Patent Publication Number: US-2022216090-A1

Title: Suction gripper for warped workpiece

Description:
FIELD OF THE INVENTION 
     The present invention relates to wafer processing. More particularly, the present invention relates to a suction gripper adapted to grip a warped workpiece. 
     BACKGROUND OF THE INVENTION 
     Many applications require the gripping and manipulation of flat, thin workpieces. For example, in the manufacture of semiconductor devices, such as integrated circuits and other electronic components, a semiconducting substrate, typically in the form of a wafer, must be manipulated in order to apply a processing step to the substrate, or to inspect the substrate. An arm that grips the substrate may be manipulated to transport the substrate to a required location for processing or inspection. 
     A typical gripping arm for gripping a semiconducting substrate may apply suction in order to hold that substrate to the arm. Typically, the suction needs to be applied at three different points on the surface of a flat substrate in order that the arm may firmly and stably grip the substrate during such manipulations as translation, rotation, or tilting of the substrate. 
     SUMMARY OF THE INVENTION 
     There is thus provided, in accordance with an embodiment of the invention, a gripper to grip a workpiece, the gripper including: a body with a flat gripper surface, the gripper surface including a plurality of openings that are distributed over the gripper surface, each of the openings connectable to a source of suction; and at least one flow restrictor located between the plurality of openings and a connector of the gripper to the source of suction to restrict the inflow through each opening of the plurality of openings, such that when the suction is applied to the plurality of openings and a part of the workpiece surface covers at least one opening of the plurality of openings and another of the openings remains uncovered, the suction force at the covered opening is sufficiently strong to grip the workpiece surface. 
     Furthermore, in accordance with an embodiment of the invention, the body includes a plurality of arms that extend from a single base. 
     Furthermore, in accordance with an embodiment of the invention, a conduit from the connector to an opening of the plurality of openings extends along an arm of the plurality of arms. 
     Furthermore, in accordance with an embodiment of the invention, the gripper surface is contiguous. 
     Furthermore, in accordance with an embodiment of the invention, the at least one flow restrictor is located between an opening of the plurality of openings and a conduit that forms a fluid connection between that opening and the connector. 
     Furthermore, in accordance with an embodiment of the invention, the flow restrictor includes a plurality of baffles. 
     Furthermore, in accordance with an embodiment of the invention, the flow restrictor includes a self adaptive segmented orifice (SASO) with internally mounted staggered opposing fins. 
     Furthermore, in accordance with an embodiment of the invention, the flow restrictor includes a constriction. 
     Furthermore, in accordance with an embodiment of the invention, the flow restrictor includes a needle valve. 
     Furthermore, in accordance with an embodiment of the invention, an opening of the plurality of openings includes elastic sealing structure that is configured to form a seal between that opening and the workpiece surface when the suction is applied to that opening when the workpiece surface is in contact with the elastic sealing structure. 
     Furthermore, in accordance with an embodiment of the invention, the elastic sealing structure includes an elastic cup. 
     Furthermore, in accordance with an embodiment of the invention, the elastic sealing structure includes a pad or a gasket. 
     Furthermore, in accordance with an embodiment of the invention, the elastic sealing structure includes a polymer. 
     Furthermore, in accordance with an embodiment of the invention, the polymer includes silicone. 
     Furthermore, in accordance with an embodiment of the invention, the elastic sealing structure is connectible to the gripper surface via a stem that is insertable into the opening of the plurality of openings. 
     Furthermore, in accordance with an embodiment of the invention, the stem is in the form of a cylinder that is laterally truncated to form a flat lateral face. 
     Furthermore, in accordance with an embodiment of the invention, the at least one flow restrictor is located in a space between the lateral face and a cylindrical surface that surrounds the stem. 
     Furthermore, in accordance with an embodiment of the invention, an opening of the plurality of openings includes an extension mechanism to extend that opening outward from the gripper surface toward the workpiece surface. 
     Furthermore, in accordance with an embodiment of the invention, the extension mechanism is configured to extend the opening outward when the opening is not in contact with the workpiece surface. 
     Furthermore, in accordance with an embodiment of the invention, the extension mechanism is configured to enable a suction force to draw the opening toward the gripper surface when the suction is applied to the opening and the opening is in contact with the surface of the workpiece. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order for the present invention to be better understood and for its practical applications to be appreciated, the following Figures are provided and referenced hereafter. It should be noted that the Figures are given as examples only and in no way limit the scope of the invention. Like components are denoted by like reference numerals. 
         FIG. 1  is a schematic top view of a workpiece gripper with arms configured to grip a warped workpiece, in accordance with an embodiment of the present invention. 
         FIG. 2  is a schematic top view of a variant of the workpiece gripper shown in  FIG. 1  with a contiguous gripper surface. 
         FIG. 3  is a schematic cross section through a suction gripper of the workpiece gripper shown in  FIG. 1 . 
         FIG. 4A  schematically illustrates a variant of the suction gripper shown in  FIG. 3 . 
         FIG. 4B  is a schematic section of the suction gripper shown in  FIG. 4A . 
         FIG. 5  is a schematic side view of a variant of a suction gripper in which the suction gripper is extendible from the workpiece gripper. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those of ordinary skill in the art that the invention may be practiced without these specific details. In other instances, well-known methods, procedures, components, modules, units and/or circuits have not been described in detail so as not to obscure the invention. 
     Although embodiments of the invention are not limited in this regard, discussions utilizing terms such as, for example, “processing,” “computing,” “calculating,” “determining,” “establishing”, “analyzing”, “checking”, or the like, may refer to operation(s) and/or process(es) of a computer, a computing platform, a computing system, or other electronic computing device, that manipulates and/or transforms data represented as physical (e.g., electronic) quantities within the computer&#39;s registers and/or memories into other data similarly represented as physical quantities within the computer&#39;s registers and/or memories or other information non-transitory storage medium (e.g., a memory) that may store instructions to perform operations and/or processes. Although embodiments of the invention are not limited in this regard, the terms “plurality” and “a plurality” as used herein may include, for example, “multiple” or “two or more”. The terms “plurality” or “a plurality” may be used throughout the specification to describe two or more components, devices, elements, units, parameters, or the like. Unless explicitly stated, the method embodiments described herein are not constrained to a particular order or sequence. Additionally, some of the described method embodiments or elements thereof can occur or be performed simultaneously, at the same point in time, or concurrently. Unless otherwise indicated, the conjunction “or” as used herein is to be understood as inclusive (any or all of the stated options). 
     In accordance with an embodiment of the invention, a gripper for gripping a workpiece includes a gripper body with a flat gripper surface on which are distributed a plurality of suction openings. In some examples, the gripper body may be divided into one or more separate arms, e.g., that extend or branch off from a single base, or otherwise. 
     Each of the suction openings is connectable to a suction source via a conduit. For example, the suction source may include a blower, vacuum pump, or other source of suction. Each suction opening may be connected to the suction source via one or more connectors. When suction is provided to a suction opening that is in contact with a surface of the workpiece, the suction force may hold the workpiece to that suction opening. 
     One or more flow restrictors are located in a fluidic path between each of the suction openings and the suction source. The flow restrictors are configured to limit the rate of inflow through each suction opening, e.g., when that suction opening is not in contact with the surface of the workpiece. Due to action of the flow restrictors, the suction openings on the gripping surface that are in contact with the workpiece surface may firmly grip a nonplanar surface of a warped workpiece, even when some of the openings are not in contact with the workpiece surface (e.g., due to the warpage or tilting of the workpiece surface). A typical workpiece may include a thin and nominally flat semiconductor substrate or wafer, a pane of glass, or another type of workpiece that is ideally flat but that may become warped, e.g., during one or more processing steps. 
     In some examples, a sufficient number of the openings may be distributed over the flat gripping surface such that it is likely that at least two of the openings (preferably at least three) are able to draw a part of the workpiece surface to the opening so as to hold that part of the workpiece surface to the arm at the opening (e.g., as a result of the suction and the generated friction between the workpiece surface and the gripping surface of the arm). Alternatively or in additions, one or more of the suction openings may be provided with an extension mechanism that is configured to extend the suction opening from the gripper surface and toward the workpiece surface. This extension may ensure that a sufficient number of the openings come into contact with, and thus grip, a warped workpiece surface. 
     When the workpiece surface is warped, at least some of the suction openings may remain uncovered by the workpiece surface. The flow restrictors of each suction opening may restrict the suction such that the inflow of air through the uncovered suction openings does not break the suction that is applied by the covered surfaces. For example, the suction may create a vacuum that is sufficiently deep to reliably grip the workpiece. Therefore, the gripper arm may continue to firmly grip the workpiece surface. Thus, manipulation of the gripper, e.g., to translate, rotate, or tilt the arm, may reliably manipulate the gripped workpiece. For example, when all the suction openings are sealed by the workpiece then the vacuum may reach a gauge pressure of −800 mbar. When all of the suction openings are uncovered by the workpiece, the vacuum may reach a gauge pressure of −100 mbar. In other examples, other vacuum levels may be attained. 
     It may be noted that in the absence of flow restrictors, the unrestricted inflow through the uncovered openings could result in a direct flow from the uncovered openings to the suction connector with little or no pressure drop. Thus, no suction would be applied at the points where the workpiece surface contacts some of the suction openings. 
     In one example, a suction conduit extends along the interior of a gripper arm from the suction connector to the location of each of the suction openings. An opening conduit between the suction conduit and each suction opening includes a flow restrictor. For example, the flow restrictor may include baffles, fins, constrictions, or other structure that restricts the rate of inflow through that suction opening. In some cases, the flow restrictor may include a self adaptive segmented orifice (SASO), which includes internally mounted staggered opposing fins that are configured to increase flow resistance by enhancing generation of vortices in the flow. The flow restrictors may include other types of flow-restricting structure and may be located elsewhere between the suction opening and a connection to the suction source. 
     Each suction opening may be provided with structure to enhance friction forces between the gripper arm and the workpiece. For example, each suction opening may be surrounded by elastic sealing structure in the form of a cup, pad, gasket, or nipple that is made of an elastic material. When the suction draws the workpiece surface toward the suction opening, the inwardly pulling force may flatten the elastic sealing structure against the surface of the gripper arm. The flattening of the elastic sealing structure against the arm surface may increasing the area of contact between the workpiece surface and the arm surface, thus increasing the friction force that resists sliding of the workpiece over the arm surface. The material of which the elastic sealing structure is made may be selected so as to provide the required friction, while minimizing the risk of contamination of the workpiece surface by adhesion of the elastic sealing structure material to the workpiece surface. For example, a suitable material may include silicone, or another polymeric or other suitable material. 
     Suction openings that are provided with flow restrictors may provide an advantageous tradeoff in the handling of warped workpieces. On the one hand, when a gripping arm or surface has a limited number of suction openings, sealing of a suction opening by contact between the warped workpiece and the suction opening cannot be ensured as the wafer may only contact portions of the gripper arm where suction openings are absent. Increasing the rate of inflow through the suction opening may draw a portion of the workpiece to the suction opening to create a seal. On the other hand, in order that the gripper arm may grip the workpiece with sufficient strength to enable reliable and precise manipulation of the workpiece, the level of vacuum at the point where the workpiece seals the suction opening must be sufficiently deep to provide the required frictional force. In order that a suction source be capable of providing the required deep vacuum to all of the suction openings, inflow through an unsealed suction opening must be limited. Thus, providing each suction opening with a flow restrictor to limit inflow through the suction opening may enable a suction opening that is sealed by contact with the warped workpiece to provide sufficient friction to hold the workpiece to the gripper arm. Thus, the workpiece may be securely and reliably maneuvered by the gripper arm when the workpiece does not seal some of the suction openings. 
       FIG. 1  is a schematic top view of a workpiece gripper configured to grip a warped workpiece, in accordance with an embodiment of the present invention. 
     Workpiece gripper  10  includes a gripper body that includes at least one flat gripper surface  13 . A plurality of suction grippers  14  are distributed on gripper surface  13 . 
     In the example shown, gripper body  11  includes two gripper arms  12  that extend outward from gripper base  20 . A plurality of suction grippers  14  are distributed along the length of each gripper arm  12 . Each suction gripper  14  includes a suction opening  16 . When a workpiece  15  is to be gripped by gripper arms  12  of workpiece gripper  10 , workpiece  15  may be placed so as to cover at some of suction grippers  14 . Suction that is applied to suction opening  16  of a suction gripper  14  when a region of the surface of workpiece  15  is in close proximity to that suction gripper  14  may draw that region toward suction opening  16  and gripper surface  13  to be gripped by suction gripper  14 . 
     In other examples, there may be more than two gripper arms  12 , or gripper surface  13  may include a contiguous surface instead of separate arms. The number of suction grippers  14  may be greater than or less than the number shown, and their distribution over each gripper arm  12  may be different than the pattern shown. The suction grippers may have different shapes, e.g., either symmetrical or asymmetrical, than the examples shown. In other examples, the suction opening of each suction gripper may not be at the center of the suction gripper. In another example, gripper body  11  may include two gripper surfaces  13 , e.g., on opposite sides of gripper body  11 . 
     In some cases, gripper arms  12  or suction grippers  14  may be arranged to enable workpiece gripper  10  to grip workpieces  15  of various sizes. For example, one set of suction grippers  14  may be arranged in an area that is the size of, or smaller than, a smaller workpiece. One or more other sets of suction grippers  14  may be arranged outside of the first set but within areas that correspond to one or more larger sizes of workpieces. For example, one or more additional gripper arms  12 , each with suction grippers  14 , may be arranged between the two gripper arms  12  of the example shown in order to enable gripper of a workpiece with a diameter or other dimension that is smaller than the distance between the two gripper arms  12  that are shown. 
     For example, a workpiece gripper may be configured to handle workpieces  15  of different sizes. In this case, some suction grippers  14  may be located on gripper surface  13  so as to grip the outer regions of a large workpiece  15  that are beyond the edge of a smaller workpiece  15 . When a small workpiece  15  is gripped, these outer suction grippers  14  may remain uncovered. However, since these outer suction grippers  14  (as well as the other suction grippers  14 ) are provided with flow restrictors, the inner suction grippers  14  that are covered by the smaller workpiece  15  may be gripped by the covered inner suction grippers  14 . 
     Workpiece gripper  10  may be connected to a manipulation mechanism via gripper base  20 . For example, a manipulation mechanism may include a mechanism that may be operated to translate, rotate, or tilt a workpiece gripper  10  that is connected to the mechanism. Suction connector  18  may be connected to a suction source. For example, a suction source may include a vacuum pump, a blower, or other suction mechanism. Suction connector  18  may be located on gripper base  20 , as shown, or may be located elsewhere on workpiece gripper  10 . 
     In some examples, workpiece gripper  10  may include two or more suction connectors  18 . For example, each gripper arm  12  of workpiece gripper  10  may be connected to a single or separate suction sources via a separate suction connector  18 . Other arrangements are possible. 
     Typically, each suction opening  16  is connected via conduits, e.g., that are interior to gripper body  11 , to suction connector  18 . One or more flow restrictors are located between each suction opening  16  and a suction source that is connected to suction connector  18 . 
     In another example, gripper surface  13  may form a single contiguous, unbranched surface, and gripper body  11  may include a single unbranched body. 
       FIG. 2  is a schematic top view of a variant of the workpiece gripper shown in  FIG. 1  with a contiguous gripper surface. 
     Gripper body  11  of workpiece gripper  40  is in the form of a contiguous body with no branching arms. Equivalently, gripper body  11  of workpiece gripper  40  may be considered to consist of a single arm. Similarly, gripper surface  13  is in the form of a contiguous surface (such that any two suction grippers  14  on gripper surface  13  may be connected by a line segment whose entire length extends along gripper surface  13  without crossing any intervening spaces). In some examples, gripper body  11  may have two similar or different gripper surfaces  13  on opposite faces of gripper body  11 . 
     In the example shown, suction grippers  14  are arranged in two rows, e.g., each row corresponding to a location along an internal suction channel. In other examples, suction grippers  14  may be otherwise arranged on gripper surface  13 . 
     Arrangement of suction grippers  14  along a contiguous gripper surface  13  may allow increased flexibility in gripping workpieces  15  of different sizes, e.g., by eliminating the possibility of a substantial part, or the whole, of a small workpiece  15  being located in a space between different arms of gripper surface  13 . As described above, the flow restrictors that limit inflow through each suction gripper  14  may limit inflow through any suction grippers  14   a  that are not covered by workpiece  15 , or through and suction grippers  14   b  that are partially covered by workpiece  15 . 
       FIG. 3  is a schematic cross section through a suction gripper of the workpiece gripper shown in  FIG. 1 . 
     In the example shown, suction gripper  14  includes a single suction opening  16 . In other examples, a suction gripper may include more than one suction opening. 
     Suction may be applied to suction opening  16  via a suction channel  26  that may extend internally to gripper body  11 , e.g., along a gripper arm  12  along which suction gripper  14  is located. For example, a proximal end of suction channel  26  may be in fluidic connection with suction connector  18  of workpiece gripper  10 . Typically, a single suction channel  26  may be connected to more than one suction opening  16 . For example, all suction openings  16  along a single gripper arm  12  may open to a single suction channel  26 . 
     In the example shown, suction opening  16  is connected to suction channel  26  (e.g., via an opening that is out of the plane of the section shown in  FIG. 3 ) via flow restrictor  24 . In the example shown, flow restrictor  24  is in the form of a labyrinthine channel within which flow is deflected by a plurality of baffles  28 . Alternatively or in addition, a flow restrictor  24  may have another form, e.g., including a SASO structure, or another arrangement of one or more baffles, fins, constrictions, curves, twists, bends, or other structure to restrict the rate of inflow of air or another fluid via suction opening  16  and flow restrictor  24 . Flow restrictor  24  may include an adjustable or nonadjustable needle valve or other constriction. 
     In the example shown, flow restrictor  24  is located between suction opening  16  and suction channel  26 . Alternatively or in addition, a flow restrictor  24  may be located within suction channel  26  or suction connector  18 , between suction channel  26  and suction connector  18 , or elsewhere in a fluidic path between suction opening  16  and suction connector  18  (e.g., at a point where a suction channel  26  branches into two or more branch channels, each connecting to a different suction opening  16 ). For example, a single flow restrictor  24  may serve to restrict the inflow via two or more suction openings  16 . Flow restrictor  24  may be located within gripper base  20 , or in a conduit between a suction source and suction connector  18 . 
     In the example shown, suction opening  16  is surrounded by elastic sealing structure in the form of elastic cup  22 . When a surface of workpiece  15  is placed in the vicinity of suction opening  16  and suction is applied to suction opening  16 , workpiece  15  may be drawn toward suction opening  16  and gripper surface  13 . When workpiece  15  contacts rim  23  of elastic cup  22 , the contact between workpiece  15  and rim  23  of elastic cup  22  may form a seal. Thus, the suction may create a partial vacuum within elastic cup  22  such that ambient air pressure pushes workpiece  15  toward suction opening  16 . 
     Further drawing of workpiece  15  toward suction opening  16  may press elastic cup  22  toward gripper surface  13 . The flexibility and elasticity of elastic cup  22  may enable elastic cup  22  to flatten between workpiece  15  and gripper surface  13 . The flattening of elastic cup  22  may increase the area of contact between elastic cup  22  and workpiece  15 . The increase in contact area may increase friction forces between workpiece  15  and the flattened elastic cup  22 , and thus between workpiece  15  and suction gripper  14 . 
     Alternative or in addition, the elastic sealing structure may have another form. For example, the elastic sealing structure may have the form of a pad, gasket, distortable nipple, or other structure that may form a seal between suction opening  16  and workpiece  15 , and facilitate friction forces between workpiece  15  and suction gripper  14 . 
     In some cases, e.g., when the surface of workpiece  15  is warped, one or more suction grippers  14  may not draw the surface of workpiece  15  sufficiently toward suction opening  16  of that suction gripper  14  in order to cover a suction opening  16  and form an airtight seal between workpiece  15  and that suction opening  16 . The presence of flow restrictor  24  in the inflow path between suction opening  16  and suction connector  18  may restrict the rate of inflow through the uncovered suction opening  16 . Since the rate of inflow through all uncovered suction openings  16 , the suction that is applied to the covered and sealed suction openings  16  (assuming that workpiece  15  surface contacts at least some, preferably at least three, of suction openings  16 ) may remain sufficient to firmly grip workpiece  15  and allow reliable and precise manipulation of workpiece  15 . (In the absence of flow restrictors, all of the suction that is applied via suction connector  18  could cause increased inflow through uncovered suction openings, breaking or preventing any application of suction via the covered suction openings.) 
     In the example shown, each suction gripper  14  and its elastic cup  22  (or similar elastic sealing structure) extends a fixed distance out of the surface of gripper arm  12  that faces workpiece  15 . The number and placement of suction grippers  14  on each gripper arm  12 , as well as the number and shapes of gripper arms  22 , may be designed to ensure that a minimum number of suction grippers  14  are able to grip the surface of a even a warped workpiece (e.g., within a range of warpage that does not render workpiece  15  unusable). For example, in some cases gripping of a workpiece by two or more suction grippers  14  may be sufficient for workpiece gripper  10  to firmly grasp a workpiece. In some cases, gripping of a workpiece by three or more suction grippers  14  may be considered sufficient to stably and reliable grip a workpiece. 
       FIG. 4A  schematically illustrates a variant of the suction gripper shown in  FIG. 3 .  FIG. 4B  is a schematic section of the suction gripper shown in  FIG. 4A . 
     Suction gripper  41  is constructed similarly to suction gripper  14  ( FIG. 3 ), including a suction opening  16  surrounded by elastic cup  22 . For example, elastic cup  22  may be formed out of a flexible and resilient polymer, such as silicone, or another elastic material. 
     In one example, suction gripper  41  may be held in a circular opening on gripper surface  13 . A region of gripper body  11  below the circular opening may include suction channel  26 . For example, the circular opening may open into suction channel  26 . The diameter of the circular opening may be less than the diameter of lip  46  but greater than the diameter of groove  48  between lip  46  and elastic cup  22 . Lip  46  may also be constructed of an elastic material. For example, lip  46  and elastic cup  22  may be manufactured, e.g., molded, as a single piece. 
     For example, in order to attach suction gripper  41  to a circular opening in gripper surface  13 , stem  42  of suction gripper  41  may be inserted into the opening until lip  46  passes through the opening (e.g., being bent or otherwise distorted during the insertion). Once lip  46  passes through the circular opening, the resilience of lip  46  may open lip  46  so as to prevent removal of suction gripper  41  from the circular opening. For example, a proximal outer surface of lip  46  may taper outward and distally toward elastic cup  22 , so as to facilitate bending or folding of lip  46  to enable insertion of lip  46  through the circular opening. The tapering may impede bending of lip  46  proximally toward stem  42 , thus impeding withdrawal of lip  46  from the circular opening. 
     In the example shown, stem  42  may be in the form of a circular cylinder that is laterally truncated to form a flat lateral face  44 . Flow restrictor  24  connects suction opening  16  at a distal end of suction gripper  41  to a region between lateral face  44  of stem  42  and cylindrical inner surface  50  of an outer piece that includes elastic cup  22  and lip  46 . Thus, cylindrical inner surface  50  surrounds at least part of the length of stem  42 . The region between lateral face  44  and cylindrical inner surface  50  forms a fluidic connection between suction opening  16  and a proximal (e.g., to lip  46 ) end of stem  42 . In the example shown, baffles  28  of flow restrictor  24  are located in the space between lateral face  42  and cylindrical inner surface  50 . 
     In the example when stem  42  is inserted into a circular opening that is open to suction conduit  26 , inflow may be blocked by a snug interface between cylindrical inner surface  50  and the cylindrical outer surface of stem  42 , except along lateral face  44 . The space between lateral face  44  and cylindrical inner surface  50  forms a fluidic connection between suction opening  16  and suction conduit  26 . The fluidic connection between suction opening  16  and suction conduit  26  includes structure (baffles  28 , in the example shown, or other structure) to form flow restrictor  24 . Thus, inflow via suction gripper  41  may be limited, even when not in contact with a surface of a workpiece  15 . 
     Other configurations of a suction gripper are possible. 
     Alternatively or in addition, one or more of the suction grippers of workpiece gripper  10  may be extendible by an adjustable length out of gripper arm  12  toward workpiece  15 . When a warped workpiece is gripped by a plurality of the extendible suction grippers and drawn toward gripper arm  12 , some of the extendible suction grippers may extend outward from gripper arm  12  so as to maintain the grip while compensating for any warpage. 
       FIG. 5  is a schematic side view of a variant of a suction gripper in which the suction gripper is extendible from the workpiece gripper. 
     Extendible suction gripper  30  includes extension mechanism  32  to extend the distal end of extendible suction gripper  30  (on which elastic cup  22  is located, in the example shown) by a variable distance from gripper arm  12 . Extension and retraction or compression of extension mechanism  32  to change the variable distance is represented by arrow  34 . 
     In the example shown, extendible suction gripper  30  is constructed similarly to suction gripper  14 , as shown in  FIG. 3 . For example, extendible suction gripper  30  includes elastic sealing structure (in the form of elastic cup  22 , as shown, or a pad, gasket, nipple, or other elastic sealing structure) that surrounds a suction opening  16  (not visible in the side view of  FIG. 5 ). Suction opening  16  is provided with one or more flow restrictors  24  between suction opening  16  and a connection to a suction source (e.g., suction connector  18  or similar structure) that applies the suction to suction opening  16  and extendible suction gripper  30 . For example, flow restrictor  24  may be located within extendible suction gripper  30 , gripper arm  12 , gripper base  20 , or elsewhere within workpiece gripper  10 . 
     In one example of extension mechanism  32 , extension mechanism  32  includes a mechanism that is configured to fully extend extendible suction gripper  30  when extendible suction gripper  30  does not grip a workpiece. Thus, until an extendible suction gripper  30  grips a part of the surface of a workpiece, extendible suction gripper  30  is normally extended. 
     In one example, extension mechanism  32  may include one or more elastic mechanical or pneumatic elements that are fully extended when not compressed. For example, an elastic element may include one or more compressible springs or bars that are fully extended when not compressed. The compressible elements may extend within extension mechanism  32  from a point near the distal end of extension mechanism  32  to a proximal point, e.g., within gripper arm  12 . When the element is compressed, part of extension mechanism  32  may be pushed into gripper arm  12 , or telescoping or foldable walls of extension mechanism  32  may shorten. 
     As another example, outer walls of extension mechanism  32  may include resilient accordion folds or are otherwise constructed to enable a compression force to crumple or crush the walls of extension mechanism  32 , while enabling the walls to recover their uncompressed extended shape when the compressing force is removed. 
     A pneumatic element may include a sealed tube with two or more telescoping segments that enable the tube to shorten when subjected to a compression force. For example, the sealed tube may extend within extension mechanism  32  from a point near the distal end of extension mechanism  32  to a proximal point, e.g., within gripper arm  12 . Alternatively or in addition, extension mechanism  32  may include a chamber into which pressurize air or another fluid may be introduced. For example, air may be blown into the chamber from an exhaust mechanism of a suction source that applies suction to suction openings  16 . 
     With a normally extended extendible suction gripper  30 , the surface of a workpiece may be made (e.g., by a robotic or other automatic mechanism, or by a manually controlled mechanism, or otherwise) to approach gripper arms  12  of a workpiece gripper  10  that includes one or more extended extendible suction grippers  30 . Typically, suction is applied to the suction opening  16  of each extendible suction gripper  30  (as well as the suction opening  16  of any suction gripper  14  that is not extendible) of workpiece gripper  10  as workpiece  15  approaches gripper arms  12 . 
     Typically, when a region of the surface of workpiece  15  is brought sufficiently near the distal end of one or more extendible suction grippers  30 , one or more regions of the surface of workpiece  15  may initially contact part of the elastic sealing structure (e.g., part of rim  23  of elastic cup  22 ) at the distal end of an extended extendible suction gripper  30 . The suction that is applied to that extendible suction gripper  30  may cause the elastic sealing structure and the surface of workpiece  15  to draw together. Thus, the elastic sealing structure may form a seal between workpiece  15  and that extendible suction gripper  30 . For example, the entire rim of an elastic sealing structure in the form of elastic cup  22  may be drawn toward and into physical contact with the surface of workpiece  15 . Thus, the interior of elastic cup  22  (or other elastic sealing structure) may form a chamber whose only opening is suction opening  16 . 
     In this case, continued application of suction to suction opening  16  may draw fluid out of the chamber formed between the surface of workpiece  15  and elastic cup  22  (or other elastic sealing structure). As a result, the suction force may overcome the distally pushing force of extension mechanism  32  and draw the distal end of extendible suction gripper  30 , with the attached surface of workpiece  15 , proximally toward gripper arm  12 . The proximal inward motion of the surface of workpiece  15  may increase the likelihood that the surface of workpiece  15  will be similarly gripped by other extendible suction grippers  30  or suction grippers  14 . 
     The presence of flow restrictors  24  may limit the inflow through any extendible suction gripper  30  or suction gripper  14  that is not in contact with, and thus not sealed by, the surface of workpiece  15 . The limited inflow may thus ensure that the suction force that is applied by any extendible suction gripper  30  or suction gripper  14  that is in contact with the surface of workpiece  15  is able to maintain the contact. 
     Alternatively or in addition, extension mechanism  32  may include a controllable motor (e.g., an electrically powered motor with a transmission that converts circular motion of the motor to linear motion of extension mechanism  32 ) or actuator (e.g., a linear actuator, e.g., electrically operated) that is operable, e.g., by a controller of workpiece gripper  10 , to extend or retract the distal end of extendible suction gripper  30 . For example, gripper arm  12 , extendible suction gripper  30 , or workpiece gripper  10  may be provided with a sensor that is configured to sense the proximity of a section of the surface of workpiece  15  that is near an extendible suction gripper  30 . When the proximity is detected, extension mechanism  32  may be operated to extend extendible suction gripper  30  toward workpiece  15 . When contact is sensed (e.g., by a sensor that detects a decrease in inflow via suction opening  16  of that extendible suction gripper  30 ), extension mechanism  32  may be operated to pull the surface of workpiece  15  toward gripper arm  12 . 
     Other types of mechanisms are possible for operation of extension mechanism  32 . Extension mechanism  32  may facilitate gripping of a workpiece by workpiece gripper  10  by enabling positioning of the distal end of extendible suction gripper  30  sufficiently close to the surface of workpiece  15  to grip the surface, despite the restricted inflow through each suction opening  16  as limited by flow restrictor  24 . 
     Different embodiments are disclosed herein. Features of certain embodiments may be combined with features of other embodiments; thus certain embodiments may be combinations of features of multiple embodiments. The foregoing description of the embodiments of the invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. It should be appreciated by persons skilled in the art that many modifications, variations, substitutions, changes, and equivalents are possible in light of the above teaching. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention. 
     While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.