Patent Publication Number: US-2006007451-A1

Title: Sensor head of reflective optical encoder

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2004-203753, filed Jul. 9, 2004, the entire contents of which are incorporated herein by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention relates to an optical encoder.  
      2. Description of the Related Art  
      Currently, so-called encoders such as optical and magnetic encoders have been used to detect displacement amounts in a linear direction for machine tool stages, three-dimensional measurement instruments, and the like, and to detect rotational angles for servo motors and the like.  
      An optical encoder generally comprises a scale fixed to a displacement measurement target such as a stage and a sensor head to detect the displacement of the scale. The scale has a movement detection pattern whose optical characteristics change periodically in the moving direction. The sensor head has a light-emitting unit to apply a light beam to the scale and a detector to detect the light beam modulated by the scale. The moving amount of the scale is calculated on the basis of a change in intensity of the light beam detected by the detector.  
      An optical encoder has characteristics such as high precision, high resolution, noncontact, and high electromagnetic interference resistance, and hence is used in various fields. As encoders demanding high precision and high resolution, in particular, optical encoders are most popular.  
     BRIEF SUMMARY OF THE INVENTION  
      The present invention is directed to a sensor head that is combined with a scale having an optical pattern to constitute a reflective optical encoder. The sensor head according to the present invention has a light source that projects a light beam to be applied to the scale, and two semiconductor substrates. The semiconductor substrates respectively have photodetectors that detect the light beam reflected and modulated by the optical pattern of the scale and electrical pads that are configured to be connected to an external circuit, and at least one of the semiconductor substrates has an electrical circuit that processes signals output from the photodetectors. The sensor head also has electric wiring lines that electrically connect the electrical pads of the two semiconductor substrates and a package that contains the light source and semiconductor substrates.  
      Advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. Advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter. 
    
    
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING  
      The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
       FIG. 1  shows a reflective optical encoder according to the first embodiment of the present invention;  
       FIG. 2  is a sectional view taken along the line II-II of the reflective optical encoder shown in  FIG. 1 ;  
       FIG. 3  is a view, seen from the scale side, of the sensor head of the reflective encoder shown in  FIG. 1 ;  
       FIG. 4  shows the circuit configuration of the sensor head shown in  FIG. 3 ;  
       FIG. 5  is a plan view of the scale shown in  FIG. 1 ;  
       FIG. 6  shows an A-phase signal, B-phase signal, and Z-phase signal detected by the sensor head;  
       FIG. 7  shows different electric wiring lines that may be employed in place of the electric wiring lines shown in  FIG. 4 ;  
       FIG. 8  shows the concept of the basic arrangement of a transmissive optical encoder that uses a Talbot image;  
       FIG. 9  shows another light source using an LED that may be employed in place of, e.g., the light source shown in  FIG. 3 ;  
       FIG. 10  is a perspective view of a sensor head in a reflective optical encoder according to the second embodiment of the present invention;  
       FIG. 11  is a sectional view of the reflective optical encoder including the sensor head shown in  FIG. 10 ;  
       FIG. 12  is a plan view, seen from the scale side, of the sensor head shown in  FIG. 10 ;  
       FIG. 13  is a sectional view of a sensor head in a reflective optical encoder according to the third embodiment of the present invention;  
       FIG. 14  is a plan view, seen from the scale side, of the sensor head shown in  FIG. 13 ; and  
       FIG. 15  is a plan view, seen from the scale side, of a portion of a sensor head according to a modification of the third embodiment, the portion including a transparent member and semiconductor substrates.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      Embodiments of the present invention will be described below with reference to the views of the accompanying drawing.  
     First Embodiment  
       FIG. 1  shows a reflective optical encoder according to the first embodiment of the present invention. As shown in  FIG. 1 , the reflective optical encoder comprises a scale  110  and a sensor head  120 . The scale  110  is attached to a movement detection target, which can move relative to the sensor head  120 . The sensor head  120  detects the movement of the scale  110 . The scale  110  can move with respect to the sensor head  120  in directions of arrows.  
      As shown in  FIG. 5 , the scale  110  has two types of optical patterns, i.e., a movement detection pattern  111  and a reference position detection pattern  112 . The movement detection pattern  111  and reference position detection pattern  112  are respectively formed on two tracks that extend adjacent to each other in the moving direction. The movement detection pattern  111  has a periodic structure in the moving direction. The reference position detection pattern  112  is provided at one portion in the moving direction.  
      The sensor head  120  comprises a light source  121  for projecting a light beam to be applied to the scale  110 , two semiconductor substrates  141  and  142 , and a package containing the light source  121  and semiconductor substrates  141  and  142 . The package comprises a box-like housing  180  and a lid member  190  having a portion that transmits light. The lid member  190  is attached to the housing  180  to close the opening of the housing  180 . The housing  180  has electrode terminals  182  that are configured to be connected to an external circuit.  
      The semiconductor substrate  141  has a moving amount detection photodetector  131  for detecting the light beam reflected and modulated by the movement detection pattern  111  of the scale  110 . The semiconductor substrate  142  has a reference position detection photodetector  132  for detecting the light beam reflected and modulated by the reference position detection pattern  112  of the scale  110 .  
       FIG. 2  is a sectional view taken along the line II-II of the reflective optical encoder shown in  FIG. 1 . As shown in  FIG. 2 , the light source  121  is fixed to the housing  180 . The semiconductor substrates  141  and  142  are fixed to the lid member  190  by, e.g., flip-chip bonding.  
       FIG. 3  is a view, seen from the scale side, of the sensor head of the reflective encoder shown in  FIG. 1 . As shown in  FIG. 3 , the semiconductor substrates  141  and  142  having the moving amount detection photodetectors  131  and  132 , respectively, are arranged on the two sides of the light source  121  in a direction substantially perpendicular to the moving direction of the scale  110 . The semiconductor substrate  141  is provided with an electrical circuit  171  on a part of its region excluding the moving amount detection photodetector  131 . The semiconductor substrate  142  is provided with an electrical circuit  172  on a part of its region excluding the reference position detection photodetector  132 . The electrical circuits  171  and  172  on the semiconductor substrates  141  and  142  are electrically connected to each other through electric wiring lines  151  and  152 .  
       FIG. 4  shows the circuit configuration of the sensor head shown in  FIG. 3 . As shown in  FIG. 4 , the electrical circuit  171  on the semiconductor substrate  141  includes an I/V converting circuit  173  that I/V-converts a photocurrent output from the moving amount detection photodetector  131  and a light source driving circuit  174  for driving the light source  121 . The electrical circuit  172  on the semiconductor substrate  142  includes an I/V converting circuit  177  that I/V-converts a photocurrent output from the reference position detection photodetector  132 , a digital circuit  175  that pulses the I/V-converted analog signal, and an interpolator  176  that interporates a periodical analog signal accompanying the movement of the scale  110  and outputs it as a pulse.  
      These electrical circuits are arranged considering their characteristics.  
      It is not preferable to transmit output signals from the moving amount detection photodetector  131  and reference position detection photodetector  132  to another semiconductor substrate as photocurrents. This is because the photocurrents may be largely influenced by noise from the connecting wiring lines. For this reason, the I/V converting circuit  173  is formed on the semiconductor substrate  141  that has the moving amount detection photodetector  131 . The I/V converting circuit  177  is formed on the semiconductor substrate  142  that has the reference position detection photodetector  132 .  
      A moving amount detection signal output from the I/V converting circuit  173  reflects the movement of the scale  110 . A reference position detection signal output from the I/V converting circuit  177  reflects the presence/absence of the reference position of the scale  110 .  
      If these signals (the moving amount detection signal and reference position detection signal) are superposed with noise from the digital circuit  175  and interpolator  176 , it appears as a sharp change. In other words, the noise from the digital circuit  175  and interpolator  176  causes a sharp change in the moving amount detection signal and reference position detection signal. A similar sharp change is also caused by the reciprocal movement of the scale  110  within a short period of time. In other words, the reciprocal movement of the scale  110  within the short period of time also causes a sharp change in the moving amount detection signal and reference position detection signal.  
      In detection of the reference position, the reciprocal movement of the scale  110  within a short period of time is not significant. Accordingly, a sharp change in the reference position detection signal may be neglected as noise. In detection of the moving amount, the reciprocal movement of the scale  110  within a short period of time is the information that should be detected. Accordingly, a sharp change in the moving amount detection signal cannot be neglected as noise.  
      In other words, the moving amount detection signal output from the I/V converting circuit  173  is readily susceptible to noise from the digital circuit  175  and interpolator  176 . Therefore, the I/V converting circuit  173  is desirably formed on a semiconductor substrate that is different from the semiconductor substrate on which the digital circuit  175  and interpolator  176  are formed. The reference position detection signal output from the I/V converting circuit  177  is not readily susceptible to noise from the digital circuit  175  and interpolator  176 . Therefore, if the I/V converting circuit  177  is formed on the same semiconductor substrate on which the digital circuit  175  and interpolator  176  are formed, it will not cause much trouble.  
      For this reason, the digital circuit  175  and interpolator  176  are formed on the semiconductor substrate  142  that is different from the semiconductor substrate  141  that has the I/V converting circuit  173 . The I/V converting circuit  177  is formed on the semiconductor substrate  142  that has the digital circuit  175 .  
      The light source driving circuit  174  is not likely to generate noise and accordingly arranged on the semiconductor substrate  141  that has the moving amount detection photodetector  131 . The light source driving circuit  174 , however, generates heat and may negligibly influence the circuit characteristics depending on the position where it is mounted. Therefore, the position to mount the light source driving circuit  174  must be selected carefully.  
      Referring back to  FIG. 3 , the electric wiring lines  151  and  152  are formed on that surface of the lid member  190  which faces the semiconductor substrates  141  and  142 . The two electrical circuits  171  and  172  are electrically connected to each other through the electric wiring lines  151  and  152 . The electric wiring lines  151  only connect the electrical circuits  171  and  172  and are not exposed outside the package. The electric wiring lines  152  are connected to the electrode terminals  182  formed on the housing  180  and configured to be connected to the external circuit. The electrical circuits  171  and  172  are designed to be operated by the same driving voltage Vcc. Hence, a ground voltage Vgnd and the driving voltage Vcc from the external circuit are applied to the electrode terminals  182  of the package. The electric wiring lines  152  from the electrode terminals  182  branch as the electric wiring lines  152  shown in  FIG. 3  and extend to the two electrical circuits  171  and  172 .  
      The electric wiring lines formed on the lid member  190  include not only the electric wiring lines  151  that merely connect the electrical circuits  171  and  172  and the electric wiring lines  152  that connect the electrical circuits  171  and  172  and are configured to be connected to the external circuit through the package but also electric wiring lines or the like that extend from the electrical circuit  171  or the electrical circuit  172  or both, as needed, to the package but not to the other one.  
      The operation of the reflective encoder will be described.  
      Referring to  FIG. 1 , part of the light beam projected from the light source  121  is reflected and modulated by the movement detection pattern  111  on the scale  110  and enters the moving amount detection photodetector  131 . The intensity of the light beam entering the moving amount detection photodetector  131  changes periodically in accordance with the movement of the movement detection pattern  111 . Hence, the moving amount detection photodetector  131  outputs a signal current that changes substantially periodically in accordance with the movement of the scale  110  to the I/V converting circuit  173  in the electrical circuit  171 . The I/V converting circuit  173  I/V-converts the input signal current into a voltage signal. This voltage signal contains periodical signals called A- and B-phase signals the phases of which are different from each other by 90°, as shown in  FIG. 6 .  
      Part of the light beam projected from the light source  121  is applied to that track on the scale  110  which the reference position detection pattern  112  is formed on. When the scale  110  is at the reference position, the reference position detection pattern  112  is located on the optical path of the light beam. The light beam reflected by the reference position detection pattern  112  enters the reference position detection photodetector  132 . When the scale  110  is not at the reference position, the reference position detection pattern  112  is off the optical path of the light beam. A very small quantity of light beam reflected by the surface of the scale  110  enters the reference position detection photodetector  132 . The reference position detection photodetector  132  outputs a signal current that changes almost in a binary manner in accordance with the presence/absence of the reference position detection pattern  112  to the I/V converting circuit  177  in the electrical circuit  172 . The I/V converting circuit  177  I/V-converts the input signal current into a voltage signal. This voltage signal is a so-called Z-phase signal and shows a high output voltage, as shown in  FIG. 6 , when the scale  110  is located near the reference position. Therefore, whether or not the scale  110  is located at the reference position can be determined on the basis of the Z-phase signal.  
      The A- and B-phase signals output from the I/V converting circuit  173  of the electrical circuit  171  are input to the digital circuit  175  in the electrical circuit  172  through the electric wiring lines  151  or  152  and converted into pulses. The Z-phase signal output from the I/V converting circuit  177  in the electrical circuit  172  is also input to the digital circuit  175  and pulsed.  
      According to this embodiment, the electrical circuits to be formed on the two semiconductor substrates  141  and  142  are arbitrarily distributed between the empty regions of the two semiconductor substrates  141  and  142  considering various respects, e.g., the functions, roles, and circuit feature sizes of the electrical circuits, and whether or not each circuit affects the other electrical circuits as a noise source. With this arrangement, the electrical circuits to be mounted on the semiconductor substrates can be optimized, and the qualities of the electrical circuits can be improved. More specifically, the I/V converting circuit  173  outputs the moving amount detection signal that is readily susceptible to the noise from the digital circuit  175  and interpolator  176 . The I/V converting circuit  173  is formed on the semiconductor substrate  141  that is different from the semiconductor substrate  142  that has the digital circuit  175  and interpolator  176 . This can minimize the influence on the I/V converting circuit  173 . Thus, a circuit configuration that is not influenced by noise as a whole can be obtained.  
      The electrical circuits on the two semiconductor substrates  141  and  142  are connected in the package to decrease the number of terminals that serve for connection with the external circuit, e.g., a power supply.  
      Since the electrical circuits  171  and  172  on the semiconductor substrates  141  and  142  are electrically connected to each other through the electric wiring lines  151  and  152 , the electrical circuits  171  and  172  can share a reference current generation circuit and reference voltage generation circuit. That is, the electrical circuits  171  and  172  can share a reference signal (e.g., a reference voltage or reference current). Therefore, the circuit feature size can be decreased. One reference voltage and one reference current can be shared by circuits so that stable circuit operation can be expected.  
      From the above description, downsizing, cost reduction, and high performance of the reflective optical encoder are enabled.  
      According to this embodiment, the sensor head  120  has the two semiconductor substrates  141  and  142 . The sensor head  120  may further comprise another semiconductor substrate that has an electrical circuit. The electrical circuit on this additional semiconductor substrate may be electrically connected to the electrical circuits  171  and  172  on the semiconductor substrates  141  and  142 .  
      According to this embodiment, the electric wiring lines  151  and  152  that electrically connect the electrical circuits  171  and  172  on the semiconductor substrates  141  and  142  are wiring patterns formed on the surface of the lid member  190 , as shown in  FIG. 3 . The electric wiring lines  151  and  152  may be of any type as far as they can be connected in the package. For example, as shown in  FIG. 7 , desired terminals of the electrical circuits  171  and  172  may be electrically connected to each other by using electric wiring lines  153  formed on the lid member  190  and electric wiring lines  154  formed on the housing  180 . Furthermore, the electric wiring lines  154  may be formed in the housing  180 .  
      According to this embodiment, the light beam that is projected from the light source  121  and then directly reflected by the scale  110  is detected. To further improve the performance, a so-called Talbot image may be employed.  
      A Talbot image will be described. For the sake of simplicity, the following description will be made on the assumption of a transmissive encoder. However, the same argument holds for a reflective encoder.  
       FIG. 8  shows the concept of the basic arrangement of a transmissive optical encoder that uses a Talbot image. Referring to  FIG. 8 , assume that z1 is the distance between the light source  121  and the movement detection pattern  111  of the scale  110 , z2 is the distance between the movement detection pattern  111  of the scale  110  and the moving amount detection photodetector  131 , p1 is the pitch of the scale pattern, and p2 is the pitch of a Talbot image projected onto the light-receiving surface of the moving amount detection photodetector  131 .  
      When z1 and z2 satisfy the relation represented by the following equation (1), a bright and dark pattern similar to the scale pattern is known to be projected on the moving amount detection photodetector  131 . The bright and dark pattern is called the Talbot image. 
 
(1 /z 1)+(1 /z 2)=λ/( k ( p 1) 2 )  (1) 
 
 where λ is the wavelength of the light beam projected from the light source  121 , and k is an integer. 
 
      To form a Talbot image, the light source  121  must substantially be a point light source. As a substantial point light source, an edge emitting laser, surface-emitting laser, current confinement type LED, or the like can be used.  
      As shown in  FIG. 9 , the light source  121  may comprise an ordinary LED  122  in place of a substantial point light source. In this case, a transparent plate  123  having a slit is arranged on the optical path along which a light beam projected from the LED  122  is directed to the scale  110 . The slit may have a single aperture, or apertures that are periodically arranged at a predetermined period. With this arrangement, even with an ordinary LED having a short coherence length, a bright and dark pattern similar to a scale pattern, like a Talbot image, can be obtained. The slit may be formed, with the same method as that for the electric wiring lines, on that portion of the lid member  190  which is located on the optical path along which the light beam projected from the LED  122  is directed to the scale.  
      The pitch of the Talbot image projected onto the moving amount detection photodetector  131  can be calculated by the following equation (2): 
 
 p 2= p 1×( z 1+ z 2)/ z 1  (2) 
 
      By using the Talbot image, a high-performance optical encoder can be formed with a comparatively simple arrangement.  
     Second Embodiment  
       FIG. 10  is a perspective view of a sensor head in a reflective optical encoder according to the second embodiment of the present invention.  FIG. 11  is a sectional view of the reflective optical encoder including the sensor head shown in  FIG. 10 . The section of  FIG. 11  corresponds to the section of  FIG. 2 .  FIG. 12  is a plan view, seen from the scale side, of the sensor head shown in  FIG. 10 . Referring to  FIGS. 10, 11 , and  12 , members indicated by the same reference numerals as those shown in  FIGS. 1 and 2  are similar members, and a detailed description thereof will be omitted.  
      The basic arrangement and operation of the reflective optical encoder of the second embodiment are similar to those of the reflective optical encoder of the first embodiment. A description will be made on portions that are different from their equivalents in the first embodiment.  
      As shown in  FIGS. 10 and 11 , a light source  121  and semiconductor substrates  141  and  142  are all fixed to a housing  180 . The electrodes on the semiconductor substrates  141  and  142  are electrically connected to the electrodes on the housing  180  through bonding wires  155 .  
      The housing  180  has a cavity to attach the light source  121 , and rest-shaped portions to attach the semiconductor substrates  141  and  142 . As shown in  FIG. 11 , the light source  121 , a moving amount detection photodetector  131 , and a reference position detection photodetector  132  are arranged so that the upper end of the light source  121  and the upper surfaces of the photodetectors  131  and  132  are substantially leveled with each other.  
      As shown in  FIG. 12 , two electrical circuits  171  and  172  are electrically connected to each other through bonding wires  156 . In other words, the electric wiring lines that connect the two electrical circuits  171  and  172  comprise the bonding wires  156 .  
      With this arrangement, a compact, high-performance encoder can be provided technically easily.  
      According to this embodiment, the electric wiring lines that connect the two electrical circuits  171  and  172  are the bonding wires  156 . Alternatively, these electric wiring lines may be electric wiring lines formed on the surface of or in the housing  180 .  
     Third Embodiment  
       FIG. 13  is a sectional view of a sensor head in a reflective optical encoder according to the third embodiment of the present invention.  FIG. 14  is a plan view, seen from the scale side, of the sensor head shown in  FIG. 13 . The section of  FIG. 13  corresponds to the section of  FIG. 2 . Referring to  FIGS. 13 and 14 , members indicated by the same reference numerals as those shown in  FIGS. 1 and 2  are similar members, and a detailed description thereof will be omitted.  
      The basic arrangement and operation of the reflective optical encoder of the third embodiment are similar to those of the reflective optical encoder of the second embodiment. A description will be made on portions that are different from their equivalents in the second embodiment.  
      As shown in  FIG. 13 , the sensor head of this embodiment has a transparent member  160 . The transparent member  160  is located between a light source  121  and a lid member  190 , and its two ends are respectively supported by semiconductor substrates  141  and  142 .  
      As shown in  FIG. 14 , the transparent member  160  comprises electric wiring lines  151  on its surface that faces the semiconductor substrates  141  and  142 . The electric wiring lines  151  connect electrical circuits  171  and  172  to each other.  
      With this arrangement, wire bonding need not be performed across the two semiconductor substrates  141  and  142 , and accordingly the electric wiring lines can be easily formed and connected simultaneously.  
      In this embodiment, a slit may be formed in the transparent member  160 , and detection using a Talbot image, which is described in the first embodiment, can be performed.  
       FIG. 15  is a plan view, seen from the scale side, of a portion of a sensor head using a Talbot image, as a modification of the third embodiment, including the transparent member  160  and semiconductor substrate  141  and  142 . According to this modification, slit apertures  161  and  162  and a light-shielding portion  163  are formed in the transparent member  160 . The slit apertures  161  and  162  are located on an optical path along which a light beam projected from the light source  121  is directed to the scale  110 . No light-shielding portions but apertures are formed above the moving amount detection photodetector  131  and reference position detection photodetector  132 .  
      With this arrangement, a Talbot image can be formed and detected easily. Therefore, a compact, low-cost, and higher-performance reflective optical encoder can be formed.  
      In all the embodiments described above, the photodetectors on the semiconductor substrates  141  and  142  are respectively a moving amount detection photodetector and reference position detection photodetector. Alternatively, the both photodetectors may be moving amount detection photodetectors, or photodetectors to detect periodic patterns having different pitches as in, e.g., a vernier absolute encoder.  
      In all the embodiments described above, by using an LED as the light source  121 , both the requirements of low cost and high performance can be met. By using an RCLED (Resonant Cavity LED) or SLD (Super Luminescent Diode) as the light source  121 , due to its further excellent coherency, the characteristics of the encoder can be further improved. In addition, by using various types of light sources such as a surface-emitting laser, edge emitting laser, current confinement type LED, or the like, an encoder can be formed in accordance with a purpose.  
      Although the embodiments of the present invention have been descried with reference to the views of the accompanying drawing, the present invention is not limited to these embodiments and may be variously modified or changed within the spirit and scope of the invention.  
      Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.