is a cross sectional view showing a sensor head according to another embodiment;  
         [0023]    FIGS.  6 A- 6 C show a process of forming a film for sliding in a sensor head according to another embodiment;  
         [0024]    [0024]FIG. 7 shows another pattern example of a film for sliding;  
         [0025]    [0025]FIG. 8 shows another pattern example of a film for sliding;  
         [0026]    [0026]FIG. 9 shows another pattern example of a film for sliding;  
         [0027]    [0027]FIG. 10 shows another pattern example of a film for sliding;  
         [0028]    [0028]FIG. 11 shows another pattern example of a film for sliding; and  
         [0029]    [0029]FIG. 12 shows another pattern example of a film for sliding.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0030]    [0030]FIG. 1 is an exploded perspective view of an electrostatic capacitive encoder according to an embodiment of the present invention and FIG. 2 is a cross sectional view thereof in a situation where a scale  1  opposes to a sensor head  2 . As shown in FIG. 1, the sensor head  2  comprises a substrate  20  such as glass. It also comprises, formed on the substrate, transmitting electrodes  21  (commonly having a two- to four-layered structure), receiving electrodes  22  (commonly having a two- to three-layered structure), and terminals  23  for connecting these electrodes to an external circuit. The surface of the sensor head  2 , which faces on the scale  1 , is such a surface that has protruded and recessed regions due to electrode arrangement (of the transmitting and receiving electrodes  21  and  22 ). Protrusions  24  for sliding are pattern-formed on a plurality of locations in a flat region, without the electrode arrangement, within the surface of the sensor head  2 , facing on the scale  1 .  
         [0031]    The protrusions  24  have slim rectangle patterns elongated along a measurement axis x and they are arrayed in two lines so as to interpose the transmitting and receiving electrode sections  21  and  22  therebetween. In the example of FIG. 1, three protrusions  24  are discretely arranged on one line and one on the other. The protrusions  24  are formed of a laminated film and have flat surfaces as described later.  
         [0032]    As shown in FIG. 2, the transmitting and receiving electrodes  21  and  22  of the sensor head  1  are covered with a protection film  25  that may be formed of a plasma-CVD silicon oxide. The protrusions  24  are formed of a laminated film on the protection film  25  ,which comprises a buffer film  24   a  of silicon oxide formed by plasma-CVD and a DLC film  24   b  formed by plasma-CVD. The DLC film  24   b  extends over other regions than the protrusions  24  and is employed as a protection film for the transmitting and receiving electrodes  21  and  22 .  
         [0033]    The scale  1  comprises a substrate  10  such as glass and a transfer electrodes  11  formed on the substrate and arranged along the measurement axis x for capacitively coupling to the transmitting and receiving electrodes  21  and  22 . A DLC film  12  is formed by plasma CVD to cover the entire surface of the substrate, on which the transfer electrodes  11  have been formed.  
         [0034]    FIGS.  3 A- 3 C show a process of forming the protrusions  24  in the sensor head  2  of this embodiment. In a film formation process, plasma CVD technology is employed, which is able to form a film at a low temperature. First, as shown in FIG. 3A, the buffer film  24   a  is deposited on the whole surface of the substrate  20 , on which the transmitting, receiving and transfer electrodes  21 ,  22  and terminals  23  have been formed. Next, the buffer film  24   a  is patterned by lithography and etching technologies, as shown in FIG. 3B, to stay only on the locations of the protrusions  24 . Subsequently, the DLC film  24   b  is deposited on the entire surface as shown in FIG. 3C. Thus, the protrusions  24  are composed of a laminated structure consisting of the buffer and DLC films  24   a  and  24   b.  The transmitting and receiving electrodes  21  and  22  are covered with the DLC film  24   b.  The terminals  23  can be exposed finally by etching or by providing a mask at the time of film deposition.  
         [0035]    In this embodiment, as the protrusions  24  have the laminated structure, they can increase their height by a thickness of the buffer film  24   a.  Accordingly, even though the DLC film  24   b  is thin, the surfaces of the protrusions  24  can be protruded upwardly beyond the surfaces of the transmitting and receiving electrodes  21  and  22 . This enables the sensor head  2  to slide relative to the scale  1  while ensuring a gap required between opposing electrodes.  
         [0036]    In this embodiment, the protrusions  24  are arranged, not on the entire surface of the sensor head  2 , but on the flat region discretely, circumventing the transmitting and receiving electrodes. Therefore, poor adhesion of the film for sliding due to steps can not occur and an excellent sliding property can be achieved. In addition, a process of planarization is not required, unlike when it is employed in forming a protection film for sliding over the entire surface of the sensor head.  
         [0037]    Further, in this embodiment, the laminated film consisting of the buffer film  24   a  and the DLC film  24   b  deposited thereon is employed as the protrusions  24 , so the DLC film can guarantee mechanical strength and wearability against sliding motions. In addition, the use of such the laminated structure can shorten the film formation time and reduce the manufacturing cost, unlike the case where the DLC film alone must ensure the height required. Further, the transmitting and receiving electrodes  21  and  22  are covered with the DLC film to achieve an excellent durability against the mixture of foreign materials.  
         [0038]    [0038]FIG. 4 shows a sensor head  2  according to another embodiment. This embodiment is similar to the preceding one with respect to the laminated structure of the protrusions  24  consisting of the buffer film  24   a  and the DLC film  24   b,  except that the transmitting and receiving electrodes  21  and  22  are not covered with the DLC film. This structure can be obtained by depositing the buffer film  24   a  and the DLC film  24   b,  and successively pattering the laminated film integrally.  
         [0039]    [0039]FIG. 5 shows a sensor head  2  according to a further embodiment. In this embodiment, a recessed region  51  is previously formed in the substrate  20  of the sensor head  2 . The transmitting and receiving electrodes  21  and  22  are formed within the recessed region  51 . The protrusions  24  are patterned only with the DLC film on a flat, protruded region at the periphery of the recessed region  51  in which the transmitting and receiving electrodes  21  and  22  are formed.  
         [0040]    In this embodiment, selection of a depth of the recessed region  51  makes the upper surfaces of the transmitting and receiving electrodes  21  and  22  be approximately equal to the flat surface of the protruded region on which no electrode are arranged. Therefore, the protrusions  24  can be formed on the protruded flat surface with a thin DLC film only and no buffer film.  
         [0041]    FIGS.  6 A- 6 C show a process of forming a sensor head  2  according to an alternative embodiment of FIGS.  3 A- 3 C. After forming the transmitting, receiving electrodes  21 ,  22  and terminals  23  on the substrate  20 , the buffer film  24   a  is deposited thereon as same as the process of FIG. 3A. Then, the upper surface of the buffer film  24   a  is planarized by CMP process as shown in FIG. 6B. The DLC film  24   b  is deposited and patterned on the planarized buffer film  24   a  as shown in FIG. 6C. The terminals  23  are finally exposed.  
         [0042]    In this embodiment, as the buffer film  24   a  is planarized, the DLC film  24   a  or the main part of the protrusion  24  has an improved adhesion. The DLC film  24   a  can be thinned and the film formation time can be reduced. The DLC film  24   a  is hard and so is difficult to be planarized. In contrast to this, the buffer film  24   a  can be planarized easily if it is composed of a CVD silicon oxide or the like.  
         [0043]    In the preceding embodiments, a film of fluororesin such as Teflon® may be employed, instead of the DLC film. The use of the fluororesin film can also achieve a small coefficient of friction and excellent wearability. Silicon nitride, polysilicon and amorphous silicon may also be employed for the buffer film as well as silicon oxide.  
         [0044]    Application of a luburicant on the surf ace of the protrusion further improves durability.  
         [0045]    FIGS.  7 - 12  show different examples of patterns and arrangements of the protrusions  24  located on the sensor head  2 . FIG. 7 exemplifies the protrusions  24  patterned in two continuous lines interposing the transmitting and receiving electrodes  21  and  22  therebetween. FIG. 8 exemplifies the protrusions  24  patterned in two discontinuous lines interposing the transmitting and receiving electrodes  21  and  22  therebetween. FIGS. 9 and 10 show examples modifying the protrusions  24  of FIGS. 7 and 8 into slim, oval patterns. FIGS. 11 and 12 show examples modifying the protrusions  24  into circular patterns.  
         [0046]    In the preceding embodiments, the protrusions are patterned on the sensor head, though they may be contrarily patterned on the scale. In the latter case, the protrusions are patterned in two lines, at an interval less than at least the dimension of the sensor head, along the longitudinal direction of the scale (the measurement axis x). In this case, it is also required to cover wiring sections on the sensor head with a protection film. Alternatively, a design is required to eliminate any wiring from a portion that contacts the protrusion of the scale through the use of the backside wires of the sensor head for connection wiring between the electrodes and terminal of the sensor head via through-holes.  
         [0047]    The present invention may also be applied to photoelectric and electromagnetic inductive encoders while the preceding embodiments exemplify the electrostatic capacitive encoder.  
         [0048]    As obvious from the forgoing, in the displacement measuring device according to the present invention, the protrusions for sliding are patterned on either the sensor head or the scale, circumventing the electrode arrangement section, using a hard film such as DLC as the film for sliding. Therefore, the device can achieve an easily adjustable gap and such a sliding property that exhibits an excellent durability.  
         [0049]    Having described the embodiments consistent with the present invention, other embodiments and variations consistent with the invention will be apparent to those skilled in the art. Therefore, the invention should not be viewed as limited to the disclosed embodiments but rather should be viewed as limited only by the spirit and scope of the appended claims.