Abstract:
A flexible optical marker is applied to an optical scale substrate to make an optical scale assembly for an optical position encoder. The marker may be a limit marker, index marker, or other type of marker. The marker substrate may be a plastic film such as polyester, singulated from a “recombine” roll created by a web process. The marker has a microstructured pattern on one surface that is covered with a reflective metal coating. The marker also has an adhesive layer and is affixed to the optical scale substrate by a process of aligning the marker to an edge of the scale and then applying pressure to the upper surface of the marker. The marker may be applied with a handle portion that is separated from the marker after the marker is affixed. The marker may be especially useful with a flexible scale substrate such as a metal tape substrate. By affixing the marker to the scale substrate as a separate step of making an encoder scale, various benefits such as reduced inventory, cost, and lead time may be achieved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Application No. 60/734,999 filed Nov. 9, 2005, the disclosure of which is hereby incorporated by reference herein. 
    
    
     BACKGROUND 
     In the field of optical position encoders, for example incremental optical position encoders, it is known to include markers on or with an optical scale that is used to derive fine-grained position information. The markers may include, for example, an index marker that establishes a “home” or index position of the encoder from which all position measurements or indications are referenced. So-called “limit” markers have also been used, especially in linear position encoders, to provide an indication when the encoder has reached a limit of its travel. Other types of markers have also been employed. 
     There are alternative techniques for incorporating a marker with an optical scale. In a typical approach, a marker is simply a special feature of the scale itself, and is formed at the same time and by the same process by which the regular scale features (e.g., regularly spaced lines) are formed. The marker may be a set of separate elements apart from the scale marks, or they may be specially formed scale marks, such as a set of relatively longer or shorter marks or some other modification of the basic mark structure. In all these cases, the marker is an integral part of the scale itself, and thus the scale as manufactured has a predetermined arrangement or marks. This makes such scales to some extent “custom”—they are designed with particular applications in mind that utilize certain specific arrangements of markers. If a scale vendor is to sell products to many customers having a variety of applications, it is necessary for the vendor to manufacture and stock a corresponding variety of scale types, which can contribute to increased cost for engineering, manufacturing, and customer support. 
     Alternatively, it has been known to manufacture scales with multiple markers formed at the same time as the scale at predetermined locations, and then for a customer/user to choose which of the markers matches his/her application and then selectively remove those that are not needed. The unused markers are either physically covered or removed. It is noted that this method is not fully-customizable, as a customer&#39;s choices for marker position are limited to those predetermined locations selected by the scale manufacturer. 
     It is also known to create a scale assembly by incorporating separate markers onto scales that are unmarked as manufactured (i.e., scales that have no markers apart from the regularly spaced scale marks). For example, in one arrangement a magnetic element is placed immediately adjacent to an edge of a linear scale of a linear optical encoder. The element can be detected by a magnetic detector that is co-located with the optical detector that reads the regular optical scale marks. Other approaches using markers entirely separate from the optical scale are also known. 
     SUMMARY 
     There are benefits to adding markers as a customization to an optical scale rather than incorporating them into the design and manufacture of the scale itself. The scale can be manufactured in larger volumes, because its non-customized nature enables its use in a variety of applications. The customization occurs as a separate step of adding the markers as dictated by the application. The benefits can include reduced cost and greater flexibility in the application of optical encoder technology. However, known techniques for adding separate markers may have significant drawbacks. For example, the above-mentioned technique of utilizing a separate magnetic element requires a generally more complex encoder design, as it must incorporate different sensing technologies (optical and magnetic). Also, magnetic position sensing generally has much lower precision than optical position sensing, and thus the use of a magnetic marker may compromise the performance of the overall position encoder. 
     In accordance with the present invention, an optical marker is disclosed that can be added as a customization to a scale of an optical encoder, along with techniques for manufacturing the optical marker and customized scale assembly. The marker may include a flexible substrate material such as plastic for relatively low cost and relatively easy application. The marker may be used in a variety of applications including with a flexible scale substrate such as an elongated metal tape. 
     A disclosed method of making a scale assembly for an optical encoder includes the step of obtaining a pre-made optical scale substrate which has a pattern of optical scale marks that are operative, in conjunction with a light source in the encoder, to produce an optical intensity distribution to be sensed by detector circuitry in the optical encoder. The pattern of scale marks may be a set of equally spaced linear marks extending crosswise on the scale substrate for example, although other types of patterns may be used. Additionally, a pre-made optical marker is obtained, which has a substrate on which a marker pattern is formed that works in conjunction with the encoder light source to produce an optical intensity distribution that is sensed by the detector circuitry. The marker substrate may be a flexible material such as polyester or polycarbonate. A rigid material such as glass or reflective metal may be used as a marker substrate in alternative embodiments. The flexible optical marker may have advantages over rigid markers including lower cost, ease of application, and wider potential use due to its ability to conform to non-planar surfaces. 
     The pre-made optical marker is applied to the pre-made optical scale at a predetermined location as determined by the application of the encoder. The marker is placed such that the resulting optical intensity distribution is detectable by the detector circuitry of the encoder as an indicator of a predefined relative position between the scale and the detector. Examples of such markers and positions include limit markers that may be placed at the left and right limits of travel of a linear scale, and an index marker placed at a predetermined location that is defined as the reference point for all displacement indications from the encoder, such as at the center of a linear scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages of the invention will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. 
         FIG. 1  is a simplified diagram of a linear optical position encoder including a scale assembly in accordance with the present invention; 
         FIG. 2  is a simplified side view of an encoder read head in the optical position encoder of  FIG. 1 ; 
         FIG. 3  is a diagram of the scale assembly of  FIG. 1  showing affixed optical reference markers in accordance with the present invention; 
         FIG. 4  is a plan view of a section of a roll of layered “recombine” from which the optical reference markers are taken during production of the scale assembly of  FIG. 3 ; 
         FIG. 5  is a plan view of an intermediate workpiece consisting of a reference marker and a handle portion as taken from the recombine of  FIG. 4  during production of the scale assembly of  FIG. 3 ; 
         FIG. 6  is a section view illustrating the layers of the recombine of  FIG. 4 ; 
         FIG. 7  is a plan view of one of the reference markers of  FIG. 3  serving as an index marker; 
         FIG. 8  is a plan view of another of the reference markers of  FIG. 3  serving as a limit marker; 
         FIG. 9  is a schematic side view of an index pattern on the index marker of  FIG. 6 ; 
         FIG. 10  is a flow diagram of a process of producing the scale assembly of  FIG. 3 ; 
         FIGS. 11-13  are diagrams illustrating a process of applying an individual marker in the scale production process of  FIG. 10 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows a linear optical position encoder, which consists of a scale assembly  10  and a source/detector assembly  13 . Linear optical encoders are used in applications in which it is necessary to know the precise relative position between two items that experience relative linear motion. The scale assembly  10  is mounted or secured to one of the items, and the source/detector assembly  13  to the other. As an example, the scale assembly  10  may be mounted to a relatively stationary frame along which a tool or other object travels in a manufacturing or assembly process. The source/detector assembly  13  is mounted to the tool or other object. As generally known in the art, the scale assembly  10  includes a scale pattern such as a series of relatively finely spaced marks extending crosswise, i.e., perpendicular to one of the long edges of the scale assembly  10 . In operation, the scale pattern is irradiated with light from a light source such as a laser diode in the source/detector assembly  13 , and the pattern of light reflected or transmitted by the scale pattern is detected by optical detector circuitry within the source/detector assembly  13 . As the scale assembly  10  moves lengthwise with respect to the light source and detector, the pattern of light incident on the detector changes correspondingly. The detector circuitry responds to the changing pattern to provide an indication of the relative motion between the scale assembly  10  and the source/detector assembly  13 . Also shown in  FIG. 1  are markers referred to generally at  11 ; these are described in more detail below. 
       FIG. 2  shows a simplified side view of the source/detector assembly  13 . In the illustrated embodiment, it includes two separate sets  15 - 1  and  15 - 2  of source/detector circuits, each set including a respective light source  17  ( 17 - 1 ,  17 - 2 ) and detector  19  ( 19 - 1 ,  19 - 2 , also collectively referred to as “detector circuitry”). For each set  15 , the respective source  17  illuminates a corresponding part of the scale assembly  10 , and the reflected light pattern is detected by the detector  19  of the same set  15 . In the illustrated embodiment, one set  15  (e.g.,  15 - 2 ) operates in conjunction with a scale pattern on the scale assembly  10  to detect incremental relative motion with high precision. The other set  15  (e.g.,  15 - 1 ) detects reference markers that are used to indicate limits of travel and to establish a reference or index for absolute position indication, as described in more detail below. 
       FIG. 3  shows a segmented view of the scale assembly  10 . A left end portion  10 - 1 , center portion  10 - 2  and right-end portion  10 - 3  are shown. In the illustrated embodiment, the scale assembly  10  has a substrate of a strong, flexible material, including for example a nickel alloy material such as known by the trade names Inconel® and Invar®. The scale assembly  10  has a scale pattern  12 , such as a diffraction grating, formed along its length. 
     The scale assembly  10  also includes three reference markers, including a left limit marker  14 , an index marker  16 , and a right limit marker  18 . The left and right limit markers  14 ,  18  provide respective areas of different reflectivity from the background reflectivity of the scale substrate. In the illustrated embodiment, the limit markers  14  and  18  operate by diffusely reflecting an incident light beam into a reflected cone of light. A particular optical power at the detector is achieved by selecting a corresponding particular cone angle, as described in more detail below. The index marker  16  has an index pattern  20 , formed as binary phase pattern, that acts as a cylindrical lens, focusing an incident light beam into a line intensity pattern at a corresponding index optical detector. The relative position at which the line intensity pattern is incident upon the index optical detector is denoted the “index position” of an incremental optical encoder. All other relative positions are identified as incrementally measured displacements from the index position. 
     It is noted that in the present description the terms “scale” may be used to refer to the underlying substrate with the scale pattern  12  formed thereon, prior to affixation of the markers  14 ,  16  and  18 . In the case of a flexible scale substrate, the term “tape” or “tape scale” may alternatively be employed. When the markers are affixed, such as shown in  FIG. 3 , the term “scale assembly” is used. 
       FIG. 4  shows a section of an article referred to as a “recombine”  22 , which is a multi-layered, elongated sheet from which individual workpieces including the markers  14 ,  16  and  18  of  FIG. 3  can be separated or “singulated”. The layer structure of the recombine  22  is shown below. The recombine  22  is typically manufactured into a roll in a web process. The recombine  22  has repeated sets  24  of various elements that are utilized in assembling the scale assembly  10 . A roll can be utilized in a process of assembling a number of scales  10  in a batch fashion. Alternatively, sections having one or more sets  24  can be separated from the roll and provided to customers as part of a kit from which the customer assembles one or more custom optical encoder scales. The customer can use the items from each set  24  to affix the markers  14 ,  16  and  18  to the customer&#39;s scale, as described in more detail below. 
     In particular, each set  24  includes a left limit marker workpiece  26 , an index marker workpiece  28 , and a right limit marker workpiece  30 . The workpieces  26 ,  28  and  30  correspond to different areas of the recombine  22 , specifically different microstructured patterns as described below. They also become physically separated from surrounding areas of the recombine  22  by die cutting as part of the web process. A set of regularly spaced registration fiducials  32  serve to facilitate aligning the die cutting apparatus with the recombine for accurate cutting. 
       FIG. 5  shows the shape and structure of the workpieces  26 ,  28  and  30  of  FIG. 4 . Each workpiece includes a marker portion  48  and a handle portion  50 . The marker portion  48  eventually becomes the respective marker  14 ,  16  or  18 . The marker portion  48  and handle portion  50  are separated by an area of perforation or one or more break-away tabs  52  to facilitate separating the handle portion  50  during assembly as described below. 
       FIG. 6  illustrates the layered construction of the recombine  22 . It includes a substrate  34  of an optically clear plastic material such as polyester or polycarbonate, one surface (bottom) of which has been patterned with an optical surface relief or “microstructured” pattern  36 . A reflective metal coating  38  is deposited on the patterned surface of the substrate  34 , and a pressure-sensitive adhesive  40  is disposed on the metal coating  38 . Not shown is a removable backing that covers the adhesive layer  40  and is removed from each individual marker prior to being affixed to the scale. On the other (top) surface of the substrate  34  is a protective film  42  to protect against contamination during manufacture of the recombine  22  and assembly of the scale assembly  10 . It is preferred that the protective film  42  have some degree of opacity, color, or other characteristic to enable an assembler to easily discern its presence so as to remove it at the appropriate assembly step. A typical thickness for the recombine  22  may be approximately 0.40 mm. 
     In the illustrated embodiment, the pattern  36  and metallization  38  are oriented on the bottom of the substrate  34  so that the polyester substrate material can act as a protective layer during operation. In this manner, both the pattern  36  and the reflectivity of metallization  38  should be more robust and less susceptible to damage. In alternative embodiments, it may be beneficial to locate the pattern  36  and metallization on the top surface of substrate  34 . 
     The microstructured pattern  36  includes individual pattern areas corresponding to the different workpieces  26 ,  28  and  30 , which are formed by UV-photopolymer casting or embossing against a die disposed on a cylindrical roller over which the substrate  34  passes in the web process. The pattern areas for the left and right limit markers  14  and  18  are so-called “tailored microdiffuser” (TMD™) patterns that provide the above-described cone-shaped light reflection pattern, although other diffusing surfaces may be used. The left limit marker  14  may have a cone angle of approximately 8 degrees in order to provide the desired 50% optical power at the detector, and the right limit marker  18  may have a cone angle of approximately 80 degrees in order to provide the desired 10% optical power at the detector. These patterns fill the entire bottom surface of the respective limit marker  14  and  18 . The pattern area for the index marker  16  has an index pattern that is described in more detail below. It will be appreciated that the microstructured pattern  36  may be metallized to create a correspondingly patterned reflective metal surface. Alternatively, the pattern may be left non-metallized for use in a transmissive optical encoder system, with the detector disposed on the opposite side of the scale from the light source. 
       FIG. 7  shows the index marker  16  with index pattern  20 . In the illustrated embodiment the index marker  16  has a rectangular shape elongated in the lengthwise direction of the scale assembly  10 . The index pattern  20  occupies only the center area  44  of the index marker  16 ; the outer areas  46  simply reflect incident light as planar mirrors, and therefore are not active components in the operation of the encoder. The outer areas  46  do serve a function during assembly, however, as described in more detail below. In one embodiment, a typical size for the index marker  16  is 20.0 mm by 2.0 mm. 
       FIG. 8  is a generalized view of the limit markers  14 ,  18 . In the illustrated embodiment, these are of generally the same shape and size as the index marker  16 . A pattern of dots represents the tailored microdiffuser pattern  36  of these markers  14 ,  18 . 
     One long edge of both the index marker  16  and the limit markers  14 ,  18  is termed a “reference edge” as shown. During a process of affixing the marker  14 ,  16 ,  18  to the scale, the reference edge is placed against a corresponding edge of an assembly tool so as to be aligned with a corresponding edge of the scale. This process is described below. 
       FIG. 9  shows the detailed profile of the microstructured pattern  36  for the index marker  16  in particular. The pattern consists of a set of grooves or depressions  54  having a rectangular cross-section. A central depression  54 - 1  is the widest, and successively outer depressions  54 - 2  through  54 - 4  are successively narrower. The specific dimensions (depth, width and separation) are chosen in conjunction with other parameters of the optical encoder (such as the spacing between the scale and the source/detector, the wavelength of the light emitted by the light source, the refractive index of the marker substrate, etc.) to yield the desired behavior of a cylindrical Fresnel zone lens as previously described. Techniques for arriving at a specific set of dimensions are generally known in the art. 
       FIG. 10  shows a process by which the scale assembly  10  is produced. In step  56 , a pre-made optical scale substrate is obtained which has a pattern of optical scale marks that are operative, in conjunction with the light source in the encoder, to produce an optical intensity distribution to be sensed by detector circuitry in the optical encoder. As mentioned above, the pattern of scale marks is typically a set of equally spaced linear marks extending crosswise on the scale substrate, although other types of patterns may be used. 
     In step  58 , a pre-made optical marker is obtained, which has a substrate on which a marker pattern is formed that works in conjunction with the encoder light source to produce an optical intensity distribution that is sensed by the detector circuitry. As described above with reference to  FIG. 2 , the detector circuitry typically includes separate optical detectors (e.g.  19 - 1  and  19 - 2 ) for the scale optical intensity distribution and the marker optical intensity distribution. Separate light sources (e.g.  17 - 1  and  17 - 2 ) may also be employed. The marker substrate may be a flexible material such as the above-described polyester substrate  34  used for the markers  14 ,  16  and  18 . A rigid material such as glass or reflective metal may be used as a marker substrate in alternative embodiments. The flexible optical marker has several advantages over rigid markers including lower cost, ease of application, and wider potential use. A flexible marker can readily conform to a non-planar scale. In particular, a flexible marker may be especially useful with a flexible scale such as the tape scale described above. If an application calls for a flexible scale to accommodate some non-planarity of the underlying item to which the scale is to be applied, then such an application may benefit from a flexible marker as well. As an example, a linear encoder can be used to measure rotation of a cylindrical object by wrapping a flexible linear scale around the object and placing the source/detector assembly opposite the scale in an orientation to detect a tangential motion. In such an encoder it may be beneficial to use a flexible index marker to conform to the cylindrical tape scale. 
     Referring again to  FIG. 10 , in step  60  the pre-made optical marker is applied to the pre-made optical scale at a predetermined location as determined by the application of the encoder. The marker is placed such that it is detectable by the detector circuitry of the encoder as an indicator of a predefined relative position between the scale and the detector. Examples are given above—i.e., limit markers  14  and  18  placed at the left and right limits of travel, and an index marker  16  placed at a location that is defined as the reference point for all displacement indications from the encoder, such as at the center of a linear scale. 
       FIGS. 11-13  illustrate the process of  FIG. 10  in conjunction with the specific markers  14 ,  16  and  18  and workpieces  26 ,  28 , and  30  described above. In  FIG. 11 , a pre-made scale  61  is placed with its reference edge against a flat vertical surface  62  of a rigid, L-shaped tool referred to as a workbench  64 . A workpiece  26 ,  28  or  30  with exposed adhesive layer  40  is placed at an angle such that its reference edge is butted against the surface  62  and rests against the reference edge of the scale  61 . As shown in  FIG. 12 , the workpiece  26 ,  28  or  30  is then rotated downward such that the marker portion  48  lies flat on the scale  61 , while the handle portion  50  is held slightly upwardly. At this point, pressure is applied to the marker portion  48  to hold it in place while the handle portion  50  is torn away (upwardly) along the perforations  52 . The result is as shown in FIG.  13 —the marker portion  48  is now adhered to the scale  61  at the desired location, with its reference edge aligned with the reference edge of the scale  61 . 
     The above illustrates the use of the outer areas  46  of the index marker  16 . A tool or an assembler&#39;s finger can be pressed against the outer area  46  to apply the hold-down pressure without having to touch the central area  44  in which the index pattern  20  is located, thus reducing the risk of contaminating or mechanically distorting the index pattern  20 . 
     Although the above description is specific to a linear scale, it will be appreciated that the presently disclosed techniques may be used with rotary scales and rotary encoders as well. The scale substrate may be flexible or rigid as mentioned above. 
     Depending on the size of the marker and the tolerance of its position on the scale, it may be installed by hand or it may be preferable to utilize a tool for greater control and/or precision. A tool with a mechanical grasping action, similar to that of a pair of tweezers, may be desirable. It may be desirable to incorporate some kind of referencing feature on the tool to enhance the accuracy of the placement of the marker. 
     It may also be desirable to incorporate additional marks or patterns on the markers that may be used to ascertain the degree of alignment of the marker, especially that of an index marker for example. The alignment information can be used to track the quality of the assembly process and also perhaps to provide corrections in an operating system to compensate for a known amount of mis-alignment. 
     Although in the above description the microstructured pattern layer  36  is formed using UV-photopolymer casting or embossing in a roll process, in alternative embodiments the patterns may be formed on a marker substrate using alternative methods such as molding, etching, laser processing, or photolithographic processing for example. Either or both of phase and/or amplitude features can be utilized. The markers may also be formed from polarizing films with differing reflection or transmission properties. The use of an adhesive layer and/or protective film are optional and may be dispensed with in alternative embodiments, although clearly there still is a requirement for a separate adhesive to affix the marker to the scale. Also, an adhesive layer if used may be any of several types, including for example pressure sensitive adhesive, epoxy, or UV-curable adhesive. 
     While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.